U.S. patent application number 12/709493 was filed with the patent office on 2010-06-17 for method of authenticating printer consumable.
This patent application is currently assigned to Silverbrook Research Pty Ltd. Invention is credited to Lapstun Paul, Simon Robert Walmsley.
Application Number | 20100153729 12/709493 |
Document ID | / |
Family ID | 24012546 |
Filed Date | 2010-06-17 |
United States Patent
Application |
20100153729 |
Kind Code |
A1 |
Walmsley; Simon Robert ; et
al. |
June 17, 2010 |
METHOD OF AUTHENTICATING PRINTER CONSUMABLE
Abstract
A method for authenticating a printer consumable in which an
encrypted random number and its first signature are passed from a
printer authentication chip to a consumable authentication chip, in
the consumable chip: the encrypted random number and first
signature are decrypted; a second signature of the random number is
calculated and compared with the first signature to produce a match
at which a first number produced by encrypting the random number
and a memory vector is passed to the printer chip, and in the
printer chip, a second number is produced by encrypting the random
number and memory vector and compared with the first number to
produce a match and valid consumable chip, or a mismatch and
invalid consumable chip. The memory vector comprises updatable
consumable state data whose manner of updating is protected by
requiring clearing of the memory vector when change of the updating
manner is attempted.
Inventors: |
Walmsley; Simon Robert;
(Balmain, AU) ; Paul; Lapstun; (Balmain,
AU) |
Correspondence
Address: |
SILVERBROOK RESEARCH PTY LTD
393 DARLING STREET
BALMAIN
2041
AU
|
Assignee: |
Silverbrook Research Pty
Ltd
|
Family ID: |
24012546 |
Appl. No.: |
12/709493 |
Filed: |
February 21, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10636283 |
Aug 8, 2003 |
7685424 |
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12709493 |
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09505951 |
Feb 15, 2000 |
7685423 |
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10636283 |
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Current U.S.
Class: |
713/172 ;
713/168; 713/169; 713/176 |
Current CPC
Class: |
G07F 7/1008 20130101;
H04L 9/3271 20130101; G06Q 20/34 20130101; H04L 9/0662 20130101;
G06Q 20/341 20130101; G06F 21/44 20130101; G06Q 20/40 20130101;
G06Q 20/40975 20130101; H04L 9/3247 20130101; H04L 9/002
20130101 |
Class at
Publication: |
713/172 ;
713/168; 713/169; 713/176 |
International
Class: |
H04L 9/32 20060101
H04L009/32; H04L 9/00 20060101 H04L009/00; G03G 15/00 20060101
G03G015/00 |
Claims
1. A method for authenticating a printer consumable comprising the
steps of: passing, from an authentication chip of the printer to an
authentication chip of the consumable, an encrypted random number
and first signature of the random number; decrypting, in the
consumable chip, the encrypted random number and first signature;
calculating, in the consumable chip, a second signature of the
decrypted random number; comparing, in the consumable chip, the
first and second signatures and if the compared signatures match,
passing, from the consumable chip to the printer chip, a first
number produced by encrypting the random number and a memory vector
of the consumable chip, the memory vector being comprised of
variables holding updatable consumable state data of the
consumable, the manner of updating the updatable consumable state
data being protected by requiring clearing of the memory vector
when access to change the updating manner is attempted; encrypting,
in the printer chip, the random number and memory vector to produce
a second number; comparing, in the printer chip, the first and
second numbers and if the comparison returns a match, considering
the consumable chip valid, and if the comparison returns a
mismatch, considering the consumable chip invalid.
2. A method according to claim 1, where the first and second
signatures are digital signatures of 160 bits.
3. A method according to claim 1, where the time taken to return an
indication the consumable chip is invalid is the same for all bad
inputs, and the time taken to return an indication the consumable
chip is valid is the same for all good inputs.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a Continuation Application of U.S. Ser.
No. 10/636,283 filed on Aug. 8, 2003, which is a Continuation
Application of U.S. Ser. No. 09/505,951, filed on Feb. 15, 2000,
all of which are herein incorporated by reference.
TECHNICAL FIELD
[0002] This invention concerns a validation protocol for
determining whether an untrusted authentication chip is valid, or
not. In another aspect it concerns a validation system for the
protocol. The protocol may be used to determine the physical
presence of a valid authentication chip. The untrusted chip may be
associated with a consumable so that validation of the untrusted
chip authenticates the consumable.
BACKGROUND ART
[0003] 1 Introduction
[0004] Manufacturers of systems that require consumables, such as a
laser printer that requires toner cartridges, have struggled with
the problem of authenticating consumables, to varying levels of
success. Most have resorted to specialized packaging. However this
does not stop home refill operations or clone manufacture. The
prevention of copying is important for two reasons: [0005] To
protect revenues [0006] To prevent poorly manufactured substitute
consumables from damaging the base system. For example, poorly
filtered ink may clog print nozzles in an ink jet printer.
[0007] 2 Scope
[0008] Authentication is an extremely large and constantly growing
field. This invention is concerned with authenticating consumables.
In most cases, there is no reason to prohibit the use of
consumables in a third party product.
[0009] The invention concerns an authentication chip that contains
an authentication code and circuit specially designed to prevent
copying. The chip is manufactured using the standard Flash memory
manufacturing process, and is low cost enough to be included in
consumables such as ink and toner cartridges.
[0010] Once programmed, the authentication chips are compliant with
the NSA export guidelines since they do not constitute an
encryption device. They can therefore be practically manufactured
in the USA (and exported) or anywhere else in the world.
[0011] 3 Concepts and Terms
[0012] This part discusses terms and concepts that are referred to
throughout the remainder of the document.
[0013] 3.1 Symbolic Nomenclature
[0014] The following symbolic nomenclature is used throughout this
document:
TABLE-US-00001 TABLE 1 Summary of Symbolic Nomenclature Symbol
Description F[X] Function F, taking a single parameter X F[X, Y]
Function F, taking two parameters, X and Y X | Y X concatenated
with Y X Y Bitwise X AND Y X Y Bitwise X OR Y (inclusive-OR) X
.sym. Y Bitwise X XOR Y (exclusive-OR) X Bitwise NOT X (complement)
X .rarw. Y X is assigned the value Y X .rarw. {Y, Z} The domain of
assignment inputs to X is Y and Z X = Y X is equal to Y X .noteq. Y
X is not equal to Y X Decrement X by 1 (floor 0) X Increment X by 1
(modulo register length) Erase X Erase Flash memory register X
SetBits[X, Y] Set the bits of the Flash memory register X based on
Y Z .rarw. ShiftRight[X, Y] Shift register X right one bit
position, taking input bit from Y and placing the output bit in
Z
[0015] 3.2 Basic Terms
[0016] A message, denoted by M, is plaintext. The process of
transforming M into ciphertext C, where the substance of M is
hidden, is called encryption. The process of transforming C back
into M is called decryption. Referring to the encryption function
as E, and the decryption function as D, we have the following
identities:
E[M]=C
D[C]=M
[0017] Therefore the following identity is true: D[E[M]]=M
[0018] 3.3 Symmetric Cryptography
[0019] A symmetric encryption algorithm is one where: [0020] the
encryption function E relies on key K.sub.1, [0021] the decryption
function D relies on key K.sub.2, [0022] K.sub.2 can be derived
from K.sub.1, and [0023] K.sub.1 can be derived from K.sub.2.
[0024] In most symmetric algorithms, K.sub.1 equals K.sub.2.
However, even if K.sub.1 does not equal
[0025] K.sub.2, given that one key can be derived from the other, a
single key K can suffice for the mathematical definition. Thus:
E.sub.K[M]=C
D.sub.K[C]=M
[0026] The security of these algorithms rests very much in the key
K. Knowledge of K allows anyone to encrypt or decrypt. Consequently
K must remain a secret for the duration of the value of M. For
example, M may be a wartime message "My current position is grid
position 123-456". Once the war is over the value of M is greatly
reduced, and if K is made public, the knowledge of the combat
unit's position may be of no relevance whatsoever. Of course if it
is politically sensitive for the combat unit's position to be known
even after the war, K may have to remain secret for a very long
time.
[0027] An enormous variety of symmetric algorithms exist, from the
textbooks of ancient history through to sophisticated modern
algorithms. Many of these are insecure, in that modern
cryptanalysis techniques (see Section 3.8) can successfully attack
the algorithm to the extent that K can be derived.
[0028] The security of the particular symmetric algorithm is a
function of two things: the strength of the algorithm and the
length of the key [78].
[0029] The strength of an algorithm is difficult to quantify,
relying on its resistance to cryptographic attacks (see Section
3.8). In addition, the longer that an algorithm has remained in the
public eye, and yet remained unbroken in the midst of intense
scrutiny, the more secure the algorithm is likely to be. By
contrast, a secret algorithm that has not been scrutinized by
cryptographic experts is unlikely to be secure.
[0030] Even if the algorithm is "perfectly" strong (the only way to
break it is to try every key--see Section 3.8.1.5), eventually the
right key will be found. However, the more keys there are, the more
keys have to be tried. If there are N keys, it will take a maximum
of N tries. If the key is N bits long, it will take a maximum of
2.sup.N tries, with a 50% chance of finding the key after only half
the attempts (2.sup.N-1). The longer N becomes, the longer it will
take to find the key, and hence the more secure it is. What makes a
good key length depends on the value of the secret and the time for
which the secret must remain secret as well as available computing
resources.
[0031] In 1996, an ad hoc group of world-renowned cryptographers
and computer scientists released a report [9] describing minimal
key lengths for symmetric ciphers to provide adequate commercial
security. They suggest an absolute minimum key length of 90 bits in
order to protect data for 20 years, and stress that increasingly,
as cryptosystems succumb to smarter attacks than brute-force key
search, even more bits may be required to account for future
surprises in cryptanalysis techniques.
[0032] We will ignore most historical symmetric algorithms on the
grounds that they are insecure, especially given modern computing
technology. Instead, we will discuss the following algorithms:
[0033] DES [0034] Blowfish [0035] IDEA
[0036] 3.3.1 DES
[0037] DES (Data Encryption Standard) [26] is a US and
international standard, where the same key is used to encrypt and
decrypt. The key length is 56 bits. It has been implemented in
hardware and software, although the original design was for
hardware only. The original algorithm used in DES was patented in
1976 (U.S. Pat. No. 3,962,539) and has since expired.
[0038] During the design of DES, the NSA (National Security Agency)
provided secret S-boxes to perform the key-dependent nonlinear
transformations of the data block. After differential cryptanalysis
was discovered outside the NSA, it was revealed that the DES
S-boxes were specifically designed to be resistant to differential
cryptanalysis.
[0039] As described in [92], using 1993 technology, a 56-bit DES
key can be recovered by a custom-designed $1 million machine
performing a brute force attack in only 35 minutes. For $10
million, the key can be recovered in only 3.5 minutes. DES is
clearly not secure now, and will become less so in the future.
[0040] A variant of DES, called triple-DES is more secure, but
requires 3 keys: K.sub.i, K.sub.2, and K.sub.3. The keys are used
in the following manner:
E.sub.K3[D.sub.K2[E.sub.K1[M]]]=C
D.sub.K3[E.sub.K2[D.sub.K1[C]]]=M
[0041] The main advantage of triple-DES is that existing DES
implementations can be used to give more security than single key
DES. Specifically, triple-DES gives protection of equivalent key
length of 112 bits [78]. Triple-DES does not give the equivalent
protection of a 168-bit key (3.times.56) as one might naively
expect.
[0042] Equipment that performs triple-DES decoding and/or encoding
cannot be exported from the United States.
[0043] 3.3.2 Blowfish
[0044] Blowfish is a symmetric block cipher first presented by
Schneier in 1994[76]. It takes a variable length key, from 32 bits
to 448 bits, is unpatented, and is both license and royalty free.
In addition, it is much faster than DES.
[0045] The Blowfish algorithm consists of two parts: a
key-expansion part and a data-encryption part. Key expansion
converts a key of at most 448 bits into several subkey arrays
totaling 4168 bytes. Data encryption occurs via a 16-round Feistel
network. All operations are XORs and additions on 32-bit words,
with four index array lookups per round.
[0046] It should be noted that decryption is the same as encryption
except that the subkey arrays are used in the reverse order.
Complexity of implementation is therefore reduced compared to other
algorithms that do not have such symmetry.
[0047] [77] describes the published attacks which have been mounted
on Blowfish, although the algorithm remains secure as of February
1998 [79]. The major finding with these attacks has been the
discovery of certain weak keys. These weak keys can be tested for
during key generation. For more information, refer to [77] and
[79].
[0048] 3.3.3 RC5
[0049] Designed by Ron Rivest in 1995, RC5 [74] has a variable
block size, key size, and number of rounds. Typically, however, it
uses a 64-bit block size and a 128-bit key.
[0050] The RC5 algorithm consists of two parts: a key-expansion
part and a data-encryption part. Key expansion converts a key into
2r+2 subkeys (where r=the number of rounds), each subkey being w
bits. For a 64-bit blocksize with 16 rounds (w=32, r=16), the
subkey arrays total 136 bytes. Data encryption uses addition mod
2w, XOR and bitwise rotation.
[0051] An initial examination by Kaliski and Yin [43] suggested
that standard linear and differential cryptanalysis appeared
impractical for the 64-bit blocksize version of the algorithm.
Their differential attacks on 9 and 12 round RC5 require 2.sup.45
and 2.sup.62 chosen plaintexts respectively, while the linear
attacks on 4, 5, and 6 round RC5 requires 2.sup.37, 2.sup.47 and
2.sup.57 known plaintexts). These two attacks are independent of
key size.
[0052] More recently however, Knudsen and Meier [47] described a
new type of differential attack on RC5 that improved the earlier
results by a factor of 128, showing that RC5 has certain weak
keys.
[0053] RC5 is protected by multiple patents owned by RSA
Laboratories. A license must be obtained to use it.
[0054] 3.3.4 IDEA
[0055] Developed in 1990 by Lai and Massey [53], the first
incarnation of the IDEA cipher was called PES. After differential
cryptanalysis was discovered by Biham and Shamir in 1991, the
algorithm was strengthened, with the result being published in 1992
as IDEA [52].
[0056] IDEA uses 128-bit keys to operate on 64-bit plaintext
blocks. The same algorithm is used for encryption and decryption.
It is generally regarded as the most secure block algorithm
available today [78][56].
[0057] The biggest drawback of IDEA is the fact that it is patented
(U.S. Pat. No. 5,214,703, issued in 1993), and a license must be
obtained from Ascom Tech AG (Bern) to use it.
[0058] 3.4 Asymmetric Cryptography
[0059] An asymmetric encryption algorithm is one where: [0060] the
encryption function E relies on key K.sub.1, [0061] the decryption
function D relies on key K.sub.2, [0062] K2 cannot be derived from
K.sub.1 in a reasonable amount of time, and [0063] K1 cannot be
derived from K.sub.2 in a reasonable amount of time.
[0064] Thus: E.sub.K1[M]=C [0065] D.sub.K2[C]=M
[0066] These algorithms are also called public-key because one key
K.sub.1 can be made public. Thus anyone can encrypt a message
(using K.sub.1) but only the person with the corresponding
decryption key (K.sub.2) can decrypt and thus read the message.
[0067] In most cases, the following identity also holds:
E.sub.K2[M]=C
D.sub.K1[C]=M
[0068] This identity is very important because it implies that
anyone with the public key K.sub.1 can see M and know that it came
from the owner of K.sub.2. No-one else could have generated C
because to do so would imply knowledge of K.sub.2. This gives rise
to a different application, unrelated to encryption--digital
signatures.
[0069] The property of not being able to derive K.sub.1 from
K.sub.2 and vice versa in a reasonable time is of course clouded by
the concept of reasonable time. What has been demonstrated time
after time, is that a calculation that was thought to require a
long time has been made possible by the introduction of faster
computers, new algorithms etc. The security of asymmetric
algorithms is based on the difficulty of one of two problems:
factoring large numbers (more specifically large numbers that are
the product of two large primes), and the difficulty of calculating
discrete logarithms in a finite field. Factoring large numbers is
conjectured to be a hard problem given today's understanding of
mathematics. The problem however, is that factoring is getting
easier much faster than anticipated. Ron Rivest in 1977 said that
factoring a 125-digit number would take 40 quadrillion years [30].
In 1994 a 129-digit number was factored [3]. According to Schneier,
you need a 1024-bit number to get the level of security today that
you got from a 512-bit number in the 1980s [78]. If the key is to
last for some years then 1024 bits may not even be enough. Rivest
revised his key length estimates in 1990: he suggests 1628 bits for
high security lasting until 2005, and 1884 bits for high security
lasting until 2015[69]. Schneier suggests 2048 bits are required in
order to protect against corporations and governments until
2015[80].
[0070] Public key cryptography was invented in 1976 by Diffie and
Hellman [15][16], and independently by Merkle [57]. Although
Diffie, Hellman and Merkle patented the concepts (U.S. Pat. Nos.
4,200,770 and 4,218,582), these patents expired in 1997.
[0071] A number of public key cryptographic algorithms exist. Most
are impractical to implement, and many generate a very large C for
a given M or require enormous keys. Still others, while secure, are
far too slow to be practical for several years. Because of this,
many public key systems are hybrid--a public key mechanism is used
to transmit a symmetric session key, and then the session key is
used for the actual messages.
[0072] All of the algorithms have a problem in terms of key
selection. A random number is simply not secure enough. The two
large primes p and q must be chosen carefully--there are certain
weak combinations that can be factored more easily (some of the
weak keys can be tested for). But nonetheless, key selection is not
a simple matter of randomly selecting 1024 bits for example.
Consequently the key selection process must also be secure.
[0073] Of the practical algorithms in use under public scrutiny,
the following are discussed: [0074] RSA [0075] DSA [0076]
ElGamal
[0077] 3.4.1 RSA
[0078] The RSA cryptosystem [75], named after Rivest, Shamir, and
Adleman, is the most widely used public key cryptosystem, and is a
de facto standard in much of the world [78].
[0079] The security of RSA depends on the conjectured difficulty of
factoring large numbers that are the product of two primes (p and
q). There are a number of restrictions on the generation of p and
q. They should both be large, with a similar number of bits, yet
not be close to one another (otherwise p=q= pq). In addition, many
authors have suggested that p and q should be strong primes [56].
The Hellman-Bach patent (U.S. Pat. No. 4,633,036) covers a method
for generating strong RSA primes p and q such that n=pq and
factoring n is believed to be computationally infeasible.
[0080] The RSA algorithm patent was issued in 1983 (U.S. Pat. No.
4,405,829). The patent expires on Sep. 20, 2000.
[0081] 3.4.2 DSA
[0082] DSA (Digital Signature Algorithm) is an algorithm designed
as part of the Digital Signature Standard (DSS) [29]. As defined,
it cannot be used for generalized encryption. In addition, compared
to RSA, DSA is 10 to 40 times slower for signature verification
[40]. DSA explicitly uses the SHA-1 hashing algorithm (see Section
3.6.3.3).
[0083] DSA key generation relies on finding two primes p and q such
that q divides p-1. According to Schneier [78], a 1024-bit p value
is required for long term DSA security. However the DSA standard
[29] does not permit values of p larger than 1024 bits (p must also
be a multiple of 64 bits).
[0084] The US Government owns the DSA algorithm and has at least
one relevant patent (U.S. Pat. No. 5,231,688 granted in 1993).
However, according to NIST [61]: [0085] "The DSA patent and any
foreign counterparts that may issue are available for use without
any written permission from or any payment of royalties to the U.S.
government."
[0086] In a much stronger declaration, NIST states in the same
document [61] that DSA does not infringe third party's rights:
[0087] "NIST reviewed all of the asserted patents and concluded
that none of them would be infringed by DSS. Extra protection will
be written into the PK1 pilot project that will prevent an
organization or individual from suing anyone except the government
for patent infringement during the course of the project."
[0088] It must however, be noted that the Schnorr authentication
algorithm [81] (U.S. Pat. No. 4,995,082) patent holder claims that
DSA infringes his patent. The Schnorr patent is not due to expire
until 2008.
[0089] 3.4.3 ElGamal
[0090] The ElGamal scheme [22][23] is used for both encryption and
digital signatures. The security is based on the conjectured
difficulty of calculating discrete logarithms in a finite
field.
[0091] Key selection involves the selection of a prime p, and two
random numbers g and x such that both g and x are less than p. Then
calculate y=gx mod p. The public key is y, g, and p. The private
key is x.
[0092] ElGamal is unpatented. Although it uses the patented
Diffie-Hellman public key algorithm [15][16], those patents expired
in 1997. ElGamal public key encryption and digital signatures can
now be safely used without infringing third party patents.
[0093] 3.5 Cryptographic Challenge-Response Protocols and Zero
Knowledge Proofs
[0094] The general principle of a challenge-response protocol is to
provide identity authentication. The simplest form of
challenge-response takes the form of a secret password. A asks B
for the secret password, and if B responds with the correct
password, A declares B authentic.
[0095] There are three main problems with this kind of simplistic
protocol. Firstly, once B has responded with the password, any
observer C will know what the password is. Secondly, A must know
the password in order to verify it. Thirdly, if C impersonates A,
then B will give the password to C (thinking C was A), thus
compromising the password.
[0096] Using a copyright text (such as a haiku) as the password is
not sufficient, because we are assuming that anyone is able to copy
the password (for example in a country where intellectual property
is not respected).
[0097] The idea of cryptographic challenge-response protocols is
that one entity (the claimant) proves its identity to another (the
verifier) by demonstrating knowledge of a secret known to be
associated with that entity, without revealing the secret itself to
the verifier during the protocol [56]. In the generalized case of
cryptographic challenge-response protocols, with some schemes the
verifier knows the secret, while in others the secret is not even
known by the verifier. A good overview of these protocols can be
found in [25], [78], and [56].
[0098] Since this document specifically concerns Authentication,
the actual cryptographic challenge-response protocols used for
authentication are detailed in the appropriate sections. However
the concept of Zero Knowledge Proofs bears mentioning here.
[0099] The Zero Knowledge Proof protocol, first described by Feige,
Fiat and Shamir in [24] is extensively used in Smart Cards for the
purpose of authentication [34][36][67]. The protocol's
effectiveness is based on the assumption that it is computationally
infeasible to compute square roots modulo a large composite integer
with unknown factorization. This is provably equivalent to the
assumption that factoring large integers is difficult.
[0100] It should be noted that there is no need for the claimant to
have significant computing power. Smart cards implement this kind
of authentication using only a few modulo multiplications
[34][36].
[0101] Finally, it should be noted that the Zero Knowledge Proof
protocol is patented [82] (U.S. Pat. No. 4,748,668, issued May 31,
1988).
[0102] 3.6 One-Way Functions
[0103] A one-way function F operates on an input X, and returns
F[X] such that X cannot be determined from F[X]. When there is no
restriction on the format of X, and F[X] contains fewer bits than
X, then collisions must exist. A collision is defined as two
different X input values producing the same F[X] value--i.e.
X.sub.i and X.sub.2 exist such that X.sub.1.noteq.X.sub.2 yet F
[X.sub.i]=F[X.sub.2].
[0104] When X contains more bits than F[X], the input must be
compressed in some way to create the output. In many cases, X is
broken into blocks of a particular size, and compressed over a
number of rounds, with the output of one round being the input to
the next. The output of the hash function is the last output once X
has been consumed. A pseudo-collision of the compression function
CF is defined as two different initial values V.sub.1 and V.sub.2
and two inputs X.sub.i and X.sub.2 (possibly identical) are given
such that CF(V.sub.1, X.sub.1)=CF(V.sub.2, X.sub.2). Note that the
existence of a pseudo-collision does not mean that it is easy to
compute an X.sub.2 for a given X.sub.1.
[0105] We are only interested in one-way functions that are fast to
compute. In addition, we are only interested in deterministic
one-way functions that are repeatable in different implementations.
Consider an example F where F[X] is the time between calls to F.
For a given F[X] X cannot be determined because X is not even used
by F. However the output from F will be different for different
implementations. This kind of F is therefore not of interest.
[0106] In the scope of this document, we are interested in the
following forms of one-way functions: [0107] Encryption using an
unknown key [0108] Random number sequences [0109] Hash Functions
[0110] Message Authentication Codes
[0111] 3.6.1 Encryption Using an Unknown Key
[0112] When a message is encrypted using an unknown key K, the
encryption function E is effectively one-way. Without the key K, it
is computationally infeasible to obtain M from EK[M]. An encryption
function is only one-way for as long as the key remains hidden.
[0113] An encryption algorithm does not create collisions, since E
creates EK[M] such that it is possible to reconstruct M using
function D. Consequently F[X] contains at least as many bits as X
(no information is lost) if the one-way function F is E.
[0114] Symmetric encryption algorithms (see Section 3.3) have the
advantage over asymmetric algorithms (see Section 3.4) for
producing one-way functions based on encryption for the following
reasons: [0115] The key for a given strength encryption algorithm
is shorter for a symmetric algorithm than an asymmetric algorithm
[0116] Symmetric algorithms are faster to compute and require less
software or silicon
[0117] Note however, that the selection of a good key depends on
the encryption algorithm chosen. Certain keys are not strong for
particular encryption algorithms, so any key needs to be tested for
strength. The more tests that need to be performed for key
selection, the less likely the key will remain hidden.
[0118] 3.6.2 Random Number Sequences
[0119] Consider a random number sequence R.sub.0, R.sub.1, . . . ,
R.sub.i, R.sub.i+1. We define the one-way function F such that F
[X] returns the X.sup.th random number in the random sequence.
However we must ensure that F[X] is repeatable for a given X on
different implementations. The random number sequence therefore
cannot be truly random. Instead, it must be pseudo-random, with the
generator making use of a specific seed.
[0120] There are a large number of issues concerned with defining
good random number generators. Knuth, in [48] describes what makes
a generator "good" (including statistical tests), and the general
problems associated with constructing them. Moreau gives a high
level survey of the current state of the field in [60].
[0121] The majority of random number generators produce the
i.sup.th random number from the i-1.sup.th state--the only way to
determine the i.sup.th number is to iterate from the 0.sup.th
number to the i.sup.th. If i is large, it may not be practical to
wait for i iterations.
[0122] However there is a type of random number generator that does
allow random access. In [10], Blum, Blum and Shub define the ideal
generator as follows: " . . . we would like a pseudo-random
sequence generator to quickly produce, from short seeds, long
sequences (of bits) that appear in every way to be generated by
successive flips of a fair coin". They defined the x.sup.2 mod n
generator [10], more commonly referred to as the BBS generator.
They showed that given certain assumptions upon which modern
cryptography relies, a BBS generator passes extremely stringent
statistical tests.
[0123] The BBS generator relies on selecting n which is a Blum
integer (n=pq where p and q are large prime numbers, p # q, p mod
4=3, and q mod 4=3). The initial state of the generator is given by
x.sub.0 where x.sub.0=x.sup.2 mod n, and x is a random integer
relatively prime to n. The i.sup.th pseudo-random bit is the least
significant bit of x.sub.i where:
x.sub.i=X.sup.2.sub.i-1 mod n
[0124] As an extra property, knowledge of p and q allows a direct
calculation of the i.sup.th number in the sequence as follows:
x.sub.i=x.sub.0.sup.y mod n where y=2.sup.i mod((p-1)(q-1))
[0125] Without knowledge of p and q, the generator must iterate
(the security of calculation relies on the conjectured difficulty
of factoring large numbers).
[0126] When first defined, the primary problem with the BBS
generator was the amount of work required for a single output bit.
The algorithm was considered too slow for most applications.
However the advent of Montgomery reduction arithmetic [58] has
given rise to more practical implementations, such as [59]. In
addition, Vazirani and Vazirani have shown in [90] that depending
on the size of n, more bits can safely be taken from x.sub.i
without compromising the security of the generator.
[0127] Assuming we only take 1 bit per x.sub.i, N bits (and hence N
iterations of the bit generator function) are needed in order to
generate an N-bit random number. To the outside observer, given a
particular set of bits, there is no way to determine the next bit
other than a 50/50 probability. If the x, p and q are hidden, they
act as a key, and it is computationally infeasible to take an
output bit stream and compute x, p, and q. It is also
computationally infeasible to determine the value of i used to
generate a given set of pseudo-random bits. This last feature makes
the generator one-way. Different values of i can produce identical
bit sequences of a given length (e.g. 32 bits of random bits). Even
if x, p and q are known, for a given F[i], i can only be derived as
a set of possibilities, not as a certain value (of course if the
domain of i is known, then the set of possibilities is reduced
further).
[0128] However, there are problems in selecting a good p and q, and
a good seed x. In particular, Ritter in [68] describes a problem in
selecting x. The nature of the problem is that a BBS generator does
not create a single cycle of known length. Instead, it creates
cycles of various lengths, including degenerate (zero-length)
cycles. Thus a BBS generator cannot be initialized with a random
state--it might be on a short cycle. Specific algorithms exist in
section 9 of [10] to determine the length of the period for a given
seed given certain strenuous conditions for n.
[0129] 3.6.3 Hash Functions
[0130] Special one-way functions, known as Hash functions, map
arbitrary length messages to fixed-length hash values. Hash
functions are referred to as H[M]. Since the input is of arbitrary
length, a hash function has a compression component in order to
produce a fixed length output. Hash functions also have an
obfuscation component in order to make it difficult to find
collisions and to determine information about M from H[M].
[0131] Because collisions do exist, most applications require that
the hash algorithm is preimage resistant, in that for a given
X.sub.i it is difficult to find X.sub.2 such that
H[X.sub.i]=H[X.sub.2]. In addition, most applications also require
the hash algorithm to be collision resistant (i.e. it should be
hard to find two messages X.sub.i and X.sub.2 such that
H[X.sub.i]=H[X.sub.2]). However, as described in [20], it is an
open problem whether a collision-resistant hash function, in the
ideal sense, can exist at all.
[0132] The primary application for hash functions is in the
reduction of an input message into a digital "fingerprint" before
the application of a digital signature algorithm. One problem of
collisions with digital signatures can be seen in the following
example. [0133] A has a long message M1 that says "I owe B $10". A
signs H[M.sub.1] using his private key. B, being greedy, then
searches for a collision message M.sub.2 where
H[M.sub.2]=H[M.sub.i] but where M.sub.2 is favorable to B, for
example "I owe B $1 million". Clearly it is in A's interest to
ensure that it is difficult to find such an M.sub.2.
[0134] Examples of collision resistant one-way hash functions are
SHA-1[28], MD5 [73] and RIPEMD-160[66], all derived from MD4
[70][72].
[0135] 3.6.3.1 MD4
[0136] Ron Rivest introduced MD4 [70][72] in 1990. It is only
mentioned here because all other one-way hash functions are derived
in some way from MD4.
[0137] MD4 is now considered completely broken [18][19] in that
collisions can be calculated instead of searched for. In the
example above, B could trivially generate a substitute message
M.sub.2 with the same hash value as the original message
M.sub.1.
[0138] 3.6.3.2 MD5
[0139] Ron Rivest introduced MD5 [73] in 1991 as a more secure MD4.
Like MD4, MD5 produces a 128-bit hash value. MD5 is not patented
[80].
[0140] Dobbertin describes the status of MD5 after recent attacks
[20]. He describes how pseudo-collisions have been found in MD5,
indicating a weakness in the compression function, and more
recently, collisions have been found. This means that MD5 should
not be used for compression in digital signature schemes where the
existence of collisions may have dire consequences. However MD5 can
still be used as a one-way function. In addition, the HMAC-MD5
construct (see Section 3.6.4.1) is not affected by these recent
attacks.
[0141] 3.6.3.3 SHA-1
[0142] SHA-1[28] is very similar to MD5, but has a 160-bit hash
value (MD5 only has 128 bits of hash value). SHA-1 was designed and
introduced by the NIST and NSA for use in the Digital Signature
Standard (DSS). The original published description was called SHA
[27], but very soon afterwards, was revised to become SHA-1[28],
supposedly to correct a security flaw in SHA (although the NSA has
not released the mathematical reasoning behind the change).
[0143] There are no known cryptographic attacks against SHA-1[78].
It is also more resistant to brute force attacks than MD4 or MD5
simply because of the longer hash result.
[0144] The US Government owns the SHA-1 and DSA algorithms (a
digital signature authentication algorithm defined as part of DSS
[29]) and has at least one relevant patent (U.S. Pat. No. 5,231,688
granted in 1993). However, according to NIST [61]: [0145] "The DSA
patent and any foreign counterparts that may issue are available
for use without any written permission from or any payment of
royalties to the U.S. government."
[0146] In a much stronger declaration, NIST states in the same
document [61] that DSA and SHA-1 do not infringe third party's
rights: [0147] "NIST reviewed all of the asserted patents and
concluded that none of them would be infringed by DSS. Extra
protection will be written into the PK1 pilot project that will
prevent an organization or individual from suing anyone except the
government for patent infringement during the course of the
project."
[0148] It must however, be noted that the Schnorr authentication
algorithm [81] (U.S. Pat. No. 4,995,082) patent holder claims that
DSA infringes his patent. The Schnorr patent is not due to expire
until 2008. Fortunately this does not affect SHA-1.
[0149] 3.6.3.4 RIPEMD-160
[0150] RIPEMD-160[66] is a hash function derived from its
predecessor RIPEMD [11] (developed for the European Community's
RIPE project in 1992). As its name suggests, RIPEMD-160 produces a
160-bit hash result. Tuned for software implementations on 32-bit
architectures, RIPEMD-160 is intended to provide a high level of
security for 10 years or more.
[0151] Although there have been no successful attacks on
RIPEMD-160, it is comparatively new and has not been extensively
cryptanalyzed. The original RIPEMD algorithm [11] was specifically
designed to resist known cryptographic attacks on MD4. The recent
attacks on MD5 (detailed in [20]) showed similar weaknesses in the
RIPEMD 128-bit hash function. Although the attacks showed only
theoretical weaknesses, Dobbertin, Preneel and Bosselaers further
strengthened RIPEMD into a new algorithm RIPEMD-160.
[0152] RIPEMD-160 is in the public domain, and requires no
licensing or royalty payments.
[0153] 3.6.4 Message Authentication Codes
[0154] The problem of message authentication can be summed up as
follows: [0155] How can A be sure that a message supposedly from B
is in fact from B?
[0156] Message authentication is different from entity
authentication (described in the section on cryptographic
challenge-response protocols). With entity authentication, one
entity (the claimant) proves its identity to another (the
verifier). With message authentication, we are concerned with
making sure that a given message is from who we think it is from
i.e. it has not been tampered with en route from the source to its
destination. While this section has a brief overview of message
authentication, a more detailed survey can be found in [86].
[0157] A one-way hash function is not sufficient protection for a
message. Hash functions such as MD5 rely on generating a hash value
that is representative of the original input, and the original
input cannot be derived from the hash value. A simple attack by E,
who is in-between A and B, is to intercept the message from B, and
substitute his own. Even if A also sends a hash of the original
message, E can simply substitute the hash of his new message. Using
a one-way hash function alone, A has no way of knowing that B's
message has been changed.
[0158] One solution to the problem of message authentication is the
Message Authentication Code, or MAC.
[0159] When B sends message M, it also sends MAC[M] so that the
receiver will know that M is actually from B. For this to be
possible, only B must be able to produce a MAC of M, and in
addition, A should be able to verify M against MAC[M]. Notice that
this is different from encryption of M-MACs are useful when M does
not have to be secret.
[0160] The simplest method of constructing a MAC from a hash
function is to encrypt the hash value with a symmetric algorithm:
[0161] 1. Hash the input message H[M] [0162] 2. Encrypt the hash
EK[H[M]]
[0163] This is more secure than first encrypting the message and
then hashing the encrypted message. Any symmetric or asymmetric
cryptographic function can be used, with the appropriate advantages
and disadvantage of each type described in Section 3.3 and Section
3.4.
[0164] However, there are advantages to using a key-dependent
one-way hash function instead of techniques that use encryption
(such as that shown above): [0165] Speed, because one-way hash
functions in general work much faster than encryption; [0166]
Message size, because EK[M] is at least the same size as M, while
H[M] is a fixed size (usually considerably smaller than M); [0167]
Hardware/software requirements--keyed one-way hash functions are
typically far less complex than their encryption-based
counterparts; and [0168] One-way hash function implementations are
not considered to be encryption or decryption devices and therefore
are not subject to US export controls.
[0169] It should be noted that hash functions were never originally
designed to contain a key or to support message authentication. As
a result, some ad hoc methods of using hash functions to perform
message authentication, including various functions that
concatenate messages with secret prefixes, suffixes, or both have
been proposed [56][78]. Most of these ad hoc methods have been
successfully attacked by sophisticated means [42][64][65].
Additional MACs have been suggested based on XOR schemes [8] and
Toeplitz matrices [49] (including the special case of LFSR-based
(Linear Feed Shift Register) constructions).
[0170] 3.6.4.1 HMAC
[0171] The HMAC construction [6][7] in particular is gaining
acceptance as a solution for Internet message authentication
security protocols. The HMAC construction acts as a wrapper, using
the underlying hash function in a black-box way. Replacement of the
hash function is straightforward if desired due to security or
performance reasons. However, the major advantage of the HMAC
construct is that it can be proven secure provided the underlying
hash function has some reasonable cryptographic strengths--that is,
HMAC's strengths are directly connected to the strength of the hash
function [6].
[0172] Since the HMAC construct is a wrapper, any iterative hash
function can be used in an HMAC. Examples include HMAC-MD5,
HMAC-SHA1, HMAC-RIPEMD160 etc.
[0173] Given the following definitions: [0174] H=the hash function
(e.g. MD5 or SHA-1) [0175] n=number of bits output from H (e.g. 160
for SHA-1, 128 bits for MD5) [0176] M=the data to which the MAC
function is to be applied [0177] K=the secret key shared by the two
parties [0178] ipad=0x36 repeated 64 times [0179] opad=0x5C
repeated 64 times
[0180] The HMAC algorithm is as follows: [0181] 1. Extend K to 64
bytes by appending 0x00 bytes to the end of K [0182] 2. XOR the 64
byte string created in (1) with ipad [0183] 3. append data stream M
to the 64 byte string created in (2) [0184] 4. Apply H to the
stream generated in (3) [0185] 5. XOR the 64 byte string created in
(1) with opad [0186] 6. Append the H result from (4) to the 64 byte
string resulting from (5) [0187] 7. Apply H to the output of (6)
and output the result
[0188] Thus:
HMAC[M]=H[(K.sym.opad)|H[(K.sym.ipad)|M]]
[0189] The recommended key length is at least n bits, although it
should not be longer than 64 bytes (the length of the hashing
block). A key longer than n bits does not add to the security of
the function.
[0190] HMAC optionally allows truncation of the final output e.g.
truncation to 128 bits from 160 bits.
[0191] The HMAC designers' Request for Comments [51] was issued in
1997, one year after the algorithm was first introduced. The
designers claimed that the strongest known attack against HMAC is
based on the frequency of collisions for the hash function H (see
Section 5.5.10), and is totally impractical for minimally
reasonable hash functions: [0192] As an example, if we consider a
hash function like MD5 where the output length is 128 bits, the
attacker needs to acquire the correct message authentication tags
computed (with the same secret key K) on about 264 known
plaintexts. This would require the processing of at least 264
blocks under H, an impossible task in any realistic scenario (for a
block length of 64 bytes this would take 250,000 years in a
continuous 1 Gbps link, and without changing the secret key K all
this time). This attack could become realistic only if serious
flaws in the collision behavior of the function H are discovered
(e.g. Collisions found after 230 messages). Such a discovery would
determine the immediate replacement of function H (the effects of
such a failure would be far more severe for the traditional uses of
H in the context of digital signatures, public key certificates
etc).
[0193] Of course, if a 160-bit hash function is used, then 2.sup.64
should be replaced with 2.sup.80.
[0194] This should be contrasted with a regular collision attack on
cryptographic hash functions where no secret key is involved and
2.sup.64 off-line parallelizable operations suffice to find
collisions.
[0195] More recently, HMAC protocols with replay prevention
components [62] have been defined in order to prevent the capture
and replay of any M, HMAC[M] combination within a given time
period.
[0196] Finally, it should be noted that HMAC is in the public
domain [50], and incurs no licensing fees. There are no known
patents infringed by HMAC.
[0197] 3.7 Random Numbers and Time Varying Messages
[0198] The use of a random number generator as a one-way function
has already been examined. However, random number generator theory
is very much intertwined with cryptography, security, and
authentication.
[0199] There are a large number of issues concerned with defining
good random number generators. Knuth, in [48] describes what makes
a generator good (including statistical tests), and the general
problems associated with constructing them. Moreau gives a high
level survey of the current state of the field in [60].
[0200] One of the uses for random numbers is to ensure that
messages vary over time. Consider a system where A encrypts
commands and sends them to B. If the encryption algorithm produces
the same output for a given input, an attacker could simply record
the messages and play them back to fool B. There is no need for the
attacker to crack the encryption mechanism other than to know which
message to play to B (while pretending to be A). Consequently
messages often include a random number and a time stamp to ensure
that the message (and hence its encrypted counterpart) varies each
time.
[0201] Random number generators are also often used to generate
keys. Although Klapper has recently shown [45] that a family of
secure feedback registers for the purposes of building key-streams
does exist, he does not give any practical construction. It is
therefore best to say at the moment that all generators are
insecure for this purpose. For example, the Berlekamp-Massey
algorithm [54], is a classic attack on an LFSR random number
generator. If the LFSR is of length n, then only 2n bits of the
sequence suffice to determine the LFSR, compromising the key
generator.
[0202] If, however, the only role of the random number generator is
to make sure that messages vary over time, the security of the
generator and seed is not as important as it is for session key
generation. If however, the random number seed generator is
compromised, and an attacker is able to calculate future "random"
numbers, it can leave some protocols open to attack. Any new
protocol should be examined with respect to this situation.
[0203] The actual type of random number generator required will
depend upon the implementation and the purposes for which the
generator is used. Generators include Blum, Blum, and Shub [10],
stream ciphers such as RC4 by Ron Rivest [71], hash functions such
as SHA-1[28] and RIPEMD-160[66], and traditional generators such
LFSRs (Linear Feedback Shift Registers) [48] and their more recent
counterpart FCSRs (Feedback with Carry Shift Registers) [44].
[0204] 3.8 Attacks
[0205] This section describes the various types of attacks that can
be undertaken to break an authentication cryptosystem. The attacks
are grouped into physical and logical attacks.
[0206] Logical attacks work on the protocols or algorithms rather
than their physical implementation, and attempt to do one of three
things: [0207] Bypass the authentication process altogether [0208]
Obtain the secret key by force or deduction, so that any question
can be answered [0209] Find enough about the nature of the
authenticating questions and answers in order to, without the key,
give the right answer to each question. The attack styles and the
forms they take are detailed below.
[0210] Regardless of the algorithms and protocol used by a security
chip, the circuitry of the authentication part of the chip can come
under physical attack. Physical attacks come in four main ways,
although the form of the attack can vary: [0211] Bypassing the
security chip altogether [0212] Physical examination of the chip
while in operation (destructive and non-destructive) [0213]
Physical decomposition of chip [0214] Physical alteration of chip
The attack styles and the forms they take are detailed below.
[0215] This section does not suggest solutions to these attacks. It
merely describes each attack type. The examination is restricted to
the context of an authentication chip (as opposed to some other
kind of system, such as Internet authentication) attached to some
System.
[0216] 3.8.1 Logical Attacks
[0217] These attacks are those which do not depend on the physical
implementation of the cryptosystem. They work against the protocols
and the security of the algorithms and random number
generators.
[0218] 3.8.1.1 Ciphertext Only Attack
[0219] This is where an attacker has one or more encrypted
messages, all encrypted using the same algorithm. The aim of the
attacker is to obtain the plaintext messages from the encrypted
messages. Ideally, the key can be recovered so that all messages in
the future can also be recovered.
[0220] 3.8.1.2 Known Plaintext Attack
[0221] This is where an attacker has both the plaintext and the
encrypted form of the plaintext. In the case of an authentication
chip, a known-plaintext attack is one where the attacker can see
the data flow between the system and the authentication chip. The
inputs and outputs are observed (not chosen by the attacker), and
can be analyzed for weaknesses (such as birthday attacks or by a
search for differentially interesting input/output pairs).
[0222] A known plaintext attack can be carried out by connecting a
logic analyzer to the connection between the system and the
authentication chip.
[0223] 3.8.1.3 Chosen Plaintext Attacks
[0224] A chosen plaintext attack describes one where a cryptanalyst
has the ability to send any chosen message to the cryptosystem, and
observe the response. If the cryptanalyst knows the algorithm,
there may be a relationship between inputs and outputs that can be
exploited by feeding a specific output to the input of another
function.
[0225] The chosen plaintext attack is much stronger than the known
plaintext attack since the attacker can choose the messages rather
than simply observe the data flow.
[0226] On a system using an embedded authentication chip, it is
generally very difficult to prevent chosen plaintext attacks since
the cryptanalyst can logically pretend he/she is the system, and
thus send any chosen bit-pattern streams to the authentication
chip.
[0227] 3.8.1.4 Adaptive Chosen Plaintext Attacks
[0228] This type of attack is similar to the chosen plaintext
attacks except that the attacker has the added ability to modify
subsequent chosen plaintexts based upon the results of previous
experiments. This is certainly the case with any
system/authentication chip scenario described for consumables such
as photocopiers and toner cartridges, especially since both systems
and consumables are made available to the public.
[0229] 3.8.1.5 Brute Force Attack
[0230] A guaranteed way to break any key-based cryptosystem
algorithm is simply to try every key. Eventually the right one will
be found. This is known as a brute force attack. However, the more
key possibilities there are, the more keys must be tried, and hence
the longer it takes (on average) to find the right one. If there
are N keys, it will take a maximum of N tries. If the key is N bits
long, it will take a maximum of 2.sup.N tries, with a 50% chance of
finding the key after only half the attempts (2.sup.N-1). The
longer N becomes, the longer it will take to find the key, and
hence the more secure the key is. Of course, an attack may guess
the key on the first try, but this is more unlikely the longer the
key is.
[0231] Consider a key length of 56 bits. In the worst case, all
2.sup.56 tests (7.2.times.10.sup.16 tests) must be made to find the
key. In 1977, Diffie and Hellman described a specialized machine
for cracking DES, consisting of one million processors, each
capable of running one million tests per second [17]. Such a
machine would take 20 hours to break any DES code.
[0232] Consider a key length of 128 bits. In the worst case, all
2.sup.128 tests (3.4.times.10.sup.38 tests) must be made to find
the key. This would take ten billion years on an array of a
trillion processors each running 1 billion tests per second.
[0233] With a long enough key length, a brute force attack takes
too long to be worth the attacker's efforts.
[0234] 3.8.1.6 Guessing Attack
[0235] This type of attack is where an attacker attempts to simply
"guess" the key. As an attack it is identical to the brute force
attack (see Section 3.8.1.5) where the odds of success depend on
the length of the key.
[0236] 3.8.1.7 Quantum Computer Attack
[0237] To break an n-bit key, a quantum computer [83] (NMR,
Optical, or Caged Atom) containing n qubits embedded in an
appropriate algorithm must be built. The quantum computer
effectively exists in 2.sup.n simultaneous coherent states. The
trick is to extract the right coherent state without causing any
decoherence. To date this has been achieved with a 2 qubit system
(which exists in 4 coherent states). It is thought possible to
extend this to 6 qubits (with 64 simultaneous coherent states)
within a few years.
[0238] Unfortunately, every additional qubit halves the relative
strength of the signal representing the key. This rapidly becomes a
serious impediment to key retrieval, especially with the long keys
used in cryptographically secure systems.
[0239] As a result, attacks on a cryptographically secure key (e.g.
160 bits) using a Quantum Computer are likely not to be feasible
and it is extremely unlikely that quantum computers will have
achieved more than 50 or so qubits within the commercial lifetime
of the authentication chips. Even using a 50 qubit quantum
computer, 2.sup.110 tests are required to crack a 160 bit key.
[0240] 3.8.1.8 Purposeful Error Attack
[0241] With certain algorithms, attackers can gather valuable
information from the results of a bad input. This can range from
the error message text to the time taken for the error to be
generated.
[0242] A simple example is that of a userid/password scheme. If the
error message usually says "Bad userid", then when an attacker gets
a message saying "Bad password" instead, then they know that the
userid is correct. If the message always says "Bad userid/password"
then much less information is given to the attacker. A more complex
example is that of the recent published method of cracking
encryption codes from secure web sites [41]. The attack involves
sending particular messages to a server and observing the error
message responses. The responses give enough information to learn
the keys--even the lack of a response gives some information.
[0243] An example of algorithmic time can be seen with an algorithm
that returns an error as soon as an erroneous bit is detected in
the input message. Depending on hardware implementation, it may be
a simple method for the attacker to time the response and alter
each bit one by one depending on the time taken for the error
response, and thus obtain the key. Certainly in a chip
implementation the time taken can be observed with far greater
accuracy than over the Internet.
[0244] 3.8.1.9 Birthday Attack
[0245] This attack is named after the famous "birthday paradox"
(which is not actually a paradox at all). The odds of one person
sharing a birthday with another, is 1 in 365 (not counting leap
years). Therefore there must be 183 people in a room for the odds
to be more than 50% that one of them shares your birthday. However,
there only needs to be 23 people in a room for there to be more
than a 50% chance that any two share a birthday, as shown in the
following relation:
Prob=1-nPr/n.sup.r=1-365P23/365.sup.23 .apprxeq.0.507
[0246] Birthday attacks are common attacks against hashing
algorithms, especially those algorithms that combine hashing with
digital signatures.
[0247] If a message has been generated and already signed, an
attacker must search for a collision message that hashes to the
same value (analogous to finding one person who shares your
birthday). However, if the attacker can generate the message, the
birthday attack comes into play. The attacker searches for two
messages that share the same hash value (analogous to any two
people sharing a birthday), only one message is acceptable to the
person signing it, and the other is beneficial for the attacker.
Once the person has signed the original message the attacker simply
claims now that the person signed the alternative
message--mathematically there is no way to tell which message was
the original, since they both hash to the same value.
[0248] Assuming a brute force attack is the only way to determine a
match, the weakening of an n-bit key by the birthday attack is
2.sup.n/2. A key length of 128 bits that is susceptible to the
birthday attack has an effective length of only 64 bits.
[0249] 3.8.1.10 Chaining Attack
[0250] These are attacks made against the chaining nature of hash
functions. They focus on the compression function of a hash
function. The idea is based on the fact that a hash function
generally takes arbitrary length input and produces a constant
length output by processing the input n bits at a time. The output
from one block is used as the chaining variable set into the next
block. Rather than finding a collision against an entire input, the
idea is that given an input chaining variable set, to find a
substitute block that will result in the same output chaining
variables as the proper message.
[0251] The number of choices for a particular block is based on the
length of the block. If the chaining variable is c bits, the
hashing function behaves like a random mapping, and the block
length is b bits, the number of such b-bit blocks is approximately
2.sup.b/2.sup.c. The challenge for finding a substitution block is
that such blocks are a sparse subset of all possible blocks.
[0252] For SHA-1, the number of 512 bit blocks is approximately
2.sup.512/2.sup.160, or 2.sup.352. The chance of finding a block by
brute force search is about 1 in 2.sup.160.
[0253] 3.8.1.11 Substitution with a Complete Lookup Table
[0254] If the number of potential messages sent to the chip is
small, then there is no need for a clone manufacturer to crack the
key. Instead, the clone manufacturer could incorporate a ROM in
their chip that had a record of all of the responses from a genuine
chip to the codes sent by the system. The larger the key, and the
larger the response, the more space is required for such a lookup
table.
[0255] 3.8.1.12 Substitution with a Sparse Lookup Table
[0256] If the messages sent to the chip are somehow predictable,
rather than effectively random, then the clone manufacturer need
not provide a complete lookup table. For example: [0257] If the
message is simply a serial number, the clone manufacturer need
simply provide a lookup table that contains values for past and
predicted future serial numbers. There are unlikely to be more than
10.sup.9 of these. [0258] If the test code is simply the date, then
the clone manufacturer can produce a lookup table using the date as
the address. [0259] If the test code is a pseudo-random number
using either the serial number or the date as a seed, then the
clone manufacturer just needs to crack the pseudo-random number
generator in the system. This is probably not difficult, as they
have access to the object code of the system. The clone
manufacturer would then produce a content addressable memory (or
other sparse array lookup) using these codes to access stored
authentication codes.
[0260] 3.8.1.13 Differential Cryptanalysis
[0261] Differential cryptanalysis describes an attack where pairs
of input streams are generated with known differences, and the
differences in the encoded streams are analyzed.
[0262] Existing differential attacks are heavily dependent on the
structure of S boxes, as used in DES and other similar algorithms.
Although other algorithms such as HMAC-SHA1 have no S boxes, an
attacker can undertake a differential-like attack by undertaking
statistical analysis of: [0263] Minimal-difference inputs, and
their corresponding outputs [0264] Minimal-difference outputs, and
their corresponding inputs
[0265] Most algorithms were strengthened against differential
cryptanalysis once the process was described. This is covered in
the specific sections devoted to each cryptographic algorithm.
However some recent algorithms developed in secret have been broken
because the developers had not considered certain styles of
differential attacks [91] and did not subject their algorithms to
public scrutiny.
[0266] 3.8.1.14 Message Substitution Attacks
[0267] In certain protocols, a man-in-the-middle can substitute
part or all of a message. This is where a real authentication chip
is plugged into a reusable clone chip within the consumable. The
clone chip intercepts all messages between the system and the
authentication chip, and can perform a number of substitution
attacks.
[0268] Consider a message containing a header followed by content.
An attacker may not be able to generate a valid header, but may be
able to substitute their own content, especially if the valid
response is something along the lines of "Yes, I received your
message". Even if the return message is "Yes, I received the
following message . . . ", the attacker may be able to substitute
the original message before sending the acknowledgment back to the
original sender.
[0269] Message Authentication Codes were developed to combat
message substitution attacks.
[0270] 3.8.1.15 Reverse Engineering the Key Generator
[0271] If a pseudo-random number generator is used to generate
keys, there is the potential for a clone manufacture to obtain the
generator program or to deduce the random seed used. This was the
way in which the security layer of the Netscape browser program was
initially broken [33].
[0272] 3.8.1.16 Bypassing the Authentication Process
[0273] It may be that there are problems in the authentication
protocols that can allow a bypass of the authentication process
altogether. With these kinds of attacks the key is completely
irrelevant, and the attacker has no need to recover it or deduce
it.
[0274] Consider an example of a system that authenticates at
power-up, but does not authenticate at any other time. A reusable
consumable with a clone authentication chip may make use of a real
authentication chip. The clone authentication chip uses the real
chip for the authentication call, and then simulates the real
authentication chip's state data after that.
[0275] Another example of bypassing authentication is if the system
authenticates only after the consumable has been used. A clone
authentication chip can accomplish a simple authentication bypass
by simulating a loss of connection after the use of the consumable
but before the authentication protocol has completed (or even
started).
[0276] One infamous attack known as the "Kentucky Fried Chip" hack
[2] involved replacing a microcontroller chip for a satellite TV
system. When a subscriber stopped paying the subscription fee, the
system would send out a "disable" message. However the new
micro-controller would simply detect this message and not pass it
on to the consumer's satellite TV system.
[0277] 3.8.1.17 Garrote/Bribe Attack
[0278] If people know the key, there is the possibility that they
could tell someone else. The telling may be due to coercion (bribe,
garrote etc.), revenge (e.g. a disgruntled employee), or simply for
principle. These attacks are usually cheaper and easier than other
efforts at deducing the key. As an example, a number of people
claiming to be involved with the development of the Divx standard
have recently (May/June 1998) been making noises on a variety of
DVD newsgroups to the effect they would like to help develop Divx
specific cracking devices--out of principle.
[0279] 3.8.2 Physical Attacks
[0280] The following attacks assume implementation of an
authentication mechanism in a silicon chip that the attacker has
physical access to. The first attack, Reading ROM, describes an
attack when keys are stored in ROM, while the remaining attacks
assume that a secret key is stored in Flash memory.
[0281] 3.8.2.1 Reading ROM
[0282] If a key is stored in ROM it can be read directly. A ROM can
thus be safely used to hold a public key (for use in asymmetric
cryptography), but not to hold a private key. In symmetric
cryptography, a ROM is completely insecure. Using a copyright text
(such as a haiku) as the key is not sufficient, because we are
assuming that the cloning of the chip is occurring in a country
where intellectual property is not respected.
[0283] 3.8.2.2 Reverse Engineering of Chip
[0284] Reverse engineering of the chip is where an attacker opens
the chip and analyzes the circuitry. Once the circuitry has been
analyzed the inner workings of the chip's algorithm can be
recovered.
[0285] Lucent Technologies have developed an active method [4]
known as TOBIC (Two photon OBIC, where OBIC stands for Optical Beam
Induced Current), to image circuits. Developed primarily for static
RAM analysis, the process involves removing any back materials,
polishing the back surface to a mirror finish, and then focusing
light on the surface. The excitation wavelength is specifically
chosen not to induce a current in the IC.
[0286] A Kerckhoffs in the nineteenth century made a fundamental
assumption about cryptanalysis: if the algorithm's inner workings
are the sole secret of the scheme, the scheme is as good as broken
[39]. He stipulated that the secrecy must reside entirely in the
key. As a result, the best way to protect against reverse
engineering of the chip is to make the inner workings
irrelevant.
[0287] 3.8.2.3 Usurping the Authentication Process
[0288] It must be assumed that any clone manufacturer has access to
both the system and consumable designs.
[0289] If the same channel is used for communication between the
system and a trusted system authentication chip, and a non-trusted
consumable authentication chip, it may be possible for the
non-trusted chip to interrogate a trusted authentication chip in
order to obtain the "correct answer". If this is so, a clone
manufacturer would not have to determine the key. They would only
have to trick the system into using the responses from the system
authentication chip.
[0290] The alternative method of usurping the authentication
process follows the same method as the logical attack described in
Section 3.8.1.16, involving simulated loss of contact with the
system whenever authentication processes take place, simulating
power-down etc.
[0291] 3.8.2.4 Modification of System
[0292] This kind of attack is where the system itself is modified
to accept clone consumables. The attack may be a change of system
ROM, a rewiring of the consumable, or, taken to the extreme case, a
completely clone system.
[0293] Note that this kind of attack requires each individual
system to be modified, and would most likely require the owner's
consent. There would usually have to be a clear advantage for the
consumer to undertake such a modification, since it would typically
void warranty and would most likely be costly. An example of such a
modification with a clear advantage to the consumer is a software
patch to change fixed-region DVD players into region-free DVD
players (although it should be noted that this is not to use clone
consumables, but rather originals from the same companies simply
targeted for sale in other countries).
[0294] 3.8.2.5 Direct Viewing of Chip Operation by Conventional
Probing
[0295] If chip operation could be directly viewed using an STM
(Scanning Tunnelling Microscope) or an electron beam, the keys
could be recorded as they are read from the internal non-volatile
memory and loaded into work registers.
[0296] These forms of conventional probing require direct access to
the top or front sides of the IC while it is powered.
[0297] 3.8.2.6 Direct Viewing of the Non-Volatile Memory
[0298] If the chip were sliced so that the floating gates of the
Flash memory were exposed, without discharging them, then the key
could probably be viewed directly using an STM or SKM (Scanning
Kelvin Microscope).
[0299] However, slicing the chip to this level without discharging
the gates is probably impossible. Using wet etching, plasma
etching, ion milling (focused ion beam etching), or chemical
mechanical polishing will almost certainly discharge the small
charges present on the floating gates.
[0300] 3.8.2.7 Viewing the Light Bursts Caused by State Changes
[0301] Whenever a gate changes state, a small amount of infrared
energy is emitted. Since silicon is transparent to infrared, these
changes can be observed by looking at the circuitry from the
underside of a chip. While the emission process is weak, it is
bright enough to be detected by highly sensitive equipment
developed for use in astronomy. The technique [89], developed by
IBM, is called PICA (Picosecond Imaging Circuit Analyzer). If the
state of a register is known at time t, then watching that register
change over time will reveal the exact value at time t+n, and if
the data is part of the key, then that part is compromised.
[0302] 3.8.2.8 Viewing the Keys Using an SEPM
[0303] A non-invasive testing device, known as a Scanning Electric
Potential Microscope (SEPM), allows the direct viewing of charges
within a chip [37]. The SEPM has a tungsten probe that is placed a
few micrometers above the chip, with the probe and circuit forming
a capacitor. Any AC signal flowing beneath the probe causes
displacement current to flow through this capacitor. Since the
value of the current change depends on the amplitude and phase of
the AC signal, the signal can be imaged. If the signal is part of
the key, then that part is compromised.
[0304] 3.8.2.9 Monitoring EMI
[0305] Whenever electronic circuitry operates, faint
electromagnetic signals are given off Relatively inexpensive
equipment can monitor these signals and could give enough
information to allow an attacker to deduce the keys.
[0306] 3.8.2.10 Viewing I.sub.dd Fluctuations
[0307] Even if keys cannot be viewed, there is a fluctuation in
current whenever registers change state. If there is a high enough
signal to noise ratio, an attacker can monitor the difference in
I.sub.dd that may occur when programming over either a high or a
low bit. The change in I.sub.dd can reveal information about the
key. Attacks such as these have already been used to break smart
cards [46].
[0308] 3.8.2.11 Differential Fault Analysis
[0309] This attack assumes introduction of a bit error by
ionization, microwave radiation, or environmental stress. In most
cases such an error is more likely to adversely affect the chip
(e.g. cause the program code to crash) rather than cause beneficial
changes which would reveal the key. Targeted faults such as ROM
overwrite, gate destruction etc. are far more likely to produce
useful results.
[0310] 3.8.2.12 Clock Glitch Attacks
[0311] Chips are typically designed to properly operate within a
certain clock speed range. Some attackers attempt to introduce
faults in logic by running the chip at extremely high clock speeds
or introduce a clock glitch at a particular time for a particular
duration [1]. The idea is to create race conditions where the
circuitry does not function properly. An example could be an AND
gate that (because of race conditions) gates through Input1 all the
time instead of the AND of Input.sub.1 and Input.sub.2.
[0312] If an attacker knows the internal structure of the chip,
they can attempt to introduce race conditions at the correct moment
in the algorithm execution, thereby revealing information about the
key (or in the worst case, the key itself).
[0313] 3.8.2.13 Power Supply Attacks
[0314] Instead of creating a glitch in the clock signal, attackers
can also produce glitches in the power supply where the power is
increased or decreased to be outside the working operating voltage
range. The net effect is the same as a clock glitch--introduction
of error in the execution of a particular instruction. The idea is
to stop the CPU from XORing the key, or from shifting the data one
bit-position etc. Specific instructions are targeted so that
information about the key is revealed.
[0315] 3.8.2.14 Overwriting ROM
[0316] Single bits in a ROM can be overwritten using a laser cutter
microscope [1], to either 1 or 0 depending on the sense of the
logic. If the ROM contains instructions, it may be a simple matter
for an attacker to change a conditional jump to a non-conditional
jump, or perhaps change the destination of a register transfer. If
the target instruction is chosen carefully, it may result in the
key being revealed.
[0317] 3.8.2.15 Modifying EEPROM/Flash
[0318] These attacks fall into two categories: [0319] those similar
to the ROM attacks except that the laser cutter microscope
technique can be used to both set and reset individual bits. This
gives much greater scope in terms of modification of algorithms.
[0320] Electron beam programming of floating gates. As described in
[87] and [32], a focused electron beam can change a gate by
depositing electrons onto it. Damage to the rest of the circuit can
be avoided, as described in [31]. This attack is potentially able
to work against multi-level flash memory.
[0321] 3.8.2.16 Gate Destruction
[0322] Anderson and Kuhn described the rump session of the 1997
workshop on Fast Software Encryption [1], where Biham and Shamir
presented an attack on DES. The attack was to use a laser cutter to
destroy an individual gate in the hardware implementation of a
known block cipher (DES). The net effect of the attack was to force
a particular bit of a register to be "stuck". Biham and Shamir
described the effect of forcing a particular register to be
affected in this way--the least significant bit of the output from
the round function is set to 0. Comparing the 6 least significant
bits of the left half and the right half can recover several bits
of the key. Damaging a number of chips in this way can reveal
enough information about the key to make complete key recovery
easy.
[0323] An encryption chip modified in this way will have the
property that encryption and decryption will no longer be
inverses.
[0324] 3.8.2.17 Overwrite Attacks
[0325] Instead of trying to read the Flash memory, an attacker may
simply set a single bit by use of a laser cutter microscope.
Although the attacker doesn't know the previous value, they know
the new value. If the chip still works, the bit's original state
must be the same as the new state. If the chip doesn't work any
longer, the bit's original state must be the logical NOT of the
current state. An attacker can perform this attack on each bit of
the key and obtain the n-bit key using at most n chips (if the new
bit matched the old bit, a new chip is not required for determining
the next bit).
[0326] 3.8.2.18 Test Circuitry Attack
[0327] Most chips contain test circuitry specifically designed to
check for manufacturing defects. This includes BIST (Built In Self
Test) and scan paths. Quite often the scan paths and test circuitry
includes access and readout mechanisms for all the embedded
latches. In some cases the test circuitry could potentially be used
to give information about the contents of particular registers.
[0328] Test circuitry is often disabled once the chip has passed
all manufacturing tests, in some cases by blowing a specific
connection within the chip. A determined attacker, however, can
reconnect the test circuitry and hence enable it.
[0329] 3.8.2.19 Memory Remanence
[0330] Values remain in RAM long after the power has been removed
[35], although they do not remain long enough to be considered
non-volatile. An attacker can remove power once sensitive
information has been moved into RAM (for example working
registers), and then attempt to read the value from RAM. This
attack is most useful against security systems that have regular
RAM chips. A classic example is cited by [1], where a security
system was designed with an automatic power-shut-off that is
triggered when the computer case is opened. The attacker was able
to simply open the case, remove the RAM chips, and retrieve the key
because the values persisted.
[0331] 3.8.2.20 Chip Theft Attack
[0332] If there are a number of stages in the lifetime of an
authentication chip, each of these stages must be examined in terms
of ramifications for security should chips be stolen. For example,
if information is programmed into the chip in stages, theft of a
chip between stages may allow an attacker to have access to key
information or reduced efforts for attack. Similarly, if a chip is
stolen directly after manufacture but before programming, does it
give an attacker any logical or physical advantage?
[0333] 3.8.2.21 Trojan Horse Attack
[0334] At some stage the authentication chips must be programmed
with a secret key. Suppose an attacker builds a clone
authentication chip and adds it to the pile of chips to be
programmed The attacker has especially built the clone chip so that
it looks and behaves just like a real authentication chip, but will
give the key out to the attacker when a special attacker-known
command is issued to the chip. Of course the attacker must have
access to the chip after the programming has taken place, as well
as physical access to add the Trojan horse authentication chip to
the genuine chips.
SUMMARY OF THE INVENTION
[0335] This invention is a validation protocol for determining
whether an untrusted authentication chip is valid, or not,
including the steps of:
[0336] Generating a secret random number and calculating a
signature for the random number using a signature function, in a
trusted authentication chip;
[0337] Encrypting the random number and the signature with a
symmetric encryption function using a first key, in the trusted
authentication chip;
[0338] Passing the encrypted random number and signature from the
trusted authentication chip to an untrusted authentication
chip;
[0339] Decrypting the encrypted random number and signature with a
symmetric decryption function using the first key, in the untrusted
authentication chip;
[0340] Calculating a signature for the decrypted random number
using the signature function, in the untrusted authentication
chip;
[0341] Comparing the signature calculated in the untrusted
authentication chip with the signature decrypted;
[0342] In the event that the two signatures match, encrypting the
decrypted random number by the symmetric encryption function using
a second key and returning it to the trusted authentication
chip;
[0343] Encrypting the random number by the symmetric encryption
function using the second key, in the trusted authentication
chip;
[0344] Comparing the two random numbers encrypted using the second
key, in the trusted authentication chip;
[0345] In the event that the two random numbers encrypted using the
second key match, considering the untrusted authentication chip to
be valid;
[0346] Otherwise considering the untrusted authentication chip to
be invalid.
[0347] The two keys are held in both the trusted and untrusted
authentication chips, and must be kept secret.
[0348] The random number may be generated only in the trusted chip,
it should be secret and be seeded with a different initial value
each time. A new random number may be generated after each
successful validation.
[0349] The symmetric encrypt function may be held in both
chips.
[0350] The symmetric decrypt function may be held only in the
untrusted chip. The signature function may be held in both chips to
generate digital signatures. The digital signature must be long
enough to counter the chances of someone generating a random
signature. 160 bits is the preferred size, giving someone 1 chance
in 2.sup.160 of generating a valid signature by random.
[0351] A prove function may be held only in the untrusted chip to
test the decrypted random number and signature. It may return the
random number encrypted with the second key if a signature
calculated from the decrypted random number matches the decrypted
signature. Otherwise it may return 0, which indicates the chip is
invalid. The time taken to return 0 must be identical for all bad
inputs. The time taken to return the random number encrypted with
the second key must be the same for all good inputs.
[0352] A test function may be held only in the trusted chip and it
may return 1 and advance the random number if the untrusted chip is
valid. Otherwise it may return 0. The time taken to return 0 must
be identical for all bad inputs. The time taken to return 1 must be
identical for all good inputs.
[0353] This protocol may be used to determine the physical presence
of a valid authentication chip. In this case a system may call the
trusted chip to generate a random number, then call the prove
function in the untrusted chip, and finally call the test function
in the trusted chip. The untrusted chip may be associated with a
consumable so that validation of the untrusted chip authenticates
the consumable.
[0354] The invention also concerns a validation system for
performing the method, including a trusted authentication chip and
an untrusted authentication chip.
[0355] The trusted authentication chip includes a random number
generator, a symmetric encryption function and two secret keys for
the function, and a signature function.
[0356] The untrusted authentication chip includes a symmetric
encryption and decryption function and two secret keys for these
functions, and signature function, and a prove function to test
data decrypted using the first key and to return data encrypted
using the second key.
[0357] The remainder of the system may be software, hardware or a
combination of both. However the trusted chip must be a physical
authentication chip. Both chips may have the same internal
structure, or they may be different.
[0358] The invention has the following advantages:
[0359] The secret keys are not revealed during the authentication
process. The time varying random number is encrypted, so that it is
not revealed during the authentication process.
[0360] An attacker cannot build a table of values for the input and
output of the encryption process. An attacker cannot call Prove
without a valid random number and signature pair encrypted with the
first key. The second key is therefore resistant to a chosen text
attack. The random number only advances with a validation, so the
first key also not susceptible to a chosen text attack.
[0361] The system is easy to design, especially in low cost systems
such as ink-jet printers, as no encryption or decryption is
required outside of the chips.
[0362] There are a number of well-documented and cryptanalyzed
symmetric algorithms to choose from for implementation, including
patent-free and license-free solutions.
[0363] A wide range of signature functions exists, from message
authentication codes to random number sequences to key-based
symmetric cryptography. Signature functions and symmetric
encryption algorithms require fewer gates and are easier to verify
than asymmetric algorithms.
[0364] Secure key size for symmetric encryption does not have to be
as large as for an asymmetric (public key) algorithm. A minimum of
128 bits can provide appropriate security for symmetric
encryption.
[0365] In another aspect the invention is a validation system for
determining whether an untrusted authentication chip is valid, the
system including a trusted authentication chip and an untrusted
authentication chip. The trusted authentication chip includes a
random number generator, a symmetric encryption function and two
keys for the function, a signature function and a test function.
The untrusted authentication chip includes a symmetric encryption
and decryption function and two keys for these functions, a
signature function, and a prove function. The prove function
operates to decrypt a random number and signature encrypted using
the first key by the trusted authentication chip, and to calculate
another signature from the decrypted random number, for comparison
with the decrypted one, and in the event that the comparison is
successful to encrypt the random number with the second key and
send it back. The test function in the trusted chip then operates
to generate an encrypted version of the random number using the
second key and to compare it with the received version to validate
the untrusted chip.
BRIEF DESCRIPTION OF THE DRAWINGS
[0366] FIG. 1 is a data flow diagram for single chip
authentication.
[0367] FIG. 2 is a data flow diagram for double chip
authentication.
[0368] FIG. 3 is a data flow diagram for Protocol P1.
[0369] FIG. 4 is a data flow diagram for Protocol P2.
[0370] FIG. 5 is a data flow diagram for Protocol P3.
[0371] FIG. 6 is a data flow diagram for read authentication using
Protocol C1.
[0372] FIG. 7 is a data flow diagram for read authentication using
Protocol C2.
[0373] FIG. 8 is a data flow diagram for read authentication using
Protocol C3.
[0374] FIG. 9 is a block diagram of a 160-bit maximal-period LFSR
random number generator.
[0375] FIG. 10 is a block diagram of a clock filter.
[0376] FIG. 11 is a circuit diagram of a tamper detection line.
[0377] FIG. 12 is a layout diagram of an oversize nMOS transistor
used as test transistors in the tamper detection line of FIG.
11.
[0378] FIG. 13 is a circuit diagram of part of the tamper detection
line of FIG. 11 including XOR gates between the two paths.
[0379] FIG. 14 is a circuit diagram of the normal FET
implementation of a CMOS inverter.
[0380] FIG. 15 is voltage/current diagram for the transistors of
the CMOS inverter of FIG. 14.
[0381] FIG. 16 is a circuit diagram of the FET implementation of a
non-flashing CMOS inverter.
[0382] FIG. 17 is impedance diagram for the transistors of the CMOS
inverter of FIG. 16.
BEST MODES OF THE INVENTION
[0383] 4 Requirements
[0384] Existing solutions to the problem of authenticating
consumables have typically relied on patents covering physical
packaging. However this does not stop home refill operations or
clone manufacture in countries with weak industrial property
protection. Consequently a much higher level of protection is
required.
[0385] The authentication mechanism is therefore built into an
authentication chip that is embedded in the consumable and allows a
system to authenticate that consumable securely and easily.
Limiting ourselves to the system authenticating consumables (we
don't consider the consumable authenticating the system), two
levels of protection can be considered:
Presence Only Authentication:
[0386] This is where only the presence of an authentication chip is
tested. The authentication chip can be removed and used in other
consumables as long as be used indefinitely.
Consumable Lifetime Authentication:
[0387] This is where not only is the presence of the authentication
chip tested for, but also the authentication chip must only last
the lifetime of the consumable. For the chip to be re-used it must
be completely erased and reprogrammed
[0388] The two levels of protection address different requirements.
We are primarily concerned with Consumable Lifetime authentication
in order to prevent cloned versions of high volume consumables. In
this case, each chip should hold secure state information about the
consumable being authenticated. It should be noted that a
Consumable Lifetime authentication chip could be used in any
situation requiring a Presence Only authentication chip.
[0389] Requirements for authentication, data storage integrity and
manufacture are considered separately. The following sections
summarize requirements of each.
[0390] 4.1 Authentication
[0391] The authentication requirements for both Presence Only and
Consumable Lifetime authentication are restricted to the case of a
system authenticating a consumable. We do not consider
bi-directional authentication where the consumable also
authenticates the system. For example, it is not necessary for a
valid toner cartridge to ensure it is being used in a valid
photocopier.
[0392] For Presence Only authentication, we must be assured that an
authentication chip is physically present. For Consumable Lifetime
authentication we also need to be assured that state data actually
came from the authentication chip, and that it has not been altered
en route. These issues cannot be separated--data that has been
altered has a new source, and if the source cannot be determined,
the question of alteration cannot be settled.
[0393] It is not enough to provide an authentication method that is
secret, relying on a home-brew security method that has not been
scrutinized by security experts. The primary requirement therefore
is to provide authentication by means that have withstood the
scrutiny of experts.
[0394] The authentication scheme used by the authentication chip
should be resistant to defeat by logical means. Logical types of
attack are extensive, and attempt to do one of three things: [0395]
Bypass the authentication process altogether [0396] Obtain the
secret key by force or deduction, so that any question can be
answered [0397] Find enough about the nature of the authenticating
questions and answers in order to, without the key, give the right
answer to each question.
[0398] The logical attack styles and the forms they take are
detailed in Section 3.8.1.
[0399] The algorithm should have a flat keyspace, allowing any
random bit string of the required length to be a possible key.
There should be no weak keys.
[0400] The examination of a solution to the requirement of
authentication is examined in Section 5.
[0401] 4.2 Data Storage Integrity
[0402] Although authentication protocols take care of ensuring data
integrity in communicated messages, data storage integrity is also
required. Two kinds of data must be stored within the
authentication chip: [0403] Authentication data, such as secret
keys [0404] Consumable state data, such as serial numbers, and
media remaining etc.
[0405] The access requirements of these two data types differ
greatly. The authentication chip therefore requires a
storage/access control mechanism that allows for the integrity
requirements of each type.
[0406] The examination of a solution to the requirement of data
storage integrity is examined in Section 7, although the
requirements of the two kinds of data are examined briefly
here.
[0407] 4.2.1 Authentication Data
[0408] Authentication data must remain confidential. It needs to be
stored in the chip during a manufacturing/programming stage of the
chip's life, but from then on must not be permitted to leave the
chip. It must be resistant to being read from non-volatile memory.
The authentication scheme is responsible for ensuring the key
cannot be obtained by deduction, and the manufacturing process is
responsible for ensuring that the key cannot be obtained by
physical means.
[0409] The size of the authentication data memory area must be
large enough to hold the necessary keys and secret information as
mandated by the authentication protocols.
[0410] 4.2.2 Consumable State Data
[0411] Consumable state data can be divided into the following
types. Depending on the application, there will be different
numbers of each of these types of data items. [0412] Read Only
[0413] ReadWrite [0414] Decrement Only [0415] Read Only data needs
to be stored in the chip during a manufacturing/programming stage
of the chip's life, but from then on should not be allowed to
change. Examples of Read Only data items are consumable batch
numbers and serial numbers. [0416] ReadWrite data is changeable
state information, for example, the last time the particular
consumable was used. ReadWrite data items can be read and written
an unlimited number of times during the lifetime of the consumable.
They can be used to store any state information about the
consumable. The only requirement for this data is that it needs to
be kept in non-volatile memory. Since an attacker can obtain access
to a system (which can write to ReadWrite data), any attacker can
potentially change data fields of this type. This data type should
not be used for secret information, and must be considered
insecure. [0417] Decrement Only data is used to count down the
availability of consumable resources. A photocopier's toner
cartridge, for example, may store the amount of toner remaining as
a Decrement Only data item. An ink cartridge for a color printer
may store the amount of each ink color as a Decrement Only data
item, requiring three (one for each of Cyan, Magenta, and Yellow),
or even as many as five or six Decrement Only data items. The
requirement for this kind of data item is that once programmed with
an initial value at the manufacturing/programming stage, it can
only reduce in value. Once it reaches the minimum value, it cannot
decrement any further. The Decrement Only data item is only
required by Consumable Lifetime authentication.
[0418] Note that the size of the consumable state data storage
required is only for that information required to be authenticated.
Information which would be of no use to an attacker, such as ink
color-curve characteristics or ink viscosity do not have to be
stored in the secure state data memory area of the authentication
chip.
[0419] 4.3 Manufacture
[0420] The authentication chip must have a low manufacturing cost
in order to be included as the authentication mechanism for low
cost consumables.
[0421] The authentication chip should use a standard manufacturing
process, such as Flash. This is necessary to: [0422] Allow a great
range of manufacturing location options [0423] Use well-defined and
well-behaved technology [0424] Reduce cost Regardless of the
authentication scheme used, the circuitry of the authentication
part of the chip must be resistant to physical attack. Physical
attack comes in four main ways, although the form of the attack can
vary: [0425] Bypassing the authentication chip altogether [0426]
Physical examination of chip while in operation (destructive and
non-destructive) [0427] Physical decomposition of chip [0428]
Physical alteration of chip
[0429] The physical attack styles and the forms they take are
detailed in Section 3.8.2.
[0430] Ideally, the chip should be exportable from the USA, so it
should not be possible to use an authentication chip as a secure
encryption device. This is low priority requirement since there are
many companies in other countries able to manufacture the
authentication chips. In any case, the export restrictions from the
USA may change.
[0431] The examination of a solution to the requirement of
manufacture is examined in Section 10.
[0432] 5 Authentication
[0433] Existing solutions to the problem of authenticating
consumables have typically relied on physical patents on packaging.
However this does not stop home refill operations or clone
manufacture in countries with weak industrial property protection.
Consequently a much higher level of protection is required.
[0434] It is not enough to provide an authentication method that is
secret, relying on a home-brew security method that has not been
scrutinized by security experts. Security systems such as
Netscape's original proprietary system and the GSM Fraud Prevention
Network used by cellular phones are examples where design secrecy
caused the vulnerability of the security [33][91]. Both security
systems were broken by conventional means that would have been
detected if the companies had followed an open design process. The
solution is to provide authentication by means that have withstood
the scrutiny of experts.
[0435] In this part, we examine a number of protocols that can be
used for consumables authentication, together with a high level
look at the advantages and disadvantages of each particular scheme.
We only use security methods that are publicly described, using
known behaviors in this new way. Readers should be familiar with
the concepts and terms described in Section 3. We avoid the Zero
Knowledge Proof protocol.
[0436] For all protocols, the security of the scheme relies on a
secret key, not a secret algorithm. The best way to protect against
reverse engineering of any authentication chip is to make the
algorithmic inner workings irrelevant (the algorithm of the inner
workings must still be must be valid, but not the actual
secret).
[0437] All the protocols rely on a time-variant challenge (i.e. the
challenge is different each time), where the response depends on
the challenge and the secret. The challenge involves a random
number so that any observer will not be able to gather useful
information about a subsequent identification.
[0438] Three protocols are presented for each of Presence Only and
Consumable Lifetime authentication. Although the protocols differ
in the number of authentication chips required for the
authentication process, in all cases the system authenticates the
consumable. Certain protocols will work with either one or two
chips, while other protocols only work with two chips. Whether one
chip or two authentication chips are used the system is still
responsible for making the authentication decision.
[0439] 5.0.1 Single Chip Authentication
[0440] When only one authentication chip is used for the
authentication protocol, a single chip 10 (referred to as ChipA) is
responsible for proving to a system 11 (referred to as System) that
it is authentic. At the start of the protocol, System 11 is unsure
of ChipA's authenticity. System 11 undertakes a challenge-response
protocol with ChipA 10, and thus determines ChipA's authenticity.
In all protocols the authenticity of the consumable 12 is directly
based on the authenticity of the chip associated with it, i.e. if
ChipA 10 is considered authentic, then the consumable 12, in which
chip 10 is placed, is considered authentic. The data flow can be
seen in FIG. 1, and involves a challenge 13 issued from the system,
and a response 14 returned by the chip 10.
[0441] In single chip authentication protocols, System 11 can be
software, hardware or a combination of both. It is important to
note that System 11 is considered insecure--it can be easily
reverse engineered by an attacker, either by examining the ROM or
by examining circuitry. System is not specially engineered to be
secure in itself.
[0442] 5.0.2 Double Chip Authentication
[0443] In other protocols, two authentication chips are required. A
single chip 20 (referred to as ChipA) is responsible for proving to
a system 21 (referred to as System) that it is authentic. ChipA 20
is associated with the consumable 22. As part of the authentication
process, System 21 makes use of a trusted authentication chip 23
(referred to as ChipT).
[0444] In double chip authentication protocols, System 21 can be
software, hardware or a combination of both. However ChipT 23 must
be a physical authentication chip. In some protocols ChipT 23 and
ChipA 20 have the same internal structure, while in others ChipT 23
and ChipA 20 have different internal structures. The data flow can
be seen in FIG. 2, and can be seen to involve a challenge 24 from
system 21 to chipA 20 and a request 25 from system 21 to chipT 23,
and a response 26 from chipA 20 to system 21 and information 27
from chipT 23 to system 21.
[0445] 5.1 Presence Only Authentication (Insecure State Data)
[0446] For this level of consumable authentication we are only
concerned about validating the presence of the authentication chip.
Although the authentication chip can contain state information, the
transmission of that state information would not be considered
secure.
[0447] Three protocols are presented. Protocols P1 and P3 require
two authentication chips, while Protocol P2 can be implemented
using either one or two authentication chips.
[0448] 5.1.1 Protocol P1
[0449] Protocol P1 is a double chip protocol (two authentication
chips are required). Each authentication chip contains the
following values: [0450] K Key for F.sub.K[X]. Must be secret.
[0451] R Current random number. Does not have to be secret, but
must be seeded with a different initial value for each chip
instance. Changes with each invocation of the Random function.
[0452] Each authentication chip contains the following logical
functions: [0453] Random[ ] Returns R, and advances R to next in
sequence. [0454] S[X] Returns S.sub.K[X], the result of applying a
digital signature function S to X based upon the secret key K. The
digital signature must be long enough to counter the chances of
someone generating a random signature. The length depends on the
signature scheme chosen (see below).
[0455] The protocol is as follows: [0456] 1. System 21 requests 30
Random[ ] from ChipT 23; [0457] 2. ChipT 23 returns 31 R to System
21; [0458] 3. System 21 requests 32 S[R] from ChipT 23 and also
requests 33 it from ChipA 20; [0459] 4. ChipT 23 returns 34
S.sub.KT[R] to System 21; [0460] 5. ChipA 20 returns 35 S.sub.A[R]
to System 21; [0461] 6. System compares S.sub.KT[R] with
S.sub.KA[R]. If they are equal, then ChipA is considered valid. If
not, then ChipA is considered invalid.
[0462] The data flow can be seen in FIG. 3:
[0463] Note that System 21 does not have to comprehend S.sub.K[R]
messages. It must merely check that the responses from ChipA and
ChipT are the same. The System 21 therefore does not require the
key.
[0464] The security of Protocol P1 lies in two places: [0465] The
security of S[X]. Only authentication chips contain the secret key,
so anything that can produce a digital signature S[X] from an X
that matches the S[X] generated by a trusted authentication chip
(ChipT) must be authentic. [0466] The domain of R generated by all
authentication chips must be large and non-deterministic. If the
domain of R generated by all authentication chips is small, then
there is no need for a clone manufacturer to crack the key.
Instead, the clone manufacturer could incorporate a ROM in their
chip that had a record of all of the responses from a genuine chip
to the codes sent by the system. The Random function does not
strictly have to be in the authentication chip, since System can
potentially generate the same random number sequence. However it
simplifies the design of System and ensures the security of the
random number generator will be the same for all implementations
that use the authentication chip, reducing possible error in system
implementation.
[0467] Protocol P1 has several advantages: [0468] K is not revealed
during the authentication process [0469] Given X, a clone chip
cannot generate S.sub.K[X] without K or access to a real
authentication Chip. [0470] System is easy to design, especially in
low cost systems such as ink-jet printers, as no encryption or
decryption is required by System itself. [0471] A wide range of
keyed signature functions exists, including symmetric cryptography,
random number sequences, and message authentication codes. [0472]
Keyed signature functions (such as one-way functions) require fewer
gates and are easier to verify than asymmetric algorithms). [0473]
Secure key size for a keyed signature functions does not have to be
as large as for an asymmetric (public key) algorithm. A key length
of 128 bits provides adequate security if S is a symmetric
cryptographic function, while a key length of 160 bits provides
adequate security if S is HMAC-SHA1.
[0474] However there are problems with this protocol: [0475] It is
susceptible to chosen text attack. An attacker can plug the chip
into their own system, generate chosen Rs, and observe the output.
In order to find the key, an attacker can also search for an R that
will generate a specific S[R] since multiple authentication chips
can be tested in parallel. [0476] Depending on the one-way function
chosen, key generation can be complicated. The method of selecting
a good key depends on the algorithm being used. Certain keys are
weak for a given algorithm. [0477] The choice of the keyed one-way
functions itself is non-trivial. Some require licensing due to
patent protection. [0478] A man-in-the middle could take action on
the plaintext message R before passing it on to ChipA--it would be
preferable if the man-in-the-middle did not see R until after ChipA
had seen it. It would be even more preferable if a
man-in-the-middle didn't see R at all. [0479] If S is symmetric
encryption, because of the 128-bit key size needed for adequate
security, the chips could not be exported from the USA since they
could be used as strong encryption devices.
[0480] If Protocol P1 is implemented with S as an asymmetric
encryption algorithm, there is no advantage over the symmetric
case--the keys needs to be longer and the encryption algorithm is
more expensive in silicon.
[0481] Protocol P1 must be implemented with two authentication
chips in order to keep the key secure. This means that each System
requires an authentication chip and each consumable requires an
authentication chip.
[0482] 5.1.2 Protocol P2
[0483] In some cases, System may contain a large amount of
processing power. Alternatively, for instances of systems that are
manufactured in large quantities, integration of ChipT into System
may be desirable. Use of an asymmetrical encryption algorithm
allows the ChipT portion of System to be insecure. Protocol P2
therefore, uses asymmetric cryptography.
[0484] For this protocol, each chip contains the following values:
[0485] K.sub.T ChipT only. Public key for encrypting. Does not have
to be secret. [0486] K.sub.A ChipA only. Private key for
decrypting. Must be secret. [0487] R ChipT only. Current random
number. Does not have to be secret, but must be seeded with a
different initial value for each chip instance. Changes with each
invocation of the Random function.
[0488] The following functions are defined: [0489] E[X] ChipT only.
Returns E.sub.KT[X] where E is asymmetric encrypt function E.
[0490] D[X] ChipA only. Returns D.sub.KA[X] where D is asymmetric
decrypt function D. [0491] Random[ ] ChipT only. Returns
R|E.sub.K[R]. Advances R to next in random number sequence.
[0492] The public key K.sub.T is in ChipT 23, while the secret key
K.sub.A is in ChipA 20. Having K.sub.T in ChipT 23 has the
advantage that ChipT can be implemented in software or hardware
(with the proviso that the seed for R is different for each chip or
system). Protocol P2 therefore can be implemented as a Single Chip
Protocol or as a Double Chip Protocol.
[0493] The protocol for authentication is as follows: [0494] 1.
System 21 calls 40 ChipT's Random function; [0495] 2. ChipT 23
returns 41 R|E.sub.KT[R] to System 21; [0496] 3. System 21 calls 42
ChipA's D function, passing in E.sub.KT[R]; [0497] 4. ChipA 20
returns 43 R, obtained by D.sub.KA[E.sub.KT[R]]; [0498] 5. System
21 compares R from ChipA 20 to the original R generated by ChipT
23. If they are equal, then ChipA 20 is considered valid. If not,
ChipA 20 is invalid.
[0499] The data flow can be seen in FIG. 4:
[0500] Protocol P2 has the following advantages: [0501] K.sub.A
(the secret key) is not revealed during the authentication process
[0502] Given E.sub.KT[X], a clone chip cannot generate X without
K.sub.A or access to a real ChipA. [0503] Since
K.sub.T.noteq.K.sub.A, ChipT can be implemented completely in
software or in insecure hardware, or as part of System. Only ChipA
(in the consumable) is required to be a secure authentication chip.
[0504] If ChipT is a physical chip, System is easy to design.
[0505] There are a number of well-documented and cryptanalyzed
asymmetric algorithms to chose from for implementation, including
patent-free and license-free solutions.
[0506] However, Protocol P2 has a number of its own problems:
[0507] For satisfactory security, each key needs to be 2048 bits
(compared to minimum 128 bits for symmetric cryptography in
Protocol P1). The associated intermediate memory used by the
encryption and decryption algorithms is correspondingly larger.
[0508] Key generation is non-trivial. Random numbers are not good
keys. [0509] If ChipT is implemented as a core, there may be
difficulties in linking it into a given System ASIC. [0510] If
ChipT is implemented as software, not only is the implementation of
System open to programming error and non-rigorous testing, but the
integrity of the compiler and mathematics primitives must be
rigorously checked for each implementation of System. This is more
complicated and costly than simply using a well-tested chip. [0511]
Although many asymmetric algorithms are specifically strengthened
to be resistant to differential cryptanalysis (which is based on
chosen text attacks), the private key K.sub.A is susceptible to a
chosen text attack [0512] It would be preferable to keep R hidden,
but since K.sub.T and in fact all of ChipT is public, R must be
public as well. [0513] If ChipA and ChipT are instances of the same
authentication chip, each chip must contain both asymmetric encrypt
and decrypt functionality. Consequently each chip is larger, more
complex, and more expensive than the chip required for Protocol P1.
[0514] If the authentication chip is broken into two chips to save
cost and reduce complexity of design/test, two chips still need to
be manufactured, reducing the economies of scale. This is offset by
the relative numbers of systems to consumables, but must still be
taken into account. [0515] Protocol P2 authentication chips could
not be exported from the USA, since they would be considered strong
encryption devices.
[0516] 5.1.3 Protocol P3
[0517] Protocol P3 attempts to solve one of the problems inherent
in Protocols P1 and P2 in that pairs of X, F.sub.K[X] can be
gathered by the attacker (where F is S or E). Protocol P1 is worse
in that it is open to a chosen text attack. It is therefore
desirable to pass the chosen random number R from ChipT to ChipA
without the intermediate System knowing the value of R. Protocol P2
cannot do this since ChipT is public and hence R is not secret. In
addition, since R is random, it is not enough to simply pass an
encrypted version of R to ChipA, since a random sequence of bits
could be substituted for a different random sequence of bits by the
attacker.
[0518] The solution is to encrypt both R and R's digital signature
so that ChipA can test if R was in fact generated by ChipT. Since
we don't want to reveal R, P3 must be a Double Chip Protocol (ChipT
cannot be incorporated into a software System or be included as an
ASIC core). Symmetric encryption can therefore be safely used.
[0519] Protocol P3 therefore uses 2 sets of keys. The first key is
used in ChipT to encrypt R and the signature of R. The encrypted R
is sent to ChipA where R is extracted and verified by ChipA. If the
R is valid, ChipA encrypts R using the second key, and outputs the
result. The System sends the output from ChipA back to ChipT where
it is compared against the known R encrypted with the second
key.
[0520] For this protocol, each chip contains the following values:
[0521] K.sub.1 Key for encrypting in ChipT and decrypting in ChipA.
Must be secret. [0522] K.sub.2 Key for encrypting in ChipA and
ChipT. Must be secret. [0523] R Current random number. Must be
secret and must be seeded with a different initial value for each
chip instance. Changes with each successful call to the Test
function.
[0524] The following functions are defined: [0525] E[X] Internal
function only. Returns E.sub.K[X] where E is symmetric encrypt
function E. [0526] D[X] Internal function ChipA only. Returns
D.sub.K[X] where D is symmetric decrypt function D. [0527] S[X]
Internal function only. Returns S[X], the digital signature for X.
The digital signature must be long enough to counter the chances of
someone generating a random signature. 160 bits is the preferred
size, giving someone 1 chance in 2.sup.160 of generating a valid
signature by random. [0528] Random[ ] ChipT only. Returns
E.sub.K1[R S[R]]. [0529] Test[X] ChipT only. Returns 1 and advances
R if E.sub.K2[R]=X. Otherwise returns 0. The time taken to return 0
must be identical for all bad inputs. The time taken to return 1
must be identical for all good inputs. [0530] Prove[X] ChipA only.
Calculates Y|Z from D.sub.K1[X]. Returns E.sub.K2[Y] if S[Y]=Z.
Otherwise returns 0. The time taken to return 0 must be identical
for all bad inputs. The time taken to return E.sub.K2[Y] must be
the same for all good inputs.
[0531] The protocol for authentication is as follows: [0532] 1.
System 21 calls 50 ChipT's Random function; [0533] 2. ChipT 23
returns 51 E.sub.K1[R|S[R]] to System 21; [0534] 3. System 21 calls
ChipA's Prove function, passing in E.sub.K1[R|S[R]]; [0535] 4.
ChipA 20 decrypts E.sub.K1[R S[R]], and calculates its own S[R]
based upon the decrypted R. If the two match, ChipA returns 53
E.sub.K2[R]. Otherwise ChipA returns 0; [0536] 5. System 21 calls
54 ChipT's Test function, passing in the returned E.sub.K2[R].
ChipT 23 generates its own E.sub.K2[R] and compares it against the
input value. If they are equal, then ChipA is considered valid and
a 1 is returned 55 to System 21. If not, ChipA 20 is considered
invalid and 0 is returned to System 21.
[0537] The data flow can be seen in FIG. 5:
[0538] Protocol P3 has the following advantages: [0539] K.sub.1 and
K.sub.2 (the secret keys) are not revealed during the
authentication process [0540] The time varying challenge R is
encrypted, so that it is not revealed during the authentication
process. An attacker cannot build a table of X, E.sub.K[X] values
for K.sub.1 or K.sub.2. [0541] An attacker cannot call Prove
without a valid RI S[R] pair encrypted with K.sub.1. K.sub.2 is
therefore resistant to a chosen text attack. R only advances with a
valid call to Test, so K.sub.1 also not susceptible to a chosen
text attack. [0542] System is easy to design, especially in low
cost systems such as ink-jet printers, as no encryption or
decryption is required by System itself [0543] There are a number
of well-documented and cryptanalyzed symmetric algorithms to chose
from for implementation of E, including patent-free and
license-free solutions. [0544] A wide range of signature functions
exists, from message authentication codes to random number
sequences to key-based symmetric cryptography. [0545] Signature
functions and symmetric encryption algorithms require fewer gates
and are easier to verify than asymmetric algorithms. [0546] Secure
key size for symmetric encryption does not have to be as large as
for an asymmetric (public key) algorithm. A minimum of 128 bits can
provide appropriate security for symmetric encryption.
[0547] However, Protocol P3 has a number of its own problems:
[0548] Although there are a large number of available functions for
E and S, the choice of E and S is non-trivial. Some require
licensing due to patent protection. [0549] Depending on the chosen
encryption algorithm, key generation can be complicated. The method
of selecting a good key depends on the algorithm being used.
Certain keys are weak for a given algorithm. [0550] If ChipA and
ChipT are instances of the same authentication chip, each chip must
contain both symmetric encrypt and decrypt functionality.
Consequently each chip is larger, more complex, and more expensive
than the chip required for Protocol P1 which only has encrypt
functionality. [0551] If the authentication chip is broken into 2
chips to save cost and reduce complexity of design/test, two chips
still need to be manufactured, reducing the economies of scale.
Unfortunately, ChipA must contain both encrypt and decrypt, making
the consumable authentication chip the larger of the two chips.
Both chips must also contain signature functions, making them more
complex than the chip required for Protocol P1. [0552] Protocol P3
authentication chips could not be exported from the USA, since they
would be considered strong encryption devices.
[0553] 5.1.4 Additional Notes
[0554] 5.1.4.1 General Comments
[0555] Protocol P3 is the most secure of the three Presence Only
authentication protocols, since nothing is revealed about the
challenge from the response. However, Protocol P3 requires
implementation of encryption, decryption and signature functions,
making it more expensive in silicon than Protocol P1. In addition,
export regulations imposed by the United States make this protocol
problematic.
[0556] With Protocol P2, even if the process of choosing a key was
straightforward, Protocol P2 is impractical at the present time due
to the high cost of silicon implementation (both key size and
functional implementation).
[0557] Protocol P1 is therefore the current protocol of choice for
Presence Only authentication. Eventually, as silicon costs come
down with Moore's Law, and USA export regulations are relaxed,
Protocol P3 will be preferable to Protocol P1. When silicon costs
are negligible or tight integration is required, Protocol P2 may be
preferable to Protocol P1, but the security protocol of choice
would still remain Protocol P3.
[0558] 5.1.4.2 Clone Consumable Using Real Authentication Chip
[0559] Protocols P1, P2 and P3 only check that ChipA is a real
authentication chip. They do not check to see if the consumable 22
itself is valid. The fundamental assumption for authentication is
that if ChipA is valid, the consumable is valid.
[0560] It is therefore possible for a clone manufacturer to insert
a real authentication chip into a clone consumable. There are two
cases to consider: [0561] In cases where state data is not written
to the authentication chip, the chip is completely reusable. Clone
manufacturers could therefore recycle a valid consumable into a
clone consumable. This may be made more difficult by melding the
authentication chip into the consumable's physical packaging, but
it would not stop refill operators. [0562] In cases where state
data is written to the authentication chip, the chip may be new,
partially used up, or completely used up. However this does not
stop a clone manufacturer from using the piggyback attack, where
the clone manufacturer builds a chip that has a real authentication
chip as a piggyback. The attacker's chip (ChipE) is therefore a
man-in-the-middle. At power up, ChipE reads all the memory state
values from the real authentication chip into its own memory. ChipE
then examines requests from System, and takes different actions
depending on the request. Authentication requests can be passed
directly to the real authentication chip, while read/write requests
can be simulated by a memory that resembles real authentication
chip behavior. In this way the authentication chip will always
appear fresh at power-up. ChipE can do this because the data access
is not authenticated.
[0563] Note that in both these cases, in order to fool System into
thinking its data accesses were successful, ChipE still requires a
real authentication chip, and in the second case, a clone chip is
required in addition to a real authentication chip. Consequently
any of these protocols can be useful in situations where it is not
cost effective for a clone manufacturer to embed a real
authentication chip into the consumable.
[0564] If the consumable cannot be recycled or refilled easily, it
may be protection enough to use a Presence Only authentication
protocol. For a clone operation to be successful each clone
consumable must include a valid authentication chip. The chips
would have to be stolen en masse, or taken from old consumables.
The quantity of these reclaimed chips (as well as the effort in
reclaiming them) should not be enough to base a business on, so the
added protection of secure data transfer (see Protocols C1-C3) may
not be useful.
[0565] 5.1.4.3 Longevity of Key
[0566] A general problem of these two protocols is that once the
authentication key is chosen, it cannot easily be changed. The
effect depends on the application of the key. In some instances, if
the key is compromised, the results are disastrous. In other cases,
it is only a minor inconvenience.
[0567] For example, in a car/car-key System/Consumable scenario,
the customer has only one set of car/car-keys. Each car has a
different authentication key. Consequently the loss of a car-key
only compromises the individual car. If the owner considers this a
problem, they must get a new lock on the car by replacing the
System chip inside the car's electronics. The owner's keys must be
reprogrammed/replaced to work with the new car System
authentication chip.
[0568] By contrast, a compromise of a key for a high volume
consumable market (for example ink cartridges in printers) would
allow a clone ink cartridge manufacturer to make their own
authentication chips. The only solution for existing systems is to
update the System authentication chips, which is a costly and
logistically difficult exercise. In any case, consumers' Systems
already work--they have no incentive to hobble their existing
equipment.
[0569] 5.2 Consumable Lifetime Authentication
[0570] In this level of consumable authentication we are concerned
with validating the existence of the authentication chip, as well
as ensuring that the authentication chip lasts only as long as the
consumable. In addition to validating that an authentication chip
is present, writes and reads of the authentication chip's memory
space must be authenticated as well. In this section we assume that
the authentication chip's data storage integrity is secure--certain
parts of memory are Read Only, others are Read/Write, while others
are Decrement Only (see Section 7 for more information).
[0571] Three protocols are presented. Protocols C1 and C3 requires
two authentication chips, while Protocol C2 can be implemented
using either one or two authentication chips.
[0572] 5.2.1 Protocol C1
[0573] This protocol is a double chip protocol (two authentication
chips are required). For this protocol, each authentication chip
contains the following values: [0574] K.sub.i Key for calculating
F.sub.K1[X]. Must be secret. [0575] K.sub.2 Key for calculating
F.sub.K2[X]. Must be secret. [0576] R Current random number. Does
not have to be secret, but must be seeded with a different initial
value for each chip instance. Changes with each successful
authentication as defined by the Test function. [0577] M Memory
vector of authentication chip. Part of this space should be
different for each chip (does not have to be a random number). Each
authentication chip contains the following logical functions:
[0578] S[X] Internal function only. Returns S.sub.K[X], the result
of applying a digital signature function S to X based upon either
secret key K.sub.1 or K.sub.2. The digital signature must be long
enough to counter the chances of someone generating a random
signature. The length depends on the signature scheme chosen (see
below). [0579] Random[ ] Returns R|S.sub.K1[R]. [0580] Test[X, Y]
Returns 1 and advances R if S.sub.K2[R|X]=Y. Otherwise returns 0.
The time taken to return 0 must be identical for all bad inputs.
The time taken to return 1 must be identical for all good inputs.
[0581] Read[X, Y] Returns M|S.sub.K2[X|M] if S.sub.K1[X]=Y.
Otherwise returns 0. The time taken to return 0 must be identical
for all bad inputs. The time taken to return M S.sub.K2[X|M] must
be identical for all good inputs. [0582] Write[X] Writes X over
those parts of M that can legitimately be written over.
[0583] To authenticate ChipA 20 and read ChipA's memory M: [0584]
1. System 21 calls 60 ChipT's Random function; [0585] 2. ChipT 23
produces R S.sub.K1[R] and returns 61 these to System; [0586] 3.
System 21 calls 62 ChipA's Read function, passing in R,
S.sub.K1[R]; [0587] 4. ChipA 20 returns 63 M and S.sub.K2[R|M];
[0588] 5. System 21 calls 64 ChipT's Test function, passing in M
and S.sub.K2[R|M]; [0589] 6. System 21 checks response 65 from
ChipT 23. If the response 65 is 1, then ChipA 20 is considered
authentic. If 0, ChipA 20 is considered invalid.
[0590] To authenticate a write of M.sub.new to ChipA's memory M:
[0591] 1. System calls ChipA's Write function, passing in
M.sub.new; [0592] 2. The authentication procedure for a Read is
carried out; [0593] 3. If ChipA is authentic and M.sub.new=M, the
write succeeded. Otherwise it failed.
[0594] The data flow for read authentication is shown in FIG.
6.
[0595] The first thing to note about Protocol C1 is that S.sub.K[X]
cannot be called directly. Instead S.sub.K[X] is called indirectly
by Random, Test and Read: [0596] Random[ ] calls S.sub.K1[X] X is
not chosen by the caller. It is chosen by the Random function. An
attacker must perform a brute force search using multiple calls to
Random, Read, and Test to obtain a desired X, S.sub.K1[X] pair.
[0597] Test[X,Y] calls S.sub.K2[R X] Does not return result
directly, but compares the result to Y and then returns 1 or 0. Any
attempt to deduce K.sub.2 by calling Test multiple times trying
different values of S.sub.K2[R X] for a given X is reduced to a
brute force search where R cannot even be chosen by the attacker.
[0598] Read[X, Y] calls S.sub.K1[X] X and S.sub.K1[X] must be
supplied by caller, so the caller must already know the X,
S.sub.K1[X] pair. Since the call returns 0 if Y.noteq.S.sub.K1[X],
an attacker is able to use the Read function for a brute force
attack on K.sub.1. [0599] Read[X, Y] calls S.sub.K2[X|M], X is
supplied by caller. However X can only be those values already
given out by the Random function (since X and Y are validated via
K.sub.1). Thus a chosen text attack must first collect pairs from
Random (effectively a brute force attack).
[0600] In addition, only part of M can be used in a chosen text
attack since some of M is constant (read-only) and the
decrement-only part of M can only be used once per consumable. In
the next consumable the read-only part of M will be different.
[0601] Having S.sub.K[X] being called indirectly prevents chosen
text attacks on the authentication chip. Since an attacker can only
obtain a chosen R, S.sub.K1[R] pair by calling Random, Read, and
Test multiple times until the desired R appears, a brute force
attack on K.sub.1 is required in order to perform a limited chosen
text attack on K.sub.2. Any attempt at a chosen text attack on
K.sub.2 would be limited since the text cannot be completely
chosen: parts of M are read-only, yet different for each
authentication chip.
[0602] The second thing to note is that two keys are used. Given
the small size of M (256 bits), two different keys K.sub.1 and
K.sub.2 are used in order to ensure there is no correlation between
S.sub.K1[R] and S.sub.K2[R|M]. K.sub.1 is therefore used to help
protect K.sub.2 against differential attacks. It is not enough to
use a single longer key since in practice, S is likely to have
limitations on key length (for example, if S is HMAC-SHA1, the key
length is a maximum of 160 bits. Adding more bits to the key adds
no protection). It is therefore safer to protect K.sub.2 from
differential attacks with K.sub.1. Otherwise it is potentially
possible that an attacker via some as-yet undiscovered technique,
could determine the effect of the limited changes in M to
particular bit combinations in R and thus calculate S.sub.K2[X|M]
based on S.sub.K1[X].
[0603] As an added precaution, the Random and Test functions in
ChipA should be disabled so that in order to generate R,
S.sub.K1[R] pairs, an attacker must use instances of ChipT, each of
which is more expensive than ChipA (since a system must be obtained
for each ChipT). Similarly, there should be a minimum delay between
calls to Random, Read and Test so that an attacker cannot call
these functions at high speed. Thus each chip can only give a
specific number of R, S.sub.K1[R] pairs away in a certain time
period. For more information, see Section 7.
[0604] The only specific timing requirement of Protocol C1 is that
the timing for good inputs must be the same regardless of the input
value, and the return value of 0 (indicating a bad input) must be
produced in the same amount of time regardless of where the error
is in the input. Attackers can therefore not learn anything about
what was bad about the input value. This is true for both Read and
Test functions.
[0605] Another thing to note about Protocol C1 is that reading data
from ChipA also requires authentication of ChipA. The System can be
sure that the contents of memory (M) is what ChipA claims it to be
if S.sub.K2[R|M] is returned correctly. A clone chip may pretend
that M is a certain value (for example it may pretend that the
consumable is full), but it cannot return S.sub.K2[R|M] for any R
passed in by System. Thus the effective signature S.sub.K2[R|M]
assures System that not only did an authentic ChipA send M, but
also that M was not altered in between ChipA and System.
[0606] Finally, the Write function as defined does not authenticate
the Write. To authenticate a write, the System must perform a Read
after each Write.
[0607] There are some basic advantages with Protocol C1: [0608]
K.sub.1 and K.sub.2 are not revealed during the authentication
process [0609] Given X, a clone chip cannot generate S.sub.K2[X|M]
without the key or access to a real authentication chip. [0610]
System is easy to design, especially in low cost systems such as
ink-jet printers, as no encryption or decryption is required by
System itself. [0611] A wide range of key based signature exists,
including symmetric cryptography, random number sequences, and
message authentication codes. [0612] Keyed signature and one-way
functions require fewer gates and are easier to verify than
asymmetric algorithms). [0613] Secure key size for a keyed
signature function does not have to be as large as for an
asymmetric (public key) algorithm. A minimum key size of 128 bits
provides appropriate security if S is a symmetric cryptographic
function, while 160 bits provides adequate security if S is
HMAC-SHA1.
[0614] Consequently, with Protocol C1, the only way to authenticate
ChipA is to read the contents of ChipA's memory.
[0615] The security of this protocol depends on the underlying
S.sub.K[X] scheme and the domain of R over the set of all
Systems.
[0616] Although S.sub.K[X] can be any keyed signature function,
there is no advantage to implement it as asymmetric encryption. The
keys for asymmetric algorithms need to be longer and the encryption
algorithm is more expensive in silicon. This leads to a second
protocol for use with asymmetric algorithms--Protocol C2.
[0617] The primary disadvantage of Protocol C1 is that the value
for R is known during the protocol. Consequently R, S.sub.K1[R]
pairs can be collected and analyzed in a form of differential
attack. It would be preferable if R were unknown, as is the case
with Protocol C3.
[0618] Protocol C1 must be implemented with two authentication
chips in order to keep the keys secure. This means that each System
requires an authentication chip and each consumable requires an
authentication chip.
[0619] 5.2.2 Protocol C2
[0620] In some cases, System may contain a large amount of
processing power. Alternatively, for instances of systems that are
manufactured in large quantities, integration of ChipT into System
may be desirable. Use of an asymmetrical encryption algorithm can
allow the ChipT portion of System to be insecure. Protocol C2
therefore, uses asymmetric cryptography.
[0621] For this protocol, each chip contains the following values:
[0622] KT ChipT only. Public key for encrypting. Does not have to
be secret. [0623] KA ChipA only. Private key for decrypting and
encrypting. Must be secret. [0624] R ChipT only. Current random
number. Does not have to be secret, but must be seeded with a
different initial value for each chip instance. Changes with each
successful authentication as defined by the Test function. [0625] M
Memory vector of authentication chip. Part of this space should be
different for each chip (does not have to be a random number).
[0626] There is no point in verifying anything in the Read
function, since anyone can encrypt using a public key. Consequently
the following functions are defined: [0627] E[X] Internal function
only. Returns E.sub.K[X] where E is asymmetric encrypt function E.
[0628] D[X] Internal function only. Returns D.sub.K[X] where D is
asymmetric decrypt function D. [0629] Random[ ] ChipT only. Returns
E.sub.KT[R]. [0630] Test[X, Y] Returns 1 and advances R if
D.sub.KT[R X]=Y. Otherwise returns 0. The time taken to return 0
must be identical for all bad inputs, and the time taken to return
1 must be the same for all good inputs. [0631] Read[X] ChipA only.
Returns M|E.sub.KA[R|M] where R=D.sub.KA[X] (does not test input
since ChipT is effectively public). [0632] Write[X] Writes X over
those parts of M that can legitimately be written over.
[0633] The public key K.sub.T is in ChipT, while the secret key
K.sub.A is in ChipA. Having K.sub.T in ChipT has the advantage that
ChipT can be implemented in software or hardware (with the proviso
that R is seeded with a different random number for each
system).
[0634] Protocol C2 requires that D.sub.KA[E.sub.KT[X]]=X and
D.sub.KT[E.sub.KA[X]]=X.
[0635] To authenticate ChipA and read ChipA's memory M: [0636] 1.
System 21 calls 70 ChipT's Random function; [0637] 2. ChipT 23
produces and returns 71 E.sub.KT[R] to System; [0638] 3. System 21
calls 72 ChipA's Read function, passing in E.sub.KT[R]; [0639] 4.
ChipA 20 returns 73 M|E.sub.KA[R|M], first obtaining R by
D.sub.KA[E.sub.KT[R]]; [0640] 5. System 21 calls 74 ChipT's Test
function, passing in M and E.sub.KA[R|M]; [0641] 6. ChipT 23
calculates D.sub.KT[E.sub.KA[R|M]] and compares it to R|M. [0642]
7. System 21 checks response 75 from ChipT 23. If the response 75
is 1, then ChipA 20 is considered authentic. If 0, ChipA 20 is
considered invalid.
[0643] To authenticate a write of M.sub.new to ChipA's memory M:
[0644] 1. System calls ChipA's Write function, passing in
M.sub.new; [0645] 2. The authentication procedure for a Read is
carried out; [0646] 3. If ChipA is authentic and M.sub.new=M, the
write succeeded. Otherwise it failed. The data flow for read
authentication is shown in FIG. 7:
[0647] Only a valid ChipA would know the value of R, since R is not
passed into the authenticate function (it is passed in as an
encrypted value). R must be obtained by decrypting E[R], which can
only be done using the secret key K.sub.A. Once obtained, R must be
appended to M and then the result re-encoded. ChipT can then verify
that the decoded form of E.sub.KA[R|M]=R|M and hence ChipA is
valid. Since K.sub.T.noteq.K.sub.A, E.sub.KT[R]1/4E.sub.KA[R].
[0648] Protocol C2 has the following advantages: [0649] K.sub.A
(the secret key) is not revealed during the authentication process
[0650] Given E.sub.KT[R], a clone chip cannot generate R without
K.sub.A or access to a real ChipA. [0651] Since K.sub.T # K.sub.A,
ChipT can be implemented completely in software or in insecure
hardware or as part of System. Only ChipA is required to be a
secure authentication chip. [0652] Since ChipT and ChipA contain
different keys, intense testing of ChipT will reveal nothing about
K.sub.A. [0653] If ChipT is a physical chip, System is easy to
design. [0654] There are a number of well-documented and
cryptanalyzed asymmetric algorithms to chose from for
implementation, including patent-free and license-free solutions.
[0655] Even if System could be rewired so that ChipA requests were
directed to ChipT,
[0656] ChipT could never answer for ChipA since K.sub.T # K.sub.A.
The attack would have to be directed at the System ROM itself to
bypass the authentication protocol.
[0657] However, Protocol C2 has a number of disadvantages: [0658]
All authentication chips need to contain both asymmetric encrypt
and decrypt functionality. Consequently each chip is larger, more
complex, and more expensive than the chip required for Protocol C2.
[0659] For satisfactory security, each key needs to be 2048 bits
(compared to a minimum of 128 bits for symmetric cryptography in
Protocol C1). The associated intermediate memory used by the
encryption and decryption algorithms is correspondingly larger.
[0660] Key generation is non-trivial. Random numbers are not good
keys. [0661] If ChipT is implemented as a core, there may be
difficulties in linking it into a given System ASIC. [0662] If
ChipT is implemented as software, not only is the implementation of
System open to programming error and non-rigorous testing, but the
integrity of the compiler and mathematics primitives must be
rigorously checked for each implementation of System. This is more
complicated and costly than simply using a well-tested chip. [0663]
Although many asymmetric algorithms are specifically strengthened
to be resistant to differential cryptanalysis (which is based on
chosen text attacks), the private key K.sub.A is susceptible to a
chosen text attack [0664] It would be preferable to keep R hidden,
but since KT and in fact all of ChipT is effectively public, R must
be public as well. [0665] Protocol C2 authentication chips could
not be exported from the USA, since they would be considered strong
encryption devices.
[0666] As with Protocol C1, the only specific timing requirement of
Protocol C2 is for returning values based on good or bad inputs.
The time taken to return a value if the input is good must be the
same regardless of the value of the input. The same is true if the
value is bad. The time taken to process good and bad inputs does
not have to be the same however. Attackers can therefore not learn
anything about what was bad (or good) about the input value. This
is true for both Read and Test functions.
[0667] 5.2.3 Protocol C3
[0668] Protocol C3 attempts to solve one of the problems inherent
in Protocols C1 and C2 in that pairs of R, F.sub.KT[R] can be
gathered by the attacker (where F is S or E). These pairs can be
used to mount a limited chosen text attack on K.sub.2, and can be
used for differential analysis of K.sub.1. It is therefore
desirable to pass the chosen random number R from ChipT to ChipA
without the intermediate System knowing the value of R. Protocol C2
cannot do this since ChipT is public and hence R is not secret. In
addition, since R is random, it is not enough to simply pass an
encrypted version of R to ChipA (as in Protocol C2), since a random
sequence of bits could be substituted for a different random
sequence of bits by the attacker.
[0669] The solution is to encrypt both R and R's digital signature
so that ChipA can test if R was in fact generated by ChipT. Since
we don't want to reveal R, C3 must be a Double Chip Protocol (ChipT
cannot be incorporated into a software System or be included as an
ASIC core). A keyed one-way function is not enough, since ChipA
must recover R and R's signature. Symmetric encryption can
therefore be safely used.
[0670] Protocol C3 therefore uses two keys. The first key is used
in ChipT to encrypt R and the signature of R. The encrypted R and
signature is sent to ChipA where R is extracted and verified by
ChipA. If the R is valid, ChipA encrypts M R using the second key,
and outputs the result. The System sends the output from ChipA back
to ChipT where it is verified against the known R encrypted with
the second key.
[0671] For this protocol, each chip contains the following values:
[0672] K.sub.1 Key for encrypting in ChipT and decrypting in ChipA.
Must be secret. [0673] K.sub.2 Key for encrypting in both ChipA and
ChipT. Must be secret. [0674] R Current random number. Must be
secret and must be seeded with a different initial value for each
chip instance. Changes with each successful call to the Test
function. [0675] M Memory vector of authentication chip. Part of
this space should be different for each chip (does not have to be a
random number).
[0676] The following functions are defined: [0677] E[X] Internal
function only. Returns E.sub.K[X] where E is symmetric encrypt
function E. [0678] D[X] Internal function ChipA only. Returns
D.sub.K[X] where D is symmetric decrypt function D. [0679] S[X]
Internal function only. Returns S[X], the digital signature for X.
The digital signature must be long enough to counter the chances of
someone generating a random signature. 128 bits is a satisfactory
size if S is symmetric encryption, while 160 bits is a satisfactory
size if S is HMAC-SHA1. [0680] Random[ ] ChipT only. Returns
E.sub.K1[R S[R]]. [0681] Test[X, Y] ChipT only. Returns 1 and
advances R if E.sub.K2[X|R]=Y. Otherwise returns 0. The time taken
to return 0 must be identical for all bad inputs. The time taken to
return 1 must be identical for all good inputs. [0682] Read[X]
ChipA only. Calculates Y|Z from D.sub.K1[X]. Returns
M|E.sub.K2[M|Y] if S[Y]=Z. Otherwise returns 0. The time taken to
return 0 must be identical for all bad inputs. The time taken to
return M|E.sub.K2[M|Y] must be the same for all good inputs.
[0683] The protocol for authentication is as follows: [0684] 1.
System 21 calls 80 ChipT's Random function; [0685] 2. ChipT 23
returns 81 E.sub.K1[R|S[R]] to System 21; [0686] 3. System 21 calls
82 ChipA's Read function, passing in E.sub.K1[R|S[R]]; [0687] 4.
ChipA 20 decrypts E.sub.K1[R S[R]], and calculates its own S[R]
based upon the decrypted R. If the two match, ChipA 20 returns 83
M, E.sub.k2[M|R]. Otherwise ChipA 20 returns 0; [0688] 5. System 21
calls 84 ChipT's Test function, passing in the returned M and
E.sub.K2[M|R]. ChipT 23 generates its own E.sub.K2[M|R] and
compares it against the input value. If they are equal, then ChipA
20 is considered valid and a 1 is returned 85 to System 21. If not,
ChipA is invalid and 0 is returned 85 to System 21.
[0689] The data flow can be seen in FIG. 8:
[0690] Protocol C3 has the following advantages: [0691] K.sub.1 and
K.sub.2 (the secret keys) are not revealed during the
authentication process The time varying challenge R is encrypted,
so that it is not revealed during the authentication process. An
attacker cannot build a table of X, E.sub.K[X] values for K.sub.1
or K.sub.2. [0692] An attacker cannot call Read without a valid RI
S[R] pair encrypted with K.sub.1. K.sub.2 is therefore resistant to
a chosen text attack. R only advances with a valid call to Test, so
K.sub.1 also not susceptible to a chosen text attack. It is true
that the E.sub.K1[R S[R]] values can be collected by an attacker,
but there is no correlation between these values and the output
value from the Read function since there are two unknowns--R and
K.sub.2. [0693] System is easy to design, especially in low cost
systems such as ink-jet printers, as no encryption or decryption is
required by System itself [0694] There are a number of
well-documented and cryptanalyzed symmetric algorithms to chose
from for implementation of E, including patent-free and
license-free solutions. [0695] A wide range of signature functions
exists, from message authentication codes to random number
sequences to key-based symmetric cryptography. [0696] Signature
functions and symmetric encryption algorithms require fewer gates
and are easier to verify than asymmetric algorithms. [0697] Secure
key size for symmetric encryption does not have to be as large as
for an asymmetric (public key) algorithm. A minimum of 128 bits can
provide appropriate security for symmetric encryption.
[0698] However, Protocol C3 has a number of its own problems:
[0699] Although there are a large number of available functions for
E and S, the choice of E and S is non-trivial. Some require
licensing due to patent protection. [0700] Depending on the chosen
encryption algorithm, key generation can be complicated. The method
of selecting a good key depends on the algorithm being used.
Certain keys are weak for a given algorithm. [0701] If ChipA and
ChipT are instances of the same authentication chip, each chip must
contain both symmetric encrypt and decrypt functionality.
Consequently each chip is larger, more complex, and more expensive
than the chip required for Protocol P1 which only has encrypt
functionality. [0702] If the authentication chip is broken into two
chips to save cost and reduce complexity of design/test, two chips
still need to be manufactured, reducing the economies of scale.
Unfortunately, ChipA must contain both encrypt and decrypt, making
the consumable authentication chip the larger of the two chips.
Both chips must also contain signature functions, making them more
complex than the chip required for Protocol C1. [0703] Protocol C3
authentication chips could not be exported from the USA, since they
are considered strong encryption devices.
[0704] 5.2.4 Additional Notes
[0705] 5.2.4.1 General Comments
[0706] Protocol C3 is the most secure of the three Consumable
Lifetime authentication protocols, since nothing is revealed about
the challenge from the response. However, Protocol C3 requires
implementation of encryption, decryption and signature functions,
making it more expensive in silicon than Protocol C1. In addition,
export regulations imposed by the United States make this protocol
problematic.
[0707] With Protocol C2, even if the process of choosing a key was
straightforward, Protocol C2 is impractical at the present time due
to the high cost of silicon implementation (both key size and
functional implementation).
[0708] Protocol C1 is therefore the current protocol of choice for
Consumable Lifetime authentication. Eventually, as silicon costs
come down with Moore's Law, and USA export regulations are relaxed,
Protocol C3 will be preferable to Protocol C1. When silicon costs
are negligible or tight integration is required, Protocol C2 may be
preferable to Protocol C1, but the security protocol of choice
would still remain Protocol C3.
[0709] 5.2.4.2 Variation on Call to Test[ ]
[0710] If there are two authentication chips used, it is
theoretically possible for a clone manufacturer to replace the
System authentication chip with one that returns 1 (success) for
each call to Test. The System can test for this by calling Test a
number of times-N times with a wrong hash value, and expect the
result to be 0. The final time that Test is called, the true
returned value from ChipA is passed, and the return value is
trusted. The question then arises of how many times to call Test.
The number of calls must be random, so that a clone chip
manufacturer cannot know the number ahead of time.
[0711] If System has a clock, bits from the clock can be used to
determine how many false calls to Test should be made. Otherwise
the returned value from ChipA can be used. In the latter case, an
attacker could still rewire the System to permit a clone ChipT to
view the returned value from ChipA, and thus know which hash value
is the correct one.
[0712] The worst case of course, is that the System can be
completely replaced by a clone System that does not require
authenticated consumables--this is the limit case of rewiring and
changing the System. For this reason, the variation on calls to
Test is optional, depending on the System, the Consumable, and how
likely modifications are to be made. Adding such logic to System
(for example in the case of a small desktop printer) may be
considered not worthwhile, as the System is made more complicated.
By contrast, adding such logic to a camera may be considered
worthwhile.
[0713] 5.2.4.3 Clone Consumable Using Real Authentication Chip
[0714] It is important to decrement the amount of consumable
remaining before use that consumable portion. If the consumable is
used first, a clone consumable could fake a loss of contact during
a write to the special known address and then appear as a fresh new
consumable. It is important to note that this attack still requires
a real authentication chip in each consumable.
[0715] 5.2.4.4 Longevity of Key
[0716] A general problem of these two protocols is that once the
authentication keys are chosen, it cannot easily be changed. In
some instances the compromise of a key could be disastrous, while
in other cases it is not a problem. See Section 5.1.4 for more
information.
[0717] 5.3 Choosing a Protocol
[0718] As described in Section 5.1.4.1 and Section 5.2.4.1,
Protocols P1 and C1 are the protocols of choice. Eventually, as
silicon costs come down with Moore's Law, and USA export
regulations are relaxed, Protocols P3 and C3 will be preferable to
Protocols P1 and C1.
[0719] However, Protocols P1 and C1 contain much of the same
components: [0720] both require read and write access; [0721] both
require implementation of a keyed one-way function; and [0722] both
require random number generation functionality
[0723] Protocol C1 requires an additional key (K.sub.2) as well as
some minimal state machine changes: [0724] a state machine
alteration to enable F.sub.K1[X] to be called during Random; [0725]
a Test function which calls F.sub.K2[X] [0726] a state machine
alteration to the Read function to call F.sub.K1[X] and F.sub.K2
[X]
[0727] Protocol C1 only requires minimal changes over Protocol P1.
It is more secure and can be used in all places where Presence Only
authentication is required (Protocol P1). It is therefore the
protocol of choice.
[0728] Given that Protocols P1 and C1 both make use of keyed
signature functions, the choice of function is examined in more
detail here. Table 2 outlines the attributes of the applicable
choices (see Section 3.3 and Section 3.6 for more information). The
attributes are phrased so that the attribute is seen as an
advantage.
TABLE-US-00002 TABLE 2 Summary of Symbolic Nomenclature Triple
Random HMAC- HMAC- DES Blowfish RC5 IDEA Sequences MD5 SHA1 HMAC-
Free of patents .cndot. .cndot. .cndot. .cndot. .cndot. .cndot.
Random key generation .cndot. .cndot. .cndot. Can be exported from
the USA .cndot. .cndot. .cndot. .cndot. Fast .cndot. .cndot.
.cndot. .cndot. Preferred Key Size (bits) for 168.sup.a 128 128 128
512 128 160 160 use in this application Block size (bits) .sup. 64
64 64 64 256 512 512 512 Cryptanalysis Attack-Free .cndot. .cndot.
.cndot. .cndot. .cndot. (apart from weak keys) Output size given
input size N .gtoreq.N .gtoreq.N .gtoreq.N .gtoreq.N 128 128 160
160 Low storage requirements .cndot. .cndot. .cndot. .cndot. Low
silicon complexity .cndot. .cndot. .cndot. .cndot. NSA designed
.cndot. .cndot. .sup.aOnly gives protection equivalent to 112-bit
DES
[0729] An examination of Table 2 shows that the choice is
effectively between the 3 HMAC constructs and the Random Sequence.
The problem of key size and key generation eliminates the Random
Sequence. Given that a number of attacks have already been carried
out on MD5 and since the hash result is only 128 bits, HMAC-MD5 is
also eliminated. The choice is therefore between HMAC-SHA1 and
HMAC-RIPEMD160. [0730] RIPEMD-160 is relatively new, and has not
been as extensively cryptanalyzed as SHA-1. However, SHA-1 was
designed by the NSA.
[0731] SHA-1 is preferred for the HMAC construct for the following
reasons: [0732] SHA-1 was designed by the NSA; [0733] SHA-1 has
been more extensively cryptanalyzed without being broken; [0734]
SHA-1 requires slightly less intermediate storage than RIPE-MD-160;
[0735] SHA-1 is algorithmically less complex than RIPE-MD-160;
[0736] Although SHA-1 is slightly faster than RIPE-MD-160, this was
not a reason for choosing SHA-1.
[0737] Protocol C1 using HMAC-SHA1 is therefore the protocol of
choice. It is examined in more detail in Section 6.
[0738] 5.4 Choosing a Random Number Generator
[0739] Each of the described protocols requires a random number
generator. The generator must be "good" in the sense that the
random numbers generated over the life of all Systems cannot be
predicted.
[0740] If the random numbers were the same for each System, an
attacker could easily record the correct responses from a real
authentication chip, and place the responses into a ROM lookup for
a clone chip. With such an attack there is no need to obtain
K.sub.1 or K.sub.2.
[0741] Therefore the random numbers from each System must be
different enough to be unpredictable, or non-deterministic. As
such, the initial value for R (the random seed) should be
programmed with a physically generated random number gathered from
a physically random phenomenon, one where there is no information
about whether a particular bit will be 1 or 0. The seed for R must
NOT be generated with a computer-run random number generator.
Otherwise the generator algorithm and seed may be compromised
enabling an attacker to generate and therefore know the set of all
R values in all Systems.
[0742] Having a different R seed in each authentication chip means
that the first R will be both random and unpredictable across all
chips. The question therefore arises of how to generate subsequent
R values in each chip. [0743] The base case is not to change R at
all. Consequently R and F.sub.K1[R] will be the same for each call
to Random[ ]. If they are the same, then F.sub.K1[R] can be a
constant rather than calculated. An attacker could then use a
single valid authentication chip to generate a valid lookup table,
and then use that lookup table in a clone chip programmed
especially for that System. A constant R is not secure. [0744] The
simplest conceptual method of changing R is to increment it by 1.
Since R is random to begin with, the values across differing
systems are still likely to be random. However given an initial R,
all subsequent R values can be determined directly (there is no
need to iterate 10,000 times-R will take on values from R.sub.0 to
R.sub.0+10000). An incrementing R is immune to the earlier attack
on a constant R. Since R is always different, there is no way to
construct a lookup table for the particular System without wasting
as many real authentication chips as the clone chip will replace.
[0745] Rather than increment using an adder, another way of
changing R is to implement it as an LFSR (Linear Feedback Shift
Register). This has the advantage of an attacker not being able to
directly determine the range of R for a particular System, since an
LFSR value-domain is determined by sequential access. To determine
which values a given initial R will generate, an attacker must
iterate through the possibilities and enumerate them. The
advantages of a changing R are also evident in the LFSR solution.
Since R is always different, there is no way to construct a lookup
table for the particular System without using up as many real
authentication chips as the clone chip will replace (and only for
that System). There is therefore no advantage in having a more
complex function to change R. Regardless of the function, it will
always be possible for an attacker to iterate through the lifetime
set of values in a simulation. The primary security lies in the
initial randomness of R. Using an LFSR to change R simply has the
advantage of not being restricted to a consecutive numeric range
(i.e. knowing R, R.sub.N cannot be directly calculated; an attacker
must iterate through the LFSR N times).
[0746] The Random number generator 90 within the authentication
chip is therefore an LFSR 91 with 160 bits and four taps 92, 93, 94
and 95, which feed an exclusive-OR gate 96, which in turn feeds
back 97 to bit.sub.159. Tap selection of the 160 bits for a
maximal-period LFSR (i.e. the LFSR will cycle through all
2.sup.160-1 states, 0 is not a valid state) yields bit.sub.s,
bit.sub.3, bit.sub.2, and bit.sub.0 [78], as shown in FIG. 9. The
example LFSR is sparse, in that not many bits are used for feedback
(only 4 out of 160 bits are used), although maximal-period LFSR
with more taps offers slightly more protection against differential
cryptanalysis on collected R, F[R] pairs.
[0747] The 160-bit seed value for R can be any random number except
0, since an LFSR filled with 0s will produce a never-ending stream
of 0s.
[0748] Since the LFSR described is a maximal-period LFSR, all 160
bits can be used directly as R.
[0749] After each successful call to Test, the random number (R)
must be advanced by XORing bits 0, 2, 3, and 5, and shifting the
result into the high order bit. The new R and corresponding
F.sub.K1[R] can be retrieved on the next call to Random.
[0750] 5.5 Holding Out Against Logical Attacks
[0751] Protocol C1 is the authentication scheme used by the
authentication chip. As such, it should be resistant to defeat by
logical means. While the effect of various types of attacks on
Protocol C1 have been mentioned in discussion, this section details
each type of attack in turn with reference to Protocol C1.
[0752] 5.5.1 Brute Force Attack
[0753] A brute force attack is guaranteed to break Protocol C1 (or
in fact, any protocol). However the length of the key means that
the time for an attacker to perform a brute force attack is too
long to be worth the effort.
[0754] An attacker only needs to break K.sub.2 to build a clone
authentication chip. K.sub.1 is merely present to strengthen
K.sub.2 against other forms of attack. A brute force attack on
K.sub.2 must therefore break a 160-bit key.
[0755] An attack against K.sub.2 requires a maximum of 2.sup.160
attempts, with a 50% chance of finding the key after only 2.sup.159
attempts. Assuming an array of a trillion processors, each running
one million tests per second, 2.sup.159 (7.3.times.1e) tests takes
2.3.times.10.sup.22 years, which is longer than the total lifetime
of the universe. There are around 100 million personal computers in
the world. Even if these were all connected in an attack (e.g. via
the Internet), this number is still 10,000 times smaller than the
trillion-processor attack described. Further, if the manufacture of
one trillion processors becomes a possibility in the age of
nanocomputers, the time taken to obtain the key is still longer
than the total lifetime of the universe.
[0756] 5.5.2 Guessing the Key Attack
[0757] It is theoretically possible that an attacker can simply
"guess the key". In fact, given enough time, and trying every
possible number, an attacker will obtain the key. This is identical
to the brute force attack described above, where 2.sup.159 attempts
must be made before a 50% chance of success is obtained.
[0758] The chances of someone simply guessing the key on the first
try is 2.sup.160. For comparison, the chance of someone winning the
top prize in a U.S. state lottery and being killed by lightning in
the same day is only 1 in 2.sup.61 [78]. The chance of someone
guessing the authentication chip key on the first go is 1 in
2.sup.16.degree., which is comparable to two people choosing
exactly the same atoms from a choice of all the atoms in the Earth
i.e. extremely unlikely.
[0759] 5.5.3 Quantum Computer Attack
[0760] To break K.sub.2, a quantum computer containing 160 qubits
embedded in an appropriate algorithm must be built. As described in
Section 3.8.1.7, an attack against a 160-bit key is not feasible.
An outside estimate of the possibility of quantum computers is that
50 qubits may be achievable within 50 years. Even using a 50 qubit
quantum computer, 2.sup.11.degree. tests are required to crack a
160 bit key. Assuming an array of 1 billion 50 qubit quantum
computers, each able to try 2.sup.50 keys in 1 microsecond (beyond
the current wildest estimates) finding the key would take an
average of 18 billion years.
[0761] 5.5.4 Ciphertext Only Attack
[0762] An attacker can launch a ciphertext only attack on K.sub.1
by monitoring calls to Random and Read, and on K.sub.2 by
monitoring calls to Read and Test. However, given that all these
calls also reveal the plaintext as well as the hashed form of the
plaintext, the attack would be transformed into a stronger form of
attack--a known plaintext attack.
[0763] 5.5.5 Known Plaintext Attack
[0764] It is easy to connect a logic analyzer to the connection
between the System and the authentication chip, and thereby monitor
the flow of data. This flow of data results in known plaintext and
the hashed form of the plaintext, which can therefore be used to
launch a known plaintext attack against both K.sub.1 and
K.sub.2.
[0765] To launch an attack against K.sub.1, multiple calls to
Random and Test must be made (with the call to Test being
successful, and therefore requiring a call to Read on a valid
chip). This is straightforward, requiring the attacker to have both
a system authentication chip and a consumable authentication chip.
For each K.sub.i: X, S.sub.K1[X] pair revealed, a K.sub.2: Y,
S.sub.K2[Y] pair is also revealed. The attacker must collect these
pairs for further analysis.
[0766] The question arises of how many pairs must be collected for
a meaningful attack to be launched with this data. An example of an
attack that requires collection of data for statistical analysis is
differential cryptanalysis (see Section 5.5.13). However, there are
no known attacks against SHA-1 or HMAC-SHA1 [7][56][78], so there
is no use for the collected data at this time.
[0767] Note that Protocol C3 is not susceptible to a plaintext
attack.
[0768] 5.5.6 Chosen Plaintext Attacks
[0769] Given that the cryptanalyst has the ability to modify
subsequent chosen plaintexts based upon the results of previous
experiments, K.sub.2 is open to a partial form of the adaptive
chosen plaintext attack, which is certainly a stronger form of
attack than a simple chosen plaintext attack.
[0770] A chosen plaintext attack is not possible against K.sub.1,
since there is no way for a caller to modify R, which used as input
to the Random function (the only function to provide the result of
hashing with K.sub.1).
[0771] 5.5.7 Adaptive Chosen Plaintext Attacks
[0772] This kind of attack is not possible against K.sub.1, since
K.sub.1 is not susceptible to chosen plaintext attacks. However, a
partial form of this attack is possible against K.sub.2, especially
since both System and consumables are typically available to the
attacker (the System may not be available to the attacker in some
instances, such as a specific car).
[0773] The HMAC construct provides security against all forms of
chosen plaintext attacks [7]. This is primarily because the HMAC
construct has two secret input variables (the result of the
original hash, and the secret key). Thus finding collisions in the
hash function itself when the input variable is secret is even
harder than finding collisions in the plain hash function. This is
because the former requires direct access to SHA-1 (not permitted
in Protocol C1) in order to generate pairs of input/output from
SHA-1.
[0774] The only values that can be collected by an attacker are
HMAC[R] and HMAC[R M]. These are not attacks against the SHA-1 hash
function itself, and reduce the attack to a differential
cryptanalysis attack (see Section 5.5.13), examining statistical
differences between collected data. Given that there is no
differential cryptanalysis attack known against SHA-1 or HMAC,
Protocol C1 is resistant to the adaptive chosen plaintext attacks.
Note that Protocol C3 is not susceptible to this attack.
[0775] 5.5.8 Purposeful Error Attack
[0776] An attacker can only launch a purposeful error attack on the
Test and Read functions, since these are the only functions that
validate input against the keys.
[0777] With both the Test and Read functions, a 0 value is produced
if an error is found in the input--no further information is given.
In addition, the time taken to produce the 0 result is independent
of the input, giving the attacker no information about which bit(s)
were wrong.
[0778] A purposeful error attack is therefore fruitless.
[0779] 5.5.9 Chaining Attack
[0780] Any form of chaining attack assumes that the message to be
hashed is over several blocks, or the input variables can somehow
be set. The HMAC-SHA1 algorithm used by Protocol C1 only ever
hashes a single 512-bit block at a time. Consequently chaining
attacks are not possible against Protocol C1.
[0781] 5.5.10 Birthday Attack
[0782] The strongest attack known against HMAC is the birthday
attack, based on the frequency of collisions for the hash function
[7][51]. However this is totally impractical for minimally
reasonable hash functions such as SHA-1. And the birthday attack is
only possible when the attacker has control over the message that
is hashed.
[0783] Protocol C1 uses hashing as a form of digital signature. The
System sends a number that must be incorporated into the response
from a valid authentication chip. Since the authentication chip
must respond with HMAC[R|M], but has no control over the input
value R, the birthday attack is not possible. This is because the
message has effectively already been generated and signed. An
attacker must instead search for a collision message that hashes to
the same value (analogous to finding one person who shares your
birthday).
[0784] The clone chip must therefore attempt to find a new value
R.sub.2 such that the hash of R.sub.2 and a chosen M.sub.2 yields
the same hash value as H[R|M]. However the System authentication
chip does not reveal the correct hash value (the Test function only
returns 1 or 0 depending on whether the hash value is correct).
Therefore the only way of finding out the correct hash value (in
order to find a collision) is to interrogate a real authentication
chip. But to find the correct value means to update M, and since
the decrement-only parts of M are one-way, and the read-only parts
of M cannot be changed, a clone consumable would have to update a
real consumable before attempting to find a collision. The
alternative is a brute force attack search on the Test function to
find a success (requiring each clone consumable to have access to a
System consumable). A brute force search, as described above, takes
longer than the lifetime of the universe, in this case, per
authentication.
[0785] Due to the fact that a timely gathering of a hash value
implies a real consumable must be decremented, there is no point
for a clone consumable to launch this kind of attack.
[0786] 5.5.11 Substitution with a Complete Lookup Table
[0787] The random number seed in each System is 160 bits. The worst
case situation for an authentication chip is that no state data is
changed. Consequently there is a constant value returned as M.
However a clone chip must still return S.sub.K2[R|M], which is a
160 bit value.
[0788] Assuming a 160-bit lookup of a 160-bit result, this requires
2.9.times.10.sup.49 bytes, or 2.6.times.10.sup.37 terabytes,
certainly more space than is feasible for the near future. This of
course does not even take into account the method of collecting the
values for the ROM. A complete lookup table is therefore completely
impossible.
[0789] 5.5.12 Substitution with a Sparse Lookup Table
[0790] A sparse lookup table is only feasible if the messages sent
to the authentication chip are somehow predictable, rather than
effectively random.
[0791] The random number R is seeded with an unknown random number,
gathered from a naturally random event. There is no possibility for
a clone manufacturer to know what the possible range of R is for
all Systems, since each bit has an unrelated chance of being 1 or
O.
[0792] Since the range of R in all systems is unknown, it is not
possible to build a sparse lookup table that can be used in all
systems. The general sparse lookup table is therefore not a
possible attack.
[0793] However, it is possible for a clone manufacturer to know
what the range of R is for a given System. This can be accomplished
by loading a LFSR with the current result from a call to a specific
System authentication chip's Random function, and iterating some
number of times into the future. If this is done, a special ROM can
be built which will only contain the responses for that particular
range of R, i.e. a ROM specifically for the consumables of that
particular System. But the attacker still needs to place correct
information in the ROM. The attacker will therefore need to find a
valid authentication chip and call it for each of the values in
R.
[0794] Suppose the clone authentication chip reports a full
consumable, and then allows a single use before simulating loss of
connection and insertion of a new full consumable. The clone
consumable would therefore need to contain responses for
authentication of a full consumable and authentication of a
partially used consumable. The worst case ROM contains entries for
full and partially used consumables for R over the lifetime of
System. However, a valid authentication chip must be used to
generate the information, and be partially used in the process. If
a given System only produces n R-values, the sparse lookup-ROM
required is 20n bytes (20=160/8) multiplied by the number of
different values for M. The time taken to build the ROM depends on
the amount of time enforced between calls to Read.
[0795] After all this, the clone manufacturer must rely on the
consumer returning for a refill, since the cost of building the ROM
in the first place consumes a single consumable. The clone
manufacturer's business in such a situation is consequently in the
refills.
[0796] The time and cost then, depends on the size of R and the
number of different values for M that must be incorporated in the
lookup. In addition, a custom clone consumable ROM must be built to
match each and every System, and a different valid authentication
chip must be used for each System (in order to provide the full and
partially used data). The use of an authentication chip in a System
must therefore be examined to determine whether or not this kind of
attack is worthwhile for a clone manufacturer.
[0797] As an example, of a camera system that has about 10,000
prints in its lifetime. Assume it has a single Decrement Only value
(number of prints remaining), and a delay of 1 second between calls
to Read. In such a system, the sparse table will take about 3 hours
to build, and consumes 100K. Remember that the construction of the
ROM requires the consumption of a valid authentication chip, so any
money charged must be worth more than a single consumable and the
clone consumable combined. Thus it is not cost effective to perform
this function for a single consumable (unless the clone consumable
somehow contained the equivalent of multiple authentic
consumables).
[0798] If a clone manufacturer is going to go to the trouble of
building a custom ROM for each owner of a System, an easier
approach would be to update System to completely ignore the
authentication chip. For more information, see Section 10.2.4.
[0799] Consequently, this attack is possible as a per-System
attack, and a decision must be made about the chance of this
occurring for a given System/Consumable combination. The chance
will depend on the cost of the consumable and authentication chips,
the longevity of the consumable, the profit margin on the
consumable, the time taken to generate the ROM, the size of the
resultant ROM, and whether customers will come back to the clone
manufacturer for refills that use the same clone chip etc.
[0800] 5.5.13 Differential Cryptanalysis
[0801] Existing differential attacks are heavily dependent on the
structure of S boxes, as used in DES and other similar algorithms.
Although other algorithms such as HMAC-SHA1 used in Protocol C1
have no S boxes, an attacker can undertake a differential-like
attack by undertaking statistical analysis of: [0802]
Minimal-difference inputs, and their corresponding outputs [0803]
Minimal-difference outputs, and their corresponding inputs
[0804] To launch an attack of this nature, sets of input/output
pairs must be collected. The collection from Protocol C1 can be via
known plaintext, or from a partially adaptive chosen plaintext
attack. Obviously the latter, being chosen, will be more
useful.
[0805] Hashing algorithms in general are designed to be resistant
to differential analysis. SHA-1 in particular has been specifically
strengthened, especially by the 80 word expansion (see Section 6)
so that minimal differences in input will still produce outputs
that vary in a larger number of bit positions (compared to 128 bit
hash functions). In addition, the information collected is not a
direct SHA-1 input/output set, due to the nature of the HMAC
algorithm. The HMAC algorithm hashes a known value with an unknown
value (the key), and the result of this hash is then rehashed with
a separate unknown value. Since the attacker does not know the
secret value, nor the result of the first hash, the inputs and
outputs from SHA-1 are not known, making any differential attack
extremely difficult.
[0806] There are no known differential attacks against SHA-1 or
HMAC-SHA-1[56][78]. Even if this does not change by the time
Protocol C3 can be affordably included in an authentication chip, a
move to the Protocol C3 will eliminate this attack, and is
therefore attractive.
[0807] The following is a more detailed discussion of minimally
different inputs and outputs from the authentication chip based on
Protocol C1.
[0808] 5.5.13.1 Minimal Difference Inputs
[0809] This is where an attacker takes a set of X, S.sub.K[X]
values where the X values are minimally different, and examines the
statistical differences between the outputs S.sub.K[X]. The attack
relies on X values that only differ by a minimal number of
bits.
The question then arises as to how to obtain minimally different X
values in order to compare the S.sub.K[X] values. [0810] K.sub.1
With K.sub.1, the attacker needs to statistically examine minimally
different X, S.sub.K1[X] pairs. However the attacker cannot choose
any X value and obtain a related S.sub.K1[X] value. Since X,
S.sub.K1[X] pairs can only be generated by calling the Random
function on a System authentication chip, the attacker must call
Random multiple times, recording each observed pair in a table. A
search must then be made through the observed values for enough
minimally different X values to undertake a statistical analysis of
the S.sub.K1[X] values. [0811] K.sub.2 With K.sub.2, the attacker
needs to statistically examine minimally different X, S.sub.K2[X]
pairs. The only way of generating X, S.sub.K2[X] pairs is via the
Read function, which produces S.sub.K2[X] for a given Y, S.sub.K1
[Y] pair, where X=Y|M. This means that Y and the changeable part of
M can be chosen to a limited extent by an attacker. The amount of
choice must therefore be limited as much as possible.
[0812] The first way of limiting an attacker's choice is to limit
Y, since Read requires an input of the format Y, S.sub.K1 [Y].
Although a valid pair can be readily obtained from the Random
function, it is a pair of Random's choosing. An attacker can only
provide their own Y if they have obtained the appropriate pair from
Random, or if they know K.sub.1. Obtaining the appropriate pair
from Random requires a brute force search. Knowing K.sub.1 is only
logically possible by performing cryptanalysis on pairs obtained
from the Random function--effectively a known text attack. Although
Random can only be called so many times per second, K.sub.1 is
common across System chips. Therefore known pairs can be generated
in parallel.
[0813] The second way to limit an attacker's choice is to limit M,
or at least the attacker's ability to choose M. The limiting of M
is done by making some parts of M Read Only, yet different for each
authentication chip, and other parts of M Decrement Only. The Read
Only parts of M should ideally be different for each authentication
chip, so could be information such as serial numbers, batch
numbers, or random numbers. The Decrement Only parts of M mean that
for an attacker to try a different M, they can only decrement those
parts of M so many times--after the Decrement Only parts of M have
been reduced to 0 those parts cannot be changed again. Obtaining a
new authentication chip provides a new M, but the Read Only
portions will be different from the previous authentication chip's
Read Only portions, thus reducing an attacker's ability to choose M
even further.
[0814] Consequently an attacker can only gain a limited number of
chances at choosing values for Y and M.
[0815] 5.5.13.2 Minimal Difference Outputs
[0816] This is where an attacker takes a set of X, S.sub.K[X]
values where the S.sub.K[X] values are minimally different, and
examines the statistical differences between the X values. The
attack relies on S.sub.K[X] values that only differ by a minimal
number of bits.
[0817] For both K.sub.1 and K.sub.2, there is no way for an
attacker to generate an X value for a given S.sub.K[X]. To do so
would violate the fact that S is a one-way function (HMAC-SHA1).
Consequently the only way for an attacker to mount an attack of
this nature is to record all observed X, S.sub.K[X] pairs in a
table. A search must then be made through the observed values for
enough minimally different S.sub.K[X] values to undertake a
statistical analysis of the X values. Given that this requires more
work than a minimally different input attack (which is extremely
limited due to the restriction on M and the choice of R), this
attack is not fruitful.
[0818] 5.5.14 Message Substitution Attacks
In order for this kind of attack to be carried out, a clone
consumable must contain a real authentication chip, but one that is
effectively reusable since it never gets decremented. The clone
authentication chip would intercept messages, and substitute its
own. However this attack does not give success to the attacker.
[0819] A clone authentication chip may choose not to pass on a
Write command to the real authentication chip. However the
subsequent Read command must return the correct response (as if the
Write had succeeded). To return the correct response, the hash
value must be known for the specific R and M. As described in the
birthday attack section, an attacker can only determine the hash
value by actually updating M in a real Chip, which the attacker
does not want to do. Even changing the R sent by System does not
help since the System authentication chip must match the R during a
subsequent Test.
[0820] A Message substitution attack would therefore be
unsuccessful. This is only true if System updates the amount of
consumable remaining before it is used.
[0821] 5.5.15 Reverse Engineering the Key Generator
[0822] If a pseudo-random number generator is used to generate
keys, there is the potential for a clone manufacture to obtain the
generator program or to deduce the random seed used. This was the
way in which the security layer of the Netscape browser was
initially broken [33].
[0823] 5.5.16 Bypassing the Authentication Process
[0824] Protocol C1 requires the System to update the consumable
state data before the consumable is used, and follow every write by
a read (to authenticate the write). Thus each use of the consumable
requires an authentication. If the System adheres to these two
simple rules, a clone manufacturer will have to simulate
authentication via a method above (such as sparse ROM lookup).
[0825] 5.5.17 Reuse of Authentication Chips
[0826] As described above, Protocol C1 requires the System to
update the consumable state data before the consumable is used, and
follow every write by a read (to authenticate the write). Thus each
use of the consumable requires an authentication.
[0827] If a consumable has been used up, then its authentication
chip will have had the appropriate state-data values decremented to
0. The chip can therefore not be used in another consumable.
[0828] Note that this only holds true for authentication chips that
hold Decrement-Only data items. If there is no state data
decremented with each usage, there is nothing stopping the reuse of
the chip. This is the basic difference between Presence-Only
authentication and Consumable Lifetime authentication. Protocol C1
allows both.
[0829] The bottom line is that if a consumable has Decrement Only
data items that are used by the System, the authentication chip
cannot be reused without being completely reprogrammed by a valid
programming station that has knowledge of the secret key.
[0830] 5.5.18 Management Decision to Omit Authentication to Save
Costs
[0831] Although not strictly an external attack, a decision to omit
authentication in future Systems in order to save costs will have
widely varying effects on different markets.
[0832] In the case of high volume consumables, it is essential to
remember that it is very difficult to introduce authentication
after the market has started, as systems requiring authenticated
consumables will not work with older consumables still in
circulation. Likewise, it is impractical to discontinue
authentication at any stage, as older Systems will not work with
the new, unauthenticated, consumables. In the second case, older
Systems can be individually altered by replacing the System
authentication chip by a simple chip that has the same programming
interface, but whose Test function always succeeds. Of course the
System may be programmed to test for an always-succeeding Test
function, and shut down.
[0833] Without any form of protection, illegal cloning of high
volume consumables is almost certain. However, with the patent and
copyright protection, the probability of illegal cloning may be,
say 50%. However, this is not the only loss possible. If a clone
manufacturer were to introduce clone consumables which caused
damage to the System (e.g. clogged nozzles in a printer due to poor
quality ink), then the loss in market acceptance, and the expense
of warranty repairs, may be significant.
[0834] In the case of a specialized pairing, such as a
car/car-keys, or door/door-key, or some other similar situation,
the omission of authentication in future systems is trivial and
without repercussions. This is because the consumer is sold the
entire set of System and Consumable authentication chips at the one
time.
[0835] 5.5.19 Garrote/Bribe Attack
[0836] This form of attack is only successful in one of two
circumstances: [0837] K.sub.1, K.sub.2, and R are already recorded
by the chip-programmer, or [0838] the attacker can coerce future
values of K.sub.1, K.sub.2, and R to be recorded.
[0839] If humans or computer systems external to the Programming
Station do not know the keys, there is no amount of force or
bribery that can reveal them. The programming of authentication
chips, described in Section 9, (and in [85], which covers the
process in more detail) is specifically designed to reduce this
possibility.
[0840] The level of security against this kind of attack is
ultimately a decision for the System/Consumable owner, to be made
according to the desired level of service.
[0841] For example, a car company may wish to keep a record of all
keys manufactured, so that a person can request a new key to be
made for their car. However this allows the potential compromise of
the entire key database, allowing an attacker to make keys for any
of the manufacturer's existing cars. It does not allow an attacker
to make keys for any new cars. Of course, the key database itself
may also be encrypted with a further key that requires a certain
number of people to combine their key portions together for access.
If no record is kept of which key is used in a particular car,
there is no way to make additional keys should one become lost.
Thus an owner will have to replace his car's authentication chip
and all his car-keys. This is not necessarily a bad situation.
[0842] By contrast, in a consumable such as a printer ink
cartridge, the one key combination is used for all Systems and all
consumables. Certainly if no backup of the keys is kept, there is
no human with knowledge of the key, and therefore no attack is
possible. However, a no-backup situation is not desirable for a
consumable such as ink cartridges, since if the key is lost no more
consumables can be made. The manufacturer should therefore keep a
backup of the key information in several parts, where a certain
number of people must together combine their portions to reveal the
full key information. This may be required if case the chip
programming station needs to be reloaded.
[0843] In any case, none of these attacks are against Protocol C1
itself, since no humans are involved in the authentication process.
Instead, it is an attack against the programming stage of the
chips. See Section 9 and [85] for more details.
[0844] 6 HMAC-SHA1
The mechanism for authentication is the HMAC-SHA1 algorithm, acting
on one of: [0845] HMAC-SHA1 (R, K.sub.1), or [0846] HMAC-SHA1 (R|M,
K.sub.2)
[0847] This part examines the HMAC-SHA1 algorithm in greater detail
than covered so far, and describes an optimization of the algorithm
that requires fewer memory resources than the original
definition.
[0848] 6.1 HMAC
[0849] The HMAC algorithm is described in Section 3.6.4.1. In
summary, given the following definitions:
H=the hash function (e.g. MD5 or SHA-1) n=number of bits output
from H (e.g. 160 for SHA-1, 128 bits for MD5) M=the data to which
the MAC function is to be applied K=the secret key shared by the
two parties ipad=0x36 repeated 64 times opad=0x5C repeated 64
times
[0850] The HMAC algorithm is as follows: [0851] 1. Extend K to 64
bytes by appending 0x00 bytes to the end of K [0852] 2. XOR the 64
byte string created in (1) with ipad [0853] 3. Append data stream M
to the 64 byte string created in (2) [0854] 4. Apply H to the
stream generated in (3) [0855] 5. XOR the 64 byte string created in
(1) with opad [0856] 6. Append the H result from (4) to the 64 byte
string resulting from (5) [0857] 7. Apply H to the output of (6)
and output the result
Thus:
[0858] HMAC[M]=H[(K.sym.opad)|H[(K.sym.ipad)|M]]
HMAC-SHA1 algorithm is simply HMAC with H=SHA-1.
[0859] 6.2 SHA-1
[0860] The SHA1 hashing algorithm is described in the context of
other hashing algorithms in Section 3.6.3.3, and completely defined
in [27]. The algorithm is summarized here.
[0861] Nine 32-bit constants are defined in Table 3. There are 5
constants used to initialize the chaining variables, and there are
4 additive constants.
TABLE-US-00003 TABLE 3 Constants used in SHA-1 Initial Chaining
Values Additive Constants h1 0x67452301 y1 0x5A827999 h2 0xEFCDAB89
y2 0x6ED9EBA1 h3 0x98BADCFE y3 0x8F1BBCDC h4 0x10325476 y4
0xCA62C1D6 h5 0xC3D2E1F0
[0862] Non-optimized SHA-1 requires a total of 2912 bits of data
storage: [0863] Five 32-bit chaining variables are defined:
H.sub.1, H.sub.2, H.sub.3, H.sub.4 and H.sub.5. [0864] Five 32-bit
working variables are defined: A, B, C, D, and E. [0865] One 32-bit
temporary variable is defined: t. [0866] Eighty 32-bit temporary
registers are defined: X.sub.0-79.
[0867] The following functions are defined for SHA-1:
TABLE-US-00004 TABLE 4 Functions used in SHA-1 Symbolic
Nomenclature Description + Addition modulo 2.sup.32 X Y Result of
rotating X left through Y bit positions f(X, Y, Z) (X Y) ( X Z)
g(X, Y, Z) (X Y) (X Z) (Y Z) h(X, Y, Z) X .sym. Y .sym. Z
[0868] The hashing algorithm consists of firstly padding the input
message to be a multiple of 512 bits and initializing the chaining
variables .sub.H1-5 with h.sub.1-5. The padded message is then
processed in 512-bit chunks, with the output hash value being the
final 160-bit value given by the concatenation of the chaining
variables: H1|H.sub.2|H.sub.3|H.sub.4|H.sub.5.
[0869] The steps of the SHA-1 algorithm are now examined in greater
detail.
[0870] 6.2.1 Step 1. Preprocessing
[0871] The first step of SHA-1 is to pad the input message to be a
multiple of 512 bits as follows and to initialize the chaining
variables.
TABLE-US-00005 TABLE 5 Steps to follow to preprocess the input
message Pad the input message Append a 1 bit to the message Append
0 bits such that the length of the padded message is 64-bits short
of a multiple of 512 bits. Append a 64-bit value containing the
length in bits of the original input message. Store the length as
most significant bit through to least significant bit. Initialize
the chaining H.sub.1 .rarw. h.sub.1, H.sub.2 {umlaut over ( )}
h.sub.2, H.sub.3 .rarw. h.sub.3, H.sub.4 .rarw. h.sub.4, H.sub.5
.rarw. h.sub.5 variables
[0872] 6.2.2 Step 2. Processing
[0873] The padded input message can now be processed.
[0874] We process the message in 512-bit blocks. Each 512-bit block
is in the form of 16.times.32-bit words, referred to as
InputWord.sub.0-15.
TABLE-US-00006 TABLE 6 Steps to follow for each 512 bit block
(InputWord.sub.0-15) Copy the 512 For j = 0 to 15 input bits
X.sub.j = InputWord.sub.j into X.sub.0-15 Expand X.sub.0-15 For j =
16 to 79 into X.sub.16-79 Xj .rarw. ((X.sub.j-3 .sym. X.sub.j-8
.sym. X.sub.j-14 .sym. X.sub.j-16) 1) Initialize working A .rarw.
H.sub.1, B .rarw. H.sub.2, C .rarw. H.sub.3, D .rarw. H.sub.4, E
.rarw. H.sub.5 variables Round 1 For j = 0 to 19 t .rarw. ((A 5) +
f(B, C, D) + E + Xj + y.sub.1) E .rarw. D, D .rarw. C, C .rarw. (B
30), B .rarw. A, A .rarw. t Round 2 For j = 20 to 39 t .rarw. ((A
5) + h(B, C, D) + E + Xj + y.sub.2) E .rarw. D, D .rarw. C, C
.rarw. (B 30), B .rarw. A, A .rarw. t Round 3 For j = 40 to 59 t t
.rarw. ((A 5) + g(B, C, D) + E + Xj + y.sub.3) E .rarw. D, D .rarw.
C, C .rarw. (B 30), B .rarw. A, A .rarw. t Round 4 For j = 60 to 79
t .rarw. ((A 5) + h(B, C, D) + E + Xj + y.sub.4) E .rarw. D, D
.rarw. C, C .rarw. (B 30), B .rarw. A, A .rarw. t Update chaining
H1 .rarw. .sub.H1 + A, H.sub.2 .rarw. H.sub.2 + B, variables
H.sub.3 .rarw. H.sub.3 + C, H.sub.4 .rarw. H.sub.4 + D, H.sub.5
.rarw. H.sub.5 + E
[0875] The bold text is to emphasize the differences between each
round.
[0876] 6.2.3 Step 3. Completion
[0877] After all the 512-bit blocks of the padded input message
have been processed, the output hash value is the final 160-bit
value given by: H.sub.1H.sub.2H.sub.3H.sub.4H.sub.5.
[0878] 6.2.4 Optimization for Hardware Implementation
[0879] The SHA-1 Step 2 procedure is not optimized for hardware. In
particular, the 80 temporary 32-bit registers use up valuable
silicon on a hardware implementation. This section describes an
optimization to the SHA-1 algorithm that only uses 16 temporary
registers. The reduction in silicon is from 2560 bits down to 512
bits, a saving of over 2000 bits. It may not be important in some
applications, but in the authentication chip storage space must be
reduced where possible.
[0880] The optimization is based on the fact that although the
original 16-word message block is expanded into an 80-word message
block, the 80 words are not updated during the algorithm. In
addition, the words rely on the previous 16 words only, and hence
the expanded words can be calculated on-the-fly during processing,
as long as we keep 16 words for the backward references. We require
rotating counters to keep track of which register we are up to
using, but the effect is to save a large amount of storage.
[0881] Rather than index X by a single value j, we use a 5 bit
counter to count through the iterations. This can be achieved by
initializing a 5-bit register with either 16 or 20, and
decrementing it until it reaches 0. In order to update the 16
temporary variables as if they were 80, we require 4 indexes, each
a 4-bit register. All 4 indexes increment (with wraparound) during
the course of the algorithm.
TABLE-US-00007 TABLE 7 Optimised Steps to follow for each 512 bit
block (InputWord.sub.0-15) Initialize working A .rarw. H.sub.1, B
.rarw. H.sub.2, C .rarw. H.sub.3, D .rarw. H.sub.4, E .rarw.
H.sub.5 variables N.sub.1 .rarw. 13, N.sub.2 .rarw. 8, N.sub.3
.rarw. 2, N.sub.4 .rarw. 0 Round 0 Do 16 times Copy the 512 input
X.sub.N4 = InputWordN.sub.4 bits into X.sub.0-15 [ N.sub.1,
N.sub.2, N.sub.3].sub.optional N.sub.4 Round 1A Do 16 times t
.rarw. ((A 5) + f(B, C, D) + E + X.sub.N4 + y1) [ N.sub.1, N.sub.2,
N ].sub.optional N.sub.4 E .rarw. D, D .rarw. C, C .rarw. (B 30), B
.rarw. A, A .rarw. t Round 1B Do 4 times X.sub.N4 .rarw. ((XN1
.sym. XN2 .sym. XN3 .sym. XN4) 1) t .rarw. ((A 5) + f(B, C, D) + E
+ X.sub.N4 + y.sub.1) N.sub.1, N.sub.2, N.sub.3, N.sub.4 E .rarw.
D, D .rarw. C, C .rarw. (B 30), B .rarw. A, A .rarw. t Round 2 Do
20 times X.sub.N4 .rarw. ((XN1 .sym. XN2 .sym. XN3 .sym. XN4) 1) t
.rarw. ((A 5) + h(B, C, D) + E + XN4 + y.sub.2) N.sub.1, N.sub.2,
N.sub.3, N.sub.4 E .rarw. D, D .rarw. C, C .rarw. (B 30), B .rarw.
A, A .rarw. t Round 3 Do 20 times XN4 .rarw. ((XN1 .sym. XN2 .sym.
XN3 .sym. XN4) 1) t .rarw. ((A 5) + g(B, C, D) + E + X.sub.N4 +
y.sub.3) N.sub.1, N.sub.2, N.sub.3, N.sub.4 E .rarw. D, D .rarw. C,
C .rarw. (B 30), B .rarw. A, A .rarw. t Round 4 Do 20 times
X.sub.N4 .rarw. ((XN1 .sym. XN2 .sym. XN3 .sym. XN4) 1) t .rarw.
((A 5) + h(B, C, D) + E + X.sub.N4 + y.sub.4) N1, N2, N3, N4 E
.rarw. D, D .rarw. C, C .rarw. (B 30), B .rarw. A, A .rarw. t
Update chaining H.sub.1 .rarw. H.sub.1 + A, H.sub.2 .rarw. H.sub.2
+ B, variables H.sub.3 .rarw. H.sub.3 + C, H4 .rarw. H.sub.4 + D,
H.sub.5 .rarw. H.sub.5 + E
[0882] The bold text is to emphasize the differences between each
round.
[0883] The incrementing of N.sub.1, N.sub.2, and N.sub.3 during
Rounds 0 and 1A is optional. A software implementation would not
increment them, since it takes time, and at the end of the 16 times
through the loop, all 4 counters will be their original values.
Designers of hardware may wish to increment all 4 counters together
to save on control logic.
Round 0 can be completely omitted if the caller loads the 512 bits
of X.sub.0-15.
[0884] 6.3 HMAC-SHA1
[0885] In the authentication chip implementation, the HMAC-SHA1
unit only ever performs hashing on two types of inputs: on R using
K.sub.1 and on R|M using K.sub.2. Since the inputs are two constant
lengths, rather than have HMAC and SHA-1 as separate entities on
chip, they can be combined and the hardware optimized. The HMAC-SHA
1 test cases described by Cheng and Glenn [14] will remain
valid.
[0886] The padding of messages in SHA-1 Step 1 (a 1 bit, a string
of 0 bits, and the length of the message) is necessary to ensure
that different messages will not look the same after padding. Since
we only deal with 2 types of messages, our padding can be constant
Os.
[0887] In addition, the optimized version of the SHA-1 algorithm is
used, where only 16 32-bit words are used for temporary storage.
These 16 registers are loaded directly by the optimized HMAC-SHA1
hardware.
[0888] The Nine 32-bit constants h.sub.1-5 and y.sub.1-4 are still
required, although the fact that they are constants is an advantage
for hardware implementation.
[0889] Hardware optimized HMAC-SHA-1 requires a total of 1024 bits
of data storage: [0890] Five 32-bit chaining variables are defined:
H.sub.1, H.sub.z, H.sub.3, H.sub.4 and H.sub.5. [0891] Five 32-bit
working variables are defined: A, B, C, D, and E. [0892] Five
32-bit variables for temporary storage and final result:
Buff160.sub.1-5 [0893] One 32 bit temporary variable is defined: t.
[0894] Sixteen 32-bit temporary registers are defined:
X.sub.0-15.
[0895] The following two sections describe the steps for the two
types of calls to HMAC-SHA1.
[0896] 6.3.1 H[R, K.sub.1]
[0897] In the case of producing the keyed hash of R using K.sub.1,
the original input message R is a constant length of 160 bits. We
can therefore take advantage of this fact during processing. Rather
than load X.sub.0-15 during the first part of the SHA-1 algorithm,
we load X.sub.0-15 directly, and thereby omit Round 0 of the
optimized Process Block (Step 2) of SHA-1. The pseudocode takes on
the following steps:
TABLE-US-00008 TABLE 8 Calculating H[R, K.sub.1] Step Description
Action 1 Process K .sym. ipad X.sub.0-4 .rarw. K.sub.1 .sym.
0x363636 . . . 2 X.sub.5-15 .rarw. 0x363636 . . . 3 H.sub.1-5
{umlaut over ( )} h.sub.1-5 4 Process Block 5 Process R X.sub.0-4
.rarw. R 6 X.sub.5-15 .rarw. 0 7 Process Block 8 Buff160.sub.1-5
.rarw. H.sub.1-5 9 Process K .sym. opad X.sub.0-4 .rarw. K.sub.1
.sym. 0x5C5C5C . . . 10 X.sub.5-15 .rarw. 0x5C5C5C . . . 11
H.sub.1-5 .rarw. h.sub.1-5 12 Process Block 13 Process previous
H[x] X.sub.0-4 .rarw. Result 14 X.sub.5-15 .rarw. 0 15 Process
Block 16 Get results Buff160.sub.1-5 .rarw. H.sub.1-5
[0898] 6.3.2 H[R|M, K.sub.2]
[0899] In the case of producing the keyed hash of R|M using
K.sub.2, the original input message is a constant length of 416
(256+160) bits. We can therefore take advantage of this fact during
processing. Rather than load X.sub.0-15 during the first part of
the SHA-1 algorithm, we load X.sub.0-15 directly, and thereby omit
Round 0 of the optimized Process Block (Step 2) of SHA-1. The
pseudocode takes on the following steps:
TABLE-US-00009 TABLE 9 Calculating H[R | M, K.sub.2] Step
Description Action 1 Process K .sym. ipad X.sub.0-4 .rarw. K.sub.2
.sym. 0x363636 . . . 2 X.sub.5-15 .rarw. 0x363636 . . . 3 H.sub.1-5
.rarw. h.sub.1-5 4 Process Block 5 Process R | M X.sub.0-4 .rarw. R
6 X.sub.5-12 .rarw. M 7 X.sub.13-15 .rarw. 0 8 Process Block 9 Temp
.rarw. H.sub.1-5 10 Process K .sym. opad X.sub.0-4 .rarw. K.sub.2
.sym. 0x5C5C5C . . . 11 X.sub.5-15 .rarw. 0x5C5C5C . . . 12
H.sub.1-5 .rarw. h.sub.1-5 13 Process Block 14 Process previous
H[x] X.sub.0-4 .rarw. Temp 15 X.sub.5-15 .rarw. 0 16 Process Block
17 Get results Result .rarw. H.sub.1-5
[0900] 7 Data Storage Integrity
[0901] Each authentication chip contains some non-volatile memory
in order to hold the variables required by Authentication Protocol
C1.
[0902] The following non-volatile variables are defined:
TABLE-US-00010 TABLE 10 Non volatile variables required by Protocol
C1 Variable Size Name (in bits) Description M[0..15] 256 16 words
(each 16 bits) containing state data such as serial numbers, media
remaining etc. K.sub.1 160 Key used to transform R during
authentication K.sub.2 160 Key used to transform M during
authentication R 160 Current random number Access Mode 32 The 16
sets of 2-bit AccessMode values [0..15] for M[n] Checksum 160
S[K.sub.1 | K.sub.2]. Used to verify that K.sub.1 and K.sub.2 have
not been tampered with. MinTicks 32 The minimum number of clock
ticks between calls to key-based functions SIWritten 1 If set, the
secret key information (K.sub.1, K.sub.2, and R) has been written
to the chip. If clear, the secret information has not been written
yet. IsTrusted 1 If set, the RND and TST functions can be called,
but RD and WR functions cannot be called. If clear, the RND and TST
functions cannot be called, but RD and WR functions can be called.
Total bits 962
[0903] Note that if these variables are in Flash memory, it is not
a simple matter to write a new value to replace the old. The memory
must be erased first, and then the appropriate bits set. This has
an effect on the algorithms used to change Flash memory based
variables. For example, Flash memory cannot easily be used as shift
registers. To update a Flash memory variable by a general
operation, it is necessary to follow these steps: [0904] 1. Read
the entire N bit value into a general purpose register; [0905] 2.
Perform the operation on the general purpose register; [0906] 3.
Erase the Flash memory corresponding to the variable; and [0907] 4.
Set the bits of the Flash memory location based on the bits set in
the general-purpose register.
[0908] A RESET of the authentication chip has no effect on these
non-volatile variables.
[0909] 7.1 M and Accessmode
Variables M[0] through M[15] are used to hold consumable state
data, such as serial numbers, batch numbers, and amount of
consumable remaining. Each M[n] register is 16 bits, making the
entire M vector 256 bits (32 bytes). Clients cannot read from or
written to individual M[n] variables. Instead, the entire vector,
referred to as M, is read or written in a single logical
access.
[0910] M can be read using the RD (read) command, and written to
via the WR (write) command. The commands only succeed if K.sub.1
and K.sub.2 are both defined (SIWritten=1) and the authentication
chip is a consumable non-trusted chip (IsTrusted=0).
[0911] Although M may contain a number of different data types,
they differ only in their write permissions. Each data type can
always be read. Once in client memory, the 256 bits can be
interpreted in any way chosen by the client. The entire 256 bits of
M are read at one time instead of in smaller amounts for reasons of
security, as described in Section 5. The different write
permissions are outlined in Table 11:
TABLE-US-00011 TABLE 11 Write Permissions Data Type Access Mode
Read Only Can never be written to ReadWrite Can always be written
to Decrement Can only be written to if the new value is less than
the Only old value. Decrement Only values are typically 16-bit or
32-bit values, but can be any multiple of 16 bits.
[0912] To accomplish the protection required for writing, a 2-bit
access mode value is defined for each M[n]. The following table
defines the interpretation of the 2-bit access mode
bit-pattern:
TABLE-US-00012 TABLE 12 Bits Op Interpretation Action taken during
Write command 00 RW ReadWrite The new 16-bit value is always
written to M[n]. 01 MSR Decrement The new 16-bit value is only
written Only (Most to M[n] if it is less than the value Significant
currently in M[n]. This is used for Region) access to the Most
Significant 16 bits of a Decrement Only number. 10 NMSR Decrement
The new 16-bit value is only written Only (Not to M[n] if M[n + 1]
can also be the Most written. The NMSR access mode allows
Significant multiple precision values of 32 bits Region) and more
(multiples of 16 bits) to decrement. 11 RO Read Only The new 16-bit
value is ignored. M[n] is left unchanged.
[0913] The 16 sets of access mode bits for the 16 M[n] registers
are gathered together in a single 32-bit AccessMode register. The
32 bits of the AccessMode register correspond to M[n] with n as
follows:
TABLE-US-00013 MSB LSB 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
[0914] Each 2-bit value is stored in hi/lo format. Consequently, if
M[0-5] were access mode MSR, with M[6-15] access mode RO, the
32-bit AccessMode register would be: [0915] 11 11 11 11 11 11 11 11
11 11 01 01 01 01 01 01
[0916] During execution of a WR (write) command, AccessMode[n] is
examined for each M[n], and a decision made as to whether the new
M[n] value will replace the old.
[0917] The AccessMode register is set using the authentication
chip's SAM (Set Access Mode) command.
[0918] Note that the Decrement Only comparison is unsigned, so any
Decrement Only values that require negative ranges must be shifted
into a positive range. For example, a consumable with a Decrement
Only data item range of -50 to 50 must have the range shifted to be
0 to 100. The System must then interpret the range 0 to 100 as
being -50 to 50. Note that most instances of Decrement Only ranges
are N to 0, so there is no range shift required.
[0919] For Decrement Only data items, arrange the data in order
from most significant to least significant 16-bit quantities from
M[n] onward. The access mode for the most significant 16 bits
(stored in M[n]) should be set to MSR. The remaining registers
(M[n+1], M[n+2] etc.) should have their access modes set to
NMSR.
[0920] If erroneously set to NMSR, with no associated MSR region,
each NMSR region will be considered independently instead of being
a multi-precision comparison.
[0921] Examples of allocating M and AccessMode bits can be found in
Section 9.
[0922] 7.2 K.sub.1
K.sub.1 is the 160-bit secret key used to transform R during the
authentication protocol. K.sub.1 is programmed along with K.sub.2,
Checksum and R with the authentication chip's SSI (Set Secret
Information) command. Since K.sub.1 must be kept secret, clients
cannot directly read K.sub.1.
[0923] The commands that make use of K.sub.1 are RND and RD. RND
returns a pair R, SK.sub.1[R] where R is a random number, while RD
requires an X, S.sub.K1[X] pair as input.
[0924] K.sub.1 is used in the keyed one-way hash function
HMAC-SHA1. As such it should be programmed with a physically
generated random number, gathered from a physically random
phenomenon. K.sub.1 must NOT be generated with a computer-run
random number generator. The security of the authentication chips
depends on K.sub.1, K.sub.2 and R being generated in a way that is
not deterministic. For example, to set K.sub.1, a person can toss a
fair coin 160 times, recording heads as 1, and tails as 0.
[0925] K.sub.1 is automatically cleared to 0 upon execution of a
CLR command. It can only be programmed to a non-zero value by the
SSI command.
[0926] 7.3 K.sub.2
[0927] K.sub.2 is the 160-bit secret key used to transform M|R
during the authentication protocol. K.sub.2 is programmed along
with K.sub.1, Checksum and R with the authentication chip's SSI
(Set Secret Information) command. Since K.sub.2 must be kept
secret, clients cannot directly read K.sub.2.
[0928] The commands that make use of K.sub.2 are RD and TST. RD
returns a pair M, S.sub.K[M X] where X was passed in as one of the
parameters to the RD function. TST requires an M, S.sub.K2[M|R]
pair as input, where R was obtained from the authentication chip's
RND function.
[0929] K.sub.2 is used in the keyed one-way hash function
HMAC-SHA1. As such it should be programmed with a physically
generated random number, gathered from a physically random
phenomenon. K.sub.2 must NOT be generated with a computer-run
random number generator. The security of the authentication chips
depends on K.sub.1, K.sub.2 and R being generated in a way that is
not deterministic. For example, to set K.sub.2, a person can toss a
fair coin 160 times, recording heads as 1, and tails as 0.
[0930] K.sub.2 is automatically cleared to 0 upon execution of a
CLR command. It can only be programmed to a non-zero value by the
SSI command.
[0931] 7.4 Checksum
[0932] The Checksum register is a 160-bit number used to verify
that K.sub.1 and K.sub.2 have not been altered by an attacker.
Checksum is programmed along with K.sub.1, K.sub.2 and R with the
authentication chip's SSI (Set Secret Information) command. Since
Checksum must be kept secret, clients cannot directly read
Checksum.
[0933] The commands that make use of Checksum are any that make use
of K.sub.1 and K.sub.2-namely RND, RD, and TST. Before calculating
any revealed value based on K.sub.1 or K.sub.2 a checksum on
K.sub.1 and K.sub.2 is calculated and compared against the stored
Checksum value. The checksum calculated is the 160-bit value
S[K.sub.1|K.sub.2].
[0934] If K.sub.1 and K.sub.2 are stored as multilevel Flash
memory, the full multi-level Flash values should be used for the
verification process instead of just the subset used to represent
valid values.
[0935] Checksum is automatically cleared to 0 upon execution of a
CLR command. It can only be programmed to a non-zero value by the
SSI command.
[0936] 7.5 R and IsTrusted
[0937] R is a 160-bit random number seed that is programmed along
with K.sub.1 and K.sub.2 with the SSI (Set Secret Information)
command. R does not have to be kept secret, since it is given
freely to callers via the RND command. However R must be changed
only by the authentication chip, and not set to any chosen value by
a caller.
[0938] R is used during the TST command to ensure that the R from
the previous call to RND was used to generate the S.sub.K2[M|R]
value in the non-trusted authentication chip (ChipA). Both RND and
TST are only used in trusted authentication chips (ChipT).
[0939] IsTrusted is a 1-bit flag register that determines whether
or not the authentication chip is a trusted chip (ChipT): [0940] If
the IsTrusted bit is set, the chip is considered to be a trusted
chip, and hence clients can call RND and TST functions (but not RD
or WR). [0941] If the IsTrusted bit is clear, the chip is not
considered to be trusted. Therefore RND and TST functions cannot be
called (but RD and WR functions can be called instead). System
never needs to call RND or TST on the consumable (since a clone
chip would simply return 1 to a function such as TST, and a
constant value for RND).
[0942] The IsTrusted bit has the added advantage of reducing the
number of available R, S.sub.K1[R] pairs obtainable by an attacker,
yet still maintain the integrity of the Authentication protocol. To
obtain valid R, S.sub.K1[R] pairs, an attacker requires a System
authentication chip, which is more expensive and less readily
available than the consumables.
[0943] Both R and the IsTrusted bit are cleared to 0 by the CLR
command. They are both written to by the issuing of the SSI
command. The IsTrusted bit can only set by storing a non-zero seed
value in R via the SSI command (R must be non-zero to be a valid
LFSR state, so this is quite reasonable). R is changed via a
160-bit maximal period LFSR with taps on bits 0, 2, 3, and 5, and
is changed only by a successful call to TST (where 1 is
returned).
[0944] Authentication chips destined to be trusted Chips used in
Systems (ChipT) should have their IsTrusted bit set during
programming, and authentication chips used in Consumables (ChipA)
should have their IsTrusted bit kept clear (by storing 0 in R via
the SSI command during programming) There is no command to read or
write the IsTrusted bit directly.
[0945] The logical security of the authentication chip does not
only rely upon the randomness of K.sub.1 and K.sub.2 and the
strength of the HMAC-SHA1 algorithm. To prevent an attacker from
building a sparse lookup table, the security of the authentication
chip also depends on the range of R over the lifetime of all
Systems. What this means is that an attacker must not be able to
deduce what values of R there are in produced and future Systems.
As such R should be programmed with a physically generated random
number, gathered from a physically random phenomenon. R must NOT be
generated with a computer-run random number generator. The
generation of R must not be deterministic. For example, to generate
an R for use in a trusted System chip, a person can toss a fair
coin 160 times, recording heads as 1, and tails as 0.0 is the only
non-valid initial value for a trusted R is 0 (or the IsTrusted bit
will not be set).
[0946] 7.6 SIWritten
[0947] The SIWritten (Secret Information Written) 1-bit register
holds the status of the secret information stored within the
authentication chip. The secret information is K.sub.1, K.sub.2 and
R.
[0948] A client cannot directly access the SIWritten bit. Instead,
it is cleared via the CLR command (which also clears K.sub.1,
K.sub.2 and R). When the authentication chip is programmed with
secret keys and random number seed using the SSI command
(regardless of the value written), the SIWritten bit is set
automatically. Although R is strictly not secret, it must be
written together with K.sub.1 and K.sub.2 to ensure that an
attacker cannot generate their own random number seed in order to
obtain chosen R, S.sub.K1[R] pairs.
[0949] The SIWritten status bit is used by all functions that
access K.sub.1, K.sub.2, or R. If the SIWritten bit is clear, then
calls to RD, WR, RND, and TST are interpreted as calls to CLR.
[0950] 7.7 MinTicks
[0951] There are two mechanisms for preventing an attacker from
generating multiple calls to TST and RD functions in a short period
of time. The first is a clock limiting hardware component that
prevents the internal clock from operating at a speed more than a
particular maximum (e.g. 10 MHz). The second mechanism is the
32-bit MinTicks register, which is used to specify the minimum
number of clock ticks that must elapse between calls to key-based
functions.
[0952] The MinTicks variable is cleared to 0 via the CLR command.
Bits can then be set via the SMT (Set MinTicks) command. The input
parameter to SMT contains the bit pattern that represents which
bits of MinTicks are to be set. The practical effect is that an
attacker can only increase the value in MinTicks (since the SMT
function only sets bits). In addition, there is no function
provided to allow a caller to read the current value of this
register.
[0953] The value of MinTicks depends on the operating clock speed
and the notion of what constitutes a reasonable time between
key-based function calls (application specific). The duration of a
single tick depends on the operating clock speed. This is the
maximum of the input clock speed and the authentication chip's
clock-limiting hardware. For example, the authentication chip's
clock-limiting hardware may be set at 10 MHz (it is not
changeable), but the input clock is 1 MHz. In this case, the value
of 1 tick is based on 1 MHz, not 10 MHz. If the input clock was 20
MHz instead of 1 MHz, the value of 1 tick is based on 10 MHz (since
the clock speed is limited to 10 MHz).
[0954] Once the duration of a tick is known, the MinTicks value can
to be set. The value for MinTicks is the minimum number of ticks
required to pass between calls to the key-based RD and TST
functions. The value is a real-time number, and divided by the
length of an operating tick.
[0955] Suppose the input clock speed matches the maximum clock
speed of 10 MHz. If we want a minimum of 1 second between calls to
key based functions, the value for MinTicks is set to 10,000,000.
Consider an attacker attempting to collect X, S.sub.K1[X] pairs by
calling RND, RD and TST multiple times. If the MinTicks value is
set such that the amount of time between calls to TST is 1 second,
then each pair requires 1 second to generate. To generate 2.sup.25
pairs (only requiring 1.25 GB of storage), an attacker requires
more than 1 year. An attack requiring 2.sup.64 pairs would require
5.84.times.10.sup.11 years using a single chip, or 584 years if 1
billion chips were used, making such an attack completely
impractical in terms of time (not to mention the storage
requirements!).
[0956] With regards to K.sub.1, it should be noted that the
MinTicks variable only slows down an attacker and causes the attack
to cost more since it does not stop an attacker using multiple
System chips in parallel. However MinTicks does make an attack on
K.sub.2 more difficult, since each consumable has a different M
(part of M is random read-only data). In order to launch a
differential attack, minimally different inputs are required, and
this can only be achieved with a single consumable (containing an
effectively constant part of M). Minimally different inputs require
the attacker to use a single chip, and MinTicks causes the use of a
single chip to be slowed down. If it takes a year just to get the
data to start searching for values to begin a differential attack
this increases the cost of attack and reduces the effective market
time of a clone consumable.
[0957] 8 Authentication Chip Commands
[0958] The System communicates with the authentication chips via a
simple operation command set. This section details the actual
commands and parameters necessary for implementation of Protocol
C1.
[0959] The authentication chip is defined here as communicating to
System via a serial interface as a minimum implementation. It is a
trivial matter to define an equivalent chip that operates over a
wider interface (such as 8, 16 or 32 bits).
[0960] Each command is defined by 3-bit opcode. The interpretation
of the opcode can depend on the current value of the IsTrusted bit
and the current value of the IsWritten bit.
[0961] The following operations are defined:
TABLE-US-00014 TABLE 13 Authentication Chip Commands Op.sup.a
T.sup.b W.sup.c Mn.sup.d Input Output Description 000 -- -- CLR --
-- Clear 001 0 0 SSI [160, 160, -- Set Secret 160, 160] Information
010 0 1 RD [160, 160] [256, 160] Read M securely 010 1 1 RND --
[160, 160] Random 011 0 1 WR [256] -- Write M 011 1 1 TST [256,
160] [1] Test 100 0 1 SAM [32] [32] Set Access Mode 101 -- 1 GIT --
[1] Get IsTrusted 110 -- 1 SMT [32] -- Set MinTicks .sup.aOpcode
.sup.bIsTrusted value .sup.cIsWritten value .sup.dMnemonic
.sup.e[n] = numer of bis requied for parameter
[0962] Any command not defined in this table (for example opcode
111) is interpreted as NOP (No Operation). This is regardless of
the IsTrusted or IsWritten value, and includes any opcode other
than SSI when IsWritten=0.
[0963] Note that the opcodes for RD and RND are the same, as are
the opcodes for WR and TST. The actual command run upon receipt of
the opcode will depend on the current value of the IsTrusted bit
(as long as IsWritten is 1). Where the IsTrusted bit is clear, RD
and WR functions will be called. Where the IsTrusted bit is set,
RND and TST functions will be called. The two sets of commands are
mutually exclusive between trusted and non-trusted authentication
chips, and the same opcodes enforces this relationship.
[0964] Each of the commands is examined in detail in the subsequent
sections. Note that some algorithms are specifically designed
because Flash memory is assumed for the implementation of
non-volatile variables.
[0965] 8.1 CLR--CLEAR
[0966] Input: None
[0967] Output: None
[0968] Changes: All
[0969] The CLR (Clear) Command is designed to completely erase the
contents of all authentication chip memory. This includes all keys
and secret information, access mode bits, and state data. After the
execution of the CLR command, an authentication chip will be in a
programmable state, just as if it had been freshly manufactured. It
can be reprogrammed with a new key and reused.
[0970] A CLR command consists of simply the CLR command opcode.
Since the authentication chip is serial, this must be transferred
one bit at a time. The bit order is LSB to MSB for each command
component. A CLR command is therefore sent as bits 0-2 of the CLR
opcode. A total of 3 bits are transferred.
[0971] The CLR command can be called directly at any time.
[0972] The order of erasure is important. SIWritten must be cleared
first, to disable further calls to key access functions (such as
RND, TST, RD and WR). If the AccessMode bits are cleared before
SIWritten, an attacker could remove power at some point after they
have been cleared, and manipulate M, thereby have a better chance
of retrieving the secret information with a partial chosen text
attack.
[0973] The CLR command is implemented with the following steps:
TABLE-US-00015 TABLE 14 Steps in CLR command Step Action 1 Erase
SIWritten, IsTrusted, K.sub.1, K.sub.2, R, M 2 Erase AccessMode,
MinTicks
[0974] Once the chip has been cleared it is ready for reprogramming
and reuse. A blank chip is of no use to an attacker, since although
they can create any value for M (M can be read from and written
to), key-based functions will not provide any information as
K.sub.1 and K.sub.2 will be incorrect.
[0975] It is not necessary to consume any input parameter bits if
CLR is called for any opcode other than CLR. An attacker will
simply have to RESET the chip. The reason for calling CLR is to
ensure that all secret information has been destroyed, making the
chip useless to an attacker.
[0976] 8.2 SSI--Set Secret Information
[0977] Input: K.sub.1, K.sub.2, Checksum, R=[160 bits, 160 bits,
160 bits, 160 bits]
[0978] Output: None
[0979] Changes: K.sub.1, K.sub.2, Checksum, R, SIWritten,
IsTrusted
[0980] The SSI (Set Secret Information) command is used to load the
K.sub.1, K.sub.2 and associated Checksum variable, the R variable,
and to set SIWritten and IsTrusted flags for later calls to RND,
TST, RD and WR commands. An SSI command consists of the SSI command
opcode followed by the secret information to be stored in the
K.sub.1, K.sub.2, Checksum and R registers. Since the
authentication chip is serial, this must be transferred one bit at
a time. The bit order is LSB to MSB for each command component.
[0981] An SSI command is therefore sent as: bits 0-2 of the SSI
opcode, followed by bits 0-159 of the new value for K.sub.1, bits
0-159 of the new value for K.sub.2, bits 0-159 of the new value for
Checksum, and finally bits 0-159 of the seed value for R. A total
of 643 bits are transferred.
[0982] The K.sub.1, K.sub.2, Checksum, R, SIWritten, and IsTrusted
registers are all cleared to 0 with a CLR command. They can only be
set using the SSI command.
[0983] The SSI command uses the flag SIWritten to store the fact
that data has been loaded into K.sub.1, K.sub.2, Checksum and R. If
the SIWritten and IsTrusted flags are clear (this is the case after
a CLR instruction), then K.sub.1, K.sub.2, Checksum and R are
loaded with the new values. If either flag is set, an attempted
call to SSI results in a CLR command being executed, since only an
attacker or an erroneous client would attempt to change keys or the
random seed without calling CLR first.
[0984] The SSI command also sets the IsTrusted flag depending on
the value for R. If R=0, then the chip is considered untrustworthy,
and therefore IsTrusted remains at 0. If R.noteq.0, then the chip
is considered trustworthy, and therefore IsTrusted is set to 1.
Note that the setting of the IsTrusted bit only occurs during the
SSI command.
[0985] If an authentication chip is to be reused, the CLR command
must be called first. The keys can then be safely reprogrammed with
an SSI command, and fresh state information loaded into M using the
SAM and WR commands.
[0986] The SSI command is implemented with the following steps:
TABLE-US-00016 TABLE 15 Steps in SSI command Step Action 1 CLR 2
K.sub.1 .rarw. Read 160 bits from client 3 K.sub.2 .rarw. Read 160
bits from client 4 Checksum .rarw. Read 160 bits from client 5 R
.rarw. Read 160 bits from client 6 IF (R .noteq. 0) IsTrusted
.rarw. 1 7 SIWritten .rarw. 1
[0987] 8.3 RD--Read
[0988] Input: X, S.sub.K1[X]=[160 bits, 160 bits]
[0989] Output: M, S.sub.K2[X|M]=[256 bits, 160 bits]
[0990] Changes:
[0991] The RD (Read) command is used to securely read the entire
256 bits of state data (M) from a non-trusted authentication chip.
Only a valid authentication chip will respond correctly to the RD
request. The output bits from the RD command can be fed as the
input bits to the TST command on a trusted authentication chip for
verification, with the first 256 bits (M) stored for later use if
(as we hope) TST returns 1.
[0992] Since the authentication chip is serial, the command and
input parameters must be transferred one bit at a time. The bit
order is LSB to MSB for each command component. A RD command is
therefore: bits 0-2 of the RD opcode, followed by bits 0-159 of X,
and bits 0-159 of S.sub.K1[X]. 323 bits are transferred in total. X
and S.sub.K1[X] are obtained by calling the trusted authentication
chip's RND command. The 320 bits output by the trusted chip's RND
command can therefore be fed directly into the non-trusted chip's
RD command, with no need for these bits to be stored by System.
[0993] The RD command can only be used when the following
conditions have been met: [0994] SIWritten=1 indicating that
K.sub.1, K.sub.2, Checksum and R have been set up via the SSI
command; and [0995] IsTrusted=0 indicating the chip is not trusted
since it is not permitted to generate random number sequences;
[0996] In addition, calls to RD must wait for the MinTicksRemaining
register to reach 0. Once it has done so, the register is reloaded
with MinTicks to ensure that a minimum time will elapse between
calls to RD.
[0997] Once MinTicksRemaining has been reloaded with MinTicks, the
RD command verifies that the keys have not been tampered with. This
is accomplished by internally generating S[K.sub.1|K.sub.2] and
comparing against Checksum. This generation and comparison must
take the same amount of time regardless of whether the keys are
correct or not. If the times are not the same, an attacker can gain
information about which bits are incorrect. If the internal
verification fails, the CLR function is called to clear all the key
information and effectively destroy the chip. If K.sub.1 and
K.sub.2 are stored as multilevel Flash memory, the full multi-level
Flash values should be used for the verification process instead of
just the subset used to represent valid values. For example, if
2-bit multi-level Flash is used, K.sub.1 and K.sub.2 are
effectively 320 bits each instead of 160 for a total of 640
bits.
[0998] Once the internal keys are known to be safe, the RD command
checks to see if the input parameters are valid. This is
accomplished by internally generating S.sub.K1[X] for the input X,
and then comparing the result against the input S.sub.K1[X]. This
generation and comparison must take the same amount of time
regardless of whether the input parameters are correct or not. If
the times are not the same, an attacker can gain information about
which bits of S.sub.K1[X] are incorrect.
[0999] The only way for the input parameters to be invalid is an
erroneous System (passing the wrong bits), a case of the wrong
consumable in the wrong System, a bad trusted chip (generating bad
pairs), or an attack on the authentication chip. A constant value
of 0 is returned when the input parameters are wrong. The time
taken for 0 to be returned must be the same for all bad inputs so
that attackers can learn nothing about what was invalid.
[1000] Once the input parameters have been verified the output
values are calculated. The 256 bit content of M are transferred in
the following order: bits 0-15 of M[0], bits 0-15 of M[1], through
to bits 0-15 of M[15]. S.sub.K2[X|M] is calculated and output as
bits 0-159.
[1001] The R register is used to store the X value during the
validation of the X, S.sub.K1[X] pair. This is because RND and RD
are mutually exclusive.
[1002] The RD command is implemented with the following steps:
TABLE-US-00017 TABLE 16 Steps in RD command Step Action 1 IF
(MinTicksRemaining .noteq. 0) GOTO 1 2 MinTicksRemaining .rarw.
MinTicks 3 Hash .rarw. Calculate S.sub.K1[K.sub.1 | K.sub.2] 4 OK
.rarw. (Hash = Checksum) Note that this operation must take
constant time so an attacker cannot determine anything about the
validity of particular bits of Hash. 5 IF ( OK) GOTO CLR 6 R .rarw.
Read 160 bits from client 7 Hash .rarw. Calculate S.sub.K1[R] 8 OK
.rarw. (Hash = next 160 bits from client) Note that this operation
must take constant time so an attacker cannot determine how much of
their guess is correct. 9 IF (OK) Output 256 bits of M to client
ELSE Output 256 bits of 0 to client 10 Hash .rarw. Calculate
S.sub.K2[R | M] 11 IF (OK) Output 160 bits of Hash to client ELSE
Output 160 bits of 0 to client
[1003] 8.4 RND--Random
[1004] Input: None
[1005] Output: R, S.sub.K1[R]=[160 bits, 160 bits]
[1006] Changes: None
[1007] The RND (Random) command is used by a client to obtain a
valid R, S.sub.K1[R] pair for use in a subsequent authentication
via the RD and TST commands. Since there are no input parameters,
an RND command is therefore simply bits 0-2 of the RND opcode.
[1008] The RND command can only be used when the following
conditions have been met: [1009] SIWritten=1 indicating that
K.sub.1, K.sub.2, Checksum and R have been set up via the SSI
command; and [1010] IsTrusted=1 indicating the chip is permitted to
generate random number sequences.
[1011] RND returns both R and S.sub.K1[R] to the caller.
[1012] The 288-bit output of the RND command can be fed straight
into the non-trusted chip's RD command as the input parameters.
There is no need for the client to store them at all, since they
are not required again. However the TST command will only succeed
if the random number passed into the RD command was obtained first
from the RND command.
[1013] If a caller only calls RND multiple times, the same R,
S.sub.K1[R] pair will be returned each time. R will only advance to
the next random number in the sequence after a successful call to
TST. See TST for more information.
[1014] Before returning any information, the RND command checks to
ensure that the keys have not been tampered with by calculating
S[K.sub.i K.sub.2] and comparing against Checksum. If the keys have
been tampered with the checksum will fail and CLR is called to
erase any key information. If K.sub.i and K.sub.2 are stored as
multilevel Flash memory, the full multi-level Flash values should
be used for the verification process instead of just the subset
used to represent valid values. For example, if 2-bit multi-level
Flash is used, K.sub.1 and K.sub.2 are effectively 320 bits each
instead of 160 for a total of 640 bits
[1015] The RND command is implemented with the following steps:
TABLE-US-00018 TABLE 17 Steps in RND command Step Action 1 Hash
.rarw. Calculate S.sub.K1[K.sub.1 | K.sub.2] 2 OK .rarw. (Hash =
Checksum) Note that this operation must take constant time so an
attacker cannot determine anything about the validity of particular
bits of Hash. 3 IF ( OK) GOTO CLR 4 Output 160 bits of R to client
5 Hash .rarw. Calculate S.sub.K1[R] 6 Output 160 bits of Hash to
client
[1016] 8.5 TST--Test
[1017] Input: X, S.sub.K[R|X]=[256 bits, 160 bits]
[1018] Output: 1 or 0=[1 bit]
[1019] Changes: M, R and MinTicksRemaining (or all registers if
attack detected)
[1020] The TST (Test) command is used to authenticate a read of M
from a non-trusted authentication chip. The TST (Test) command
consists of the TST command opcode followed by input parameters: X
and S.sub.K[R|X]. Since the authentication chip is serial, this
must be transferred one bit at a time. The bit order is LSB to MSB
for each command component.
[1021] A TST command is therefore: bits 0-2 of the TST opcode,
followed by bits 0-255 of M, bits 0-159 of S.sub.K2[R|M]. 419 bits
are transferred in total. Since the last 416 input bits are
obtained as the output bits from a RD command to a non-trusted
authentication chip, the entire data does not even have to be
stored by the client. Instead, the bits can be passed directly to
the trusted authentication chip's TST command. Only the 256 bits of
M should be kept from a RD command.
[1022] The TST command can only be used when the following
conditions have been met: [1023] SIWritten=1 indicating that
K.sub.1, K.sub.2, Checksum and R have been set up via the SSI
command; and [1024] IsTrusted=1 indicating the chip is permitted to
generate random number sequences.
[1025] In addition, calls to TST must wait for the
MinTicksRemaining register to reach 0. Once it has done so, the
register is reloaded with MinTicks to ensure that a minimum time
will elapse between calls to TST.
[1026] The TST command then checks to make sure that the keys have
not been tampered. This is accomplished by internally generating
S[K.sub.1|K.sub.2] and comparing against Checksum. This generation
and comparison must take the same amount of time regardless of
whether the keys are correct or not. If the times are not the same,
an attacker can gain information about which bits are incorrect. If
the internal verification fails, the CLR function is called to
clear all the key information and effectively destroy the chip. If
K.sub.1 and K.sub.2 are stored as multilevel Flash memory, the full
multi-level Flash values should be used for the verification
process instead of just the subset used to represent valid values.
For example, if 2-bit multi-level Flash is used, K.sub.1 and
K.sub.2 are effectively 320 bits each instead of 160 for a total of
640 bits
[1027] TST causes the internal M value to be replaced by the input
M value. S.sub.K2[M|R] is then calculated, and compared against the
160 bit input hash value. A single output bit is produced: 1 if
they are the same, and 0 if they are different. The use of the
internal M value is to save space on chip, and is the reason why RD
and TST are mutually exclusive commands. If the output bit is 1, R
is updated to be the next random number in the sequence. This
forces the caller to use a new random number each time RD and TST
are called.
[1028] The resultant output bit is not output until the entire
input string has been compared, so that the time to evaluate the
comparison in the TST function is always the same. Thus no attacker
can compare execution times or number of bits processed before an
output is given.
[1029] The next random number is generated from R using a 160-bit
maximal period LFSR (tap selections on bits 5, 3, 2, and 0). The
initial 160-bit value for R is set up via the SSI command, and can
be any random number except 0 (an LFSR filled with 0s will produce
a never-ending stream of 0s). R is transformed by XORing bits 0, 2,
3, and 5 together, and shifting all 160 bits right 1 bit using the
XOR result as the input bit to b.sub.159. The new R will be
returned on the next call to RND. The LFSR is the same as that
shown in FIG. 9.
[1030] Note that the time taken for 0 to be returned from TST must
be the same for all bad inputs so that attackers can learn nothing
about what was invalid about the input.
[1031] The TST command is implemented with the following steps:
TABLE-US-00019 TABLE 18 Steps in TST command Step Action 1 IF
(MinTicksRemaining .noteq. 0) GOTO 1 2 MinTicksRemaining .rarw.
MinTicks 3 Hash .rarw. Calculate S.sub.K1[K.sub.1 | K.sub.2] 4 OK
.rarw. (Hash = Checksum) Note that this operation must take
constant time so an attacker cannot determine anything about the
validity of particular bits of Hash 5 IF (( OK) OR (R = 0)) GOTO
CLR 6 M .rarw. Read 256 bits from client 7 Hash .rarw. Calculate
S.sub.K2[R | M] 8 Hash {umlaut over ( )} (Hash = next 160 bits from
client) Note that this operation must take constant time so an
attacker cannot determine how much of their guess is correct. 9 IF
(OK) Temp .rarw. R Erase .rarw. R Advance TEMP via LFSR R .rarw.
Temp 10 Output 1 bit of OK to client
[1032] Note that we can't simply advance R directly in Step 9 since
R is Flash memory, and must be erased in order for any set bit to
become 0. If power is removed from the authentication chip during
Step 9 after erasing the old value of R, but before the new value
for R has been written, then R will be erased but not reprogrammed
We therefore have the situation of IsTrusted=1, yet R=0, a
situation only possible due to an attacker. Step 5 detects this
event (as well as the check of K.sub.1 and K.sub.2), and takes
action if the attack is detected.
[1033] The problem can be avoided by having a second 160-bit Flash
register for R and a Validity Bit, toggled after the new value has
been loaded. It has not been included in this implementation for
reasons of space, but if chip space allows it, an extra 160-bit
Flash register would be useful for this purpose.
[1034] 8.6 WR--Write
[1035] Input: M.sub.new=[256 bits]
[1036] Output: None
[1037] Changes:
[1038] A WR (Write) command is used to update the writable parts of
M containing authentication chip state data. The WR command by
itself is not secure. It must be followed by an authenticated read
of M (via a RD command) to ensure that the change was made as
specified.
[1039] The WR command is called by passing the WR command opcode
followed by the new 256 bits of data to be written to M. Since the
authentication chip is serial, the new value for M must be
transferred one bit at a time. The bit order is LSB to MSB for each
command component. A WR command is therefore: bits 0-2 of the WR
opcode, followed by bits 0-15 of M[0], bits 0-15 of M[1], through
to bits 0-15 of M[15]. 259 bits are transferred in total.
[1040] The WR command can only be used when SIWritten=1, indicating
that K.sub.1, K.sub.2, Checksum and R have been set up via the SSI
command (if SIWritten is 0, then K.sub.1, K.sub.2, Checksum and R
have not been setup yet, and the CLR command is called
instead).
[1041] The ability to write to a specific M[n] is governed by the
corresponding Access Mode bits as stored in the AccessMode
register. The AccessMode bits can be set using the SAM command
[1042] When writing the new value to M[n] the fact that M[n] is
Flash memory must be taken into account. All the bits of M[n] must
be erased, and then the appropriate bits set. Since these two steps
occur on different cycles, it leaves the possibility of attack
open. An attacker can remove power after erasure, but before
programming with the new value. However, there is no advantage to
an attacker in doing this: [1043] A Read/Write M[n] changed to 0 by
this means is of no advantage since the attacker could have written
any value using the WR command anyway. [1044] A Read Only M[n]
changed to 0 by this means allows an additional known text pair
(where the M[n] is 0 instead of the original value). For future use
M[n] values, they are already 0, so no information is given. [1045]
A Decrement Only M[n] changed to 0 simply speeds up the time in
which the consumable is used up. It does not give any new
information to an attacker that using the consumable would
give.
[1046] The WR command is implemented with the following steps:
TABLE-US-00020 TABLE 19 Steps in WR command Step Action 1
DecEncountered .rarw.0 EqEncountered .rarw. 0 n .rarw. 15 2 Temp
.rarw. Read 16 bits from client 3 AM .rarw. AccessMode[ n] Compare
to the previous value 4 LT .rarw. (Temp < M[ n]) [comparison is
unsigned] EQ .rarw. (Temp = M[ n]) 5 WE .rarw. (AM = RW) ((AM =
MSR) LT) ((AM = NMSR) (DecEncountered LT)) 6 DecEncountered .rarw.
((AM = MSR) LT) ((AM = NMSR) DecEncountered) ((AM = NMSR)
EqEncountered LT) EqEncountered .rarw. ((AM = MSR) EQ) ((AM = NMSR)
EqEncountered EQ) Advance to the next Access Mode set and write the
new M[ n] if applicable 7 IF (WE) Erase M[ n] M[ n] .rarw. Temp 8 n
9 IF (n .noteq. 0) GOTO 2
[1047] 8.7 SAM--Set AccessMode
[1048] Input: AccessMode.sub.new=[32 bits]
[1049] Output: AccessMode=[32 bits]
[1050] Changes: AccessMode
[1051] The SAM (Set Access Mode) command is used to set the 32 bits
of the AccessMode register, and is only available for use in
consumable authentication chips (where the IsTrusted flag=0).
[1052] The SAM command is called by passing the SAM command opcode
followed by a 32-bit value that is used to set bits in the
AccessMode register. Since the authentication chip is serial, the
data must be transferred one bit at a time. The bit order is LSB to
MSB for each command component. A SAM command is therefore: bits
0-2 of the SAM opcode, followed by bits 0-31 of bits to be set in
AccessMode. 35 bits are transferred in total.
[1053] The AccessMode register is only cleared to 0 upon execution
of a CLR command. Since an access mode of 00 indicates an access
mode of RW (read/write), not setting any AccessMode bits after a
CLR means that all of M can be read from and written to.
[1054] The SAM command only sets bits in the AccessMode register.
Consequently a client can change the access mode bits for M[n] from
RW to RO (read only) by setting the appropriate bits in a 32-bit
word, and calling SAM with that 32-bit value as the input
parameter. This allows the programming of the access mode bits at
different times, perhaps at different stages of the manufacturing
process. For example, the read only random data can be written to
during the initial key programming stage, while allowing a second
programming stage for items such as consumable serial numbers.
[1055] Since the SAM command only sets bits, the effect is to allow
the access mode bits corresponding to M[n] to progress from RW to
either MSR, NMSR, or RO. It should be noted that an access mode of
MSR can be changed to RO, but this would not help an attacker,
since the authentication of M after a write to a doctored
authentication chip would detect that the write was not successful
and hence abort the operation. The setting of bits corresponds to
the way that Flash memory works best.
[1056] The only way to clear bits in the AccessMode register, for
example to change a Decrement Only M[n] to be Read/Write, is to use
the CLR command. The CLR command not only erases (clears) the
AccessMode register, but also clears the keys and all of M.
[1057] Thus the AccessMode[n] bits corresponding to M[n] can only
usefully be changed once between CLR commands.
[1058] The SAM command returns the new value of the AccessMode
register (after the appropriate bits have been set due to the input
parameter). By calling SAM with an input parameter of 0, AccessMode
will not be changed, and therefore the current value of AccessMode
will be returned to the caller.
[1059] The SAM command is implemented with the following steps:
TABLE-US-00021 TABLE 20 Steps in SAM command Step Action 1 Temp
.rarw. Read 32 bits from client 2 SetBits(AccessMode, Temp) 3
Output 32 bits of AccessMode to client
[1060] 8.8 GIT--Get IsTrusted
[1061] Input: None
[1062] Output: IsTrusted=[1 bit]
[1063] Changes: None
[1064] The GIT (Get IsTrusted) command is used to read the current
value of the IsTrusted bit on the authentication chip. If the bit
returned is 1, the authentication chip is a trusted System
authentication chip. If the bit returned is 0, the authentication
chip is a consumable authentication chip.
[1065] A GIT command consists of simply the GIT command opcode.
Since the authentication chip is serial, this must be transferred
one bit at a time. The bit order is LSB to MSB for each command
component. A GIT command is therefore sent as bits 0-2 of the GIT
opcode. A total of 3 bits are transferred.
[1066] The GIT command is implemented with the following step:
TABLE-US-00022 TABLE 21 Steps in GIT command Step Action 1 Output
IsTrusted bit to client
[1067] 8.9 SMT--Set MinTicks
[1068] Input: MinTicks.sub.new=[32 bits]
[1069] Output: None
[1070] Changes: MinTicks
[1071] The SMT (Set MinTicks) command is used to set bits in the
MinTicks register and hence define the minimum number of ticks that
must pass in between calls to TST and RD. The SMT command is called
by passing the SMT command opcode followed by a 32-bit value that
is used to set bits in the MinTicks register. Since the
authentication chip is serial, the data must be transferred one bit
at a time. The bit order is LSB to MSB for each command component.
An SMT command is therefore: bits 0-2 of the SMT opcode, followed
by bits 0-31 of bits to be set in MinTicks. 35 bits are transferred
in total.
[1072] The MinTicks register is only cleared to 0 upon execution of
a CLR command. A value of 0 indicates that no ticks need to pass
between calls to key-based functions. The functions may therefore
be called as frequently as the clock speed limiting hardware allows
the chip to run.
[1073] Since the SMT command only sets bits, the effect is to allow
a client to set a value, and only increase the time delay if
further calls are made. Setting a bit that is already set has no
effect, and setting a bit that is clear only serves to slow the
chip down further. The setting of bits corresponds to the way that
Flash memory works best.
[1074] The only way to clear bits in the MinTicks register, for
example to change a value of 10 ticks to a value of 4 ticks, is to
use the CLR command. However the CLR command clears the MinTicks
register to 0 as well as clearing all keys and M. It is therefore
useless for an attacker.
[1075] Thus the MinTicks register can only usefully be changed once
between CLR commands.
[1076] The SMT command is implemented with the following steps:
TABLE-US-00023 TABLE 22 Steps in SMT command Step Action 1 Temp
.rarw. Read 32 bits from client 2 SetBits(MinTicks, Temp)
[1077] 9 Programming Authentication Chips
[1078] Authentication chips must be programmed with logically
secure information in a physically secure environment. Consequently
the programming procedures cover both logical and physical
security.
[1079] Logical security is the process of ensuring that K.sub.1,
K.sub.2, R, and the random M[n] values are generated by a
physically random process, and not by a computer. It is also the
process of ensuring that the order in which parts of the chip are
programmed is the most logically secure.
[1080] Physical security is the process of ensuring that the
programming station is physically secure, so that K.sub.1 and
K.sub.2 remain secret, both during the key generation stage and
during the lifetime of the storage of the keys. In addition, the
programming station must be resistant to physical attempts to
obtain or destroy the keys. The authentication chip has its own
security mechanisms for ensuring that K.sub.1, K.sub.2, and
Checksum are kept secret, but the Programming Station must also
keep K.sub.1 and K.sub.2 safe. The physical security of the
programming station is mentioned briefly here, but has an entire
document of its own [85].
[1081] 9.1 Overview
[1082] After manufacture, an authentication chip must be programmed
before it can be used. In all chips values for K.sub.1 and K.sub.2
must be established. If the chip is destined to be a System
authentication chip, the initial value for R must be determined If
the chip is destined to be a consumable authentication chip, R must
be set to 0, and initial values for M and AccessMode must be set
up.
[1083] The following stages are therefore identified: [1084] 0.
Manufacture [1085] 1. Determine Interaction between Systems and
Consumables [1086] 2. Determine Keys for Systems and Consumables
[1087] 3. Determine MinTicks for Systems and Consumables [1088] 4.
Program Keys, Random Seed, MinTicks and Unused M [1089] 5. Program
State Data and Access Modes
[1090] Once the consumable or system is no longer required, the
attached authentication chip can be reused. This is easily
accomplished by reprogrammed the chip starting at Stage 4
again.
[1091] Each of the stages is examined in the subsequent
sections.
[1092] 9.2 Stage 0: Manufacture
[1093] Although the manufacture of authentication chips is outlined
in Section 10, a number of points can be made here.
[1094] The algorithms and chip process is not special, and requires
no special security. Standard Flash processes are used.
[1095] At the end of the manufacturing stage, the authentication
chips are tested by being programmed with particular test programs.
There is no JTAG test mechanism.
[1096] A theft of authentication chips between the chip
manufacturer and programming station would only provide the clone
manufacturer with blank chips. This merely compromises the sale of
authentication chips, not anything authenticated by authentication
chips. Since the programming station is the only mechanism with
consumable and system product keys, a clone manufacturer would not
be able to program the chips with the correct key. Clone
manufacturers would be able to program the blank chips for their
own systems and consumables, but it would be difficult to place
these items on the market without detection. In addition, a single
theft would be difficult to base a business around.
[1097] 9.3 Stage 1: Determine Interaction Between Systems and
Consumables
[1098] The decision of what is a System and what is a Consumable
needs to be determined before any authentication chips can be
programmed A decision needs to be made about which Consumables can
be used in which Systems, since all connected Systems and
Consumables must share the same key information. They also need to
share state-data usage mechanisms even if some of the
interpretations of that data have not yet been determined
[1099] A simple example is that of a car and car-keys. The car
itself is the System, and the car-keys are the consumables. There
are several car-keys for each car, each containing the same key
information as the specific car. However each car (System) would
contain a different key (shared by its car-keys), since we don't
want car-keys from one car working in another.
[1100] Another example is that of a photocopier that requires a
particular toner cartridge. In simple terms the photocopier is the
System, and the toner cartridge is the consumable. However the
decision must be made as to what compatibility there is to be
between cartridges and photocopiers. The decision has historically
been made in terms of the physical packaging of the toner
cartridge: certain cartridges will or won't fit in a new model
photocopier based on the design decisions for that copier. When
authentication chips are used, the components that must work
together must share the same key information.
[1101] In addition, each type of consumable requires a different
way of dividing M (the state data). Although the way in which M is
used will vary from application to application, the method of
allocating M[n] and AccessMode[n] will be the same: [1102] Define
the consumable state data for specific use [1103] Set some M[n]
registers aside for future use (if required). Set these to be 0 and
Read Only. The value can be tested for in Systems to maintain
compatibility. [1104] Set the remaining M[n] registers (at least
one, but it does not have to be M[15]) to be Read Only, with the
contents of each M[n] completely random. This is to make it more
difficult for a clone manufacturer to attack the authentication
keys (see Section 5).
[1105] The following examples show ways in which the state data may
be organized.
[1106] 9.3.1 Example 1
[1107] Suppose we have a car with associated car-keys. A 16-bit key
number is more than enough to uniquely identify each car-key for a
given car.
[1108] The 256 bits of M could be divided up as follows:
TABLE-US-00024 TABLE 23 Car's 256 M bits M[n] Access Description 0
RO Key number (16 bits) 1-4 RO Car engine number (64 bits) 5-8 RO
For future expansion = 0 (64 bits) 9-15 RO Random bit data (112
bits)
[1109] If the car manufacturer keeps all logical keys for all cars,
it is a trivial matter to manufacture a new physical car-key for a
given car should one be lost. The new car-key would contain a new
Key Number in M[0], but have the same K.sub.1 and K.sub.2 as the
car's authentication chip.
[1110] Car Systems could allow specific key numbers to be
invalidated (for example if a key is lost). Such a system might
require Key 0 (the master key) to be inserted first, then all valid
keys, then Key 0 again. Only those valid keys would now work with
the car. In the worst case, for example if all car-keys are lost,
then a new set of logical keys could be generated for the car and
its associated physical car-keys if desired.
[1111] The Car engine number would be used to tie the key to the
particular car.
[1112] Future use data may include such things as rental
information, such as driver/renter details.
[1113] 9.3.2 Example 2
[1114] Suppose we have a photocopier image unit which should be
replaced every 100,000 copies. 32 bits are required to store the
number of pages remaining.
[1115] The 256 bits of M could be divided up as follows:
TABLE-US-00025 TABLE 24 Photocopier's 256 M bits M[n] Access
Description 0 RO Serial number (16 bits) 1 RO Batch number (16
bits) 2 MSR Page Count Remaining (32 bits, hi/lo) 3 NMSR 4-7 RO For
future expansion = 0 (64 bits) 8-15 RO Random bit data (128
bits)
[1116] If a lower quality image unit is made that must be replaced
after only 10,000 copies, the 32-bit page count can still be used
for compatibility with existing photocopiers. This allows several
consumable types to be used with the same system.
[1117] 9.3.3 Example 3
[1118] Consider a Polaroid camera consumable containing 25 photos.
A 16-bit countdown is all that is required to store the number of
photos remaining.
[1119] The 256 bits of M could be divided up as follows:
TABLE-US-00026 TABLE 25 Camera 256 M bits M[n] Access Description 0
RO Serial number (16 bits) 1 RO Batch number (16 bits) 2 MSR Photos
Remaining (16 bits) 3-6 RO For future expansion = 0 (64 bits) 7-15
RO Random bit data (144 bits)
[1120] The Photos Remaining value at M[2] allows a number of
consumable types to be built for use with the same camera System.
For example, a new consumable with 36 photos is trivial to
program.
[1121] Suppose 2 years after the introduction of the camera, a new
type of camera was introduced. It is able to use the old
consumable, but also can process a new film type. M[3] can be used
to define Film Type. Old film types would be 0, and the new film
types would be some new value. New Systems can take advantage of
this. Original systems would detect a non-zero value at M[3] and
realize incompatibility with new film types. New Systems would
understand the value of M[3] and so react appropriately. To
maintain compatibility with the old consumable, the new consumable
and System needs to have the same key information as the old one.
To make a clean break with a new System and its own special
consumables, a new key set would be required.
[1122] 9.3.4 Example 4
[1123] Consider a printer consumable containing 3 inks: cyan,
magenta, and yellow. Each ink amount can be decremented
separately.
[1124] The 256 bits of M could be divided up as follows:
TABLE-US-00027 TABLE 26 Printer's 256 M bits M[n] Access
Description 0 RO Serial number (16 bits) 1 RO Batch number (16
bits) 2 MSR Cyan Remaining (32 bits, hi/lo) 3 NMSR 4 MSR Magenta
Remaining (32 bits, hi/lo) 5 NMSR 6 MSR Yellow Remaining (32 bits,
hi/lo) 7 NMSR 8-11 RO For future expansion = 0 (64 bits) 12-15 RO
Random bit data (64 bits)
[1125] 9.4 Stage 2: Determine Keys for Systems and Consumables
[1126] Once the decision has been made as to which Systems and
consumables are to share the same keys, those keys must be defined.
The values for K.sub.1, K.sub.2 and their corresponding Checksum
must therefore be determined
[1127] In most cases, K.sub.1 and K.sub.2 will be generated once
for all time. All Systems and consumables that have to work
together (both now and in the future) need to have the same K.sub.1
and K.sub.2 values. K.sub.1 and K.sub.2 must therefore be kept
secret since the entire security mechanism for the
System/Consumable combination is made void if the keys are
compromised. If the keys are compromised, the damage depends on the
number of systems and consumables, and the ease to which they can
be reprogrammed with new non-compromised keys: [1128] In the case
of a photocopier with toner cartridges, the worst case is that a
clone manufacturer could then manufacture their own authentication
chips (or worse, buy them), program the chips with the known keys,
and then insert them into their own consumables. [1129] In the case
of a car with car-keys, each car has a different set of keys. This
leads to two possible general scenarios. The first is that after
the car and car-keys are programmed with the keys, K.sub.1 and
K.sub.2 are deleted so no record of their values are kept, meaning
that there is no way to compromise K.sub.1 and K.sub.2. However no
more car-keys can be made for that car without reprogramming the
car's authentication chip. The second scenario is that the car
manufacturer keeps K.sub.1 and K.sub.2, and new keys can be made
for the car. A compromise of K.sub.1 and K.sub.2 means that someone
could make a car-key specifically for a particular car.
[1130] The keys and random data used in the authentication chips
must therefore be generated by a means that is non-deterministic (a
completely computer generated pseudo-random number cannot be used
because it is deterministic--knowledge of the generator's seed
gives all future numbers). K.sub.1 and K.sub.2 should be generated
by a physically random process, and not by a computer.
[1131] However, random bit generators based on natural sources of
randomness are subject to influence by external factors and also to
malfunction. It is imperative that such devices be tested
periodically for statistical randomness.
[1132] A simple yet useful source of random numbers is the
Lavarand.RTM. system from SGI [55]. This generator uses a digital
camera to photograph six lava lamps every few minutes. Lava lamps
contain chaotic turbulent systems. The resultant digital images are
fed into an SHA-1 implementation that produces a 7-way hash,
resulting in a 160-bit value from every 7th bye from the digitized
image. These 7 sets of 160 bits total 140 bytes. The 140 byte value
is fed into a BBS generator (see Section 3.6.2 for more information
on the Blum-Blum-Shub generator) to position the start of the
output bitstream. The output 160 bits from the BBS would be the key
or the authentication chip.
[1133] An extreme example of a non-deterministic random process is
someone flipping a coin 160 times for K.sub.1 and 160 times for
K.sub.2 in a clean room. With each head or tail, a 1 or 0 is
entered on a panel of a Key Programmer Device. The process must be
undertaken with several observers (for verification) in silence
(someone may have a hidden microphone). The point to be made is
that secure data entry and storage is not as simple as it sounds.
The physical security of the Key Programmer Device and accompanying
Programming Station requires an entire document of its own
[85].
[1134] Once keys K.sub.1 and K.sub.2 have been determined, and the
checksum calculated, they must be kept for as long as
authentication chips need to be made that use the key. In the first
car/car-key scenario K.sub.1 and K.sub.2 are destroyed after a
single System chip and a few consumable chips have been programmed
In the case of the photocopier/toner cartridge, K.sub.1 and K.sub.2
must be retained for as long as the toner-cartridges are being made
for the photocopiers. The keys must be kept securely. See [85] for
more information.
[1135] 9.5 Stage 3: Determine MinTicks For Systems and
Consumables
[1136] The value of MinTicks depends on the operating clock speed
of the authentication chip (System specific) and the notion of what
constitutes a reasonable time between RD or TST function calls
(application specific). The duration of a single tick depends on
the operating clock speed. This is the maximum of the input clock
speed and the authentication chip's clock-limiting hardware. For
example, the authentication chip's clock-limiting hardware may be
set at 10 MHz (it is not changeable), but the input clock is 1 MHz.
In this case, the value of 1 tick is based on 1 MHz, not 10 MHz. If
the input clock was 20 MHz instead of 1 MHz, the value of 1 tick is
based on 10 MHz (since the clock speed is limited to 10 MHz).
[1137] Once the duration of a tick is known, the MinTicks value can
be set. The value for MinTicks is the minimum number of ticks
required to pass between calls to RD or RND key-based
functions.
[1138] Suppose the input clock speed matches the maximum clock
speed of 10 MHz. If we want a minimum of 1 second between calls to
TST, the value for MinTicks is set to 10,000,000. Even a value such
as 2 seconds might be a completely reasonable value for a System
such as a printer (one authentication per page, and one page
produced every 2 or 3 seconds).
[1139] 9.6 Stage 4: Program Keys, Random Seed, MinTicks and Unused
M
[1140] Authentication chips are in an unknown state after
manufacture. Alternatively, they have already been used in one
consumable, and must be reprogrammed for use in another. Each
authentication chip must be physically validated (to ensure it is
not a Trojan horse authentication chip--see Section 10.2.20),
cleared, and programmed with new keys and new state data.
[1141] Validation, clearing and subsequent programming of
authentication chips must take place in a secure Programming
Station environment. See [85] for more information about the
physical nature of the programming environment. For this section,
the Programming Station is considered physically secure.
[1142] 9.6.1 Programming a Trusted System Authentication Chip
[1143] If the chip is to be a trusted System chip, a seed value for
R must be generated. It must be a random number derived from a
physically random process, and must not be 0. The following tasks
must be undertaken, in the following order, and in a secure
programming environment: [1144] 1. RESET the chip [1145] 2. CLR[ ]
[1146] 3. Load R (160 bit register) with physically random data
[1147] 4. SSI[K.sub.i, K.sub.2, Checksum, R] [1148] 5.
SMT[MinTicks.sub.System]
[1149] The authentication chip is now ready for insertion into a
System. It has been completely programmed
[1150] If the System authentication chips are stolen at this point,
a clone manufacturer could use them to generate R, F.sub.K1[R]
pairs in order to launch a known text attack on K.sub.1, or to use
for launching a partially chosen-text attack on K.sub.2. This is no
different to the purchase of a number of Systems, each containing a
trusted authentication chip. The security relies on the strength of
the Authentication protocols and the randomness of K.sub.1 and
K.sub.2.
[1151] 9.6.2 Programming a Non-Trusted Consumable Authentication
Chip
[1152] If the chip is to be a non-trusted Consumable authentication
chip, the programming is slightly different to that of the trusted
System authentication chip. Firstly, the seed value for R must be
0. It must have additional programming for M and the AccessMode
values. The future use M[n] must be programmed with 0, and the
random M[n] must be programmed with random data. The following
tasks must be undertaken, in the following order, and in a secure
programming environment: [1153] 1. RESET the chip [1154] 2. CLR[ ]
[1155] 3. Load R (160 bit register) with 0 [1156] 4. SSI[K.sub.1,
K.sub.2, Checksum, R] [1157] 5. Load X (256 bit register) with 0
[1158] 6. Set bits in X corresponding to appropriate M[n] with
physically random data [1159] 7. WR[X] [1160] 8. Load Y (32 bit
register) with 0 [1161] 9. Set bits in Y corresponding to
appropriate M[n] with Read Only Access Modes [1162] 10. SAM[Y]
[1163] 11. SMT[MinTicks.sub.Consumable]
[1164] The non-trusted consumable chip is now ready to be
programmed with the general state data.
[1165] If the authentication chips are stolen at this point, an
attacker could perform a limited chosen text attack. In the best
situation, parts of M are Read Only (0 and random data), with the
remainder of M completely chosen by an attacker (via the WR
command). A number of RD calls by an attacker obtains F.sub.K2[M|R]
for a limited M. In the worst situation, M can be completely chosen
by an attacker (since all 256 bits are used for state data). In
both cases however, the attacker cannot choose any value for R
since it is supplied by calls to RND from a System authentication
chip. The only way to obtain a chosen R is by a brute force
attack.
[1166] It should be noted that if Stages 4 and 5 are carried out on
the same Programming Station (the preferred and ideal situation),
authentication chips cannot be removed in between the stages. Hence
there is no possibility of the authentication chips being stolen at
this point. The decision to program the authentication chips at one
or two times depends on the requirements of the System/Consumable
manufacturer. This decision is examined more in Stage 5, and in
[85].
[1167] 9.7 Stage 5: Program State Data and Access Modes
[1168] This stage is only required for consumable authentication
chips, since M and AccessMode registers cannot be altered on System
authentication chips.
[1169] The future use and random values of M[n] have already been
programmed in Stage 4. The remaining state data values need to be
programmed and the associated Access Mode values need to be set.
Bear in mind that the speed of this stage will be limited by the
value stored in the MinTicks register.
[1170] This stage is separated from Stage 4 on account of the
differences either in physical location or in time between
where/when Stage 4 is performed, and where/when Stage 5 is
performed. Ideally, Stages 4 and 5 are performed at the same time
in the same Programming Station.
[1171] Stage 4 produces valid authentication chips, but does not
load them with initial state values (other than 0). This is to
allow the programming of the chips to coincide with production line
runs of consumables. Although Stage 5 can be run multiple times,
each time setting a different state data value and Access Mode
value, it is more likely to be run a single time, setting all the
remaining state data values and setting all the remaining Access
Mode values. For example, a production line can be set up where the
batch number and serial number of the authentication chip is
produced according to the physical consumable being produced. This
is much harder to match if the state data is loaded at a physically
different factory.
[1172] The Stage 5 process involves first checking to ensure the
chip is a valid consumable chip, which includes a RD to gather the
data from the authentication chip, followed by a WR of the initial
data values, and then a SAM to permanently set the new data values.
The steps are outlined here: [1173] 1. IsTrusted=GIT[ ] [1174] 2.
If (IsTrusted), exit with error (wrong kind of chip!) [1175] 3.
Call RND on a valid System chip to get a valid input pair [1176] 4.
Call RD on chip to be programmed, passing in valid input pair
[1177] 5. Load X (256 bit register) with results from a RD of
authentication chip [1178] 6. Call TST on valid System chip to
ensure X and consumable chip are valid [1179] 7. If (TST returns
0), exit with error (wrong consumable chip for system) [1180] 8.
Set bits of X to initial state values [1181] 9. WR[X] [1182] 10.
Load Y (32 bit register) with 0 [1183] 11. Set bits of Y
corresponding to Access Modes for new state values [1184] 12.
SAM[Y]
[1185] Of course the validation (Steps 1 to 7) does not have to
occur if Stage 4 and 5 follow on from one another on the same
Programming Station. But it should occur in all other situations
where Stage 5 is run as a separate programming process from Stage
4.
[1186] If these authentication chips are now stolen, they are
already programmed for use in a particular consumable. An attacker
could place the stolen chips into a clone consumable. Such a theft
would limit the number of cloned products to the number of chips
stolen. A single theft should not create a supply constant enough
to provide clone manufacturers with a cost-effective business. The
alternative use for the chips is to save the attacker from
purchasing the same number of consumables, each with an
authentication chip, in order to launch a partially chosen text
attack or brute force attack. There is no special security breach
of the keys if such an attack were to occur.
[1187] 10 Manufacture
[1188] This part makes some general comments about the manufacture
and implementation of authentication chips. While the comments
presented here are general, see [84] for a detailed description of
an authentication chip for Protocol C1.
[1189] The authentication chip algorithms do not constitute a
strong encryption device. The net effect is that they can be safely
manufactured in any country (including the USA) and exported to
anywhere in the world.
[1190] The circuitry of the authentication chip must be resistant
to physical attack. A summary of manufacturing implementation
guidelines is presented, followed by specification of the chip's
physical defenses (ordered by attack).
[1191] Note that manufacturing comments are in addition to any
legal protection undertaken, such as patents, copyright, and
license agreements (for example, penalties if caught reverse
engineering the authentication chip).
[1192] 10.1 Guidelines for Manufacturing
[1193] The following are general guidelines for implementation of
an authentication chip in terms of manufacture (see [84] for a
detailed description of an authentication chip based on Protocol
C1). No special security is required during the manufacturing
process. [1194] Standard process [1195] Minimum size (if possible)
[1196] Clock Filter [1197] Noise Generator [1198] Tamper Prevention
and Detection circuitry [1199] Protected memory with tamper
detection [1200] Boot circuitry for loading program code [1201]
Special implementation of FETs for key data paths [1202] Data
connections in polysilicon layers where possible [1203]
OverUnderPower Detection Unit [1204] No test circuitry [1205]
Transparent epoxy packaging
[1206] Finally, as a general note to manufacturers of Systems, the
data line to the System authentication chip and the data line to
the Consumable authentication chip must not be the same line. See
Section 10.2.3.
[1207] 10.1.1 Standard Process
[1208] The authentication chip should be implemented with a
standard manufacturing process (such as Flash). This is necessary
to: [1209] allow a great range of manufacturing location options
[1210] take advantage of well-defined and well-behaved technology
[1211] reduce cost
[1212] Note that the standard process still allows physical
protection mechanisms.
[1213] 10.1.2 Minimum Size
[1214] The authentication chip must have a low manufacturing cost
in order to be included as the authentication mechanism for low
cost consumables. It is therefore desirable to keep the chip size
as low as reasonably possible.
[1215] Each authentication chip requires 962 bits of non-volatile
memory. In addition, the storage required for optimized HMAC-SHA1
is 1024 bits. The remainder of the chip (state machine, processor,
CPU or whatever is chosen to implement Protocol C1) must be kept to
a minimum in order that the number of transistors is minimized and
thus the cost per chip is minimized The circuit areas that process
the secret key information or could reveal information about the
key should also be minimized (see Section 10.1.8 for special data
paths).
[1216] 10.1.3 Clock Filter
[1217] The authentication chip circuitry is designed to operate
within a specific clock speed range. Since the user directly
supplies the clock signal, it is possible for an attacker to
attempt to introduce race-conditions in the circuitry at specific
times during processing. An example of this is where a high clock
speed (higher than the circuitry is designed for) may prevent an
XOR from working properly, and of the two inputs, the first may
always be returned. These styles of transient fault attacks can be
very efficient at recovering secret key information, and have been
documented in [5] and [1]. The lesson to be learned from this is
that the input clock signal cannot be trusted.
[1218] Since the input clock signal cannot be trusted, it must be
limited to operate up to a maximum frequency. This can be achieved
a number of ways.
[1219] In clock filter 100 an edge detect unit 101 passes the edge
on to a delay 102, which in turn enables a gate 103 so that the
clock signal is able to pass from the input port 104 to the output
105.
[1220] FIG. 10 shows the Clock Filter:
[1221] The delay should be set so that the maximum clock speed is a
particular frequency (e.g. about 4 MHz). Note that this delay is
not programmable--it is fixed.
[1222] The filtered clock signal would be further divided
internally as required.
[1223] 10.1.4 Noise Generator
[1224] Each authentication chip should contain a noise generator
that generates continuous circuit noise. The noise will interfere
with other electromagnetic emissions from the chip's regular
activities and add noise to the Idd signal. Placement of the noise
generator is not an issue on an authentication chip due to the
length of the emission wavelengths.
[1225] The noise generator is used to generate electronic noise,
multiple state changes each clock cycle, and as a source of
pseudo-random bits for the Tamper Prevention and Detection
circuitry (see Section 10.1.5).
[1226] A simple implementation of a noise generator is a 64-bit
maximal period LFSR seeded with a non-zero number. The clock used
for the noise generator should be running at the maximum clock rate
for the chip in order to generate as much noise as possible.
[1227] 10.1.5 Tamper Prevention and Detection circuitry
[1228] A set of circuits is required to test for and prevent
physical attacks on the authentication chip. However what is
actually detected as an attack may not be an intentional physical
attack. It is therefore important to distinguish between these two
types of attacks in an authentication chip: [1229] where you can be
certain that a physical attack has occurred. [1230] where you
cannot be certain that a physical attack has occurred.
[1231] The two types of detection differ in what is performed as a
result of the detection. In the first case, where the circuitry can
be certain that a true physical attack has occurred, erasure of
Flash memory key information is a sensible action. In the second
case, where the circuitry cannot be sure if an attack has occurred,
there is still certainly something wrong. Action must be taken, but
the action should not be the erasure of secret key information. A
suitable action to take in the second case is a chip RESET. If what
was detected was an attack that has permanently damaged the chip,
the same conditions will occur next time and the chip will RESET
again. If, on the other hand, what was detected was part of the
normal operating environment of the chip, a RESET will not harm the
key.
[1232] A good example of an event that circuitry cannot have
knowledge about, is a power glitch. The glitch may be an
intentional attack, attempting to reveal information about the key.
It may, however, be the result of a faulty connection, or simply
the start of a power-down sequence. It is therefore best to only
RESET the chip, and not erase the key. If the chip was powering
down, nothing is lost. If the System is faulty, repeated RESETs
will cause the consumer to get the System repaired. In both cases
the consumable is still intact.
[1233] A good example of an event that circuitry can have knowledge
about, is the cutting of a data line within the chip. If this
attack is somehow detected, it could only be a result of a faulty
chip (manufacturing defect) or an attack. In either case, the
erasure of the secret information is a sensible step to take.
[1234] Consequently each authentication chip should have 2 Tamper
Detection Lines--one for definite attacks, and one for possible
attacks. Connected to these Tamper Detection Lines would be a
number of Tamper Detection test units, each testing for different
forms of tampering. In addition, we want to ensure that the Tamper
Detection Lines and Circuits themselves cannot also be tampered
with.
[1235] At one end of the Tamper Detection Line 110 is a source of
pseudo-random bits 111 (clocking at high speed compared to the
general operating circuitry). The Noise Generator circuit described
above is an adequate source. The generated bits pass through two
different paths--one 112 carries the original data, and the other
113 carries the inverse of the data; it having passed through an
inverter 114. The wires carrying these bits are in the layer above
the general chip circuitry (for example, the memory, the key
manipulation circuitry etc.). The wires must also cover the random
bit generator. The bits are recombined at a number of places via an
XOR gate 115. If the bits are different (they should be), a 1 is
output, and used by the particular unit (for example, each output
bit from a memory read should be ANDed with this bit value). The
lines finally come together at the Flash memory Erase circuit,
where a complete erasure is triggered by a 0 from the XOR. Attached
to the line is a number of triggers, each detecting a physical
attack on the chip. Each trigger has oversize nMOS transistors,
such as 116, attached to GND. The Tamper Detection Line physically
goes through these nMOS transistors. If the test fails, the trigger
causes the Tamper Detect Line to become 0. The XOR test will
therefore fail on either this clock cycle or the next one (on
average), thus RESETing or erasing the chip.
[1236] FIG. 11 illustrates the basic circuitry of a Tamper
Detection Line with its output connected to either the Erase or
RESET circuitry.
[1237] The Tamper Detection Line must go through the drain 120 of
an output transistor 116 for each test, as illustrated by FIG.
12:
[1238] It is not possible to break the Tamper Detect Line since
this would stop the flow of 1s and 0s from the random source. The
XOR tests would therefore fail. As the Tamper Detect Line
physically passes through each test, it is not possible to
eliminate any particular test without breaking the Tamper Detect
Line.
[1239] It is important that the XORs take values from a variety of
places along the Tamper Detect Lines in order to reduce the chances
of an attack. FIG. 13 illustrates the taking of multiple XORs,
indicated generally at 130, from the Tamper Detect Line 110 to be
used in the different parts of the chip. Each of these XORs 130 can
be considered to be generating a ChipOK bit that can be used within
each unit or sub-unit.
[1240] A sample usage would be to have an OK bit in each unit that
is ANDed with a given ChipOK bit each cycle. The OK bit is loaded
with 1 on a RESET. If OK is 0, that unit will fail until the next
RESET. If the Tamper Detect Line is functioning correctly, the chip
will either RESET or erase all key information. If the RESET or
erase circuitry has been destroyed, then this unit will not
function, thus thwarting an attacker.
[1241] The destination of the RESET and Erase line and associated
circuitry is very context sensitive. It needs to be protected in
much the same way as the individual tamper tests. There is no point
generating a RESET pulse if the attacker can simply cut the wire
leading to the RESET circuitry. The actual implementation will
depend very much on what is to be cleared at RESET, and how those
items are cleared.
[1242] The Tamper Lines cover the noise generator circuitry of the
chip. The generator and NOT gate are on one level, while the Tamper
Detect Lines run on a level above the generator.
[1243] 10.1.6 Protected Memory with Tamper Detection
[1244] It is not enough to simply store secret information or
program code in Flash memory. The Flash memory and RAM must be
protected from an attacker who would attempt to modify (or set) a
particular bit of program code or key information. The mechanism
used must conform to being used in the Tamper Detection Circuitry
(described above).
[1245] The first part of the solution is to ensure that the Tamper
Detection Line passes directly above each Flash or RAM bit. This
ensures that an attacker cannot probe the contents of Flash or RAM.
A breach of the covering wire is a break in the Tamper Detection
Line. The breach causes the Erase signal to be set, thus deleting
any contents of the memory. The high frequency noise on the Tamper
Detection Line also obscures passive observation.
[1246] The second part of the solution for Flash is to use
multi-level data storage, but only to use a subset of those
multiple levels for valid bit representations. Normally, when
multi-level Flash storage is used, a single floating gate holds
more than one bit. For example, a 4-voltage-state transistor can
represent two bits. Assuming a minimum and maximum voltage
representing 00 and 11 respectively, the two middle voltages
represent 01 and 10. In the authentication chip, we can use the two
middle voltages to represent a single bit, and consider the two
extremes to be invalid states. If an attacker attempts to force the
state of a bit one way or the other by closing or cutting the
gate's circuit, an invalid voltage (and hence invalid state)
results.
[1247] The second part of the solution for RAM is to use a parity
bit. The data part of the register can be checked against the
parity bit (which will not match after an attack).
[1248] The bits coming from Flash and RAM can therefore be
validated by a number of test units (one per bit) connected to the
common Tamper Detection Line. The Tamper Detection circuitry would
be the first circuitry the data passes through (thus stopping an
attacker from cutting the data lines).
[1249] While the multi-level Flash protection is enough for
non-secret information, such as program code, R, and MinTicks, it
is not sufficient for protecting K.sub.1 and K.sub.2. If an
attacker adds electrons to a gate (see Section 3.8.2.15)
representing a single bit of K.sub.1, and the chip boots up yet
doesn't activate the Tamper Detection Line, the key bit must have
been a 0. If it does activate the Tamper Detection Line, it must
have been a 1. For this reason, all other non-volatile memory can
activate the Tamper Detection Line, but K.sub.1 and K.sub.2 must
not. Consequently Checksum is used to check for tampering of
K.sub.1 and K.sub.2. A signature of the expanded form of K.sub.1
and K.sub.2 (i.e. 320 bits instead of 160 bits for each of K.sub.1
and K.sub.2) is produced, and the result compared against the
Checksum. Any non-match causes a clear of all key information.
[1250] 10.1.7 Boot Circuitry for Loading Program Code
[1251] Program code should be kept in multi-level Flash instead of
ROM, since ROM is subject to being altered in a non-testable way. A
boot mechanism is therefore required to load the program code into
Flash memory (Flash memory is in an indeterminate state after
manufacture).
[1252] The boot circuitry must not be in ROM--a small state-machine
would suffice. Otherwise the boot code could be modified in an
undetectable way.
[1253] The boot circuitry must erase all Flash memory, check to
ensure the erasure worked, and then load the program code. Flash
memory must be erased before loading the program code. Otherwise an
attacker could put the chip into the boot state, and then load
program code that simply extracted the existing keys. The state
machine must also check to ensure that all Flash memory has been
cleared (to ensure that an attacker has not cut the Erase line)
before loading the new program code.
[1254] The loading of program code must be undertaken by the secure
Programming Station before secret information (such as keys) can be
loaded. This step must be undertaken as the first part of the
programming process described in Section 9.6.
[1255] 10.1.8 Special Implementation of FETs for Key Data Paths
[1256] The normal situation for FET implementation for the case of
a CMOS Inverter 140, which involves a pMOS transistor 141 combined
with an nMOS transistor 142 as shown in FIG. 14.
[1257] FIG. 15 is the voltage/current diagram for the CMOS inverter
140. During the transition, there is a small period of time 150
where both the nMOS transistor 142 and the pMOS transistor 141 have
an intermediate resistance. The resultant power-ground short
circuit causes a temporary increase in the current, and in fact
accounts for the majority of current consumed by a CMOS device. A
small amount of infrared light is emitted during the short circuit,
and can be viewed through the silicon substrate (silicon is
transparent to infrared light). A small amount of light is also
emitted during the charging and discharging of the transistor gate
capacitance and transmission line capacitance.
[1258] For circuitry that manipulates secret key information, such
information must be kept hidden. An alternative non-flashing CMOS
160 implementation should therefore be used for all data paths that
manipulate the key or a partially calculated value that is based on
the key.
[1259] The use of two non-overlapping clocks .phi.1 and .phi.2 can
provide a non-flashing mechanism .phi.1 is connected to a second
gate 161 of all nMOS transistors 162, and .phi.2 is connected to a
second gate 163 of all pMOS transistors 164. The transition can
only take place in combination with the clock. Since .phi.1 and
.phi.2 are non-overlapping, the pMOS and nMOS transistors will not
have a simultaneous intermediate resistance. The setup is shown in
FIG. 16, and the impedance diagram in FIG. 17.
[1260] Finally, regular CMOS inverters can be positioned near
critical non-Flashing CMOS components. These inverters should take
their input signal from the Tamper Detection Line above. Since the
Tamper Detection Line operates multiple times faster than the
regular operating circuitry, the net effect will be a high rate of
light-bursts next to each non-Flashing CMOS component. Since a
bright light overwhelms observation of a nearby faint light, an
observer will not be able to detect what switching operations are
occurring in the chip proper. These regular CMOS inverters will
also effectively increase the amount of circuit noise, reducing the
SNR and obscuring useful EMI.
[1261] There are a number of side effects due to the use of
non-Flashing CMOS: [1262] The effective speed of the chip is
reduced by twice the rise time of the clock per clock cycle. This
is not a problem for an authentication chip. [1263] The amount of
current drawn by the non-Flashing CMOS is reduced (since the short
circuits do not occur). However, this is offset by the use of
regular CMOS inverters. [1264] Routing of the clocks increases chip
area, especially since multiple versions of .phi.1 and .phi.2 are
required to cater for different levels of propagation. The
estimation of chip area is double that of a regular implementation.
[1265] Design of the non-Flashing areas of the authentication chip
are slightly more complex than to do the same with a with a regular
CMOS design. In particular, standard cell components cannot be
used, making these areas full custom. This is not a problem for
something as small as an authentication chip, particularly when the
entire chip does not have to be protected in this manner.
[1266] 10.1.9 Connections in Polysilicon Layers where Possible
[1267] Wherever possible, the connections along which the key or
secret data flows, should be made in the polysilicon layers. Where
necessary, they can be in metal 1, but must never be in the top
metal layer (containing the Tamper Detection Lines).
[1268] 10.1.10 OverUnderPower Detection Unit
[1269] Each authentication chip requires an OverUnderPower
Detection Unit to prevent Power Supply Attacks. An OverUnderPower
Detection Unit detects power glitches and tests the power level
against a Voltage Reference to ensure it is within a certain
tolerance. The Unit contains a single Voltage Reference and two
comparators. The OverUnderPower Detection Unit would be connected
into the RESET Tamper Detection Line, thus causing a RESET when
triggered.
[1270] A side effect of the OverUnderPower Detection Unit is that
as the voltage drops during a power-down, a RESET is triggered,
thus erasing any work registers.
[1271] 10.1.11 No Test Circuitry
[1272] Test hardware on an authentication chip could very easily
introduce vulnerabilities. As a result, the authentication chip
should not contain any BIST or scan paths.
[1273] The authentication chip must therefore be testable with
external test vectors. This should be possible since the
authentication chip is not complex.
[1274] 10.1.12 Transparent Epoxy Packaging
[1275] The authentication chip needs to be packaged in transparent
epoxy so it can be photo-imaged by the programming station to
prevent Trojan horse attacks. The transparent packaging does not
compromise the security of the authentication chip since an
attacker can fairly easily remove a chip from its packaging. For
more information see Section 10.2.20 and [85].
[1276] 10.2 Resistance To Physical Attacks
[1277] While this part only describes manufacture in general terms
(since this document does not cover a specific implementation of a
Protocol C1 authentication chip), we can still make some
observations about such a chip's resistance to physical attack. A
description of the general form of each physical attack can be
found in Section 3.8.2.
[1278] 10.2.1 Reading ROM
[1279] This attack depends on the key being stored in an
addressable ROM. Since each authentication chip stores its
authentication keys in internal Flash memory and not in an
addressable ROM, this attack is irrelevant.
[1280] 10.2.2 Reverse Engineering the Chip
[1281] Reverse engineering a chip is only useful when the security
of authentication lies in the algorithm alone. However our
authentication chips rely on a secret key, and not in the secrecy
of the algorithm. Our authentication algorithm is, by contrast,
public, and in any case, an attacker of a high volume consumable is
assumed to have been able to obtain detailed plans of the internals
of the chip.
[1282] In light of these factors, reverse engineering the chip
itself, as opposed to the stored data, poses no threat.
[1283] 10.2.3 Usurping the Authentication Process
[1284] There are several forms this attack can take, each with
varying degrees of success. In all cases, it is assumed that a
clone manufacturer will have access to both the System and the
consumable designs.
[1285] An attacker may attempt to build a chip that tricks the
System into returning a valid code instead of generating an
authentication code. This attack is not possible for two reasons.
The first reason is that System authentication chips and Consumable
authentication chips, although physically identical, are programmed
differently. In particular, the RD opcode and the RND opcode are
the same, as are the WR and TST opcodes. A System authentication
Chip cannot perform a RD command since every call is interpreted as
a call to RND instead. The second reason this attack would fail is
that separate serial data lines are provided from the System to the
System and Consumable authentication chips. Consequently neither
chip can see what is being transmitted to or received from the
other.
[1286] If the attacker builds a clone chip that ignores WR commands
(which decrement the consumable remaining), Protocol C1 ensures
that the subsequent RD will detect that the WR did not occur. The
System will therefore not go ahead with the use of the consumable,
thus thwarting the attacker. The same is true if an attacker
simulates loss of contact before authentication--since the
authentication does not take place, the use of the consumable
doesn't occur.
[1287] An attacker is therefore limited to modifying each System in
order for clone consumables to be accepted (see Section 10.2.4 for
details of resistance this attack).
[1288] 10.2.4 Modification of System
[1289] The simplest method of modification is to replace the
System's authentication chip with one that simply reports success
for each call to TST. This can be thwarted by System calling TST
several times for each authentication, with the first few times
providing false values, and expecting a fail from TST. The final
call to TST would be expected to succeed. The number of false calls
to TST could be determined by some part of the returned result from
RD or from the system clock. Unfortunately an attacker could simply
rewire System so that the new System clone authentication chip can
monitor the returned result from the consumable chip or clock. The
clone System authentication chip would only return success when
that monitored value is presented to its TST function. Clone
consumables could then return any value as the hash result for RD,
as the clone System chip would declare that value valid. There is
therefore no point for the System to call the System authentication
chip multiple times, since a rewiring attack will only work for the
System that has been rewired, and not for all Systems. For more
information see Section 5.2.4.
[1290] A similar form of attack on a System is a replacement of the
System ROM. The ROM program code can be altered so that the
Authentication never occurs. There is nothing that can be done
about this, since the System remains in the hands of a consumer. Of
course this would void any warranty, but the consumer may consider
the alteration worthwhile if the clone consumable were extremely
cheap and more readily available than the original item.
[1291] The System/consumable manufacturer must therefore determine
how likely an attack of this nature is. Such a study must include
given the pricing structure of Systems and Consumables, frequency
of System service, advantage to the consumer of having a physical
modification performed, and where consumers would go to get the
modification performed.
[1292] The likelihood of physical alteration increases with the
perceived artificiality of the consumable marketing scheme. It is
one thing for a consumable to be protected against clone
manufacturers. It is quite another for a consumable's market to be
protected by a form of exclusive licensing arrangement that creates
what is viewed by consumers as artificial markets. In the former
case, owners are not so likely to go to the trouble of modifying
their system to allow a clone manufacturer's goods. In the latter
case, consumers are far more likely to modify their System. A case
in point is DVD. Each DVD is marked with a region code, and will
only play in a DVD player from that region. Thus a DVD from the USA
will not play in an Australian player, and a DVD from Japan, Europe
or Australia will not play in a USA DVD player. Given that certain
DVD titles are not available in all regions, or because of quality
differences, pricing differences or timing of releases, many
consumers have had their DVD players modified to accept DVDs from
any region. The modification is usually simple (it often involves
soldering a single wire), voids the owner's warranty, and often
costs the owner some money. But the interesting thing to note is
that the change is not made so the consumer can use clone
consumables--the consumer will still only buy real consumables, but
from different regions. The modification is performed to remove
what is viewed as an artificial barrier, placed on the consumer by
the movie companies. In the same way, a System/Consumable scheme
that is viewed as unfair will result in people making modifications
to their Systems.
[1293] The limit case of modifying a system is for a clone
manufacturer to provide a completely clone System which takes clone
consumables. This may be simple competition or violation of
patents. Either way, it is beyond the scope of the authentication
chip and depends on the technology or service being cloned.
[1294] 10.2.5 Direct Viewing of Chip Operation by Conventional
Probing
[1295] In order to view the chip operation, the chip must be
operating. However, the Tamper Prevention and Detection circuitry
covers those sections of the chip that process or hold the key. It
is not possible to view those sections through the Tamper
Prevention lines.
[1296] An attacker cannot simply slice the chip past the Tamper
Prevention layer, for this will break the Tamper Detection Lines
and cause an erasure of all keys at power-up. Simply destroying the
erasure circuitry is not sufficient, since the multiple ChipOK bits
(now all 0) feeding into multiple units within the authentication
chip will cause the chip's regular operating circuitry to stop
functioning.
[1297] To set up the chip for an attack, then, requires the
attacker to delete the Tamper Detection lines, stop the Erasure of
Flash memory, and somehow rewire the components that relied on the
ChipOK lines. Even if all this could be done, the act of slicing
the chip to this level will most likely destroy the charge patterns
in the non-volatile memory that holds the keys, making the process
fruitless.
[1298] 10.2.6 Direct Viewing of the Non-Volatile Memory
[1299] If the authentication chip were sliced so that the floating
gates of the Flash memory were exposed, without discharging them,
then the keys could probably be viewed directly using an STM or
SKM.
[1300] However, slicing the chip to this level without discharging
the gates is probably impossible. Using wet etching, plasma
etching, ion milling, or chemical mechanical polishing will almost
certainly discharge the small charges present on the floating
gates. This is true of regular Flash memory, but even more so of
multi-level Flash memory.
[1301] 10.2.7 Viewing the Light Bursts Caused by State Changes
[1302] All sections of circuitry that manipulate secret key
information are implemented in the non-Flashing CMOS described
above. This prevents the emission of the majority of light bursts.
Regular CMOS inverters placed in close proximity to the
non-Flashing CMOS will hide any faint emissions caused by capacitor
charge and discharge. The inverters are connected to the Tamper
Detection circuitry, so they change state many times (at the high
clock rate) for each non-Flashing CMOS state change.
[1303] 10.2.8 Viewing the Keys Using an SEPM
[1304] An SEPM attack can be simply thwarted by adding a metal
layer to cover the circuitry. However an attacker could etch a hole
in the layer, so this is not an appropriate defense.
[1305] The Tamper Detection circuitry described above will shield
the signal as well as cause circuit noise. The noise will actually
be a greater signal than the one that the attacker is looking for.
If the attacker attempts to etch a hole in the noise circuitry
covering the protected areas, the chip will not function, and the
SEPM will not be able to read any data.
[1306] An SEPM attack is therefore fruitless.
[1307] 10.2.9 Monitoring EMI
[1308] The Noise Generator described above will cause circuit
noise. The noise will interfere with other electromagnetic
emissions from the chip's regular activities and thus obscure any
meaningful reading of internal data transfers.
[1309] 10.2.10 Viewing I.sub.dd Fluctuations
[1310] The solution against this kind of attack is to decrease the
SNR in the Idd signal. This is accomplished by increasing the
amount of circuit noise and decreasing the amount of signal.
[1311] The Noise Generator circuit (which also acts as a defense
against EMI attacks) will also cause enough state changes each
cycle to obscure any meaningful information in the Idd signal.
[1312] In addition, the special Non-Flashing CMOS implementation of
the key-carrying data paths of the chip prevents current from
flowing when state changes occur. This has the benefit of reducing
the amount of signal.
[1313] 10.2.11 Differential Fault Analysis
[1314] Differential fault bit errors are introduced in a
non-targeted fashion by ionization, microwave radiation, and
environmental stress. The most likely effect of an attack of this
nature is a change in Flash memory (causing an invalid state) or
RAM (bad parity). Invalid states and bad parity are detected by the
Tamper Detection Circuitry, and cause an erasure of the key.
[1315] Since the Tamper Detection Lines cover the key manipulation
circuitry, any error introduced in the key manipulation circuitry
will be mirrored by an error in a Tamper Detection Line. If the
Tamper Detection Line is affected, the chip will either continually
RESET or simply erase the key upon a power-up, rendering the attack
fruitless.
[1316] Rather than relying on a non-targeted attack and hoping that
"just the right part of the chip is affected in just the right
way", an attacker is better off trying to introduce a targeted
fault (such as overwrite attacks, gate destruction etc.). For
information on these targeted fault attacks, see the relevant
sections below.
[1317] 10.2.12 Clock Glitch Attacks
[1318] The Clock Filter (described above) eliminates the
possibility of clock glitch attacks.
[1319] 10.2.13 Power Supply Attacks
[1320] The OverUnderPower Detection Unit (described above)
eliminates the possibility of power supply attacks.
[1321] 10.2.14 Overwriting ROM
[1322] Authentication chips store program code, keys and secret
information in Flash memory, and not in ROM. This attack is
therefore not possible.
[1323] 10.2.15 Modifying EEPROM/Flash
[1324] Authentication chips store program code, keys and secret
information in multi-level Flash memory. However the Flash memory
is covered by two Tamper Prevention and Detection Lines. If either
of these lines is broken (in the process of destroying a gate via a
laser-cutter) the attack will be detected on power-up, and the chip
will either RESET (continually) or erase the keys from Flash
memory. This process is described in Section 10.1.6.
[1325] Even if an attacker is able to somehow access the bits of
Flash and destroy or short out the gate holding a particular bit,
this will force the bit to have no charge or a full charge. These
are both invalid states for the authentication chip's usage of the
multi-level Flash memory (only the two middle states are valid).
When that data value is transferred from Flash, detection circuitry
will cause the Erasure Tamper Detection Line to be
triggered--thereby erasing the remainder of Flash memory and
RESETing the chip. This is true for program code, and non-secret
information. As key data is read from multi-level flash memory, it
is not immediately checked for validity (otherwise information
about the key is given away). Instead, a specific key validation
mechanism is used to protect the secret key information.
[1326] An attacker could theoretically etch off the upper levels of
the chip, and deposit enough electrons to change the state of the
multi-level Flash memory by 1/3. If the beam is high enough energy
it might be possible to focus the electron beam through the Tamper
Prevention and Detection Lines. As a result, the authentication
chip must perform a validation of the keys before replying to the
Random, Test or Random commands. The SHA-1 algorithm must be run on
the keys, and the results compared against an internal checksum
value. This gives an attacker a 1 in 2.sup.160 chance of tricking
the chip, which is the same chance as guessing either of the
keys.
[1327] A Modify EEPROM/Flash attack is therefore fruitless.
[1328] 10.2.16 Gate Destruction Attacks
[1329] Gate Destruction Attacks rely on the ability of an attacker
to modify a single gate to cause the chip to reveal information
during operation. However any circuitry that manipulates secret
information is covered by one of the two Tamper Prevention and
Detection lines. If either of these lines is broken (in the process
of destroying a gate) the attack will be detected on power-up, and
the chip will either RESET (continually) or erase the keys from
Flash memory.
[1330] To launch this kind of attack, an attacker must first
reverse-engineer the chip to determine which gate(s) should be
targeted. Once the location of the target gates has been
determined, the attacker must break the covering Tamper Detection
line, stop the Erasure of Flash memory, and somehow rewire the
components that rely on the ChipOK lines. Rewiring the circuitry
cannot be done without slicing the chip, and even if it could be
done, the act of slicing the chip to this level will most likely
destroy the charge patterns in the non-volatile memory that holds
the keys, making the process fruitless.
[1331] 10.2.17 Overwrite Attack
[1332] An overwrite attack relies on being able to set individual
bits of the key without knowing the previous value. It relies on
probing the chip, as in the conventional probing attack and
destroying gates as in the gate destruction attack. Both of these
attacks (as explained in their respective sections), will not
succeed due to the use of the Tamper Prevention and Detection
Circuitry and ChipOK lines.
[1333] However, even if the attacker is able to somehow access the
bits of Flash and destroy or short out the gate holding a
particular bit, this will force the bit to have no charge or a full
charge. These are both invalid states for the authentication chip's
usage of the multi-level Flash memory (only the two middle states
are valid). When that data value is transferred from Flash
detection circuitry will cause the Erasure Tamper Detection Line to
be triggered--thereby erasing the remainder of Flash memory and
RESETing the chip. In the same way, a parity check on tampered
values read from RAM will cause the Erasure Tamper Detection Line
to be triggered.
[1334] An overwrite attack is therefore fruitless.
[1335] 10.2.18 Memory Remanence Attack
[1336] Any working registers or RAM within the authentication chip
may be holding part of the authentication keys when power is
removed. The working registers and RAM would continue to hold the
information for some time after the removal of power. If the chip
were sliced so that the gates of the registers/RAM were exposed,
without discharging them, then the data could probably be viewed
directly using an STM.
[1337] The first defense can be found above, in the description of
defense against power glitch attacks. When power is removed, all
registers and RAM are cleared, just as the RESET condition causes a
clearing of memory.
[1338] The chances then, are less for this attack to succeed than
for a reading of the Flash memory. RAM charges (by nature) are more
easily lost than Flash memory. The slicing of the chip to reveal
the RAM will certainly cause the charges to be lost (if they
haven't been lost simply due to the memory not being refreshed and
the time taken to perform the slicing).
[1339] This attack is therefore fruitless.
[1340] 10.2.19 Chip Theft Attack
[1341] There are distinct phases in the lifetime of an
authentication chip. Chips can be stolen when at any of these
stages: [1342] After manufacture, but before programming of key
[1343] After programming of key, but before programming of state
data [1344] After programming of state data, but before insertion
into the consumable or system [1345] After insertion into the
system or consumable
[1346] A theft in between the chip manufacturer and programming
station would only provide the clone manufacturer with blank chips.
This merely compromises the sale of authentication chips, not
anything authenticated by the authentication chips. Since the
programming station is the only mechanism with consumable and
system product keys, a clone manufacturer would not be able to
program the chips with the correct key. Clone manufacturers would
be able to program the blank chips for their own Systems and
Consumables, but it would be difficult to place these items on the
market without detection.
[1347] The second form of theft can only happen in a situation
where an authentication chip passes through two or more distinct
programming phases. This is possible, but unlikely. In any case,
the worst situation is where no state data has been programmed, so
all of M is read/write. If this were the case, an attacker could
attempt to launch an adaptive chosen text attack on the chip. The
HMAC-SHA1 algorithm is resistant to such attacks. For more
information see Section 5.5.
[1348] The third form of theft would have to take place in between
the programming station and the installation factory. The
authentication chips would already be programmed for use in a
particular system or for use in a particular consumable. The only
use these chips have to a thief is to place them into a clone
System or clone Consumable. Clone systems are irrelevant--a cloned
System would not even require an authentication chip. For clone
Consumables, such a theft would limit the number of cloned products
to the number of chips stolen. A single theft should not create a
supply constant enough to provide clone manufacturers with a
cost-effective business.
[1349] The final form of theft is where the System or Consumable
itself is stolen. When the theft occurs at the manufacturer,
physical security protocols must be enhanced. If the theft occurs
anywhere else, it is a matter of concern only for the owner of the
item and the police or insurance company. The security mechanisms
that the authentication chip uses assume that the consumables and
systems are in the hands of the public. Consequently, having them
stolen makes no difference to the security of the keys.
[1350] 10.2.20 Trojan Horse Attack
[1351] A Trojan horse attack involves an attacker inserting a fake
authentication chip into the programming station and retrieving the
same chip after it has been programmed with the secret key
information. The difficulty of these two tasks depends on both
logical and physical security, but is an expensive attack--the
attacker has to manufacture a false authentication chip, and it
will only be useful where the effort is worth the gain. For
example, obtaining the secret key for a specific car's
authentication chip is most likely not worth an attacker's efforts,
while the key for a printer's ink cartridge may be very
valuable.
[1352] The problem arises if the programming station is unable to
tell a Trojan horse authentication chip from a real one--which is
the problem of authenticating the authentication chip.
[1353] One solution to the authentication problem is for the
manufacturer to have a programming station attached to the end of
the production line. Chips passing the manufacture QA tests are
programmed with the manufacturer's secret key information. The chip
can therefore be verified by the C1 authentication protocol, and
give information such as the expected batch number, serial number
etc. The information can be verified and recorded, and the valid
chip can then be reprogrammed with the System or Consumable key and
state data. An attacker would have to substitute an authentication
chip with a Trojan horse programmed with the manufacturer's secret
key information and copied batch number data from the removed
authentication chip. This is only possible if the manufacturer's
secret key is compromised (the key is changed regularly and not
known by a human) or if the physical security at the manufacturing
plant is compromised at the end of the manufacturing chain.
[1354] Even if the solution described were to be undertaken, the
possibility of a Trojan horse attack does not go away--it merely is
removed to the manufacturer's physical location. A better solution
requires no physical security at the manufacturing location.
[1355] The preferred solution then, is to use transparent epoxy on
the chip's packaging and to image the chip before programming it.
Once the chip has been mounted for programming it is in a known
fixed orientation. It can therefore be high resolution photo-imaged
and X-rayed from multiple directions, and the images compared
against "signature" images. Any chip not matching the image
signature is treated as a Trojan horse and rejected.
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* * * * *
References