U.S. patent application number 09/816684 was filed with the patent office on 2002-01-31 for method and apparatus for roaming use of cryptographic values.
Invention is credited to Freund, Peter C., Huang, Stuart T.F., Sudia, Frank W..
Application Number | 20020013898 09/816684 |
Document ID | / |
Family ID | 26858080 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020013898 |
Kind Code |
A1 |
Sudia, Frank W. ; et
al. |
January 31, 2002 |
Method and apparatus for roaming use of cryptographic values
Abstract
A multi-step signing system and method uses multiple signing
devices to affix a single signature which can be verified using a
single public verification key. Each signing device posesses a
share of the signature key and affixes a partial signature in
response to authorization from a plurality of authorizing agents.
In a serial embodiment, after a first partial signature has been
affixed, a second signing device exponentiates the first partial
signature. In a parallel embodiment, each signing device affixes a
partial signature, and the plurality of partial signatures are
multiplied together to form the final signature. Security of the
system is enhanced by distributing capability to affix signatures
among a plurality of signing devices and by distributing authority
us affix a partial signature among a plurality of authorizing
agents.
Inventors: |
Sudia, Frank W.; (New York,
NY) ; Freund, Peter C.; (New York, NY) ;
Huang, Stuart T.F.; (Washington, DC) |
Correspondence
Address: |
STEPTOE & JOHNSON LLP
Attn. : Stuart T.F. Huang
1330 Connecticut Avenue, NW
Washington
DC
20036
US
|
Family ID: |
26858080 |
Appl. No.: |
09/816684 |
Filed: |
March 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09816684 |
Mar 26, 2001 |
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09161741 |
Sep 29, 1998 |
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6209091 |
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09161741 |
Sep 29, 1998 |
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08869253 |
Jun 4, 1997 |
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5825880 |
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Current U.S.
Class: |
713/155 ;
380/278 |
Current CPC
Class: |
G06Q 20/02 20130101;
H04L 9/085 20130101; G06Q 20/3829 20130101; G06F 7/725 20130101;
H04L 9/3255 20130101 |
Class at
Publication: |
713/155 ;
380/278 |
International
Class: |
H04L 009/00 |
Claims
What is claimed is:
1. A digital signing method comprising steps of: generating shares
of a private signature key; storing shares in separate electronic
signing devices; certifying multiple authorizing agents for signing
devices; and for each of a plurality of signing devices, affixing a
partial signature to an electronic message in response to
authorization from a minimum number of authorizing agents; wherein
a plurality of partial signatures constitutes a digital
signature.
2. A system for affixing digital signatures to electronic documents
comprising: a plurality of intercommunicative signing devices, each
signing device comprising an electronic device programmed to
receive an electronic document and to affix a partial signature
using a signature key share in response to a predetermined number
of authorizations; and a plurality of authorizing agents, each
agent communicative with an associated signing device, each anent
comprising an electronic device programmned to provide an
authorization to an associated signing device.
3. A system of interlocked rings of signing devices for affixing
digital signatures to electronic documents comprising: a first set
of signing devices, said first set comprising a plurality of
electronic devices, each device programmed-to receive an electronic
document and affix a partial signature for a first signature key, a
plurality of said partial signatures comprising a first digital
signature; a second set of signing devices, said second set
comprising a plurality of electronic devices, each device
programmed to receive an electronic document and affix a partial
signature for a second signature key, a plurality of said partial
signatures comprising a second digital signature; wherein said
first includes at least one member which is not in said second set,
and said first and second sets include at least one common
member.
4. An electronic method for delegated use of an electronic key
comprising steps storing said key in a first electronic device;
commnunicating an electronic delegation certificate to a delegate:
sending a request and the delegation certificate from the delegate
to the first electronic device; and using said first electronic
device to use the electronic key in response to the request and the
delegation certificate.
Description
[0001] This application is a continuation in part of U.S. patent
application Nos. 08/181,859, CRYPTOGRAPHIC SYSTEM WITH KEY ESCROW
FEATURE, and U.S. Patent application Nos. 08/272,203, ENHANCED
CRYPTOGRAPHIC SYSTEM AND METHOD WITH KEY ESCROW FEATURE, both of
which are incorporated here by reference.
BACKGROUND
[0002] Public key certificates are electronic documents signed by a
trusted issuer and used to attest to the binding of a user's name
to a public key and other related data. Certificates provide
assurance to the public that the public key identified in the
certificate is owned by the user whose name is in the certificate.
Major standards which describe public key certificate systems
include ITU-T X.509 The Directory-Authentication Framework, and
American Bankers Association ANSI X9.30-Part 3: Certificate
Management for DSA (draft). Many implementations impose a
hierarchical structure in which each trusted issuer, referred to as
a Certification Authority (CA) certifies keys for entities that are
subordinate to it. The CA affixes a digital signature to the
electronic document in a way that is verifiable (one can prove that
the CA signed the document) and cannot be forged (one can be
assured to a high level of confidence that no one other than the CA
signed the document). For example, at the top of the CA hierarchy
there may be relatively few "root" CAs, perhaps one per country
which certify subordinate CAs. Below the root CAs in the hierarchy,
high level CAs perhaps banks) certify lower level CAs beneath them
(e.g., companies), which in turn sign individual user
certificates.
[0003] A CA's signature becomes more valuable as it creates a large
hierarchy of users beneath it and uses its signature key to sign
the certificates of both high-value users and subordinate CAs. The
CA's signature key then also becomes a more likely target for
terrorists, criminals bent on economic gain, and foreign military
and espionage services bent on economic spying or de-stabilizing
the economy via information warfare. All these issues also apply
with equal force to keys used to sign electronic representations of
money.
[0004] Thus far, the need for security of a CA's private signature
key has been addressed by providing a "certificate signing unit"
(CSU), which is a tamper-proof secure module satisfying standards
set forth in Federal Information Processing Standard (FIPS) PUB
140-1, level 3 or 4 as issued by the U.S. Dept. of Commerce,
National Institute of Standards and Technology (NIST). Such a CSU
generates its public/private signature key pair internally,
"confines" the private signature key securely and permanently
inside an area of the device that cannot be read externally, and
outputs only the corresponding public key, which will be used to
verify its signatures. One CSU available from Bolt, Baranek, and
Newman of Boston, MA (BBN) is configured to allow a back-up version
of its private signature key to be created using a "K-of-N
threshold" scheme, in which the private key is split into N shares
and placed on small plastic data-keys, each of which contains a
memory chip. The data-keys are a patented product of Datakey, Inc.
of Burnsville, Minn. Then, should the CSU device be destroyed, a
quorum of at least K data-keys can reconstruct the private key.
[0005] At least one major security standards body, the American
Bankers Association ANSI X9.F1 committee on cryptographic security
in wholesale banking applications has recommended that CSU's should
be designed to forbid any export of the private key from the device
in any form in order to prevent any possible unauthorized theft and
use of the key. This approach would require an elaborate procedure
for disaster recovery, involving the use of several key pairs
simultaneously. Because a single key would exist only in a single
CSU at a single site, the loss of a CSU or of a site would force
the CA to use another key pair in order to continue business. This
would require the CA to publicize and/or securely distribute
several (at least two or three) public keys, each identified by a
distinct code number (e.g., BT01, BT02, BT03), so that users could
continue to verify the signatures that the CA would issue after one
CSU (possibly containing the private key for BT01) had been
destroyed. See X9.30-Part 3 concerning procedures for disaster
recovery.
SUMMARY
[0006] An object of the present invention is to provide a digital
signing system ("signing system") for certificates and other high
value documents (including contracts, electronic representations of
currency, negotiable documents, etc.) with improved security and
flexibility.
[0007] A further object of the present invention is to provide a
signing system in which a digital signature verifiably relates to a
signature key, and in which no single signing device needs to
contain the signature key during the document signing
operation.
[0008] A further object of the present invention is to provide a
signing system which permits loss or compromise of one or more
signing devices while maintaining available, uncompromised signing
services.
[0009] A further object of the present invention is to provide a
signing system in which multiple signing devices each create,
modify, or combine one or more partial signatures, and the result
of operations by multiple signing devices produces a single digital
signature.
[0010] A further object of the present invention is to provide a
signing system in which multiple authorizing agents directly or
indirectly authorize each individual signing device to affix or
modify a partial signature.
[0011] A further object of the present invention is to provide a
robust and easy-to-use mechanism in which authorizing agents can
temporarily delegate their authorizing capability.
[0012] The multi-step signing system described here uses a public
key cryptosystem approach to sign an electronic document such that
a recipient of the document can verify the signature using a public
verification key of the signer. The private signature key which
corresponds to the public verification key is not permitted to
exist in whole, available form in one place at any time during
normal signing operations. Instead, a private signature key
consists of "operational shares" which can be used to affix or
modify a partial signature, and sequential operation of multiple
shares produces a signature that can be verified using the public
verification key. The full signature is not completed until all, or
some quorum, of the signing devices have signed. Each signing
device in turn requires authorization from all, or some quorum, of
its associated authorizing agents before participating in the
signature process.
[0013] If, during the initial generation of operational shares, a
whole signature key is generated, the whole signature key is
destroyed after shares are distributed. Because the risk of loss
from the theft or compromise of any one device is now greatly
reduced, the information content of each signing device can be now
duplicated (e.g., for remote backup or for a plug-in replacement or
"hot" standby) so that if any device fails, it can be replaced (or
reconstituted) and service can resume quickly. The consequence of
subversion of any individual signing device is lowered, because the
signing operation cannot be completed with a single device.
[0014] A multi-layered authorization management system is
established, such that each signing device has registered within it
a number of individuals (or external smart cards used by designated
individuals), and the signing device participates in the signing
operation only upon authorization from a quorum of registered
individuals. A quorum of these individuals (called authorizing
agents) are also required to authorize changes to the system, such
as registering additional authorizing agents, deleting authorizing
agents, altering the quorum requirements for any of the various
actions that the signing devices can perform, or generating and
distributing additional or substitute key sets.
[0015] In this way, a signature can be applied that can be verified
using a public verification key, but no private signature key
exists at a single location where it may be subject to compromise
or catastrophe. Multiple sites must fail or be compromised before
interrupting signing services or before an adversary acquires
sufficient information to forge signatures. Individual signing
devices need not be as be as highly secure for a CSU using a single
whole key. A relatively inexpensive device meeting the standards of
FIPS 140-1 level 3 may be used (i.e., a device that is tamper
resistant), thus avoiding the need to use a relatively expensive
level 4 device (which takes active measures to destroy or safeguard
internal information when tampering is detected).
[0016] An authorization delegation mechanism allows an authorizing
agent to let a delegate, or quorum of delegates, authorize his
smart card to affix his/her signature during temporary periods of
time.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The invention will be described below with reference to
attached drawings in which:
[0018] FIG. 1 illustrates an overview of a basic architecture for
an operational signing system in accordance with the present
invention;
[0019] FIG. 2 shows a preferred architecture for a data center
having a signing device;
[0020] FIG. 3 illustrates a preferred architecture for a trusted
device used by an authorizing agent;
[0021] FIG. 4 illustrates a process for temporarily certifying
uninitiated signing devices, during system startup and
initialization;
[0022] FIG. 5 illustrates a process for generating and distributing
operational shares of a system wide authority key;
[0023] FIG. 6 illustrates a multi-step signature procedure for
recertifying a signing device;
[0024] FIG. 7 shows an overall system architecture for certifying
and registering authorizing agents;
[0025] FIG. 8 illustrates a multi-step signature procedure using
authorizing agents;
[0026] FIG. 9 illustrates the flow of a document through various
authorizing agents and signing devices during routine multi-step
signature operations;
[0027] FIG. 10 illustrates the evolution of signatures on a
document during routine multi-step signature operations.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The most direct explanation of the multi-step signature
method begins with a discussion of several relevant mathematical
processes.
[0029] A. Multiplicative Scheme with Sequential Partial Signing
[0030] First, a secret signature key "K.sup.SWA" of a
public/private key pair belonging to a "system-wide authority" is
represented as number ("n0") of shares ("a.sub.i") in such a way
that the signature key K.sub.SWA can be computed as the product of
any-threshold number ("t0") of shares, where t0 is
less-than-or-equal-to n0. The representation is done such that it
is difficult or impossible to recover the signature key K.sub.SWA
when possessing fewer than t0 shares. This can be accomplished by,
for example: 1) using a Shanir-type secret sharing scheme (A.
Shamir, "How to Share a Secret," Communications of the ACM, Nov.
1979, V. 22, n. 11), 2) using a Blakley-type secret sharing scheme
(G. R. Blakley, "Safeguarding Cryptographic Keys," Proceedings of
the National Computer Conference, 1979, American Federation of
Information Processing Societies, V. 48, 1979, pp. 242-268); 3)
factoring the key; or 4) generating the key as a product of known
factors. All that is necessary is that the private key is
represented as:
K.sup.-.sub.SWA=a.sub.1* a.sub.2* . . . a.sub.t0(mod 2N)
[0031] where K.sub.SWA is the signature key and a, are any
combination of t0 shares.
[0032] Second, a signature is formed using multiple devices by
having each device exponentiate a partial signature left by a prior
device, using one share ai of the private key. When using "modulo
N" arithmetic (wherein an arithmetic operation concludes by
dividing the result by a modulus N and taking the remainder as the
modulo N result), the following relationship between multiplication
of exponents and sequential exponentiation is true:
(x.sup.a1*a2)(mod N)=((X.sup.a1).sup.a2)(mod
N)=((x.sup.a2).sup.a1)(mod N)
[0033] Stated another way, if a base value x is exponentiated by
the product of two factors a1 and a2, the result is the same as if
the base were exponentiated by a first factor a1, and that result
exponentiated by the second factor a2. Furthermore, the order of
exponentiation may be reversed, so that the result will be the same
if the base is first exponentiated by the second factor a2, and
that result exponentiated by the first factor a1. This relationship
can be generalized to exponentiation by three or more factors.
Unless stated otherwise, all arithmetic operations are to be
considered modulo N.
[0034] In the multi-step signature method, shares of a signature
key a.sub.1, a.sub.2, . . . , a.sub.n0 are distributed to separate
devices. A first device affixes a partial signature to a document
by hashing the document (the symbol "H" will be used to designate
the result of the hash operation) and exponentiating the hash
as:
first partial signature=(H).sup.a1 (mod N)
[0035] A second device advances the signature by exponentiating the
first partial signature using a second share a.sub.2 as:
second partial signature=((H).sup.a1).sup.a2 (mod N)
[0036] The process repeats until "t0" devices have exponentiated
the hash using each of "to" separate shares, to produce a final
signature that can be verified using the public
K.sup.-.sub.SWA.
[0037] B. Additive Scheme with Asynchronous Partial Signing
[0038] An alternative way to accomplish a similar result involves
dividing the private key of the signing authority into shares which
can be added (modulo N) to yield the private key.
K=a.sub.1+a.sub.2+. . . a.sub.t(mod N)
[0039] This in turn permits the multi-step signing to be performed
an in an asynchronous manner by separately generating intermediate
values (H).sup.ai by exponentiating the hash by each of the shares,
and then multiplying tile resulting intermediate values, such as
the following:
S=H.sup.a1* H.sup.a2 * . . . H.sup.a3(mod N)
[0040] This can have considerable operational advantages over the
sequential method described above, because it is not necessary to
route the message sequentially from one location to another.
Instead, a central administrator can, in a straightforward manner,
simply send the same message (or hash) directly to each location
for partial signing, and then combine the resulting partial
signatures to produce the final desired official signature. This
final combining operation does not require any special security,
because it does not add any information not already contained in
the partial signatures, thus allowing the administrator to work
from a desktop. Indeed, the partial signatures could conceivably be
even be left for later combining by the recipient who verifies the
transaction! This burdens the recipient with additional processing
workload, but does not weaken the security of the official
signature.
[0041] Signature schemes based on exponentiation which can be
modified to permit multi-step signing include: R. Rivest, A. Shamir
and L. Adleman ("RSA"), "A method for Obtaining Digital Signatures
an Public Key Cryptosystems," Communications of the ACM, v.21, n.2,
pp.120-126, February 1978); D. Kravitz, Digital Signature Algorithm
("DSA"), U.S. Pat. No. 5,231,668; Desmet, Y. Frankel, "Threshold
Cryptosystems," CRYPTO '89, pp.307-15, 1989; Taher El-Gamal, "A
Public Key Cryptosystem and a Signature Scheme Based on Discrete
Logarithms" ("El-Gamal Signature Algorithm"), IEEE Transactions on
Information Theory, Vol. IT-31, No. 4, Jul. 1985; S. Micali, "A
Secure and Efficient Digital Signature System," MIT/LCS/TM-501,
Massachusetts Institute of Technology, Laborator for Computer
Science, March 1994; A. Menezes et al., "Elliptic Curve Public Key
Crypto Systems," 1993.
[0042] System Overview
[0043] FIG. 1 illustrates an overview of an architecture for a
signing system in accordance with the present invention. The
architecture includes multiple signing devices 11, 13, 15, 17, 19
interconnected by a wide area network (WAN) or local area network
(LAN) 21. Individual signing devices 11, 13, 15, 17, 19 are
dispersed geographically as widely as the WAN/LAN permits, such as
in separate continents, separate cites or at least in separate
parts of a single metropolitan area.
[0044] In FIG. 1, Signing Device 2 has been illustrated in greater
detail as an example. Each signing device is assigned a permanent
identification code (e.g., unique serial number) and a logical name
(e.g., "Signing Device X"), along with a public/private device key
pair 12a, 12b, for encrypting/decrypting communications and a
separate public/private device key pair 14a, 14b, for
verifying/signing signatures. In addition, each signing device
receives the public encryption keys 16 and public verification keys
18 for all other signing devices.
[0045] Hereafter, encryption/decryption keys are designated as
"KE," while "KS" designates signature/verification keys. A plus
("+") superscript indicates a public key, and a minus ("-")
superscript indicates a private key. Subscripts indicate the owners
of the private keys of respective key pairs.
[0046] Groups of authorizing agents 23, 25, 27, 29, 31 are also
interconnected through the network to one another and to the
signing devices 11, 13, 15, 17, 19. Each authorizing agent is a
person acting through a trusted computer device (such as a
tamper-resistant smart card, or other trusted device) as will be
discussed more fully below. Authorizing agents may be dispersed to
the full extent of the LAN/WAN 21, but it is assumed that groups of
authorizing agents will be located in proximity to corresponding
signing devices at most times for the convenience of the
organization managing the signing system.
[0047] In FIG. 1, Authorizing Agent 2a (item 25) has been
illustrated by way of example and using the same notation for keys
as discussed above in relation to keys held by Signing Device 2.
Each authorizing agent's trusted device is assigned a unique name,
along with a public/private device key pair 20a, 20b for
encrypting/decrypting communications and a separate public/private
device key pair 22a, 22b for verifying/signing signatures. If the
RSA public key cryptosystem is employed, then one such pair could
be used for both signatures and encryption at the same time.
Authorizing agents also receive public encryption keys 24 and
public verification keys 26 of all other authorizing agents.
[0048] Signing devices also receive the public encryption keys 24
and public verification keys 26 for all authorizing agents.
Similarly, Authorizing agents' trusted devices receive public
encryption keys 28 and public verification keys 30 for all signing
devices.
[0049] For ease of explanation of the multi-step signature process
which follows, it will be assumed that all communications on the
network are encrypted using a standard Public Key Cryptosystem
("PKC") scheme, such as RSA-key-transport. It will also be assumed
that commands sent from one network entity to another are signed by
the sender using a standard (PKC) scheme, such as RSA-signature
with MD5 message digest. In future drawings, device
encryption/decryption keys, and device signature/verification keys
may be omitted, but should be understood as present in all devices
as discussed above.
[0050] FIG. 2 shows a preferred architecture for a secure data
center computer configuration 48, where each signing device of FIG.
1 preferably will be found. In addition to a signing device 29,
each data center configuration 48 additionally contains a separate
message server 47. The signing device 39 is dedicated to signing
operations and is located in a physically secure location, such as
a vault. There is no direct connection between the signing device
and the external computer network. As will be discussed more fully
below, the signing device 39 will be provided with a key share for
multi-step signing 36, its own device signature key 37, table 38
identifying its authorizing agents, and a certificate for its
public verification key 40, a public key chosen to match its key
share 36 (where the certificate is signed by the full KS.sub.swa
via the multi-step method).
[0051] During the multi-step signing processes, a signing device 39
will receive requests through the message server 47. The message
server performs routine communication processes, such as stripping
off routine privacy envelopes which may have been affixed by
intermediates (the server 47 does not possess the signing device's
private decryption key), and queuing the inputs in case they are
presented faster than they can be processed. The message server
presents messages to the signing device for signing, receives the
signed (or partially signed) result, and either (a) returns the
partially signed result to the requester, or (b) routes the result
to the next device in the protocol. In order to receive and
participate in ordinary communication protocols, the message server
also possesses a public-private key pair 32, 33 for signing its own
messages, and another 34, 35 for encryption, to enable it to
receive and open encrypted messages-thereby freeing the signing
device of this routine burden without significantly affecting the
security of the secure signing process.
[0052] The message server 47 may be a comparatively less secure
computer in a lower security environment such as an ordinary secure
data center. The message server 47 connects to the LAN/WAN 21 and
provides document queuing and communications services for the
signing device 39. The message server 47 includes a system log 49
that maintains an audit trail of messages and documents sent to and
from the signing device. As shown, a signing device and its
associated message server preferably are divided into two,
physically separate computers. Although less preferred, the signing
device 39 and message server 47 could be implemented as separate
tasks on a single computer in a highly secure environment.
[0053] The message server can also provide a layer of protection,
known as a "firewall," that separately validates all transactional
inputs prior to passing them on to the signing devices. Otherwise
an "on-line" signing device accessible to a public network would be
open to unlimited hacking attempts, as well as to network
saturation attacks aimed at denial of service. Denial attacks may
disrupt daily certificate issuance, but would not cripple users who
rely on previously signed documents (which is the vast majority of
the anticipated user population). However, hack attempts will
always pose a threat, especially if hackers identify some hidden
flaw. The message server can verify all messages against a list of
authorized devices (signing devices and authorizing agents), as
well as more complex strategies to identify possible attacks, deny
access after a number of failed attempts, and undertake
sophisticated actions to track down the source of any false data
inputs. This will allow the signing device's firmware to remain
simpler and easier to validate, while also allowing the system
operators to modify their detection and evasion strategies in
accord with the current state of network security.
[0054] FIG. 3 illustrates a working station for authorizing agents.
The human operators who act as authorizing agents may work in
relatively unsecured areas at desk-top computers or terminals 51
typically found in a business office. Each such computer or
terminal will have a card reader 53, and each operator will have a
secure "smart card" 55. Each smart card 55 securely contains a
private decryption key and a private signature key which are unique
to that smart card. The human operator can use the card to issue
signing instructions. Such a trusted device may be implemented
using a FIPS level-3 device, such as an Power card from National
Semiconductor Corp. of Santa Clara, Calif., which can be readily
reprogrammed at the firmware level to allow for progressive
evolution of new methods and procedures for secure signing and
authorization without needing to replace the physical devices. Each
authorizing agent's trusted device must have at least a private
signature key. Preferably, the private signature key is installed
in the device at time of manufacturer, and the corresponding public
verification key is "certified" by the manufacturer. Certification
here means that the manufacturer has included, with the trusted
device, an electronic message containing the device's serial number
and public key, along with its model number and other evidence of
its trusted characteristics, and that message (certificate) has
been signed by the manufacturer.
[0055] The human operators use their desk-top computers to read and
generate messages. When a human operator wishes to sign a message,
the desk-top computer sends the message to the trusted device,
which appends a digital signature using the device private
signature key. In the preferred embodiment, this signature is the
signature of a second signature key pair which has been
specifically generated for and certified as belonging to the
specified user. In this manner, the system can continue to use the
device's signature to verify the trust level of the device on any
given transaction, while using the user's signature to attest to
the user's identity and consent to the transaction. This allows the
user key to be generated and revoked remotely, depending possibly
on various administrative facts about the user's identity or
authority, while also allowing the device to be reused, or to host
several other user key pairs which the user may wish to use for
other unrelated purposes.
[0056] FIG. 3 also illustrates a preferred architecture for a
possible trusted device to be used by an authorizing agent. It
comprises a single micro-chip encased on a card in a configuration
known as a "smart card." The micro-chip device has input/output
circuitry 42 for power and communications, and a microcontroller 44
for executing firmware programs. Memory 52 contains system firmware
43 to operate the hardware of the microchip (similar to a simple
operating system). Memory 52 also includes areas for storing
manufacturer-installed device keys 45, user keys 47 received as
part of the protocol described herein, and application firmware 49
for executing the network protocols described herein. Additional
unused memory is provided as a work area 54 for temporary storage
as required. The micro-chip may also include an optional
"crypto-unit" 46, which is a special purpose arithmetic accelerator
unit having hardware for performing accelerated exponentiation and
other arithmetic operations of encryption/decryption and signature
processes. The micro-chip further includes an optional trusted
time-clock 48 (assuming the presence of suitable battery power)
initialized by the manufacture and useful for time stamping
signatures. The micro-chip further includes an optional random
number generator 50 to be used with encryption/decryption
processes. The smart card may also include an optional noise source
(not shown), such as a diode, that is either internal or external
to the micro-chip, for use in generating random numbers.
[0057] The signing device previously shown in FIG. 2 may also be a
smart card having the same general design as the trusted devices of
the authorizing agents.
[0058] Devices in the network will be initialized in a series of
stages as follows:
[0059] 1) encryption key distribution;
[0060] 2) signing device temporary certification;
[0061] 3) key share distribution;
[0062] 4) signing device recertification; and
[0063] 5) authorizing agent certification.
[0064] Each will be discussed in turn. Following the discussion of
system initialization, the preferred methods of use for signing
highly secure certificates and other documents will be explained,
as well as further variations and enhancements.
[0065] Encryption Key Distribution
[0066] Each signing device, and each authorizing agents' smart card
is assumed to be a "trusted device" in that it is a
tamper-resistant device that functions only in accord with stated
characteristics, and whose manufacturer has endowed it with a
device signature key pair and a device encryption key pair stored
in a protected memory. At a minimum, the manufacturer of such a
device will attest that the device will not divulge either its own
or its user's private key(s) without an expensive tampering effort.
Each device also has an electronic certificate, signed by the
manufacturer, containing: 1) the device serial number; 2) the
device's public signature verification key; and 3) the device's
public encryption key. The manufacture may install two separate
certificates, one for the signature verification key and one for
the encryption key. Signing devices encrypt their communications
using a public/private cryptographic scheme. In the alternative,
the method can proceed without manufacturer certificates by
providing physical protection for all devices, such as conducting
the initialization tasks in a secure vault where a small (notebook)
computer is used in lieu of a trusted signing device.
[0067] It is assumed that each trusted device begins with certain
basic functionality, such as software conferring the ability to
initiate and receive messages through a network or an electronic
mail system, that lets it communicate with other trusted devices.
It is also assumed that at least one signing device, designated as
the "lead" device, is capable of receiving information about the
initial state of the system from human operators responsible for
initializing the system.
[0068] The next step in preparing the system is for devices to
exchange device keys. Key distribution proceeds as follows.
[0069] 1) One signing device, designated as the "lead," receives
from human operators the identities of other signing devices in the
system. The lead device-sends its public encryption key and public
signature verification key to the other signing devices.
Optionally, the lead device may also send a message for validating
the firmware under which it is operating, for example, by hashing
its firmware, signing the hash value using its device signature key
and sending the signed hash value to the other devices.
[0070] 2) After other signing devices receive the lead device's
public encryption key, each other signing device sends its
respective public signature verification key and public encryption
key certificate(s) back to the lead device. If the lead device sent
a hash of its firmware, each other signing device hashes its own
firmware and compares the two. Both hashes must match, otherwise,
the respective signing device stops participating in the protocol
and notifies its operators. This comparison of hash values ensures
that all signing devices use identical firmware, which acts as a
check that the lead device is not an "impostor." Each signing
device optionally returns a hash of its respective firmware to the
lead device.
[0071] 3) The lead device compares the hashes of the respective
other devices' firmware against its own hash, which acts as a check
that none of the other devices is an impostor.
[0072] All signing devices now have received public encryption and
signature verification keys for the other devices. It will be
understood that all future messages will be signed by the sender's
private signature key and verified by the recipient using the
sender's public verification key. It will also be understood that
all communications will be encrypted using the recipient's public
encryption key and decrypted using the recipient's private
decryption key.
[0073] These additional signature keys are not used for multi-step
signing (which will be discussed below), but are used instead for
encrypting and signing routine communications among network
entities as proof of a device's individual identity. Such proofs of
identity and membership in the group are of critical importance
when generating and distributing the master key fragments for use
in the actual multi-step protocol.
[0074] Signing Device Temporary Certification
[0075] FIG. 4 illustrates temporary certification of uninitiated
signing devices. During this process, signing device's public key
certificates (which were unsigned or signed by the device
manufacturer) will be replaced by certificates signed by a
temporary administrator ("the administrator") 61. The administrator
preferably is a human operator responsible for initializing the
system and acting through the administrator's personal smart card.
This temporary certification establishes an increased level of
security among signing devices (as belonging to the target group)
for use while they generate signature keys for multi-step signing.
During actual use, it is anticipated that the temporary
administrator would be operating with multiple human witnesses to
assure correct procedures, and that the temporary certification
would be effective only for the minimal time (a few minutes or
hours, at most) necessary to perform the complete master key
generation protocol. Temporary certification proceeds as
follows:
[0076] 1) The administrator 61 generates a private signature key 63
and a corresponding public verification key 65.
[0077] 2) The administrator 61 communicates its public signature
verification key 65 to each of the signing devices 11, 13, 15, 17,
19.
[0078] 3) Each signing device 11, 13, 15, 17, 19 generates a
private signature key 67, 69, 71, 73, 75 and a public verification
key (not shown), and sends a signature key certification request to
the administrator 61. The signature key certification request is an
electronic message containing the name of the signing device (e.g.,
a device serial number and/or a logical name, such as "SD1"), the
device's newly generated public signature verification key, and
other administrative information as desirable.
[0079] 4) The administrator signs each certification request using
the administrator's private signature key.
[0080] 5) The administrator returns the signed signature key
certificates 68, 70, 72, 74, 76 to the respective signing devices
11, 13, 15, 17, 19. Signed certificates 68, 70, 72, 74, 76 are
illustrated as symbols for public signature keys (KS+) with
appropriate subscripts and, attached below, the administrator's
signature ("-ADMIN"). Such certificates will, of course, also
include information on device identity and type (not shown).
[0081] 6) The signing devices exchange their new temporary public
signature verification key certificates among one another.
[0082] Each signing device now possesses: a) the administrator's
public verification key; b) its own temporary private signature
key; 3) its own temporary certificate, signed by the administrator
and bearing the signing device's temporary public signature
verification key; and 4) the temporary signature verification key
certificates of the other signing devices. Each signing device can
use the administrator's verification key to verify the
administrator's signature on the temporary certificates received
from the other signing devices.
[0083] Each signing device may now advance to a more tightly
controlled phase of the protocol by exchanging messages using the
signature keys that have been certified by the temporary
administrator. For ease of explanation, it will be assumed that
communications on the network involved in the multi-signature
operations from this point until the end of device recertification
are signed using a signature key that has been certified by the
temporary administrator, and that each recipient verifies the
sender's signature of the sender. If a message is not properly
signed, the message will be rejected and the protocol will fail to
continue unless a conforming message is supplied. It is further
contemplated that some form of threat analysis or threat response
may be undertaken when an improperly signed or unsigned message is
received during the multi-step initialization and signature
operations.
[0084] Authorizing Agent Temporary Certification
[0085] FIG. 4 illustrates temporary certification of authorizing
agents. As will be discussed more fully below, a signing device
affixes a partial signature only in response to authorization from
a quorum of authorizing agents. Signing devices operating under the
authorization of the temporary administrator also require a quorum
of authorizing agents. Temporary certification of authorizing
agents assure that only designated human agents may authorize
signing devices during the initiation process.
[0086] The procedure for temporarily certifying authorizing agents
is similar to the procedure above for temporarily certifying
signing devices, and proceeds as follows:
[0087] 1) The administrator 61 communicates its public signature
verification key 65 to each of the authorizing agents 23, 25, 27,
29, 31.
[0088] 2) Each authorizing agent generates a private signature key
certification request to the administrator 61. The signature key
certification request contains at least the following information:
a) authorizing agent name (human's distinguished name); b)
identification code for the agent's trusted device (e.g., smart
card serial number and model number); c) signature verification key
for the human agent; and d) signature verification key for the
agent's trusted device (which serves as an assurance that the
trusted device is of a known type).
[0089] 3) The administrator signs each certification request using
the administrator's private signature key.
[0090] 4) The administrator returns the signed signature key
certificates to the respective authorizing agents.
[0091] Key Share Distribution
[0092] FIG. 5 illustrates generation and distribution of
"operational shares" of a system wide authority (SWA) "official"
signature key. One signing device, here Signing Device 1 (item 11),
is designated as a "lead" device. Human operators provide to this
lead signing device at least the following information:
[0093] a) The threshold parameters for splitting a key into shares,
i.e., the total number of shares to be generated and the minimum
number needed to affix the SWA signature.
[0094] b) A key identification number and/or logical name to be
assigned to the public/private key pair, e.g., key serial number
"KS-01234," or logical name BT01.
[0095] c) Key share identification numbers and/or logical names to
be assigned to the respective shares, e.g., "SWA-SHR-56789," or
"BT01."
[0096] d) The device certificates of authorizing agents who will
initially be permitted to authorize that particular signature for
each device.
[0097] The human operators may additionally provide a number that
limits the total number of fragments that can reside in a single
signing device, which can be used when a signing device has
multiple master keys as discussed more fully below.
[0098] The next step is to generate shares for a signature key,
called the "system wide authority" (SWA) key, which will be used to
administer the system. The public SWA public signature key and
corresponding private SWA key shares are generated and distributed
as follows.
[0099] 1) Each signing device 11, 13, 15, 17, 19 transmits an
encrypted string of random "seed" information to the lead signing
device 11.
[0100] 2) The lead device 11 combines the seed information and uses
it to generate a public system wide authority signature
verification key (KS.sub.SWA+) 91, which ultimately will be used to
verify official signatures.
[0101] 3) The lead device 11 generates operational shares 93, 95,
97, 99, 101 of a private SWA signature key. This may be
accomplished by first generating a whole private/public key pair
using well known prior art key generation methods and then
splitting the private signature key 92 into shares using one of
several well known private signature key splitting methods. The
generation of shares carries with it a requirement that a minimum
number of separate shares n0 be sufficient to complete a system
wide authority signature.
[0102] 4) The lead device 11 transmits the SWA public verification
key 91 and one private signature key share 95, 97, 99, 101 to each
other signing device, while retaining a copy of the SWA public
verification key 91 and one share of the SWA private signature key
93 for itself. Each SWA private signature key share is transmitted
with the following additional information:
[0103] a) a type code identifying the key as a signature key share
(also indicating the length of the share);
[0104] b) a unique identification code for the SWA public
verification key;
[0105] c) a unique identification code for each respective SWA
private signature key share;
[0106] d) the total number of SWA private signature key shares
distributed;
[0107] e) the minimum number of SWA private signature key shares
needed to complete a SWA signature;
[0108] f) the identities of signing devices receiving other SWA
private signature key shares; and
[0109] g) certificates of authorizing agents who will be permitted
initially to authorize use of each SWA private signature key share
on the target signing device.
[0110] The lead device 11 will encrypt each SWA private signature
key share using the certified public encryption key of the
respective signing device for which it is intended.
[0111] 5) The lead device 11 outputs the public SWA verification
key for the human operators and erases the following
information:
[0112] a) the whole private SWA signature key (if at any time
during the generation process the whole private SWA signature key
was stored); and
[0113] b) all shares of the SWA private signature key (except for
one share which it retains for its own use).
[0114] 6) Each recipient signing device installs its SWA private
signature key share in a tamper-proof memory area, along with the
certificates of the initial human authorizers for that device.
[0115] It is preferred that the private SWA signature key exist at
most only in the lead signing device 11, and then only for the
minimum time necessary to generate and distribute shares. In this
way, the whole private SWA signature key simply does not exist for
operational use, and is susceptible to attack for only a short
period of time during the generation process.
[0116] At this stage, each signing device now additionally has
securely received: a) a copy of the public SWA signature
verification key; and b) a private SWA signature key share.
[0117] For the purpose of illustrating an example in the following
discussion, it will be assumed (for the sake of simplicity) that
the minimum number of shares n0 needed to affix the SWA signature
is two out of five shares. It should be understood that a higher
number may be chosen, most probably at least three, which will
increase security, but which will also increase the number of steps
in the signing process.
[0118] Signing Device Recertification
[0119] During previous steps of the initialization protocol, a
temporary administrator 61 certified device signature verification
keys under the authority of the temporary administrator 61, and the
signing device certificates were signed by the administrator's
temporary signature key. During recertification, each signing
device will circulate a new certificate request for its own public
key among the other signing devices to be certified under the
system wide authority key using multi-step signing.
[0120] FIG. 6 illustrates steps for recertifying Signing Device 1.
The other signing devices will recertify themselves by repeating
the process for each device. The process for Signing Device 1
proceeds as follows:
[0121] 1) Signing Device 1 generates an unsigned certificate 103
and transmits that certificate to Signing Device 2. The certificate
includes at least: a) the signing device's identity (e.g., serial
number and/or device logical name); and b) a public signature
verification key for the device's signature key. The key which is
to be recertified is the same public key which was originally
generated by the device at the start of the protocol, and first
temporarily certified by the administrator. This key will now
become the device's permanent indicia of membership in the family
of signing devices handling the shares of this particular SWA key.
(The device signing key and its associated manufacturer's
certificate remain unchanged during this process, and are retained
permanently as proof of the device's origin and underlying
characteristics.)
[0122] 2) Signing Device 2 affixes a partial SWA signature using
its SWA signature key share 93. The partial signature is formed in
two steps. First, Signing Device 2 applies a "hash" function (such
as MD5 or SHA) that generates a reduced-length string that is
verifiably related to the un-hashed certificate. This string is
expressed as binary digits which can be manipulated as a numerical
(large integer) value. Second, Signing Device 2 forms a partial
signature by exponentiating the hash string with its SWA signature
key share. That is, Signing Device 2 calculates a numerical value,
which becomes the partial signature, according to the formula:
-SD2=(HASH(CERT)).sup.[KEY SHARE 2] modulo N
[0123] (Note that in both text and drawings, a string of bits that
constitutes a signature block is typically indicated by placimg a
long dash in front of the signer's identifying label. The resulting
block is typically appended to the bottom of the block of data that
was signed, or is otherwise obvious from the context.)
[0124] 3) Signing Device 2 sends the partially signed certificate
105 to Signing Device 3.
[0125] 4) Signing Device 3 completes the system wide authority
signature by exponentiating the already-applied partial
signature-SD2. That is, Signing Device 3 calculates a numerical
value according to the formula: 1 -- SD3 = [ -- SD2 ] [ KEY SHARE 3
] modulo N = ( HASH ( CERT ) exp KEY SHARE 2 ) exp KEY SHARE 3 ) =
-- SWA
[0126] The partial signature affixed by Signing Device 2 may be
allowed to remain attached to the document as an audit trail. Note
that only 2 partial signatures were required in this simplified
example.
[0127] 5) Signing Device 3 returns the signed certificate to
Signing Device 1, which then distributes copies of the certificate
to the other signing devices, thereby allowing them to verify its
future signatures.
[0128] In this example, signing devices 2 and 3 affixed signatures
in that order. Any combination of signing devices may sign in any
order (as long as the number exceeds the minimum t0), producing the
same signature.
[0129] Recertification is important, because future operations
performed by the full system of signing devices will preferably be
performed only in response to requests from devices (e.g., of the
authorizers, as discussed below) that have been certified by the
SWA signature. Signing devices themselves may make requests to
other signing devices. By this procedure, the signing devices
themselves become the first devices certified by the system wide
authority (SWA) as a whole, using the herein defined multi-step
signature process.
[0130] In an alternative embodiment of the foregoing
recertification process, the group of target devices might submit
their recertification requests (unsigned certificates) prior to the
initial key generation by the lead device. The lead device would
sign these certificates at the time it creates the SWA private
signing key prior to splitting it into fragments and erasing the
whole key. There does not seem to be any major advantage in doing
this, as the main function of the resulting system is to sign such
certificates in a highly controlled yet efficient manner.
[0131] Authorizing Agent Recertification
[0132] FIGS. 7 and 8 illustrate steps for certifying and
registering authorizing agents. FIG. 7 shows an overall system
architecture, while FIG. 8 illustrates the processing sequence for
a certification request. Signing devices will affix the system wide
authority official signature to authorizing agent certificates,
thus certifying a public signature verification key for each
authorizing agent. In the registration process, each signing device
will also update an internally-stored table of particular
authorizing agents who will be empowered to instruct the signing
device to apply its partial signature. During routine operation, a
signing device will affix its partial signature only if the request
is signed by a minimum number of temporarily certified or SWA
certified authorizing agents (or if a minimum number of
individually signed messages are receives) as discussed more fully
below. An example of the process for certifying Authorizing Agent
3a (AA3a) and registering AA3a with Signing Device 3 proceeds as
follows.
[0133] For purpose of illustration, it will be assumed that Signing
Devices 3 and 1 (FIG. 7, items 15 and 11) are the 2 of 5 signing
devices selected to affix the SWA signature.
[0134] 1) Authorizing Agent 3a submits a re-certification request
for himself (FIG. 8, item 121) to Signing Device 3 through the
LAN/WAN 21. (Alternately, authorization and/or registration can be
restricted to direct input to the signing device through a limited
access communication channel, e.g., direct connection to a
stand-alone personal computer). The certification request includes
at least the following information: a) authorizing agent name
(human's distinguished name); b) identification code for the
agent's trusted device (e.g., smart card serial number and model
number); c) a signature verification key for the human agent (as
initially signed by the temporary administrator); and d) a
signature verification key for the agent's trusted device, which
serves as an assurance that his device is of a known type. Such
assurances are particularly critical when all or substantially all
operations are performed at widely separated locations, such that
the system operators cannot verify anything via visual
inspection.
[0135] 2) Signing Device 3 affixes a partial SWA signature (-SD3)
to the certificate 121, and transmits the partially-signed
certificate 123 to another of the signing devices.
[0136] 3) Signing Device 1 authorizes that the partial certificate
can now be sent to SDI.
[0137] 4) Signing Device 1 completes the signature process using
its share 93 of the SWA signature key.
[0138] 5) Signing device 1 returns the fully-signed certificate 125
to Signing Device 3.
[0139] 6) Signing Device 3 retains a copy of the signed certificate
111, enters AA3a in a log of authorizing agents 113, and returns
the signed certificate 125 to the Authorizing Agent 3a.
[0140] The process is repeated for all authorizing agents 101 which
are to be registered with Signing Device 3, leaving each
authorizing agent 101 with a signed certificate and leaving Signing
Device 3 with a log 113 of all certificates. The process is
repeated for all authorizing agents of the other signing devices
11, 13, 17, 19.
[0141] Multi-Step Signing
[0142] At this stage, signing devices have been initialized with
shares of the SWA private signature key. Signing devices have
recertified themselves, and authorizing agents have been both
recertified and registered with their respective signing devices.
The system is now ready to enter routine service for both system
administration and official certification functions. In the
following discussion, multi-step signing will be described for the
system wide authority key, which typically will be used for system
administration. As will be discussed below, additional "master
keys" will also be generated and used for multi-step signing within
the same family of devices, in the same way as for the system wide
authority key, except that the content of messages to be signed by
such master keys may not be administrative in nature.
[0143] FIGS. 9 and 10 illustrate multi-step signing using the
system wide authority key. FIG. 9 illustrates the flow of a
document ("DOC") through various authorizing agents and signing
devices, while FIG. 10 illustrates the evolution of signatures on
the document. This example assumes that Authorizing Agents 1a and
1b authorize Signing Device 1 to affix a partial signature, and
that Authorizing Agents 2a and 2b authorize Signing Device 2 to
complete the SWA signature. For simplicity, we assume that any two
authorizing agents are needed to activate each signing device. The
sequence proceeds as follows.
[0144] 1) Authorizing Agent la receives a request for a signature
through the WAN/LAN. The request is an electronic message 131
having a header 133 and the document to be signed 135. The header
will contain a command code that designates the message as a
signing request.
[0145] 2) Authorizing Agent la (FIG. 9, item 132) strips off the
header and performs a number of procedural checks to determine
whether the document should be signed. The specific procedural
checks, which may include the judgment of the human operator AA1a
and which may vary depending on the underlying purpose of the
document, are not germane to the multi-step signature process
itself. When satisfied that the document should be signed,
Authorizing Agent la signs the document using the agent's secret
signature key (which was re-certified under the SWA signature). As
shown in FIG. 10, Authorizing Agent 1a's signature (-AA1a) is
determined by hashing the document and exponentiating the hash
using AA1a's secret signature key. AA1a then affixes a new header
and sends the signed certificate 137 to Authorizing Agent 1b
(another agent for the same signing device as Authorizing Agent
1a).
[0146] 3) Authorizing Agent 1b (FIG. 9, item 138) strips off the
header and performs a number of procedural checks (not germane to
multi-step signing) to determine whether the document should be
signed. When satisfied that the certificate should be signed,
Authorizing Agent 1b also signs the document. As shown in FIG. 10,
AA1b's signature (-AA1b) is determined by: 1) hashing the
concatenated combination of the document and AA1b's signature; and
b) exponentiating the hash using AA1b's signature key. AA1a's
signature is left on the document as an audit trail. AA1b then
affixes a new header and sends the twice-signed document 139 to
Signing Device 1 (FIG. 9, item 11).
[0147] 4) Signing Device 1 receives the twice-signed document 139,
strips off the header and verifies that the document bears the
necessary number of signatures of its registered authorizing agents
(in this example, two). If so, Signing Device 1 strips off the
signatures of authorizing agents and affixes a partial SWA
signature. As shown in FIG. 10, the partial SWA signature (-SD1) is
determined by hashing the base document (without authorizing agents
signatures) and exponentiating the hash using Signature Device 1's
SWA signature key share 93. Signing Device 1 then affixes a new
header, and sends the partially signed document 141 to an
authorizing agent for another signing device, here Authorizing
Agent 2a of Signing Device 2.
[0148] 5) Authorizing Agent 2a (FIG. 9, item 143) strips off the
header and performs a number of procedural checks (not germane to
multi-step signing) to determine whether the document should be
signed. When satisfied that the certificate should be signed,
Authorizing Agent 2a signs the document. As shown in FIG. 10,
AA2a's signature (-AA2a) is determined by: 1) hashing the
concatenated combination of the certificate and the partial SWA
signature (-SD1); and b) exponentiating the hash using AA2a's
re-certified signature key. The partial SWA signature of SD1 is
left on the document. AA2a then affixes a new header and sends the
signed certificate 145 to Authorizing Agent 2b (FIG. 9, item
147).
[0149] 6) Authorizing Agent 2b (FIG. 9, item 147) strips off the
header and performs a number of procedural checks (not germane to
multi-step signing) to determine whether the document should be
signed. When satisfied that the document should be signed,
Authorizing Agent 2b signs the document. As shown in FIG. 10,
AA2b's signature (-AA2b) is determined by: 1) hashing the
concatenated combination of the certificate, the partial SWA
signature and AA1a's signature; and b) exponentiating the hash
using AA2b's re-certified signature key. The partial SWA signature
and AAla's signature are left on the document. AA1b then affixes a
new header and sends the signed certificate 149 to Signing Device 2
(FIG. 9, item 13).
[0150] 7) Signing Device 2 receives the signed document 149, strips
off the header and verifies that the certificate bears the
necessary number of signatures of its registered authorizing agents
(in this example, two). If so, Signing Device 2 strips off the
signatures of its authorizing agents and modifies the partial SWA
signature to complete the SWA signature. As shown in FIG. 10, the
completed SWA signature (-SWA) is determined by exponentiating the
partial signature affixed by Signature Device 1 (-SDI) using
Signature Device 2's SWA signature key share 95. Signing Device 2
then affixes a new header, and sends the partially signed
certificate 151 to AAla (the originating authorizing agent).
[0151] In the example described above, two signing devices were
necessary to affix the system wide authority signature, and each
signing device required authorization from two authorizing agents.
The total number of signing devices needed to complete a signature
in the system may be adjusted at the time the key shares are
generated, and threshold numbers of authorizing agents for each
signing device may also vary. For example, it may require 3 signing
devices of five to complete the system wide authority signature,
and the number of authorizing agents necessary to authorize a
signing device may vary for each signing device, depending on the
level of human review desired for security purposes.
[0152] After having established a multi-step signing process as
discussed above, certain core administrative actions can be taken
conditioned on the "assent" of a quorum of other signing devices as
authorized by the presence of the system wide authority key. Some
of these administrative actions are discussed below. To effectuate
such actions and decisions, the firmware inside each tamper
resistant signing device will be programmed to respond only to
commands signed:
[0153] 1. in the case of partial signing requests, by a proper
quorum of authorizing agents; and
[0154] 2. in the case of system administrative changes, by the
systemwide authority itself.
[0155] That is, in the preferred embodiment, no changes can be made
in the list of authorizers or related requirements on any signing
device by other than the consent of a quorum of authorizers on a
quorum of all signing devices. In some cases, it may be deemed
unduly burdensome to obtain the consent of the entire system for
certain minor changes, such as authority to perform encrypted
backups. However, it is anticipated that such administrative
changes will generally be relatively few and infrequent, in
contrast to the volume of official business, and that the security
of the system demands that such consent should be normally obtained
in all cases. Note that in the example, only 4 human signatures
were required to (re)certify and (re)register a user.
[0156] Parallel Signing
[0157] FIG. 11 illustrates the flow of a document during a parallel
embodiment of the multi-step signing system. In this illustration,
it will be assumed that there are a total of three signing devices
169a, 169b, 169c in the system, and that all three signing devices
are required to complete the system wide authority (SWA) signature.
It will be understood that parallel signing can be adapted to
differing numbers of signing devices.
[0158] In the parallel method, a document coordinator 161 ("the
coordinator") receives a document to be signed 163. The coordinator
may but need not be an authorizing agent for one of the signing
devices, but the coordinator is illustrated as a separate entity
for generality.
[0159] The document coordinator 161 generates three copies (or in
the alternative, three copies of a hash of the document) 165a,
165b, 165c of the document to be signed 163. Each copy is sent to a
first authorizing agent 167a, 167b, 167c, then to a second
authorizing agent 171a, 171b, 171c, then to one of the three
signing devices 169a, 169b, 169c, and finally is returned back to
the coordinator 161. In a manner discussed more fully below, the
document coordinator combines the separate signatures of the three
signing devices and produces a system wide authority signature
(-SWA) which is affixed to the original document 163 to produce a
signed document 173.
[0160] FIG. 12 illustrates the processing of one of the copies, and
the combination of three partial signatures into the system wide
authority signature. It should be understood that each of the
copies undergoes processing that is essentially the same, except
that differing authorizing agents and signing devices will affix
signatures, or partial signatures, according to their individual
signature keys.
[0161] In this example, two authorizing agents are required to
authorize their respective signing device 169a to affix its
signature. The coordinator 161 sends a first copy 165a of the
document to be signed, along with a routing and information header
(not shown) to a first authorizing agent 167, who affixes his
signature (-AAla) and sends the signed copy 175a to a second
authorizing agent 171a. The second authorizing agent 171a adds a
second authorizing signature and sends the (twice signed) document
179a to the signing device. The signing device 169a verifies the
two authorizing signature, affixes its partial signature (-SD1) to
the copy, and returns the signed copy 181a to the coordinator
161.
[0162] Two other signing devices (not shown) affix partial
signatures to copies of the document to be signed and return the
signed copies 181b, 181c to the coordinator. All three copies may
be processed in parallel.
[0163] After the coordinator has received all three copies 181a,
181b, 181c of the document to be singed, the coordinator multiplies
together the three partial signatures (-SD1, -SD2, -SD3). The
product of the three partial signatures is the system wide
authority signature (-SWA).
[0164] The signing device and the smart cards of the authorizing
agents will be trusted devices. The security of this parallel
multi-step signing method does not depend on the physical security
of the coordinator's workstation. The coordinator need not possess
any secret keys for authorizing the signing devices (although it
will likely have routing encryption and signature keys for privacy
and identification purposes).
[0165] The functions of the coordinator may spread among
authorizing agents. A first authorizing agent may receive the
original document to be signed and designate another authorizing
agent (or even another entity which is not an authorizing agent,
such as a server for one of the signing devices) to receive and
combine the partial signatures. It is expected that the normal
operation of the organization will make it preferable to have the
coordinator both receive the document to be signed, and then be
responsible for delivering the signed document to its ultimate
recipient.
[0166] Adding/Deleting Authorizing Agents
[0167] Each signing device has an associated group of authorizing
agents. Because people come and go in organization, the system
includes provisions to add and delete authorizers dynamically by
adding and deleting the public keys of the authorizing agents'
trusted devices. Adding, or deleting an authorizing agent is
accomplished by submitting, to a signing device, a command to add
or delete a public key of the agent. The command takes the form of
an electronic message having a code for the add/delete command,
additional information (discussed below) and authorizing
signatures.
[0168] The authorizing signatures may be from other authorizing
agents of the same signing device, and the add/delete process can
be completed locally by a single signing device. In an alternate
version, the add/delete procedure may require the signature of the
system wide authority key, thus requiring quorums of authorizing
agents on a quorum of related signing devices to approve and
authorize the change. In yet another alternative, different
authorizing agents may have differing capabilities, and some more
powerful authorizers may be added or deleted under the system wide
authority key, while less capable authorizers may be added or
deleted locally under the authority of a local quorum. Preferably,
the addition or deletion of authorizing agents requires the
signature of the system wide authority key.
[0169] FIG. 13 illustrates a command 201 for deleting an
authorizing agent. The additional information with the command 203
includes: a) the agent's name 205; b) the agent's title 207; c) the
ID number 209 of the signing device from which the agent will be
deleted; and d) the identification code 211 of the trusted device
associated with the authorizing agent to be deleted. After
receiving a properly signed command, the signing device deletes the
authorizing agent's public verification key from its internal lists
of authorizing agents.
[0170] FIG. 14 illustrates a command 213 adding an authorizing
agent. The additional information includes: a) the agent's name
217; b) the agent's title 219; c) the ID number 221 of the signing
device for which the agent is authorized 221; d) an administrative
class 225 indicating powers for which the agent is authorized; e)
an expiration date 223 for the new agent's authority; f)
identification codes 227 for the master key or keys which the
authorizing agent may instruct the signing device to apply; g) ID
code 229 of the agent's trusted device; and h) a certificate 231
with the trusted device's public signature verification key.
Preferably, the pubic key of the new agent is certified 233 under
the authority of the SWA signature key and the certificate is
included with the command. The device certificate 231, signed by
the manufacturer of the trusted device associated with the
authorizing agent, also includes an assurance that the authorizing
agent's private signature key is permanently confined in a smart
card or other trusted device having approved minimum security
properties. (Preferably, the device's minimum security properties
will also include the fact that biometric information is used to
link the smart card to a physical characteristic of the human user.
For example, the manufacturer might state that the card will not
crete its user signatures unless the user activates an attached
fingerprint reader, where the matching fingerprint data is stored
inside the card and used to activate it.) After receiving a
properly-signed request (i.e., after SWA multi-step signing has
been completed), the signing device will add the new agent's
information to its internal lists of authorizing agents.
[0171] Add/Delete Card Manufacturers And Models
[0172] As discussed above, authorizing agents act through trusted
devices, which may be smart cards manufactured with predetermined
security properties. As a condition for adding an authorizing
agent, the agent's trusted device must be of an approved model.
During the initiation of the system, model numbers of trusted
devices that would be acceptable for use in the system were input.
Over time, new models will become available, and security
procedures may be tightened such that older models may no longer be
acceptable. All signing devices maintain an internal table of
accepted models.
[0173] New manufacturers may be added by circulating an electronic
request among all the signing devices to add a new manufacturer.
FIG. 15 illustrates a sample request. The request includes a
command 243 along with the manufacturer's name 245, the model name
or code 247, and a public signature verification key 249, bound
together in a message 241 signed by the system wide authority
key.
[0174] Old manufacturers may be deleted by circulating an
electronic request, signed by the SWA key, to remove the
manufacturer's public verification key from the tables of the
signing devices. FIG. 16 illustrates a sample request 251 which
includes a command 253 and the manufacturer's name 255. These
add/delete requests, once signed by a quorum of devices, are then
sent to all devices, which then verify them using K.sup.+.sub.SWA
and act upon them.
[0175] New models for an already-approved manufacturer may be added
by submitting an electronic request, signed by the SWA key, to add
a new model. FIG. 17 illustrates a sample request 261. The request
will include a command 263; the manufacturer's name 265; the model
number 267 and a certificate 269, signed by the manufacturer, that
the particular model meets certain security standards (e.g., a
certificate that a model satisfies FIPS level 3 requirements).
[0176] Old models may be deleted by submitting an electronic
request, signed by the SWA key, to remove the model from the tables
of the signing devices. FIG. 18 illustrates a sample request 271,
which includes: a command 273; the manufacturer's name 275; and the
model number 277.
[0177] Adding /Deleteting Signing Devices
[0178] Over time, it will be desirable to add or delete signing
devices from the system. Each signing device contains a table of
other signing devices in the system that hold shares of the SWA key
(or shares of another master key for multi-step signing as
discussed more fully below). The identity of each signing device is
defined by: 1) the device identification number (e.g., serial
number); 2) the device public verification key (installed by a
manufacturer and certified under the manufacturer's signature, or a
similar key recertified by the SWA signature); 3) the device public
encryption key (used to send encrypted messages to the device); and
4) any subsequent certified public keys uniquely in its
possession.
[0179] New signing devices are added to the system by circulating
an unsigned certificate among other devices to receive the SWA
signature and then circulating the signed certificate. The
certificate contains the identifying information as discussed
above. After the certificate has been signed by the SWA key, the
certificate is sent to all other signing devices with an
instruction to add the new device to the other signing device's
internal tables. FIG. 19 illustrates a sample instruction 281,
which includes a command 283 and a certificate 282. The certificate
includes: the new signing device ID code 285; a signature
verification key certificate 286 of the signing device (singed by
the manufacturer); and an encryption key certificate 289 of the
signing device (also signed by the device manufacturer). The
signature verification key and encryption key cold also be in a
single certificate. Other information must be circulated among
other signing devices, such as the identities of key shares 291
used by the new signing device and shares of decryption keys 292
escrowed with the new device. Once a signing device is added to the
group, it can: 1) participate in protocols to generate a new master
key and receive a share of it; 2) serve as a backup unit to receive
the contents of a signing SD; or 3) serve as a replacement unit to
receive the restored contents of a revision backed up signing
device that has either been destroyed or removed from service.
[0180] FIG. 20 illustrates a message 293 for removing a signing
device. The message 293 includes a command 295 and the device ID
code 297.
[0181] Copy Key Shares
[0182] The risk (consequences) of theft or destruction of signing
devices has been reduced by virtue of the multi-step signing
process and the fact that no single signing device is capable of
forging a signature or divulging information sufficient to forge a
signature. The information content of a signing device, including
the SWA key share, can therefore be transferred to another device,
e.g., when upgrading signing device hardware or for back-up
purposes.
[0183] Copying of key shares and other information is accomplished
by submitting a request, signed by the SWA key, to copy all or some
of the information in a particular signing device to a second
device. FIG. 21a illustrates a sample request to a sending device
to copy its key share(s). The request 301 preferably includes: a
command 303, signed by the SWA key, identifying the second device
by manufacturer 305 (which must already be included in the signing
devices list of approved manufacturer), model number 307 (which
must already be an the approved list of models), and serial number
309; a certificate 311 with a public encryption key for receiving
device; ID codes 313 of the key shares (or other designation of
information) to be copied; and the sending device ID 315. When the
signed request is received by the proper sending device, the
sending device encrypts the identified key share(s) and related
information using the public encryption key of the receiving
device, and then the sending device outputs the encrypted
information as an "add key(s)" message to the receiving device.
FIG. 21(b) illustrates a sample message from a sending device to a
receiving device. The request 314 preferably includes: a command
316, signed by the sending device (-SD); the receiving device ID
317; the sending device ID 318; the e ID codes of the encrypted key
shares 319; and the ID code of the key share owner 320. The receive
share command could also specify a quorum (or other authorization
details) for use on the receiving device, but preferably, the
received key will be used in accord with default quorum of the
receiving device. As a typical operative procedure, all systems
operators and authorities would be informed that a copy has been
made, along with the identity of the device or storage medium
holding the copy.
[0184] Alternately, the information may be copied to a. storage
device which is kept physically secure (e.g., stored in vault) and
off line (not subject to remote attack) in encrypted form for use
as backup.
[0185] Change Quorum Requirements
[0186] The quorum of signing devices needed to affix the SWA key is
a system design parameter used by the lead device when generating
key shares. This quorum can be changed by re-combining the key
shares to recover the whole signature key, and then splitting the
key into an increased number of shares which are then
re-distributed as with the original key shares, but with a new
quorum requirement.
[0187] The quorum of authorizing agents needed to authorize a
particular signing device to affix a partial signature can be
changed without reinitializing the system. Such a change preferably
is accomplished by submitting a request to the respective signing
device signed by the SWA key. Alternately, authorizing agents of a
particular signing device may change the local quorum by submitting
a request signed only by local authorizing agents. The number of
signatures needed to change the quorum may be the same as or
different from the number needed to authorizing the signing device
to affix the SWA signature. Note that if SWA key shares are stored
within signing devices in encrypted form and if authorizers hold
decryption key shares as discussed below, the quorum needed for
authorizing a signature should not be reduced to less than the
number of shares needed to decrypt the SWA key share. In normal
banking practice, the N of authorities must not be less than 2 per
signing device, although some authorizers may have rights on
multiple signing devices.
[0188] Encrypting Stored Key Shares
[0189] In this variation, shown in FIG. 22, each SWA key share 323
stored within a signing device 321 is stored in an encrypted form
323. The decryption key ("KEY") is split into shares, and each
authorizing agent's trusted device 325, 327, 329 stores a share of
the decryption key. As discussed above, each request for the
signing device to affix a partial signature must be accompanied by
signatures of a quorum of authorizing agents. Under this variation,
the authorizing agents additionally send a share of the decryption
key 331, 333, 335 to the signing device 321. The signing device
then:
[0190] 1) combines the decryption key shares 337 to recover the
decryption key 347;
[0191] 2) decrypts 339 its share of the SWA key;
[0192] 3) uses the plaintext SWA share 341 to affix a partial
signature 343 to a document 345;
[0193] 4) erases the decryption key 347;
[0194] 5) erases the shares 331, 333, 335 of the decryption key;
and
[0195] 6) erases 342 the plaintext SWA key share 341.
[0196] When sending a document to a signing device for signature,
an authorizing agent includes that agent's share of the decryption
key and signs the message. In normal operation, the decryption key
shares are protected due to the fact that all communications on the
network are encrypted using the public encryption key of the
recipient (i.e., of another authorizing agent when a document is
being circulated for agents signatures, or of a signing device when
submitted for signing). Alternately, each authorizing agent may
develop a session key for each message in order to protect the
decryption key shares. (That is, each time a key-containing message
passes from an authorizing agent to another authorizing agent or to
a singing device, a new session encryption key is used.) The entire
message is then encrypted under the session key.
[0197] In this way, the plaintext SWA key share exists only
transiently during the time that it is being used to affix a
partial signature. Furthermore, the decryption key, and a complete
assembly of shares of the decryption key exist only transiently. If
a signing device is stolen, thieves would at best be able to
recover the encrypted form of the SWA key share.
[0198] The process for generating and distributing encrypted key
shares and shares of decryption keys would proceed as follows and
illustrated in FIG. 23.
[0199] 1) The lead device generates a public SWA verification key
351 and shares 353, 355, 357 of a private SWA signature key as
discussed above for the basic variation.
[0200] 2) The lead device generates a separate public/private
encryption key pair 359, 361 for each private share of the SWA
signature key (one SWA share 357 is illustrated, and it should be
understood that other shares are processed similarly).
[0201] 3) For each private encryption key, the lead device splits
the private decryption key into shares 363a, . . . , 363m using an
L of M split where M is the total number of shares and L is the
minimum number of shares needed to reconstruct the private
decryption key. M may be chosen to equal the total number of
authorizers on a signing device, while L equals the quorum of
authorizing agents needed to authorize a signature on the
respective SWA key share.
[0202] 4) The lead device encrypts each share of the SWA signature
key 357 under the associated public encryption key 359, and sends
an encrypted share 365 of the SWA signature key to a respective
signing device along with M shares of the respective private
decryption key.
[0203] 5) The private decryption key shares for the SWA key shares
may also be escrowed (distributed for safe keeping) among the other
signing devices such that any private decryption key can be
recovered from the signing devices, but no one signing device
contains enough information to recover any decryption key for
another device. Such general shares for any given signing device
would be released and upon consent of a quorum of authorities on
several other SDs.
[0204] 6) The lead device erases the private decryption keys, the
private decryption key shares, and the whole private SWA signature
key (if it still exists) from memory.
[0205] When each signing device registers its respective
authorizing agents, the signing device additionally sends each
authorizing agent a decryption key share, identified by: 1) an
identification number for the decryption key share; and 2) the
identification number for the associated SWA key share.
[0206] For example, if there were five SWA signature key shares,
(with three needed for a signature) and each SWA key share were
encrypted under a separate public encryption key, and each SWA key
share required three of five authorizing agents, then each
decryption key could be divided into five shares with any three
capable of recovering the decryption key. There would be twenty
five decryption key shares, with each signing device having
distributed five to its authorizing agents (for its own key) and
holding one share of each of the decryption keys for the other four
devices.
[0207] In this way, the quorum of authorizing agents needed to
authorize a signing device to affix a partial signature will also
have a sufficient number of decryption key shares to allow the
signing device to decrypt the SWA key share transiently for each
signing operation.
[0208] If one or more of the authorizing agents lose their keys
(e.g., loose their trusted device smart cards), then new smart
cards would be registered on the same signing device. The
decryption key shares could be recovered from other signing devices
and could be reinstated to the newly-registered smart cards by
submitting an electronic message, signed by the SWA signature key,
for the signature devices to transfer shares of the decryption key
to the newly registered devices. As an alternate method, subject to
the consent of the SWA, a given device could receive all
description shares, decrypt its signing share, generate a new
encryption key pair, reencrypt the signing share under the public
key, divide the new private decryption key into new shares and
redistribute these shares to the trusted devices of the relevant
authorities, taking care to encrypt them under the public
encryption keys of those receiving authorities'trusted devices.
[0209] As an alternate back-up method, up the decryption key shares
can be escrowed off-line with an independent trust institution as
described in copending U.S. patent application Nos. 08/181,859 and
08/277,438.
[0210] Cryptographic Heartbeat
[0211] As a further protective measure, each signing device
receives a periodic data input ("heartbeat") which, if interrupted,
causes the signing device to become deactivated. The heartbeat
would be generated from a location separate from signing device so
that, if thieves attempt to steal a signing device, they must also
penetrate a separate room or vault to get the heartbeat source. If
they fail to acquire the heartbeat source, the signing device
becomes inactive and is useless.
[0212] In one implementation, each signing device provides an
encryption key to a heartbeat source. The heartbeat source
periodically sends encrypted messages to the signing device. If the
signing device fails to receive a minimum number of messages over a
period of time from the heartbeat source, then the signing device
erases its internal memory or takes other evasive action. The
messages may be empty messages or simple messages, which must be
encrypted by the heartbeat source using the public even key given
to it by the SD. Alternately, the messages could be a pseudo random
string generated in the heartbeat source by a pseudo random number
generator (RNG) and verified by a synchronized (RNG) in the signing
device.
[0213] Multiple heartbeat sources could be established so that a
signing device must receive messages from at least one (or a
minimum number) over a period of time. If one heartbeat source goes
off line due to equipment failure or power outage, it will not
trigger premature erasure of signing device memories Keys used in
heartbeat communications may be backed up in shares to multiple
locations.
[0214] In a second implementation, each signing device may send a
query to a group of associated ("satellite") devices on the
network, and continue operation only if at least a quorum of
associated devices responds. Requiring a quorum allows operations
to continue during inevitable outages and repairs to
communications.
[0215] Use of satellite devices, while more complex, adds physical
security and can be used at locations having less secure
environments, rather then upgrading these facilities with vaults,
guards, cameras, etc.
[0216] The communication link between a signing device and its
heartbeat source or satellite device may be a public network. If a
signing device is reported stolen, its associated satellite units
can be deactivated by the system operators to prevent thieves from
tapping communication lines and re-routing the heartbeats to the
stolen device.
[0217] For example, the signing device may be in the United States
and its associated satellite device in Europe. When the signing
device is stolen, the European satellite device is taken off line
by its operators. Liability of the European agent for any erroneous
action would be minimal, because the removal of the satellite only
interferes with new signing operations for a short time. Previously
signed signatures remain in force Alternately secure physical
wiring can be provided between a signing device and its satellite
or heart-beat source in lieu of a public network.
[0218] Generating Additional Master Keys
[0219] Having established a secure, multi-step signing system with
a SWA key, it is a simple matter to generate a number of additional
"master" keys to be used for other purposes. While the SWA
signature key controls system administration, master keys can be
used to sign other certified messages or documents by use on behalf
of other legal entities. The generation and administration of other
master keys is similar to the SWA key but without intermediate
temporary certification steps. The method proceeds as follows:
[0220] 1) Designate one signing device as "lead" (it need not be
the same "lead" that generated the SWA signature key.
[0221] 2) Input a list public key certificates of signing devices
to receive shares of the master key.
[0222] 3) Input an identification code for the master key and a
logical name.
[0223] 4) Establish secure communication channels among signing
devices (preferably using the encryption key certificates of each
related signing device).
[0224] 5) Optionally obtain random material from each signing
device.
[0225] 6) Generate a new "master" public private key pair.
[0226] 7) Distribute private keys shares (optionally encrypting
each share and distributing shares of decryption key).
[0227] 8) Erase the whole master private key (if it was stored),
and erase all shares not retained by the lead signing device.
[0228] This process may also be used to replace the SWA signature,
by additionally sending each signing device a command, signed by
the (old) SWA signature key to install the new master key as the
SWA signature key. Generally, the master key will have separate
uses from the SWA key and the shares of many master keys may
coexist in the signing devices. A previously generated master key
(other than a SWA signature key) can be deleted from the system by
submitting a message, signed by the SWA signature key, to delete
the master key fragments.
[0229] Document And Signature Tracking
[0230] It is desirable to assign a unique identification code to
each document to be signed in order to assist in managing the flow
of documents through the system. The following information may be
included in the headers of each document for use by message servers
and authorizers:
[0231] 1) The signature key identification code of the key to be
used to sign the document.
[0232] 2) The total number of partial signatures needed to complete
the signature and/or the number of partial signatures already
applied.
[0233] 3) The key fragment identification codes that have already
been used to sign.
[0234] 4) The identities of the signing devices that have already
signed (e.g., the logical device names).
[0235] Interlocking Rings of Signing Services
[0236] A root CA, using a multi-step signing system as described
above, will generally certify subordinate CAs located in other
business and government organizations. Hypothetically, a large
money center bank might certify a major agency of a state
government. The state agency, in turn, might certify a corporation.
This distributes the certification process flexibly in a way which
can conform to existing political, economic and social
organizations.
[0237] However, each mid-tier CA must maintain strong security over
its signature key. Few such organizations, other than banks, some
large corporations, and some government agencies, have
traditionally maintained multiple highly secure data processing
facilities and storage vaults. For example, a mid-tier CA may
possess at least one nominally secure physical location, such as a
data center or vault operation, but lack the funds to serve
multiple sites for the multi-device schemes described above. In the
alternative, the mid-tier CA may have no truly secure location.
[0238] Less secure mid-tier CA's (such as a corporate CA) may
nevertheless set up their own signature-rings (as described above)
and interlock these mid-tier rings with the more highly secure ring
of a parental CA (such as a bank or secure government agency). This
can be done while separating the issues of: (1) key ownership and
official control, (2) administrative and backup responsibility, and
(3) physical possession of the devices.
[0239] An interlocking ring architecture can be created as shown in
FIG. 24 by having a mid-tier CA 371 maintaining one or more
mid-tier signing devices 373, 375, 377 in its own secure locations.
Additional mid-tier signing devices 379, 381 will be maintained at
the secure locations of a parent CA 383 and may even include some
or all of the same devices 379, 381 that make up the parent (root)
CA ring 383 (hence "interlocking rings"). The parent CA could
maintain several signing devices 385, 387, 389 that are independent
from those of any given mid-tier CA 383. The signing devices
described above require no additional modification to hold
additional master keys, each under different ownership and control
by respective authority agents 391a, 391b, with supplemental master
keys grouped in different ways.
[0240] The mid-tier CA initiates the key generation and share
distribution protocol outlined above using one of its own signing
devices as a "lead" device, and authorizes its own officers as
authorizing agents 391b. Some shares of the new CA master key would
reside on its own signing device(s) 373, 375, 377, while others
would reside on signing devices of its parent CA 379, 381. The
authority to issue signatures can remain vested solely in the
officers of the key owner, although they could also delegate some
of this authority to some officers of the parent CA institution, in
case of emergency.
[0241] Thereafter, the mid-tier CA would initiate multi-step
signing of the CA's signatures based on signatures generated by
smart cards possessed by their officers, and route those requests
to their own signing devices and/or to devices in the possession of
the parent CA. Indeed, signing devices need not be located with the
parent CA, but could be sited at any other CA also having a secure
location and communication access.
[0242] Fully Leased Services
[0243] An organization that does not possess even one secure
facility might still wish to generate certificates and can still
become a CA. The organization can lease use of signing devices
located in secure locations already established by various banks or
other CAs. The organization takes possession of smart cards for its
authorizing agents, and routes signing requests to signing devices
through a communication network. The processes of generating keys,
issuing signatures, and performing other administrative tasks can
therefore occur within devices under local bank physical control in
accord with contractual trust arrangements with the owner.
[0244] The organization's officers would go to the local secure
(banking) facility to witness the key generation protocol by which
their new signature key is created, divided, and distributed to
each of a number of host facilities (possibly other banks or other
locations of the same bank) that they have selected. At that time
they could also assign the appropriate administrative backup powers
as needed.
[0245] The organization could then issue official signatures and
certifications, without the need of establishing their own secure
data center or vault facilities, while still achieving
substantially all the security benefits of the system as
described.
[0246] Signature Delegation
[0247] When an authorizing agent becomes temporarily unavailable
(due to being on vacation, incapacitated, etc.), some form of
delegation of signatory authority is desirable. It is undesirable
for a human operator to loan his/her smart card-and an associated
pin number or key-to another, because that creates an un-managed
security risk.
[0248] One alternate delegation mechanism is for an original
authorizing agent ("primary user") to issue a specialized
"delegation" certificate to a substitute authorizing agent
("delegate"). The certificate, signed by the primary user, would
identify the delegate and the delegate's public signature
verification key. The delegation certificate would also contain a
time limit during which the delegation certificate (and hence the
delegate's authority) would be valid. (See Sudia & Ankney,
"Commercialization of Digital Signatures," 1993.) A delegate, using
his/her personal smart card, would sign a document using the
delegate's personal signature key and would attach the delegation
certificate. Resulting documents would be signed by the delegate,
not the primary user, and a document recipient must undertake
additional steps to verify the delegate's signature and the
delegate certificate. This relies, in part, on an ability for all
public users of a system to have such verification capability and,
to have good access to a source of revocation information (or "hot
list"), in case the authority must be cancelled before it expires A
preferred approach is to allow a delegate to use the primary user's
smart card in a secure way that, in effect, substitutes the human
delegate for the human primary user vis--vis the primary user's
smart card. Then, the delegate would use the primary user's smart
card to affix the primary user's signature, and the universe of
document recipients is spared the additional burden of verifying
and evaluating another complex certificate.
[0249] When the primary user wishes to delegate signatory
authority, the primary user issues a "substitution" certificate 409
to the delegate as illustrated in FIG. 25. The substitution
certificate identifies the primary user ID 411, the delegate ID
413, a means for the primary smart card to recognize the delegate
(most likely the delegate's public verification key 417), and a
time limit 415 during which the substitution certificate 409 (and
hence the delegate's authority) is valid. The primary user may
identify multiple individuals, any one of whom can authorize the
smart card, or a group of individuals of whom multiple ones must
jointly authorize the smart card. The antecedents of such methods
are discussed in U.S. Pat. Nos. 4,868,877, 5,005,200, and 5,214,702
by Addison Fischer.
[0250] As shown in FIG. 25, when a delegate wants to sign a
document 403 on behalf of the primary user, the delegate 401
prepares and signs a request 405 in a specified format to be
communicated to the primary user's card 407. Attached to, or
otherwise included in the message is the substitution certificte
409. If multiple delegates need to authorize the primary user's
card, they may sequentially sign the request in a similar manner to
the way multiple authorizing agents sign a request submitted to a
signing device as discussed above. Upon receipt of the signature
request, the primary user's card will verify that the requesting
user's signature(s) match(es) the public key(s) that were
originally specified in the substitution certificate, apply the
primary user's signature 419, and forward the signed document on to
a signing device 421 (or other destination) in the usual
manner.
[0251] The primary user's smart card 407 may be given physically to
a delegate. The presence of a time limit for the delegate's
authority provide a "time lock" so that delegates can only use the
primary user's smart card during a limited period. As discussed
above, the primary user's authority is also limited to a fixed time
period. These limits reduce the consequences of theft, and allow
primary users and delegates to store the primary user's card in a
relatively non-secure office environment. After the time period had
expired, the smart card would not be vulnerable to any key-guessing
attacks. (In fact, it would be immune from attack even if the
primary user or delegate had written his/her pin directly onto the
card.) Additional protection against loss or physical attack can be
achieved by placing the smart card into a vault or other locked
environment, and inserting the card into a card reader where it can
be accessed electronically but not physically. In this manner, all
the actions described above may be carried out, but no one will
have physical possession of the card.
[0252] For example, a primary user might be a vice-president in
charge of purchasing, who wishes to delegate his specific signature
authority to his secretary while he travels to negotiate a pending
deal. The substitution certificate might specify that his smart
card is to issue the vice president's signature only upon receipt
of a signature request signed by: (a) the secretary, as designated
by-his substitution certificate; and (b) co-signed by any other
person with primary signing authority in the purchasing department.
The vice-president places his card in a card reader in a locked
vault and leaves.
[0253] To obtain the vice-president's signature, the secretary
would prepare the document to be signed and compute its associated
hash using her desk-top computer terminal. She would then sign the
hash, attach the vice-president's public key certificate, the final
recipient will need it and then send them in a message to another
purchasing agent. The other purchasing agent co-signs the same hash
and attaches his public key certificate, along with his
authorization certificate which grants him his purchasing
authority. The other purchasing agent sends them in a message to
the vice-president's smart card through a local area network. Given
that the vice-president's card also contains trusted copies of the
public keys of the certifying authorities which created these
certificates, such as the SWA, the vice-president's card determines
that the signatures and certificates are all valid and affixes the
vice-president's signature to the document. The card might also
request that all these certificates be accompanied by recently
signed CRL's or certificates of good standing from a locally
recognized CRL handler.
[0254] This delegation mechanism takes advantage of an ability to
re-program the primary user's smart card. The primary user's smart
card is trusted device having known security characteristics, one
of which must be a capability to engage in a secure download of new
instructions (e.g., substitution certificates), as described for
example in co-pending U.S. patent applications Nos.08/181,859 and
08/272,203 (Sudia key escrow parent and Sudia key escrow CIP).
[0255] The foregoing delegation mechanism may be generalized
such-that many high-value end-user digital signature keys are in
fact generated and used within tamper resistant secure modules
(TRSMs) that are stored inside secure vaults or data centers, while
the authorization for such signatures comes from signature request
messages signed by approved users who are given unofficial (time
locked) smart cards to carry around with them. These TRSMs would
remain secure against tampering, to prevent any data center
personnel from ever having access to user private keys, but could
be designed to contain the keys of many different users, each of
which might be authorized to act based on some single non-official
signature, or some prearranged combination of signatures and
authorizations.
[0256] Another use for the delegation mechanism, apart from simple
delegation from users on temporary leaves of absence, would be a
system or method whereby such a programmatic signature request
would be made to a card (or to a key contained with a common TRSM)
to perform the signature of a major "desk" or other role within a
financial or corporate environment.
[0257] After learning of the embodiments described above, people
practicing in this art will be able to make variations that fall
within the spirit and scope of the invention. The embodiments
described above are exemplary but not intended to limit unduly the
scope of the invention as defined by the following claims.
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