U.S. patent application number 11/433232 was filed with the patent office on 2006-11-30 for method to prevent power dissipation attacks on a cryptographic algorithm by implementing a random transformation step.
Invention is credited to Patrick Salle.
Application Number | 20060271795 11/433232 |
Document ID | / |
Family ID | 9524129 |
Filed Date | 2006-11-30 |
United States Patent
Application |
20060271795 |
Kind Code |
A1 |
Salle; Patrick |
November 30, 2006 |
Method to prevent power dissipation attacks on a cryptographic
algorithm by implementing a random transformation step
Abstract
The invention relates to a data protection method using a
cryptographic algorithm comprising at least one execution cycle of
repetitive operations for processing data elements (K2, R1) so as
to generate encrypted information (C), this method comprising at
least one step (120, 220) for randomly modifying the execution of
at least one operation from one cycle to another, or at least one
of the data elements, so that the encrypted information is
unchanged by this random modification.
Inventors: |
Salle; Patrick; (Verrieres
Le Buisson, FR) |
Correspondence
Address: |
ANDERSON & JANSSON L.L.P.
9501 N. CAPITAL OF TX HWY #202
AUSTIN
TX
78759
US
|
Family ID: |
9524129 |
Appl. No.: |
11/433232 |
Filed: |
May 12, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09646640 |
Nov 9, 2000 |
7073072 |
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PCT/FR99/00613 |
Mar 17, 1999 |
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11433232 |
May 12, 2006 |
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Current U.S.
Class: |
713/193 |
Current CPC
Class: |
G06F 2207/7219 20130101;
H04L 9/003 20130101; H04L 2209/08 20130101; H04L 9/0625
20130101 |
Class at
Publication: |
713/193 |
International
Class: |
G06F 12/14 20060101
G06F012/14 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 17, 1998 |
FR |
98/03242 |
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. Data protection method for operating a microprocessor of a chip
card to protect data elements contained in a memory of the chip
card from discovery by analysis of the electric power consumption
of the microprocessor, said method using a symmetric cryptographic
algorithm of the DES-type with a permutation step for executing
operations for processing data elements so as to generate encrypted
information, said method comprising: operating the microprocessor
to randomly determine a processing order of the bits of an input
data for the execution of the permutation step, thereby protecting
said data elements from discovery by analysis of the
microprocessor's electric power consumption.
10. The data protection method of claim 9 wherein the cryptographic
algorithm for executing operations for processing data elements
includes a group of operations executed repeatedly.
11. The data protection method of claim 9 wherein said data
elements are keys.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] 1. Field of the Invention
[0002] The present invention relates to a data protection method,
for example designed to be implemented by the microprocessor of a
bank card or an access authorization card during a connection to an
authenticating computer terminal.
[0003] 2. Background of the Invention
[0004] The known types of data protection methods use a
cryptographic algorithm comprising execution cycles of repetitive
operations for processing data elements contained in a memory of
the card so as to generate encrypted information intended to be
communicated to the computer terminal.
[0005] The execution of the method by the microprocessor of the
card results in the sending of derivative signals such as peaks in
the level of the microprocessor's electric power consumption, or
variations in the electromagnetic radiation such that the envelope
of electromagnetic radiation is indicative of the data processed.
An attacker seeking to use the microprocessor cards in an
unauthorized way can trigger the execution of the method repeatedly
and analyze the derivative signals emitted in order to determine
correspondences between the various processing operations and each
signal or series of signals. From these correspondences, and for
example by subjecting the card to electromagnetic disturbances or
voltage drops at precise moments in the execution of the algorithm,
the attacker can study the encrypted information obtained and the
differences, or lack of differences, between the derivative signals
emitted, in order to discover the data contained in the memory of
the card.
[0006] To complicate this type of analysis of the derivative
signals, it has been suggested that parasitic signals be generated
and added to the derivative signals emitted during the execution of
the method. The extraction of the signals that correspond to the
execution of the method is then more difficult, but it is still
possible. It has also been suggested that the electronic components
of the card and the program for executing the method be designed so
that the derivative signals emitted are independent of the value of
the sensitive data. However, this complicates the production of the
cards without providing satisfactory protection of the data.
SUMMARY OF THE INVENTION
[0007] One object of the invention is to offer an effective
protection method that does not have the aforementioned
disadvantages.
[0008] In order to achieve this object, the invention provides a
data protection method using a cryptographic algorithm for
executing operations for processing data elements so as to generate
encrypted information, this method comprising at least one step for
the random transformation of the execution of at least one
operation from one cycle to another, or for the random
transformation of at least one of the data elements, so that the
encrypted information is unchanged by this random
transformation.
[0009] Random transformation of the execution of at least one
operation is intended to mean a modification of the order of
execution of operations or parts of operations, or a modification
of the execution of a single operation. Thus, at least one
operation and/or at least one of the pieces of data processed is
randomly modified, which randomly affects the derivative signals
emitted. This makes it very difficult for an attacker to
distinguish between the various processing operations and to
discover the data from the derivative signals. Moreover, the random
modification does not affect the encrypted information, so it can
be used in the normal way after it is generated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Other characteristics and advantages of the invention will
emerge through the reading of the following description of a
particular non-limiting embodiment of the invention, in connection
with the FIGS. 1-5, illustrating, in the form of block diagrams,
different permutations of the execution of operations according to
embodiments of the invention, and an exemplary hardware embodiment
of the invention.
DETAILED DESCRIPTION OF THE DRAWING
[0011] The protection method according to the invention described
herein uses a symmetric cryptographic algorithm of the DES (DATA
ENCRYPTION STANDARD) type to generate 64-bit encrypted information
C from a message block M and a secret key K1, both 64-bit. A
description of the algorithms used in DES is presented in the
document Federal Information Processing Standards Publication 46-2,
Dec. 30, 1993 issued by the National Bureau of Standards, and its
content is hereby incorporated by reference.
[0012] The method begins with the permutation 10 of the bits of the
message block M with one another, in order to form the block
M0.
[0013] The block M0 is then divided into two 32-bit blocks M1 and
M2 during a division step 20.
[0014] It then performs the expansion 30 of the block M2 to form a
48-bit block M3. This expansion 30 is performed, for example, by
partitioning the block M2 into eight quartets, and by adding to
each quartet the adjacent end bit of the quartets framing the
quartet in question (the end quartets being considered to be
adjacent).
[0015] In parallel with these operations, a permutation 110 is
performed on the bits of the key K1 to form the key K2. The
insignificant bits of the key K1 are simultaneously deleted so that
the key K2 has only 56 bits.
[0016] According to the invention, the bits of the key K2 are then
randomly modified during a transformation 120. The bits of the key
K3 corresponding to the modified bits of the key K2, here marked
with a star, are stored. The transformation 120 is for example
performed by associating with the key K2, by means of a logical
operator of the exclusive-OR type, a random number generated by an
unpredictable number generator of the card.
[0017] A key K4 is obtained through the rotation 130 of the bits of
the key K3. Then, a permutation 140 is performed on the bits of the
key K4 to form the key K5. Simultaneously with the permutation 140,
the insignificant bits of the key K4 are eliminated so that the key
K5 comprises 48 bits.
[0018] The method continues with the association 210 of the block
M3 and the key K5 by means of a logic operator of the exclusive--OR
type. The result of this association is the block R1.
[0019] The inverse transformation of the bits of the block R1
corresponding to the bits modified by the transformation 120 is
then performed in order to form the block R2. The purpose of this
inverse transformation 220 of the transformation 120 is to return
the bits of the block R1 corresponding to the bits marked with a
star to the state in which they would have been without the
transformation 120.
[0020] The method then continues, in a conventional way, with the
division and the processing 230 of the block R2, the permutation
240 of the bits of the block R3 formed in step 230, and the
association 250 of the block R4 resulting from step 240 with the
block M1 by means of an exclusive-OR operator, in order to form the
block R5.
[0021] The group of operations designated overall by the reference
270 is then re-executed five times assigning, with each execution,
the value of the block M1 to the block M2 and the value of the
block R5 to the block M1 during an assignment step 260.
[0022] The method ends with the operation 300 for obtaining the
encrypted information C through the inverse permutation and the
combining of the last block M2 and the last block R5 obtained.
[0023] It is understood that the step for randomly modifying the
key K2 comprises the transformation phase 120 and the inverse
transformation phase 220. These two phases make it possible to
obtain encrypted information C that is not affected by this random
modification.
[0024] It would also be possible, in the same way, to perform a
random modification of the block M2 and/or of another piece of
data.
[0025] According to another embodiment of the invention, which can
be associated with a modification step like the one described
above, the execution of at least one operation can be randomly
modified from one cycle to another, a cycle being a complete
execution cycle of the algorithm or an intermediate execution cycle
of a group of operations.
[0026] For example, a random determination of the order of
execution of certain operations can be made during an execution
cycle of the algorithm. The operations retained are the ones whose
order of execution relative to the others does not affect the
result. To make this determination, it is possible to perform, at
the end of the chosen operations, a conditional jump to certain
operations as a function of the value of a random number or to
define a table of the addresses of the various operations, scanned
randomly.
[0027] For example, the permutation 10 of the bits of the message
block M could be performed after the permutation 110 of the bits of
the key K1, or vice versa.
[0028] Likewise, it is possible to provide for a random
determination of the order of execution of the operations of the
group 270 for each intermediate execution cycle of the latter (16
intermediate execution cycles of these operations for one complete
execution cycle of the algorithm). Here again, the order of
execution of these operations is chosen so as not to affect the
result.
[0029] Furthermore, for certain operations, the data are processed
in elements. Thus, during the expansion 30, the blocks M2 are
processed in quartets. During this operation, it is possible to
provide for a random determination of the processing order of the
various quartets. Likewise, during the permutation 140, the bits of
the key K4 are processed individually. A step for randomly
determining the processing order of the bits can also be provided
for the execution of this permutation. The quartets of the block M2
can also be processed alternately with the bits of the key K4,
meaning for example that a first quartet of the block M2 is
processed, followed by a bit string of the key K4, followed by a
second quartet of the block M2, etc., each time storing the data
elements processed in order to verify that all of the required
operations are actually executed.
[0030] Of course, the invention is not limited to the embodiment
just described, but on the contrary encompasses any variant that
retains, with equivalent means, its essential characteristics.
[0031] In particular, although the invention has been described in
connection with an algorithm of the DES type, the invention can be
applied to other symmetric algorithms that work by modifying bits.
Thus, the modification being performed by means of a logical
operator of the exclusive-OR type, the length of the
non-transformed data elements is identical to the length of these
data elements transformed.
[0032] Furthermore, the numbers of bits of the data are only
mentioned as an example and can be modified in order to be adapted
to the degree of protection sought.
[0033] It will also be noted that all of the data elements M, M0,
M1, M2, M3, K1, K2, K3, K4, K5, R1, R2, R3, R4 and R5 can be
transformed by associating a random number with them, by means of
the exclusive-OR logical operator, bearing in mind that after this
random transformation step, an inverse transformation step is
performed so that the encrypted information C is unchanged by said
transformations.
[0034] In particular, the data elements can be keys K1, K2, K3, K4,
K5 or message blocks M, M0, M1, M2, M3, or message blocks
associated with a key by a logical operator of the exclusive-OR
type R1, R2, R3, R4, R5.
[0035] FIG. 2 is a block diagram illustrating an alternative
embodiment in which the randomly transformed data element is a
message block. FIG. 3 is a block diagram illustrating an
alternative embodiment in which the randomly transformed data
element is a message block associated with a key by logical
operator of the exclusive-OR type.
[0036] Finally, it will be noted that if the random transformation
step is a step that precedes the group of operations executed
repeatedly, and if the inverse transformation step is a step that
follows said group of operations, generating a random number once
and processing the message block M with the algorithm is enough to
obtain the encrypted information, all the data elements of the
block being modified. The data string is protected from end to end.
Moreover, by not multiplying the transformation steps and the
number of random numbers generated, the algorithm is executed
quickly, which is necessary in the case of a chip card, in which
the execution time of an algorithm should be minimal.
[0037] FIG. 4 is a block diagram illustrating an embodiment in
which the random transformation step is a step that precedes the
group of operations (270) executed repeatedly and in which the
inverse transformation step follow the group of repeated operations
(270).
[0038] FIG. 5 is a schematic illustration showing a microprocessor
505 of a chip card 501, for example, a bank card or access
authorization card connected to an authenticating computer terminal
507. The data protection method of the present invention is, for
example, designed to be implemented on a microprocessor 505 of a
chip card 501 during a connection to an authenticating terminal
507. The chip card 501 also contains a memory 509 having therein
some data, for example, keys, which is protected by the method of
the present invention.
* * * * *