Abstract:
A method and a circuit for protecting against possible fault injections a calculation successively performed by several hardware cells of a same electronic element, including: starting a first execution of the calculation; starting a second execution of the same calculation once the first execution has freed a first cell and goes on in a second cell; synchronizing the executions so that the second execution uses a cell only when the first execution has passed to the next cell; and verifying the identity between the two results at the end of the execution of the two calculations.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to the execution of calculation functions by an integrated circuit. The present invention more specifically relates to the control of the execution of an algorithm, especially of cryptography, against attacks by injection of faults aiming at discovering the secret (generally, a key) handled by this algorithm. 
   2. Discussion of the Related Art 
   An attack by fault injection consists of introducing a fault into the executed program (for example, blowing up the program counter) or into the handled data to obtain an erroneous result. This fault injection is repeated several times and ends by enabling the attacker to discover the handled secret quantity. For example, for cryptography algorithms (DSA, RSA, DES, AES, etc.), the secret keys can be discovered by means of a pirating causing instruction jumps. 
   A known technique to protect a program against fault injections consists of calculating a signature (application of a ciphering algorithm to at least a portion of the software code) upon installation or writing of the program. This signature is then stored inside or outside of the integrated circuit executing the program. Then, in the execution of the software code, the exploitation system recalculates a signature based on the same algorithm as that having been used to generate the initial signature. The current signature is then compared with the predetermined signature. A divergence between these two signatures means that the stored program has been modified and thus enables identifying a potential attack, voluntary or incidental. An example of such an integrity control method is described in U.S. Pat. No. 5,442,645, which is incorporated hereby by reference. 
   Such a solution protects the program, but not the data and, especially, not the handled secret keys. Further, attacks by fault injection such as described in document “DFA of DES with single injection faults” by M. Witterman in IBM Workshop on Security—April 2000, remain efficient on algorithms such as the DES. 
   To protect the data, a known technique consists of applying a function C for calculating an error-correction code to data D being processed. Before starting a given operation O of the program, this calculation function is applied to the data to be processed to obtain an initial code C(D). At the end of the processing of the data by the operation, the same function C is applied to the result data O(D) and operation O of the program is applied to the initial code C(D). The data have not been modified during the processing if the two results C(O(D)) and O(C(D)) are identical. 
   A disadvantage of this technique is that it is not applicable to all operations. In particular, it requires for the operation handling the data and for the code calculation function to respect, for valid data, condition C(O(D))=O(C(D)). 
   Another known solution to control the execution of a program is to perform certain operations twice, to have a redundancy on the data to check the consistency between the two executions. A disadvantage of such a solution is that it requires either doubling the execution time, or doubling the hardware calculation elements. 
   SUMMARY OF THE INVENTION 
   The present invention aims at overcoming the disadvantages of known solutions of protection against fault injections in the execution of programs and, more specifically, detecting possible fault injections, be it in the program or in the data. 
   The present invention also aims at providing a solution which is compatible with an execution on a microcontroller of limited power of smart card type. 
   To achieve these and other objects, the present invention provides a method for protecting against possible fault injections a calculation successively performed by several hardware cells of a same electronic element, comprising:
         starting a first execution of the calculation;   starting a second execution of the same calculation once the first execution has freed a first cell and goes on in a second cell;   synchronizing the executions so that the second execution uses a cell only when the first execution has passed on to the next cell; and   verifying the identity between the two results at the end of the execution of the two calculations.       

   According to an embodiment of the present invention, a temporary storage element for storing intermediary results provided by said cells is assigned to each execution. 
   According to an embodiment of the present invention, at least one temporary storage element is assigned to each hardware cell to store a value to be processed, the result of each cell being stored in a memorization element forming, preferably, the input element of the next cell. 
   According to an embodiment of the present invention, an error processing is implemented if the two results at the end of the execution are different from each other. 
   According to an embodiment of the present invention, the method is applied to a DES-type algorithm. 
   The present invention also provides a microcontroller comprising means for executing the method and a smart card comprising such a microcontroller. 
   The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flowchart of an embodiment of the method according to the present invention. 
   

   DETAILED DESCRIPTION 
   For clarity, only those elements and steps which are necessary to the understanding of the present invention will be described hereafter. In particular, the operators or instructions concerned by the implementation of the present invention have not been described in detail, the present invention applying whatever the type of operator or of instruction. Further, all the components necessary to the execution of a program, be it for a software implementation or by means of a state machine in wired logic, have not been described in detail, their forming using conventional elements. 
   A feature of the present invention is to execute twice the same calculation with the same data by using, as soon as it is freed by a first execution, a same hardware cell for the same execution. In other words, the two executions provided by the present invention are time-shifted by one cell. 
   The present invention takes advantage from the fact that the algorithms that handle data, and especially the cryptography algorithms, successively use several hardware cells that they free along the execution of the algorithm. 
   “Hardware cell” is used to designate, in the meaning of the present invention, a wired OR operation or function, that is, which uses dedicated components. This also assumes that the input-output values of this operation or function are storable, for example, in a temporary register. 
   According to a preferred embodiment of the present invention, at least one first temporary storage element (for example, one or several registers) is assigned to each of the executions to store, in particular, intermediary results. If necessary, a storage element is assigned to each result to enable the comparison. As an alternative, the first element of each execution stores its result. The number of storage elements is to be adapted to the algorithm and depends, in particular, on the possible need for taking into account one or several previous intermediary results. In practice, the implementation of the present invention doubles the number of intermediary state storage elements with respect to a conventional execution of the considered algorithm. Accordingly, the smaller this number (ideally, a single one), the less the implementation of the present invention is resource consuming. 
   Designating with F an algorithm executing the n operations f 1 , . . . , f i , . . . f n  (i designating the rank of the operation in a first execution), with R 1  and R 2  two registers (or sets of registers) assigned to the respective intermediary results of two executions of the algorithm, and with D init  the data to be processed which must result in data D fin  at the end of the processing, the implementation of the present invention may be written as follows. 
   For simplification, each operation f is assimilated to a time period t j . In fact, the respective durations of operations f may be different from one another, but this has no incidence provided that the same operation is not simultaneously applied to the respective contents of the two registers R 1  and R 2 . It is assumed that registers R 1  and R 2  contain the data being processed at each end of an operation. The respective contents of registers R 1  and R 2  at the end of period j will be designated as R 1   j  and R 2   j . In fact, due to the execution shift, ranks i and j are equal for the first execution (R 1 ). 
     FIG. 1  is a flowchart of an embodiment of the method according to the present invention. 
   Initially (before period t 1 ), at least register R 1  contains value D init  (block  10 , D init —&gt;R 1 ). 
   During period t 1  (block  11 , f 1 (R 1 ), D init —&gt;R 2 ), first operation f 1  is applied to data D init  contained in register R 1  and, at the end of period t 1 , register R 1  contains data R 1   1 =f 1 (D init ). During this period t 1 , no processing is applied to register R 2 , lest possibly its loading with data D init . Accordingly, R 2   1 =D init . 
   During period t 2  (block  12 , f 2 (R 1 ), f 1 (R 2 )), second operation f 2  is applied to the data of register R 1  which, at the end of the period, contains data R 1   2 =f 2 (R 1   1 ). In parallel, first operation f 1  is applied to initial data D init  contained in register R 2  which, at the end of period t 2 , then contains data R 2   2 =f 1 (D init ). 
   After, at the end of each time phase t j  (block  13 , f j (R 1   j-1 ), f j-1 (R 2   j-1 )), registers R 1  and R 2  respectively contain values R 1   j =f j (R 1   j-1 ) and R 2   j =f j-1 (R 2   j-1 ). 
   At the end of period t n  (block  14 , f n (R 1   n-1 ), f n-1 (R 2   n-1 )), the first execution is over and register R 1  contains data R 1   n =f n (R 1   n-1 ). If the execution has occurred properly, R 1   n =D fin . On the side of register R 2 , the penultimate operation f n-1  is applied during period t n  to result in data R 2   n =f n-1 (R 2   n-1 ). 
   The implementation of the present invention requires at least one additional period t (block  15 , f n (R 2   n )) to end the second execution. At the end of this additional period t n+1 , register R 2  contains data R 2   n+1 =f n (R 2   n ). Here again, if the execution has occurred properly, R 2   n+1 =D fin . 
   It is then enough, at the end of period t n+1 , to compare (block  16 , R 1 =R 2 ?) the respective contents of registers R 1  and R 2 . In case of an identity (output Y), it can be concluded therefrom that the result is reliable OK (that the executions have not been disturbed). In the opposite case (output N), this means that an attack (or an incidental error) ERROR has occurred. 
   An example of implementation of the present invention will be described hereafter in relation with a specific example of application to the DES algorithm. However, the present invention more specifically applies to any algorithm successively using several hardware cells. For example, the present invention also applies to the AES algorithm. 
   An algorithm of DES type decomposes in ciphering rounds (ROUND) in which data and keys are handled. This algorithm is described, for example, in document “Federal Information Processing Standards Publication” 46-2, Dec. 30, 1993, which is incorporated hereby by reference. 
   Each round can be broken-up as follows, noting i the rank of the round, K the key, R the right-hand portion of the data word to be processed, L the left-hand portion of the data word, and T the variable being processed.
         T=E®, where E designates the expansion of data R over all the bits of the word;   T=T xor K(i), where xor designates the XOR function and K(i) designates the key of round i;   T=Sbox(T), where Sbox designates a substitution table;   T=P(T), where P designates a permutation;   T=T xor L;   L=R; and   R=T.       

   In terms of time, the execution of a round of rank i can be represented as in table 1 hereafter, where t designates the clock cycle. 
   
     
       
             
             
           
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
               Content of the registers 
             
           
        
         
             
               Operation 
               temporary register 
               R 
                 
             
             
                 
             
             
                 
               R i−1   
               R i−1   
               i − 1 
             
             
               T = E(R i−1 ) 
               E(R i−1 ) 
             
             
               T = T xor 
               E(R i−1 ) xor K(i) 
             
             
               K(i) 
             
             
               T = Sbox( 
               Sbox(E(R i−1 ) xor K(i)) 
             
             
               T) 
             
             
               T = P(T) 
               P(Sbox(E(R i−1 ) xor K(i))) 
             
             
               T = T xor L 
               P(Sbox(E(R i−1 ) xor K(i))) 
             
             
                 
               xor L 
             
             
               L = R 
                 
                 
               i − 1 
             
             
               R = T 
                 
               P(Sbox(E(R i−1 ) 
             
             
                 
                 
               xor K(i))) xor L 
             
             
                 
             
           
        
       
     
   
   Table 2 hereafter illustrates the implementation of the present invention on a ciphering round of the DES algorithm. The same notations as previously have been used by being assigned with a 1 for the first execution and with a 2 for the second execution. 
   
     
       
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 2 
             
           
           
             
                 
                 
             
             
                 
               Execution 1 
               Execution 2 
               Register content 
             
           
        
         
             
               ycle 
               Operation 
               Operation 
               T1 
               T2 
               R 
                 
             
             
                 
             
             
                 
                 
                 
               R i−1   
               R i−1   
               R i−1   
               i − 1 
             
             
                 
               T1 = E(T1) 
                 
               E(R i−1 ) 
             
             
                 
               T1 = T1 xor 
               T2 = E(T2) 
               E(R i−1 ) xor K(i) 
               E(R i−1 ) 
             
             
                 
               K(i) 
             
             
                 
               T1 = Sbox(T1) 
               T2 = T2 xor 
               Sbox(E(R i−1 )) xor 
               E(R i−1 ) xor 
             
             
                 
                 
               K(i) 
               K(i)) 
               K(i) 
             
             
                 
               T1 = P(T1) 
               T2 = Sbox(T2) 
               P(Sbox(E(R i−1 )) 
               Sbox(E(R i−1 )) 
             
             
                 
                 
                 
               xor K(i)) 
               xor K(i)) 
             
             
                 
               T1 = T1 xor L 
               T2 = P(T2) 
               P(Sbox(E(R i−1 )) 
               P(Sbox(E(R i−1 )) 
             
             
                 
                 
                 
               xor K(i)) xor L 
               xor K(i)) 
             
             
                 
                 
               T2 = T2 xor L 
                 
               P(Sbox(E(R i−1 )) 
             
             
                 
                 
                 
                 
               xor K(i)) xor L 
             
             
                 
               T2 = T1 ? 
             
             
                 
               L = R 
                 
                 
                 
                 
               i − 1 
             
             
                 
               R = T1 
                 
                 
                 
               P(Sbox(E(R i−1 )) 
             
             
                 
                 
                 
                 
                 
               xor K(i)) xor L 
             
             
                 
             
           
        
       
     
   
   As compared to the conventional execution, the identity of the contents of registers T 1  and T 2  is tested before updating registers R and L. As can be seen, two additional cycles are necessary (one for the testing and one to support the shift). In table 2, it is assumed that the testing of cycle 7 validates the execution. In the opposite case, cycles 8 and 9 are not executed. 
   As an alternative, the testing is not performed at the end of each round i, but at the end of the execution of all rounds. In this case, the number of additional cycles is limited to n+1 (n being the number of rounds) instead of 2n. 
   It should be noted that the implementation of the present invention is compatible with an algorithm using more than once a same cell. If this use is sufficiently spaced apart in time, this requires no specific precaution. However, if this use is close, for example, if a same cell is used twice successively, the two uses are then considered as a single one and it is awaited for this cell to be freed. As an alternative, this cell is used a first time for the first execution. Then it is awaited for it to have been used by the second execution before using it back for the first one. Each execution is then temporarily put to wait while the cell is used for the other one. The selection of the implementation mode depends on the duration of execution of the cell with respect to the previous or next cells. In particular, using it twice successively for a same execution does not lengthen the method if the previous step alone is twice as long (the data of the second execution would then anyway not be ready before the end of the two uses for the first execution). 
   After detection of two different results, any conventional action may be taken. For example, the final result is not taken into account for the rest of the application. According to another example, the electronic element, for example, the smart card, is blocked. 
   An advantage of the present invention is that its implementation requires but little additional (time or hardware) resources with respect to an unprotected execution. Indeed, the lengthening of the total duration, linked to the implementation of the present invention with respect to two executions in parallel, is limited to the time of execution of a single cell (that which takes the most time). Further, the extra hardware elements, with respect to two successive executions, are at most one element for storing the intermediary results of the second execution (the storage of the results of the two executions for comparison being already present for two successive executions). 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the dividing of the algorithm (the selection of the hardware cells) is within the abilities of those skilled in the art according to the application and based on the functional indications given hereabove. Further, the practical implementation of the present invention by software means to execute the verification method and, especially, manage the registers and the operation sequencing is within the abilities of those skilled in the art and calls forth usual programming notions. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.