Abstract:
A method for protecting a ciphering algorithm executing looped operations on bits of a first quantity and on a first variable initialized by a second quantity, wherein, for each bit of the first quantity, a random number is added to the state of this bit to update a second variable maintained between two thresholds.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French Patent Application number 13/58271, filed on Aug. 29, 2013, which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND 
       [0002]    Technical Field 
         [0003]    The present disclosure generally relates to electronic circuits and, more specifically, to circuits performing ciphering calculations. The present disclosure more specifically relates to the protection of a calculation executed by an electronic circuit against side-channel attacks. 
         [0004]    Discussion of the Related Art 
         [0005]    Many techniques for protecting a ciphering calculation against side-channel attacks are known. Such techniques typically use one or several random numbers for masking quantities manipulated by the calculation. 
         [0006]    Be they calculations based on elliptic curves, using add and double operations, or modular exponentiation calculations based on a so-called square multiplication technique, a difficulty is to eliminate the disturbance introduced by the random number to restore the result. 
       SUMMARY 
       [0007]    An embodiment of the present disclosure provides a technique for protecting a calculation executed by an electronic circuit which overcomes all or part of the disadvantages of usual solutions. 
         [0008]    Another embodiment provides a technique compatible with double-and-add operations on elliptic curves or square-and-multiply operations for modular exponentiation calculations. 
         [0009]    Another embodiment is more specifically adapted to side-channel attacks. 
         [0010]    An embodiment provides a method for protecting a ciphering algorithm executing looped operations on bits of a first quantity and on a first variable initialized by a second quantity, wherein, for each bit of the first quantity, a random number is added to the state of this bit to update a second variable maintained between two thresholds. 
         [0011]    According to an embodiment, for each bit of the first quantity, the second variable is updated with: 
         [0012]    its value plus the random number and minus the state of the bit if the result of this calculation ranges between said two thresholds; or 
         [0013]    the random number minus the state of the concerned bit in the opposite case. 
         [0014]    According to an embodiment, said second variable is used in one of the operations performed for each bit of the first quantity. 
         [0015]    According to an embodiment, for each bit of the first quantity, the first variable is updated with: 
         [0016]    the result of a calculation taking into account the second quantity and the random number if the second variable, plus the random number and minus the state of the concerned bit, ranges between said two thresholds; or 
         [0017]    the result of a calculation further taking into account said second variable in the opposite case. 
         [0018]    According to an embodiment, said calculation comprises a sum and a product. 
         [0019]    According to an embodiment, said calculation comprises a product and an exponentiation. 
         [0020]    According to an embodiment, the random number is drawn for each bit of the first quantity. 
         [0021]    According to an embodiment, at the end of a calculation, the first variable is updated by a calculation taking into account said second variable and said second quantity. 
         [0022]    According to an embodiment, the second variable is at most over 8 bits. 
         [0023]    According to an embodiment, said operations comprise an addition and a doubling, said second variable being taken into account during the addition step. 
         [0024]    According to an embodiment, the method is applied to a multiplication of a point of an elliptic curve by a scalar number, said scalar number representing the first quantity and said point representing the second quantity. 
         [0025]    According to an embodiment, said operations comprise a squaring and a multiplication, said second variable being taken into account in the multiplication step. 
         [0026]    According to an embodiment, the method is applied to a modular exponentiation of the second quantity, the first quantity representing the exponent. 
         [0027]    An embodiment also provides an electronic circuit capable of implementing the method. 
         [0028]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a very simplified representation in the form of blocks of an example of electronic circuit of the type to which the embodiments which will be described apply; 
           [0030]      FIG. 2  illustrates an example of usual protection of a multiplication on an elliptic curve; 
           [0031]      FIG. 3  illustrates an example of usual protection of a modular exponentiation; 
           [0032]      FIG. 4  schematically shows in the form of blocks an embodiment of the method for protecting a multiplication on an elliptic curve; and 
           [0033]      FIG. 5  schematically shows in the form of blocks an embodiment of the method for protecting a modular exponentiation. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those steps and elements which are useful to the understanding of the embodiments which will be described have been illustrated and will be detailed. In particular, the nature of the quantities manipulated by the different calculations have not been detailed, the described embodiments being compatible with usual natures of such numbers. Further, the origin of the numbers to be submitted to the calculations which will be described, as well as the destination of the results, more specifically in ciphering applications, has not been detailed either, the described embodiments being here again compatible with the usual origins and destinations of such numbers. 
         [0035]      FIG. 1  very schematically shows, in the form of blocks, an example of an electronic circuit  1  of the type to which the embodiments which will be described apply. Such a circuit aims at encryption applications and generally comprises a processing unit  12  (CPU) capable of communicating, over one or several address, data, and control buses  13  with different elements, among which at least one input/output element  14  (I/O), for communicating with the outside of circuit  1 , one or several volatile memory circuits  15  (MEM), one or several non-volatile memory circuits  16  (NVM), and one or several functions symbolized by blocks  17  (FCT) depending on the application of the circuit. Although reference has been made hereabove to an electronic circuit, it should be understood that the embodiments which will be described apply to a hardware implementation by a state machine in wired logic or to a software implementation having instructions executed by processing unit  12 . 
         [0036]    In ciphering applications, calculations are generally performed over numbers of significant size (from a few hundreds to a few thousands of bits) and the most current ciphering algorithms to which the present description relates comprise either a modular exponentiation (typically, the RSA algorithm), or a multiplication of a point of an elliptic curve by a scalar number. The exponent in the case of the modular exponentiation or the scalar number in the case of the multiplication on an elliptic curve generally forms a so-called secret quantity which should not be made accessible to non-authorized circuits or users. 
         [0037]    To protect the secret quantities against side-channel attacks in ciphering calculations, random numbers are generally used. 
         [0038]      FIG. 2  schematically shows a usual example of protection by a random number of a multiplication of a point P of an elliptic curve by a scalar number k. In the example of  FIG. 2 , it is assumed that the calculations are performed modulo an integer n which represents the size of the manipulated numbers. 
         [0039]    It is started by calculating (block  21 , +) the sum of number k generally representing the secret quantity with a random number r. The size of the random number generally corresponds to at least that of key k since the size of this number conditions the security, that is, the number of masked bits. In the example of  FIG. 2 , quantities k and r over n bits are assumed. 
         [0040]    The obtained number k+r is then used in a so-called calculation of addition/doubling (Add/Doub) of a quantity P which is desired to be ciphered. Quantity P represents a point of an elliptic curve. A calculation variable T is initialized with the value of point P. Then, for each bit (k+r), of number k+r, the content of variable T is doubled (bloc  23 , T=2T) after which, for the bits (k+r), of number k+r of value 1, point P is added to variable T (T=T+(k+r) i ·P). 
         [0041]    Once all the bits of number k+r have been processed, the contribution of random number r is eliminated by performing a last operation (bloc  25 , T=T−r·P) of subtraction of quantity r·P to the intermediate result. Variable T then contains result k·P mod(n). 
         [0042]    Most often, to protect the calculation against so-called horizontal attacks, typically single power analysis attacks (SPA), the same number of calculation steps is carried out whatever the state of the bit of number k+r. This generally results in using an additional variable which does not take part in the final result. 
         [0043]    The protection implemented by the algorithm described in relation with  FIG. 2  imposes, to unmask the calculated quantity at the end of the calculation, performing an additional multiplication by a scalar number (product r·P) to be able to subtract it from the obtained result. 
         [0044]      FIG. 3  illustrates a usual example of modular exponentiation calculation implementing a random number r to mask calculations exploiting a secret quantity d. 
         [0045]    As in the previous case, it is started (block  31 ) by adding quantity d, which represent the exponent of the modular exponentiation, to a random number r, to obtain a value d+r. Here again, the number (for example, n) of bits of random number r corresponds at least to the number of bits of value d to obtain an efficient protection. 
         [0046]    For each bit (d+r)i of number d+r, a so-called square-and-multiply operation is then performed. A variable T is initialized by message M to be submitted to the modular exponentiation. Then, the content of variable T is multiplied by itself (squaring). Then, for all the bits (d+r) i  of value d+r of value 1, variable T is updated by multiplying its content by message M. 
         [0047]    Once all the bits of the exponent have been processed, the contribution of the random number is eliminated by performing an operation (block  35 , T=T/M r ) where intermediate result T is divided by value M exponent r. One then obtains, in variable T, result M d  mod(n). 
         [0048]    As in the case of  FIG. 2 , when the bits of the exponent (value d+r) are at zero, an operation which is not useful for the calculation is generally performed to protect it against SPA-type attacks. 
         [0049]    As for a double-and-add calculation, the introduction of a random number of the size of the quantity to be protected generates, at the end of an operation, a resource-intensive calculation (intensive in terms of time or circuit capacity). 
         [0050]    According to an embodiment, it is provided to take advantage of the fact that the calculations are performed in a loop for the different bits of the quantity to be protected to maintain the drift introduced by the random number within a range smaller than the number of bits of the quantity to be protected. More specifically, it is provided to define a window (in number of bits) where the drift is limited. 
         [0051]      FIG. 4  very schematically shows in the form of blocks an embodiment applied to the multiplication of a point P of an elliptic curve by a scalar number k, by the double-and-add technique. 
         [0052]    Different variables used in the calculation (block  41 , T=P; drift=0; i=0) are initialized. For example, a variable T is intended to contain an intermediate result and is initialized with value P of the point of the elliptic curve, a drift value used to mask the calculation is set to 0. An index of current bit i is set, for example, to 0, to start the calculation from right to left (from the most significant bit to the least significant bit) but may as a variation be set to n, representing the number of bits of quantity k for a calculation from left to right. A calculation modulo n is arbitrarily assumed, but the modulo may be different from the size of quantity k. 
         [0053]    For each bit k i  of quantity k, an operation (block  42 , T=2T) of doubling of the content of variable T is performed. Then, a random number r (block  43 ), −X′≦r≦X′) selected to ranges between two values −X′ and X′ is drawn. In practice, limit X′ corresponds to the number of bits over which the random number is drawn. 
         [0054]    A test (block  44 , −X≦drift+r−k i ≦X?) is then performed on the sum of the content of the drift variable, of random number r, minus the state (1 or 0) of current bit k i  of number k. 
         [0055]    If (output N of block  44 ) the result exceeds the authorized range (from −X to X), variable T is updated by an operation (block  45 , T=T+/−(r−drift·P)) of addition or subtraction (according to the direction in which the limit is exceeded) of quantity r−drift·P. Then, variable drift takes value r−k i  (block  46 , drift=r−k i ). 
         [0056]    In the opposite case (output Y of block  44 ), variable T is updated (block  45 ′, T=T+r·P) by being added the product of quantity r by value P. Then, quantity r−k i  is added to the drift variable (block  46 ′, drift =drift+r−k i ). 
         [0057]    As a variation, the order of steps  45  and  46  ( 45 ′ and  46 ′) may be modified. In this case, variable T is updated with value T+drift·P whatever the state of bit k i . 
         [0058]    The drift variable, which takes into account a random element and the state of the current bit, is always contained in the window defined by number X. Further, the addition calculation is well protected by the random quantity. 
         [0059]    Limit X′ of the random drawing is preferably identical to limit X authorized for the drift. For the case where different limits X and X′ are selected, it will be ascertained to select limit X′ of the random selection smaller than limit X of the drift, to avoid too frequent an exceeding of the range of values tested at block  44 . 
         [0060]    Steps  42  to  46  are repeated as long as all the bits of value k have not been processed (test  47 , i=n?). As long as there remains a bit to be processed (output N of block  47 ), it is proceeded to the next bit (block  48 , i=i+1) and it is returned to the input of block  42 . Once all the bits have been processed (output Y of block  48 ), there only remains to cancel the last contribution of the random number (block  49 , T=T−drift·P) to obtain the final result in variable T (k·P mod(n)). 
         [0061]    It can be seen that multiplication drift·P to be performed in order to eliminate the contribution of random number only concerns a number of small size which is at most X. Thus, it is no longer needed to select a random number r having the size of quantity k. Indeed, since the random number is introduced at the level of the bit taken into account in the calculation loops, a random number over one bit is sufficient (values 0, 1). 
         [0062]    Preferably, a random number r over a few bits, typically between 3 and 8 bits, will be selected, which amounts to up to 256 possible values for the random number and provides a satisfactory masking security. 
         [0063]    The process described in relation with  FIG. 4  provides a masking against side-channel attacks, for example, of DPA, electromagnetic signature analysis, or other type. Further, it also provides a protection against so-called horizontal attacks (for example, of SPA type). Indeed, the number of performed operations is identical whatever the state of the bit of the secret quantity. 
         [0064]    Preferably to spare calculation time, the different values capable of being taken by quantity drift·P (block  45 ) are pre-calculated. Thus, as illustrated by block  40  in  FIG. 4 , several products Z·P are pre-calculated for any possible value of Z ranging between −X and X. This set of pre-calculated values is then exploited by block  45  according to value drift which is necessarily within the range defined by value X. It can be seen that the number of values to be pre-calculated is relatively limited. For a calculation on an elliptic curve and a 3-bit window, values −8P to 8P are calculated. Knowing that 0·P is a point at infinity defined by convention, this results in 17 values to be stored, but only in 15 values to be calculated (P being known). 
         [0065]    The selection of the size of the window depends on the calculation and storage capacities of the circuit. 
         [0066]    Further, the security level becomes parameterizable in the algorithm. Indeed, value X may be defined according to the desired security level. Thus, a same encryption calculation cell may be used with different security levels according to the manipulated quantities. 
         [0067]      FIG. 5  shows an embodiment applied to a modular exponentiation. 
         [0068]    It shows most of the steps described in relation with  FIG. 4  to within the following differences: 
         [0069]    steps  50 ,  51 , and  59 , corresponding to steps  40 ,  41 , and  49 , take into account value M (modulo n) to be submitted to the modular exponentiation (for example, the message to be ciphered) instead of point P; 
         [0070]    step  42  is replaced with a squaring step  52  (block  52 , T=T·T); 
         [0071]    steps  54 ,  56 , and  56 ′, corresponding to steps  44 ,  46 , and  46 ′, take into account the n bits d i  of exponent d instead of bits k i  of scalar number k; 
         [0072]    at steps  55  (T=T·M (r−drift) ),  55 ′ (T=T·M r ) and  59  (T=T/M drift ), corresponding to steps  45 ,  45 ′, and  49 , the product of point P is replaced with an exponentiation of value M, the addition (+) is replaced with a multiplication (·), the subtraction (−) is replaced with a division (/). 
         [0073]    Preferably, random number r is drawn at each round (for each bit of the secret quantity). However, in a simplified embodiment (providing a lesser protection), it may be provided to use a same random number r for the entire calculation. 
         [0074]    It can be seen that at steps  45 ,  46 ,  45 ′,  46 ′,  55 ,  56 ,  55 ′,  56 ′, the calculation differs according to the state of the bit of the secret quantity. Accordingly, the executed calculation is in accordance with a double-and-add operation on an elliptic curve or a square-and-multiply operation for the modular exponentiation. 
         [0075]    Various embodiments have been described. Various alterations and modifications will occur to those skilled in the art. In particular, although the embodiments have been described in relation with a multiplication on an elliptic curve and a modular exponentiation, they more generally apply whatever the form of the algorithm used, provided for it to comprise an double-and-add or a square-and-multiply operation according to the bits of one of the operands. In particular, a so-called Montgomery representation may be used in the context of the modular exponentiation. Further, the practical implementation of the described embodiments is within the abilities of those skilled in the art based on the functional indications given hereabove and by using tools usual per se. 
         [0076]    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 disclosure. Accordingly, the foregoing description is by way of example only and is not intended to be limiting.