Abstract:
A device for calculating a result or an integer multiple of the result of a division of a numerator by a denominator includes a unit for providing a factor which is selected such that a product of the factor and the denominator is greater than the result. The device further includes a unit for modularly reducing a first product of the numerator and the factor using a modulus equaling a sum of a second product of the denominator and the factor and of an integer to obtain an auxiliary quantity having the result. A unit is used to extract the result or the integer multiple of the result from the auxiliary quantity. A division is thus reduced to a modular reduction and an extraction which is uncomplicated as far as calculation is concerned so that, in particular in long-number division tasks, the speed on the one hand and the safety on the other hand are increased.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of copending International Application No. PCT/EP03/00669, filed Jan. 23, 2003, which designated the United States and was not published in English, and is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to cryptographic algorithms and, in particular, to division algorithms suitable for cryptographic applications. 
   2. Description of the Related Art 
   The division of two long numbers is often required in cryptographic algorithms. In the RSA algorithm, for example, the modulus N is a product of two prime numbers p, q, wherein q is obtained when N is divided by p, or p is obtained when N is divided by q. 
   If a division routine is not incorporated on a cryptographic coprocessor used for this as an explicit command consisting of micro-commands which are internally processed quickly, the division must take place by means of software. Conventional division routines for this are slow on the one hand and not safe against SPA attacks (SPA=Simple Power Analysis) on the other hand. 
   Usual division routines, as are, for example, described in “Computer Arithmetic”, Henessy and Patterson, Morgan Kaufmann Publishers, Inc., 1996, such as, e.g., the restoring division, the non-restoring division etc., are based on register shifts taking place and subtractions or additions being performed, depending on whether certain bits have certain values. Routines of this kind are susceptible to SPA attacks since the current or power consumption and, additionally, the time consumption depend on the numbers to be processed. An attacker could thus draw conclusions as to the numbers processed from the current or time profile and thus for example spy out a secret key of a public-key crypto algorithm. 
   In order to tackle this problem, so-called dummy operations by which a homogenization of the current profile can be obtained are incorporated. The incorporation of dummy operations, however, results in an additional performance loss which can amount to 33%. 
   SUMMARY OF THE INVENTION 
   It is the object of the present invention to provide a more efficient and safer concept for calculating a division. 
   In accordance with a first aspect, the present invention provides a device for calculating a result or an integer multiple of the result of a division of a numerator by a denominator, having: a unit for providing a factor selected such that a product of the factor and the denominator is greater than the result; a unit for modularly reducing a first product of the numerator and the factor using a modulus equaling a sum of a second product of the denominator and the factor and of an integer to obtain an auxiliary quantity having the result; and a unit for extracting the result or the integer multiple of the result from the auxiliary quantity. 
   In accordance with a second aspect, the present invention provides a method for calculating a result or an integer multiple of the result of a division of a numerator by a denominator, having the following steps: providing a factor which is selected such that a product of the factor and the denominator is greater than the result; modularly reducing a first product of the numerator and the factor using a modulus equaling a sum of a second product of the denominator and the factor and of an integer to obtain an auxiliary quantity having the result; and extracting the result or the integer multiple of the result from the auxiliary quantity. 
   The present invention is based on the finding that, in particular for cryptographic purposes, classic division routines must be dropped to perform a division. According to the invention, a division is reduced to a modular reduction by introducing a factor which is selected such that a product of the factor and the denominator for the division is greater than the result of the division. The modular reduction is executed on a first product of the numerator and the factor, wherein a modulus equaling a sum of a second product of the denominator and the factor and of an integer is used to obtain an auxiliary quantity comprising the result of the division. Finally, the result can be extracted from the auxiliary quantity without great calculating complexity. 
   The inventive concept is of advantage in that the current and/or time profile are independent from the quantities to be processed, i.e. the numerator and the denominator. In addition, the modular reduction to which the division to be calculated is finally reduced can be executed in a calculating-efficient way. In particular in crypto processors, the modular reduction is a frequently used operation for which efficient algorithms are typically implemented as hardware in a crypto coprocessor. The modular reduction can thus be calculated in a fast and efficient way. 
   Depending on the hardware implementation, the extraction of the result of the division from the auxiliary quantity can either be read directly from a long number register when a register of sufficient length is present. 
   Alternatively, another modular reduction and a subtraction can be performed for extracting the result, wherein in this case the calculating complexity also remains within sensitive limits since the other modular reduction, too, is executed in a fast and safe way using efficient reduction circuits which are present on crypto coprocessors anyway. 
   The inventive concept provides an essential acceleration of the division with simultaneously increasing the safety. A division of long numbers, in a well-known processor ACE (ACE=Advanced Crypto Engine), available from Infineon Technologies AG, Munich, Germany, requires about 27 clocks per bit. The inventive division, on the same processor, only requires 6 clocks per bit, which corresponds to an acceleration of 4.5 times. 
   The inventive division concept is, at the same time, safe against SPA attacks since the current or time consumption is independent of the special bit pattern of the numbers processed, i.e. of the numerator and the denominator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be detailed subsequently referring to the appendage drawings, in which: 
       FIG. 1  is a block diagram of an inventive device for calculating a division; 
       FIG. 2  is a block diagram of an inventive method according to a preferred embodiment; and 
       FIG. 3  is an illustration of the auxiliary quantity in a binary long-number register for explaining an extraction of the result from the auxiliary quantity. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Before the figures will be explained in greater detail, the derivation of the inventive division concept which bases on a modular reduction of a first product of the numerator and the factor will be explained, wherein the modulus equals a sum of a second product of the denominator and the factor and an integer. 
   The result Q of a division of a numerator A by a denominator N according to the following equation is sought:
 
 Q=A/N   (1)
 
   Without limiting the generality, it is assumed that both the numerator A and the denominator N are binary numbers, so that the following applies:
 
2 a−1 ≦A&lt;2 a   (2a)
 
2 n−1 ≦N&lt;2 n   (2b)
 
   Equations 2a and 2b indicate the orders of magnitude of the numerator A and the denominator N. 
   Equation 1 can be transformed as follows:
 
 A=Q·N+H   (3a)
 
   The value H of equation 3 is calculated as follows:
 
 H=A−Q·N   (3b)
 
wherein the value H is greater than or equal to 0 and smaller than N.
 
   It becomes evident from equation 3a that the result Q of the division which will subsequently be of interest is the integer result while the quantity H is the remainder. The result Q of the division of A and N thus represents the result of the so-called DIV operation, while the remainder H is obtained by the modular reduction of the numerator A and the denominator N as a modulus:
 
H=A mod N.   (4)
 
   It is to be pointed out that every floating-point division can be reduced to an integer division, namely, for example, by shifting the point and by rounding to the next integer. Usually, floating-point divisions are reduced to integer divisions within a calculating unit. 
   According to the invention, a factor F is introduced which, in the case of a binary number system, is defined as follows:
 
F=2 e   (5)
 
   The number 2 is the base since exemplarily only a binary number system is considered here, while the factor F results when the base 2 is raised to the power of an exponent e. According to the invention, the factor F must fulfil the following condition:
 
 N·F&gt;Q   (6)
 
or, when equation 5 is inserted into equation 6:
 
 N· 2 e   &gt;Q   (7)
 
   Thus, the factor is determined such that the product of the factor F and the denominator of the division (equation 1) is greater than the result Q of the division sought. 
   It is to be pointed out that the exact result Q of the division need not be known for this consideration because this is what is to be calculated. Only the order of magnitude of Q must be known in order to correctly dimension the factor F. 
   Usually it is, however, no problem to estimate the order of magnitude of the result of the division of the numerator and the denominator, in particular as equation 6 only includes a greater-than condition, so that a correct procedure of the algorithm is always ensured when the factor F is selected to be very large. 
   It is, however, preferred to rather select the factor to be smaller since the factor determines the length of the registers required for calculating the division. If the factor is selected to be very large, very long registers will be required, while shorter registers are sufficient when the factor F is selected to be smaller. The following equation 8 indicates a preferred dimensioning of the quantity e for the binary case (equation 5):
 
 e≧a +2−2 n   (8)
 
   Equation 8 only contains information on the numerator A (equation 2a) and information on the denominator N (equation 2b). If e is dimensioned as in equation 8, the condition for the factor of equation 6 will always be fulfilled. 
   When equation 3 is multiplied by the factor F, the following equation results:
 
 A·F=Q·N·F+H·F   (9)
 
   When additionally equation 4 is also multiplied by the factor F on both sides, equation 10 will be the result:
 
 H·F=A·F  mod ( N·F )  (10)
 
   In addition, the following applies:
 
0 ≦H·F&lt;N·F   (11)
 
   Equation 11 indicates that the result of the modular reduction of equation 10 must be within the remainder class of the modulus N·F, i.e. larger than or equal to 0 and smaller than N·F. 
   The result Q is then added to and simultaneously subtracted from the right side of equation 9, which corresponds to the following equation:
 
 A·F=Q·N·F+Q+H·F−Q   (12)
 
   When equation 12 is transformed such that the result Q of the first two terms on the right side of equation 12 is factored out, the following expression results:
 
 A·F=Q ( N·F +1)+ H·F−Q   (13)
 
   Alternatively, equation 12 can also be transformed such that the sum of HF and Q and not the difference of the two terms is formed:
 
 A·F=Q ( N·F −1)+ H·F+Q   (13′)
 
   A transformation of equation 13 or 13′ such that the difference H·F−Q (or the sum of this) is on the left side of the equation, results in the following expression:
 
 H·F−Q=A·F−Q ( N·F +1)  (14)
 
   The following results for the “sum alternative”:
 
 H·F+Q=A·F−Q ( N·F −1)  (14′)
 
   When equation 14 or 14′ is then compared to equations 3a and 3b, it becomes evident that equation 14 is a new determination equation for a new division, wherein the difference or sum on the left side of equation 14 or 14′, i.e. the auxiliary quantity (H·F−Q) or (H·F+Q), in which the result Q sought for is contained, corresponds to the remainder of an integer division of a numerator A·F by a denominator (N·F+1) or (N·F−1). 
   The remainder of this division, i.e. the auxiliary quantity on the left side of equation 14, can be calculated by the following equation 15 in analogy to equation 4:
 
 H·F−Q=A·F  mod ( N·F +1)  (15)
 
   Equation 15 thus represent the modular reduction which gives the auxiliary quantity H·F−Q as the result, from which, as will be illustrated subsequently, the result Q sought for can be extracted in different manners without considerable complexity. Equation 15 thus is the central modular reduction to which the division (equation 1) has been reduced. It is also to be noted that the difference on the left side of the previous equation could also be negative. In this case, the modulus is added in order for the equation to be true, in particular as the result of a modular reduction, as the definition says, may not be negative. 
   The following equation results for the “sum alternative”:
 
 H·F+Q=A·F  mod ( N·F −1)  (15′)
 
   As will be explained subsequently, there are different possibilities to extract the result Q sought from the auxiliary quantity H·F+/−Q. 
   For this, reference will first be made to  FIG. 3  to show a manner for extracting the result Q of the division of the numerator A by the denominator N.  FIG. 3  shows a binary long-number register  300  into which the result of the modular reduction of the right side of equation 15 has been stored. The long-number register has an msb side and an lsb side (msb=most significant bit; lsb=least significant bit). 
   The numbers H·F and Q are then in the register  300  as will be explained subsequently. The number H·F is a large number and, as regards its bit pattern, corresponds to the number H as is illustrated in  FIG. 3 , since the number H·F is obtained from the number H when the number H is shifted by i positions to the left in the long-number register, wherein the factor F is selected to be 2 e . 
   In addition, a, compared to the number H·F, small number “+/−Q” is contained in the binary long-number register  300  of  FIG. 3 , i.e. the negative or positive of the result Q sought. If the long-number register  300  is large and the factor F has been selected to be as large that the numbers H and −Q and +Q, respectively, do not overlap in the register  300 , wherein such a case is illustrated in  FIG. 3 , the number −Q sought can be read out directly from the register  300 . The number Q results after inverting −Q. For this, the corresponding bits on the lsb side of the register are to be considered (which results in −Q). Subsequently, in the usual usage of the two&#39;s complement, the bit pattern contained therein is inverted, after which a one is added to the inverted bits to obtain the result Q sought. 
   Thus, only a simple arithmetic operation in the form of adding a one to the inverted bits is required. No larger arithmetic operations, such as, e.g., a subtraction using the register contents etc., are required. Due to the size differences of the numbers H·F and Q, it is easily possible to somehow read Q separately from the register  300 , i.e. extract it from the auxiliary quantity (the left side of equation 15). 
   It is to be pointed out that the factor F need not be selected to be so great that the numbers H and −Q in the register shown in  FIG. 3  do not have overlapping regions. Even if these numbers do have an overlapping region, it is also possible, as will be explained subsequently, to extract the number Q from the auxiliary quantity. Another modular reduction is executed for this, as is illustrated in equation 16:
 
 H·F=A·F  mod ( N·F )  (16)
 
   Equation 16 corresponds to equation 4, wherein the factor F is, however, taken into consideration here. 
   The result Q sought in this case results by subtracting equation 15 from the result of equation 16:
 
 H·F −( H·F−Q )= Q   (17)
 
   The following differentiation of cases when the auxiliary quantity, i.e. the difference H·F−Q, is negative is to be pointed out. If the difference H·F−Q in equation 15 is negative, the modulus (N·F+1) is added to the left side of equation 15, since, according to definition, the result of a modular reduction is always to be positive. If the auxiliary quantity is negative so that a modulus is added to the left side in equation 15, this will be taken into consideration when subtracting equation 15 from equation 16 as follows:
 
 Q=A·F  mod ( N·F )− A·F  mod ( N·F +1)+ N·F +1  (18)
 
   Subsequently,  FIG. 1  will be dealt with to explain a block diagram of a preferred device for calculating a result or, as will be explained later, an integer multiple of the result Q of a division of a numerator by a denominator. The determination equations, for reasons of clarity, are illustrated in  FIG. 1  in a block  10 . The inventive device includes means  12  for providing a factor and, in particular, a number e which forms the factor as the exponent of the base 2, so that equation 6 and equation 7, respectively, are fulfilled. 
   The inventive device further includes means  14  for calculating the auxiliary quantity, i.e. for carrying out equation 15. Finally, the inventive device includes means  16  for extracting Q from the auxiliary quantity in one of different manners, such as, e.g., by the mechanism described in  FIG. 3  or by calculating another modular reduction (equation 16) and by subtracting the result of the two modular reductions, as is illustrated by equation 17. 
   Subsequently, reference will be made to  FIG. 2  to illustrate a preferred method which can do with four registers only, i.e. a first register for the numerator A, a second register for the denominator N, a third register for the first auxiliary quantity Hi and a fourth register H 2  for the second auxiliary quantity. Optionally, a fifth result register can be used, or the numerator register, the denominator register or the third register for the first auxiliary quantity can be used as the result register when desired. 
   In a step  20 , the value e is at first selected according to equation 8. Subsequently, the numerator register is loaded with the first product A·F (step  22 ). Then, the denominator register is also re-loaded, with the second product ( 24 ). In a step  26 , a modular reduction according to equation 16 is calculated. Subsequently, after the calculation in step  26 , the denominator is incremented by 1 (step  28 ) to calculate the central reduction equation 15 in step  30 . In step  32 , a subtraction of the two relevant equations 15 and 16 is performed, as is illustrated by equation 17. After calculating the difference in step  32 , step  34  checks whether the result is negative. If this is the case, the modulus will be added (step  36 ) to obtain the result Q of the division (step  38 ). 
   If it is, however, determined in step  34  that the result obtained by step  32  is larger than 0, this result will be output directly as the result of the division (step  38 ′). 
   It is to be pointed out that the embodiment of the present invention shown in  FIG. 2  can be used with special advantage when the numbers H and Q are overlapping in the binary long-number register  300  of  FIG. 3  since in this case the processes described referring to  FIG. 3  of reading out the lowest bits of the register  300  and subsequently of inverting to obtain the result Q do not lead to correct results. In the embodiment of the present invention shown in  FIG. 2 , means  16  for extracting the result Q from the difference H·2 e −Q includes the functionality of steps  26 ,  32 ,  34  and  36 . 
   As will be explained subsequently, the inventive concept, without increased complexity, can also be used to calculate not only the result of a division but also the integer multiple of the division. This can easily be obtained when an integer x&gt;1 is inserted in the modulus on the right side of equation 15 instead of the number “1”, wherein at the same time the result Q is also multiplied by the integer x on the left side of equation 15, so that the following equation 19 results:
 
 H·F−Q·x=A·F  mod ( N·F+x )  (19)
 
   If a number x larger than 1 is used, this must also be taken into consideration in equation 6 when selecting the factor F in that the factor, compared to x=1, must be x times the result. 
   When considering equation 19, it shows that the result Q·x, which can, for example, be obtained by reading out the register, can either be used directly when a subsequent algorithm step does not require the result Q but x times the result Q. 
   When the result Q is required, when, however, the modular reduction with the modulus (N·F+x) for some reason can be calculated more easily than for the case x=1, this can alternatively be obtained by again dividing Q by x. In particular in the case in which x is an integer multiple of 2 and the system is a binary system, this can be obtained by shifting the register by a corresponding number of positions to the right. 
   When equation 19 is evaluated in analogy to equations 16 and 17, x times Q will be obtained. 
   Another alternative for extracting Q or a multiple of Q from equation 19 is to use the following equation 20 for evaluation, wherein equation 20 basically corresponds to equation 19, wherein this time the integer y is to be selected to be different from x. When equation 20 is then subtracted from equation 19, the result will be equation 21. It is not the result Q but an integer multiple of the result Q, namely the difference of y and x that results on the left side of equation 21. Q can be obtained from equation 21 by performing a division by (y−x). This division can be dispensed with when y and x are selected such that the difference thereof equals 1.
 
 H·F−Q·y=A·F  mod ( N·F+y )  (20)
 
 Q · ( y−x )= A·F  mod ( N·F+x )− A·F  mod ( N·F+y )  (21)
 
   It is to be mentioned that the parameters x and y may also be negative, in analogy to the procedure which has been set forth in connection with equations (13′) to (15′). 
   The present invention, due to its flexibility, safety and performance, is suitable in particular for cryptographic algorithms and for cryptographic coprocessors on which a safe and efficient implementation of the modular reduction is typically implemented by means of a circuit. 
   While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.