denyhosts/clamav/libclamav/tomsfastmath/mont/fp_montgomery_setup.c

49 lines
1.1 KiB
C

/* TomsFastMath, a fast ISO C bignum library.
*
* This project is meant to fill in where LibTomMath
* falls short. That is speed ;-)
*
* This project is public domain and free for all purposes.
*
* Tom St Denis, tomstdenis@gmail.com
*/
#include <tfm_private.h>
/* setups the montgomery reduction */
int fp_montgomery_setup(fp_int *a, fp_digit *rho)
{
fp_digit x, b;
/* fast inversion mod 2**k
*
* Based on the fact that
*
* XA = 1 (mod 2**n) => (X(2-XA)) A = 1 (mod 2**2n)
* => 2*X*A - X*X*A*A = 1
* => 2*(1) - (1) = 1
*/
b = a->dp[0];
if ((b & 1) == 0) {
return FP_VAL;
}
x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */
x *= 2 - b * x; /* here x*a==1 mod 2**8 */
x *= 2 - b * x; /* here x*a==1 mod 2**16 */
x *= 2 - b * x; /* here x*a==1 mod 2**32 */
#ifdef FP_64BIT
x *= 2 - b * x; /* here x*a==1 mod 2**64 */
#endif
/* rho = -1/m mod b */
*rho = (((fp_word) 1 << ((fp_word) DIGIT_BIT)) - ((fp_word)x));
return FP_OKAY;
}
/* $Source$ */
/* $Revision$ */
/* $Date$ */