78 lines
1.6 KiB
C
78 lines
1.6 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>
|
|
|
|
/* Miller-Rabin test of "a" to the base of "b" as described in
|
|
* HAC pp. 139 Algorithm 4.24
|
|
*
|
|
* Sets result to 0 if definitely composite or 1 if probably prime.
|
|
* Randomly the chance of error is no more than 1/4 and often
|
|
* very much lower.
|
|
*/
|
|
void fp_prime_miller_rabin (fp_int * a, fp_int * b, int *result)
|
|
{
|
|
fp_int n1, y, r;
|
|
int s, j;
|
|
|
|
/* default */
|
|
*result = FP_NO;
|
|
|
|
/* ensure b > 1 */
|
|
if (fp_cmp_d(b, 1) != FP_GT) {
|
|
return;
|
|
}
|
|
|
|
/* get n1 = a - 1 */
|
|
fp_init_copy(&n1, a);
|
|
fp_sub_d(&n1, 1, &n1);
|
|
|
|
/* set 2**s * r = n1 */
|
|
fp_init_copy(&r, &n1);
|
|
|
|
/* count the number of least significant bits
|
|
* which are zero
|
|
*/
|
|
s = fp_cnt_lsb(&r);
|
|
|
|
/* now divide n - 1 by 2**s */
|
|
fp_div_2d (&r, s, &r, NULL);
|
|
|
|
/* compute y = b**r mod a */
|
|
fp_init(&y);
|
|
fp_exptmod(b, &r, a, &y);
|
|
|
|
/* if y != 1 and y != n1 do */
|
|
if (fp_cmp_d (&y, 1) != FP_EQ && fp_cmp (&y, &n1) != FP_EQ) {
|
|
j = 1;
|
|
/* while j <= s-1 and y != n1 */
|
|
while ((j <= (s - 1)) && fp_cmp (&y, &n1) != FP_EQ) {
|
|
fp_sqrmod (&y, a, &y);
|
|
|
|
/* if y == 1 then composite */
|
|
if (fp_cmp_d (&y, 1) == FP_EQ) {
|
|
return;
|
|
}
|
|
++j;
|
|
}
|
|
|
|
/* if y != n1 then composite */
|
|
if (fp_cmp (&y, &n1) != FP_EQ) {
|
|
return;
|
|
}
|
|
}
|
|
|
|
/* probably prime now */
|
|
*result = FP_YES;
|
|
}
|
|
|
|
/* $Source$ */
|
|
/* $Revision$ */
|
|
/* $Date$ */
|