78 lines
1.6 KiB
C
78 lines
1.6 KiB
C
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/* TomsFastMath, a fast ISO C bignum library.
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*
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* This project is meant to fill in where LibTomMath
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* falls short. That is speed ;-)
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*
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* This project is public domain and free for all purposes.
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*
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* Tom St Denis, tomstdenis@gmail.com
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*/
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#include <tfm_private.h>
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/* Miller-Rabin test of "a" to the base of "b" as described in
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* HAC pp. 139 Algorithm 4.24
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*
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* Sets result to 0 if definitely composite or 1 if probably prime.
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* Randomly the chance of error is no more than 1/4 and often
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* very much lower.
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*/
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void fp_prime_miller_rabin (fp_int * a, fp_int * b, int *result)
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{
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fp_int n1, y, r;
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int s, j;
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/* default */
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*result = FP_NO;
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/* ensure b > 1 */
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if (fp_cmp_d(b, 1) != FP_GT) {
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return;
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}
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/* get n1 = a - 1 */
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fp_init_copy(&n1, a);
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fp_sub_d(&n1, 1, &n1);
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/* set 2**s * r = n1 */
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fp_init_copy(&r, &n1);
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/* count the number of least significant bits
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* which are zero
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*/
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s = fp_cnt_lsb(&r);
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/* now divide n - 1 by 2**s */
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fp_div_2d (&r, s, &r, NULL);
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/* compute y = b**r mod a */
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fp_init(&y);
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fp_exptmod(b, &r, a, &y);
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/* if y != 1 and y != n1 do */
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if (fp_cmp_d (&y, 1) != FP_EQ && fp_cmp (&y, &n1) != FP_EQ) {
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j = 1;
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/* while j <= s-1 and y != n1 */
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while ((j <= (s - 1)) && fp_cmp (&y, &n1) != FP_EQ) {
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fp_sqrmod (&y, a, &y);
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/* if y == 1 then composite */
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if (fp_cmp_d (&y, 1) == FP_EQ) {
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return;
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}
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++j;
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}
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/* if y != n1 then composite */
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if (fp_cmp (&y, &n1) != FP_EQ) {
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return;
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}
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}
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/* probably prime now */
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*result = FP_YES;
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}
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/* $Source$ */
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/* $Revision$ */
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/* $Date$ */
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