92 lines
2.0 KiB
C
92 lines
2.0 KiB
C
#include "tommath_private.h"
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#ifdef MP_PRIME_MILLER_RABIN_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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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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mp_err mp_prime_miller_rabin(const mp_int *a, const mp_int *b, bool *result)
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{
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mp_int n1, y, r;
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mp_err err;
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int s, j;
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/* ensure b > 1 */
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if (mp_cmp_d(b, 1uL) != MP_GT) {
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return MP_VAL;
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}
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/* get n1 = a - 1 */
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if ((err = mp_init_copy(&n1, a)) != MP_OKAY) {
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return err;
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}
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if ((err = mp_sub_d(&n1, 1uL, &n1)) != MP_OKAY) {
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goto LBL_ERR1;
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}
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/* set 2**s * r = n1 */
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if ((err = mp_init_copy(&r, &n1)) != MP_OKAY) {
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goto LBL_ERR1;
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}
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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 = mp_cnt_lsb(&r);
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/* now divide n - 1 by 2**s */
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if ((err = mp_div_2d(&r, s, &r, NULL)) != MP_OKAY) {
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goto LBL_ERR2;
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}
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/* compute y = b**r mod a */
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if ((err = mp_init(&y)) != MP_OKAY) {
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goto LBL_ERR2;
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}
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if ((err = mp_exptmod(b, &r, a, &y)) != MP_OKAY) {
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goto LBL_END;
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}
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/* if y != 1 and y != n1 do */
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if ((mp_cmp_d(&y, 1uL) != MP_EQ) && (mp_cmp(&y, &n1) != MP_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)) && (mp_cmp(&y, &n1) != MP_EQ)) {
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if ((err = mp_sqrmod(&y, a, &y)) != MP_OKAY) {
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goto LBL_END;
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}
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/* if y == 1 then composite */
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if (mp_cmp_d(&y, 1uL) == MP_EQ) {
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*result = false;
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goto LBL_END;
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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 (mp_cmp(&y, &n1) != MP_EQ) {
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*result = false;
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goto LBL_END;
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}
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}
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/* probably prime now */
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*result = true;
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LBL_END:
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mp_clear(&y);
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LBL_ERR2:
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mp_clear(&r);
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LBL_ERR1:
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mp_clear(&n1);
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return err;
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}
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#endif
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