48 lines
1.0 KiB
C
48 lines
1.0 KiB
C
#include "tommath_private.h"
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#ifdef BN_MP_PRIME_FERMAT_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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/* performs one Fermat test.
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*
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* If "a" were prime then b**a == b (mod a) since the order of
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* the multiplicative sub-group would be phi(a) = a-1. That means
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* it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
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*
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* Sets result to 1 if the congruence holds, or zero otherwise.
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*/
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mp_err mp_prime_fermat(const mp_int *a, const mp_int *b, mp_bool *result)
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{
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mp_int t;
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mp_err err;
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/* default to composite */
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*result = MP_NO;
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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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/* init t */
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if ((err = mp_init(&t)) != MP_OKAY) {
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return err;
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}
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/* compute t = b**a mod a */
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if ((err = mp_exptmod(b, a, a, &t)) != MP_OKAY) {
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goto LBL_T;
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}
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/* is it equal to b? */
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if (mp_cmp(&t, b) == MP_EQ) {
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*result = MP_YES;
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}
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err = MP_OKAY;
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LBL_T:
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mp_clear(&t);
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return err;
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}
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#endif
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