tommath/bn_mp_prime_fermat.c

61 lines
1.4 KiB
C
Raw Normal View History

#include "tommath_private.h"
2004-10-29 18:07:18 -04:00
#ifdef BN_MP_PRIME_FERMAT_C
2003-03-22 10:10:20 -05:00
/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
2003-08-04 21:24:44 -04:00
* LibTomMath is a library that provides multiple-precision
2003-03-22 10:10:20 -05:00
* integer arithmetic as well as number theoretic functionality.
*
2003-08-04 21:24:44 -04:00
* The library was designed directly after the MPI library by
2003-03-22 10:10:20 -05:00
* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
2018-12-29 11:56:20 -05:00
* SPDX-License-Identifier: Unlicense
2003-03-22 10:10:20 -05:00
*/
/* performs one Fermat test.
2017-08-29 23:51:11 -04:00
*
2003-07-02 11:39:39 -04:00
* If "a" were prime then b**a == b (mod a) since the order of
2003-03-22 10:10:20 -05:00
* the multiplicative sub-group would be phi(a) = a-1. That means
2003-07-02 11:39:39 -04:00
* it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
2003-03-22 10:10:20 -05:00
*
* Sets result to 1 if the congruence holds, or zero otherwise.
*/
2017-09-20 10:59:43 -04:00
int mp_prime_fermat(const mp_int *a, const mp_int *b, int *result)
2003-03-22 10:10:20 -05:00
{
2017-08-30 13:13:53 -04:00
mp_int t;
int err;
2003-03-22 10:10:20 -05:00
2017-08-30 13:13:53 -04:00
/* default to composite */
*result = MP_NO;
2003-03-22 10:10:20 -05:00
2017-08-30 13:13:53 -04:00
/* ensure b > 1 */
2017-10-15 10:11:09 -04:00
if (mp_cmp_d(b, 1uL) != MP_GT) {
2017-08-30 13:13:53 -04:00
return MP_VAL;
}
2003-07-12 10:31:43 -04:00
2017-08-30 13:13:53 -04:00
/* init t */
if ((err = mp_init(&t)) != MP_OKAY) {
return err;
}
2003-03-22 10:10:20 -05:00
2017-08-30 13:13:53 -04:00
/* compute t = b**a mod a */
if ((err = mp_exptmod(b, a, a, &t)) != MP_OKAY) {
goto LBL_T;
}
2003-03-22 10:10:20 -05:00
2017-08-30 13:13:53 -04:00
/* is it equal to b? */
if (mp_cmp(&t, b) == MP_EQ) {
*result = MP_YES;
}
2003-03-22 10:10:20 -05:00
2017-08-30 13:13:53 -04:00
err = MP_OKAY;
2017-08-28 16:34:46 -04:00
LBL_T:
2017-08-30 13:13:53 -04:00
mp_clear(&t);
return err;
2003-03-22 10:10:20 -05:00
}
2004-10-29 18:07:18 -04:00
#endif
2005-08-01 12:37:28 -04:00
2017-08-28 10:27:26 -04:00
/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */