1 /* $NetBSD: bn_mp_prime_fermat.c,v 1.1.1.2 2014/04/24 12:45:31 pettai Exp $ */ 2 3 #include <tommath.h> 4 #ifdef BN_MP_PRIME_FERMAT_C 5 /* LibTomMath, multiple-precision integer library -- Tom St Denis 6 * 7 * LibTomMath is a library that provides multiple-precision 8 * integer arithmetic as well as number theoretic functionality. 9 * 10 * The library was designed directly after the MPI library by 11 * Michael Fromberger but has been written from scratch with 12 * additional optimizations in place. 13 * 14 * The library is free for all purposes without any express 15 * guarantee it works. 16 * 17 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org 18 */ 19 20 /* performs one Fermat test. 21 * 22 * If "a" were prime then b**a == b (mod a) since the order of 23 * the multiplicative sub-group would be phi(a) = a-1. That means 24 * it would be the same as b**(a mod (a-1)) == b**1 == b (mod a). 25 * 26 * Sets result to 1 if the congruence holds, or zero otherwise. 27 */ 28 int mp_prime_fermat (mp_int * a, mp_int * b, int *result) 29 { 30 mp_int t; 31 int err; 32 33 /* default to composite */ 34 *result = MP_NO; 35 36 /* ensure b > 1 */ 37 if (mp_cmp_d(b, 1) != MP_GT) { 38 return MP_VAL; 39 } 40 41 /* init t */ 42 if ((err = mp_init (&t)) != MP_OKAY) { 43 return err; 44 } 45 46 /* compute t = b**a mod a */ 47 if ((err = mp_exptmod (b, a, a, &t)) != MP_OKAY) { 48 goto LBL_T; 49 } 50 51 /* is it equal to b? */ 52 if (mp_cmp (&t, b) == MP_EQ) { 53 *result = MP_YES; 54 } 55 56 err = MP_OKAY; 57 LBL_T:mp_clear (&t); 58 return err; 59 } 60 #endif 61 62 /* Source: /cvs/libtom/libtommath/bn_mp_prime_fermat.c,v */ 63 /* Revision: 1.4 */ 64 /* Date: 2006/12/28 01:25:13 */ 65