1 #include <tommath.h> 2 #ifdef BN_MP_PRIME_FERMAT_C 3 /* LibTomMath, multiple-precision integer library -- Tom St Denis 4 * 5 * LibTomMath is a library that provides multiple-precision 6 * integer arithmetic as well as number theoretic functionality. 7 * 8 * The library was designed directly after the MPI library by 9 * Michael Fromberger but has been written from scratch with 10 * additional optimizations in place. 11 * 12 * The library is free for all purposes without any express 13 * guarantee it works. 14 * 15 * Tom St Denis, tomstdenis (at) gmail.com, http://math.libtomcrypt.com 16 */ 17 18 /* performs one Fermat test. 19 * 20 * If "a" were prime then b**a == b (mod a) since the order of 21 * the multiplicative sub-group would be phi(a) = a-1. That means 22 * it would be the same as b**(a mod (a-1)) == b**1 == b (mod a). 23 * 24 * Sets result to 1 if the congruence holds, or zero otherwise. 25 */ 26 int mp_prime_fermat (mp_int * a, mp_int * b, int *result) 27 { 28 mp_int t; 29 int err; 30 31 /* default to composite */ 32 *result = MP_NO; 33 34 /* ensure b > 1 */ 35 if (mp_cmp_d(b, 1) != MP_GT) { 36 return MP_VAL; 37 } 38 39 /* init t */ 40 if ((err = mp_init (&t)) != MP_OKAY) { 41 return err; 42 } 43 44 /* compute t = b**a mod a */ 45 if ((err = mp_exptmod (b, a, a, &t)) != MP_OKAY) { 46 goto LBL_T; 47 } 48 49 /* is it equal to b? */ 50 if (mp_cmp (&t, b) == MP_EQ) { 51 *result = MP_YES; 52 } 53 54 err = MP_OKAY; 55 LBL_T:mp_clear (&t); 56 return err; 57 } 58 #endif 59 60 /* $Source: /cvs/libtom/libtommath/bn_mp_prime_fermat.c,v $ */ 61 /* $Revision: 1.3 $ */ 62 /* $Date: 2006/03/31 14:18:44 $ */ 63
