## Mercurial > dropbear

### view libtommath/bn_mp_prime_fermat.c @ 1470:8bba51a55704

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Update to libtommath v1.0.1

author | Matt Johnston <matt@ucc.asn.au> |
---|---|

date | Thu, 08 Feb 2018 23:11:40 +0800 |

parents | 60fc6476e044 |

children | f52919ffd3b1 |

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#include <tommath_private.h> #ifdef BN_MP_PRIME_FERMAT_C /* LibTomMath, multiple-precision integer library -- Tom St Denis * * LibTomMath is a library that provides multiple-precision * integer arithmetic as well as number theoretic functionality. * * The library was designed directly after the MPI library by * Michael Fromberger but has been written from scratch with * additional optimizations in place. * * The library is free for all purposes without any express * guarantee it works. * * Tom St Denis, [email protected], http://libtom.org */ /* performs one Fermat test. * * If "a" were prime then b**a == b (mod a) since the order of * the multiplicative sub-group would be phi(a) = a-1. That means * it would be the same as b**(a mod (a-1)) == b**1 == b (mod a). * * Sets result to 1 if the congruence holds, or zero otherwise. */ int mp_prime_fermat (mp_int * a, mp_int * b, int *result) { mp_int t; int err; /* default to composite */ *result = MP_NO; /* ensure b > 1 */ if (mp_cmp_d(b, 1) != MP_GT) { return MP_VAL; } /* init t */ if ((err = mp_init (&t)) != MP_OKAY) { return err; } /* compute t = b**a mod a */ if ((err = mp_exptmod (b, a, a, &t)) != MP_OKAY) { goto LBL_T; } /* is it equal to b? */ if (mp_cmp (&t, b) == MP_EQ) { *result = MP_YES; } err = MP_OKAY; LBL_T:mp_clear (&t); return err; } #endif /* ref: $Format:%D$ */ /* git commit: $Format:%H$ */ /* commit time: $Format:%ai$ */