### comparison libtommath/bn_mp_prime_fermat.c @ 389:5ff8218bcee9

author Matt Johnston Thu, 11 Jan 2007 03:14:55 +0000 eed26cff980b 60fc6476e044
comparison
equal inserted replaced
388:fb54020f78e1 389:5ff8218bcee9
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, [email protected], 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 \$ */