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comparison bn_mp_prime_fermat.c @ 1:22d5cf7d4b1a libtommath

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Renaming branch
author Matt Johnston <matt@ucc.asn.au>
date Mon, 31 May 2004 18:23:46 +0000
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-1:000000000000 1:22d5cf7d4b1a
1 /* LibTomMath, multiple-precision integer library -- Tom St Denis
2 *
3 * LibTomMath is a library that provides multiple-precision
4 * integer arithmetic as well as number theoretic functionality.
5 *
6 * The library was designed directly after the MPI library by
7 * Michael Fromberger but has been written from scratch with
8 * additional optimizations in place.
9 *
10 * The library is free for all purposes without any express
11 * guarantee it works.
12 *
13 * Tom St Denis, [email protected], http://math.libtomcrypt.org
14 */
15 #include <tommath.h>
16
17 /* performs one Fermat test.
18 *
19 * If "a" were prime then b**a == b (mod a) since the order of
20 * the multiplicative sub-group would be phi(a) = a-1. That means
21 * it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
22 *
23 * Sets result to 1 if the congruence holds, or zero otherwise.
24 */
25 int mp_prime_fermat (mp_int * a, mp_int * b, int *result)
26 {
27 mp_int t;
28 int err;
29
30 /* default to composite */
31 *result = MP_NO;
32
33 /* ensure b > 1 */
34 if (mp_cmp_d(b, 1) != MP_GT) {
35 return MP_VAL;
36 }
37
38 /* init t */
39 if ((err = mp_init (&t)) != MP_OKAY) {
40 return err;
41 }
42
43 /* compute t = b**a mod a */
44 if ((err = mp_exptmod (b, a, a, &t)) != MP_OKAY) {
45 goto __T;
46 }
47
48 /* is it equal to b? */
49 if (mp_cmp (&t, b) == MP_EQ) {
50 *result = MP_YES;
51 }
52
53 err = MP_OKAY;
54 __T:mp_clear (&t);
55 return err;
56 }