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comparison tomsfastmath/src/numtheory/fp_prime_miller_rabin.c @ 643:a362b62d38b2 dropbear-tfm

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Add tomsfastmath from git rev bfa4582842bc3bab42e4be4aed5703437049502a with Makefile.in renamed
author Matt Johnston <matt@ucc.asn.au>
date Wed, 23 Nov 2011 18:10:20 +0700
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642:33fd2f3499d2 643:a362b62d38b2
1 /* TomsFastMath, a fast ISO C bignum library.
2 *
3 * This project is meant to fill in where LibTomMath
4 * falls short. That is speed ;-)
5 *
6 * This project is public domain and free for all purposes.
7 *
8 * Tom St Denis, [email protected]
9 */
10 #include <tfm.h>
11
12 /* Miller-Rabin test of "a" to the base of "b" as described in
13 * HAC pp. 139 Algorithm 4.24
14 *
15 * Sets result to 0 if definitely composite or 1 if probably prime.
16 * Randomly the chance of error is no more than 1/4 and often
17 * very much lower.
18 */
19 void fp_prime_miller_rabin (fp_int * a, fp_int * b, int *result)
20 {
21 fp_int n1, y, r;
22 int s, j;
23
24 /* default */
25 *result = FP_NO;
26
27 /* ensure b > 1 */
28 if (fp_cmp_d(b, 1) != FP_GT) {
29 return;
30 }
31
32 /* get n1 = a - 1 */
33 fp_init_copy(&n1, a);
34 fp_sub_d(&n1, 1, &n1);
35
36 /* set 2**s * r = n1 */
37 fp_init_copy(&r, &n1);
38
39 /* count the number of least significant bits
40 * which are zero
41 */
42 s = fp_cnt_lsb(&r);
43
44 /* now divide n - 1 by 2**s */
45 fp_div_2d (&r, s, &r, NULL);
46
47 /* compute y = b**r mod a */
48 fp_init(&y);
49 fp_exptmod(b, &r, a, &y);
50
51 /* if y != 1 and y != n1 do */
52 if (fp_cmp_d (&y, 1) != FP_EQ && fp_cmp (&y, &n1) != FP_EQ) {
53 j = 1;
54 /* while j <= s-1 and y != n1 */
55 while ((j <= (s - 1)) && fp_cmp (&y, &n1) != FP_EQ) {
56 fp_sqrmod (&y, a, &y);
57
58 /* if y == 1 then composite */
59 if (fp_cmp_d (&y, 1) == FP_EQ) {
60 return;
61 }
62 ++j;
63 }
64
65 /* if y != n1 then composite */
66 if (fp_cmp (&y, &n1) != FP_EQ) {
67 return;
68 }
69 }
70
71 /* probably prime now */
72 *result = FP_YES;
73 }
74
75 /* $Source$ */
76 /* $Revision$ */
77 /* $Date$ */