Line data Source code
1 : #include <tommath.h>
2 : #ifdef BN_MP_PRIME_MILLER_RABIN_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@gmail.com, http://libtom.org
16 : */
17 :
18 : /* Miller-Rabin test of "a" to the base of "b" as described in
19 : * HAC pp. 139 Algorithm 4.24
20 : *
21 : * Sets result to 0 if definitely composite or 1 if probably prime.
22 : * Randomly the chance of error is no more than 1/4 and often
23 : * very much lower.
24 : */
25 0 : int mp_prime_miller_rabin (mp_int * a, mp_int * b, int *result)
26 : {
27 : mp_int n1, y, r;
28 : int s, j, err;
29 :
30 : /* default */
31 0 : *result = MP_NO;
32 :
33 : /* ensure b > 1 */
34 0 : if (mp_cmp_d(b, 1) != MP_GT) {
35 0 : return MP_VAL;
36 : }
37 :
38 : /* get n1 = a - 1 */
39 0 : if ((err = mp_init_copy (&n1, a)) != MP_OKAY) {
40 0 : return err;
41 : }
42 0 : if ((err = mp_sub_d (&n1, 1, &n1)) != MP_OKAY) {
43 0 : goto LBL_N1;
44 : }
45 :
46 : /* set 2**s * r = n1 */
47 0 : if ((err = mp_init_copy (&r, &n1)) != MP_OKAY) {
48 0 : goto LBL_N1;
49 : }
50 :
51 : /* count the number of least significant bits
52 : * which are zero
53 : */
54 0 : s = mp_cnt_lsb(&r);
55 :
56 : /* now divide n - 1 by 2**s */
57 0 : if ((err = mp_div_2d (&r, s, &r, NULL)) != MP_OKAY) {
58 0 : goto LBL_R;
59 : }
60 :
61 : /* compute y = b**r mod a */
62 0 : if ((err = mp_init (&y)) != MP_OKAY) {
63 0 : goto LBL_R;
64 : }
65 0 : if ((err = mp_exptmod (b, &r, a, &y)) != MP_OKAY) {
66 0 : goto LBL_Y;
67 : }
68 :
69 : /* if y != 1 and y != n1 do */
70 0 : if (mp_cmp_d (&y, 1) != MP_EQ && mp_cmp (&y, &n1) != MP_EQ) {
71 0 : j = 1;
72 : /* while j <= s-1 and y != n1 */
73 0 : while ((j <= (s - 1)) && mp_cmp (&y, &n1) != MP_EQ) {
74 0 : if ((err = mp_sqrmod (&y, a, &y)) != MP_OKAY) {
75 0 : goto LBL_Y;
76 : }
77 :
78 : /* if y == 1 then composite */
79 0 : if (mp_cmp_d (&y, 1) == MP_EQ) {
80 0 : goto LBL_Y;
81 : }
82 :
83 0 : ++j;
84 : }
85 :
86 : /* if y != n1 then composite */
87 0 : if (mp_cmp (&y, &n1) != MP_EQ) {
88 0 : goto LBL_Y;
89 : }
90 : }
91 :
92 : /* probably prime now */
93 0 : *result = MP_YES;
94 0 : LBL_Y:mp_clear (&y);
95 0 : LBL_R:mp_clear (&r);
96 0 : LBL_N1:mp_clear (&n1);
97 0 : return err;
98 : }
99 : #endif
100 :
101 : /* $Source: /cvs/libtom/libtommath/bn_mp_prime_miller_rabin.c,v $ */
102 : /* $Revision: 1.4 $ */
103 : /* $Date: 2006/12/28 01:25:13 $ */
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