Mercurial > dropbear
comparison bn_mp_prime_miller_rabin.c @ 1:22d5cf7d4b1a libtommath
Renaming branch
author | Matt Johnston <matt@ucc.asn.au> |
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date | Mon, 31 May 2004 18:23:46 +0000 |
parents | |
children | d29b64170cf0 |
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-1:000000000000 | 1:22d5cf7d4b1a |
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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 /* Miller-Rabin test of "a" to the base of "b" as described in | |
18 * HAC pp. 139 Algorithm 4.24 | |
19 * | |
20 * Sets result to 0 if definitely composite or 1 if probably prime. | |
21 * Randomly the chance of error is no more than 1/4 and often | |
22 * very much lower. | |
23 */ | |
24 int mp_prime_miller_rabin (mp_int * a, mp_int * b, int *result) | |
25 { | |
26 mp_int n1, y, r; | |
27 int s, j, err; | |
28 | |
29 /* default */ | |
30 *result = MP_NO; | |
31 | |
32 /* ensure b > 1 */ | |
33 if (mp_cmp_d(b, 1) != MP_GT) { | |
34 return MP_VAL; | |
35 } | |
36 | |
37 /* get n1 = a - 1 */ | |
38 if ((err = mp_init_copy (&n1, a)) != MP_OKAY) { | |
39 return err; | |
40 } | |
41 if ((err = mp_sub_d (&n1, 1, &n1)) != MP_OKAY) { | |
42 goto __N1; | |
43 } | |
44 | |
45 /* set 2**s * r = n1 */ | |
46 if ((err = mp_init_copy (&r, &n1)) != MP_OKAY) { | |
47 goto __N1; | |
48 } | |
49 | |
50 /* count the number of least significant bits | |
51 * which are zero | |
52 */ | |
53 s = mp_cnt_lsb(&r); | |
54 | |
55 /* now divide n - 1 by 2**s */ | |
56 if ((err = mp_div_2d (&r, s, &r, NULL)) != MP_OKAY) { | |
57 goto __R; | |
58 } | |
59 | |
60 /* compute y = b**r mod a */ | |
61 if ((err = mp_init (&y)) != MP_OKAY) { | |
62 goto __R; | |
63 } | |
64 if ((err = mp_exptmod (b, &r, a, &y)) != MP_OKAY) { | |
65 goto __Y; | |
66 } | |
67 | |
68 /* if y != 1 and y != n1 do */ | |
69 if (mp_cmp_d (&y, 1) != MP_EQ && mp_cmp (&y, &n1) != MP_EQ) { | |
70 j = 1; | |
71 /* while j <= s-1 and y != n1 */ | |
72 while ((j <= (s - 1)) && mp_cmp (&y, &n1) != MP_EQ) { | |
73 if ((err = mp_sqrmod (&y, a, &y)) != MP_OKAY) { | |
74 goto __Y; | |
75 } | |
76 | |
77 /* if y == 1 then composite */ | |
78 if (mp_cmp_d (&y, 1) == MP_EQ) { | |
79 goto __Y; | |
80 } | |
81 | |
82 ++j; | |
83 } | |
84 | |
85 /* if y != n1 then composite */ | |
86 if (mp_cmp (&y, &n1) != MP_EQ) { | |
87 goto __Y; | |
88 } | |
89 } | |
90 | |
91 /* probably prime now */ | |
92 *result = MP_YES; | |
93 __Y:mp_clear (&y); | |
94 __R:mp_clear (&r); | |
95 __N1:mp_clear (&n1); | |
96 return err; | |
97 } |