Mercurial > dropbear

comparison libtommath/bn_mp_reduce.c @ 389:5ff8218bcee9

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 2af95f00ebd5bb7a28b3817db1218442c935388e) to branch 'au.asn.ucc.matt.dropbear' (head ecd779509ef23a8cdf64888904fc9b31d78aa933)
author Matt Johnston <matt@ucc.asn.au>
date Thu, 11 Jan 2007 03:14:55 +0000
parents eed26cff980b
children 60fc6476e044
comparison
equal deleted inserted replaced
388:fb54020f78e1 389:5ff8218bcee9
1 #include <tommath.h>
2 #ifdef BN_MP_REDUCE_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 /* reduces x mod m, assumes 0 < x < m**2, mu is
19 * precomputed via mp_reduce_setup.
20 * From HAC pp.604 Algorithm 14.42
21 */
22 int mp_reduce (mp_int * x, mp_int * m, mp_int * mu)
23 {
24 mp_int q;
25 int res, um = m->used;
26
27 /* q = x */
28 if ((res = mp_init_copy (&q, x)) != MP_OKAY) {
29 return res;
30 }
31
32 /* q1 = x / b**(k-1) */
33 mp_rshd (&q, um - 1);
34
35 /* according to HAC this optimization is ok */
36 if (((unsigned long) um) > (((mp_digit)1) << (DIGIT_BIT - 1))) {
37 if ((res = mp_mul (&q, mu, &q)) != MP_OKAY) {
38 goto CLEANUP;
39 }
40 } else {
41 #ifdef BN_S_MP_MUL_HIGH_DIGS_C
42 if ((res = s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) {
43 goto CLEANUP;
44 }
45 #elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C)
46 if ((res = fast_s_mp_mul_high_digs (&q, mu, &q, um)) != MP_OKAY) {
47 goto CLEANUP;
48 }
49 #else
50 {
51 res = MP_VAL;
52 goto CLEANUP;
53 }
54 #endif
55 }
56
57 /* q3 = q2 / b**(k+1) */
58 mp_rshd (&q, um + 1);
59
60 /* x = x mod b**(k+1), quick (no division) */
61 if ((res = mp_mod_2d (x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
62 goto CLEANUP;
63 }
64
65 /* q = q * m mod b**(k+1), quick (no division) */
66 if ((res = s_mp_mul_digs (&q, m, &q, um + 1)) != MP_OKAY) {
67 goto CLEANUP;
68 }
69
70 /* x = x - q */
71 if ((res = mp_sub (x, &q, x)) != MP_OKAY) {
72 goto CLEANUP;
73 }
74
75 /* If x < 0, add b**(k+1) to it */
76 if (mp_cmp_d (x, 0) == MP_LT) {
77 mp_set (&q, 1);
78 if ((res = mp_lshd (&q, um + 1)) != MP_OKAY)
79 goto CLEANUP;
80 if ((res = mp_add (x, &q, x)) != MP_OKAY)
81 goto CLEANUP;
82 }
83
84 /* Back off if it's too big */
85 while (mp_cmp (x, m) != MP_LT) {
86 if ((res = s_mp_sub (x, m, x)) != MP_OKAY) {
87 goto CLEANUP;
88 }
89 }
90
91 CLEANUP:
92 mp_clear (&q);
93
94 return res;
95 }
96 #endif
97
98 /* $Source: /cvs/libtom/libtommath/bn_mp_reduce.c,v $ */
99 /* $Revision: 1.3 $ */
100 /* $Date: 2006/03/31 14:18:44 $ */