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comparison libtommath/bn_mp_reduce.c @ 284:eed26cff980b

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propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 6c790cad5a7fa866ad062cb3a0c279f7ba788583) to branch 'au.asn.ucc.matt.dropbear' (head fff0894a0399405a9410ea1c6d118f342cf2aa64)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:23:49 +0000
parents
children 5ff8218bcee9
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283:bd240aa12ba7 284:eed26cff980b
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.org
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