1 /* $NetBSD: bn_mp_reduce_2k_l.c,v 1.1.1.2 2014/04/24 12:45:31 pettai Exp $ */
2
3 #include <tommath.h>
4 #ifdef BN_MP_REDUCE_2K_L_C
5 /* LibTomMath, multiple-precision integer library -- Tom St Denis
6 *
7 * LibTomMath is a library that provides multiple-precision
8 * integer arithmetic as well as number theoretic functionality.
9 *
10 * The library was designed directly after the MPI library by
11 * Michael Fromberger but has been written from scratch with
12 * additional optimizations in place.
13 *
14 * The library is free for all purposes without any express
15 * guarantee it works.
16 *
17 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
18 */
19
20 /* reduces a modulo n where n is of the form 2**p - d
21 This differs from reduce_2k since "d" can be larger
22 than a single digit.
23 */
mp_reduce_2k_l(mp_int * a,mp_int * n,mp_int * d)24 int mp_reduce_2k_l(mp_int *a, mp_int *n, mp_int *d)
25 {
26 mp_int q;
27 int p, res;
28
29 if ((res = mp_init(&q)) != MP_OKAY) {
30 return res;
31 }
32
33 p = mp_count_bits(n);
34 top:
35 /* q = a/2**p, a = a mod 2**p */
36 if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
37 goto ERR;
38 }
39
40 /* q = q * d */
41 if ((res = mp_mul(&q, d, &q)) != MP_OKAY) {
42 goto ERR;
43 }
44
45 /* a = a + q */
46 if ((res = s_mp_add(a, &q, a)) != MP_OKAY) {
47 goto ERR;
48 }
49
50 if (mp_cmp_mag(a, n) != MP_LT) {
51 s_mp_sub(a, n, a);
52 goto top;
53 }
54
55 ERR:
56 mp_clear(&q);
57 return res;
58 }
59
60 #endif
61
62 /* Source: /cvs/libtom/libtommath/bn_mp_reduce_2k_l.c,v */
63 /* Revision: 1.4 */
64 /* Date: 2006/12/28 01:25:13 */
65