1 /* $NetBSD: bn_mp_reduce_2k.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_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 */
mp_reduce_2k(mp_int * a,mp_int * n,mp_digit d)21 int mp_reduce_2k(mp_int *a, mp_int *n, mp_digit d)
22 {
23 mp_int q;
24 int p, res;
25
26 if ((res = mp_init(&q)) != MP_OKAY) {
27 return res;
28 }
29
30 p = mp_count_bits(n);
31 top:
32 /* q = a/2**p, a = a mod 2**p */
33 if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
34 goto ERR;
35 }
36
37 if (d != 1) {
38 /* q = q * d */
39 if ((res = mp_mul_d(&q, d, &q)) != MP_OKAY) {
40 goto ERR;
41 }
42 }
43
44 /* a = a + q */
45 if ((res = s_mp_add(a, &q, a)) != MP_OKAY) {
46 goto ERR;
47 }
48
49 if (mp_cmp_mag(a, n) != MP_LT) {
50 s_mp_sub(a, n, a);
51 goto top;
52 }
53
54 ERR:
55 mp_clear(&q);
56 return res;
57 }
58
59 #endif
60
61 /* Source: /cvs/libtom/libtommath/bn_mp_reduce_2k.c,v */
62 /* Revision: 1.4 */
63 /* Date: 2006/12/28 01:25:13 */
64