### comparison libtommath/bn_mp_reduce_2k.c @ 389:5ff8218bcee9

author Matt Johnston Thu, 11 Jan 2007 03:14:55 +0000 eed26cff980b 60fc6476e044
comparison
equal inserted replaced
388:fb54020f78e1 389:5ff8218bcee9
1 #include <tommath.h>
2 #ifdef BN_MP_REDUCE_2K_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 a modulo n where n is of the form 2**p - d */
19 int mp_reduce_2k(mp_int *a, mp_int *n, mp_digit d)
20 {
21 mp_int q;
22 int p, res;
23
24 if ((res = mp_init(&q)) != MP_OKAY) {
25 return res;
26 }
27
28 p = mp_count_bits(n);
29 top:
30 /* q = a/2**p, a = a mod 2**p */
31 if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
32 goto ERR;
33 }
34
35 if (d != 1) {
36 /* q = q * d */
37 if ((res = mp_mul_d(&q, d, &q)) != MP_OKAY) {
38 goto ERR;
39 }
40 }
41
42 /* a = a + q */
43 if ((res = s_mp_add(a, &q, a)) != MP_OKAY) {
44 goto ERR;
45 }
46
47 if (mp_cmp_mag(a, n) != MP_LT) {
48 s_mp_sub(a, n, a);
49 goto top;
50 }
51
52 ERR:
53 mp_clear(&q);
54 return res;
55 }
56
57 #endif
58
59 /* \$Source: /cvs/libtom/libtommath/bn_mp_reduce_2k.c,v \$ */
60 /* \$Revision: 1.3 \$ */
61 /* \$Date: 2006/03/31 14:18:44 \$ */