1#include <tommath.h>
2#ifdef BN_MP_REDUCE_2K_L_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, tomstdenis@gmail.com, http://math.libtomcrypt.com
16 */
17
18/* reduces a modulo n where n is of the form 2**p - d
19   This differs from reduce_2k since "d" can be larger
20   than a single digit.
21*/
22int mp_reduce_2k_l(mp_int *a, mp_int *n, mp_int *d)
23{
24   mp_int q;
25   int    p, res;
26
27   if ((res = mp_init(&q)) != MP_OKAY) {
28      return res;
29   }
30
31   p = mp_count_bits(n);
32top:
33   /* q = a/2**p, a = a mod 2**p */
34   if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
35      goto ERR;
36   }
37
38   /* q = q * d */
39   if ((res = mp_mul(&q, d, &q)) != MP_OKAY) {
40      goto ERR;
41   }
42
43   /* a = a + q */
44   if ((res = s_mp_add(a, &q, a)) != MP_OKAY) {
45      goto ERR;
46   }
47
48   if (mp_cmp_mag(a, n) != MP_LT) {
49      s_mp_sub(a, n, a);
50      goto top;
51   }
52
53ERR:
54   mp_clear(&q);
55   return res;
56}
57
58#endif
59
60/* $Source: /cvs/libtom/libtommath/bn_mp_reduce_2k_l.c,v $ */
61/* $Revision: 1.3 $ */
62/* $Date: 2006/03/31 14:18:44 $ */
63