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, tomstdenis@gmail.com, http://math.libtomcrypt.com
16 */
17
18/* reduces a modulo n where n is of the form 2**p - d */
19int 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);
29top:
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
52ERR:
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 $ */
62