1 #include <tommath.h>
2 #ifdef BN_MP_MOD_2D_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 /* calc a value mod 2**b */
19 int
mp_mod_2d(mp_int * a,int b,mp_int * c)20 mp_mod_2d (mp_int * a, int b, mp_int * c)
21 {
22 int x, res;
23
24 /* if b is <= 0 then zero the int */
25 if (b <= 0) {
26 mp_zero (c);
27 return MP_OKAY;
28 }
29
30 /* if the modulus is larger than the value than return */
31 if (b >= (int) (a->used * DIGIT_BIT)) {
32 res = mp_copy (a, c);
33 return res;
34 }
35
36 /* copy */
37 if ((res = mp_copy (a, c)) != MP_OKAY) {
38 return res;
39 }
40
41 /* zero digits above the last digit of the modulus */
42 for (x = (b / DIGIT_BIT) + ((b % DIGIT_BIT) == 0 ? 0 : 1); x < c->used; x++) {
43 c->dp[x] = 0;
44 }
45 /* clear the digit that is not completely outside/inside the modulus */
46 c->dp[b / DIGIT_BIT] &=
47 (mp_digit) ((((mp_digit) 1) << (((mp_digit) b) % DIGIT_BIT)) - ((mp_digit) 1));
48 mp_clamp (c);
49 return MP_OKAY;
50 }
51 #endif
52
53 /* $Source: /cvs/libtom/libtommath/bn_mp_mod_2d.c,v $ */
54 /* $Revision: 1.3 $ */
55 /* $Date: 2006/03/31 14:18:44 $ */
56