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comparison bn_mp_mod_2d.c @ 1:22d5cf7d4b1a libtommath

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Renaming branch
author Matt Johnston <matt@ucc.asn.au>
date Mon, 31 May 2004 18:23:46 +0000
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-1:000000000000 1:22d5cf7d4b1a
1 /* LibTomMath, multiple-precision integer library -- Tom St Denis
2 *
3 * LibTomMath is a library that provides multiple-precision
4 * integer arithmetic as well as number theoretic functionality.
5 *
6 * The library was designed directly after the MPI library by
7 * Michael Fromberger but has been written from scratch with
8 * additional optimizations in place.
9 *
10 * The library is free for all purposes without any express
11 * guarantee it works.
12 *
13 * Tom St Denis, [email protected], http://math.libtomcrypt.org
14 */
15 #include <tommath.h>
16
17 /* calc a value mod 2**b */
18 int
19 mp_mod_2d (mp_int * a, int b, mp_int * c)
20 {
21 int x, res;
22
23 /* if b is <= 0 then zero the int */
24 if (b <= 0) {
25 mp_zero (c);
26 return MP_OKAY;
27 }
28
29 /* if the modulus is larger than the value than return */
30 if (b > (int) (a->used * DIGIT_BIT)) {
31 res = mp_copy (a, c);
32 return res;
33 }
34
35 /* copy */
36 if ((res = mp_copy (a, c)) != MP_OKAY) {
37 return res;
38 }
39
40 /* zero digits above the last digit of the modulus */
41 for (x = (b / DIGIT_BIT) + ((b % DIGIT_BIT) == 0 ? 0 : 1); x < c->used; x++) {
42 c->dp[x] = 0;
43 }
44 /* clear the digit that is not completely outside/inside the modulus */
45 c->dp[b / DIGIT_BIT] &=
46 (mp_digit) ((((mp_digit) 1) << (((mp_digit) b) % DIGIT_BIT)) - ((mp_digit) 1));
47 mp_clamp (c);
48 return MP_OKAY;
49 }