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comparison libtommath/bn_mp_reduce_2k_l.c @ 284:eed26cff980b

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propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 6c790cad5a7fa866ad062cb3a0c279f7ba788583) to branch 'au.asn.ucc.matt.dropbear' (head fff0894a0399405a9410ea1c6d118f342cf2aa64)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:23:49 +0000
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children 5ff8218bcee9
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283:bd240aa12ba7 284:eed26cff980b
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, [email protected], http://math.libtomcrypt.org
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 */
22 int 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);
32 top:
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
53 ERR:
54 mp_clear(&q);
55 return res;
56 }
57
58 #endif