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comparison libtommath/bn_mp_montgomery_calc_normalization.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_MONTGOMERY_CALC_NORMALIZATION_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 /*
19 * shifts with subtractions when the result is greater than b.
20 *
21 * The method is slightly modified to shift B unconditionally upto just under
22 * the leading bit of b. This saves alot of multiple precision shifting.
23 */
24 int mp_montgomery_calc_normalization (mp_int * a, mp_int * b)
25 {
26 int x, bits, res;
27
28 /* how many bits of last digit does b use */
29 bits = mp_count_bits (b) % DIGIT_BIT;
30
31 if (b->used > 1) {
32 if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) {
33 return res;
34 }
35 } else {
36 mp_set(a, 1);
37 bits = 1;
38 }
39
40
41 /* now compute C = A * B mod b */
42 for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
43 if ((res = mp_mul_2 (a, a)) != MP_OKAY) {
44 return res;
45 }
46 if (mp_cmp_mag (a, b) != MP_LT) {
47 if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {
48 return res;
49 }
50 }
51 }
52
53 return MP_OKAY;
54 }
55 #endif