Mercurial > dropbear

comparison bn_mp_montgomery_calc_normalization.c @ 1:22d5cf7d4b1a libtommath

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
Renaming branch
author Matt Johnston <matt@ucc.asn.au>
date Mon, 31 May 2004 18:23:46 +0000
parents
children d29b64170cf0
comparison
equal deleted inserted replaced
-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 /* calculates a = B^n mod b for Montgomery reduction
18 * Where B is the base [e.g. 2^DIGIT_BIT].
19 * B^n mod b is computed by first computing
20 * A = B^(n-1) which doesn't require a reduction but a simple OR.
21 * then C = A * B = B^n is computed by performing upto DIGIT_BIT
22 * shifts with subtractions when the result is greater than b.
23 *
24 * The method is slightly modified to shift B unconditionally upto just under
25 * the leading bit of b. This saves alot of multiple precision shifting.
26 */
27 int
28 mp_montgomery_calc_normalization (mp_int * a, mp_int * b)
29 {
30 int x, bits, res;
31
32 /* how many bits of last digit does b use */
33 bits = mp_count_bits (b) % DIGIT_BIT;
34
35 /* compute A = B^(n-1) * 2^(bits-1) */
36 if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) {
37 return res;
38 }
39
40 /* now compute C = A * B mod b */
41 for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
42 if ((res = mp_mul_2 (a, a)) != MP_OKAY) {
43 return res;
44 }
45 if (mp_cmp_mag (a, b) != MP_LT) {
46 if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {
47 return res;
48 }
49 }
50 }
51
52 return MP_OKAY;
53 }