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comparison bn_mp_dr_reduce.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 /* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
18 *
19 * Based on algorithm from the paper
20 *
21 * "Generating Efficient Primes for Discrete Log Cryptosystems"
22 * Chae Hoon Lim, Pil Loong Lee,
23 * POSTECH Information Research Laboratories
24 *
25 * The modulus must be of a special format [see manual]
26 *
27 * Has been modified to use algorithm 7.10 from the LTM book instead
28 *
29 * Input x must be in the range 0 <= x <= (n-1)**2
30 */
31 int
32 mp_dr_reduce (mp_int * x, mp_int * n, mp_digit k)
33 {
34 int err, i, m;
35 mp_word r;
36 mp_digit mu, *tmpx1, *tmpx2;
37
38 /* m = digits in modulus */
39 m = n->used;
40
41 /* ensure that "x" has at least 2m digits */
42 if (x->alloc < m + m) {
43 if ((err = mp_grow (x, m + m)) != MP_OKAY) {
44 return err;
45 }
46 }
47
48 /* top of loop, this is where the code resumes if
49 * another reduction pass is required.
50 */
51 top:
52 /* aliases for digits */
53 /* alias for lower half of x */
54 tmpx1 = x->dp;
55
56 /* alias for upper half of x, or x/B**m */
57 tmpx2 = x->dp + m;
58
59 /* set carry to zero */
60 mu = 0;
61
62 /* compute (x mod B**m) + k * [x/B**m] inline and inplace */
63 for (i = 0; i < m; i++) {
64 r = ((mp_word)*tmpx2++) * ((mp_word)k) + *tmpx1 + mu;
65 *tmpx1++ = (mp_digit)(r & MP_MASK);
66 mu = (mp_digit)(r >> ((mp_word)DIGIT_BIT));
67 }
68
69 /* set final carry */
70 *tmpx1++ = mu;
71
72 /* zero words above m */
73 for (i = m + 1; i < x->used; i++) {
74 *tmpx1++ = 0;
75 }
76
77 /* clamp, sub and return */
78 mp_clamp (x);
79
80 /* if x >= n then subtract and reduce again
81 * Each successive "recursion" makes the input smaller and smaller.
82 */
83 if (mp_cmp_mag (x, n) != MP_LT) {
84 s_mp_sub(x, n, x);
85 goto top;
86 }
87 return MP_OKAY;
88 }