Mercurial > dropbear

comparison libtommath/bn_s_mp_mul_digs.c @ 389:5ff8218bcee9

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 2af95f00ebd5bb7a28b3817db1218442c935388e) to branch 'au.asn.ucc.matt.dropbear' (head ecd779509ef23a8cdf64888904fc9b31d78aa933)
author Matt Johnston <matt@ucc.asn.au>
date Thu, 11 Jan 2007 03:14:55 +0000
parents eed26cff980b
children 60fc6476e044
comparison
equal deleted inserted replaced
388:fb54020f78e1 389:5ff8218bcee9
1 #include <tommath.h>
2 #ifdef BN_S_MP_MUL_DIGS_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.com
16 */
17
18 /* multiplies |a| * |b| and only computes upto digs digits of result
19 * HAC pp. 595, Algorithm 14.12 Modified so you can control how
20 * many digits of output are created.
21 */
22 int s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
23 {
24 mp_int t;
25 int res, pa, pb, ix, iy;
26 mp_digit u;
27 mp_word r;
28 mp_digit tmpx, *tmpt, *tmpy;
29
30 /* can we use the fast multiplier? */
31 if (((digs) < MP_WARRAY) &&
32 MIN (a->used, b->used) <
33 (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
34 return fast_s_mp_mul_digs (a, b, c, digs);
35 }
36
37 if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
38 return res;
39 }
40 t.used = digs;
41
42 /* compute the digits of the product directly */
43 pa = a->used;
44 for (ix = 0; ix < pa; ix++) {
45 /* set the carry to zero */
46 u = 0;
47
48 /* limit ourselves to making digs digits of output */
49 pb = MIN (b->used, digs - ix);
50
51 /* setup some aliases */
52 /* copy of the digit from a used within the nested loop */
53 tmpx = a->dp[ix];
54
55 /* an alias for the destination shifted ix places */
56 tmpt = t.dp + ix;
57
58 /* an alias for the digits of b */
59 tmpy = b->dp;
60
61 /* compute the columns of the output and propagate the carry */
62 for (iy = 0; iy < pb; iy++) {
63 /* compute the column as a mp_word */
64 r = ((mp_word)*tmpt) +
65 ((mp_word)tmpx) * ((mp_word)*tmpy++) +
66 ((mp_word) u);
67
68 /* the new column is the lower part of the result */
69 *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
70
71 /* get the carry word from the result */
72 u = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
73 }
74 /* set carry if it is placed below digs */
75 if (ix + iy < digs) {
76 *tmpt = u;
77 }
78 }
79
80 mp_clamp (&t);
81 mp_exch (&t, c);
82
83 mp_clear (&t);
84 return MP_OKAY;
85 }
86 #endif
87
88 /* $Source: /cvs/libtom/libtommath/bn_s_mp_mul_digs.c,v $ */
89 /* $Revision: 1.3 $ */
90 /* $Date: 2006/03/31 14:18:44 $ */