### view libtommath/bn_s_mp_mul_digs.c @ 487:2e17f82a7330

Add multihop 'onion-forwarding' mode. Needs refinement, works.
author Matt Johnston Thu, 18 Sep 2008 14:47:02 +0000 5ff8218bcee9 60fc6476e044
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```#include <tommath.h>
#ifdef BN_S_MP_MUL_DIGS_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is a library that provides multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library was designed directly after the MPI library by
* Michael Fromberger but has been written from scratch with
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, [email protected], http://math.libtomcrypt.com
*/

/* multiplies |a| * |b| and only computes upto digs digits of result
* HAC pp. 595, Algorithm 14.12  Modified so you can control how
* many digits of output are created.
*/
int s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
{
mp_int  t;
int     res, pa, pb, ix, iy;
mp_digit u;
mp_word r;
mp_digit tmpx, *tmpt, *tmpy;

/* can we use the fast multiplier? */
if (((digs) < MP_WARRAY) &&
MIN (a->used, b->used) <
(1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
return fast_s_mp_mul_digs (a, b, c, digs);
}

if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
return res;
}
t.used = digs;

/* compute the digits of the product directly */
pa = a->used;
for (ix = 0; ix < pa; ix++) {
/* set the carry to zero */
u = 0;

/* limit ourselves to making digs digits of output */
pb = MIN (b->used, digs - ix);

/* setup some aliases */
/* copy of the digit from a used within the nested loop */
tmpx = a->dp[ix];

/* an alias for the destination shifted ix places */
tmpt = t.dp + ix;

/* an alias for the digits of b */
tmpy = b->dp;

/* compute the columns of the output and propagate the carry */
for (iy = 0; iy < pb; iy++) {
/* compute the column as a mp_word */
r       = ((mp_word)*tmpt) +
((mp_word)tmpx) * ((mp_word)*tmpy++) +
((mp_word) u);

/* the new column is the lower part of the result */
*tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));

/* get the carry word from the result */
u       = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
}
/* set carry if it is placed below digs */
if (ix + iy < digs) {
*tmpt = u;
}
}

mp_clamp (&t);
mp_exch (&t, c);

mp_clear (&t);
return MP_OKAY;
}
#endif

/* \$Source: /cvs/libtom/libtommath/bn_s_mp_mul_digs.c,v \$ */
/* \$Revision: 1.3 \$ */
/* \$Date: 2006/03/31 14:18:44 \$ */
```