98 lines
2.5 KiB
C
98 lines
2.5 KiB
C
#include <tommath.h>
|
|
#ifdef BN_FAST_S_MP_MUL_HIGH_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
|
|
* additional optimizations in place.
|
|
*
|
|
* The library is free for all purposes without any express
|
|
* guarantee it works.
|
|
*
|
|
* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
|
|
*/
|
|
|
|
/* this is a modified version of fast_s_mul_digs that only produces
|
|
* output digits *above* digs. See the comments for fast_s_mul_digs
|
|
* to see how it works.
|
|
*
|
|
* This is used in the Barrett reduction since for one of the multiplications
|
|
* only the higher digits were needed. This essentially halves the work.
|
|
*
|
|
* Based on Algorithm 14.12 on pp.595 of HAC.
|
|
*/
|
|
int fast_s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
|
|
{
|
|
int olduse, res, pa, ix, iz;
|
|
mp_digit W[MP_WARRAY];
|
|
mp_word _W;
|
|
|
|
/* grow the destination as required */
|
|
pa = a->used + b->used;
|
|
if (c->alloc < pa) {
|
|
if ((res = mp_grow (c, pa)) != MP_OKAY) {
|
|
return res;
|
|
}
|
|
}
|
|
|
|
/* number of output digits to produce */
|
|
pa = a->used + b->used;
|
|
_W = 0;
|
|
for (ix = digs; ix < pa; ix++) {
|
|
int tx, ty, iy;
|
|
mp_digit *tmpx, *tmpy;
|
|
|
|
/* get offsets into the two bignums */
|
|
ty = MIN(b->used-1, ix);
|
|
tx = ix - ty;
|
|
|
|
/* setup temp aliases */
|
|
tmpx = a->dp + tx;
|
|
tmpy = b->dp + ty;
|
|
|
|
/* this is the number of times the loop will iterrate, essentially its
|
|
while (tx++ < a->used && ty-- >= 0) { ... }
|
|
*/
|
|
iy = MIN(a->used-tx, ty+1);
|
|
|
|
/* execute loop */
|
|
for (iz = 0; iz < iy; iz++) {
|
|
_W += ((mp_word)*tmpx++)*((mp_word)*tmpy--);
|
|
}
|
|
|
|
/* store term */
|
|
W[ix] = ((mp_digit)_W) & MP_MASK;
|
|
|
|
/* make next carry */
|
|
_W = _W >> ((mp_word)DIGIT_BIT);
|
|
}
|
|
|
|
/* store final carry */
|
|
W[ix] = _W & MP_MASK;
|
|
|
|
/* setup dest */
|
|
olduse = c->used;
|
|
c->used = pa;
|
|
|
|
{
|
|
register mp_digit *tmpc;
|
|
|
|
tmpc = c->dp + digs;
|
|
for (ix = digs; ix <= pa; ix++) {
|
|
/* now extract the previous digit [below the carry] */
|
|
*tmpc++ = W[ix];
|
|
}
|
|
|
|
/* clear unused digits [that existed in the old copy of c] */
|
|
for (; ix < olduse; ix++) {
|
|
*tmpc++ = 0;
|
|
}
|
|
}
|
|
mp_clamp (c);
|
|
return MP_OKAY;
|
|
}
|
|
#endif
|