96 lines
2.7 KiB
C
96 lines
2.7 KiB
C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is library that provides for multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library is designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://math.libtomcrypt.org
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*/
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#include <tommath.h>
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/* this is a modified version of fast_s_mp_mul_digs that only produces
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* output digits *above* digs. See the comments for fast_s_mp_mul_digs
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* to see how it works.
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*
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* This is used in the Barrett reduction since for one of the multiplications
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* only the higher digits were needed. This essentially halves the work.
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*
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* Based on Algorithm 14.12 on pp.595 of HAC.
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*/
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int
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fast_s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
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{
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int oldused, newused, res, pa, pb, ix;
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mp_word W[MP_WARRAY];
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/* calculate size of product and allocate more space if required */
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newused = a->used + b->used + 1;
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if (c->alloc < newused) {
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if ((res = mp_grow (c, newused)) != MP_OKAY) {
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return res;
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}
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}
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/* like the other comba method we compute the columns first */
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pa = a->used;
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pb = b->used;
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memset (W + digs, 0, (pa + pb + 1 - digs) * sizeof (mp_word));
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for (ix = 0; ix < pa; ix++) {
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{
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register mp_digit tmpx, *tmpy;
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register int iy;
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register mp_word *_W;
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/* work todo, that is we only calculate digits that are at "digs" or above */
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iy = digs - ix;
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/* copy of word on the left of A[ix] * B[iy] */
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tmpx = a->dp[ix];
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/* alias for right side */
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tmpy = b->dp + iy;
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/* alias for the columns of output. Offset to be equal to or above the
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* smallest digit place requested
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*/
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_W = W + digs;
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/* skip cases below zero where ix > digs */
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if (iy < 0) {
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iy = abs(iy);
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tmpy += iy;
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_W += iy;
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iy = 0;
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}
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/* compute column products for digits above the minimum */
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for (; iy < pb; iy++) {
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*_W++ += ((mp_word) tmpx) * ((mp_word) * tmpy++);
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}
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}
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}
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/* setup dest */
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oldused = c->used;
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c->used = newused;
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/* now convert the array W downto what we need */
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for (ix = digs + 1; ix < newused; ix++) {
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W[ix] += (W[ix - 1] >> ((mp_word) DIGIT_BIT));
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c->dp[ix - 1] = (mp_digit) (W[ix - 1] & ((mp_word) MP_MASK));
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}
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c->dp[(pa + pb + 1) - 1] = (mp_digit) (W[(pa + pb + 1) - 1] & ((mp_word) MP_MASK));
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for (; ix < oldused; ix++) {
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c->dp[ix] = 0;
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}
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mp_clamp (c);
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return MP_OKAY;
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}
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