Mercurial > dropbear
diff libtommath/bn_s_mp_mul_digs_fast.c @ 1739:13d834efc376 fuzz
merge from main
author | Matt Johnston <matt@ucc.asn.au> |
---|---|
date | Thu, 15 Oct 2020 19:55:15 +0800 |
parents | 1051e4eea25a |
children |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/libtommath/bn_s_mp_mul_digs_fast.c Thu Oct 15 19:55:15 2020 +0800 @@ -0,0 +1,90 @@ +#include "tommath_private.h" +#ifdef BN_S_MP_MUL_DIGS_FAST_C +/* LibTomMath, multiple-precision integer library -- Tom St Denis */ +/* SPDX-License-Identifier: Unlicense */ + +/* Fast (comba) multiplier + * + * This is the fast column-array [comba] multiplier. It is + * designed to compute the columns of the product first + * then handle the carries afterwards. This has the effect + * of making the nested loops that compute the columns very + * simple and schedulable on super-scalar processors. + * + * This has been modified to produce a variable number of + * digits of output so if say only a half-product is required + * you don't have to compute the upper half (a feature + * required for fast Barrett reduction). + * + * Based on Algorithm 14.12 on pp.595 of HAC. + * + */ +mp_err s_mp_mul_digs_fast(const mp_int *a, const mp_int *b, mp_int *c, int digs) +{ + int olduse, pa, ix, iz; + mp_err err; + mp_digit W[MP_WARRAY]; + mp_word _W; + + /* grow the destination as required */ + if (c->alloc < digs) { + if ((err = mp_grow(c, digs)) != MP_OKAY) { + return err; + } + } + + /* number of output digits to produce */ + pa = MP_MIN(digs, a->used + b->used); + + /* clear the carry */ + _W = 0; + for (ix = 0; ix < pa; ix++) { + int tx, ty; + int iy; + mp_digit *tmpx, *tmpy; + + /* get offsets into the two bignums */ + ty = MP_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 + while (tx++ < a->used && ty-- >= 0) { ... } + */ + iy = MP_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)MP_DIGIT_BIT; + } + + /* setup dest */ + olduse = c->used; + c->used = pa; + + { + mp_digit *tmpc; + tmpc = c->dp; + for (ix = 0; 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] */ + MP_ZERO_DIGITS(tmpc, olduse - ix); + } + mp_clamp(c); + return MP_OKAY; +} +#endif