Mercurial > dropbear
view libtommath/bn_s_mp_mul_high_digs_fast.c @ 1910:54a85bbe5017
Fix dropbearconvert for dropbearmulti test
author | Matt Johnston <matt@ucc.asn.au> |
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date | Tue, 29 Mar 2022 22:36:30 +0800 |
parents | 1051e4eea25a |
children |
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#include "tommath_private.h" #ifdef BN_S_MP_MUL_HIGH_DIGS_FAST_C /* LibTomMath, multiple-precision integer library -- Tom St Denis */ /* SPDX-License-Identifier: Unlicense */ /* 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. */ mp_err s_mp_mul_high_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 */ pa = a->used + b->used; if (c->alloc < pa) { if ((err = mp_grow(c, pa)) != MP_OKAY) { return err; } } /* 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 = 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 its 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 + 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] */ MP_ZERO_DIGITS(tmpc, olduse - ix); } mp_clamp(c); return MP_OKAY; } #endif