| #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@gmail.com, http://math.libtomcrypt.com |
| */ |
| |
| /* 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); |
| } |
| |
| /* 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 |
| |
| /* $Source: /cvs/libtom/libtommath/bn_fast_s_mp_mul_high_digs.c,v $ */ |
| /* $Revision: 1.5 $ */ |
| /* $Date: 2006/11/14 03:46:25 $ */ |