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      1 #include <tommath.h>
      2 #ifdef BN_FAST_S_MP_MUL_HIGH_DIGS_C
      3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
      4  *
      5  * LibTomMath is a library that provides multiple-precision
      6  * integer arithmetic as well as number theoretic functionality.
      7  *
      8  * The library was designed directly after the MPI library by
      9  * Michael Fromberger but has been written from scratch with
     10  * additional optimizations in place.
     11  *
     12  * The library is free for all purposes without any express
     13  * guarantee it works.
     14  *
     15  * Tom St Denis, tomstdenis (at) gmail.com, http://math.libtomcrypt.com
     16  */
     17 
     18 /* this is a modified version of fast_s_mul_digs that only produces
     19  * output digits *above* digs.  See the comments for fast_s_mul_digs
     20  * to see how it works.
     21  *
     22  * This is used in the Barrett reduction since for one of the multiplications
     23  * only the higher digits were needed.  This essentially halves the work.
     24  *
     25  * Based on Algorithm 14.12 on pp.595 of HAC.
     26  */
     27 int fast_s_mp_mul_high_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
     28 {
     29   int     olduse, res, pa, ix, iz;
     30   mp_digit W[MP_WARRAY];
     31   mp_word  _W;
     32 
     33   /* grow the destination as required */
     34   pa = a->used + b->used;
     35   if (c->alloc < pa) {
     36     if ((res = mp_grow (c, pa)) != MP_OKAY) {
     37       return res;
     38     }
     39   }
     40 
     41   /* number of output digits to produce */
     42   pa = a->used + b->used;
     43   _W = 0;
     44   for (ix = digs; ix < pa; ix++) {
     45       int      tx, ty, iy;
     46       mp_digit *tmpx, *tmpy;
     47 
     48       /* get offsets into the two bignums */
     49       ty = MIN(b->used-1, ix);
     50       tx = ix - ty;
     51 
     52       /* setup temp aliases */
     53       tmpx = a->dp + tx;
     54       tmpy = b->dp + ty;
     55 
     56       /* this is the number of times the loop will iterrate, essentially its
     57          while (tx++ < a->used && ty-- >= 0) { ... }
     58        */
     59       iy = MIN(a->used-tx, ty+1);
     60 
     61       /* execute loop */
     62       for (iz = 0; iz < iy; iz++) {
     63          _W += ((mp_word)*tmpx++)*((mp_word)*tmpy--);
     64       }
     65 
     66       /* store term */
     67       W[ix] = ((mp_digit)_W) & MP_MASK;
     68 
     69       /* make next carry */
     70       _W = _W >> ((mp_word)DIGIT_BIT);
     71   }
     72 
     73   /* setup dest */
     74   olduse  = c->used;
     75   c->used = pa;
     76 
     77   {
     78     register mp_digit *tmpc;
     79 
     80     tmpc = c->dp + digs;
     81     for (ix = digs; ix < pa; ix++) {
     82       /* now extract the previous digit [below the carry] */
     83       *tmpc++ = W[ix];
     84     }
     85 
     86     /* clear unused digits [that existed in the old copy of c] */
     87     for (; ix < olduse; ix++) {
     88       *tmpc++ = 0;
     89     }
     90   }
     91   mp_clamp (c);
     92   return MP_OKAY;
     93 }
     94 #endif
     95 
     96 /* $Source: /cvs/libtom/libtommath/bn_fast_s_mp_mul_high_digs.c,v $ */
     97 /* $Revision: 1.5 $ */
     98 /* $Date: 2006/11/14 03:46:25 $ */
     99