release/src/router/dropbear/libtommath/bn_s_mp_mul_digs.c

   1 #include <tommath.h>
   2 #ifdef BN_S_MP_MUL_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@gmail.com, http://math.libtomcrypt.com
  16  */
  17
  18 /* multiplies |a| * |b| and only computes upto digs digits of result
  19  * HAC pp. 595, Algorithm 14.12  Modified so you can control how
  20  * many digits of output are created.
  21  */
  22 int s_mp_mul_digs (mp_int * a, mp_int * b, mp_int * c, int digs)
  23 {
  24   mp_int  t;
  25   int     res, pa, pb, ix, iy;
  26   mp_digit u;
  27   mp_word r;
  28   mp_digit tmpx, *tmpt, *tmpy;
  29
  30   /* can we use the fast multiplier? */
  31   if (((digs) < MP_WARRAY) &&
  32       MIN (a->used, b->used) <
  33           (1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT)))) {
  34     return fast_s_mp_mul_digs (a, b, c, digs);
  35   }
  36
  37   if ((res = mp_init_size (&t, digs)) != MP_OKAY) {
  38     return res;
  39   }
  40   t.used = digs;
  41
  42   /* compute the digits of the product directly */
  43   pa = a->used;
  44   for (ix = 0; ix < pa; ix++) {
  45     /* set the carry to zero */
  46     u = 0;
  47
  48     /* limit ourselves to making digs digits of output */
  49     pb = MIN (b->used, digs - ix);
  50
  51     /* setup some aliases */
  52     /* copy of the digit from a used within the nested loop */
  53     tmpx = a->dp[ix];
  54
  55     /* an alias for the destination shifted ix places */
  56     tmpt = t.dp + ix;
  57
  58     /* an alias for the digits of b */
  59     tmpy = b->dp;
  60
  61     /* compute the columns of the output and propagate the carry */
  62     for (iy = 0; iy < pb; iy++) {
  63       /* compute the column as a mp_word */
  64       r       = ((mp_word)*tmpt) +
  65                 ((mp_word)tmpx) * ((mp_word)*tmpy++) +
  66                 ((mp_word) u);
  67
  68       /* the new column is the lower part of the result */
  69       *tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
  70
  71       /* get the carry word from the result */
  72       u       = (mp_digit) (r >> ((mp_word) DIGIT_BIT));
  73     }
  74     /* set carry if it is placed below digs */
  75     if (ix + iy < digs) {
  76       *tmpt = u;
  77     }
  78   }
  79
  80   mp_clamp (&t);
  81   mp_exch (&t, c);
  82
  83   mp_clear (&t);
  84   return MP_OKAY;
  85 }
  86 #endif
  87
  88 /* $Source: /cvs/libtom/libtommath/bn_s_mp_mul_digs.c,v $ */
  89 /* $Revision: 1.3 $ */
  90 /* $Date: 2006/03/31 14:18:44 $ */