shotgun/external_libs/libtommath/bn_mp_montgomery_calc_normalization.c

   1 #include <tommath.h>
   2 #ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_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://libtom.org
  16  */
  17
  18 /*
  19  * shifts with subtractions when the result is greater than b.
  20  *
  21  * The method is slightly modified to shift B unconditionally upto just under
  22  * the leading bit of b.  This saves alot of multiple precision shifting.
  23  */
  24 int mp_montgomery_calc_normalization (mp_int * a, mp_int * b)
  25 {
  26   int     x, bits, res;
  27
  28   /* how many bits of last digit does b use */
  29   bits = mp_count_bits (b) % DIGIT_BIT;
  30
  31   if (b->used > 1) {
  32      if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) {
  33         return res;
  34      }
  35   } else {
  36      mp_set(a, 1);
  37      bits = 1;
  38   }
  39
  40
  41   /* now compute C = A * B mod b */
  42   for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
  43     if ((res = mp_mul_2 (a, a)) != MP_OKAY) {
  44       return res;
  45     }
  46     if (mp_cmp_mag (a, b) != MP_LT) {
  47       if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {
  48         return res;
  49       }
  50     }
  51   }
  52
  53   return MP_OKAY;
  54 }
  55 #endif
  56
  57 /* $Source: /cvs/libtom/libtommath/bn_mp_montgomery_calc_normalization.c,v $ */
  58 /* $Revision: 1.4 $ */
  59 /* $Date: 2006/12/28 01:25:13 $ */