libtommath/bn_mp_rand.c

#include <tommath_private.h>
#ifdef BN_MP_RAND_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, tstdenis82@gmail.com, http://libtom.org
 */

#if MP_GEN_RANDOM_MAX == 0xffffffff
#define MP_GEN_RANDOM_SHIFT  32
#elif MP_GEN_RANDOM_MAX == 32767
/* SHRT_MAX */
#define MP_GEN_RANDOM_SHIFT  15
#elif MP_GEN_RANDOM_MAX == 2147483647
/* INT_MAX */
#define MP_GEN_RANDOM_SHIFT  31
#elif !defined(MP_GEN_RANDOM_SHIFT)
#error Thou shalt define their own valid MP_GEN_RANDOM_SHIFT
#endif

/* makes a pseudo-random int of a given size */
static mp_digit s_gen_random(void)
{
   mp_digit d = 0, msk = 0;
   do {
      d <<= MP_GEN_RANDOM_SHIFT;
      d |= ((mp_digit) MP_GEN_RANDOM());
      msk <<= MP_GEN_RANDOM_SHIFT;
      msk |= (MP_MASK & MP_GEN_RANDOM_MAX);
   } while ((MP_MASK & msk) != MP_MASK);
   d &= MP_MASK;
   return d;
}

int mp_rand(mp_int *a, int digits)
{
   int     res;
   mp_digit d;

   mp_zero(a);
   if (digits <= 0) {
      return MP_OKAY;
   }

   /* first place a random non-zero digit */
   do {
      d = s_gen_random();
   } while (d == 0);

   if ((res = mp_add_d(a, d, a)) != MP_OKAY) {
      return res;
   }

   while (--digits > 0) {
      if ((res = mp_lshd(a, 1)) != MP_OKAY) {
         return res;
      }

      if ((res = mp_add_d(a, s_gen_random(), a)) != MP_OKAY) {
         return res;
      }
   }

   return MP_OKAY;
}
#endif

/* ref:         $Format:%D$ */
/* git commit:  $Format:%H$ */
/* commit time: $Format:%ai$ */