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

   1 #include <tommath.h>
   2 #ifdef BN_MP_PRIME_FERMAT_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 /* performs one Fermat test.
  19  *
  20  * If "a" were prime then b**a == b (mod a) since the order of
  21  * the multiplicative sub-group would be phi(a) = a-1.  That means
  22  * it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
  23  *
  24  * Sets result to 1 if the congruence holds, or zero otherwise.
  25  */
  26 int mp_prime_fermat (mp_int * a, mp_int * b, int *result)
  27 {
  28   mp_int  t;
  29   int     err;
  30
  31   /* default to composite  */
  32   *result = MP_NO;
  33
  34   /* ensure b > 1 */
  35   if (mp_cmp_d(b, 1) != MP_GT) {
  36      return MP_VAL;
  37   }
  38
  39   /* init t */
  40   if ((err = mp_init (&t)) != MP_OKAY) {
  41     return err;
  42   }
  43
  44   /* compute t = b**a mod a */
  45   if ((err = mp_exptmod (b, a, a, &t)) != MP_OKAY) {
  46     goto LBL_T;
  47   }
  48
  49   /* is it equal to b? */
  50   if (mp_cmp (&t, b) == MP_EQ) {
  51     *result = MP_YES;
  52   }
  53
  54   err = MP_OKAY;
  55 LBL_T:mp_clear (&t);
  56   return err;
  57 }
  58 #endif
  59
  60 /* $Source: /cvs/libtom/libtommath/bn_mp_prime_fermat.c,v $ */
  61 /* $Revision: 1.3 $ */
  62 /* $Date: 2006/03/31 14:18:44 $ */