c3-lang/libtommath/bn_mp_prime_miller_rabin.c

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• Hash : 079b0f65
  Author :
  Date : 2015-10-30T17:55:29
  

  Getting ready for v0.43.0 release Also updated my email address... Signed-off-by: Tom St Denis <tstdenis82@gmail.com>

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• bn_mp_prime_miller_rabin.c

• #include <tommath.h>
  #ifdef BN_MP_PRIME_MILLER_RABIN_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
   */
  
  /* Miller-Rabin test of "a" to the base of "b" as described in 
   * HAC pp. 139 Algorithm 4.24
   *
   * Sets result to 0 if definitely composite or 1 if probably prime.
   * Randomly the chance of error is no more than 1/4 and often 
   * very much lower.
   */
  int mp_prime_miller_rabin (mp_int * a, mp_int * b, int *result)
  {
    mp_int  n1, y, r;
    int     s, j, err;
  
    /* default */
    *result = MP_NO;
  
    /* ensure b > 1 */
    if (mp_cmp_d(b, 1) != MP_GT) {
       return MP_VAL;
    }     
  
    /* get n1 = a - 1 */
    if ((err = mp_init_copy (&n1, a)) != MP_OKAY) {
      return err;
    }
    if ((err = mp_sub_d (&n1, 1, &n1)) != MP_OKAY) {
      goto LBL_N1;
    }
  
    /* set 2**s * r = n1 */
    if ((err = mp_init_copy (&r, &n1)) != MP_OKAY) {
      goto LBL_N1;
    }
  
    /* count the number of least significant bits
     * which are zero
     */
    s = mp_cnt_lsb(&r);
  
    /* now divide n - 1 by 2**s */
    if ((err = mp_div_2d (&r, s, &r, NULL)) != MP_OKAY) {
      goto LBL_R;
    }
  
    /* compute y = b**r mod a */
    if ((err = mp_init (&y)) != MP_OKAY) {
      goto LBL_R;
    }
    if ((err = mp_exptmod (b, &r, a, &y)) != MP_OKAY) {
      goto LBL_Y;
    }
  
    /* if y != 1 and y != n1 do */
    if (mp_cmp_d (&y, 1) != MP_EQ && mp_cmp (&y, &n1) != MP_EQ) {
      j = 1;
      /* while j <= s-1 and y != n1 */
      while ((j <= (s - 1)) && mp_cmp (&y, &n1) != MP_EQ) {
        if ((err = mp_sqrmod (&y, a, &y)) != MP_OKAY) {
           goto LBL_Y;
        }
  
        /* if y == 1 then composite */
        if (mp_cmp_d (&y, 1) == MP_EQ) {
           goto LBL_Y;
        }
  
        ++j;
      }
  
      /* if y != n1 then composite */
      if (mp_cmp (&y, &n1) != MP_EQ) {
        goto LBL_Y;
      }
    }
  
    /* probably prime now */
    *result = MP_YES;
  LBL_Y:mp_clear (&y);
  LBL_R:mp_clear (&r);
  LBL_N1:mp_clear (&n1);
    return err;
  }
  #endif
  
  /* $Source$ */
  /* $Revision$ */
  /* $Date$ */
  

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