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

   1 #include <tommath.h>
   2 #ifdef BN_MP_PRIME_MILLER_RABIN_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 /* Miller-Rabin test of "a" to the base of "b" as described in
  19  * HAC pp. 139 Algorithm 4.24
  20  *
  21  * Sets result to 0 if definitely composite or 1 if probably prime.
  22  * Randomly the chance of error is no more than 1/4 and often
  23  * very much lower.
  24  */
  25 int mp_prime_miller_rabin (mp_int * a, mp_int * b, int *result)
  26 {
  27   mp_int  n1, y, r;
  28   int     s, j, err;
  29
  30   /* default */
  31   *result = MP_NO;
  32
  33   /* ensure b > 1 */
  34   if (mp_cmp_d(b, 1) != MP_GT) {
  35      return MP_VAL;
  36   }
  37
  38   /* get n1 = a - 1 */
  39   if ((err = mp_init_copy (&n1, a)) != MP_OKAY) {
  40     return err;
  41   }
  42   if ((err = mp_sub_d (&n1, 1, &n1)) != MP_OKAY) {
  43     goto LBL_N1;
  44   }
  45
  46   /* set 2**s * r = n1 */
  47   if ((err = mp_init_copy (&r, &n1)) != MP_OKAY) {
  48     goto LBL_N1;
  49   }
  50
  51   /* count the number of least significant bits
  52    * which are zero
  53    */
  54   s = mp_cnt_lsb(&r);
  55
  56   /* now divide n - 1 by 2**s */
  57   if ((err = mp_div_2d (&r, s, &r, NULL)) != MP_OKAY) {
  58     goto LBL_R;
  59   }
  60
  61   /* compute y = b**r mod a */
  62   if ((err = mp_init (&y)) != MP_OKAY) {
  63     goto LBL_R;
  64   }
  65   if ((err = mp_exptmod (b, &r, a, &y)) != MP_OKAY) {
  66     goto LBL_Y;
  67   }
  68
  69   /* if y != 1 and y != n1 do */
  70   if (mp_cmp_d (&y, 1) != MP_EQ && mp_cmp (&y, &n1) != MP_EQ) {
  71     j = 1;
  72     /* while j <= s-1 and y != n1 */
  73     while ((j <= (s - 1)) && mp_cmp (&y, &n1) != MP_EQ) {
  74       if ((err = mp_sqrmod (&y, a, &y)) != MP_OKAY) {
  75          goto LBL_Y;
  76       }
  77
  78       /* if y == 1 then composite */
  79       if (mp_cmp_d (&y, 1) == MP_EQ) {
  80          goto LBL_Y;
  81       }
  82
  83       ++j;
  84     }
  85
  86     /* if y != n1 then composite */
  87     if (mp_cmp (&y, &n1) != MP_EQ) {
  88       goto LBL_Y;
  89     }
  90   }
  91
  92   /* probably prime now */
  93   *result = MP_YES;
  94 LBL_Y:mp_clear (&y);
  95 LBL_R:mp_clear (&r);
  96 LBL_N1:mp_clear (&n1);
  97   return err;
  98 }
  99 #endif
 100
 101 /* $Source: /cvs/libtom/libtommath/bn_mp_prime_miller_rabin.c,v $ */
 102 /* $Revision: 1.3 $ */
 103 /* $Date: 2006/03/31 14:18:44 $ */