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

   1 #include <tommath.h>
   2 #ifdef BN_MP_MONTGOMERY_SETUP_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 /* setups the montgomery reduction stuff */
  19 int
  20 mp_montgomery_setup (mp_int * n, mp_digit * rho)
  21 {
  22   mp_digit x, b;
  23
  24 /* fast inversion mod 2**k
  25  *
  26  * Based on the fact that
  27  *
  28  * XA = 1 (mod 2**n)  =>  (X(2-XA)) A = 1 (mod 2**2n)
  29  *                    =>  2*X*A - X*X*A*A = 1
  30  *                    =>  2*(1) - (1)     = 1
  31  */
  32   b = n->dp[0];
  33
  34   if ((b & 1) == 0) {
  35     return MP_VAL;
  36   }
  37
  38   x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */
  39   x *= 2 - b * x;               /* here x*a==1 mod 2**8 */
  40 #if !defined(MP_8BIT)
  41   x *= 2 - b * x;               /* here x*a==1 mod 2**16 */
  42 #endif
  43 #if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT))
  44   x *= 2 - b * x;               /* here x*a==1 mod 2**32 */
  45 #endif
  46 #ifdef MP_64BIT
  47   x *= 2 - b * x;               /* here x*a==1 mod 2**64 */
  48 #endif
  49
  50   /* rho = -1/m mod b */
  51   *rho = (unsigned long)(((mp_word)1 << ((mp_word) DIGIT_BIT)) - x) & MP_MASK;
  52
  53   return MP_OKAY;
  54 }
  55 #endif
  56
  57 /* $Source: /cvs/libtom/libtommath/bn_mp_montgomery_setup.c,v $ */
  58 /* $Revision: 1.4 $ */
  59 /* $Date: 2006/12/04 21:34:03 $ */