kc3-lang/libtommath/mp_montgomery_setup.c

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• Hash : 7a68f128
  Author :
  Date : 2019-10-19T16:24:39
  

  Execute move.sh - Rename files from bn_* to match the function names. * git blame <renamed-file> is not affected * git log --follow <renamed-file> can be used to show log across renames

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• mp_montgomery_setup.c

• #include "tommath_private.h"
  #ifdef MP_MONTGOMERY_SETUP_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* setups the montgomery reduction stuff */
  mp_err mp_montgomery_setup(const mp_int *n, mp_digit *rho)
  {
     mp_digit x, b;
  
     /* fast inversion mod 2**k
      *
      * Based on the fact that
      *
      * XA = 1 (mod 2**n)  =>  (X(2-XA)) A = 1 (mod 2**2n)
      *                    =>  2*X*A - X*X*A*A = 1
      *                    =>  2*(1) - (1)     = 1
      */
     b = n->dp[0];
  
     if ((b & 1u) == 0u) {
        return MP_VAL;
     }
  
     x = (((b + 2u) & 4u) << 1) + b; /* here x*a==1 mod 2**4 */
     x *= 2u - (b * x);              /* here x*a==1 mod 2**8 */
     x *= 2u - (b * x);              /* here x*a==1 mod 2**16 */
  #if defined(MP_64BIT) || !(defined(MP_16BIT))
     x *= 2u - (b * x);              /* here x*a==1 mod 2**32 */
  #endif
  #ifdef MP_64BIT
     x *= 2u - (b * x);              /* here x*a==1 mod 2**64 */
  #endif
  
     /* rho = -1/m mod b */
     *rho = (mp_digit)(((mp_word)1 << (mp_word)MP_DIGIT_BIT) - x) & MP_MASK;
  
     return MP_OKAY;
  }
  #endif
  

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