kc3-lang/libtommath/mp_montgomery_reduce.c

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• Hash : 93f8e760
  Author :
  Date : 2019-11-05T18:04:05
  

  rename MP_MAXFAST to MP_MAX_COMBA

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• mp_montgomery_reduce.c

• #include "tommath_private.h"
  #ifdef MP_MONTGOMERY_REDUCE_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* computes xR**-1 == x (mod N) via Montgomery Reduction */
  mp_err mp_montgomery_reduce(mp_int *x, const mp_int *n, mp_digit rho)
  {
     mp_err err;
     int ix, digs;
  
     /* can the fast reduction [comba] method be used?
      *
      * Note that unlike in mul you're safely allowed *less*
      * than the available columns [255 per default] since carries
      * are fixed up in the inner loop.
      */
     digs = (n->used * 2) + 1;
     if ((digs < MP_WARRAY) &&
         (x->used <= MP_WARRAY) &&
         (n->used < MP_MAX_COMBA)) {
        return s_mp_montgomery_reduce_comba(x, n, rho);
     }
  
     /* grow the input as required */
     if ((err = mp_grow(x, digs)) != MP_OKAY) {
        return err;
     }
     x->used = digs;
  
     for (ix = 0; ix < n->used; ix++) {
        int iy;
        mp_digit u, mu;
  
        /* mu = ai * rho mod b
         *
         * The value of rho must be precalculated via
         * montgomery_setup() such that
         * it equals -1/n0 mod b this allows the
         * following inner loop to reduce the
         * input one digit at a time
         */
        mu = (mp_digit)(((mp_word)x->dp[ix] * (mp_word)rho) & MP_MASK);
  
        /* a = a + mu * m * b**i */
  
        /* Multiply and add in place */
        u = 0;
        for (iy = 0; iy < n->used; iy++) {
           /* compute product and sum */
           mp_word r = ((mp_word)mu * (mp_word)n->dp[iy]) +
                       (mp_word)u + (mp_word)x->dp[ix + iy];
  
           /* get carry */
           u       = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
  
           /* fix digit */
           x->dp[ix + iy] = (mp_digit)(r & (mp_word)MP_MASK);
        }
        /* At this point the ix'th digit of x should be zero */
  
        /* propagate carries upwards as required*/
        while (u != 0u) {
           x->dp[ix + iy]   += u;
           u        = x->dp[ix + iy] >> MP_DIGIT_BIT;
           x->dp[ix + iy] &= MP_MASK;
           ++iy;
        }
     }
  
     /* at this point the n.used'th least
      * significant digits of x are all zero
      * which means we can shift x to the
      * right by n.used digits and the
      * residue is unchanged.
      */
  
     /* x = x/b**n.used */
     mp_clamp(x);
     mp_rshd(x, n->used);
  
     /* if x >= n then x = x - n */
     if (mp_cmp_mag(x, n) != MP_LT) {
        return s_mp_sub(x, n, x);
     }
  
     return MP_OKAY;
  }
  #endif
  

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