tomsfastmath/fp_montgomery_calc_normalization.c

/* TomsFastMath, a fast ISO C bignum library.
 * 
 * This project is meant to fill in where LibTomMath
 * falls short.  That is speed ;-)
 *
 * This project is public domain and free for all purposes.
 * 
 * Tom St Denis, tomstdenis@gmail.com
 */
#include <tfm.h>

/* computes a = B**n mod b without division or multiplication useful for
 * normalizing numbers in a Montgomery system.
 */
void fp_montgomery_calc_normalization(fp_int *a, fp_int *b)
{
  int     x, bits;

  /* how many bits of last digit does b use */
  bits = fp_count_bits (b) % DIGIT_BIT;

  /* compute A = B^(n-1) * 2^(bits-1) */
  if (b->used > 1) {
     fp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1);
  } else {
     fp_set(a, 1);
     bits = 1;
  }

  /* now compute C = A * B mod b */
  for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
    fp_mul_2 (a, a);
    if (fp_cmp_mag (a, b) != FP_LT) {
      s_fp_sub (a, b, a);
    }
  }
}


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