kc3-lang/libtommath/mp_montgomery_calc_normalization.c

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• Hash : c7686f24
  Author :
  Date : 2022-10-02T12:58:53
  

  slightly edit, update and run typos.sh Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

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• mp_montgomery_calc_normalization.c

• #include "tommath_private.h"
  #ifdef MP_MONTGOMERY_CALC_NORMALIZATION_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /*
   * shifts with subtractions when the result is greater than b.
   *
   * The method is slightly modified to shift B unconditionally upto just under
   * the leading bit of b.  This saves a lot of multiple precision shifting.
   */
  mp_err mp_montgomery_calc_normalization(mp_int *a, const mp_int *b)
  {
     int    x, bits;
     mp_err err;
  
     /* how many bits of last digit does b use */
     bits = mp_count_bits(b) % MP_DIGIT_BIT;
  
     if (b->used > 1) {
        if ((err = mp_2expt(a, ((b->used - 1) * MP_DIGIT_BIT) + bits - 1)) != MP_OKAY) {
           return err;
        }
     } else {
        mp_set(a, 1uL);
        bits = 1;
     }
  
     /* now compute C = A * B mod b */
     for (x = bits - 1; x < (int)MP_DIGIT_BIT; x++) {
        if ((err = mp_mul_2(a, a)) != MP_OKAY) {
           return err;
        }
        if (mp_cmp_mag(a, b) != MP_LT) {
           if ((err = s_mp_sub(a, b, a)) != MP_OKAY) {
              return err;
           }
        }
     }
  
     return MP_OKAY;
  }
  #endif
  

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