kc3-lang/libtommath/bn_mp_reduce.c

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• Hash : e45f75fd
  Author :
  Date : 2019-04-13T08:46:57
  

  deprecate DIGIT_BIT, use MP_DIGIT_BIT

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• bn_mp_reduce.c

• #include "tommath_private.h"
  #ifdef BN_MP_REDUCE_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* reduces x mod m, assumes 0 < x < m**2, mu is
   * precomputed via mp_reduce_setup.
   * From HAC pp.604 Algorithm 14.42
   */
  int mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu)
  {
     mp_int  q;
     int     res, um = m->used;
  
     /* q = x */
     if ((res = mp_init_copy(&q, x)) != MP_OKAY) {
        return res;
     }
  
     /* q1 = x / b**(k-1)  */
     mp_rshd(&q, um - 1);
  
     /* according to HAC this optimization is ok */
     if ((mp_digit)um > ((mp_digit)1 << (MP_DIGIT_BIT - 1))) {
        if ((res = mp_mul(&q, mu, &q)) != MP_OKAY) {
           goto CLEANUP;
        }
     } else {
  #ifdef BN_S_MP_MUL_HIGH_DIGS_C
        if ((res = s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
           goto CLEANUP;
        }
  #elif defined(BN_S_MP_MUL_HIGH_DIGS_FAST_C)
        if ((res = s_mp_mul_high_digs_fast(&q, mu, &q, um)) != MP_OKAY) {
           goto CLEANUP;
        }
  #else
        {
           res = MP_VAL;
           goto CLEANUP;
        }
  #endif
     }
  
     /* q3 = q2 / b**(k+1) */
     mp_rshd(&q, um + 1);
  
     /* x = x mod b**(k+1), quick (no division) */
     if ((res = mp_mod_2d(x, MP_DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
        goto CLEANUP;
     }
  
     /* q = q * m mod b**(k+1), quick (no division) */
     if ((res = s_mp_mul_digs(&q, m, &q, um + 1)) != MP_OKAY) {
        goto CLEANUP;
     }
  
     /* x = x - q */
     if ((res = mp_sub(x, &q, x)) != MP_OKAY) {
        goto CLEANUP;
     }
  
     /* If x < 0, add b**(k+1) to it */
     if (mp_cmp_d(x, 0uL) == MP_LT) {
        mp_set(&q, 1uL);
        if ((res = mp_lshd(&q, um + 1)) != MP_OKAY)
           goto CLEANUP;
        if ((res = mp_add(x, &q, x)) != MP_OKAY)
           goto CLEANUP;
     }
  
     /* Back off if it's too big */
     while (mp_cmp(x, m) != MP_LT) {
        if ((res = s_mp_sub(x, m, x)) != MP_OKAY) {
           goto CLEANUP;
        }
     }
  
  CLEANUP:
     mp_clear(&q);
  
     return res;
  }
  #endif
  

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