kc3-lang/libtommath/s_mp_invmod_odd.c

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• Hash : 5c335f84
  Author :
  Date : 2019-10-30T17:26:27
  

  rename mul/sqr functions for consistency, comba instead of fast suffix

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• s_mp_invmod_odd.c

• #include "tommath_private.h"
  #ifdef S_MP_INVMOD_ODD_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* computes the modular inverse via binary extended euclidean algorithm,
   * that is c = 1/a mod b
   *
   * Based on slow invmod except this is optimized for the case where b is
   * odd as per HAC Note 14.64 on pp. 610
   */
  mp_err s_mp_invmod_odd(const mp_int *a, const mp_int *b, mp_int *c)
  {
     mp_int  x, y, u, v, B, D;
     mp_sign neg;
     mp_err  err;
  
     /* 2. [modified] b must be odd   */
     if (mp_iseven(b)) {
        return MP_VAL;
     }
  
     /* init all our temps */
     if ((err = mp_init_multi(&x, &y, &u, &v, &B, &D, NULL)) != MP_OKAY) {
        return err;
     }
  
     /* x == modulus, y == value to invert */
     if ((err = mp_copy(b, &x)) != MP_OKAY)                         goto LBL_ERR;
  
     /* we need y = |a| */
     if ((err = mp_mod(a, b, &y)) != MP_OKAY)                       goto LBL_ERR;
  
     /* if one of x,y is zero return an error! */
     if (mp_iszero(&x) || mp_iszero(&y)) {
        err = MP_VAL;
        goto LBL_ERR;
     }
  
     /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
     if ((err = mp_copy(&x, &u)) != MP_OKAY)                        goto LBL_ERR;
     if ((err = mp_copy(&y, &v)) != MP_OKAY)                        goto LBL_ERR;
     mp_set(&D, 1uL);
  
     do {
        /* 4.  while u is even do */
        while (mp_iseven(&u)) {
           /* 4.1 u = u/2 */
           if ((err = mp_div_2(&u, &u)) != MP_OKAY)                    goto LBL_ERR;
  
           /* 4.2 if B is odd then */
           if (mp_isodd(&B)) {
              if ((err = mp_sub(&B, &x, &B)) != MP_OKAY)               goto LBL_ERR;
           }
           /* B = B/2 */
           if ((err = mp_div_2(&B, &B)) != MP_OKAY)                    goto LBL_ERR;
        }
  
        /* 5.  while v is even do */
        while (mp_iseven(&v)) {
           /* 5.1 v = v/2 */
           if ((err = mp_div_2(&v, &v)) != MP_OKAY)                    goto LBL_ERR;
  
           /* 5.2 if D is odd then */
           if (mp_isodd(&D)) {
              /* D = (D-x)/2 */
              if ((err = mp_sub(&D, &x, &D)) != MP_OKAY)               goto LBL_ERR;
           }
           /* D = D/2 */
           if ((err = mp_div_2(&D, &D)) != MP_OKAY)                    goto LBL_ERR;
        }
  
        /* 6.  if u >= v then */
        if (mp_cmp(&u, &v) != MP_LT) {
           /* u = u - v, B = B - D */
           if ((err = mp_sub(&u, &v, &u)) != MP_OKAY)                  goto LBL_ERR;
  
           if ((err = mp_sub(&B, &D, &B)) != MP_OKAY)                  goto LBL_ERR;
        } else {
           /* v - v - u, D = D - B */
           if ((err = mp_sub(&v, &u, &v)) != MP_OKAY)                  goto LBL_ERR;
  
           if ((err = mp_sub(&D, &B, &D)) != MP_OKAY)                  goto LBL_ERR;
        }
  
        /* if not zero goto step 4 */
     } while (!mp_iszero(&u));
  
     /* now a = C, b = D, gcd == g*v */
  
     /* if v != 1 then there is no inverse */
     if (mp_cmp_d(&v, 1uL) != MP_EQ) {
        err = MP_VAL;
        goto LBL_ERR;
     }
  
     /* b is now the inverse */
     neg = a->sign;
     while (D.sign == MP_NEG) {
        if ((err = mp_add(&D, b, &D)) != MP_OKAY)                   goto LBL_ERR;
     }
  
     /* too big */
     while (mp_cmp_mag(&D, b) != MP_LT) {
        if ((err = mp_sub(&D, b, &D)) != MP_OKAY)                   goto LBL_ERR;
     }
  
     mp_exch(&D, c);
     c->sign = neg;
     err = MP_OKAY;
  
  LBL_ERR:
     mp_clear_multi(&x, &y, &u, &v, &B, &D, NULL);
     return err;
  }
  #endif
  

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