kc3-lang/libtommath/bn_s_mp_invmod_slow.c

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• Hash : 4534056c
  Author :
  Date : 2019-05-13T00:22:18
  

  use enums mp_err, mp_ord, mp_bool, mp_sign * MP_USE_ENUMS enables enums * Wc++-compat catches some implicit conversions if MP_USE_ENUMS is defined * 100% backwards compatible API/ABI if MP_USE_ENUMS is not defined

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• bn_s_mp_invmod_slow.c

• #include "tommath_private.h"
  #ifdef BN_S_MP_INVMOD_SLOW_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* hac 14.61, pp608 */
  mp_err s_mp_invmod_slow(const mp_int *a, const mp_int *b, mp_int *c)
  {
     mp_int  x, y, u, v, A, B, C, D;
     mp_err  res;
  
     /* b cannot be negative */
     if ((b->sign == MP_NEG) || MP_IS_ZERO(b)) {
        return MP_VAL;
     }
  
     /* init temps */
     if ((res = mp_init_multi(&x, &y, &u, &v,
                              &A, &B, &C, &D, NULL)) != MP_OKAY) {
        return res;
     }
  
     /* x = a, y = b */
     if ((res = mp_mod(a, b, &x)) != MP_OKAY) {
        goto LBL_ERR;
     }
     if ((res = mp_copy(b, &y)) != MP_OKAY) {
        goto LBL_ERR;
     }
  
     /* 2. [modified] if x,y are both even then return an error! */
     if (MP_IS_EVEN(&x) && MP_IS_EVEN(&y)) {
        res = MP_VAL;
        goto LBL_ERR;
     }
  
     /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
     if ((res = mp_copy(&x, &u)) != MP_OKAY) {
        goto LBL_ERR;
     }
     if ((res = mp_copy(&y, &v)) != MP_OKAY) {
        goto LBL_ERR;
     }
     mp_set(&A, 1uL);
     mp_set(&D, 1uL);
  
  top:
     /* 4.  while u is even do */
     while (MP_IS_EVEN(&u)) {
        /* 4.1 u = u/2 */
        if ((res = mp_div_2(&u, &u)) != MP_OKAY) {
           goto LBL_ERR;
        }
        /* 4.2 if A or B is odd then */
        if (MP_IS_ODD(&A) || MP_IS_ODD(&B)) {
           /* A = (A+y)/2, B = (B-x)/2 */
           if ((res = mp_add(&A, &y, &A)) != MP_OKAY) {
              goto LBL_ERR;
           }
           if ((res = mp_sub(&B, &x, &B)) != MP_OKAY) {
              goto LBL_ERR;
           }
        }
        /* A = A/2, B = B/2 */
        if ((res = mp_div_2(&A, &A)) != MP_OKAY) {
           goto LBL_ERR;
        }
        if ((res = mp_div_2(&B, &B)) != MP_OKAY) {
           goto LBL_ERR;
        }
     }
  
     /* 5.  while v is even do */
     while (MP_IS_EVEN(&v)) {
        /* 5.1 v = v/2 */
        if ((res = mp_div_2(&v, &v)) != MP_OKAY) {
           goto LBL_ERR;
        }
        /* 5.2 if C or D is odd then */
        if (MP_IS_ODD(&C) || MP_IS_ODD(&D)) {
           /* C = (C+y)/2, D = (D-x)/2 */
           if ((res = mp_add(&C, &y, &C)) != MP_OKAY) {
              goto LBL_ERR;
           }
           if ((res = mp_sub(&D, &x, &D)) != MP_OKAY) {
              goto LBL_ERR;
           }
        }
        /* C = C/2, D = D/2 */
        if ((res = mp_div_2(&C, &C)) != MP_OKAY) {
           goto LBL_ERR;
        }
        if ((res = 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, A = A - C, B = B - D */
        if ((res = mp_sub(&u, &v, &u)) != MP_OKAY) {
           goto LBL_ERR;
        }
  
        if ((res = mp_sub(&A, &C, &A)) != MP_OKAY) {
           goto LBL_ERR;
        }
  
        if ((res = mp_sub(&B, &D, &B)) != MP_OKAY) {
           goto LBL_ERR;
        }
     } else {
        /* v - v - u, C = C - A, D = D - B */
        if ((res = mp_sub(&v, &u, &v)) != MP_OKAY) {
           goto LBL_ERR;
        }
  
        if ((res = mp_sub(&C, &A, &C)) != MP_OKAY) {
           goto LBL_ERR;
        }
  
        if ((res = mp_sub(&D, &B, &D)) != MP_OKAY) {
           goto LBL_ERR;
        }
     }
  
     /* if not zero goto step 4 */
     if (!MP_IS_ZERO(&u))
        goto top;
  
     /* now a = C, b = D, gcd == g*v */
  
     /* if v != 1 then there is no inverse */
     if (mp_cmp_d(&v, 1uL) != MP_EQ) {
        res = MP_VAL;
        goto LBL_ERR;
     }
  
     /* if its too low */
     while (mp_cmp_d(&C, 0uL) == MP_LT) {
        if ((res = mp_add(&C, b, &C)) != MP_OKAY) {
           goto LBL_ERR;
        }
     }
  
     /* too big */
     while (mp_cmp_mag(&C, b) != MP_LT) {
        if ((res = mp_sub(&C, b, &C)) != MP_OKAY) {
           goto LBL_ERR;
        }
     }
  
     /* C is now the inverse */
     mp_exch(&C, c);
     res = MP_OKAY;
  LBL_ERR:
     mp_clear_multi(&x, &y, &u, &v, &A, &B, &C, &D, NULL);
     return res;
  }
  #endif
  

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