kc3-lang/libtommath/s_mp_invmod.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.c

• #include "tommath_private.h"
  #ifdef S_MP_INVMOD_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* hac 14.61, pp608 */
  mp_err s_mp_invmod(const mp_int *a, const mp_int *b, mp_int *c)
  {
     mp_int  x, y, u, v, A, B, C, D;
     mp_err  err;
  
     /* b cannot be negative */
     if ((b->sign == MP_NEG) || mp_iszero(b)) {
        return MP_VAL;
     }
  
     /* init temps */
     if ((err = mp_init_multi(&x, &y, &u, &v,
                              &A, &B, &C, &D, NULL)) != MP_OKAY) {
        return err;
     }
  
     /* x = a, y = b */
     if ((err = mp_mod(a, b, &x)) != MP_OKAY)                       goto LBL_ERR;
     if ((err = mp_copy(b, &y)) != MP_OKAY)                         goto LBL_ERR;
  
     /* 2. [modified] if x,y are both even then return an error! */
     if (mp_iseven(&x) && mp_iseven(&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(&A, 1uL);
     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 A or B is odd then */
           if (mp_isodd(&A) || mp_isodd(&B)) {
              /* A = (A+y)/2, B = (B-x)/2 */
              if ((err = mp_add(&A, &y, &A)) != MP_OKAY)               goto LBL_ERR;
              if ((err = mp_sub(&B, &x, &B)) != MP_OKAY)               goto LBL_ERR;
           }
           /* A = A/2, B = B/2 */
           if ((err = mp_div_2(&A, &A)) != MP_OKAY)                    goto LBL_ERR;
           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 C or D is odd then */
           if (mp_isodd(&C) || mp_isodd(&D)) {
              /* C = (C+y)/2, D = (D-x)/2 */
              if ((err = mp_add(&C, &y, &C)) != MP_OKAY)               goto LBL_ERR;
              if ((err = mp_sub(&D, &x, &D)) != MP_OKAY)               goto LBL_ERR;
           }
           /* C = C/2, D = D/2 */
           if ((err = mp_div_2(&C, &C)) != MP_OKAY)                    goto LBL_ERR;
           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, A = A - C, B = B - D */
           if ((err = mp_sub(&u, &v, &u)) != MP_OKAY)                  goto LBL_ERR;
  
           if ((err = mp_sub(&A, &C, &A)) != MP_OKAY)                  goto LBL_ERR;
  
           if ((err = mp_sub(&B, &D, &B)) != MP_OKAY)                  goto LBL_ERR;
        } else {
           /* v - v - u, C = C - A, D = D - B */
           if ((err = mp_sub(&v, &u, &v)) != MP_OKAY)                  goto LBL_ERR;
  
           if ((err = mp_sub(&C, &A, &C)) != 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;
     }
  
     /* if its too low */
     while (mp_cmp_d(&C, 0uL) == MP_LT) {
        if ((err = mp_add(&C, b, &C)) != MP_OKAY)                   goto LBL_ERR;
     }
  
     /* too big */
     while (mp_cmp_mag(&C, b) != MP_LT) {
        if ((err = mp_sub(&C, b, &C)) != MP_OKAY)                   goto LBL_ERR;
     }
  
     /* C is now the inverse */
     mp_exch(&C, c);
  
  LBL_ERR:
     mp_clear_multi(&x, &y, &u, &v, &A, &B, &C, &D, NULL);
     return err;
  }
  #endif
  

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