c3-lang/libtommath/bn_fast_mp_invmod.c

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• Hash : 4e3f1344
  Author :
  Date : 2015-11-12T01:49:07
  

  provide tommath_private.h tommath.h contains declarations for the public part of the library. tommath_private.h contains the functions which are private to ltm and should not be exposed to the public.

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• bn_fast_mp_invmod.c

• #include <tommath_private.h>
  #ifdef BN_FAST_MP_INVMOD_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis
   *
   * LibTomMath is a library that provides multiple-precision
   * integer arithmetic as well as number theoretic functionality.
   *
   * The library was designed directly after the MPI library by
   * Michael Fromberger but has been written from scratch with
   * additional optimizations in place.
   *
   * The library is free for all purposes without any express
   * guarantee it works.
   *
   * Tom St Denis, tstdenis82@gmail.com, http://libtom.org
   */
  
  /* 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
   */
  int fast_mp_invmod (mp_int * a, mp_int * b, mp_int * c)
  {
    mp_int  x, y, u, v, B, D;
    int     res, neg;
  
    /* 2. [modified] b must be odd   */
    if (mp_iseven (b) == MP_YES) {
      return MP_VAL;
    }
  
    /* init all our temps */
    if ((res = mp_init_multi(&x, &y, &u, &v, &B, &D, NULL)) != MP_OKAY) {
       return res;
    }
  
    /* x == modulus, y == value to invert */
    if ((res = mp_copy (b, &x)) != MP_OKAY) {
      goto LBL_ERR;
    }
  
    /* we need y = |a| */
    if ((res = mp_mod (a, b, &y)) != MP_OKAY) {
      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 (&D, 1);
  
  top:
    /* 4.  while u is even do */
    while (mp_iseven (&u) == MP_YES) {
      /* 4.1 u = u/2 */
      if ((res = mp_div_2 (&u, &u)) != MP_OKAY) {
        goto LBL_ERR;
      }
      /* 4.2 if B is odd then */
      if (mp_isodd (&B) == MP_YES) {
        if ((res = mp_sub (&B, &x, &B)) != MP_OKAY) {
          goto LBL_ERR;
        }
      }
      /* B = B/2 */
      if ((res = mp_div_2 (&B, &B)) != MP_OKAY) {
        goto LBL_ERR;
      }
    }
  
    /* 5.  while v is even do */
    while (mp_iseven (&v) == MP_YES) {
      /* 5.1 v = v/2 */
      if ((res = mp_div_2 (&v, &v)) != MP_OKAY) {
        goto LBL_ERR;
      }
      /* 5.2 if D is odd then */
      if (mp_isodd (&D) == MP_YES) {
        /* D = (D-x)/2 */
        if ((res = mp_sub (&D, &x, &D)) != MP_OKAY) {
          goto LBL_ERR;
        }
      }
      /* D = D/2 */
      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, B = B - D */
      if ((res = mp_sub (&u, &v, &u)) != MP_OKAY) {
        goto LBL_ERR;
      }
  
      if ((res = mp_sub (&B, &D, &B)) != MP_OKAY) {
        goto LBL_ERR;
      }
    } else {
      /* v - v - u, D = D - B */
      if ((res = mp_sub (&v, &u, &v)) != 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_iszero (&u) == MP_NO) {
      goto top;
    }
  
    /* now a = C, b = D, gcd == g*v */
  
    /* if v != 1 then there is no inverse */
    if (mp_cmp_d (&v, 1) != MP_EQ) {
      res = MP_VAL;
      goto LBL_ERR;
    }
  
    /* b is now the inverse */
    neg = a->sign;
    while (D.sign == MP_NEG) {
      if ((res = mp_add (&D, b, &D)) != MP_OKAY) {
        goto LBL_ERR;
      }
    }
    mp_exch (&D, c);
    c->sign = neg;
    res = MP_OKAY;
  
  LBL_ERR:mp_clear_multi (&x, &y, &u, &v, &B, &D, NULL);
    return res;
  }
  #endif
  
  /* $Source$ */
  /* $Revision$ */
  /* $Date$ */
  

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