C++ s_mp_sub函数代码示例

/* high level subtraction (handles signs) */
int
mp_sub (mp_int * a, mp_int * b, mp_int * c)
{
  int     sa, sb, res;

  sa = a->sign;
  sb = b->sign;

  if (sa != sb) {
    /* subtract a negative from a positive, OR */
    /* subtract a positive from a negative. */
    /* In either case, ADD their magnitudes, */
    /* and use the sign of the first number. */
    c->sign = sa;
    res = s_mp_add (a, b, c);
  } else {
    /* subtract a positive from a positive, OR */
    /* subtract a negative from a negative. */
    /* First, take the difference between their */
    /* magnitudes, then... */
    if (mp_cmp_mag (a, b) != MP_LT) {
      /* Copy the sign from the first */
      c->sign = sa;
      /* The first has a larger or equal magnitude */
      res = s_mp_sub (a, b, c);
    } else {
      /* The result has the *opposite* sign from */
      /* the first number. */
      c->sign = (sa == MP_ZPOS) ? MP_NEG : MP_ZPOS;
      /* The second has a larger magnitude */
      res = s_mp_sub (b, a, c);
    }
  }
  return res;
}

/* high level addition (handles signs) */
int mp_add(const mp_int *a, const mp_int *b, mp_int *c)
{
   int     sa, sb, res;

   /* get sign of both inputs */
   sa = a->sign;
   sb = b->sign;

   /* handle two cases, not four */
   if (sa == sb) {
      /* both positive or both negative */
      /* add their magnitudes, copy the sign */
      c->sign = sa;
      res = s_mp_add(a, b, c);
   } else {
      /* one positive, the other negative */
      /* subtract the one with the greater magnitude from */
      /* the one of the lesser magnitude.  The result gets */
      /* the sign of the one with the greater magnitude. */
      if (mp_cmp_mag(a, b) == MP_LT) {
         c->sign = sb;
         res = s_mp_sub(b, a, c);
      } else {
         c->sign = sa;
         res = s_mp_sub(a, b, c);
      }
   }
   return res;
}

/* determines the setup value */
int 
mp_reduce_2k_setup(mp_int *a, mp_digit *d)
{
   int res, p;
   mp_int tmp;
   
   if ((res = mp_init(&tmp)) != MP_OKAY) {
      return res;
   }
   
   p = mp_count_bits(a);
   if ((res = mp_2expt(&tmp, p)) != MP_OKAY) {
      mp_clear(&tmp);
      return res;
   }
   
   if ((res = s_mp_sub(&tmp, a, &tmp)) != MP_OKAY) {
      mp_clear(&tmp);
      return res;
   }
   
   *d = tmp.dp[0];
   mp_clear(&tmp);
   return MP_OKAY;
}

/* computes T = REDC(T), 2^b == R */
mp_err s_mp_redc(mp_int *T, mp_mont_modulus *mmm)
{
  mp_err res;
  mp_size i;

  i = MP_USED(T) + MP_USED(&mmm->N) + 2;
  MP_CHECKOK( s_mp_pad(T, i) );
  for (i = 0; i < MP_USED(&mmm->N); ++i ) {
    mp_digit m_i = MP_DIGIT(T, i) * mmm->n0prime;
    /* T += N * m_i * (MP_RADIX ** i); */
    MP_CHECKOK( s_mp_mul_d_add_offset(&mmm->N, m_i, T, i) );
  }
  s_mp_clamp(T);

  /* T /= R */
  s_mp_div_2d(T, mmm->b); 

  if ((res = s_mp_cmp(T, &mmm->N)) >= 0) {
    /* T = T - N */
    MP_CHECKOK( s_mp_sub(T, &mmm->N) );
#ifdef DEBUG
    if ((res = mp_cmp(T, &mmm->N)) >= 0) {
      res = MP_UNDEF;
      goto CLEANUP;
    }
#endif
  }
  res = MP_OKAY;
CLEANUP:
  return res;
}

/* reduces a modulo n where n is of the form 2**p - d 
   This differs from reduce_2k since "d" can be larger
   than a single digit.
*/
int mp_reduce_2k_l(mp_int *a, mp_int *n, mp_int *d)
{
   mp_int q;
   int    p, res;
   
   if ((res = mp_init(&q)) != MP_OKAY) {
      return res;
   }
   
   p = mp_count_bits(n);    
top:
   /* q = a/2**p, a = a mod 2**p */
   if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
      goto LBL_ERR;
   }
   
   /* q = q * d */
   if ((res = mp_mul(&q, d, &q)) != MP_OKAY) { 
      goto LBL_ERR;
   }
   
   /* a = a + q */
   if ((res = s_mp_add(a, &q, a)) != MP_OKAY) {
      goto LBL_ERR;
   }
   
   if (mp_cmp_mag(a, n) != MP_LT) {
      s_mp_sub(a, n, a);
      goto top;
   }
   
LBL_ERR:
   mp_clear(&q);
   return res;
}

/*
 * shifts with subtractions when the result is greater than b.
 *
 * The method is slightly modified to shift B unconditionally upto just under
 * the leading bit of b.  This saves alot of multiple precision shifting.
 */
int mp_montgomery_calc_normalization (mp_int * a, mp_int * b)
{
  int     x, bits, res;

  /* how many bits of last digit does b use */
  bits = mp_count_bits (b) % DIGIT_BIT;

  if (b->used > 1) {
     if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) {
        return res;
     }
  } else {
     mp_set(a, 1);
     bits = 1;
  }

  /* now compute C = A * B mod b */
  for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
    if ((res = mp_mul_2 (a, a)) != MP_OKAY) {
      return res;
    }
    if (mp_cmp_mag (a, b) != MP_LT) {
      if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {
        return res;
      }
    }
  }

  return MP_OKAY;
}

/* reduce "x" in place modulo "n" using the Diminished Radix algorithm.
 *
 * Based on algorithm from the paper
 *
 * "Generating Efficient Primes for Discrete Log Cryptosystems"
 *                 Chae Hoon Lim, Pil Joong Lee,
 *          POSTECH Information Research Laboratories
 *
 * The modulus must be of a special format [see manual]
 *
 * Has been modified to use algorithm 7.10 from the LTM book instead
 *
 * Input x must be in the range 0 <= x <= (n-1)**2
 */
int
mp_dr_reduce(mp_int *x, mp_int *n, mp_digit k) {
    int      err, i, m;
    mp_word  r;
    mp_digit mu, *tmpx1, *tmpx2;

    /* m = digits in modulus */
    m = n->used;

    /* ensure that "x" has at least 2m digits */
    if (x->alloc < (m + m)) {
        if ((err = mp_grow(x, m + m)) != MP_OKAY) {
            return err;
        }
    }

/* top of loop, this is where the code resumes if
 * another reduction pass is required.
 */
top:
    /* aliases for digits */
    /* alias for lower half of x */
    tmpx1 = x->dp;

    /* alias for upper half of x, or x/B**m */
    tmpx2 = x->dp + m;

    /* set carry to zero */
    mu = 0;

    /* compute (x mod B**m) + k * [x/B**m] inline and inplace */
    for (i = 0; i < m; i++) {
        r        = (((mp_word) * tmpx2++) * (mp_word)k) + *tmpx1 + mu;
        *tmpx1++ = (mp_digit)(r & MP_MASK);
        mu       = (mp_digit)(r >> ((mp_word)DIGIT_BIT));
    }

    /* set final carry */
    *tmpx1++ = mu;

    /* zero words above m */
    for (i = m + 1; i < x->used; i++) {
        *tmpx1++ = 0;
    }

    /* clamp, sub and return */
    mp_clamp(x);

    /* if x >= n then subtract and reduce again
     * Each successive "recursion" makes the input smaller and smaller.
     */
    if (mp_cmp_mag(x, n) != MP_LT) {
        if ((err = s_mp_sub(x, n, x)) != MP_OKAY) {
            return err;
        }
        goto top;
    }
    return MP_OKAY;
}

/*! c <- REDC( a * b ) mod N
    \param a < N  i.e. "reduced"
    \param b < N  i.e. "reduced"
    \param mmm modulus N and n0' of N
*/
mp_err
s_mp_mul_mont(const mp_int *a, const mp_int *b, mp_int *c,
              mp_mont_modulus *mmm)
{
    mp_digit *pb;
    mp_digit m_i;
    mp_err res;
    mp_size ib; /* "index b": index of current digit of B */
    mp_size useda, usedb;

    ARGCHK(a != NULL && b != NULL && c != NULL, MP_BADARG);

    if (MP_USED(a) < MP_USED(b)) {
        const mp_int *xch = b; /* switch a and b, to do fewer outer loops */
        b = a;
        a = xch;
    }

    MP_USED(c) = 1;
    MP_DIGIT(c, 0) = 0;
    ib = (MP_USED(&mmm->N) << 1) + 1;
    if ((res = s_mp_pad(c, ib)) != MP_OKAY)
        goto CLEANUP;

    useda = MP_USED(a);
    pb = MP_DIGITS(b);
    s_mpv_mul_d(MP_DIGITS(a), useda, *pb++, MP_DIGITS(c));
    s_mp_setz(MP_DIGITS(c) + useda + 1, ib - (useda + 1));
    m_i = MP_DIGIT(c, 0) * mmm->n0prime;
    s_mp_mul_d_add_offset(&mmm->N, m_i, c, 0);

    /* Outer loop:  Digits of b */
    usedb = MP_USED(b);
    for (ib = 1; ib < usedb; ib++) {
        mp_digit b_i = *pb++;

        /* Inner product:  Digits of a */
        if (b_i)
            s_mpv_mul_d_add_prop(MP_DIGITS(a), useda, b_i, MP_DIGITS(c) + ib);
        m_i = MP_DIGIT(c, ib) * mmm->n0prime;
        s_mp_mul_d_add_offset(&mmm->N, m_i, c, ib);
    }
    if (usedb < MP_USED(&mmm->N)) {
        for (usedb = MP_USED(&mmm->N); ib < usedb; ++ib) {
            m_i = MP_DIGIT(c, ib) * mmm->n0prime;
            s_mp_mul_d_add_offset(&mmm->N, m_i, c, ib);
        }
    }
    s_mp_clamp(c);
    s_mp_rshd(c, MP_USED(&mmm->N)); /* c /= R */
    if (s_mp_cmp(c, &mmm->N) >= 0) {
        MP_CHECKOK(s_mp_sub(c, &mmm->N));
    }
    res = MP_OKAY;

CLEANUP:
    return res;
}

/* determines the setup value */
int mp_reduce_2k_setup_l(mp_int *a, mp_int *d)
{
   int    res;
   mp_int tmp;
   
   if ((res = mp_init(&tmp)) != MP_OKAY) {
      return res;
   }
   
   if ((res = mp_2expt(&tmp, mp_count_bits(a))) != MP_OKAY) {
      goto ERR;
   }
   
   if ((res = s_mp_sub(&tmp, a, d)) != MP_OKAY) {
      goto ERR;
   }
   
ERR:
   mp_clear(&tmp);
   return res;
}

/* reduces a modulo n where n is of the form 2**p - k */
int
mp_reduce_2k(mp_int *a, mp_int *n, mp_digit k)
{
   mp_int q;
   int    p, res;
   
   if ((res = mp_init(&q)) != MP_OKAY) {
      return res;
   }
   
   p = mp_count_bits(n);    
top:
   /* q = a/2**p, a = a mod 2**p */
   if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
      goto ERR;
   }
   
   if (k != 1) {
      /* q = q * k */
      if ((res = mp_mul_d(&q, k, &q)) != MP_OKAY) { 
         goto ERR;
      }
   }
   
   /* a = a + q */
   if ((res = s_mp_add(a, &q, a)) != MP_OKAY) {
      goto ERR;
   }
   
   if (mp_cmp_mag(a, n) != MP_LT) {
      s_mp_sub(a, n, a);
      goto top;
   }
   
ERR:
   mp_clear(&q);
   return res;
}

/* Karatsuba squaring, computes b = a*a using three
 * half size squarings
 *
 * See comments of karatsuba_mul for details.  It
 * is essentially the same algorithm but merely
 * tuned to perform recursive squarings.
 */
int mp_karatsuba_sqr(const mp_int *a, mp_int *b)
{
   mp_int  x0, x1, t1, t2, x0x0, x1x1;
   int     B, err;

   err = MP_MEM;

   /* min # of digits */
   B = a->used;

   /* now divide in two */
   B = B >> 1;

   /* init copy all the temps */
   if (mp_init_size(&x0, B) != MP_OKAY)
      goto LBL_ERR;
   if (mp_init_size(&x1, a->used - B) != MP_OKAY)
      goto X0;

   /* init temps */
   if (mp_init_size(&t1, a->used * 2) != MP_OKAY)
      goto X1;
   if (mp_init_size(&t2, a->used * 2) != MP_OKAY)
      goto T1;
   if (mp_init_size(&x0x0, B * 2) != MP_OKAY)
      goto T2;
   if (mp_init_size(&x1x1, (a->used - B) * 2) != MP_OKAY)
      goto X0X0;

   {
      int x;
      mp_digit *dst, *src;

      src = a->dp;

      /* now shift the digits */
      dst = x0.dp;
      for (x = 0; x < B; x++) {
         *dst++ = *src++;
      }

      dst = x1.dp;
      for (x = B; x < a->used; x++) {
         *dst++ = *src++;
      }
   }

   x0.used = B;
   x1.used = a->used - B;

   mp_clamp(&x0);

   /* now calc the products x0*x0 and x1*x1 */
   if (mp_sqr(&x0, &x0x0) != MP_OKAY)
      goto X1X1;           /* x0x0 = x0*x0 */
   if (mp_sqr(&x1, &x1x1) != MP_OKAY)
      goto X1X1;           /* x1x1 = x1*x1 */

   /* now calc (x1+x0)**2 */
   if (s_mp_add(&x1, &x0, &t1) != MP_OKAY)
      goto X1X1;           /* t1 = x1 - x0 */
   if (mp_sqr(&t1, &t1) != MP_OKAY)
      goto X1X1;           /* t1 = (x1 - x0) * (x1 - x0) */

   /* add x0y0 */
   if (s_mp_add(&x0x0, &x1x1, &t2) != MP_OKAY)
      goto X1X1;           /* t2 = x0x0 + x1x1 */
   if (s_mp_sub(&t1, &t2, &t1) != MP_OKAY)
      goto X1X1;           /* t1 = (x1+x0)**2 - (x0x0 + x1x1) */

   /* shift by B */
   if (mp_lshd(&t1, B) != MP_OKAY)
      goto X1X1;           /* t1 = (x0x0 + x1x1 - (x1-x0)*(x1-x0))<<B */
   if (mp_lshd(&x1x1, B * 2) != MP_OKAY)
      goto X1X1;           /* x1x1 = x1x1 << 2*B */

   if (mp_add(&x0x0, &t1, &t1) != MP_OKAY)
      goto X1X1;           /* t1 = x0x0 + t1 */
   if (mp_add(&t1, &x1x1, b) != MP_OKAY)
      goto X1X1;           /* t1 = x0x0 + t1 + x1x1 */

   err = MP_OKAY;

X1X1:
   mp_clear(&x1x1);
X0X0:
   mp_clear(&x0x0);
T2:
   mp_clear(&t2);
T1:
   mp_clear(&t1);
X1:
   mp_clear(&x1);
//.........这里部分代码省略.........

//.........这里部分代码省略.........
  for (ix = 0; ix < n->used; ix++) {
    /* mu = ai * m' mod b
     *
     * We avoid a double precision multiplication (which isn't required)
     * by casting the value down to a mp_digit.  Note this requires
     * that W[ix-1] have  the carry cleared (see after the inner loop)
     */
    register mp_digit mu;
    mu = (mp_digit) (((W[ix] & MP_MASK) * rho) & MP_MASK);

    /* a = a + mu * m * b**i
     *
     * This is computed in place and on the fly.  The multiplication
     * by b**i is handled by offseting which columns the results
     * are added to.
     *
     * Note the comba method normally doesn't handle carries in the
     * inner loop In this case we fix the carry from the previous
     * column since the Montgomery reduction requires digits of the
     * result (so far) [see above] to work.  This is
     * handled by fixing up one carry after the inner loop.  The
     * carry fixups are done in order so after these loops the
     * first m->used words of W[] have the carries fixed
     */
    {
      register int iy;
      register mp_digit *tmpn;
      register mp_word *_W;

      /* alias for the digits of the modulus */
      tmpn = n->dp;

      /* Alias for the columns set by an offset of ix */
      _W = W + ix;

      /* inner loop */
      for (iy = 0; iy < n->used; iy++) {
          *_W++ += ((mp_word)mu) * ((mp_word)*tmpn++);
      }
    }

    /* now fix carry for next digit, W[ix+1] */
    W[ix + 1] += W[ix] >> ((mp_word) DIGIT_BIT);
  }

  /* now we have to propagate the carries and
   * shift the words downward [all those least
   * significant digits we zeroed].
   */
  {
    register mp_digit *tmpx;
    register mp_word *_W, *_W1;

    /* nox fix rest of carries */

    /* alias for current word */
    _W1 = W + ix;

    /* alias for next word, where the carry goes */
    _W = W + ++ix;

    for (; ix <= n->used * 2 + 1; ix++) {
      *_W++ += *_W1++ >> ((mp_word) DIGIT_BIT);
    }

    /* copy out, A = A/b**n
     *
     * The result is A/b**n but instead of converting from an
     * array of mp_word to mp_digit than calling mp_rshd
     * we just copy them in the right order
     */

    /* alias for destination word */
    tmpx = x->dp;

    /* alias for shifted double precision result */
    _W = W + n->used;

    for (ix = 0; ix < n->used + 1; ix++) {
      *tmpx++ = (mp_digit)(*_W++ & ((mp_word) MP_MASK));
    }

    /* zero oldused digits, if the input a was larger than
     * m->used+1 we'll have to clear the digits
     */
    for (; ix < olduse; ix++) {
      *tmpx++ = 0;
    }
  }

  /* set the max used and clamp */
  x->used = n->used + 1;
  mp_clamp (x);

  /* if A >= m then A = A - m */
  if (mp_cmp_mag (x, n) != MP_LT) {
    return s_mp_sub (x, n, x);
  }
  return MP_OKAY;
}

//.........这里部分代码省略.........

    /* init temps */
    if (mp_init_size (&t1, B * 2) != MP_OKAY)
        goto Y1;
    if (mp_init_size (&x0y0, B * 2) != MP_OKAY)
        goto T1;
    if (mp_init_size (&x1y1, B * 2) != MP_OKAY)
        goto X0Y0;

    /* now shift the digits */
    x0.used = y0.used = B;
    x1.used = a->used - B;
    y1.used = b->used - B;

    {
        register int x;
        register mp_digit *tmpa, *tmpb, *tmpx, *tmpy;

        /* we copy the digits directly instead of using higher level functions
         * since we also need to shift the digits
         */
        tmpa = a->dp;
        tmpb = b->dp;

        tmpx = x0.dp;
        tmpy = y0.dp;
        for (x = 0; x < B; x++) {
            *tmpx++ = *tmpa++;
            *tmpy++ = *tmpb++;
        }

        tmpx = x1.dp;
        for (x = B; x < a->used; x++) {
            *tmpx++ = *tmpa++;
        }

        tmpy = y1.dp;
        for (x = B; x < b->used; x++) {
            *tmpy++ = *tmpb++;
        }
    }

    /* only need to clamp the lower words since by definition the
     * upper words x1/y1 must have a known number of digits
     */
    mp_clamp (&x0);
    mp_clamp (&y0);

    /* now calc the products x0y0 and x1y1 */
    /* after this x0 is no longer required, free temp [x0==t2]! */
    if (mp_mul (&x0, &y0, &x0y0) != MP_OKAY)
        goto X1Y1;          /* x0y0 = x0*y0 */
    if (mp_mul (&x1, &y1, &x1y1) != MP_OKAY)
        goto X1Y1;          /* x1y1 = x1*y1 */

    /* now calc x1+x0 and y1+y0 */
    if (s_mp_add (&x1, &x0, &t1) != MP_OKAY)
        goto X1Y1;          /* t1 = x1 - x0 */
    if (s_mp_add (&y1, &y0, &x0) != MP_OKAY)
        goto X1Y1;          /* t2 = y1 - y0 */
    if (mp_mul (&t1, &x0, &t1) != MP_OKAY)
        goto X1Y1;          /* t1 = (x1 + x0) * (y1 + y0) */

    /* add x0y0 */
    if (mp_add (&x0y0, &x1y1, &x0) != MP_OKAY)
        goto X1Y1;          /* t2 = x0y0 + x1y1 */
    if (s_mp_sub (&t1, &x0, &t1) != MP_OKAY)
        goto X1Y1;          /* t1 = (x1+x0)*(y1+y0) - (x1y1 + x0y0) */

    /* shift by B */
    if (mp_lshd (&t1, B) != MP_OKAY)
        goto X1Y1;          /* t1 = (x0y0 + x1y1 - (x1-x0)*(y1-y0))<<B */
    if (mp_lshd (&x1y1, B * 2) != MP_OKAY)
        goto X1Y1;          /* x1y1 = x1y1 << 2*B */

    if (mp_add (&x0y0, &t1, &t1) != MP_OKAY)
        goto X1Y1;          /* t1 = x0y0 + t1 */
    if (mp_add (&t1, &x1y1, c) != MP_OKAY)
        goto X1Y1;          /* t1 = x0y0 + t1 + x1y1 */

    /* Algorithm succeeded set the return code to MP_OKAY */
    err = MP_OKAY;

X1Y1:
    mp_clear (&x1y1);
X0Y0:
    mp_clear (&x0y0);
T1:
    mp_clear (&t1);
Y1:
    mp_clear (&y1);
Y0:
    mp_clear (&y0);
X1:
    mp_clear (&x1);
X0:
    mp_clear (&x0);
ERR:
    return err;
}

/* Karatsuba squaring, computes b = a*a using three 
 * half size squarings
 *
 * See comments of karatsuba_mul for details.  It 
 * is essentially the same algorithm but merely 
 * tuned to perform recursive squarings.
 */
int mp_karatsuba_sqr (mp_int * a, mp_int * b)
{
  mp_int  x0, x1, t1, t2, x0x0, x1x1;
  int     B, err;

  err = MP_MEM;

  /* min # of digits */
  B = USED(a);

  /* now divide in two */
  B = B >> 1;

  /* init copy all the temps */
  if (mp_init_size (&x0, B) != MP_OKAY)
    goto ERR;
  if (mp_init_size (&x1, USED(a) - B) != MP_OKAY)
    goto X0;

  /* init temps */
  if (mp_init_size (&t1, USED(a) * 2) != MP_OKAY)
    goto X1;
  if (mp_init_size (&t2, USED(a) * 2) != MP_OKAY)
    goto T1;
  if (mp_init_size (&x0x0, B * 2) != MP_OKAY)
    goto T2;
  if (mp_init_size (&x1x1, (USED(a) - B) * 2) != MP_OKAY)
    goto X0X0;

  {
    register int x;
    register mp_digit *dst, *src;

    src = DIGITS(a);

    /* now shift the digits */
    dst = DIGITS(&x0);
    for (x = 0; x < B; x++) {
      *dst++ = *src++;
    }

    dst = DIGITS(&x1);
    for (x = B; x < USED(a); x++) {
      *dst++ = *src++;
    }
  }

  SET_USED(&x0,B);
  SET_USED(&x1,USED(a) - B);

  mp_clamp (&x0);

  /* now calc the products x0*x0 and x1*x1 */
  if (mp_sqr (&x0, &x0x0) != MP_OKAY)
    goto X1X1;           /* x0x0 = x0*x0 */
  if (mp_sqr (&x1, &x1x1) != MP_OKAY)
    goto X1X1;           /* x1x1 = x1*x1 */

  /* now calc (x1+x0)**2 */
  if (s_mp_add (&x1, &x0, &t1) != MP_OKAY)
    goto X1X1;           /* t1 = x1 - x0 */
  if (mp_sqr (&t1, &t1) != MP_OKAY)
    goto X1X1;           /* t1 = (x1 - x0) * (x1 - x0) */

  /* add x0y0 */
  if (s_mp_add (&x0x0, &x1x1, &t2) != MP_OKAY)
    goto X1X1;           /* t2 = x0x0 + x1x1 */
  if (s_mp_sub (&t1, &t2, &t1) != MP_OKAY)
    goto X1X1;           /* t1 = (x1+x0)**2 - (x0x0 + x1x1) */

  /* shift by B */
  if (mp_lshd (&t1, B) != MP_OKAY)
    goto X1X1;           /* t1 = (x0x0 + x1x1 - (x1-x0)*(x1-x0))<<B */
  if (mp_lshd (&x1x1, B * 2) != MP_OKAY)
    goto X1X1;           /* x1x1 = x1x1 << 2*B */

  if (mp_add (&x0x0, &t1, &t1) != MP_OKAY)
    goto X1X1;           /* t1 = x0x0 + t1 */
  if (mp_add (&t1, &x1x1, b) != MP_OKAY)
    goto X1X1;           /* t1 = x0x0 + t1 + x1x1 */

  err = MP_OKAY;

X1X1:mp_clear (&x1x1);
X0X0:mp_clear (&x0x0);
T2:mp_clear (&t2);
T1:mp_clear (&t1);
X1:mp_clear (&x1);
X0:mp_clear (&x0);
ERR:
  return err;
}

/* 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, mp_int * m, mp_int * mu)
{
  mp_int  q;
  int     res, um = USED(m);

  /* 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 (((unsigned long) um) > (((mp_digit)1) << (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_FAST_S_MP_MUL_HIGH_DIGS_C)
    if ((res = fast_s_mp_mul_high_digs (&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, 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, 0) == MP_LT) {
    mp_set (&q, 1);
    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;
}