pow_1.c

a very popular packet of cryptography tools,it encloses the most common used algorithm and protocols

/* mpn_pow_1 -- Compute powers R = U^exp.

   THE FUNCTIONS IN THIS FILE ARE FOR INTERNAL USE ONLY.  THEY'RE ALMOST
   CERTAIN TO BE SUBJECT TO INCOMPATIBLE CHANGES OR DISAPPEAR COMPLETELY IN
   FUTURE GNU MP RELEASES.

Copyright 2002 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by the
Free Software Foundation; either version 2.1 of the License, or (at your
option) any later version.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
for more details.

You should have received a copy of the GNU Lesser General Public License along
with the GNU MP Library; see the file COPYING.LIB.  If not, write to the Free
Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
USA.  */


#include "gmp.h"
#include "gmp-impl.h"
#include "longlong.h"

mp_size_t
mpn_pow_1 (mp_ptr rp, mp_srcptr bp, mp_size_t bn, mp_limb_t exp, mp_ptr tp)
{
  mp_limb_t x;
  int cnt, i;
  mp_size_t rn;
  int par;

  if (exp <= 1)
    {
      if (exp == 0)
	{
	  rp[0] = 1;
	  return 1;
	}
      else
	{
	  MPN_COPY (rp, bp, bn);
	  return bn;
	}
    }

  /* Count number of bits in exp, and compute where to put initial square in
     order to magically get results in the entry rp.  Use simple code,
     optimized for small exp.  For large exp, the bignum operations will take
     so much time that the slowness of this code will be negligible.  */
  par = 0;
  cnt = GMP_LIMB_BITS;
  for (x = exp; x != 0; x >>= 1)
    {
      par ^= x & 1;
      cnt--;
    }
  exp <<= cnt;

  if (bn == 1)
    {
      mp_limb_t bl = bp[0];

      if ((cnt & 1) != 0)
	MP_PTR_SWAP (rp, tp);

      mpn_sqr_n (rp, bp, bn);
      rn = 2 * bn; rn -= rp[rn - 1] == 0;

      for (i = GMP_LIMB_BITS - cnt - 1;;)
	{
	  exp <<= 1;
	  if ((exp & GMP_LIMB_HIGHBIT) != 0)
	    {
	      rp[rn] = mpn_mul_1 (rp, rp, rn, bl);
	      rn += rp[rn] != 0;
	    }

	  if (--i == 0)
	    break;

	  mpn_sqr_n (tp, rp, rn);
	  rn = 2 * rn; rn -= tp[rn - 1] == 0;
	  MP_PTR_SWAP (rp, tp);
	}
    }
  else
    {
      if (((par ^ cnt) & 1) == 0)
	MP_PTR_SWAP (rp, tp);

      mpn_sqr_n (rp, bp, bn);
      rn = 2 * bn; rn -= rp[rn - 1] == 0;

      for (i = GMP_LIMB_BITS - cnt - 1;;)
	{
	  exp <<= 1;
	  if ((exp & GMP_LIMB_HIGHBIT) != 0)
	    {
	      rn = rn + bn - (mpn_mul (tp, rp, rn, bp, bn) == 0);
	      MP_PTR_SWAP (rp, tp);
	    }

	  if (--i == 0)
	    break;

	  mpn_sqr_n (tp, rp, rn);
	  rn = 2 * rn; rn -= tp[rn - 1] == 0;
	  MP_PTR_SWAP (rp, tp);
	}
    }

  return rn;
}