gnupg/mpi/mpi-pow.c

/* mpi-pow.c  -  MPI functions
 * Copyright (C) 1994, 1996, 1998, 2000 Free Software Foundation, Inc.
 * Copyright (C) 2013 Werner Koch
 *
 * This file is part of GnuPG.
 *
 * GnuPG is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3 of the License, or
 * (at your option) any later version.
 *
 * GnuPG 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, see <http://www.gnu.org/licenses/>.
 *
 * Note: This code is heavily based on the GNU MP Library.
 *	 Actually it's the same code with only minor changes in the
 *	 way the data is stored; this is to support the abstraction
 *	 of an optional secure memory allocation which may be used
 *	 to avoid revealing of sensitive data due to paging etc.
 *	 The GNU MP Library itself is published under the LGPL;
 *	 however I decided to publish this code under the plain GPL.
 */

#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "mpi-internal.h"
#include "longlong.h"
#include <assert.h>

/*
 * When you need old implementation, please add compilation option
 * -DUSE_ALGORITHM_SIMPLE_EXPONENTIATION
 * or expose this line:
#define USE_ALGORITHM_SIMPLE_EXPONENTIATION 1
 */

#if defined(USE_ALGORITHM_SIMPLE_EXPONENTIATION)
/****************
 * RES = BASE ^ EXP mod MOD
 */
void
mpi_powm( MPI res, MPI base, MPI exponent, MPI mod)
{
    mpi_ptr_t  rp, ep, mp, bp;
    mpi_size_t esize, msize, bsize, rsize;
    int               msign, bsign, rsign;
    int        esec,  msec,  bsec,  rsec;
    mpi_size_t size;
    int mod_shift_cnt;
    int negative_result;
    mpi_ptr_t mp_marker=NULL, bp_marker=NULL, ep_marker=NULL;
    mpi_ptr_t xp_marker=NULL;
    int assign_rp=0;
    mpi_ptr_t tspace = NULL;
    mpi_size_t tsize=0;   /* to avoid compiler warning */
			  /* fixme: we should check that the warning is void*/

    esize = exponent->nlimbs;
    msize = mod->nlimbs;
    size = 2 * msize;
    msign = mod->sign;

    esec = mpi_is_secure(exponent);
    msec = mpi_is_secure(mod);
    bsec = mpi_is_secure(base);
    rsec = mpi_is_secure(res);

    rp = res->d;
    ep = exponent->d;

    if( !msize )
	msize = 1 / msize;	    /* provoke a signal */

    if( !esize ) {
	/* Exponent is zero, result is 1 mod MOD, i.e., 1 or 0
	 * depending on if MOD equals 1.  */
	rp[0] = 1;
	res->nlimbs = (msize == 1 && mod->d[0] == 1) ? 0 : 1;
	res->sign = 0;
	goto leave;
    }

    /* Normalize MOD (i.e. make its most significant bit set) as required by
     * mpn_divrem.  This will make the intermediate values in the calculation
     * slightly larger, but the correct result is obtained after a final
     * reduction using the original MOD value.	*/
    mp = mp_marker = mpi_alloc_limb_space(msize, msec);
    count_leading_zeros( mod_shift_cnt, mod->d[msize-1] );
    if( mod_shift_cnt )
	mpihelp_lshift( mp, mod->d, msize, mod_shift_cnt );
    else
	MPN_COPY( mp, mod->d, msize );

    bsize = base->nlimbs;
    bsign = base->sign;
    if( bsize > msize ) { /* The base is larger than the module. Reduce it. */
	/* Allocate (BSIZE + 1) with space for remainder and quotient.
	 * (The quotient is (bsize - msize + 1) limbs.)  */
	bp = bp_marker = mpi_alloc_limb_space( bsize + 1, bsec );
	MPN_COPY( bp, base->d, bsize );
	/* We don't care about the quotient, store it above the remainder,
	 * at BP + MSIZE.  */
	mpihelp_divrem( bp + msize, 0, bp, bsize, mp, msize );
	bsize = msize;
	/* Canonicalize the base, since we are going to multiply with it
	 * quite a few times.  */
	MPN_NORMALIZE( bp, bsize );
    }
    else
	bp = base->d;

    if( !bsize ) {
	res->nlimbs = 0;
	res->sign = 0;
	goto leave;
    }

    if( res->alloced < size ) {
	/* We have to allocate more space for RES.  If any of the input
	 * parameters are identical to RES, defer deallocation of the old
	 * space.  */
	if( rp == ep || rp == mp || rp == bp ) {
	    rp = mpi_alloc_limb_space( size, rsec );
	    assign_rp = 1;
	}
	else {
	    mpi_resize( res, size );
	    rp = res->d;
	}
    }
    else { /* Make BASE, EXPONENT and MOD not overlap with RES.  */
	if( rp == bp ) {
	    /* RES and BASE are identical.  Allocate temp. space for BASE.  */
	    assert( !bp_marker );
	    bp = bp_marker = mpi_alloc_limb_space( bsize, bsec );
	    MPN_COPY(bp, rp, bsize);
	}
	if( rp == ep ) {
	    /* RES and EXPONENT are identical.
               Allocate temp. space for EXPONENT.  */
	    ep = ep_marker = mpi_alloc_limb_space( esize, esec );
	    MPN_COPY(ep, rp, esize);
	}
	if( rp == mp ) {
	    /* RES and MOD are identical.  Allocate temporary space for MOD.*/
	    assert( !mp_marker );
	    mp = mp_marker = mpi_alloc_limb_space( msize, msec );
	    MPN_COPY(mp, rp, msize);
	}
    }

    MPN_COPY( rp, bp, bsize );
    rsize = bsize;
    rsign = bsign;

    {
	mpi_size_t i;
	mpi_ptr_t xp = xp_marker = mpi_alloc_limb_space( 2 * (msize + 1), msec );
	int c;
	mpi_limb_t e;
	mpi_limb_t carry_limb;
	struct karatsuba_ctx karactx;

	memset( &karactx, 0, sizeof karactx );
	negative_result = (ep[0] & 1) && base->sign;

	i = esize - 1;
	e = ep[i];
	count_leading_zeros (c, e);
	e = (e << c) << 1;     /* shift the exp bits to the left, lose msb */
	c = BITS_PER_MPI_LIMB - 1 - c;

	/* Main loop.
	 *
	 * Make the result be pointed to alternately by XP and RP.  This
	 * helps us avoid block copying, which would otherwise be necessary
	 * with the overlap restrictions of mpihelp_divmod. With 50% probability
	 * the result after this loop will be in the area originally pointed
	 * by RP (==RES->d), and with 50% probability in the area originally
	 * pointed to by XP.
	 */

	for(;;) {
	    while( c ) {
		mpi_ptr_t tp;
		mpi_size_t xsize;

		/*mpihelp_mul_n(xp, rp, rp, rsize);*/
		if( rsize < KARATSUBA_THRESHOLD )
		    mpih_sqr_n_basecase( xp, rp, rsize );
		else {
		    if( !tspace ) {
			tsize = 2 * rsize;
			tspace = mpi_alloc_limb_space( tsize, 0 );
		    }
		    else if( tsize < (2*rsize) ) {
			mpi_free_limb_space( tspace );
			tsize = 2 * rsize;
			tspace = mpi_alloc_limb_space( tsize, 0 );
		    }
		    mpih_sqr_n( xp, rp, rsize, tspace );
		}

		xsize = 2 * rsize;
		if( xsize > msize ) {
		    mpihelp_divrem(xp + msize, 0, xp, xsize, mp, msize);
		    xsize = msize;
		}

		tp = rp; rp = xp; xp = tp;
		rsize = xsize;

                /* To mitigate the Yarom/Falkner flush+reload cache
                 * side-channel attack on the RSA secret exponent, we
                 * do the multiplication regardless of the value of
                 * the high-bit of E.  But to avoid this performance
                 * penalty we do it only if the exponent has been
                 * stored in secure memory and we can thus assume it
                 * is a secret exponent.  */
                if (esec || (mpi_limb_signed_t)e < 0) {
		    /*mpihelp_mul( xp, rp, rsize, bp, bsize );*/
		    if( bsize < KARATSUBA_THRESHOLD ) {
			mpihelp_mul( xp, rp, rsize, bp, bsize );
		    }
		    else {
			mpihelp_mul_karatsuba_case(
				     xp, rp, rsize, bp, bsize, &karactx );
		    }

		    xsize = rsize + bsize;
		    if( xsize > msize ) {
			mpihelp_divrem(xp + msize, 0, xp, xsize, mp, msize);
			xsize = msize;
		    }
                }
		if ((mpi_limb_signed_t)e < 0) {
		    tp = rp; rp = xp; xp = tp;
		    rsize = xsize;
		}
		e <<= 1;
		c--;
	    }

	    i--;
	    if( i < 0 )
		break;
	    e = ep[i];
	    c = BITS_PER_MPI_LIMB;
	}

	/* We shifted MOD, the modulo reduction argument, left MOD_SHIFT_CNT
	 * steps.  Adjust the result by reducing it with the original MOD.
	 *
	 * Also make sure the result is put in RES->d (where it already
	 * might be, see above).
	 */
	if( mod_shift_cnt ) {
	    carry_limb = mpihelp_lshift( res->d, rp, rsize, mod_shift_cnt);
	    rp = res->d;
	    if( carry_limb ) {
		rp[rsize] = carry_limb;
		rsize++;
	    }
	}
	else {
	    MPN_COPY( res->d, rp, rsize);
	    rp = res->d;
	}

	if( rsize >= msize ) {
	    mpihelp_divrem(rp + msize, 0, rp, rsize, mp, msize);
	    rsize = msize;
	}

	/* Remove any leading zero words from the result.  */
	if( mod_shift_cnt )
	    mpihelp_rshift( rp, rp, rsize, mod_shift_cnt);
	MPN_NORMALIZE (rp, rsize);

	mpihelp_release_karatsuba_ctx( &karactx );
    }

    if( negative_result && rsize ) {
	if( mod_shift_cnt )
	    mpihelp_rshift( mp, mp, msize, mod_shift_cnt);
	mpihelp_sub( rp, mp, msize, rp, rsize);
	rsize = msize;
	rsign = msign;
	MPN_NORMALIZE(rp, rsize);
    }
    res->nlimbs = rsize;
    res->sign = rsign;

  leave:
    if( assign_rp ) mpi_assign_limb_space( res, rp, size );
    if( mp_marker ) mpi_free_limb_space( mp_marker );
    if( bp_marker ) mpi_free_limb_space( bp_marker );
    if( ep_marker ) mpi_free_limb_space( ep_marker );
    if( xp_marker ) mpi_free_limb_space( xp_marker );
    if( tspace )    mpi_free_limb_space( tspace );
}
#else /*!USE_ALGORITHM_SIMPLE_EXPONENTIATION */

/**
 * Internal function to compute
 *
 *    X = R * S mod M
 *
 * and set the size of X at the pointer XSIZE_P.
 * Use karatsuba structure at KARACTX_P.
 *
 * Condition:
 *   RSIZE >= SSIZE
 *   Enough space for X is allocated beforehand.
 *
 * For generic cases, we can/should use mpi_mulm.
 * This function is use for specific internal case.
 */
static void
mul_mod (mpi_ptr_t xp, mpi_size_t *xsize_p,
         mpi_ptr_t rp, mpi_size_t rsize,
         mpi_ptr_t sp, mpi_size_t ssize,
         mpi_ptr_t mp, mpi_size_t msize,
         struct karatsuba_ctx *karactx_p)
{
  if( ssize < KARATSUBA_THRESHOLD )
    mpihelp_mul ( xp, rp, rsize, sp, ssize );
  else
    mpihelp_mul_karatsuba_case (xp, rp, rsize, sp, ssize, karactx_p);

   if (rsize + ssize > msize)
    {
      mpihelp_divrem (xp + msize, 0, xp, rsize + ssize, mp, msize);
      *xsize_p = msize;
    }
   else
     *xsize_p = rsize + ssize;
}

#define SIZE_PRECOMP ((1 << (5 - 1)))

/****************
 * RES = BASE ^ EXPO mod MOD
 *
 * To mitigate the Yarom/Falkner flush+reload cache side-channel
 * attack on the RSA secret exponent, we don't use the square
 * routine but multiplication.
 *
 * Reference:
 *   Handbook of Applied Cryptography
 *       Algorithm 14.83: Modified left-to-right k-ary exponentiation
 */
void
mpi_powm (MPI res, MPI base, MPI expo, MPI mod)
{
  /* Pointer to the limbs of the arguments, their size and signs. */
  mpi_ptr_t  rp, ep, mp, bp;
  mpi_size_t esize, msize, bsize, rsize;
  int               msign, bsign, rsign;
  /* Flags telling the secure allocation status of the arguments.  */
  int        esec,  msec,  bsec;
  /* Size of the result including space for temporary values.  */
  mpi_size_t size;
  /* Helper.  */
  int mod_shift_cnt;
  int negative_result;
  mpi_ptr_t mp_marker = NULL;
  mpi_ptr_t bp_marker = NULL;
  mpi_ptr_t ep_marker = NULL;
  mpi_ptr_t xp_marker = NULL;
  mpi_ptr_t precomp[SIZE_PRECOMP]; /* Pre-computed array: BASE^1, ^3, ^5, ... */
  mpi_size_t precomp_size[SIZE_PRECOMP];
  mpi_size_t W;
  mpi_ptr_t base_u;
  mpi_size_t base_u_size;
  mpi_size_t max_u_size;

  esize = expo->nlimbs;
  msize = mod->nlimbs;
  size = 2 * msize;
  msign = mod->sign;

  if (esize * BITS_PER_MPI_LIMB > 512)
    W = 5;
  else if (esize * BITS_PER_MPI_LIMB > 256)
    W = 4;
  else if (esize * BITS_PER_MPI_LIMB > 128)
    W = 3;
  else if (esize * BITS_PER_MPI_LIMB > 64)
    W = 2;
  else
    W = 1;

  esec = mpi_is_secure(expo);
  msec = mpi_is_secure(mod);
  bsec = mpi_is_secure(base);

  rp = res->d;
  ep = expo->d;

  if (!msize)
     msize = 1 / msize;	    /* provoke a signal */

  if (!esize)
    {
      /* Exponent is zero, result is 1 mod MOD, i.e., 1 or 0 depending
         on if MOD equals 1.  */
      res->nlimbs = (msize == 1 && mod->d[0] == 1) ? 0 : 1;
      if (res->nlimbs)
        {
          RESIZE_IF_NEEDED (res, 1);
          rp = res->d;
          rp[0] = 1;
        }
      res->sign = 0;
      goto leave;
    }

  /* Normalize MOD (i.e. make its most significant bit set) as
     required by mpn_divrem.  This will make the intermediate values
     in the calculation slightly larger, but the correct result is
     obtained after a final reduction using the original MOD value. */
  mp = mp_marker = mpi_alloc_limb_space(msize, msec);
  count_leading_zeros (mod_shift_cnt, mod->d[msize-1]);
  if (mod_shift_cnt)
    mpihelp_lshift (mp, mod->d, msize, mod_shift_cnt);
  else
    MPN_COPY( mp, mod->d, msize );

  bsize = base->nlimbs;
  bsign = base->sign;
  if (bsize > msize)
    {
      /* The base is larger than the module.  Reduce it.

         Allocate (BSIZE + 1) with space for remainder and quotient.
         (The quotient is (bsize - msize + 1) limbs.)  */
      bp = bp_marker = mpi_alloc_limb_space( bsize + 1, bsec );
      MPN_COPY ( bp, base->d, bsize );
      /* We don't care about the quotient, store it above the
       * remainder, at BP + MSIZE.  */
      mpihelp_divrem( bp + msize, 0, bp, bsize, mp, msize );
      bsize = msize;
      /* Canonicalize the base, since we are going to multiply with it
         quite a few times.  */
      MPN_NORMALIZE( bp, bsize );
    }
  else
    bp = base->d;

  if (!bsize)
    {
      res->nlimbs = 0;
      res->sign = 0;
      goto leave;
    }


  /* Make BASE, EXPO and MOD not overlap with RES.  */
  if ( rp == bp )
    {
      /* RES and BASE are identical.  Allocate temp. space for BASE.  */
      assert (!bp_marker);
      bp = bp_marker = mpi_alloc_limb_space( bsize, bsec );
      MPN_COPY(bp, rp, bsize);
    }
  if ( rp == ep )
    {
      /* RES and EXPO are identical.  Allocate temp. space for EXPO.  */
      ep = ep_marker = mpi_alloc_limb_space( esize, esec );
      MPN_COPY(ep, rp, esize);
    }
  if ( rp == mp )
    {
      /* RES and MOD are identical.  Allocate temporary space for MOD.*/
      assert (!mp_marker);
      mp = mp_marker = mpi_alloc_limb_space( msize, msec );
      MPN_COPY(mp, rp, msize);
    }

  /* Copy base to the result.  */
  if (res->alloced < size)
    {
      mpi_resize (res, size);
      rp = res->d;
    }

  /* Main processing.  */
  {
    mpi_size_t i, j, k;
    mpi_ptr_t xp;
    mpi_size_t xsize;
    int c;
    mpi_limb_t e;
    mpi_limb_t carry_limb;
    struct karatsuba_ctx karactx;
    mpi_ptr_t tp;

    xp = xp_marker = mpi_alloc_limb_space( 2 * (msize + 1), msec );

    memset( &karactx, 0, sizeof karactx );
    negative_result = (ep[0] & 1) && bsign;

    /* Precompute PRECOMP[], BASE^(2 * i + 1), BASE^1, ^3, ^5, ... */
    if (W > 1)                  /* X := BASE^2 */
      mul_mod (xp, &xsize, bp, bsize, bp, bsize, mp, msize, &karactx);
    base_u = precomp[0] = mpi_alloc_limb_space (bsize, esec);
    base_u_size = max_u_size = precomp_size[0] = bsize;
    MPN_COPY (precomp[0], bp, bsize);
    for (i = 1; i < (1 << (W - 1)); i++)
      {                         /* PRECOMP[i] = BASE^(2 * i + 1) */
        if (xsize >= base_u_size)
          mul_mod (rp, &rsize, xp, xsize, base_u, base_u_size,
                   mp, msize, &karactx);
        else
          mul_mod (rp, &rsize, base_u, base_u_size, xp, xsize,
                   mp, msize, &karactx);
        base_u = precomp[i] = mpi_alloc_limb_space (rsize, esec);
        base_u_size = precomp_size[i] = rsize;
        if (max_u_size < base_u_size)
          max_u_size = base_u_size;
        MPN_COPY (precomp[i], rp, rsize);
      }

    base_u = mpi_alloc_limb_space (max_u_size, esec);

    i = esize - 1;

    /* Main loop.

       Make the result be pointed to alternately by XP and RP.  This
       helps us avoid block copying, which would otherwise be
       necessary with the overlap restrictions of mpihelp_divmod. With
       50% probability the result after this loop will be in the area
       originally pointed by RP (==RES->d), and with 50% probability
       in the area originally pointed to by XP. */
    rsign = 0;
    if (W == 1)
      {
        rsize = bsize;
      }
    else
      {
        rsize = msize;
        MPN_ZERO (rp, rsize);
      }
    MPN_COPY ( rp, bp, bsize );

    e = ep[i];
    count_leading_zeros (c, e);
    e = (e << c) << 1;
    c = BITS_PER_MPI_LIMB - 1 - c;

    j = 0;

    for (;;)
      if (e == 0)
        {
          j += c;
          i--;
          if ( i < 0 )
            {
              c = 0;
              break;
            }

          e = ep[i];
          c = BITS_PER_MPI_LIMB;
        }
      else
        {
          int c0;
          mpi_limb_t e0;

          count_leading_zeros (c0, e);
          e = (e << c0);
          c -= c0;
          j += c0;

          if (c >= W)
            {
              e0 = (e >> (BITS_PER_MPI_LIMB - W));
              e = (e << W);
              c -= W;
            }
          else
            {
              i--;
              if ( i < 0 )
                {
                  e = (e >> (BITS_PER_MPI_LIMB - c));
                  break;
                }

              c0 = c;
              e0 = (e >> (BITS_PER_MPI_LIMB - W))
                | (ep[i] >> (BITS_PER_MPI_LIMB - W + c0));
              e = (ep[i] << (W - c0));
              c = BITS_PER_MPI_LIMB - W + c0;
            }

          count_trailing_zeros (c0, e0);
          e0 = (e0 >> c0) >> 1;

          for (j += W - c0; j; j--)
            {
              mul_mod (xp, &xsize, rp, rsize, rp, rsize, mp, msize, &karactx);
              tp = rp; rp = xp; xp = tp;
              rsize = xsize;
            }

          /*
           *  base_u <= precomp[e0]
           *  base_u_size <= precomp_size[e0];
           */
          base_u_size = 0;
          for (k = 0; k < (1<< (W - 1)); k++)
            {
              struct gcry_mpi w, u;
              w.alloced = w.nlimbs = precomp_size[k];
              u.alloced = u.nlimbs = precomp_size[k];
              w.nbits = w.nlimbs * BITS_PER_MPI_LIMB;
              u.nbits = u.nlimbs * BITS_PER_MPI_LIMB;
              w.sign = u.sign = 0;
              w.flags = u.flags = 0;
              w.d = base_u;
              u.d = precomp[k];

              mpi_set_cond (&w, &u, k == e0);
              base_u_size |= (precomp_size[k] & ((mpi_size_t)0 - (k == e0)) );
            }
          mul_mod (xp, &xsize, rp, rsize, base_u, base_u_size,
                   mp, msize, &karactx);
          tp = rp; rp = xp; xp = tp;
          rsize = xsize;

          j = c0;
        }

    if (c != 0)
      {
        j += c;
        count_trailing_zeros (c, e);
        e = (e >> c);
        j -= c;
      }

    while (j--)
      {
        mul_mod (xp, &xsize, rp, rsize, rp, rsize, mp, msize, &karactx);
        tp = rp; rp = xp; xp = tp;
        rsize = xsize;
      }

    if (e != 0)
      {
        base_u_size = 0;
        for (k = 0; k < (1<< (W - 1)); k++)
          {
            struct gcry_mpi w, u;
            w.alloced = w.nlimbs = precomp_size[k];
            u.alloced = u.nlimbs = precomp_size[k];
            w.nbits = w.nlimbs * BITS_PER_MPI_LIMB;
            u.nbits = u.nlimbs * BITS_PER_MPI_LIMB;
            w.sign = u.sign = 0;
            w.flags = u.flags = 0;
            w.d = base_u;
            u.d = precomp[k];

            mpi_set_cond (&w, &u, k == (e>>1));
            base_u_size |= (precomp_size[k] & ((mpi_size_t)0 - (k == (e>>1))) );
          }

        mul_mod (xp, &xsize, rp, rsize, base_u, base_u_size,
                 mp, msize, &karactx);
        tp = rp; rp = xp; xp = tp;
        rsize = xsize;

        for (; c; c--)
          {
            mul_mod (xp, &xsize, rp, rsize, rp, rsize, mp, msize, &karactx);
            tp = rp; rp = xp; xp = tp;
            rsize = xsize;
          }
      }

    /* We shifted MOD, the modulo reduction argument, left
       MOD_SHIFT_CNT steps.  Adjust the result by reducing it with the
       original MOD.

       Also make sure the result is put in RES->d (where it already
       might be, see above).  */
    if ( mod_shift_cnt )
      {
        carry_limb = mpihelp_lshift( res->d, rp, rsize, mod_shift_cnt);
        rp = res->d;
        if ( carry_limb )
          {
            rp[rsize] = carry_limb;
            rsize++;
          }
      }
    else if (res->d != rp)
      {
        MPN_COPY (res->d, rp, rsize);
        rp = res->d;
      }

    if ( rsize >= msize )
      {
        mpihelp_divrem(rp + msize, 0, rp, rsize, mp, msize);
        rsize = msize;
      }

    /* Remove any leading zero words from the result.  */
    if ( mod_shift_cnt )
      mpihelp_rshift (rp, rp, rsize, mod_shift_cnt);
    MPN_NORMALIZE (rp, rsize);

    mpihelp_release_karatsuba_ctx (&karactx );
    for (i = 0; i < (1 << (W - 1)); i++)
      mpi_free_limb_space (precomp[i]);
    mpi_free_limb_space (base_u);
  }

  /* Fixup for negative results.  */
  if ( negative_result && rsize )
    {
      if ( mod_shift_cnt )
        mpihelp_rshift (mp, mp, msize, mod_shift_cnt);
      mpihelp_sub (rp, mp, msize, rp, rsize);
      rsize = msize;
      rsign = msign;
      MPN_NORMALIZE(rp, rsize);
    }
  assert (res->d == rp);
  res->nlimbs = rsize;
  res->sign = rsign;

 leave:
  if (mp_marker)
    mpi_free_limb_space (mp_marker);
  if (bp_marker)
    mpi_free_limb_space (bp_marker);
  if (ep_marker)
    mpi_free_limb_space (ep_marker);
  if (xp_marker)
    mpi_free_limb_space (xp_marker);
}
#endif /*!USE_ALGORITHM_SIMPLE_EXPONENTIATION */