/* mpi-pow.c  -  MPI functions for exponentiation

 * Copyright (C) 1994, 1996, 1998, 2000, 2002

 *               2003  Free Software Foundation, Inc.

 *               2013  g10 Code GmbH

 *

 * This file is part of Libgcrypt.

 *

 * Libgcrypt 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.

 *

 * Libgcrypt 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 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.

 */

#include <config.h>

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include "mpi-internal.h"

#include "longlong.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 ^ EXPO mod MOD

 */

void

_gcry_mpi_powm (gcry_mpi_t res,

                gcry_mpi_t base, gcry_mpi_t expo, gcry_mpi_t 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;

  unsigned int mp_nlimbs = 0;

  unsigned int bp_nlimbs = 0;

  unsigned int ep_nlimbs = 0;

  unsigned int xp_nlimbs = 0;

  mpi_ptr_t tspace = NULL;

  mpi_size_t tsize = 0;

  esize = expo->nlimbs;

  msize = mod->nlimbs;

  size = 2 * msize;

  msign = mod->sign;

  esec = mpi_is_secure(expo);

  msec = mpi_is_secure(mod);

  bsec = mpi_is_secure(base);

  rp = res->d;

  ep = expo->d;

  MPN_NORMALIZE(ep, esize);

  if (!msize)

    _gcry_divide_by_zero();

  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_nlimbs = msec? msize:0;

  mp = mp_marker = mpi_alloc_limb_space(msize, msec);

  count_leading_zeros (mod_shift_cnt, mod->d[msize-1]);

  if (mod_shift_cnt)

    _gcry_mpih_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_nlimbs = bsec ? (bsize + 1):0;

      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.  */

      _gcry_mpih_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.  */

      gcry_assert (!bp_marker);

      bp_nlimbs = bsec? bsize:0;

      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_nlimbs = esec? esize:0;

      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.*/

      gcry_assert (!mp_marker);

      mp_nlimbs = msec?msize:0;

      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;

    }

  MPN_COPY ( rp, bp, bsize );

  rsize = bsize;

  rsign = 0;

  /* Main processing.  */

  {

    mpi_size_t i;

    mpi_ptr_t xp;

    int c;

    mpi_limb_t e;

    mpi_limb_t carry_limb;

    struct karatsuba_ctx karactx;

    struct gcry_mpi w, u;

    xp_nlimbs = msec? size:0;

    xp = xp_marker = mpi_alloc_limb_space( size, msec );

    w.sign = u.sign = 0;

    w.flags = u.flags = 0;

    w.alloced = w.nlimbs = size; /* RES->alloc may be longer.  */

    u.alloced = u.nlimbs = size;

    memset( &karactx, 0, sizeof karactx );

    negative_result = (ep[0] & 1) && bsign;

    i = esize - 1;

    e = ep[i];

    count_leading_zeros (c, e);

    e = (e << c) << 1;     /* Shift the expo 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

       _gcry_mpih_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;

            /*mpih_mul_n(xp, rp, rp, rsize);*/

            if ( rsize < KARATSUBA_THRESHOLD )

              _gcry_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) )

                  {

                    _gcry_mpi_free_limb_space (tspace, 0);

                    tsize = 2 * rsize;

                    tspace = mpi_alloc_limb_space (tsize, 0 );

                  }

                _gcry_mpih_sqr_n (xp, rp, rsize, tspace);

              }

            xsize = 2 * rsize;

            if ( xsize > msize )

              {

                _gcry_mpih_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)

              {

                /*mpih_mul( xp, rp, rsize, bp, bsize );*/

                if( bsize < KARATSUBA_THRESHOLD )

                  _gcry_mpih_mul ( xp, rp, rsize, bp, bsize );

                else

                  _gcry_mpih_mul_karatsuba_case (xp, rp, rsize, bp, bsize,

                                                 &karactx);

                xsize = rsize + bsize;

                if ( xsize > msize )

                  {

                    _gcry_mpih_divrem(xp + msize, 0, xp, xsize, mp, msize);

                    xsize = msize;

                  }

              }

            w.d = rp;

            u.d = xp;

            mpi_set_cond (&w, &u, ((mpi_limb_signed_t)e < 0));

            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 = _gcry_mpih_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 )

      {

        _gcry_mpih_divrem(rp + msize, 0, rp, rsize, mp, msize);

        rsize = msize;

      }

    /* Remove any leading zero words from the result.  */

    if ( mod_shift_cnt )

      _gcry_mpih_rshift( rp, rp, rsize, mod_shift_cnt);

    MPN_NORMALIZE (rp, rsize);

    _gcry_mpih_release_karatsuba_ctx (&karactx );

  }

  /* Fixup for negative results.  */

  if ( negative_result && rsize )

    {

      if ( mod_shift_cnt )

        _gcry_mpih_rshift( mp, mp, msize, mod_shift_cnt);

      _gcry_mpih_sub( rp, mp, msize, rp, rsize);

      rsize = msize;

      rsign = msign;

      MPN_NORMALIZE(rp, rsize);

    }

  gcry_assert (res->d == rp);

  res->nlimbs = rsize;

  res->sign = rsign;

 leave:

  if (mp_marker)

    _gcry_mpi_free_limb_space( mp_marker, mp_nlimbs );

  if (bp_marker)

    _gcry_mpi_free_limb_space( bp_marker, bp_nlimbs );

  if (ep_marker)

    _gcry_mpi_free_limb_space( ep_marker, ep_nlimbs );

  if (xp_marker)

    _gcry_mpi_free_limb_space( xp_marker, xp_nlimbs );

  if (tspace)

    _gcry_mpi_free_limb_space( tspace, 0 );

}

#else

/**

 * 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 gcry_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 )

    _gcry_mpih_mul ( xp, rp, rsize, sp, ssize );

  else

    _gcry_mpih_mul_karatsuba_case (xp, rp, rsize, sp, ssize, karactx_p);

   if (rsize + ssize > msize)

    {

      _gcry_mpih_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

_gcry_mpi_powm (gcry_mpi_t res,

                gcry_mpi_t base, gcry_mpi_t expo, gcry_mpi_t 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;

  unsigned int mp_nlimbs = 0;

  unsigned int bp_nlimbs = 0;

  unsigned int ep_nlimbs = 0;

  unsigned int xp_nlimbs = 0;

  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;

  ep = expo->d;

  MPN_NORMALIZE(ep, esize);

  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;

  if (!msize)

    _gcry_divide_by_zero();

  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_nlimbs = msec? msize:0;

  mp = mp_marker = mpi_alloc_limb_space(msize, msec);

  count_leading_zeros (mod_shift_cnt, mod->d[msize-1]);

  if (mod_shift_cnt)

    _gcry_mpih_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_nlimbs = bsec ? (bsize + 1):0;

      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.  */

      _gcry_mpih_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 not overlap with RES.  We don't need to check MOD

     because that has already been copied to the MP var.  */

  if ( rp == bp )

    {

      /* RES and BASE are identical.  Allocate temp. space for BASE.  */

      gcry_assert (!bp_marker);

      bp_nlimbs = bsec? bsize:0;

      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_nlimbs = esec? esize:0;

      ep = ep_marker = mpi_alloc_limb_space( esize, esec );

      MPN_COPY(ep, rp, esize);

    }

  /* 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 = 0;

    int c;

    mpi_limb_t e;

    mpi_limb_t carry_limb;

    struct karatsuba_ctx karactx;

    mpi_ptr_t tp;

    xp_nlimbs = msec? size:0;

    xp = xp_marker = mpi_alloc_limb_space( size, 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);

      }

    if (msize > max_u_size)

      max_u_size = msize;

    base_u = mpi_alloc_limb_space (max_u_size, esec);

    MPN_ZERO (base_u, max_u_size);

    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

       _gcry_mpih_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;

          if ( --i < 0 )

            break;

          e = ep[i];

          c = BITS_PER_MPI_LIMB;

        }

      else

        {

          int c0;

          mpi_limb_t e0;

          struct gcry_mpi w, u;

          w.sign = u.sign = 0;

          w.flags = u.flags = 0;

          w.d = base_u;

          count_leading_zeros (c0, e);

          e = (e << c0);

          c -= c0;

          j += c0;

          e0 = (e >> (BITS_PER_MPI_LIMB - W));

          if (c >= W)

            c0 = 0;

          else

            {

              if ( --i < 0 )

                {

                  e0 = (e >> (BITS_PER_MPI_LIMB - c));

                  j += c - W;

                  goto last_step;

                }

              else

                {

                  c0 = c;

                  e = ep[i];

                  c = BITS_PER_MPI_LIMB;

                  e0 |= (e >> (BITS_PER_MPI_LIMB - (W - c0)));

                }

            }

          e = e << (W - c0);

          c -= (W - c0);

        last_step:

          count_trailing_zeros (c0, e0);

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

          for (j += W - c0; j >= 0; j--)

            {

              /*

               *  base_u <= precomp[e0]

               *  base_u_size <= precomp_size[e0]

               */

              base_u_size = 0;

              for (k = 0; k < (1<< (W - 1)); k++)

                {

                  w.alloced = w.nlimbs = precomp_size[k];

                  u.alloced = u.nlimbs = precomp_size[k];

                  u.d = precomp[k];

                  mpi_set_cond (&w, &u, k == e0);

                  base_u_size |= ( precomp_size[k] & (0UL - (k == e0)) );

                }

              w.alloced = w.nlimbs = rsize;

              u.alloced = u.nlimbs = rsize;

              u.d = rp;

              mpi_set_cond (&w, &u, j != 0);

              base_u_size ^= ((base_u_size ^ rsize)  & (0UL - (j != 0)));

              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 ( i < 0 )

            break;

        }

    while (j--)

      {

        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 = _gcry_mpih_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 )

      {

        _gcry_mpih_divrem(rp + msize, 0, rp, rsize, mp, msize);

        rsize = msize;

      }

    /* Remove any leading zero words from the result.  */

    if ( mod_shift_cnt )

      _gcry_mpih_rshift( rp, rp, rsize, mod_shift_cnt);

    MPN_NORMALIZE (rp, rsize);

    _gcry_mpih_release_karatsuba_ctx (&karactx );

    for (i = 0; i < (1 << (W - 1)); i++)

      _gcry_mpi_free_limb_space( precomp[i], esec ? precomp_size[i] : 0 );

    _gcry_mpi_free_limb_space (base_u, esec ? max_u_size : 0);

  }

  /* Fixup for negative results.  */

  if ( negative_result && rsize )

    {

      if ( mod_shift_cnt )

        _gcry_mpih_rshift( mp, mp, msize, mod_shift_cnt);

      _gcry_mpih_sub( rp, mp, msize, rp, rsize);

      rsize = msize;

      rsign = msign;

      MPN_NORMALIZE(rp, rsize);

    }

  gcry_assert (res->d == rp);

  res->nlimbs = rsize;

  res->sign = rsign;

 leave:

  if (mp_marker)

    _gcry_mpi_free_limb_space( mp_marker, mp_nlimbs );

  if (bp_marker)

    _gcry_mpi_free_limb_space( bp_marker, bp_nlimbs );

  if (ep_marker)

    _gcry_mpi_free_limb_space( ep_marker, ep_nlimbs );

  if (xp_marker)

    _gcry_mpi_free_limb_space( xp_marker, xp_nlimbs );

}

#endif