/* Elgamal.c  -  Elgamal Public Key encryption

 * Copyright (C) 1998, 2000, 2001, 2002, 2003,

 *               2008  Free Software Foundation, Inc.

 * Copyright (C) 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/>.

 *

 * For a description of the algorithm, see:

 *   Bruce Schneier: Applied Cryptography. John Wiley & Sons, 1996.

 *   ISBN 0-471-11709-9. Pages 476 ff.

 */

#include <config.h>

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include "g10lib.h"

#include "mpi.h"

#include "cipher.h"

#include "pubkey-internal.h"

#include "const-time.h"

/* Blinding is used to mitigate side-channel attacks.  You may undef

   this to speed up the operation in case the system is secured

   against physical and network mounted side-channel attacks.  */

#define USE_BLINDING 1

typedef struct

{

  gcry_mpi_t p;     /* prime */

  gcry_mpi_t g;     /* group generator */

  gcry_mpi_t y;     /* g^x mod p */

} ELG_public_key;

typedef struct

{

  gcry_mpi_t p;     /* prime */

  gcry_mpi_t g;     /* group generator */

  gcry_mpi_t y;     /* g^x mod p */

  gcry_mpi_t x;     /* secret exponent */

} ELG_secret_key;

static const char *elg_names[] =

  {

    "elg",

    "openpgp-elg",

    "openpgp-elg-sig",

    NULL,

  };

static int test_keys (ELG_secret_key *sk, unsigned int nbits, int nodie);

static gcry_mpi_t gen_k (gcry_mpi_t p);

static gcry_err_code_t generate (ELG_secret_key *sk, unsigned nbits,

                                 gcry_mpi_t **factors);

static int  check_secret_key (ELG_secret_key *sk);

static void do_encrypt (gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input,

                        ELG_public_key *pkey);

static void decrypt (gcry_mpi_t output, gcry_mpi_t a, gcry_mpi_t b,

                     ELG_secret_key *skey);

static void sign (gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input,

                  ELG_secret_key *skey);

static int  verify (gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input,

                    ELG_public_key *pkey);

static unsigned int elg_get_nbits (gcry_sexp_t parms);

static void (*progress_cb) (void *, const char *, int, int, int);

static void *progress_cb_data;

void

_gcry_register_pk_elg_progress (void (*cb) (void *, const char *,

                                            int, int, int),

        void *cb_data)

{

  progress_cb = cb;

  progress_cb_data = cb_data;

}

static void

progress (int c)

{

  if (progress_cb)

    progress_cb (progress_cb_data, "pk_elg", c, 0, 0);

}

/****************

 * Michael Wiener's table on subgroup sizes to match field sizes.

 * (floating around somewhere, probably based on the paper from

 * Eurocrypt 96, page 332)

 */

static unsigned int

wiener_map( unsigned int n )

{

  static struct { unsigned int p_n, q_n; } t[] =

    { /*   p    q  attack cost */

      {  512, 119 },  /* 9 x 10^17 */

      {  768, 145 },  /* 6 x 10^21 */

      { 1024, 165 },  /* 7 x 10^24 */

      { 1280, 183 },  /* 3 x 10^27 */

      { 1536, 198 },  /* 7 x 10^29 */

      { 1792, 212 },  /* 9 x 10^31 */

      { 2048, 225 },  /* 8 x 10^33 */

      { 2304, 237 },  /* 5 x 10^35 */

      { 2560, 249 },  /* 3 x 10^37 */

      { 2816, 259 },  /* 1 x 10^39 */

      { 3072, 269 },  /* 3 x 10^40 */

      { 3328, 279 },  /* 8 x 10^41 */

      { 3584, 288 },  /* 2 x 10^43 */

      { 3840, 296 },  /* 4 x 10^44 */

      { 4096, 305 },  /* 7 x 10^45 */

      { 4352, 313 },  /* 1 x 10^47 */

      { 4608, 320 },  /* 2 x 10^48 */

      { 4864, 328 },  /* 2 x 10^49 */

      { 5120, 335 },  /* 3 x 10^50 */

      { 0, 0 }

    };

  int i;

  for(i=0; t[i].p_n; i++ )

    {

      if( n <= t[i].p_n )

        return t[i].q_n;

    }

  /* Not in table - use an arbitrary high number. */

  return  n / 8 + 200;

}

static int

test_keys ( ELG_secret_key *sk, unsigned int nbits, int nodie )

{

  ELG_public_key pk;

  gcry_mpi_t test   = mpi_new ( 0 );

  gcry_mpi_t out1_a = mpi_new ( nbits );

  gcry_mpi_t out1_b = mpi_new ( nbits );

  gcry_mpi_t out2   = mpi_new ( nbits );

  int failed = 0;

  pk.p = sk->p;

  pk.g = sk->g;

  pk.y = sk->y;

  _gcry_mpi_randomize ( test, nbits, GCRY_WEAK_RANDOM );

  do_encrypt ( out1_a, out1_b, test, &pk );

  decrypt ( out2, out1_a, out1_b, sk );

  if ( mpi_cmp( test, out2 ) )

    failed |= 1;

  sign ( out1_a, out1_b, test, sk );

  if ( !verify( out1_a, out1_b, test, &pk ) )

    failed |= 2;

  _gcry_mpi_release ( test );

  _gcry_mpi_release ( out1_a );

  _gcry_mpi_release ( out1_b );

  _gcry_mpi_release ( out2 );

  if (failed && !nodie)

    log_fatal ("Elgamal test key for %s %s failed\n",

               (failed & 1)? "encrypt+decrypt":"",

               (failed & 2)? "sign+verify":"");

  if (failed && DBG_CIPHER)

    log_debug ("Elgamal test key for %s %s failed\n",

               (failed & 1)? "encrypt+decrypt":"",

               (failed & 2)? "sign+verify":"");

  return failed;

}

/****************

 * Generate a random secret exponent k from prime p, so that k is

 * relatively prime to p-1.

 */

static gcry_mpi_t

gen_k( gcry_mpi_t p )

{

  gcry_mpi_t k = mpi_alloc_secure( 0 );

  gcry_mpi_t temp = mpi_alloc( mpi_get_nlimbs(p) );

  gcry_mpi_t p_1 = mpi_copy(p);

  unsigned int orig_nbits = mpi_get_nbits(p);

  unsigned int nbits, nbytes;

  char *rndbuf = NULL;

  nbits = orig_nbits;

  nbytes = (nbits+7)/8;

  if( DBG_CIPHER )

    log_debug("choosing a random k\n");

  mpi_sub_ui( p_1, p, 1);

  for(;;)

    {

      if( !rndbuf || nbits < 32 )

        {

          xfree(rndbuf);

          rndbuf = _gcry_random_bytes_secure( nbytes, GCRY_STRONG_RANDOM );

        }

      else

        {

          /* Change only some of the higher bits.  We could improve

             this by directly requesting more memory at the first call

             to get_random_bytes() and use this the here maybe it is

             easier to do this directly in random.c Anyway, it is

             highly inlikely that we will ever reach this code. */

          char *pp = _gcry_random_bytes_secure( 4, GCRY_STRONG_RANDOM );

          memcpy( rndbuf, pp, 4 );

          xfree(pp);

  }

      _gcry_mpi_set_buffer( k, rndbuf, nbytes, 0 );

      for(;;)

        {

          if( !(mpi_cmp( k, p_1 ) < 0) )  /* check: k < (p-1) */

            {

              if( DBG_CIPHER )

                progress('+');

              break; /* no  */

            }

          if( !(mpi_cmp_ui( k, 0 ) > 0) )  /* check: k > 0 */

            {

              if( DBG_CIPHER )

                progress('-');

              break; /* no */

            }

          if (mpi_gcd( temp, k, p_1 ))

            goto found;  /* okay, k is relative prime to (p-1) */

          mpi_add_ui( k, k, 1 );

          if( DBG_CIPHER )

            progress('.');

  }

    }

 found:

  xfree (rndbuf);

  if( DBG_CIPHER )

    progress('\n');

  mpi_free(p_1);

  mpi_free(temp);

  return k;

}

/****************

 * Generate a key pair with a key of size NBITS

 * Returns: 2 structures filled with all needed values

 *      and an array with n-1 factors of (p-1)

 */

static gcry_err_code_t

generate ( ELG_secret_key *sk, unsigned int nbits, gcry_mpi_t **ret_factors )

{

  gcry_err_code_t rc;

  gcry_mpi_t p;    /* the prime */

  gcry_mpi_t p_min1;

  gcry_mpi_t g;

  gcry_mpi_t x;    /* the secret exponent */

  gcry_mpi_t y;

  unsigned int qbits;

  unsigned int xbits;

  byte *rndbuf;

  p_min1 = mpi_new ( nbits );

  qbits = wiener_map( nbits );

  if( qbits & 1 ) /* better have a even one */

    qbits++;

  g = mpi_alloc(1);

  rc = _gcry_generate_elg_prime (0, nbits, qbits, g, &p, ret_factors);

  if (rc)

    {

      mpi_free (p_min1);

      mpi_free (g);

      return rc;

    }

  mpi_sub_ui(p_min1, p, 1);

  /* Select a random number which has these properties:

   *   0 < x < p-1

   * This must be a very good random number because this is the

   * secret part.  The prime is public and may be shared anyway,

   * so a random generator level of 1 is used for the prime.

   *

   * I don't see a reason to have a x of about the same size

   * as the p.  It should be sufficient to have one about the size

   * of q or the later used k plus a large safety margin. Decryption

   * will be much faster with such an x.

   */

  xbits = qbits * 3 / 2;

  if( xbits >= nbits )

    BUG();

  x = mpi_snew ( xbits );

  if( DBG_CIPHER )

    log_debug("choosing a random x of size %u\n", xbits );

  rndbuf = NULL;

  do

    {

      if( DBG_CIPHER )

        progress('.');

      if( rndbuf )

        { /* Change only some of the higher bits */

          if( xbits < 16 ) /* should never happen ... */

            {

              xfree(rndbuf);

              rndbuf = _gcry_random_bytes_secure ((xbits+7)/8,

                                                  GCRY_VERY_STRONG_RANDOM);

            }

          else

            {

              char *r = _gcry_random_bytes_secure (2, GCRY_VERY_STRONG_RANDOM);

              memcpy(rndbuf, r, 2 );

              xfree (r);

            }

  }

      else

        {

          rndbuf = _gcry_random_bytes_secure ((xbits+7)/8,

                                              GCRY_VERY_STRONG_RANDOM );

  }

      _gcry_mpi_set_buffer( x, rndbuf, (xbits+7)/8, 0 );

      mpi_clear_highbit( x, xbits+1 );

    }

  while( !( mpi_cmp_ui( x, 0 )>0 && mpi_cmp( x, p_min1 )<0 ) );

  xfree(rndbuf);

  y = mpi_new (nbits);

  mpi_powm( y, g, x, p );

  if( DBG_CIPHER )

    {

      progress ('\n');

      log_mpidump ("elg  p", p );

      log_mpidump ("elg  g", g );

      log_mpidump ("elg  y", y );

      log_mpidump ("elg  x", x );

    }

  /* Copy the stuff to the key structures */

  sk->p = p;

  sk->g = g;

  sk->y = y;

  sk->x = x;

  _gcry_mpi_release ( p_min1 );

  /* Now we can test our keys (this should never fail!) */

  test_keys ( sk, nbits - 64, 0 );

  return 0;

}

/* Generate a key pair with a key of size NBITS not using a random

   value for the secret key but the one given as X.  This is useful to

   implement a passphrase based decryption for a public key based

   encryption.  It has appliactions in backup systems.

   Returns: A structure filled with all needed values and an array

      with n-1 factors of (p-1).  */

static gcry_err_code_t

generate_using_x (ELG_secret_key *sk, unsigned int nbits, gcry_mpi_t x,

                  gcry_mpi_t **ret_factors )

{

  gcry_err_code_t rc;

  gcry_mpi_t p;      /* The prime.  */

  gcry_mpi_t p_min1; /* The prime minus 1.  */

  gcry_mpi_t g;      /* The generator.  */

  gcry_mpi_t y;      /* g^x mod p.  */

  unsigned int qbits;

  unsigned int xbits;

  sk->p = NULL;

  sk->g = NULL;

  sk->y = NULL;

  sk->x = NULL;

  /* Do a quick check to see whether X is suitable.  */

  xbits = mpi_get_nbits (x);

  if ( xbits < 64 || xbits >= nbits )

    return GPG_ERR_INV_VALUE;

  p_min1 = mpi_new ( nbits );

  qbits  = wiener_map ( nbits );

  if ( (qbits & 1) ) /* Better have an even one.  */

    qbits++;

  g = mpi_alloc (1);

  rc = _gcry_generate_elg_prime (0, nbits, qbits, g, &p, ret_factors );

  if (rc)

    {

      mpi_free (p_min1);

      mpi_free (g);

      return rc;

    }

  mpi_sub_ui (p_min1, p, 1);

  if (DBG_CIPHER)

    log_debug ("using a supplied x of size %u", xbits );

  if ( !(mpi_cmp_ui ( x, 0 ) > 0 && mpi_cmp ( x, p_min1 ) <0 ) )

    {

      _gcry_mpi_release ( p_min1 );

      _gcry_mpi_release ( p );

      _gcry_mpi_release ( g );

      return GPG_ERR_INV_VALUE;

    }

  y = mpi_new (nbits);

  mpi_powm ( y, g, x, p );

  if ( DBG_CIPHER )

    {

      progress ('\n');

      log_mpidump ("elg  p", p );

      log_mpidump ("elg  g", g );

      log_mpidump ("elg  y", y );

      log_mpidump ("elg  x", x );

    }

  /* Copy the stuff to the key structures */

  sk->p = p;

  sk->g = g;

  sk->y = y;

  sk->x = mpi_copy (x);

  _gcry_mpi_release ( p_min1 );

  /* Now we can test our keys. */

  if ( test_keys ( sk, nbits - 64, 1 ) )

    {

      _gcry_mpi_release ( sk->p ); sk->p = NULL;

      _gcry_mpi_release ( sk->g ); sk->g = NULL;

      _gcry_mpi_release ( sk->y ); sk->y = NULL;

      _gcry_mpi_release ( sk->x ); sk->x = NULL;

      return GPG_ERR_BAD_SECKEY;

    }

  return 0;

}

/****************

 * Test whether the secret key is valid.

 * Returns: if this is a valid key.

 */

static int

check_secret_key( ELG_secret_key *sk )

{

  int rc;

  gcry_mpi_t y = mpi_alloc( mpi_get_nlimbs(sk->y) );

  mpi_powm (y, sk->g, sk->x, sk->p);

  rc = !mpi_cmp( y, sk->y );

  mpi_free( y );

  return rc;

}

static void

do_encrypt(gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input, ELG_public_key *pkey )

{

  gcry_mpi_t k;

  /* Note: maybe we should change the interface, so that it

   * is possible to check that input is < p and return an

   * error code.

   */

  k = gen_k( pkey->p );

  mpi_powm (a, pkey->g, k, pkey->p);

  /* b = (y^k * input) mod p

   *   = ((y^k mod p) * (input mod p)) mod p

   * and because input is < p

   *   = ((y^k mod p) * input) mod p

   */

  mpi_powm (b, pkey->y, k, pkey->p);

  mpi_mulm (b, b, input, pkey->p);

#if 0

  if( DBG_CIPHER )

    {

      log_mpidump("elg encrypted y", pkey->y);

      log_mpidump("elg encrypted p", pkey->p);

      log_mpidump("elg encrypted k", k);

      log_mpidump("elg encrypted M", input);

      log_mpidump("elg encrypted a", a);

      log_mpidump("elg encrypted b", b);

    }

#endif

  mpi_free(k);

}

static void

decrypt (gcry_mpi_t output, gcry_mpi_t a, gcry_mpi_t b, ELG_secret_key *skey )

{

  gcry_mpi_t t1, t2, r, r1, h;

  unsigned int nbits = mpi_get_nbits (skey->p);

  gcry_mpi_t x_blind;

  mpi_normalize (a);

  mpi_normalize (b);

  t1 = mpi_snew (nbits);

#ifdef USE_BLINDING

  t2 = mpi_snew (nbits);

  r  = mpi_new (nbits);

  r1 = mpi_new (nbits);

  h  = mpi_new (nbits);

  x_blind = mpi_snew (nbits);

  /* We need a random number of about the prime size.  The random

     number merely needs to be unpredictable; thus we use level 0.  */

  _gcry_mpi_randomize (r, nbits, GCRY_WEAK_RANDOM);

  /* Also, exponent blinding: x_blind = x + (p-1)*r1 */

  _gcry_mpi_randomize (r1, nbits, GCRY_WEAK_RANDOM);

  mpi_set_highbit (r1, nbits - 1);

  mpi_sub_ui (h, skey->p, 1);

  mpi_mul (x_blind, h, r1);

  mpi_add (x_blind, skey->x, x_blind);

  /* t1 = r^x mod p */

  mpi_powm (t1, r, x_blind, skey->p);

  /* t2 = (a * r)^-x mod p */

  mpi_mulm (t2, a, r, skey->p);

  mpi_powm (t2, t2, x_blind, skey->p);

  mpi_invm (t2, t2, skey->p);

  /* t1 = (t1 * t2) mod p*/

  mpi_mulm (t1, t1, t2, skey->p);

  mpi_free (x_blind);

  mpi_free (h);

  mpi_free (r1);

  mpi_free (r);

  mpi_free (t2);

#else /*!USE_BLINDING*/

  /* output = b/(a^x) mod p */

  mpi_powm (t1, a, skey->x, skey->p);

  mpi_invm (t1, t1, skey->p);

#endif /*!USE_BLINDING*/

  mpi_mulm (output, b, t1, skey->p);

#if 0

  if( DBG_CIPHER )

    {

      log_mpidump ("elg decrypted x", skey->x);

      log_mpidump ("elg decrypted p", skey->p);

      log_mpidump ("elg decrypted a", a);

      log_mpidump ("elg decrypted b", b);

      log_mpidump ("elg decrypted M", output);

    }

#endif

  mpi_free (t1);

}

/****************

 * Make an Elgamal signature out of INPUT

 */

static void

sign(gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input, ELG_secret_key *skey )

{

    gcry_mpi_t k;

    gcry_mpi_t t   = mpi_alloc( mpi_get_nlimbs(a) );

    gcry_mpi_t inv = mpi_alloc( mpi_get_nlimbs(a) );

    gcry_mpi_t p_1 = mpi_copy(skey->p);

   /*

    * b = (t * inv) mod (p-1)

    * b = (t * inv(k,(p-1),(p-1)) mod (p-1)

    * b = (((M-x*a) mod (p-1)) * inv(k,(p-1),(p-1))) mod (p-1)

    *

    */

    mpi_sub_ui(p_1, p_1, 1);

    k = gen_k( skey->p );

    mpi_powm( a, skey->g, k, skey->p );

    mpi_mul(t, skey->x, a );

    mpi_subm(t, input, t, p_1 );

    mpi_invm(inv, k, p_1 );

    mpi_mulm(b, t, inv, p_1 );

#if 0

    if( DBG_CIPHER )

      {

  log_mpidump ("elg sign p", skey->p);

  log_mpidump ("elg sign g", skey->g);

  log_mpidump ("elg sign y", skey->y);

  log_mpidump ("elg sign x", skey->x);

  log_mpidump ("elg sign k", k);

  log_mpidump ("elg sign M", input);

  log_mpidump ("elg sign a", a);

  log_mpidump ("elg sign b", b);

      }

#endif

    mpi_free(k);

    mpi_free(t);

    mpi_free(inv);

    mpi_free(p_1);

}

/****************

 * Returns true if the signature composed of A and B is valid.

 */

static int

verify(gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input, ELG_public_key *pkey )

{

  int rc;

  gcry_mpi_t t1;

  gcry_mpi_t t2;

  gcry_mpi_t base[4];

  gcry_mpi_t ex[4];

  if( !(mpi_cmp_ui( a, 0 ) > 0 && mpi_cmp( a, pkey->p ) < 0) )

    return 0; /* assertion  0 < a < p  failed */

  t1 = mpi_alloc( mpi_get_nlimbs(a) );

  t2 = mpi_alloc( mpi_get_nlimbs(a) );

#if 0

  /* t1 = (y^a mod p) * (a^b mod p) mod p */

  gcry_mpi_powm( t1, pkey->y, a, pkey->p );

  gcry_mpi_powm( t2, a, b, pkey->p );

  mpi_mulm( t1, t1, t2, pkey->p );

  /* t2 = g ^ input mod p */

  gcry_mpi_powm( t2, pkey->g, input, pkey->p );

  rc = !mpi_cmp( t1, t2 );

#elif 0

  /* t1 = (y^a mod p) * (a^b mod p) mod p */

  base[0] = pkey->y; ex[0] = a;

  base[1] = a;       ex[1] = b;

  base[2] = NULL;    ex[2] = NULL;

  mpi_mulpowm( t1, base, ex, pkey->p );

  /* t2 = g ^ input mod p */

  gcry_mpi_powm( t2, pkey->g, input, pkey->p );

  rc = !mpi_cmp( t1, t2 );

#else

  /* t1 = g ^ - input * y ^ a * a ^ b  mod p */

  mpi_invm(t2, pkey->g, pkey->p );

  base[0] = t2     ; ex[0] = input;

  base[1] = pkey->y; ex[1] = a;

  base[2] = a;       ex[2] = b;

  base[3] = NULL;    ex[3] = NULL;

  mpi_mulpowm( t1, base, ex, pkey->p );

  rc = !mpi_cmp_ui( t1, 1 );

#endif

  mpi_free(t1);

  mpi_free(t2);

  return rc;

}

/*********************************************

 **************  interface  ******************

 *********************************************/

static gpg_err_code_t

elg_generate (const gcry_sexp_t genparms, gcry_sexp_t *r_skey)

{

  gpg_err_code_t rc;

  unsigned int nbits;

  ELG_secret_key sk;

  gcry_mpi_t xvalue = NULL;

  gcry_sexp_t l1;

  gcry_mpi_t *factors = NULL;

  gcry_sexp_t misc_info = NULL;

  memset (&sk, 0, sizeof sk);

  rc = _gcry_pk_util_get_nbits (genparms, &nbits);

  if (rc)

    return rc;

  /* Parse the optional xvalue element. */

  l1 = sexp_find_token (genparms, "xvalue", 0);

  if (l1)

    {

      xvalue = sexp_nth_mpi (l1, 1, 0);

      sexp_release (l1);

      if (!xvalue)

        return GPG_ERR_BAD_MPI;

    }

  if (xvalue)

    {

      rc = generate_using_x (&sk, nbits, xvalue, &factors);

      mpi_free (xvalue);

    }

  else

    {

      rc = generate (&sk, nbits, &factors);

    }

  if (rc)

    goto leave;

  if (factors && factors[0])

    {

      int nfac;

      void **arg_list;

      char *buffer, *p;

      for (nfac = 0; factors[nfac]; nfac++)

        ;

      arg_list = xtrycalloc (nfac+1, sizeof *arg_list);

      if (!arg_list)

        {

          rc = gpg_err_code_from_syserror ();

          goto leave;

        }

      buffer = xtrymalloc (30 + nfac*2 + 2 + 1);

      if (!buffer)

        {

          rc = gpg_err_code_from_syserror ();

          xfree (arg_list);

          goto leave;

        }

      p = stpcpy (buffer, "(misc-key-info(pm1-factors");

      for(nfac = 0; factors[nfac]; nfac++)

        {

          p = stpcpy (p, "%m");

          arg_list[nfac] = factors + nfac;

        }

      p = stpcpy (p, "))");

      rc = sexp_build_array (&misc_info, NULL, buffer, arg_list);

      xfree (arg_list);

      xfree (buffer);

      if (rc)

        goto leave;

    }

  rc = sexp_build (r_skey, NULL,

                   "(key-data"

                   " (public-key"

                   "  (elg(p%m)(g%m)(y%m)))"

                   " (private-key"

                   "  (elg(p%m)(g%m)(y%m)(x%m)))"

                   " %S)",

                   sk.p, sk.g, sk.y,

                   sk.p, sk.g, sk.y, sk.x,

                   misc_info);

 leave:

  mpi_free (sk.p);

  mpi_free (sk.g);

  mpi_free (sk.y);

  mpi_free (sk.x);

  sexp_release (misc_info);

  if (factors)

    {

      gcry_mpi_t *mp;

      for (mp = factors; *mp; mp++)

        mpi_free (*mp);

      xfree (factors);

    }

  return rc;

}

static gcry_err_code_t

elg_check_secret_key (gcry_sexp_t keyparms)

{

  gcry_err_code_t rc;

  ELG_secret_key sk = {NULL, NULL, NULL, NULL};

  rc = sexp_extract_param (keyparms, NULL, "pgyx",

                           &sk.p, &sk.g, &sk.y, &sk.x,

                           NULL);

  if (rc)

    goto leave;

  if (!check_secret_key (&sk))

    rc = GPG_ERR_BAD_SECKEY;

 leave:

  _gcry_mpi_release (sk.p);

  _gcry_mpi_release (sk.g);

  _gcry_mpi_release (sk.y);

  _gcry_mpi_release (sk.x);

  if (DBG_CIPHER)

    log_debug ("elg_testkey    => %s\n", gpg_strerror (rc));

  return rc;

}

static gcry_err_code_t

elg_encrypt (gcry_sexp_t *r_ciph, gcry_sexp_t s_data, gcry_sexp_t keyparms)

{

  gcry_err_code_t rc;

  struct pk_encoding_ctx ctx;

  gcry_mpi_t mpi_a = NULL;

  gcry_mpi_t mpi_b = NULL;

  gcry_mpi_t data = NULL;

  ELG_public_key pk = { NULL, NULL, NULL };

  _gcry_pk_util_init_encoding_ctx (&ctx, PUBKEY_OP_ENCRYPT,

                                   elg_get_nbits (keyparms));

  /* Extract the data.  */

  rc = _gcry_pk_util_data_to_mpi (s_data, &data, &ctx);

  if (rc)

    goto leave;

  if (DBG_CIPHER)

    log_mpidump ("elg_encrypt data", data);

  if (mpi_is_opaque (data))

    {

      rc = GPG_ERR_INV_DATA;

      goto leave;

    }

  /* Extract the key.  */

  rc = sexp_extract_param (keyparms, NULL, "pgy",

                           &pk.p, &pk.g, &pk.y, NULL);

  if (rc)

    goto leave;

  if (DBG_CIPHER)

    {

      log_mpidump ("elg_encrypt  p", pk.p);

      log_mpidump ("elg_encrypt  g", pk.g);

      log_mpidump ("elg_encrypt  y", pk.y);

    }

  /* Do Elgamal computation and build result.  */

  mpi_a = mpi_new (0);

  mpi_b = mpi_new (0);

  do_encrypt (mpi_a, mpi_b, data, &pk);

  rc = sexp_build (r_ciph, NULL, "(enc-val(elg(a%m)(b%m)))", mpi_a, mpi_b);

 leave:

  _gcry_mpi_release (mpi_a);

  _gcry_mpi_release (mpi_b);

  _gcry_mpi_release (pk.p);

  _gcry_mpi_release (pk.g);

  _gcry_mpi_release (pk.y);

  _gcry_mpi_release (data);

  _gcry_pk_util_free_encoding_ctx (&ctx);

  if (DBG_CIPHER)

    log_debug ("elg_encrypt   => %s\n", gpg_strerror (rc));

  return rc;

}

static gcry_err_code_t

elg_decrypt (gcry_sexp_t *r_plain, gcry_sexp_t s_data, gcry_sexp_t keyparms)

{

  gpg_err_code_t rc, rc_sexp;

  struct pk_encoding_ctx ctx;

  gcry_sexp_t l1 = NULL;

  gcry_mpi_t data_a = NULL;

  gcry_mpi_t data_b = NULL;

  ELG_secret_key sk = {NULL, NULL, NULL, NULL};

  gcry_mpi_t plain = NULL;

  unsigned char *unpad = NULL;

  size_t unpadlen = 0;

  gcry_sexp_t result = NULL;

  gcry_sexp_t dummy = NULL;

  _gcry_pk_util_init_encoding_ctx (&ctx, PUBKEY_OP_DECRYPT,

                                   elg_get_nbits (keyparms));

  /* Extract the data.  */

  rc = _gcry_pk_util_preparse_encval (s_data, elg_names, &l1, &ctx);

  if (rc)

    goto leave;

  rc = sexp_extract_param (l1, NULL, "ab", &data_a, &data_b, NULL);

  if (rc)

    goto leave;

  if (DBG_CIPHER)

    {

      log_printmpi ("elg_decrypt  d_a", data_a);

      log_printmpi ("elg_decrypt  d_b", data_b);