libgcrypt-1.4.6/cipher/elgamal.c

/* Elgamal.c  -  Elgamal Public Key encryption
 * Copyright (C) 1998, 2000, 2001, 2002, 2003,
 *               2008  Free Software Foundation, Inc.
 *
 * 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"

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 int test_keys (ELG_secret_key *sk, unsigned int nbits, int nodie);
static gcry_mpi_t gen_k (gcry_mpi_t p, int small_k);
static void 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 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 = gcry_mpi_new ( 0 );
  gcry_mpi_t out1_a = gcry_mpi_new ( nbits );
  gcry_mpi_t out1_b = gcry_mpi_new ( nbits );
  gcry_mpi_t out2 = gcry_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.  With SMALL_K set, k will be selected for
 * better encryption performance - this must never be used signing!
 */
static gcry_mpi_t
gen_k( gcry_mpi_t p, int small_k )
{
  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;

  if (small_k)
    {
      /* Using a k much lesser than p is sufficient for encryption and
       * it greatly improves the encryption performance.  We use
       * Wiener's table and add a large safety margin. */
      nbits = wiener_map( orig_nbits ) * 3 / 2;
      if( nbits >= orig_nbits )
        BUG();
    }
  else
    nbits = orig_nbits;


  nbytes = (nbits+7)/8;
  if( DBG_CIPHER )
    log_debug("choosing a random k ");
  mpi_sub_ui( p_1, p, 1);
  for(;;) 
    {
      if( !rndbuf || nbits < 32 ) 
        {
          gcry_free(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 );
          gcry_free(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 (gcry_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:
  gcry_free(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 void
generate ( ELG_secret_key *sk, unsigned int nbits, gcry_mpi_t **ret_factors )
{
  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 = gcry_mpi_new ( nbits );
  qbits = wiener_map( nbits );
  if( qbits & 1 ) /* better have a even one */
    qbits++;
  g = mpi_alloc(1);
  p = _gcry_generate_elg_prime( 0, nbits, qbits, g, ret_factors );
  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 = gcry_mpi_snew ( xbits );
  if( DBG_CIPHER )
    log_debug("choosing a random x of size %u", xbits );
  rndbuf = NULL;
  do 
    {
      if( DBG_CIPHER )
        progress('.');
      if( rndbuf )
        { /* Change only some of the higher bits */
          if( xbits < 16 ) /* should never happen ... */
            {
              gcry_free(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 );
              gcry_free(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 ) );
  gcry_free(rndbuf);

  y = gcry_mpi_new (nbits);
  gcry_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 );
}


/* 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_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 = gcry_mpi_new ( nbits );
  qbits  = wiener_map ( nbits );
  if ( (qbits & 1) ) /* Better have an even one.  */
    qbits++;
  g = mpi_alloc (1);
  p = _gcry_generate_elg_prime ( 0, nbits, qbits, g, ret_factors );
  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 = gcry_mpi_new (nbits);
  gcry_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 = gcry_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) );

  gcry_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, 1 );
  gcry_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
   */
  gcry_mpi_powm( b, pkey->y, k, pkey->p );
  gcry_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 = mpi_alloc_secure( mpi_get_nlimbs( skey->p ) );

  /* output = b/(a^x) mod p */
  gcry_mpi_powm( t1, a, skey->x, skey->p );
  mpi_invm( t1, t1, skey->p );
  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, 0 /* no small K ! */ );
    gcry_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_ext (int algo, unsigned int nbits, unsigned long evalue,
                  const gcry_sexp_t genparms,
                  gcry_mpi_t *skey, gcry_mpi_t **retfactors,
                  gcry_sexp_t *r_extrainfo)
{
  gpg_err_code_t ec;
  ELG_secret_key sk;
  gcry_mpi_t xvalue = NULL;
  gcry_sexp_t l1;

  (void)algo;
  (void)evalue;
  (void)r_extrainfo;

  if (genparms)
    {
      /* Parse the optional xvalue element. */
      l1 = gcry_sexp_find_token (genparms, "xvalue", 0);
      if (l1)
        {
          xvalue = gcry_sexp_nth_mpi (l1, 1, 0);
          gcry_sexp_release (l1);
          if (!xvalue)
            return GPG_ERR_BAD_MPI;
        }
    }

  if (xvalue)
    ec = generate_using_x (&sk, nbits, xvalue, retfactors);
  else
    {
      generate (&sk, nbits, retfactors);
      ec = 0;
    }

  skey[0] = sk.p;
  skey[1] = sk.g;
  skey[2] = sk.y;
  skey[3] = sk.x;
  
  return ec;
}


static gcry_err_code_t
elg_generate (int algo, unsigned int nbits, unsigned long evalue,
              gcry_mpi_t *skey, gcry_mpi_t **retfactors)
{
  ELG_secret_key sk;

  (void)algo;
  (void)evalue;

  generate (&sk, nbits, retfactors);
  skey[0] = sk.p;
  skey[1] = sk.g;
  skey[2] = sk.y;
  skey[3] = sk.x;
  
  return GPG_ERR_NO_ERROR;
}


static gcry_err_code_t
elg_check_secret_key (int algo, gcry_mpi_t *skey)
{
  gcry_err_code_t err = GPG_ERR_NO_ERROR;
  ELG_secret_key sk;

  (void)algo;

  if ((! skey[0]) || (! skey[1]) || (! skey[2]) || (! skey[3]))
    err = GPG_ERR_BAD_MPI;
  else
    {
      sk.p = skey[0];
      sk.g = skey[1];
      sk.y = skey[2];
      sk.x = skey[3];
      
      if (! check_secret_key (&sk))
	err = GPG_ERR_BAD_SECKEY;
    }

  return err;
}


static gcry_err_code_t
elg_encrypt (int algo, gcry_mpi_t *resarr,
             gcry_mpi_t data, gcry_mpi_t *pkey, int flags)
{
  gcry_err_code_t err = GPG_ERR_NO_ERROR;
  ELG_public_key pk;

  (void)algo;
  (void)flags;

  if ((! data) || (! pkey[0]) || (! pkey[1]) || (! pkey[2]))
    err = GPG_ERR_BAD_MPI;
  else
    {
      pk.p = pkey[0];
      pk.g = pkey[1];
      pk.y = pkey[2];
      resarr[0] = mpi_alloc (mpi_get_nlimbs (pk.p));
      resarr[1] = mpi_alloc (mpi_get_nlimbs (pk.p));
      do_encrypt (resarr[0], resarr[1], data, &pk);
    }
  return err;
}


static gcry_err_code_t
elg_decrypt (int algo, gcry_mpi_t *result,
             gcry_mpi_t *data, gcry_mpi_t *skey, int flags)
{
  gcry_err_code_t err = GPG_ERR_NO_ERROR;
  ELG_secret_key sk;

  (void)algo;
  (void)flags;

  if ((! data[0]) || (! data[1])
      || (! skey[0]) || (! skey[1]) || (! skey[2]) || (! skey[3]))
    err = GPG_ERR_BAD_MPI;
  else
    {
      sk.p = skey[0];
      sk.g = skey[1];
      sk.y = skey[2];
      sk.x = skey[3];
      *result = mpi_alloc_secure (mpi_get_nlimbs (sk.p));
      decrypt (*result, data[0], data[1], &sk);
    }
  return err;
}


static gcry_err_code_t
elg_sign (int algo, gcry_mpi_t *resarr, gcry_mpi_t data, gcry_mpi_t *skey)
{
  gcry_err_code_t err = GPG_ERR_NO_ERROR;
  ELG_secret_key sk;

  (void)algo;

  if ((! data)
      || (! skey[0]) || (! skey[1]) || (! skey[2]) || (! skey[3]))
    err = GPG_ERR_BAD_MPI;
  else
    {
      sk.p = skey[0];
      sk.g = skey[1];
      sk.y = skey[2];
      sk.x = skey[3];
      resarr[0] = mpi_alloc (mpi_get_nlimbs (sk.p));
      resarr[1] = mpi_alloc (mpi_get_nlimbs (sk.p));
      sign (resarr[0], resarr[1], data, &sk);
    }
  
  return err;
}


static gcry_err_code_t
elg_verify (int algo, gcry_mpi_t hash, gcry_mpi_t *data, gcry_mpi_t *pkey,
            int (*cmp) (void *, gcry_mpi_t), void *opaquev)
{
  gcry_err_code_t err = GPG_ERR_NO_ERROR;
  ELG_public_key pk;

  (void)algo;
  (void)cmp;
  (void)opaquev;

  if ((! data[0]) || (! data[1]) || (! hash)
      || (! pkey[0]) || (! pkey[1]) || (! pkey[2]))
    err = GPG_ERR_BAD_MPI;
  else
    {
      pk.p = pkey[0];
      pk.g = pkey[1];
      pk.y = pkey[2];
      if (! verify (data[0], data[1], hash, &pk))
	err = GPG_ERR_BAD_SIGNATURE;
    }

  return err;
}


static unsigned int
elg_get_nbits (int algo, gcry_mpi_t *pkey)
{
  (void)algo;

  return mpi_get_nbits (pkey[0]);
}


static const char *elg_names[] =
  {
    "elg",
    "openpgp-elg",
    "openpgp-elg-sig",
    NULL,
  };


gcry_pk_spec_t _gcry_pubkey_spec_elg =
  {
    "ELG", elg_names,
    "pgy", "pgyx", "ab", "rs", "pgy",
    GCRY_PK_USAGE_SIGN | GCRY_PK_USAGE_ENCR,
    elg_generate,
    elg_check_secret_key,
    elg_encrypt,
    elg_decrypt,
    elg_sign,
    elg_verify,
    elg_get_nbits
  };

pk_extra_spec_t _gcry_pubkey_extraspec_elg = 
  {
    NULL,
    elg_generate_ext,
    NULL
  };