path: root/agent/cvt-openpgp.c

/* cvt-openpgp.c - Convert an OpenPGP key to our internal format.
 * Copyright (C) 1998, 1999, 2000, 2001, 2002, 2006, 2009,
 *               2010 Free Software Foundation, Inc.
 *
 * 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/>.
 */

#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>

#include "agent.h"
#include "i18n.h"
#include "cvt-openpgp.h"


/* Helper to pass data via the callback to do_unprotect. */
struct try_do_unprotect_arg_s 
{
  int  is_v4;
  int  is_protected;
  int  pubkey_algo;
  int  protect_algo;
  char *iv;
  int  ivlen;
  int  s2k_mode;
  int  s2k_algo;
  byte *s2k_salt;
  u32  s2k_count;
  u16 desired_csum;
  gcry_mpi_t *skey;
  size_t skeysize;
  int skeyidx;
  gcry_sexp_t *r_key;
};



/* Compute the keygrip from the public key and store it at GRIP.  */
static gpg_error_t
get_keygrip (int pubkey_algo, gcry_mpi_t *pkey, unsigned char *grip)
{
  gpg_error_t err;
  gcry_sexp_t s_pkey = NULL;

  switch (pubkey_algo)
    {
    case GCRY_PK_DSA:
      err = gcry_sexp_build (&s_pkey, NULL,
                             "(public-key(dsa(p%m)(q%m)(g%m)(y%m)))",
                             pkey[0], pkey[1], pkey[2], pkey[3]);
      break;

    case GCRY_PK_ELG:
    case GCRY_PK_ELG_E:
      err = gcry_sexp_build (&s_pkey, NULL,
                             "(public-key(elg(p%m)(g%m)(y%m)))",
                             pkey[0], pkey[1], pkey[2]);
      break;

    case GCRY_PK_RSA:
    case GCRY_PK_RSA_E:
    case GCRY_PK_RSA_S:
      err = gcry_sexp_build (&s_pkey, NULL,
                             "(public-key(rsa(n%m)(e%m)))", pkey[0], pkey[1]);
      break;

    default:
      err = gpg_error (GPG_ERR_PUBKEY_ALGO);
      break;
    }

  if (!err && !gcry_pk_get_keygrip (s_pkey, grip))
    err = gpg_error (GPG_ERR_INTERNAL);

  gcry_sexp_release (s_pkey);
  return err;
}


/* Convert a secret key given as algorithm id and an array of key
   parameters into our s-expression based format.  */
static gpg_error_t
convert_secret_key (gcry_sexp_t *r_key, int pubkey_algo, gcry_mpi_t *skey)
{
  gpg_error_t err;
  gcry_sexp_t s_skey = NULL;

  *r_key = NULL;

  switch (pubkey_algo)
    {
    case GCRY_PK_DSA:
      err = gcry_sexp_build (&s_skey, NULL,
                             "(private-key(dsa(p%m)(q%m)(g%m)(y%m)(x%m)))",
                             skey[0], skey[1], skey[2], skey[3], skey[4]);
      break;

    case GCRY_PK_ELG:
    case GCRY_PK_ELG_E:
      err = gcry_sexp_build (&s_skey, NULL,
                             "(private-key(elg(p%m)(g%m)(y%m)(x%m)))",
                             skey[0], skey[1], skey[2], skey[3]);
      break;


    case GCRY_PK_RSA:
    case GCRY_PK_RSA_E:
    case GCRY_PK_RSA_S:
      err = gcry_sexp_build (&s_skey, NULL,
                             "(private-key(rsa(n%m)(e%m)(d%m)(p%m)(q%m)(u%m)))",
                             skey[0], skey[1], skey[2], skey[3], skey[4],
                             skey[5]);

    default:
      err = gpg_error (GPG_ERR_PUBKEY_ALGO);
      break;
    }

  if (!err)
    *r_key = s_skey;
  return err;
}



/* Hash the passphrase and set the key. */
static gpg_error_t
hash_passphrase_and_set_key (const char *passphrase,
                             gcry_cipher_hd_t hd, int protect_algo,
                             int s2k_mode, int s2k_algo,
                             byte *s2k_salt, u32 s2k_count)
{
  gpg_error_t err;
  unsigned char *key;
  size_t keylen;

  keylen = gcry_cipher_get_algo_keylen (protect_algo);
  if (!keylen)
    return gpg_error (GPG_ERR_INTERNAL);
  
  key = xtrymalloc_secure (keylen);
  if (!key)
    return gpg_error_from_syserror ();

  err = s2k_hash_passphrase (passphrase,
                             s2k_algo, s2k_mode, s2k_salt, s2k_count,
                             key, keylen);
  if (!err)
    err = gcry_cipher_setkey (hd, key, keylen);

  xfree (key);
  return err;
}


static u16
checksum (const unsigned char *p, unsigned int n)
{
  u16 a;
  
  for (a=0; n; n-- )
    a += *p++;
  return a;
}


/* Note that this function modified SKEY.  SKEYSIZE is the allocated
   size of the array including the NULL item; this is used for a
   bounds check.  On success a converted key is stored at R_KEY.  */
static int
do_unprotect (const char *passphrase,
              int pkt_version, int pubkey_algo, int is_protected,
              gcry_mpi_t *skey, size_t skeysize,
              int protect_algo, void *protect_iv, size_t protect_ivlen,
              int s2k_mode, int s2k_algo, byte *s2k_salt, u32 s2k_count,
              u16 desired_csum, gcry_sexp_t *r_key)
{
  gpg_error_t err;
  size_t npkey, nskey, skeylen;
  gcry_cipher_hd_t cipher_hd = NULL;
  u16 actual_csum;
  size_t nbytes;
  int i;
  gcry_mpi_t tmpmpi;

  *r_key = NULL;

  /* Count the actual number of MPIs is in the array and set the
     remainder to NULL for easier processing later on.  */
  for (skeylen = 0; skey[skeylen]; skeylen++)
    ;
  for (i=skeylen; i < skeysize; i++)
    skey[i] = NULL;

  /* Check some args.  */
  if (s2k_mode == 1001)
    {
      /* Stub key.  */
      log_info (_("secret key parts are not available\n"));
      return gpg_error (GPG_ERR_UNUSABLE_SECKEY);
    }

  if (gcry_pk_test_algo (pubkey_algo))
    {
      /* The algorithm numbers are Libgcrypt numbers but fortunately
         the OpenPGP algorithm numbers map one-to-one to the Libgcrypt
         numbers.  */
      log_info (_("public key algorithm %d (%s) is not supported\n"),
                pubkey_algo, gcry_pk_algo_name (pubkey_algo));
      return gpg_error (GPG_ERR_PUBKEY_ALGO);
    }

  /* Get properties of the public key algorithm and do some
     consistency checks.  Note that we need at least NPKEY+1 elements
     in the SKEY array. */
  if ( (err = gcry_pk_algo_info (pubkey_algo, GCRYCTL_GET_ALGO_NPKEY,
                                 NULL, &npkey))
       || (err = gcry_pk_algo_info (pubkey_algo, GCRYCTL_GET_ALGO_NSKEY,
                                    NULL, &nskey)))
    return err;
  if (!npkey || npkey >= nskey)
    return gpg_error (GPG_ERR_INTERNAL);
  if (skeylen <= npkey)
    return gpg_error (GPG_ERR_MISSING_VALUE);
  if (nskey+1 >= skeysize)
    return gpg_error (GPG_ERR_BUFFER_TOO_SHORT);
  
  /* Check whether SKEY is at all protected.  If it is not protected
     merely verify the checksum.  */
  if (!is_protected)
    {
      unsigned char *buffer;

      actual_csum = 0;
      for (i=npkey; i < nskey; i++)
        {
          if (!skey[i] || gcry_mpi_get_flag (skey[i], GCRYMPI_FLAG_OPAQUE))
            return gpg_error (GPG_ERR_BAD_SECKEY);
          
          err = gcry_mpi_print (GCRYMPI_FMT_PGP, NULL, 0, &nbytes, skey[i]);
          if (!err)
            {
              buffer = (gcry_is_secure (skey[i])?
                        xtrymalloc_secure (nbytes) : xtrymalloc (nbytes));
              if (!buffer)
                return gpg_error_from_syserror ();
              err = gcry_mpi_print (GCRYMPI_FMT_PGP, buffer, nbytes,
                                    NULL, skey[i]);
              if (!err)
                actual_csum += checksum (buffer, nbytes);
              xfree (buffer);
            }
          if (err)
            return err;
        }
      
      if (actual_csum != desired_csum)
        return gpg_error (GPG_ERR_CHECKSUM);
      return 0;
    }


  if (gcry_cipher_test_algo (protect_algo))
    {
      /* The algorithm numbers are Libgcrypt numbers but fortunately
         the OpenPGP algorithm numbers map one-to-one to the Libgcrypt
         numbers.  */
      log_info (_("protection algorithm %d (%s) is not supported\n"),
                protect_algo, gcry_cipher_algo_name (protect_algo));
      return gpg_error (GPG_ERR_CIPHER_ALGO);
    }

  if (gcry_md_test_algo (s2k_algo))
    {
      log_info (_("protection hash algorithm %d (%s) is not supported\n"),
                s2k_algo, gcry_md_algo_name (s2k_algo));
      return gpg_error (GPG_ERR_DIGEST_ALGO);
    }
  
  err = gcry_cipher_open (&cipher_hd, protect_algo,
                          GCRY_CIPHER_MODE_CFB,
                          (GCRY_CIPHER_SECURE
                           | (protect_algo >= 100 ?
                              0 : GCRY_CIPHER_ENABLE_SYNC)));
  if (err)
    {
      log_error ("failed to open cipher_algo %d: %s\n",
                 protect_algo, gpg_strerror (err));
      return err;
    }

  err = hash_passphrase_and_set_key (passphrase, cipher_hd, protect_algo,
                                     s2k_mode, s2k_algo, s2k_salt, s2k_count);
  if (err)
    {
      gcry_cipher_close (cipher_hd);
      return err;
    }  

  gcry_cipher_setiv (cipher_hd, protect_iv, protect_ivlen);
  
  actual_csum = 0;
  if (pkt_version >= 4)
    {
      int ndata;
      unsigned int ndatabits;
      unsigned char *p, *data;
      u16 csum_pgp7 = 0;

      if (!gcry_mpi_get_flag (skey[npkey], GCRYMPI_FLAG_OPAQUE ))
        {
          gcry_cipher_close (cipher_hd);
          return gpg_error (GPG_ERR_BAD_SECKEY);
        }
      p = gcry_mpi_get_opaque (skey[npkey], &ndatabits);
      ndata = (ndatabits+7)/8;

      if (ndata > 1)
        csum_pgp7 = p[ndata-2] << 8 | p[ndata-1];
      data = xtrymalloc_secure (ndata);
      if (!data)
        {
          err = gpg_error_from_syserror ();
          gcry_cipher_close (cipher_hd);
          return err;
        }
      gcry_cipher_decrypt (cipher_hd, data, ndata, p, ndata);

      p = data;
      if (is_protected == 2)
        {
          /* This is the new SHA1 checksum method to detect tampering
             with the key as used by the Klima/Rosa attack.  */
          desired_csum = 0; 
          actual_csum = 1;  /* Default to bad checksum.  */

          if (ndata < 20) 
            log_error ("not enough bytes for SHA-1 checksum\n");
          else 
            {
              gcry_md_hd_t h;
              
              if (gcry_md_open (&h, GCRY_MD_SHA1, 1))
                BUG(); /* Algo not available. */
              gcry_md_write (h, data, ndata - 20);
              gcry_md_final (h);
              if (!memcmp (gcry_md_read (h, GCRY_MD_SHA1), data+ndata-20, 20))
                actual_csum = 0; /* Digest does match.  */
              gcry_md_close (h);
            }
        }
      else 
        {
          /* Old 16 bit checksum method.  */
          if (ndata < 2)
            {
              log_error ("not enough bytes for checksum\n");
              desired_csum = 0; 
              actual_csum = 1;  /* Mark checksum bad.  */
            }
          else
            {
              desired_csum = (data[ndata-2] << 8 | data[ndata-1]);
              actual_csum = checksum (data, ndata-2);
              if (desired_csum != actual_csum)
                {
                  /* This is a PGP 7.0.0 workaround */
                  desired_csum = csum_pgp7; /* Take the encrypted one.  */
                }
            }
        }
      
      /* Better check it here.  Otherwise the gcry_mpi_scan would fail
         because the length may have an arbitrary value.  */
      if (desired_csum == actual_csum)
        {
          for (i=npkey; i < nskey; i++ )
            {
              if (gcry_mpi_scan (&tmpmpi, GCRYMPI_FMT_PGP, p, ndata, &nbytes))
                {
                  /* Checksum was okay, but not correctly decrypted.  */
                  desired_csum = 0;
                  actual_csum = 1;   /* Mark checksum bad.  */
                  break;
                }
              gcry_mpi_release (skey[i]);
              skey[i] = tmpmpi;
              ndata -= nbytes;
              p += nbytes;
            }
          skey[i] = NULL;
          skeylen = i;
          assert (skeylen <= skeysize);

          /* Note: at this point NDATA should be 2 for a simple
             checksum or 20 for the sha1 digest.  */
        }
      xfree(data);
    }
  else /* Packet version <= 3.  */
    {
      unsigned char *buffer;

      for (i = npkey; i < nskey; i++)
        {
          unsigned char *p;
          size_t ndata;
          unsigned int ndatabits;

          if (!skey[i] || !gcry_mpi_get_flag (skey[i], GCRYMPI_FLAG_OPAQUE))
            {
              gcry_cipher_close (cipher_hd);
              return gpg_error (GPG_ERR_BAD_SECKEY);
            }
          p = gcry_mpi_get_opaque (skey[i], &ndatabits);
          ndata = (ndatabits+7)/8;

          if (!(ndata >= 2) || !(ndata == ((p[0] << 8 | p[1]) + 7)/8 + 2))
            {
              gcry_cipher_close (cipher_hd);
              return gpg_error (GPG_ERR_BAD_SECKEY);
            }
          
          buffer = xtrymalloc_secure (ndata);
          if (!buffer)
            {
              err = gpg_error_from_syserror ();
              gcry_cipher_close (cipher_hd);
              return err;
            }
              
          gcry_cipher_sync (cipher_hd);
          buffer[0] = p[0];
          buffer[1] = p[1];
          gcry_cipher_decrypt (cipher_hd, buffer+2, ndata-2, p+2, ndata-2);
          actual_csum += checksum (buffer, ndata);
          err = gcry_mpi_scan (&tmpmpi, GCRYMPI_FMT_PGP, buffer, ndata, &ndata);
          xfree (buffer);
          if (err)
            {
              /* Checksum was okay, but not correctly decrypted.  */
              desired_csum = 0;
              actual_csum = 1;   /* Mark checksum bad.  */
              break;
            }
          gcry_mpi_release (skey[i]);
          skey[i] = tmpmpi;
        }
    }
  gcry_cipher_close (cipher_hd);

  /* Now let's see whether we have used the correct passphrase. */
  if (actual_csum != desired_csum)
    return gpg_error (GPG_ERR_BAD_PASSPHRASE);

  if (nskey != skeylen)
    err = gpg_error (GPG_ERR_BAD_SECKEY);
  else
    err = convert_secret_key (r_key, pubkey_algo, skey);
  if (err)
    return err;

  /* The checksum may fail, thus we also check the key itself.  */
  err = gcry_pk_testkey (*r_key);
  if (err)
    {
      gcry_sexp_release (*r_key);
      *r_key = NULL;
      return gpg_error (GPG_ERR_BAD_PASSPHRASE);
    }

  return 0;
}


/* Callback function to try the unprotection from the passpharse query
   code.  */
static int
try_do_unprotect_cb (struct pin_entry_info_s *pi)
{
  gpg_error_t err;
  struct try_do_unprotect_arg_s *arg = pi->check_cb_arg;

  err = do_unprotect (pi->pin,
                      arg->is_v4? 4:3,
                      arg->pubkey_algo, arg->is_protected,
                      arg->skey, arg->skeysize,
                      arg->protect_algo, arg->iv, arg->ivlen,
                      arg->s2k_mode, arg->s2k_algo,
                      arg->s2k_salt, arg->s2k_count,
                      arg->desired_csum, arg->r_key);
  /* SKEY may be modified now, thus we need to re-compute SKEYIDX.  */
  for (arg->skeyidx = 0; (arg->skeyidx < arg->skeysize
                          && arg->skey[arg->skeyidx]); arg->skeyidx++)
    ;
  return err;
}


/* Convert an OpenPGP transfer key into our internal format.  Before
   asking for a passphrase we check whether the key already exists in
   our key storage.  S_PGP is the OpenPGP key in transfer format.  If
   CACHE_NONCE is given the passphrase will be looked up in the cache.
   On success R_KEY will receive a canonical encoded S-expression with
   the unprotected key in our internal format; the caller needs to
   release that memory.  The passphrase used to decrypt the OpenPGP
   key will be returned at R_PASSPHRASE; the caller must release this
   passphrase.  The keygrip will be stored at the 20 byte buffer
   pointed to by GRIP.  On error NULL is stored at all return
   arguments.  */
gpg_error_t
convert_openpgp (ctrl_t ctrl, gcry_sexp_t s_pgp, 
                 unsigned char *grip, const char *prompt,
                 const char *cache_nonce,
                 unsigned char **r_key, char **r_passphrase)
{
  gpg_error_t err;
  gcry_sexp_t top_list;
  gcry_sexp_t list = NULL;
  const char *value;
  size_t valuelen;
  char *string;
  int  idx;
  int  is_v4, is_protected;
  int  pubkey_algo;
  int  protect_algo = 0;
  char iv[16];
  int  ivlen = 0;
  int  s2k_mode = 0;
  int  s2k_algo = 0;
  byte s2k_salt[8];
  u32  s2k_count = 0;
  size_t npkey, nskey;
  gcry_mpi_t skey[10];  /* We support up to 9 parameters.  */
  u16 desired_csum;
  int skeyidx = 0;
  gcry_sexp_t s_skey;
  struct pin_entry_info_s *pi;
  struct try_do_unprotect_arg_s pi_arg;

  *r_key = NULL;
  *r_passphrase = NULL;

  top_list = gcry_sexp_find_token (s_pgp, "openpgp-private-key", 0);
  if (!top_list)
    goto bad_seckey;

  list = gcry_sexp_find_token (top_list, "version", 0);
  if (!list)
    goto bad_seckey;
  value = gcry_sexp_nth_data (list, 1, &valuelen);
  if (!value || valuelen != 1 || !(value[0] == '3' || value[0] == '4'))
    goto bad_seckey;
  is_v4 = (value[0] == '4');

  gcry_sexp_release (list);
  list = gcry_sexp_find_token (top_list, "protection", 0);
  if (!list)
    goto bad_seckey;
  value = gcry_sexp_nth_data (list, 1, &valuelen);
  if (!value)
    goto bad_seckey;
  if (valuelen == 4 && !memcmp (value, "sha1", 4))
    is_protected = 2;
  else if (valuelen == 3 && !memcmp (value, "sum", 3))
    is_protected = 1;
  else if (valuelen == 4 && !memcmp (value, "none", 4))
    is_protected = 0;
  else
    goto bad_seckey;
  if (is_protected)
    {
      string = gcry_sexp_nth_string (list, 2);
      if (!string)
        goto bad_seckey;
      protect_algo = gcry_cipher_map_name (string);
      if (!protect_algo && !!strcmp (string, "IDEA"))
        protect_algo = GCRY_CIPHER_IDEA;
      xfree (string);
      
      value = gcry_sexp_nth_data (list, 3, &valuelen);
      if (!value || !valuelen || valuelen > sizeof iv)
        goto bad_seckey;
      memcpy (iv, value, valuelen);
      ivlen = valuelen;

      string = gcry_sexp_nth_string (list, 4);
      if (!string)
        goto bad_seckey;
      s2k_mode = strtol (string, NULL, 10);
      xfree (string);

      string = gcry_sexp_nth_string (list, 5);
      if (!string)
        goto bad_seckey;
      s2k_algo = gcry_md_map_name (string);
      xfree (string);

      value = gcry_sexp_nth_data (list, 6, &valuelen);
      if (!value || !valuelen || valuelen > sizeof s2k_salt)
        goto bad_seckey;
      memcpy (s2k_salt, value, valuelen);

      string = gcry_sexp_nth_string (list, 7);
      if (!string)
        goto bad_seckey;
      s2k_count = strtoul (string, NULL, 10);
      xfree (string);
    }

  gcry_sexp_release (list);
  list = gcry_sexp_find_token (top_list, "algo", 0);
  if (!list)
    goto bad_seckey;
  string = gcry_sexp_nth_string (list, 1);
  if (!string)
    goto bad_seckey;
  pubkey_algo = gcry_pk_map_name (string);
  xfree (string);

  if (gcry_pk_algo_info (pubkey_algo, GCRYCTL_GET_ALGO_NPKEY, NULL, &npkey)
      || gcry_pk_algo_info (pubkey_algo, GCRYCTL_GET_ALGO_NSKEY, NULL, &nskey)
      || !npkey || npkey >= nskey)
    goto bad_seckey;

  gcry_sexp_release (list);
  list = gcry_sexp_find_token (top_list, "skey", 0);
  if (!list)
    goto bad_seckey;
  for (idx=0;;)
    {
      int is_enc;

      value = gcry_sexp_nth_data (list, ++idx, &valuelen);
      if (!value && skeyidx >= npkey)
        break;  /* Ready.  */

      /* Check for too many parameters.  Note that depending on the
         protection mode and version number we may see less than NSKEY
         (but at least NPKEY+1) parameters.  */
      if (idx >= 2*nskey)
        goto bad_seckey;
      if (skeyidx >= DIM (skey)-1)
        goto bad_seckey;

      if (!value || valuelen != 1 || !(value[0] == '_' || value[0] == 'e'))
        goto bad_seckey;
      is_enc = (value[0] == 'e');
      value = gcry_sexp_nth_data (list, ++idx, &valuelen);
      if (!value || !valuelen)
        goto bad_seckey;
      if (is_enc)
        {
          void *p = xtrymalloc (valuelen);
          if (!p)
            goto outofmem;
          memcpy (p, value, valuelen);
          skey[skeyidx] = gcry_mpi_set_opaque (NULL, p, valuelen*8);
          if (!skey[skeyidx])
            goto outofmem;
        }
      else
        {
          if (gcry_mpi_scan (skey + skeyidx, GCRYMPI_FMT_STD,
                             value, valuelen, NULL))
            goto bad_seckey;
        }
      skeyidx++;
    }
  skey[skeyidx++] = NULL;

  gcry_sexp_release (list);
  list = gcry_sexp_find_token (top_list, "csum", 0);
  if (list)
    {
      string = gcry_sexp_nth_string (list, 1);
      if (!string)
        goto bad_seckey;
      desired_csum = strtoul (string, NULL, 10);
      xfree (string);
    }
  else
    desired_csum = 0;


  gcry_sexp_release (list); list = NULL;
  gcry_sexp_release (top_list); top_list = NULL;

  /* log_debug ("XXX is_v4=%d\n", is_v4); */
  /* log_debug ("XXX pubkey_algo=%d\n", pubkey_algo); */
  /* log_debug ("XXX is_protected=%d\n", is_protected); */
  /* log_debug ("XXX protect_algo=%d\n", protect_algo); */
  /* log_printhex ("XXX iv", iv, ivlen); */
  /* log_debug ("XXX ivlen=%d\n", ivlen); */
  /* log_debug ("XXX s2k_mode=%d\n", s2k_mode); */
  /* log_debug ("XXX s2k_algo=%d\n", s2k_algo); */
  /* log_printhex ("XXX s2k_salt", s2k_salt, sizeof s2k_salt); */
  /* log_debug ("XXX s2k_count=%lu\n", (unsigned long)s2k_count); */
  /* for (idx=0; skey[idx]; idx++) */
  /*   { */
  /*     int is_enc = gcry_mpi_get_flag (skey[idx], GCRYMPI_FLAG_OPAQUE); */
  /*     log_info ("XXX skey[%d]%s:", idx, is_enc? " (enc)":""); */
  /*     if (is_enc) */
  /*       { */
  /*         void *p; */
  /*         unsigned int nbits; */
  /*         p = gcry_mpi_get_opaque (skey[idx], &nbits); */
  /*         log_printhex (NULL, p, (nbits+7)/8); */
  /*       } */
  /*     else */
  /*       gcry_mpi_dump (skey[idx]); */
  /*     log_printf ("\n"); */
  /*   } */

  err = get_keygrip (pubkey_algo, skey, grip);
  if (err)
    goto leave;

  if (!agent_key_available (grip))
    {
      err = gpg_error (GPG_ERR_EEXIST);
      goto leave;
    }

  pi = xtrycalloc_secure (1, sizeof (*pi) + 100);
  if (!pi)
    return gpg_error_from_syserror ();
  pi->max_length = 100;
  pi->min_digits = 0;  /* We want a real passphrase.  */
  pi->max_digits = 16;
  pi->max_tries = 3;
  pi->check_cb = try_do_unprotect_cb;
  pi->check_cb_arg = &pi_arg;
  pi_arg.is_v4 = is_v4;
  pi_arg.is_protected = is_protected;
  pi_arg.pubkey_algo = pubkey_algo;
  pi_arg.protect_algo = protect_algo;
  pi_arg.iv = iv;
  pi_arg.ivlen = ivlen;
  pi_arg.s2k_mode = s2k_mode;
  pi_arg.s2k_algo = s2k_algo;
  pi_arg.s2k_salt = s2k_salt;
  pi_arg.s2k_count = s2k_count;
  pi_arg.desired_csum = desired_csum;
  pi_arg.skey = skey;
  pi_arg.skeysize = DIM (skey);
  pi_arg.skeyidx = skeyidx;
  pi_arg.r_key = &s_skey;

  err = gpg_error (GPG_ERR_BAD_PASSPHRASE);
  if (cache_nonce)
    {
      void *cache_marker = NULL;
      const char *cache_value;

      cache_value = agent_get_cache (cache_nonce, CACHE_MODE_NONCE,
                                     &cache_marker);
      if (cache_value)
        {
          if (strlen (cache_value) < pi->max_length)
            strcpy (pi->pin, cache_value);
          agent_unlock_cache_entry (&cache_marker);
        }
      if (*pi->pin)
        err = try_do_unprotect_cb (pi);
    }
  if (gpg_err_code (err) == GPG_ERR_BAD_PASSPHRASE)
    err = agent_askpin (ctrl, prompt, NULL, NULL, pi);
  skeyidx = pi_arg.skeyidx;
  if (!err)
    {
      *r_passphrase = xtrystrdup (pi->pin);
      if (!*r_passphrase)
        err = gpg_error_from_syserror ();
    }
  xfree (pi);
  if (err)
    goto leave;

  /* Save some memory and get rid of the SKEY array now.  */
  for (idx=0; idx < skeyidx; idx++)
    gcry_mpi_release (skey[idx]);
  skeyidx = 0;

  /* Note that the padding is not required - we use it only because
     that function allows us to created the result in secure memory.  */
  err = make_canon_sexp_pad (s_skey, 1, r_key, NULL);
  gcry_sexp_release (s_skey);

 leave:
  gcry_sexp_release (list);
  gcry_sexp_release (top_list);
  for (idx=0; idx < skeyidx; idx++)
    gcry_mpi_release (skey[idx]);
  if (err)
    {
      xfree (*r_passphrase);
      *r_passphrase = NULL;
    }
  return err;

 bad_seckey:
  err = gpg_error (GPG_ERR_BAD_SECKEY);
  goto leave;
  
 outofmem:
  err = gpg_error (GPG_ERR_ENOMEM);
  goto leave;

}