/*
     This file is part of GNUnet.
     (C) 2012, 2013 Christian Grothoff (and other contributing authors)

     GNUnet 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, or (at your
     option) any later version.

     GNUnet 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 GNUnet; see the file COPYING.  If not, write to the
     Free Software Foundation, Inc., 59 Temple Place - Suite 330,
     Boston, MA 02111-1307, USA.
*/

/**
 * @file util/crypto_ecc.c
 * @brief public key cryptography (ECC) with libgcrypt
 * @author Christian Grothoff
 */
#include "platform.h"
#include <gcrypt.h>
#include "gnunet_util_lib.h"

#define EXTRA_CHECKS ALLOW_EXTRA_CHECKS

/**
 * Name of the curve we are using.  Note that we have hard-coded
 * structs that use 256 bits, so using a bigger curve will require
 * changes that break stuff badly.  The name of the curve given here
 * must be agreed by all peers and be supported by libgcrypt.
 *
 * NOTE: this will change to Curve25519 before GNUnet 0.10.0.
 */
#define CURVE "NIST P-256"

#define LOG(kind,...) GNUNET_log_from (kind, "util", __VA_ARGS__)

#define LOG_STRERROR(kind,syscall) GNUNET_log_from_strerror (kind, "util", syscall)

#define LOG_STRERROR_FILE(kind,syscall,filename) GNUNET_log_from_strerror_file (kind, "util", syscall, filename)

/**
 * Log an error message at log-level 'level' that indicates
 * a failure of the command 'cmd' with the message given
 * by gcry_strerror(rc).
 */
#define LOG_GCRY(level, cmd, rc) do { LOG(level, _("`%s' failed at %s:%d with error: %s\n"), cmd, __FILE__, __LINE__, gcry_strerror(rc)); } while(0)


/**
 * Extract values from an S-expression.
 *
 * @param array where to store the result(s)
 * @param sexp S-expression to parse
 * @param topname top-level name in the S-expression that is of interest
 * @param elems names of the elements to extract
 * @return 0 on success
 */
static int
key_from_sexp (gcry_mpi_t * array, gcry_sexp_t sexp, const char *topname,
               const char *elems)
{
  gcry_sexp_t list;
  gcry_sexp_t l2;
  const char *s;
  unsigned int i;
  unsigned int idx;

  list = gcry_sexp_find_token (sexp, topname, 0);
  if (! list)
    return 1;
  l2 = gcry_sexp_cadr (list);
  gcry_sexp_release (list);
  list = l2;
  if (! list)
    return 2;

  idx = 0;
  for (s = elems; *s; s++, idx++)
  {
    l2 = gcry_sexp_find_token (list, s, 1);
    if (! l2)
    {
      for (i = 0; i < idx; i++)
      {
        gcry_free (array[i]);
        array[i] = NULL;
      }
      gcry_sexp_release (list);
      return 3;                 /* required parameter not found */
    }
    array[idx] = gcry_sexp_nth_mpi (l2, 1, GCRYMPI_FMT_USG);
    gcry_sexp_release (l2);
    if (! array[idx])
    {
      for (i = 0; i < idx; i++)
      {
        gcry_free (array[i]);
        array[i] = NULL;
      }
      gcry_sexp_release (list);
      return 4;                 /* required parameter is invalid */
    }
  }
  gcry_sexp_release (list);
  return 0;
}


/**
 * If target != size, move @a target bytes to the end of the size-sized
 * buffer and zero out the first @a target - @a size bytes.
 *
 * @param buf original buffer
 * @param size number of bytes in @a buf
 * @param target target size of the buffer
 */
static void
adjust (unsigned char *buf,
	size_t size,
	size_t target)
{
  if (size < target)
  {
    memmove (&buf[target - size], buf, size);
    memset (buf, 0, target - size);
  }
}


/**
 * Output the given MPI value to the given buffer.
 *
 * @param buf where to output to
 * @param size number of bytes in @a buf
 * @param val value to write to @a buf
 */
static void
mpi_print (unsigned char *buf,
	   size_t size,
	   gcry_mpi_t val)
{
  size_t rsize;

  rsize = size;
  GNUNET_assert (0 ==
                 gcry_mpi_print (GCRYMPI_FMT_USG, buf, rsize, &rsize,
                                 val));
  adjust (buf, rsize, size);
}


/**
 * Convert data buffer into MPI value.
 *
 * @param result where to store MPI value (allocated)
 * @param data raw data (GCRYMPI_FMT_USG)
 * @param size number of bytes in data
 */
static void
mpi_scan (gcry_mpi_t *result,
	  const unsigned char *data,
	  size_t size)
{
  int rc;

  if (0 != (rc = gcry_mpi_scan (result,
				GCRYMPI_FMT_USG,
				data, size, &size)))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "gcry_mpi_scan", rc);
    GNUNET_assert (0);
  }
}


/**
 * Convert the given private key from the network format to the
 * S-expression that can be used by libgcrypt.
 *
 * @param priv private key to decode
 * @return NULL on error
 */
static gcry_sexp_t
decode_private_key (const struct GNUNET_CRYPTO_EccPrivateKey *priv)
{
  gcry_sexp_t result;
  gcry_mpi_t d;
  int rc;

  mpi_scan (&d,
	    priv->d,
	    sizeof (priv->d));
  rc = gcry_sexp_build (&result, NULL,
			"(private-key(ecdsa(curve \"" CURVE "\")(d %m)))",
			d);
  gcry_mpi_release (d);
  if (0 != rc)
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "gcry_sexp_build", rc);
    GNUNET_assert (0);
  }
#if EXTRA_CHECKS
  if (0 != (rc = gcry_pk_testkey (result)))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "gcry_pk_testkey", rc);
    GNUNET_assert (0);
  }
#endif
  return result;
}


/**
 * Initialize public key struct from the respective point
 * on the curve.
 *
 * @param q point on curve
 * @param pub public key struct to initialize
 * @param ctx context to use for ECC operations
 */
static void
point_to_public_sign_key (gcry_mpi_point_t q,
			  gcry_ctx_t ctx,
			  struct GNUNET_CRYPTO_EccPublicSignKey *pub)
{
  gcry_mpi_t q_x;
  gcry_mpi_t q_y;

  q_x = gcry_mpi_new (256);
  q_y = gcry_mpi_new (256);
  if (gcry_mpi_ec_get_affine (q_x, q_y, q, ctx))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "get_affine failed", 0);
    return;
  }

  mpi_print (pub->q_x, sizeof (pub->q_x), q_x);
  mpi_print (pub->q_y, sizeof (pub->q_y), q_y);
  gcry_mpi_release (q_x);
  gcry_mpi_release (q_y);
}


/**
 * Initialize public key struct from the respective point
 * on the curve.
 *
 * @param q point on curve
 * @param pub public key struct to initialize
 * @param ctx context to use for ECC operations
 */
static void
point_to_public_encrypt_key (gcry_mpi_point_t q,
			     gcry_ctx_t ctx,
			     struct GNUNET_CRYPTO_EccPublicEncryptKey *pub)
{
  gcry_mpi_t q_x;
  gcry_mpi_t q_y;

  q_x = gcry_mpi_new (256);
  q_y = gcry_mpi_new (256);
  if (gcry_mpi_ec_get_affine (q_x, q_y, q, ctx))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "get_affine failed", 0);
    return;
  }

  mpi_print (pub->q_x, sizeof (pub->q_x), q_x);
  mpi_print (pub->q_y, sizeof (pub->q_y), q_y);
  gcry_mpi_release (q_x);
  gcry_mpi_release (q_y);
}


/**
 * Extract the public key for the given private key.
 *
 * @param priv the private key
 * @param pub where to write the public key
 */
void
GNUNET_CRYPTO_ecc_key_get_public_for_signature (const struct GNUNET_CRYPTO_EccPrivateKey *priv,
						struct GNUNET_CRYPTO_EccPublicSignKey *pub)
{
  gcry_sexp_t sexp;
  gcry_ctx_t ctx;
  gcry_mpi_point_t q;

  sexp = decode_private_key (priv);
  GNUNET_assert (NULL != sexp);
  GNUNET_assert (0 == gcry_mpi_ec_new (&ctx, sexp, NULL));
  gcry_sexp_release (sexp);
  q = gcry_mpi_ec_get_point ("q", ctx, 0);
  point_to_public_sign_key (q, ctx, pub);
  gcry_ctx_release (ctx);
  gcry_mpi_point_release (q);
}


/**
 * Extract the public key for the given private key.
 *
 * @param priv the private key
 * @param pub where to write the public key
 */
void
GNUNET_CRYPTO_ecc_key_get_public_for_encryption (const struct GNUNET_CRYPTO_EccPrivateKey *priv,
						 struct GNUNET_CRYPTO_EccPublicEncryptKey *pub)
{
  gcry_sexp_t sexp;
  gcry_ctx_t ctx;
  gcry_mpi_point_t q;

  sexp = decode_private_key (priv);
  GNUNET_assert (NULL != sexp);
  GNUNET_assert (0 == gcry_mpi_ec_new (&ctx, sexp, NULL));
  gcry_sexp_release (sexp);
  q = gcry_mpi_ec_get_point ("q", ctx, 0);
  point_to_public_encrypt_key (q, ctx, pub);
  gcry_ctx_release (ctx);
  gcry_mpi_point_release (q);
}


/**
 * Convert a public key to a string.
 *
 * @param pub key to convert
 * @return string representing @a pub
 */
char *
GNUNET_CRYPTO_ecc_public_sign_key_to_string (const struct GNUNET_CRYPTO_EccPublicSignKey *pub)
{
  char *pubkeybuf;
  size_t keylen = (sizeof (struct GNUNET_CRYPTO_EccPublicSignKey)) * 8;
  char *end;

  if (keylen % 5 > 0)
    keylen += 5 - keylen % 5;
  keylen /= 5;
  pubkeybuf = GNUNET_malloc (keylen + 1);
  end = GNUNET_STRINGS_data_to_string ((unsigned char *) pub,
				       sizeof (struct GNUNET_CRYPTO_EccPublicSignKey),
				       pubkeybuf,
				       keylen);
  if (NULL == end)
  {
    GNUNET_free (pubkeybuf);
    return NULL;
  }
  *end = '\0';
  return pubkeybuf;
}


/**
 * Convert a string representing a public key to a public key.
 *
 * @param enc encoded public key
 * @param enclen number of bytes in @a enc (without 0-terminator)
 * @param pub where to store the public key
 * @return #GNUNET_OK on success
 */
int
GNUNET_CRYPTO_ecc_public_sign_key_from_string (const char *enc,
					  size_t enclen,
					  struct GNUNET_CRYPTO_EccPublicSignKey *pub)
{
  size_t keylen = (sizeof (struct GNUNET_CRYPTO_EccPublicSignKey)) * 8;

  if (keylen % 5 > 0)
    keylen += 5 - keylen % 5;
  keylen /= 5;
  if (enclen != keylen)
    return GNUNET_SYSERR;

  if (GNUNET_OK != GNUNET_STRINGS_string_to_data (enc, enclen,
						  pub,
						  sizeof (struct GNUNET_CRYPTO_EccPublicSignKey)))
    return GNUNET_SYSERR;
  return GNUNET_OK;
}


/**
 * Convert the given public key from the network format to the
 * S-expression that can be used by libgcrypt.
 *
 * @param pub public key to decode
 * @return NULL on error
 */
static gcry_sexp_t
decode_public_sign_key (const struct GNUNET_CRYPTO_EccPublicSignKey *pub)
{
  gcry_sexp_t pub_sexp;
  gcry_mpi_t q_x;
  gcry_mpi_t q_y;
  gcry_mpi_point_t q;
  gcry_ctx_t ctx;

  mpi_scan (&q_x, pub->q_x, sizeof (pub->q_x));
  mpi_scan (&q_y, pub->q_y, sizeof (pub->q_y));
  q = gcry_mpi_point_new (256);
  gcry_mpi_point_set (q, q_x, q_y, GCRYMPI_CONST_ONE);
  gcry_mpi_release (q_x);
  gcry_mpi_release (q_y);

  /* initialize 'ctx' with 'q' */
  GNUNET_assert (0 == gcry_mpi_ec_new (&ctx, NULL, CURVE));
  gcry_mpi_ec_set_point ("q", q, ctx);
  gcry_mpi_point_release (q);

  /* convert 'ctx' to 'sexp' */
  GNUNET_assert (0 == gcry_pubkey_get_sexp (&pub_sexp, GCRY_PK_GET_PUBKEY, ctx));
  gcry_ctx_release (ctx);
  return pub_sexp;
}


/**
 * @ingroup crypto
 * Clear memory that was used to store a private key.
 *
 * @param pk location of the key
 */
void
GNUNET_CRYPTO_ecc_key_clear (struct GNUNET_CRYPTO_EccPrivateKey *pk)
{
  memset (pk, 0, sizeof (struct GNUNET_CRYPTO_EccPrivateKey));
}


/**
 * Create a new private key. Caller must free return value.
 *
 * @return fresh private key
 */
struct GNUNET_CRYPTO_EccPrivateKey *
GNUNET_CRYPTO_ecc_key_create ()
{
  struct GNUNET_CRYPTO_EccPrivateKey *priv;
  gcry_sexp_t priv_sexp;
  gcry_sexp_t s_keyparam;
  gcry_mpi_t d;
  int rc;

  if (0 != (rc = gcry_sexp_build (&s_keyparam, NULL,
                                  "(genkey(ecdsa(curve \"" CURVE "\")))")))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "gcry_sexp_build", rc);
    return NULL;
  }
  if (0 != (rc = gcry_pk_genkey (&priv_sexp, s_keyparam)))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "gcry_pk_genkey", rc);
    gcry_sexp_release (s_keyparam);
    return NULL;
  }
  gcry_sexp_release (s_keyparam);
#if EXTRA_CHECKS
  if (0 != (rc = gcry_pk_testkey (priv_sexp)))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "gcry_pk_testkey", rc);
    gcry_sexp_release (priv_sexp);
    return NULL;
  }
#endif
  if (0 != (rc = key_from_sexp (&d, priv_sexp, "private-key", "d")))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "key_from_sexp", rc);
    gcry_sexp_release (priv_sexp);
    return NULL;
  }
  gcry_sexp_release (priv_sexp);
  priv = GNUNET_new (struct GNUNET_CRYPTO_EccPrivateKey);
  mpi_print (priv->d, sizeof (priv->d), d);
  gcry_mpi_release (d);
  return priv;
}


/**
 * Get the shared private key we use for anonymous users.
 *
 * @return "anonymous" private key
 */
const struct GNUNET_CRYPTO_EccPrivateKey *
GNUNET_CRYPTO_ecc_key_get_anonymous ()
{
  /**
   * 'anonymous' pseudonym (global static, d=1, public key = G
   * (generator).
   */
  static struct GNUNET_CRYPTO_EccPrivateKey anonymous;
  static int once;

  if (once)
    return &anonymous;
  mpi_print (anonymous.d,
	     sizeof (anonymous.d),
	     GCRYMPI_CONST_ONE);
  once = 1;
  return &anonymous;
}


/**
 * Wait for a short time (we're trying to lock a file or want
 * to give another process a shot at finishing a disk write, etc.).
 * Sleeps for 100ms (as that should be long enough for virtually all
 * modern systems to context switch and allow another process to do
 * some 'real' work).
 */
static void
short_wait ()
{
  struct GNUNET_TIME_Relative timeout;

  timeout = GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MILLISECONDS, 100);
  (void) GNUNET_NETWORK_socket_select (NULL, NULL, NULL, timeout);
}


/**
 * Create a new private key by reading it from a file.  If the
 * files does not exist, create a new key and write it to the
 * file.  Caller must free return value.  Note that this function
 * can not guarantee that another process might not be trying
 * the same operation on the same file at the same time.
 * If the contents of the file
 * are invalid the old file is deleted and a fresh key is
 * created.
 *
 * @param filename name of file to use to store the key
 * @return new private key, NULL on error (for example,
 *   permission denied)
 */
struct GNUNET_CRYPTO_EccPrivateKey *
GNUNET_CRYPTO_ecc_key_create_from_file (const char *filename)
{
  struct GNUNET_CRYPTO_EccPrivateKey *priv;
  struct GNUNET_DISK_FileHandle *fd;
  unsigned int cnt;
  int ec;
  uint64_t fs;

  if (GNUNET_SYSERR == GNUNET_DISK_directory_create_for_file (filename))
    return NULL;
  while (GNUNET_YES != GNUNET_DISK_file_test (filename))
  {
    fd = GNUNET_DISK_file_open (filename,
                                GNUNET_DISK_OPEN_WRITE | GNUNET_DISK_OPEN_CREATE
                                | GNUNET_DISK_OPEN_FAILIFEXISTS,
                                GNUNET_DISK_PERM_USER_READ |
                                GNUNET_DISK_PERM_USER_WRITE);
    if (NULL == fd)
    {
      if (EEXIST == errno)
      {
        if (GNUNET_YES != GNUNET_DISK_file_test (filename))
        {
          /* must exist but not be accessible, fail for good! */
          if (0 != ACCESS (filename, R_OK))
            LOG_STRERROR_FILE (GNUNET_ERROR_TYPE_ERROR, "access", filename);
          else
            GNUNET_break (0);   /* what is going on!? */
          return NULL;
        }
        continue;
      }
      LOG_STRERROR_FILE (GNUNET_ERROR_TYPE_ERROR, "open", filename);
      return NULL;
    }
    cnt = 0;
    while (GNUNET_YES !=
           GNUNET_DISK_file_lock (fd, 0,
                                  sizeof (struct GNUNET_CRYPTO_EccPrivateKey),
                                  GNUNET_YES))
    {
      short_wait ();
      if (0 == ++cnt % 10)
      {
        ec = errno;
        LOG (GNUNET_ERROR_TYPE_ERROR,
             _("Could not acquire lock on file `%s': %s...\n"), filename,
             STRERROR (ec));
      }
    }
    LOG (GNUNET_ERROR_TYPE_INFO,
         _("Creating a new private key.  This may take a while.\n"));
    priv = GNUNET_CRYPTO_ecc_key_create ();
    GNUNET_assert (NULL != priv);
    GNUNET_assert (sizeof (*priv) ==
                   GNUNET_DISK_file_write (fd, priv, sizeof (*priv)));
    GNUNET_DISK_file_sync (fd);
    if (GNUNET_YES !=
        GNUNET_DISK_file_unlock (fd, 0,
                                 sizeof (struct GNUNET_CRYPTO_EccPrivateKey)))
      LOG_STRERROR_FILE (GNUNET_ERROR_TYPE_WARNING, "fcntl", filename);
    GNUNET_assert (GNUNET_YES == GNUNET_DISK_file_close (fd));
    return priv;
  }
  /* key file exists already, read it! */
  fd = GNUNET_DISK_file_open (filename, GNUNET_DISK_OPEN_READ,
                              GNUNET_DISK_PERM_NONE);
  if (NULL == fd)
  {
    LOG_STRERROR_FILE (GNUNET_ERROR_TYPE_ERROR, "open", filename);
    return NULL;
  }
  cnt = 0;
  while (1)
  {
    if (GNUNET_YES !=
        GNUNET_DISK_file_lock (fd, 0,
                               sizeof (struct GNUNET_CRYPTO_EccPrivateKey),
                               GNUNET_NO))
    {
      if (0 == ++cnt % 60)
      {
        ec = errno;
        LOG (GNUNET_ERROR_TYPE_ERROR,
             _("Could not acquire lock on file `%s': %s...\n"), filename,
             STRERROR (ec));
        LOG (GNUNET_ERROR_TYPE_ERROR,
             _
             ("This may be ok if someone is currently generating a private key.\n"));
      }
      short_wait ();
      continue;
    }
    if (GNUNET_YES != GNUNET_DISK_file_test (filename))
    {
      /* eh, what!? File we opened is now gone!? */
      LOG_STRERROR_FILE (GNUNET_ERROR_TYPE_WARNING, "stat", filename);
      if (GNUNET_YES !=
          GNUNET_DISK_file_unlock (fd, 0,
                                   sizeof (struct GNUNET_CRYPTO_EccPrivateKey)))
        LOG_STRERROR_FILE (GNUNET_ERROR_TYPE_WARNING, "fcntl", filename);
      GNUNET_assert (GNUNET_OK == GNUNET_DISK_file_close (fd));

      return NULL;
    }
    if (GNUNET_OK != GNUNET_DISK_file_size (filename, &fs, GNUNET_YES, GNUNET_YES))
      fs = 0;
    if (fs < sizeof (struct GNUNET_CRYPTO_EccPrivateKey))
    {
      /* maybe we got the read lock before the key generating
       * process had a chance to get the write lock; give it up! */
      if (GNUNET_YES !=
          GNUNET_DISK_file_unlock (fd, 0,
                                   sizeof (struct GNUNET_CRYPTO_EccPrivateKey)))
        LOG_STRERROR_FILE (GNUNET_ERROR_TYPE_WARNING, "fcntl", filename);
      if (0 == ++cnt % 10)
      {
        LOG (GNUNET_ERROR_TYPE_ERROR,
             _
             ("When trying to read key file `%s' I found %u bytes but I need at least %u.\n"),
             filename, (unsigned int) fs,
             (unsigned int) sizeof (struct GNUNET_CRYPTO_EccPrivateKey));
        LOG (GNUNET_ERROR_TYPE_ERROR,
             _
             ("This may be ok if someone is currently generating a key.\n"));
      }
      short_wait ();                /* wait a bit longer! */
      continue;
    }
    break;
  }
  fs = sizeof (struct GNUNET_CRYPTO_EccPrivateKey);
  priv = GNUNET_malloc (fs);
  GNUNET_assert (fs == GNUNET_DISK_file_read (fd, priv, fs));
  if (GNUNET_YES !=
      GNUNET_DISK_file_unlock (fd, 0,
                               sizeof (struct GNUNET_CRYPTO_EccPrivateKey)))
    LOG_STRERROR_FILE (GNUNET_ERROR_TYPE_WARNING, "fcntl", filename);
  GNUNET_assert (GNUNET_YES == GNUNET_DISK_file_close (fd));
  return priv;
}


/**
 * Create a new private key by reading our peer's key from
 * the file specified in the configuration.
 *
 * @param cfg the configuration to use
 * @return new private key, NULL on error (for example,
 *   permission denied)
 */
struct GNUNET_CRYPTO_EccPrivateKey *
GNUNET_CRYPTO_ecc_key_create_from_configuration (const struct GNUNET_CONFIGURATION_Handle *cfg)
{
  struct GNUNET_CRYPTO_EccPrivateKey *priv;
  char *fn;

  if (GNUNET_OK !=
      GNUNET_CONFIGURATION_get_value_filename (cfg, "PEER", "PRIVATE_KEY", &fn))
    return NULL;
  priv = GNUNET_CRYPTO_ecc_key_create_from_file (fn);
  GNUNET_free (fn);
  return priv;
}


/**
 * Setup a key file for a peer given the name of the
 * configuration file (!).  This function is used so that
 * at a later point code can be certain that reading a
 * key is fast (for example in time-dependent testcases).
 *
 * @param cfg_name name of the configuration file to use
 */
void
GNUNET_CRYPTO_ecc_setup_key (const char *cfg_name)
{
  struct GNUNET_CONFIGURATION_Handle *cfg;
  struct GNUNET_CRYPTO_EccPrivateKey *priv;

  cfg = GNUNET_CONFIGURATION_create ();
  (void) GNUNET_CONFIGURATION_load (cfg, cfg_name);
  priv = GNUNET_CRYPTO_ecc_key_create_from_configuration (cfg);
  if (NULL != priv)
    GNUNET_free (priv);
  GNUNET_CONFIGURATION_destroy (cfg);
}


/**
 * Retrieve the identity of the host's peer.
 *
 * @param cfg configuration to use
 * @param dst pointer to where to write the peer identity
 * @return #GNUNET_OK on success, #GNUNET_SYSERR if the identity
 *         could not be retrieved
 */
int
GNUNET_CRYPTO_get_peer_identity (const struct GNUNET_CONFIGURATION_Handle *cfg,
                                 struct GNUNET_PeerIdentity *dst)
{
  struct GNUNET_CRYPTO_EccPrivateKey *priv;

  if (NULL == (priv = GNUNET_CRYPTO_ecc_key_create_from_configuration (cfg)))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                _("Could not load peer's private key\n"));
    return GNUNET_SYSERR;
  }
  GNUNET_CRYPTO_ecc_key_get_public_for_signature (priv, &dst->public_key);
  GNUNET_free (priv);
  return GNUNET_OK;
}


/**
 * Convert the data specified in the given purpose argument to an
 * S-expression suitable for signature operations.
 *
 * @param purpose data to convert
 * @return converted s-expression
 */
static gcry_sexp_t
data_to_pkcs1 (const struct GNUNET_CRYPTO_EccSignaturePurpose *purpose)
{
  struct GNUNET_HashCode hc;
  gcry_sexp_t data;
  int rc;

  GNUNET_CRYPTO_hash (purpose, ntohl (purpose->size), &hc);
  if (0 != (rc = gcry_sexp_build (&data, NULL,
				  "(data(flags rfc6979)(hash %s %b))",
				  "sha512",
				  sizeof (hc),
				  &hc)))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "gcry_sexp_build", rc);
    return NULL;
  }
  return data;
}


/**
 * Sign a given block.
 *
 * @param priv private key to use for the signing
 * @param purpose what to sign (size, purpose)
 * @param sig where to write the signature
 * @return #GNUNET_SYSERR on error, #GNUNET_OK on success
 */
int
GNUNET_CRYPTO_ecc_sign (const struct GNUNET_CRYPTO_EccPrivateKey *priv,
                        const struct GNUNET_CRYPTO_EccSignaturePurpose *purpose,
                        struct GNUNET_CRYPTO_EccSignature *sig)
{
  gcry_sexp_t priv_sexp;
  gcry_sexp_t sig_sexp;
  gcry_sexp_t data;
  int rc;
  gcry_mpi_t rs[2];

  priv_sexp = decode_private_key (priv);
  data = data_to_pkcs1 (purpose);
  if (0 != (rc = gcry_pk_sign (&sig_sexp, data, priv_sexp)))
  {
    LOG (GNUNET_ERROR_TYPE_WARNING,
         _("ECC signing failed at %s:%d: %s\n"), __FILE__,
         __LINE__, gcry_strerror (rc));
    gcry_sexp_release (data);
    gcry_sexp_release (priv_sexp);
    return GNUNET_SYSERR;
  }
  gcry_sexp_release (priv_sexp);
  gcry_sexp_release (data);

  /* extract 'r' and 's' values from sexpression 'sig_sexp' and store in
     'signature' */
  if (0 != (rc = key_from_sexp (rs, sig_sexp, "sig-val", "rs")))
  {
    GNUNET_break (0);
    gcry_sexp_release (sig_sexp);
    return GNUNET_SYSERR;
  }
  gcry_sexp_release (sig_sexp);
  mpi_print (sig->r, sizeof (sig->r), rs[0]);
  mpi_print (sig->s, sizeof (sig->s), rs[1]);
  gcry_mpi_release (rs[0]);
  gcry_mpi_release (rs[1]);
  return GNUNET_OK;
}


/**
 * Verify signature.
 *
 * @param purpose what is the purpose that the signature should have?
 * @param validate block to validate (size, purpose, data)
 * @param sig signature that is being validated
 * @param pub public key of the signer
 * @returns #GNUNET_OK if ok, #GNUNET_SYSERR if invalid
 */
int
GNUNET_CRYPTO_ecc_verify (uint32_t purpose,
                          const struct GNUNET_CRYPTO_EccSignaturePurpose
                          *validate,
                          const struct GNUNET_CRYPTO_EccSignature *sig,
                          const struct GNUNET_CRYPTO_EccPublicSignKey *pub)
{
  gcry_sexp_t data;
  gcry_sexp_t sig_sexpr;
  gcry_sexp_t pub_sexpr;
  int rc;
  gcry_mpi_t r;
  gcry_mpi_t s;

  if (purpose != ntohl (validate->purpose))
    return GNUNET_SYSERR;       /* purpose mismatch */

  /* build s-expression for signature */
  mpi_scan (&r, sig->r, sizeof (sig->r));
  mpi_scan (&s, sig->s, sizeof (sig->s));
  if (0 != (rc = gcry_sexp_build (&sig_sexpr, NULL,
				  "(sig-val(ecdsa(r %m)(s %m)))",
                                  r, s)))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "gcry_sexp_build", rc);
    gcry_mpi_release (r);
    gcry_mpi_release (s);
    return GNUNET_SYSERR;
  }
  gcry_mpi_release (r);
  gcry_mpi_release (s);
  data = data_to_pkcs1 (validate);
  if (! (pub_sexpr = decode_public_sign_key (pub)))
  {
    gcry_sexp_release (data);
    gcry_sexp_release (sig_sexpr);
    return GNUNET_SYSERR;
  }
  rc = gcry_pk_verify (sig_sexpr, data, pub_sexpr);
  gcry_sexp_release (pub_sexpr);
  gcry_sexp_release (data);
  gcry_sexp_release (sig_sexpr);
  if (0 != rc)
  {
    LOG (GNUNET_ERROR_TYPE_INFO,
         _("ECC signature verification failed at %s:%d: %s\n"), __FILE__,
         __LINE__, gcry_strerror (rc));
    return GNUNET_SYSERR;
  }
  return GNUNET_OK;
}


/**
 * Convert the given public key from the network format to the
 * S-expression that can be used by libgcrypt.
 *
 * @param pub public key to decode
 * @return NULL on error
 */
static gcry_sexp_t
decode_public_encrypt_key (const struct GNUNET_CRYPTO_EccPublicEncryptKey *pub)
{
  gcry_sexp_t pub_sexp;
  gcry_mpi_t q_x;
  gcry_mpi_t q_y;
  gcry_mpi_point_t q;
  gcry_ctx_t ctx;

  mpi_scan (&q_x, pub->q_x, sizeof (pub->q_x));
  mpi_scan (&q_y, pub->q_y, sizeof (pub->q_y));
  q = gcry_mpi_point_new (256);
  gcry_mpi_point_set (q, q_x, q_y, GCRYMPI_CONST_ONE);
  gcry_mpi_release (q_x);
  gcry_mpi_release (q_y);

  /* initialize 'ctx' with 'q' */
  GNUNET_assert (0 == gcry_mpi_ec_new (&ctx, NULL, CURVE));
  gcry_mpi_ec_set_point ("q", q, ctx);
  gcry_mpi_point_release (q);

  /* convert 'ctx' to 'sexp' */
  GNUNET_assert (0 == gcry_pubkey_get_sexp (&pub_sexp, GCRY_PK_GET_PUBKEY, ctx));
  gcry_ctx_release (ctx);
  return pub_sexp;
}


/**
 * Derive key material from a public and a private ECC key.
 *
 * @param priv private key to use for the ECDH (x)
 * @param pub public key to use for the ECDH (yG)
 * @param key_material where to write the key material (xyG)
 * @return #GNUNET_SYSERR on error, #GNUNET_OK on success
 */
int
GNUNET_CRYPTO_ecc_ecdh (const struct GNUNET_CRYPTO_EccPrivateKey *priv,
                        const struct GNUNET_CRYPTO_EccPublicEncryptKey *pub,
                        struct GNUNET_HashCode *key_material)
{
  gcry_mpi_point_t result;
  gcry_mpi_point_t q;
  gcry_mpi_t d;
  gcry_ctx_t ctx;
  gcry_sexp_t pub_sexpr;
  gcry_mpi_t result_x;
  gcry_mpi_t result_y;
  unsigned char xbuf[256 / 8];

  /* first, extract the q = dP value from the public key */
  if (! (pub_sexpr = decode_public_encrypt_key (pub)))
    return GNUNET_SYSERR;
  GNUNET_assert (0 == gcry_mpi_ec_new (&ctx, pub_sexpr, NULL));
  gcry_sexp_release (pub_sexpr);
  q = gcry_mpi_ec_get_point ("q", ctx, 0);

  /* second, extract the d value from our private key */
  mpi_scan (&d, priv->d, sizeof (priv->d));

  /* then call the 'multiply' function, to compute the product */
  result = gcry_mpi_point_new (0);
  gcry_mpi_ec_mul (result, d, q, ctx);
  gcry_mpi_point_release (q);
  gcry_mpi_release (d);

  /* finally, convert point to string for hashing */
  result_x = gcry_mpi_new (256);
  result_y = gcry_mpi_new (256);
  if (gcry_mpi_ec_get_affine (result_x, result_y, result, ctx))
  {
    LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "get_affine failed", 0);
    gcry_mpi_point_release (result);
    gcry_ctx_release (ctx);
    return GNUNET_SYSERR;
  }
  gcry_mpi_point_release (result);
  gcry_ctx_release (ctx);

  mpi_print (xbuf, sizeof (xbuf), result_x);
  GNUNET_CRYPTO_hash (xbuf, sizeof (xbuf), key_material);
  gcry_mpi_release (result_x);
  gcry_mpi_release (result_y);
  return GNUNET_OK;
}


/**
 * Derive the 'h' value for key derivation, where
 * 'h = H(l,P)'.
 *
 * @param pub public key for deriviation
 * @param label label for deriviation
 * @param context additional context to use for HKDF of 'h';
 *        typically the name of the subsystem/application
 * @return h value
 */
static gcry_mpi_t
derive_h (const struct GNUNET_CRYPTO_EccPublicSignKey *pub,
	  const char *label,
	  const char *context)
{
  gcry_mpi_t h;
  struct GNUNET_HashCode hc;

  GNUNET_CRYPTO_kdf (&hc, sizeof (hc),
		     "key-derivation", strlen ("key-derivation"),
		     pub, sizeof (*pub),
		     label, strlen (label),
		     context, strlen (context),
		     NULL, 0);
  mpi_scan (&h, (unsigned char *) &hc, sizeof (hc));
  return h;
}


/**
 * Derive a private key from a given private key and a label.
 * Essentially calculates a private key 'd = H(l,P) * x mod n'
 * where n is the size of the ECC group and P is the public
 * key associated with the private key 'd'.
 *
 * @param priv original private key
 * @param label label to use for key deriviation
 * @param context additional context to use for HKDF of 'h';
 *        typically the name of the subsystem/application
 * @return derived private key
 */
struct GNUNET_CRYPTO_EccPrivateKey *
GNUNET_CRYPTO_ecc_key_derive (const struct GNUNET_CRYPTO_EccPrivateKey *priv,
			      const char *label,
			      const char *context)
{
  struct GNUNET_CRYPTO_EccPublicSignKey pub;
  struct GNUNET_CRYPTO_EccPrivateKey *ret;
  gcry_mpi_t h;
  gcry_mpi_t x;
  gcry_mpi_t d;
  gcry_mpi_t n;
  gcry_ctx_t ctx;

  GNUNET_assert (0 == gcry_mpi_ec_new (&ctx, NULL, CURVE));
  n = gcry_mpi_ec_get_mpi ("n", ctx, 1);
  GNUNET_CRYPTO_ecc_key_get_public_for_signature (priv, &pub);
  h = derive_h (&pub, label, context);
  mpi_scan (&x, priv->d, sizeof (priv->d));
  d = gcry_mpi_new (256);
  gcry_mpi_mulm (d, h, x, n);
  gcry_mpi_release (h);
  gcry_mpi_release (x);
  gcry_mpi_release (n);
  gcry_ctx_release (ctx);
  ret = GNUNET_new (struct GNUNET_CRYPTO_EccPrivateKey);
  mpi_print (ret->d, sizeof (ret->d), d);
  gcry_mpi_release (d);
  return ret;
}


/**
 * Derive a public key from a given public key and a label.
 * Essentially calculates a public key 'V = H(l,P) * P'.
 *
 * @param pub original public key
 * @param label label to use for key deriviation
 * @param context additional context to use for HKDF of 'h';
 *        typically the name of the subsystem/application
 * @param result where to write the derived public key
 */
void
GNUNET_CRYPTO_ecc_public_key_derive (const struct GNUNET_CRYPTO_EccPublicSignKey *pub,
				     const char *label,
				     const char *context,
				     struct GNUNET_CRYPTO_EccPublicSignKey *result)
{
  gcry_ctx_t ctx;
  gcry_mpi_t h;
  gcry_mpi_t n;
  gcry_mpi_t h_mod_n;
  gcry_mpi_t q_x;
  gcry_mpi_t q_y;
  gcry_mpi_point_t q;
  gcry_mpi_point_t v;

  GNUNET_assert (0 == gcry_mpi_ec_new (&ctx, NULL, CURVE));

  /* obtain point 'q' from original public key */
  mpi_scan (&q_x, pub->q_x, sizeof (pub->q_x));
  mpi_scan (&q_y, pub->q_y, sizeof (pub->q_y));

  q = gcry_mpi_point_new (0);
  gcry_mpi_point_set (q, q_x, q_y, GCRYMPI_CONST_ONE);
  gcry_mpi_release (q_x);
  gcry_mpi_release (q_y);

  /* calulcate h_mod_n = h % n */
  h = derive_h (pub, label, context);
  n = gcry_mpi_ec_get_mpi ("n", ctx, 1);
  h_mod_n = gcry_mpi_new (256);
  gcry_mpi_mod (h_mod_n, h, n);
  /* calculate v = h_mod_n * q */
  v = gcry_mpi_point_new (0);
  gcry_mpi_ec_mul (v, h_mod_n, q, ctx);
  gcry_mpi_release (h_mod_n);
  gcry_mpi_release (h);
  gcry_mpi_release (n);
  gcry_mpi_point_release (q);
  /* convert point 'v' to public key that we return */
  point_to_public_sign_key (v, ctx, result);
  gcry_mpi_point_release (v);
  gcry_ctx_release (ctx);
}


/* end of crypto_ecc.c */