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/*
     This file is part of GNUnet.
     (C) 2012 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 secretsharing/secretsharing_api.c
 * @brief
 * @author Florian Dold
 */
#include "platform.h"
#include "gnunet_util_lib.h"
#include "gnunet_secretsharing_service.h"
#include "secretsharing.h"
#include <gcrypt.h>


#define LOG(kind,...) GNUNET_log_from (kind, "secretsharing-api",__VA_ARGS__)

/**
 * Session that will eventually establish a shared secred between
 * the involved peers and allow encryption and cooperative decryption.
 */
struct GNUNET_SECRETSHARING_Session
{
  /**
   * Client connected to the secretsharing service.
   */
  struct GNUNET_CLIENT_Connection *client;

  /**
   * Message queue for 'client'.
   */
  struct GNUNET_MQ_Handle *mq;

  /**
   * Called when the secret sharing is done.
   */
  GNUNET_SECRETSHARING_SecretReadyCallback secret_ready_cb;

  /**
   * Closure for 'secret_ready_cb'.
   */
  void *secret_ready_cls;
};


struct GNUNET_SECRETSHARING_DecryptionHandle
{
  /**
   * Client connected to the secretsharing service.
   */
  struct GNUNET_CLIENT_Connection *client;

  /**
   * Message queue for 'client'.
   */
  struct GNUNET_MQ_Handle *mq;

  /**
   * Called when the secret sharing is done.
   */
  GNUNET_SECRETSHARING_DecryptCallback decrypt_cb;

  /**
   * Closure for 'decrypt_cb'.
   */
  void *decrypt_cls;
};


/**
 * The ElGamal prime field order as libgcrypt mpi.
 * Initialized in #init_crypto_constants.
 */
static gcry_mpi_t elgamal_q;

/**
 * Modulus of the prime field used for ElGamal.
 * Initialized in #init_crypto_constants.
 */
static gcry_mpi_t elgamal_p;

/**
 * Generator for prime field of order 'elgamal_q'.
 * Initialized in #init_crypto_constants.
 */
static gcry_mpi_t elgamal_g;


static void
ensure_elgamal_initialized (void)
{
  if (NULL != elgamal_q)
    return; /* looks like crypto is already initialized */

  GNUNET_assert (0 == gcry_mpi_scan (&elgamal_q, GCRYMPI_FMT_HEX,
                                     GNUNET_SECRETSHARING_ELGAMAL_Q_HEX, 0, NULL));
  GNUNET_assert (0 == gcry_mpi_scan (&elgamal_p, GCRYMPI_FMT_HEX,
                                     GNUNET_SECRETSHARING_ELGAMAL_P_HEX, 0, NULL));
  GNUNET_assert (0 == gcry_mpi_scan (&elgamal_g, GCRYMPI_FMT_HEX,
                                     GNUNET_SECRETSHARING_ELGAMAL_G_HEX, 0, NULL));
}


static void
handle_session_client_error (void *cls, enum GNUNET_MQ_Error error)
{
  struct GNUNET_SECRETSHARING_Session *s = cls;

  s->secret_ready_cb (s->secret_ready_cls, NULL, NULL, 0, NULL);
}


static void
handle_decrypt_client_error (void *cls, enum GNUNET_MQ_Error error)
{
  struct GNUNET_SECRETSHARING_DecryptionHandle *dh = cls;
  
  dh->decrypt_cb (dh->decrypt_cls, NULL);
}


static void
handle_secret_ready (void *cls, const struct GNUNET_MessageHeader *msg)
{
  struct GNUNET_SECRETSHARING_Session *session = cls;
  struct GNUNET_SECRETSHARING_Share *share;
  const struct GNUNET_SECRETSHARING_SecretReadyMessage *m = (const void *) msg;
  size_t share_size;

  share_size = ntohs (m->header.size) - sizeof *m;

  share = GNUNET_SECRETSHARING_share_read (&m[1], share_size, NULL);

  session->secret_ready_cb (session->secret_ready_cls,
                      share, /* FIXME */
                      &share->public_key,
                      share->num_peers,
                      (struct GNUNET_PeerIdentity *) &m[1]);

  GNUNET_SECRETSHARING_session_destroy (session);
}


void
GNUNET_SECRETSHARING_session_destroy (struct GNUNET_SECRETSHARING_Session *session)
{
  GNUNET_MQ_destroy (session->mq);
  session->mq = NULL;
  GNUNET_CLIENT_disconnect (session->client);
  session->client = NULL;
  GNUNET_free (session);
}


struct GNUNET_SECRETSHARING_Session *
GNUNET_SECRETSHARING_create_session (const struct GNUNET_CONFIGURATION_Handle *cfg,
                                     unsigned int num_peers,
                                     const struct GNUNET_PeerIdentity *peers,
                                     const struct GNUNET_HashCode *session_id,
                                     struct GNUNET_TIME_Absolute deadline,
                                     unsigned int threshold,
                                     GNUNET_SECRETSHARING_SecretReadyCallback cb,
                                     void *cls)
{
  struct GNUNET_SECRETSHARING_Session *s;
  struct GNUNET_MQ_Envelope *ev;
  struct GNUNET_SECRETSHARING_CreateMessage *msg;
  static const struct GNUNET_MQ_MessageHandler mq_handlers[] = {
    {handle_secret_ready, GNUNET_MESSAGE_TYPE_SECRETSHARING_CLIENT_SECRET_READY, 0},
    GNUNET_MQ_HANDLERS_END
  };


  s = GNUNET_new (struct GNUNET_SECRETSHARING_Session);
  s->client = GNUNET_CLIENT_connect ("secretsharing", cfg);
  s->secret_ready_cb = cb;
  s->secret_ready_cls = cls;
  GNUNET_assert (NULL != s->client);

  s->mq = GNUNET_MQ_queue_for_connection_client (s->client, mq_handlers,
                                                   handle_session_client_error, s);
  GNUNET_assert (NULL != s->mq);

  ev = GNUNET_MQ_msg_extra (msg,
                            num_peers * sizeof (struct GNUNET_PeerIdentity),
                            GNUNET_MESSAGE_TYPE_SECRETSHARING_CLIENT_GENERATE);

  msg->threshold = htons (threshold);
  msg->num_peers = htons (num_peers);
  msg->session_id = *session_id;
  msg->deadline = GNUNET_TIME_absolute_hton (deadline);
  memcpy (&msg[1], peers, num_peers * sizeof (struct GNUNET_PeerIdentity));

  GNUNET_MQ_send (s->mq, ev);

  LOG (GNUNET_ERROR_TYPE_DEBUG, "secretsharing session created with %u peers\n",
       num_peers);
  return s;
}


static void
handle_decrypt_done (void *cls, const struct GNUNET_MessageHeader *msg)
{
  struct GNUNET_SECRETSHARING_DecryptionHandle *dh = cls;
  const struct GNUNET_SECRETSHARING_DecryptResponseMessage *m =
      (const void *) msg;

  const struct GNUNET_SECRETSHARING_Plaintext *plaintext;

  if (m->success == 0)
    plaintext = NULL;
  else
    plaintext = (void *) &m->plaintext;

  dh->decrypt_cb (dh->decrypt_cls, plaintext);

  GNUNET_SECRETSHARING_decrypt_cancel (dh);
}


/**
 * Publish the given ciphertext for decryption.  Once a sufficient (>=k) number of peers has
 * published the same value, it will be decrypted.
 *
 * When the operation is canceled, the decrypt_cb is not called anymore, but the calling
 * peer may already have irrevocably contributed his share for the decryption of the value.
 *
 * @param share our secret share to use for decryption
 * @param ciphertext ciphertext to publish in order to decrypt it (if enough peers agree)
 * @param decrypt_cb callback called once the decryption succeeded
 * @param decrypt_cb_cls closure for @a decrypt_cb
 * @return handle to cancel the operation
 */
struct GNUNET_SECRETSHARING_DecryptionHandle *
GNUNET_SECRETSHARING_decrypt (const struct GNUNET_CONFIGURATION_Handle *cfg,
                              struct GNUNET_SECRETSHARING_Share *share,
                              const struct GNUNET_SECRETSHARING_Ciphertext *ciphertext,
                              struct GNUNET_TIME_Absolute deadline,
                              GNUNET_SECRETSHARING_DecryptCallback decrypt_cb,
                              void *decrypt_cb_cls)
{
  struct GNUNET_SECRETSHARING_DecryptionHandle *s;
  struct GNUNET_MQ_Envelope *ev;
  struct GNUNET_SECRETSHARING_DecryptRequestMessage *msg;
  static const struct GNUNET_MQ_MessageHandler mq_handlers[] = {
    {handle_decrypt_done, GNUNET_MESSAGE_TYPE_SECRETSHARING_CLIENT_DECRYPT_DONE, 0},
    GNUNET_MQ_HANDLERS_END
  };
  size_t share_size;


  s = GNUNET_new (struct GNUNET_SECRETSHARING_DecryptionHandle);
  s->client = GNUNET_CLIENT_connect ("secretsharing", cfg);
  s->decrypt_cb = decrypt_cb;
  s->decrypt_cls = decrypt_cb_cls;
  GNUNET_assert (NULL != s->client);

  s->mq = GNUNET_MQ_queue_for_connection_client (s->client, mq_handlers,
                                                 handle_decrypt_client_error, s);
  GNUNET_assert (NULL != s->mq);

  GNUNET_assert (GNUNET_OK == GNUNET_SECRETSHARING_share_write (share, NULL, 0, &share_size));

  ev = GNUNET_MQ_msg_extra (msg,
                            share_size,
                            GNUNET_MESSAGE_TYPE_SECRETSHARING_CLIENT_DECRYPT);

  GNUNET_assert (GNUNET_OK == GNUNET_SECRETSHARING_share_write (share, &msg[1], share_size, NULL));

  msg->deadline = GNUNET_TIME_absolute_hton (deadline);
  msg->ciphertext = *ciphertext;

  GNUNET_MQ_send (s->mq, ev);

  LOG (GNUNET_ERROR_TYPE_DEBUG, "decrypt session created\n");
  return s;
}


int
GNUNET_SECRETSHARING_plaintext_generate_i (struct GNUNET_SECRETSHARING_Plaintext *plaintext,
                                           int64_t exponent)
{
  int negative;
  gcry_mpi_t x;

  ensure_elgamal_initialized ();

  GNUNET_assert (NULL != (x = gcry_mpi_new (0)));

  negative = GNUNET_NO;
  if (exponent < 0)
  {
    negative = GNUNET_YES;
    exponent = -exponent;
  }

  gcry_mpi_set_ui (x, exponent);

  gcry_mpi_powm (x, elgamal_g, x, elgamal_p);

  if (GNUNET_YES == negative)
  {
    int res;
    res = gcry_mpi_invm (x, x, elgamal_p);
    if (0 == res)
      return GNUNET_SYSERR;
  }

  GNUNET_CRYPTO_mpi_print_unsigned (plaintext, sizeof (struct GNUNET_SECRETSHARING_Plaintext), x);

  return GNUNET_OK;
}


/**
 * Encrypt a value.  This operation is executed locally, no communication is
 * necessary.
 *
 * This is a helper function, encryption can be done soley with a session's public key
 * and the crypto system parameters.
 *
 * @param public_key public key to use for decryption
 * @param message message to encrypt
 * @param message_size number of bytes in @a message
 * @param result_ciphertext pointer to store the resulting ciphertext
 * @return #GNUNET_YES on succes, #GNUNET_SYSERR if the message is invalid (invalid range)
 */
int
GNUNET_SECRETSHARING_encrypt (const struct GNUNET_SECRETSHARING_PublicKey *public_key,
                              const struct GNUNET_SECRETSHARING_Plaintext *plaintext,
                              struct GNUNET_SECRETSHARING_Ciphertext *result_ciphertext)
{
  /* pubkey */
  gcry_mpi_t h;
  /* nonce */
  gcry_mpi_t y;
  /* plaintext message */
  gcry_mpi_t m;
  /* temp value */
  gcry_mpi_t tmp;

  ensure_elgamal_initialized ();

  GNUNET_assert (NULL != (h = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (y = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (tmp = gcry_mpi_new (0)));

  GNUNET_CRYPTO_mpi_scan_unsigned (&h, public_key, sizeof *public_key);
  GNUNET_CRYPTO_mpi_scan_unsigned (&m, plaintext, sizeof *plaintext);

  // Randomize y such that 0 < y < elgamal_q.
  // The '- 1' is necessary as bitlength(q) = bitlength(p) - 1.
  do 
  {
    gcry_mpi_randomize (y, GNUNET_SECRETSHARING_ELGAMAL_BITS - 1, GCRY_WEAK_RANDOM);
  } while ((gcry_mpi_cmp_ui (y, 0) == 0) || (gcry_mpi_cmp (y, elgamal_q) >= 0));

  // tmp <- g^y
  gcry_mpi_powm (tmp, elgamal_g, y, elgamal_p);
  // write tmp to c1
  GNUNET_CRYPTO_mpi_print_unsigned (&result_ciphertext->c1_bits,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8, tmp);
  
  // tmp <- h^y
  gcry_mpi_powm (tmp, h, y, elgamal_p);
  // tmp <- tmp * m 
  gcry_mpi_mulm (tmp, tmp, m, elgamal_p);
  // write tmp to c2
  GNUNET_CRYPTO_mpi_print_unsigned (&result_ciphertext->c2_bits,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8, tmp);

  return GNUNET_OK;
}


void
GNUNET_SECRETSHARING_decrypt_cancel (struct GNUNET_SECRETSHARING_DecryptionHandle *h)
{
  GNUNET_MQ_destroy (h->mq);
  h->mq = NULL;
  GNUNET_CLIENT_disconnect (h->client);
  h->client = NULL;
  GNUNET_free (h);
}