/*
     This file is part of GNUnet.
     Copyright (C) 2013 GNUnet e.V.

     GNUnet is free software: you can redistribute it and/or modify it
     under the terms of the GNU Affero General Public License as published
     by the Free Software Foundation, either version 3 of the License,
     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
     Affero General Public License for more details.

     You should have received a copy of the GNU Affero General Public License
     along with this program.  If not, see <http://www.gnu.org/licenses/>.

     SPDX-License-Identifier: AGPL3.0-or-later
 */

/**
 * @file secretsharing/gnunet-service-secretsharing.c
 * @brief secret sharing service
 * @author Florian Dold
 */
#include "platform.h"
#include "gnunet_util_lib.h"
#include "gnunet_time_lib.h"
#include "gnunet_signatures.h"
#include "gnunet_consensus_service.h"
#include "secretsharing.h"
#include "secretsharing_protocol.h"
#include <gcrypt.h>


#define EXTRA_CHECKS 1


/**
 * Info about a peer in a key generation session.
 */
struct KeygenPeerInfo
{
  /**
   * Peer identity of the peer.
   */
  struct GNUNET_PeerIdentity peer;

  /**
   * The peer's paillier public key.
   * Freshly generated for each keygen session.
   */
  struct GNUNET_CRYPTO_PaillierPublicKey paillier_public_key;

  /**
   * The peer's commitment to its presecret.
   */
  gcry_mpi_t presecret_commitment;

  /**
   * Commitment to the preshare that is
   * intended for our peer.
   */
  gcry_mpi_t preshare_commitment;

  /**
   * Sigma (exponentiated share) for this peer.
   */
  gcry_mpi_t sigma;

  /**
   * Did we successfully receive the round1 element
   * of the peer?
   */
  int round1_valid;

  /**
   * Did we successfully receive the round2 element
   * of the peer?
   */
  int round2_valid;
};


/**
 * Information about a peer in a decrypt session.
 */
struct DecryptPeerInfo
{
  /**
   * Identity of the peer.
   */
  struct GNUNET_PeerIdentity peer;

  /**
   * Original index in the key generation round.
   * Necessary for computing the lagrange coefficients.
   */
  unsigned int original_index;

  /**
   * Set to the partial decryption of
   * this peer, or NULL if we did not
   * receive a partial decryption from this
   * peer or the zero knowledge proof failed.
   */
  gcry_mpi_t partial_decryption;
};


/**
 * State we keep per client.
 */
struct ClientState;


/**
 * Session to establish a threshold-shared secret.
 */
struct KeygenSession
{
  /**
   * Current consensus, used for both DKG rounds.
   */
  struct GNUNET_CONSENSUS_Handle *consensus;

  /**
   * Which client is this for?
   */
  struct ClientState *cs;

  /**
   * Randomly generated coefficients of the polynomial for sharing our
   * pre-secret, where 'preshares[0]' is our pre-secret.  Contains 'threshold'
   * elements, thus represents a polynomial of degree 'threshold-1', which can
   * be interpolated with 'threshold' data points.
   *
   * The pre-secret-shares 'i=1,...,num_peers' are given by evaluating this
   * polyomial at 'i' for share i.
   */
  gcry_mpi_t *presecret_polynomial;

  /**
   * Minimum number of shares required to restore the secret.
   * Also the number of coefficients for the polynomial representing
   * the sharing.  Obviously, the polynomial then has degree threshold-1.
   */
  unsigned int threshold;

  /**
   * Total number of peers.
   */
  unsigned int num_peers;

  /**
   * Index of the local peer.
   */
  unsigned int local_peer;

  /**
   * Information about all participating peers.
   * Array of size 'num_peers'.
   */
  struct KeygenPeerInfo *info;

  /**
   * List of all peers involved in the secret sharing session.
   */
  struct GNUNET_PeerIdentity *peers;

  /**
   * Identifier for this session.
   */
  struct GNUNET_HashCode session_id;

  /**
   * Paillier private key of our peer.
   */
  struct GNUNET_CRYPTO_PaillierPrivateKey paillier_private_key;

  /**
   * When would we like the key to be established?
   */
  struct GNUNET_TIME_Absolute deadline;

  /**
   * When does the DKG start?  Necessary to compute fractions of the
   * operation's desired time interval.
   */
  struct GNUNET_TIME_Absolute start_time;

  /**
   * Index of the local peer in the ordered list
   * of peers in the session.
   */
  unsigned int local_peer_idx;

  /**
   * Share of our peer.  Once preshares from other peers are received, they
   * will be added to 'my'share.
   */
  gcry_mpi_t my_share;

  /**
   * Public key, will be updated when a round2 element arrives.
   */
  gcry_mpi_t public_key;
};


/**
 * Session to cooperatively decrypt a value.
 */
struct DecryptSession
{
  /**
   * Handle to the consensus over partial decryptions.
   */
  struct GNUNET_CONSENSUS_Handle *consensus;

  /**
   * Which client is this for?
   */
  struct ClientState *cs;

  /**
   * When should we start communicating for decryption?
   */
  struct GNUNET_TIME_Absolute start;

  /**
   * When would we like the ciphertext to be
   * decrypted?
   */
  struct GNUNET_TIME_Absolute deadline;

  /**
   * Ciphertext we want to decrypt.
   */
  struct GNUNET_SECRETSHARING_Ciphertext ciphertext;

  /**
   * Share of the local peer.
   * Contains other important information, such as
   * the list of other peers.
   */
  struct GNUNET_SECRETSHARING_Share *share;

  /**
   * State information about other peers.
   */
  struct DecryptPeerInfo *info;
};


/**
 * State we keep per client.
 */
struct ClientState
{
  /**
   * Decrypt session of the client, if any.
   */
  struct DecryptSession *decrypt_session;

  /**
   * Keygen session of the client, if any.
   */
  struct KeygenSession *keygen_session;

  /**
   * Client this is about.
   */
  struct GNUNET_SERVICE_Client *client;

  /**
   * MQ to talk to @a client.
   */
  struct GNUNET_MQ_Handle *mq;
};


/**
 * 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;

/**
 * Peer that runs this service.
 */
static struct GNUNET_PeerIdentity my_peer;

/**
 * Peer that runs this service.
 */
static struct GNUNET_CRYPTO_EddsaPrivateKey *my_peer_private_key;

/**
 * Configuration of this service.
 */
static const struct GNUNET_CONFIGURATION_Handle *cfg;


/**
 * Get the peer info belonging to a peer identity in a keygen session.
 *
 * @param ks The keygen session.
 * @param peer The peer identity.
 * @return The Keygen peer info, or NULL if the peer could not be found.
 */
static struct KeygenPeerInfo *
get_keygen_peer_info (const struct KeygenSession *ks,
                      const struct GNUNET_PeerIdentity *peer)
{
  unsigned int i;

  for (i = 0; i < ks->num_peers; i++)
    if (0 == GNUNET_memcmp (peer, &ks->info[i].peer))
      return &ks->info[i];
  return NULL;
}


/**
 * Get the peer info belonging to a peer identity in a decrypt session.
 *
 * @param ds The decrypt session.
 * @param peer The peer identity.
 * @return The decrypt peer info, or NULL if the peer could not be found.
 */
static struct DecryptPeerInfo *
get_decrypt_peer_info (const struct DecryptSession *ds,
                       const struct GNUNET_PeerIdentity *peer)
{
  unsigned int i;

  for (i = 0; i < ds->share->num_peers; i++)
    if (0 == GNUNET_memcmp (peer, &ds->info[i].peer))
      return &ds->info[i];
  return NULL;
}


/**
 * Interpolate between two points in time.
 *
 * @param start start time
 * @param end end time
 * @param num numerator of the scale factor
 * @param denum denumerator of the scale factor
 */
static struct GNUNET_TIME_Absolute
time_between (struct GNUNET_TIME_Absolute start,
              struct GNUNET_TIME_Absolute end,
              int num, int denum)
{
  struct GNUNET_TIME_Absolute result;
  uint64_t diff;

  GNUNET_assert (start.abs_value_us <= end.abs_value_us);
  diff = end.abs_value_us - start.abs_value_us;
  result.abs_value_us = start.abs_value_us + ((diff * num) / denum);

  return result;
}


/**
 * Compare two peer identities.  Intended to be used with qsort or bsearch.
 *
 * @param p1 Some peer identity.
 * @param p2 Some peer identity.
 * @return 1 if p1 > p2, -1 if p1 < p2 and 0 if p1 == p2.
 */
static int
peer_id_cmp (const void *p1, const void *p2)
{
  return memcmp (p1,
                 p2,
                 sizeof(struct GNUNET_PeerIdentity));
}


/**
 * Get the index of a peer in an array of peers
 *
 * @param haystack Array of peers.
 * @param n Size of @a haystack.
 * @param needle Peer to find
 * @return Index of @a needle in @a haystack, or -1 if peer
 *         is not in the list.
 */
static int
peer_find (const struct GNUNET_PeerIdentity *haystack, unsigned int n,
           const struct GNUNET_PeerIdentity *needle)
{
  unsigned int i;

  for (i = 0; i < n; i++)
    if (0 == GNUNET_memcmp (&haystack[i],
                            needle))
      return i;
  return -1;
}


/**
 * Normalize the given list of peers, by including the local peer
 * (if it is missing) and sorting the peers by their identity.
 *
 * @param listed Peers in the unnormalized list.
 * @param num_listed Peers in the un-normalized list.
 * @param[out] num_normalized Number of peers in the normalized list.
 * @param[out] my_peer_idx Index of the local peer in the normalized list.
 * @return Normalized list, must be free'd by the caller.
 */
static struct GNUNET_PeerIdentity *
normalize_peers (struct GNUNET_PeerIdentity *listed,
                 unsigned int num_listed,
                 unsigned int *num_normalized,
                 unsigned int *my_peer_idx)
{
  unsigned int local_peer_in_list;
  /* number of peers in the normalized list */
  unsigned int n;
  struct GNUNET_PeerIdentity *normalized;

  local_peer_in_list = GNUNET_YES;
  n = num_listed;
  if (peer_find (listed, num_listed, &my_peer) < 0)
  {
    local_peer_in_list = GNUNET_NO;
    n += 1;
  }

  normalized = GNUNET_new_array (n,
                                 struct GNUNET_PeerIdentity);

  if (GNUNET_NO == local_peer_in_list)
    normalized[n - 1] = my_peer;

  GNUNET_memcpy (normalized,
                 listed,
                 num_listed * sizeof(struct GNUNET_PeerIdentity));
  qsort (normalized,
         n,
         sizeof(struct GNUNET_PeerIdentity),
         &peer_id_cmp);

  if (NULL != my_peer_idx)
    *my_peer_idx = peer_find (normalized, n, &my_peer);
  if (NULL != num_normalized)
    *num_normalized = n;

  return normalized;
}


/**
 * Get a the j-th lagrange coefficient for a set of indices.
 *
 * @param[out] coeff the lagrange coefficient
 * @param j lagrange coefficient we want to compute
 * @param indices indices
 * @param num number of indices in @a indices
 */
static void
compute_lagrange_coefficient (gcry_mpi_t coeff, unsigned int j,
                              unsigned int *indices,
                              unsigned int num)
{
  unsigned int i;
  /* numerator */
  gcry_mpi_t n;
  /* denominator */
  gcry_mpi_t d;
  /* temp value for l-j */
  gcry_mpi_t tmp;

  GNUNET_assert (0 != coeff);

  GNUNET_assert (0 != (n = gcry_mpi_new (0)));
  GNUNET_assert (0 != (d = gcry_mpi_new (0)));
  GNUNET_assert (0 != (tmp = gcry_mpi_new (0)));

  gcry_mpi_set_ui (n, 1);
  gcry_mpi_set_ui (d, 1);

  for (i = 0; i < num; i++)
  {
    unsigned int l = indices[i];
    if (l == j)
      continue;
    gcry_mpi_mul_ui (n, n, l + 1);
    // d <- d * (l-j)
    gcry_mpi_set_ui (tmp, l + 1);
    gcry_mpi_sub_ui (tmp, tmp, j + 1);
    gcry_mpi_mul (d, d, tmp);
  }

  // gcry_mpi_invm does not like negative numbers ...
  gcry_mpi_mod (d, d, elgamal_q);

  GNUNET_assert (gcry_mpi_cmp_ui (d, 0) > 0);

  // now we do the actual division, with everything mod q, as we
  // are not operating on elements from <g>, but on exponents
  GNUNET_assert (0 != gcry_mpi_invm (d, d, elgamal_q));

  gcry_mpi_mulm (coeff, n, d, elgamal_q);

  gcry_mpi_release (n);
  gcry_mpi_release (d);
  gcry_mpi_release (tmp);
}


/**
 * Destroy a decrypt session, removing it from
 * the linked list of decrypt sessions.
 *
 * @param ds decrypt session to destroy
 */
static void
decrypt_session_destroy (struct DecryptSession *ds)
{
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "destroying decrypt session\n");
  if (NULL != ds->cs)
  {
    ds->cs->decrypt_session = NULL;
    ds->cs = NULL;
  }
  if (NULL != ds->consensus)
  {
    GNUNET_CONSENSUS_destroy (ds->consensus);
    ds->consensus = NULL;
  }

  if (NULL != ds->info)
  {
    for (unsigned int i = 0; i < ds->share->num_peers; i++)
    {
      if (NULL != ds->info[i].partial_decryption)
      {
        gcry_mpi_release (ds->info[i].partial_decryption);
        ds->info[i].partial_decryption = NULL;
      }
    }
    GNUNET_free (ds->info);
    ds->info = NULL;
  }
  if (NULL != ds->share)
  {
    GNUNET_SECRETSHARING_share_destroy (ds->share);
    ds->share = NULL;
  }

  GNUNET_free (ds);
}


static void
keygen_info_destroy (struct KeygenPeerInfo *info)
{
  if (NULL != info->sigma)
  {
    gcry_mpi_release (info->sigma);
    info->sigma = NULL;
  }
  if (NULL != info->presecret_commitment)
  {
    gcry_mpi_release (info->presecret_commitment);
    info->presecret_commitment = NULL;
  }
  if (NULL != info->preshare_commitment)
  {
    gcry_mpi_release (info->preshare_commitment);
    info->preshare_commitment = NULL;
  }
}


static void
keygen_session_destroy (struct KeygenSession *ks)
{
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "destroying keygen session\n");

  if (NULL != ks->cs)
  {
    ks->cs->keygen_session = NULL;
    ks->cs = NULL;
  }
  if (NULL != ks->info)
  {
    for (unsigned int i = 0; i < ks->num_peers; i++)
      keygen_info_destroy (&ks->info[i]);
    GNUNET_free (ks->info);
    ks->info = NULL;
  }

  if (NULL != ks->consensus)
  {
    GNUNET_CONSENSUS_destroy (ks->consensus);
    ks->consensus = NULL;
  }

  if (NULL != ks->presecret_polynomial)
  {
    for (unsigned int i = 0; i < ks->threshold; i++)
    {
      GNUNET_assert (NULL != ks->presecret_polynomial[i]);
      gcry_mpi_release (ks->presecret_polynomial[i]);
      ks->presecret_polynomial[i] = NULL;
    }
    GNUNET_free (ks->presecret_polynomial);
    ks->presecret_polynomial = NULL;
  }
  if (NULL != ks->my_share)
  {
    gcry_mpi_release (ks->my_share);
    ks->my_share = NULL;
  }
  if (NULL != ks->public_key)
  {
    gcry_mpi_release (ks->public_key);
    ks->public_key = NULL;
  }
  if (NULL != ks->peers)
  {
    GNUNET_free (ks->peers);
    ks->peers = NULL;
  }
  GNUNET_free (ks);
}


/**
 * Task run during shutdown.
 *
 * @param cls unused
 */
static void
cleanup_task (void *cls)
{
  /* Nothing to do! */
}


/**
 * Generate the random coefficients of our pre-secret polynomial
 *
 * @param ks the session
 */
static void
generate_presecret_polynomial (struct KeygenSession *ks)
{
  int i;
  gcry_mpi_t v;

  GNUNET_assert (NULL == ks->presecret_polynomial);
  ks->presecret_polynomial = GNUNET_new_array (ks->threshold,
                                               gcry_mpi_t);
  for (i = 0; i < ks->threshold; i++)
  {
    v = ks->presecret_polynomial[i] = gcry_mpi_new (
      GNUNET_SECRETSHARING_ELGAMAL_BITS);
    GNUNET_assert (NULL != v);
    // Randomize v such that 0 < v < elgamal_q.
    // The '- 1' is necessary as bitlength(q) = bitlength(p) - 1.
    do
    {
      gcry_mpi_randomize (v, GNUNET_SECRETSHARING_ELGAMAL_BITS - 1,
                          GCRY_WEAK_RANDOM);
    }
    while ((gcry_mpi_cmp_ui (v, 0) == 0) || (gcry_mpi_cmp (v, elgamal_q) >= 0));
  }
}


/**
 * Consensus element handler for round one.
 * We should get one ephemeral key for each peer.
 *
 * @param cls Closure (keygen session).
 * @param element The element from consensus, or
 *                NULL if consensus failed.
 */
static void
keygen_round1_new_element (void *cls,
                           const struct GNUNET_SET_Element *element)
{
  const struct GNUNET_SECRETSHARING_KeygenCommitData *d;
  struct KeygenSession *ks = cls;
  struct KeygenPeerInfo *info;

  if (NULL == element)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING, "round1 consensus failed\n");
    return;
  }

  /* elements have fixed size */
  if (element->size != sizeof(struct GNUNET_SECRETSHARING_KeygenCommitData))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "keygen commit data with wrong size (%u) in consensus, %u expected\n",
                (unsigned int) element->size,
                (unsigned int) sizeof(struct
                                      GNUNET_SECRETSHARING_KeygenCommitData));
    return;
  }

  GNUNET_log (GNUNET_ERROR_TYPE_INFO, "got round1 element\n");

  d = element->data;
  info = get_keygen_peer_info (ks, &d->peer);

  if (NULL == info)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "keygen commit data with wrong peer identity (%s) in consensus\n",
                GNUNET_i2s (&d->peer));
    return;
  }

  /* Check that the right amount of data has been signed. */
  if (d->purpose.size !=
      htonl (element->size - offsetof (struct
                                       GNUNET_SECRETSHARING_KeygenCommitData,
                                       purpose)))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "keygen commit data with wrong signature purpose size in consensus\n");
    return;
  }

  if (GNUNET_OK != GNUNET_CRYPTO_eddsa_verify_ (
        GNUNET_SIGNATURE_PURPOSE_SECRETSHARING_DKG1,
        &d->purpose, &d->signature,
        &d->peer.public_key))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "keygen commit data with invalid signature in consensus\n");
    return;
  }
  info->paillier_public_key = d->pubkey;
  GNUNET_CRYPTO_mpi_scan_unsigned (&info->presecret_commitment, &d->commitment,
                                   512 / 8);
  info->round1_valid = GNUNET_YES;
}


/**
 * Evaluate the polynomial with coefficients @a coeff at @a x.
 * The i-th element in @a coeff corresponds to the coefficient of x^i.
 *
 * @param[out] z result of the evaluation
 * @param coeff array of coefficients
 * @param num_coeff number of coefficients
 * @param x where to evaluate the polynomial
 * @param m what group are we operating in?
 */
static void
horner_eval (gcry_mpi_t z, gcry_mpi_t *coeff, unsigned int num_coeff, gcry_mpi_t
             x, gcry_mpi_t m)
{
  unsigned int i;

  gcry_mpi_set_ui (z, 0);
  for (i = 0; i < num_coeff; i++)
  {
    // z <- zx + c
    gcry_mpi_mul (z, z, x);
    gcry_mpi_addm (z, z, coeff[num_coeff - i - 1], m);
  }
}


static void
keygen_round2_conclude (void *cls)
{
  struct KeygenSession *ks = cls;
  struct GNUNET_SECRETSHARING_SecretReadyMessage *m;
  struct GNUNET_MQ_Envelope *ev;
  size_t share_size;
  unsigned int i;
  unsigned int j;
  struct GNUNET_SECRETSHARING_Share *share;

  GNUNET_log (GNUNET_ERROR_TYPE_INFO, "round2 conclude\n");

  GNUNET_CONSENSUS_destroy (ks->consensus);
  ks->consensus = NULL;

  share = GNUNET_new (struct GNUNET_SECRETSHARING_Share);

  share->num_peers = 0;

  for (i = 0; i < ks->num_peers; i++)
    if (GNUNET_YES == ks->info[i].round2_valid)
      share->num_peers++;

  share->peers = GNUNET_new_array (share->num_peers,
                                   struct GNUNET_PeerIdentity);
  share->sigmas =
    GNUNET_new_array (share->num_peers,
                      struct GNUNET_SECRETSHARING_FieldElement);
  share->original_indices = GNUNET_new_array (share->num_peers,
                                              uint16_t);

  /* maybe we're not even in the list of peers? */
  share->my_peer = share->num_peers;

  j = 0; /* running index of valid peers */
  for (i = 0; i < ks->num_peers; i++)
  {
    if (GNUNET_YES == ks->info[i].round2_valid)
    {
      share->peers[j] = ks->info[i].peer;
      GNUNET_CRYPTO_mpi_print_unsigned (&share->sigmas[j],
                                        GNUNET_SECRETSHARING_ELGAMAL_BITS / 8,
                                        ks->info[i].sigma);
      share->original_indices[i] = j;
      if (0 == GNUNET_memcmp (&share->peers[i], &my_peer))
        share->my_peer = j;
      j += 1;
    }
  }

  if (share->my_peer == share->num_peers)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_INFO, "P%u: peer identity not in share\n",
                ks->local_peer_idx);
  }

  GNUNET_CRYPTO_mpi_print_unsigned (&share->my_share,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8,
                                    ks->my_share);
  GNUNET_CRYPTO_mpi_print_unsigned (&share->public_key,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8,
                                    ks->public_key);

  GNUNET_log (GNUNET_ERROR_TYPE_INFO, "keygen completed with %u peers\n",
              share->num_peers);

  /* Write the share. If 0 peers completed the dkg, an empty
   * share will be sent. */

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

  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "writing share of size %u\n",
              (unsigned int) share_size);

  ev = GNUNET_MQ_msg_extra (m, share_size,
                            GNUNET_MESSAGE_TYPE_SECRETSHARING_CLIENT_SECRET_READY);

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

  GNUNET_SECRETSHARING_share_destroy (share);
  share = NULL;

  GNUNET_MQ_send (ks->cs->mq,
                  ev);
}


static void
restore_fair (const struct GNUNET_CRYPTO_PaillierPublicKey *ppub,
              const struct GNUNET_SECRETSHARING_FairEncryption *fe,
              gcry_mpi_t x, gcry_mpi_t xres)
{
  gcry_mpi_t a_1;
  gcry_mpi_t a_2;
  gcry_mpi_t b_1;
  gcry_mpi_t b_2;
  gcry_mpi_t big_a;
  gcry_mpi_t big_b;
  gcry_mpi_t big_t;
  gcry_mpi_t n;
  gcry_mpi_t t_1;
  gcry_mpi_t t_2;
  gcry_mpi_t t;
  gcry_mpi_t r;
  gcry_mpi_t v;


  GNUNET_assert (NULL != (n = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (t = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (t_1 = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (t_2 = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (r = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (big_t = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (v = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (big_a = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (big_b = gcry_mpi_new (0)));

  // a = (N,0)^T
  GNUNET_CRYPTO_mpi_scan_unsigned (&a_1,
                                   ppub,
                                   sizeof(struct
                                          GNUNET_CRYPTO_PaillierPublicKey));
  GNUNET_assert (NULL != (a_2 = gcry_mpi_new (0)));
  gcry_mpi_set_ui (a_2, 0);
  // b = (x,1)^T
  GNUNET_assert (NULL != (b_1 = gcry_mpi_new (0)));
  gcry_mpi_set (b_1, x);
  GNUNET_assert (NULL != (b_2 = gcry_mpi_new (0)));
  gcry_mpi_set_ui (b_2, 1);

  // A = a DOT a
  gcry_mpi_mul (t, a_1, a_1);
  gcry_mpi_mul (big_a, a_2, a_2);
  gcry_mpi_add (big_a, big_a, t);

  // B = b DOT b
  gcry_mpi_mul (t, b_1, b_1);
  gcry_mpi_mul (big_b, b_2, b_2);
  gcry_mpi_add (big_b, big_b, t);

  while (1)
  {
    // n = a DOT b
    gcry_mpi_mul (t, a_1, b_1);
    gcry_mpi_mul (n, a_2, b_2);
    gcry_mpi_add (n, n, t);

    // r = nearest(n/B)
    gcry_mpi_div (r, NULL, n, big_b, 0);

    // T := A - 2rn + rrB
    gcry_mpi_mul (v, r, n);
    gcry_mpi_mul_ui (v, v, 2);
    gcry_mpi_sub (big_t, big_a, v);
    gcry_mpi_mul (v, r, r);
    gcry_mpi_mul (v, v, big_b);
    gcry_mpi_add (big_t, big_t, v);

    if (gcry_mpi_cmp (big_t, big_b) >= 0)
    {
      break;
    }

    // t = a - rb
    gcry_mpi_mul (v, r, b_1);
    gcry_mpi_sub (t_1, a_1, v);
    gcry_mpi_mul (v, r, b_2);
    gcry_mpi_sub (t_2, a_2, v);

    // a = b
    gcry_mpi_set (a_1, b_1);
    gcry_mpi_set (a_2, b_2);
    // b = t
    gcry_mpi_set (b_1, t_1);
    gcry_mpi_set (b_2, t_2);

    gcry_mpi_set (big_a, big_b);
    gcry_mpi_set (big_b, big_t);
  }

  gcry_mpi_set (xres, b_2);
  gcry_mpi_invm (xres, xres, elgamal_q);
  gcry_mpi_mulm (xres, xres, b_1, elgamal_q);

  gcry_mpi_release (a_1);
  gcry_mpi_release (a_2);
  gcry_mpi_release (b_1);
  gcry_mpi_release (b_2);
  gcry_mpi_release (big_a);
  gcry_mpi_release (big_b);
  gcry_mpi_release (big_t);
  gcry_mpi_release (n);
  gcry_mpi_release (t_1);
  gcry_mpi_release (t_2);
  gcry_mpi_release (t);
  gcry_mpi_release (r);
  gcry_mpi_release (v);
}


static void
get_fair_encryption_challenge (const struct
                               GNUNET_SECRETSHARING_FairEncryption *fe,
                               gcry_mpi_t *e)
{
  struct
  {
    struct GNUNET_CRYPTO_PaillierCiphertext c;
    char h[GNUNET_SECRETSHARING_ELGAMAL_BITS / 8];
    char t1[GNUNET_SECRETSHARING_ELGAMAL_BITS / 8];
    char t2[GNUNET_CRYPTO_PAILLIER_BITS * 2 / 8];
  } hash_data;
  struct GNUNET_HashCode e_hash;

  memset (&hash_data,
          0,
          sizeof(hash_data));
  GNUNET_memcpy (&hash_data.c, &fe->c, sizeof(struct
                                              GNUNET_CRYPTO_PaillierCiphertext));
  GNUNET_memcpy (&hash_data.h, &fe->h, GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
  GNUNET_memcpy (&hash_data.t1, &fe->t1, GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
  GNUNET_memcpy (&hash_data.t2, &fe->t2, GNUNET_CRYPTO_PAILLIER_BITS * 2 / 8);
  GNUNET_CRYPTO_hash (&hash_data,
                      sizeof(hash_data),
                      &e_hash);
  /* This allocates "e" */
  GNUNET_CRYPTO_mpi_scan_unsigned (e,
                                   &e_hash,
                                   sizeof(struct GNUNET_HashCode));
  gcry_mpi_mod (*e, *e, elgamal_q);
}


static int
verify_fair (const struct GNUNET_CRYPTO_PaillierPublicKey *ppub,
             const struct GNUNET_SECRETSHARING_FairEncryption *fe)
{
  gcry_mpi_t n;
  gcry_mpi_t n_sq;
  gcry_mpi_t z;
  gcry_mpi_t t1;
  gcry_mpi_t t2;
  gcry_mpi_t e;
  gcry_mpi_t w;
  gcry_mpi_t tmp1;
  gcry_mpi_t tmp2;
  gcry_mpi_t y;
  gcry_mpi_t big_y;
  int res;

  GNUNET_assert (NULL != (n_sq = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (tmp1 = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (tmp2 = gcry_mpi_new (0)));

  get_fair_encryption_challenge (fe,
                                 &e /* this allocates e */);

  GNUNET_CRYPTO_mpi_scan_unsigned (&n,
                                   ppub,
                                   sizeof(struct
                                          GNUNET_CRYPTO_PaillierPublicKey));
  GNUNET_CRYPTO_mpi_scan_unsigned (&t1, fe->t1, GNUNET_CRYPTO_PAILLIER_BITS
                                   / 8);
  GNUNET_CRYPTO_mpi_scan_unsigned (&z, fe->z,
                                   GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
  GNUNET_CRYPTO_mpi_scan_unsigned (&y, fe->h,
                                   GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
  GNUNET_CRYPTO_mpi_scan_unsigned (&w, fe->w, GNUNET_CRYPTO_PAILLIER_BITS / 8);
  GNUNET_CRYPTO_mpi_scan_unsigned (&big_y, fe->c.bits,
                                   GNUNET_CRYPTO_PAILLIER_BITS * 2 / 8);
  GNUNET_CRYPTO_mpi_scan_unsigned (&t2, fe->t2, GNUNET_CRYPTO_PAILLIER_BITS
                                   * 2 / 8);
  gcry_mpi_mul (n_sq, n, n);

  // tmp1 = g^z
  gcry_mpi_powm (tmp1, elgamal_g, z, elgamal_p);
  // tmp2 = y^{-e}
  gcry_mpi_powm (tmp1, y, e, elgamal_p);
  gcry_mpi_invm (tmp1, tmp1, elgamal_p);
  // tmp1 = tmp1 * tmp2
  gcry_mpi_mulm (tmp1, tmp1, tmp2, elgamal_p);

  if (0 == gcry_mpi_cmp (t1, tmp1))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR, "fair encryption invalid (t1)\n");
    res = GNUNET_NO;
    goto cleanup;
  }

  gcry_mpi_powm (big_y, big_y, e, n_sq);
  gcry_mpi_invm (big_y, big_y, n_sq);

  gcry_mpi_add_ui (tmp1, n, 1);
  gcry_mpi_powm (tmp1, tmp1, z, n_sq);

  gcry_mpi_powm (tmp2, w, n, n_sq);

  gcry_mpi_mulm (tmp1, tmp1, tmp2, n_sq);
  gcry_mpi_mulm (tmp1, tmp1, big_y, n_sq);


  if (0 == gcry_mpi_cmp (t2, tmp1))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR, "fair encryption invalid (t2)\n");
    res = GNUNET_NO;
    goto cleanup;
  }

  res = GNUNET_YES;

cleanup:

  gcry_mpi_release (n);
  gcry_mpi_release (n_sq);
  gcry_mpi_release (z);
  gcry_mpi_release (t1);
  gcry_mpi_release (t2);
  gcry_mpi_release (e);
  gcry_mpi_release (w);
  gcry_mpi_release (tmp1);
  gcry_mpi_release (tmp2);
  gcry_mpi_release (y);
  gcry_mpi_release (big_y);
  return res;
}


/**
 * Create a fair Paillier encryption of then given ciphertext.
 *
 * @param v the ciphertext
 * @param[out] fe the fair encryption
 */
static void
encrypt_fair (gcry_mpi_t v,
              const struct GNUNET_CRYPTO_PaillierPublicKey *ppub,
              struct GNUNET_SECRETSHARING_FairEncryption *fe)
{
  gcry_mpi_t r;
  gcry_mpi_t s;
  gcry_mpi_t t1;
  gcry_mpi_t t2;
  gcry_mpi_t z;
  gcry_mpi_t w;
  gcry_mpi_t n;
  gcry_mpi_t e;
  gcry_mpi_t n_sq;
  gcry_mpi_t u;
  gcry_mpi_t Y;
  gcry_mpi_t G;
  gcry_mpi_t h;

  GNUNET_assert (NULL != (r = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (s = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (t1 = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (t2 = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (z = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (w = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (n_sq = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (u = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (Y = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (G = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (h = gcry_mpi_new (0)));

  GNUNET_CRYPTO_mpi_scan_unsigned (&n,
                                   ppub,
                                   sizeof(struct
                                          GNUNET_CRYPTO_PaillierPublicKey));
  gcry_mpi_mul (n_sq, n, n);
  gcry_mpi_add_ui (G, n, 1);

  do
  {
    gcry_mpi_randomize (u, GNUNET_CRYPTO_PAILLIER_BITS, GCRY_WEAK_RANDOM);
  }
  while (gcry_mpi_cmp (u, n) >= 0);

  gcry_mpi_powm (t1, G, v, n_sq);
  gcry_mpi_powm (t2, u, n, n_sq);
  gcry_mpi_mulm (Y, t1, t2, n_sq);

  GNUNET_CRYPTO_mpi_print_unsigned (fe->c.bits,
                                    sizeof fe->c.bits,
                                    Y);


  gcry_mpi_randomize (r, 2048, GCRY_WEAK_RANDOM);
  do
  {
    gcry_mpi_randomize (s, GNUNET_CRYPTO_PAILLIER_BITS, GCRY_WEAK_RANDOM);
  }
  while (gcry_mpi_cmp (s, n) >= 0);

  // compute t1
  gcry_mpi_mulm (t1, elgamal_g, r, elgamal_p);
  // compute t2 (use z and w as temp)
  gcry_mpi_powm (z, G, r, n_sq);
  gcry_mpi_powm (w, s, n, n_sq);
  gcry_mpi_mulm (t2, z, w, n_sq);


  gcry_mpi_powm (h, elgamal_g, v, elgamal_p);

  GNUNET_CRYPTO_mpi_print_unsigned (fe->h,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8,
                                    h);

  GNUNET_CRYPTO_mpi_print_unsigned (fe->t1,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8,
                                    t1);

  GNUNET_CRYPTO_mpi_print_unsigned (fe->t2,
                                    GNUNET_CRYPTO_PAILLIER_BITS * 2 / 8,
                                    t2);

  get_fair_encryption_challenge (fe,
                                 &e /* This allocates "e" */);

  // compute z
  gcry_mpi_mul (z, e, v);
  gcry_mpi_addm (z, z, r, elgamal_q);
  // compute w
  gcry_mpi_powm (w, u, e, n);
  gcry_mpi_mulm (w, w, s, n);

  GNUNET_CRYPTO_mpi_print_unsigned (fe->z,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8,
                                    z);

  GNUNET_CRYPTO_mpi_print_unsigned (fe->w,
                                    GNUNET_CRYPTO_PAILLIER_BITS / 8,
                                    w);

  gcry_mpi_release (n);
  gcry_mpi_release (r);
  gcry_mpi_release (s);
  gcry_mpi_release (t1);
  gcry_mpi_release (t2);
  gcry_mpi_release (z);
  gcry_mpi_release (w);
  gcry_mpi_release (e);
  gcry_mpi_release (n_sq);
  gcry_mpi_release (u);
  gcry_mpi_release (Y);
  gcry_mpi_release (G);
  gcry_mpi_release (h);
}


/**
 * Insert round 2 element in the consensus, consisting of
 * (1) The exponentiated pre-share polynomial coefficients A_{i,l}=g^{a_{i,l}}
 * (2) The exponentiated pre-shares y_{i,j}=g^{s_{i,j}}
 * (3) The encrypted pre-shares Y_{i,j}
 * (4) The zero knowledge proof for fairness of
 *     the encryption
 *
 * @param ks session to use
 */
static void
insert_round2_element (struct KeygenSession *ks)
{
  struct GNUNET_SET_Element *element;
  struct GNUNET_SECRETSHARING_KeygenRevealData *d;
  unsigned char *pos;
  unsigned char *last_pos;
  size_t element_size;
  unsigned int i;
  gcry_mpi_t idx;
  gcry_mpi_t v;

  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "P%u: Inserting round2 element\n",
              ks->local_peer_idx);

  GNUNET_assert (NULL != (v = gcry_mpi_new (
                            GNUNET_SECRETSHARING_ELGAMAL_BITS)));
  GNUNET_assert (NULL != (idx = gcry_mpi_new (
                            GNUNET_SECRETSHARING_ELGAMAL_BITS)));

  element_size = (sizeof(struct GNUNET_SECRETSHARING_KeygenRevealData)
                  + sizeof(struct GNUNET_SECRETSHARING_FairEncryption)
                  * ks->num_peers
                  + GNUNET_SECRETSHARING_ELGAMAL_BITS / 8 * ks->threshold);

  element = GNUNET_malloc (sizeof(struct GNUNET_SET_Element) + element_size);
  element->size = element_size;
  element->data = (void *) &element[1];

  d = (void *) element->data;
  d->peer = my_peer;

  // start inserting vector elements
  // after the fixed part of the element's data
  pos = (void *) &d[1];
  last_pos = pos + element_size;

  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "P%u: computed exp preshares\n",
              ks->local_peer_idx);

  // encrypted pre-shares
  // and fair encryption proof
  {
    for (i = 0; i < ks->num_peers; i++)
    {
      ptrdiff_t remaining = last_pos - pos;
      struct GNUNET_SECRETSHARING_FairEncryption *fe = (void *) pos;

      GNUNET_assert (remaining > 0);
      memset (fe, 0, sizeof *fe);
      if (GNUNET_YES == ks->info[i].round1_valid)
      {
        gcry_mpi_set_ui (idx, i + 1);
        // evaluate the polynomial
        horner_eval (v, ks->presecret_polynomial, ks->threshold, idx,
                     elgamal_q);
        // encrypt the result
        encrypt_fair (v, &ks->info[i].paillier_public_key, fe);
      }
      pos += sizeof *fe;
    }
  }

  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "P%u: computed enc preshares\n",
              ks->local_peer_idx);

  // exponentiated coefficients
  for (i = 0; i < ks->threshold; i++)
  {
    ptrdiff_t remaining = last_pos - pos;
    GNUNET_assert (remaining > 0);
    gcry_mpi_powm (v, elgamal_g, ks->presecret_polynomial[i], elgamal_p);
    GNUNET_CRYPTO_mpi_print_unsigned (pos, GNUNET_SECRETSHARING_ELGAMAL_BITS
                                      / 8, v);
    pos += GNUNET_SECRETSHARING_ELGAMAL_BITS / 8;
  }

  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "P%u: computed exp coefficients\n",
              ks->local_peer_idx);


  d->purpose.size = htonl (element_size - offsetof (struct
                                                    GNUNET_SECRETSHARING_KeygenRevealData,
                                                    purpose));
  d->purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_SECRETSHARING_DKG2);
  GNUNET_assert (GNUNET_OK ==
                 GNUNET_CRYPTO_eddsa_sign_ (my_peer_private_key,
                                            &d->purpose,
                                            &d->signature));

  GNUNET_CONSENSUS_insert (ks->consensus, element, NULL, NULL);
  GNUNET_free (element);  /* FIXME: maybe stack-allocate instead? */

  gcry_mpi_release (v);
  gcry_mpi_release (idx);
}


static gcry_mpi_t
keygen_reveal_get_exp_coeff (struct KeygenSession *ks,
                             const struct
                             GNUNET_SECRETSHARING_KeygenRevealData *d,
                             unsigned int idx)
{
  unsigned char *pos;
  gcry_mpi_t exp_coeff;

  GNUNET_assert (idx < ks->threshold);

  pos = (void *) &d[1];
  // skip encrypted pre-shares
  pos += sizeof(struct GNUNET_SECRETSHARING_FairEncryption) * ks->num_peers;
  // skip exp. coeffs we are not interested in
  pos += GNUNET_SECRETSHARING_ELGAMAL_BITS / 8 * idx;
  // the first exponentiated coefficient is the public key share
  GNUNET_CRYPTO_mpi_scan_unsigned (&exp_coeff, pos,
                                   GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
  return exp_coeff;
}


static struct GNUNET_SECRETSHARING_FairEncryption *
keygen_reveal_get_enc_preshare (struct KeygenSession *ks,
                                const struct
                                GNUNET_SECRETSHARING_KeygenRevealData *d,
                                unsigned int idx)
{
  unsigned char *pos;

  GNUNET_assert (idx < ks->num_peers);

  pos = (void *) &d[1];
  // skip encrypted pre-shares we're not interested in
  pos += sizeof(struct GNUNET_SECRETSHARING_FairEncryption) * idx;
  return (struct GNUNET_SECRETSHARING_FairEncryption *) pos;
}


static gcry_mpi_t
keygen_reveal_get_exp_preshare (struct KeygenSession *ks,
                                const struct
                                GNUNET_SECRETSHARING_KeygenRevealData *d,
                                unsigned int idx)
{
  gcry_mpi_t exp_preshare;
  struct GNUNET_SECRETSHARING_FairEncryption *fe;

  GNUNET_assert (idx < ks->num_peers);
  fe = keygen_reveal_get_enc_preshare (ks, d, idx);
  GNUNET_CRYPTO_mpi_scan_unsigned (&exp_preshare, fe->h,
                                   GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
  return exp_preshare;
}


static void
keygen_round2_new_element (void *cls,
                           const struct GNUNET_SET_Element *element)
{
  struct KeygenSession *ks = cls;
  const struct GNUNET_SECRETSHARING_KeygenRevealData *d;
  struct KeygenPeerInfo *info;
  size_t expected_element_size;
  unsigned int j;
  int cmp_result;
  gcry_mpi_t tmp;
  gcry_mpi_t public_key_share;
  gcry_mpi_t preshare;

  if (NULL == element)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING, "round2 consensus failed\n");
    return;
  }

  expected_element_size = (sizeof(struct GNUNET_SECRETSHARING_KeygenRevealData)
                           + sizeof(struct
                                    GNUNET_SECRETSHARING_FairEncryption)
                           * ks->num_peers
                           + GNUNET_SECRETSHARING_ELGAMAL_BITS / 8
                           * ks->threshold);

  if (element->size != expected_element_size)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "keygen round2 data with wrong size (%u) in consensus, %u expected\n",
                (unsigned int) element->size,
                (unsigned int) expected_element_size);
    return;
  }

  d = (const void *) element->data;

  info = get_keygen_peer_info (ks, &d->peer);

  if (NULL == info)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "keygen commit data with wrong peer identity (%s) in consensus\n",
                GNUNET_i2s (&d->peer));
    return;
  }

  if (GNUNET_NO == info->round1_valid)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "ignoring round2 element from peer with invalid round1 element (%s)\n",
                GNUNET_i2s (&d->peer));
    return;
  }

  if (GNUNET_YES == info->round2_valid)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "ignoring duplicate round2 element (%s)\n",
                GNUNET_i2s (&d->peer));
    return;
  }

  GNUNET_log (GNUNET_ERROR_TYPE_INFO, "got round2 element\n");

  if (ntohl (d->purpose.size) !=
      element->size - offsetof (struct GNUNET_SECRETSHARING_KeygenRevealData,
                                purpose))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "keygen reveal data with wrong signature purpose size in consensus\n");
    return;
  }

  if (GNUNET_OK != GNUNET_CRYPTO_eddsa_verify_ (
        GNUNET_SIGNATURE_PURPOSE_SECRETSHARING_DKG2,
        &d->purpose, &d->signature,
        &d->peer.public_key))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "keygen reveal data with invalid signature in consensus\n");
    return;
  }

  public_key_share = keygen_reveal_get_exp_coeff (ks, d, 0);
  info->preshare_commitment = keygen_reveal_get_exp_preshare (ks, d,
                                                              ks->local_peer_idx);

  if (NULL == ks->public_key)
  {
    GNUNET_assert (NULL != (ks->public_key = gcry_mpi_new (0)));
    gcry_mpi_set_ui (ks->public_key, 1);
  }
  gcry_mpi_mulm (ks->public_key, ks->public_key, public_key_share, elgamal_p);

  gcry_mpi_release (public_key_share);
  public_key_share = NULL;

  {
    struct GNUNET_SECRETSHARING_FairEncryption *fe =
      keygen_reveal_get_enc_preshare (ks, d, ks->local_peer_idx);
    GNUNET_assert (NULL != (preshare = gcry_mpi_new (0)));
    GNUNET_CRYPTO_paillier_decrypt (&ks->paillier_private_key,
                                    &ks->info[ks->local_peer_idx].
                                    paillier_public_key,
                                    &fe->c,
                                    preshare);

    // FIXME: not doing the restoration is less expensive
    restore_fair (&ks->info[ks->local_peer_idx].paillier_public_key,
                  fe,
                  preshare,
                  preshare);
  }

  GNUNET_assert (NULL != (tmp = gcry_mpi_new (0)));
  gcry_mpi_powm (tmp, elgamal_g, preshare, elgamal_p);

  cmp_result = gcry_mpi_cmp (tmp, info->preshare_commitment);
  gcry_mpi_release (tmp);
  tmp = NULL;
  if (0 != cmp_result)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "P%u: Got invalid presecret from P%u\n",
                (unsigned int) ks->local_peer_idx, (unsigned int) (info
                                                                   - ks->info));
    return;
  }

  if (NULL == ks->my_share)
  {
    GNUNET_assert (NULL != (ks->my_share = gcry_mpi_new (0)));
  }
  gcry_mpi_addm (ks->my_share, ks->my_share, preshare, elgamal_q);

  for (j = 0; j < ks->num_peers; j++)
  {
    gcry_mpi_t presigma;
    if (NULL == ks->info[j].sigma)
    {
      GNUNET_assert (NULL != (ks->info[j].sigma = gcry_mpi_new (0)));
      gcry_mpi_set_ui (ks->info[j].sigma, 1);
    }
    presigma = keygen_reveal_get_exp_preshare (ks, d, j);
    gcry_mpi_mulm (ks->info[j].sigma, ks->info[j].sigma, presigma, elgamal_p);
    gcry_mpi_release (presigma);
  }

  gcry_mpi_t prod;
  GNUNET_assert (NULL != (prod = gcry_mpi_new (0)));
  gcry_mpi_t j_to_k;
  GNUNET_assert (NULL != (j_to_k = gcry_mpi_new (0)));
  // validate that the polynomial sharing matches the additive sharing
  for (j = 0; j < ks->num_peers; j++)
  {
    unsigned int k;
    int cmp_result;
    gcry_mpi_t exp_preshare;
    gcry_mpi_set_ui (prod, 1);
    for (k = 0; k < ks->threshold; k++)
    {
      // Using pow(double,double) is a bit sketchy.
      // We count players from 1, but shares from 0.
      gcry_mpi_t tmp;
      gcry_mpi_set_ui (j_to_k, (unsigned int) pow (j + 1, k));
      tmp = keygen_reveal_get_exp_coeff (ks, d, k);
      gcry_mpi_powm (tmp, tmp, j_to_k, elgamal_p);
      gcry_mpi_mulm (prod, prod, tmp, elgamal_p);
      gcry_mpi_release (tmp);
    }
    exp_preshare = keygen_reveal_get_exp_preshare (ks, d, j);
    gcry_mpi_mod (exp_preshare, exp_preshare, elgamal_p);
    cmp_result = gcry_mpi_cmp (prod, exp_preshare);
    gcry_mpi_release (exp_preshare);
    exp_preshare = NULL;
    if (0 != cmp_result)
    {
      GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                  "P%u: reveal data from P%u incorrect\n",
                  ks->local_peer_idx, j);
      /* no need for further verification, round2 stays invalid ... */
      return;
    }
  }

  // TODO: verify proof of fair encryption (once implemented)
  for (j = 0; j < ks->num_peers; j++)
  {
    struct GNUNET_SECRETSHARING_FairEncryption *fe =
      keygen_reveal_get_enc_preshare (ks, d, j);
    if (GNUNET_YES != verify_fair (&ks->info[j].paillier_public_key, fe))
    {
      GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                  "P%u: reveal data from P%u incorrect (fair encryption)\n",
                  ks->local_peer_idx, j);
      return;
    }
  }

  info->round2_valid = GNUNET_YES;

  gcry_mpi_release (preshare);
  gcry_mpi_release (prod);
  gcry_mpi_release (j_to_k);
}


/**
 * Called when the first consensus round has concluded.
 * Will initiate the second round.
 *
 * @param cls closure
 */
static void
keygen_round1_conclude (void *cls)
{
  struct KeygenSession *ks = cls;

  GNUNET_CONSENSUS_destroy (ks->consensus);

  ks->consensus = GNUNET_CONSENSUS_create (cfg, ks->num_peers, ks->peers,
                                           &ks->session_id,
                                           time_between (ks->start_time,
                                                         ks->deadline, 1, 2),
                                           ks->deadline,
                                           keygen_round2_new_element, ks);

  insert_round2_element (ks);

  GNUNET_CONSENSUS_conclude (ks->consensus,
                             keygen_round2_conclude,
                             ks);
}


/**
 * Insert the ephemeral key and the presecret commitment
 * of this peer in the consensus of the given session.
 *
 * @param ks session to use
 */
static void
insert_round1_element (struct KeygenSession *ks)
{
  struct GNUNET_SET_Element *element;
  struct GNUNET_SECRETSHARING_KeygenCommitData *d;
  // g^a_{i,0}
  gcry_mpi_t v;
  // big-endian representation of 'v'
  unsigned char v_data[GNUNET_SECRETSHARING_ELGAMAL_BITS / 8];

  element = GNUNET_malloc (sizeof *element + sizeof *d);
  d = (void *) &element[1];
  element->data = d;
  element->size = sizeof *d;

  d->peer = my_peer;

  GNUNET_assert (0 != (v = gcry_mpi_new (GNUNET_SECRETSHARING_ELGAMAL_BITS)));

  gcry_mpi_powm (v, elgamal_g, ks->presecret_polynomial[0], elgamal_p);

  GNUNET_CRYPTO_mpi_print_unsigned (v_data, GNUNET_SECRETSHARING_ELGAMAL_BITS
                                    / 8, v);

  GNUNET_CRYPTO_hash (v_data, GNUNET_SECRETSHARING_ELGAMAL_BITS / 8,
                      &d->commitment);

  d->pubkey = ks->info[ks->local_peer_idx].paillier_public_key;

  d->purpose.size = htonl ((sizeof *d) - offsetof (struct
                                                   GNUNET_SECRETSHARING_KeygenCommitData,
                                                   purpose));
  d->purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_SECRETSHARING_DKG1);
  GNUNET_assert (GNUNET_OK ==
                 GNUNET_CRYPTO_eddsa_sign_ (my_peer_private_key,
                                            &d->purpose,
                                            &d->signature));

  GNUNET_CONSENSUS_insert (ks->consensus, element, NULL, NULL);

  gcry_mpi_release (v);
  GNUNET_free (element);
}


/**
 * Check that @a msg is well-formed.
 *
 * @param cls identification of the client
 * @param msg the actual message
 * @return #GNUNET_OK if @a msg is well-formed
 */
static int
check_client_keygen (void *cls,
                     const struct GNUNET_SECRETSHARING_CreateMessage *msg)
{
  unsigned int num_peers = ntohs (msg->num_peers);

  if (ntohs (msg->header.size) - sizeof(*msg) !=
      num_peers * sizeof(struct GNUNET_PeerIdentity))
  {
    GNUNET_break (0);
    return GNUNET_SYSERR;
  }
  return GNUNET_OK;
}


/**
 * Functions with this signature are called whenever a message is
 * received.
 *
 * @param cls identification of the client
 * @param msg the actual message
 */
static void
handle_client_keygen (void *cls,
                      const struct GNUNET_SECRETSHARING_CreateMessage *msg)
{
  struct ClientState *cs = cls;
  struct KeygenSession *ks;

  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
              "client requested key generation\n");
  if (NULL != cs->keygen_session)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (cs->client);
    return;
  }
  ks = GNUNET_new (struct KeygenSession);
  ks->cs = cs;
  cs->keygen_session = ks;
  ks->deadline = GNUNET_TIME_absolute_ntoh (msg->deadline);
  ks->threshold = ntohs (msg->threshold);
  ks->num_peers = ntohs (msg->num_peers);

  ks->peers = normalize_peers ((struct GNUNET_PeerIdentity *) &msg[1],
                               ks->num_peers,
                               &ks->num_peers,
                               &ks->local_peer_idx);


  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
              "first round of consensus with %u peers\n",
              ks->num_peers);
  ks->consensus = GNUNET_CONSENSUS_create (cfg,
                                           ks->num_peers,
                                           ks->peers,
                                           &msg->session_id,
                                           GNUNET_TIME_absolute_ntoh (
                                             msg->start),
                                           GNUNET_TIME_absolute_ntoh (
                                             msg->deadline),
                                           keygen_round1_new_element,
                                           ks);

  ks->info = GNUNET_new_array (ks->num_peers,
                               struct KeygenPeerInfo);

  for (unsigned int i = 0; i < ks->num_peers; i++)
    ks->info[i].peer = ks->peers[i];

  GNUNET_CRYPTO_paillier_create (
    &ks->info[ks->local_peer_idx].paillier_public_key,
    &ks->paillier_private_key);

  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "P%u: Generated paillier key pair\n",
              ks->local_peer_idx);
  generate_presecret_polynomial (ks);
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "P%u: Generated presecret polynomial\n",
              ks->local_peer_idx);
  insert_round1_element (ks);
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "P%u: Concluding for round 1\n",
              ks->local_peer_idx);
  GNUNET_CONSENSUS_conclude (ks->consensus,
                             keygen_round1_conclude,
                             ks);
  GNUNET_SERVICE_client_continue (cs->client);
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "P%u: Waiting for round 1 elements ...\n",
              ks->local_peer_idx);
}


/**
 * Called when the partial decryption consensus concludes.
 */
static void
decrypt_conclude (void *cls)
{
  struct DecryptSession *ds = cls;
  struct GNUNET_SECRETSHARING_DecryptResponseMessage *msg;
  struct GNUNET_MQ_Envelope *ev;
  gcry_mpi_t lagrange;
  gcry_mpi_t m;
  gcry_mpi_t tmp;
  gcry_mpi_t c_2;
  gcry_mpi_t prod;
  unsigned int *indices;
  unsigned int num;
  unsigned int i;
  unsigned int j;

  GNUNET_CONSENSUS_destroy (ds->consensus);
  ds->consensus = NULL;

  GNUNET_assert (0 != (lagrange = gcry_mpi_new (0)));
  GNUNET_assert (0 != (m = gcry_mpi_new (0)));
  GNUNET_assert (0 != (tmp = gcry_mpi_new (0)));
  GNUNET_assert (0 != (prod = gcry_mpi_new (0)));

  num = 0;
  for (i = 0; i < ds->share->num_peers; i++)
    if (NULL != ds->info[i].partial_decryption)
      num++;

  indices = GNUNET_new_array (num,
                              unsigned int);
  j = 0;
  for (i = 0; i < ds->share->num_peers; i++)
    if (NULL != ds->info[i].partial_decryption)
      indices[j++] = ds->info[i].original_index;

  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
              "P%u: decrypt conclude, with %u peers\n",
              ds->share->my_peer,
              num);

  gcry_mpi_set_ui (prod, 1);
  for (i = 0; i < num; i++)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_INFO,
                "P%u: index of %u: %u\n",
                ds->share->my_peer, i, indices[i]);
    compute_lagrange_coefficient (lagrange, indices[i], indices, num);
    // w_i^{\lambda_i}
    gcry_mpi_powm (tmp, ds->info[indices[i]].partial_decryption, lagrange,
                   elgamal_p);

    // product of all exponentiated partiel decryptions ...
    gcry_mpi_mulm (prod, prod, tmp, elgamal_p);
  }

  GNUNET_CRYPTO_mpi_scan_unsigned (&c_2, ds->ciphertext.c2_bits,
                                   GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);

  GNUNET_assert (0 != gcry_mpi_invm (prod, prod, elgamal_p));
  gcry_mpi_mulm (m, c_2, prod, elgamal_p);
  ev = GNUNET_MQ_msg (msg,
                      GNUNET_MESSAGE_TYPE_SECRETSHARING_CLIENT_DECRYPT_DONE);
  GNUNET_CRYPTO_mpi_print_unsigned (&msg->plaintext,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8, m);
  msg->success = htonl (1);
  GNUNET_MQ_send (ds->cs->mq,
                  ev);

  GNUNET_log (GNUNET_ERROR_TYPE_INFO, "sent decrypt done to client\n");

  GNUNET_free (indices);

  gcry_mpi_release (lagrange);
  gcry_mpi_release (m);
  gcry_mpi_release (tmp);
  gcry_mpi_release (prod);
  gcry_mpi_release (c_2);

  // FIXME: what if not enough peers participated?
}


/**
 * Get a string representation of an MPI.
 * The caller must free the returned string.
 *
 * @param mpi mpi to convert to a string
 * @return string representation of @a mpi, must be free'd by the caller
 */
static char *
mpi_to_str (gcry_mpi_t mpi)
{
  unsigned char *buf;

  GNUNET_assert (0 == gcry_mpi_aprint (GCRYMPI_FMT_HEX, &buf, NULL, mpi));
  return (char *) buf;
}


/**
 * Called when a new partial decryption arrives.
 */
static void
decrypt_new_element (void *cls,
                     const struct GNUNET_SET_Element *element)
{
  struct DecryptSession *session = cls;
  const struct GNUNET_SECRETSHARING_DecryptData *d;
  struct DecryptPeerInfo *info;
  struct GNUNET_HashCode challenge_hash;

  /* nizk response */
  gcry_mpi_t r;
  /* nizk challenge */
  gcry_mpi_t challenge;
  /* nizk commit1, g^\beta */
  gcry_mpi_t commit1;
  /* nizk commit2, c_1^\beta */
  gcry_mpi_t commit2;
  /* homomorphic commitment to the peer's share,
   * public key share */
  gcry_mpi_t sigma;
  /* partial decryption we received */
  gcry_mpi_t w;
  /* ciphertext component #1 */
  gcry_mpi_t c1;
  /* temporary variable (for comparison) #1 */
  gcry_mpi_t tmp1;
  /* temporary variable (for comparison) #2 */
  gcry_mpi_t tmp2;

  if (NULL == element)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR, "decryption failed\n");
    /* FIXME: destroy */
    return;
  }

  if (element->size != sizeof *d)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "element of wrong size in decrypt consensus\n");
    return;
  }

  d = element->data;

  info = get_decrypt_peer_info (session, &d->peer);

  if (NULL == info)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "decrypt element from invalid peer (%s)\n",
                GNUNET_i2s (&d->peer));
    return;
  }

  if (NULL != info->partial_decryption)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR, "decrypt element duplicate\n");
    return;
  }

  if (0 != GNUNET_memcmp (&d->ciphertext, &session->ciphertext))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "P%u: got decrypt element with non-matching ciphertext from P%u\n",
                (unsigned int) session->share->my_peer, (unsigned int) (info
                                                                        -
                                                                        session
                                                                        ->info));

    return;
  }


  GNUNET_CRYPTO_hash (offsetof (struct GNUNET_SECRETSHARING_DecryptData,
                                ciphertext) + (char *) d,
                      offsetof (struct GNUNET_SECRETSHARING_DecryptData,
                                nizk_response)
                      - offsetof (struct GNUNET_SECRETSHARING_DecryptData,
                                  ciphertext),
                      &challenge_hash);

  GNUNET_CRYPTO_mpi_scan_unsigned (&challenge, &challenge_hash,
                                   sizeof(struct GNUNET_HashCode));

  GNUNET_CRYPTO_mpi_scan_unsigned (&sigma, &session->share->sigmas[info
                                                                   - session->
                                                                   info],
                                   sizeof(struct
                                          GNUNET_SECRETSHARING_FieldElement));

  GNUNET_CRYPTO_mpi_scan_unsigned (&c1, session->ciphertext.c1_bits,
                                   sizeof(struct
                                          GNUNET_SECRETSHARING_FieldElement));

  GNUNET_CRYPTO_mpi_scan_unsigned (&commit1, &d->nizk_commit1,
                                   sizeof(struct
                                          GNUNET_SECRETSHARING_FieldElement));

  GNUNET_CRYPTO_mpi_scan_unsigned (&commit2, &d->nizk_commit2,
                                   sizeof(struct
                                          GNUNET_SECRETSHARING_FieldElement));

  GNUNET_CRYPTO_mpi_scan_unsigned (&r, &d->nizk_response,
                                   sizeof(struct
                                          GNUNET_SECRETSHARING_FieldElement));

  GNUNET_CRYPTO_mpi_scan_unsigned (&w, &d->partial_decryption,
                                   sizeof(struct
                                          GNUNET_SECRETSHARING_FieldElement));

  GNUNET_assert (NULL != (tmp1 = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (tmp2 = gcry_mpi_new (0)));

  // tmp1 = g^r
  gcry_mpi_powm (tmp1, elgamal_g, r, elgamal_p);

  // tmp2 = g^\beta * \sigma^challenge
  gcry_mpi_powm (tmp2, sigma, challenge, elgamal_p);
  gcry_mpi_mulm (tmp2, tmp2, commit1, elgamal_p);

  if (0 != gcry_mpi_cmp (tmp1, tmp2))
  {
    char *tmp1_str;
    char *tmp2_str;

    tmp1_str = mpi_to_str (tmp1);
    tmp2_str = mpi_to_str (tmp2);
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "P%u: Received invalid partial decryption from P%u (eqn 1), expected %s got %s\n",
                session->share->my_peer,
                (unsigned int) (info - session->info),
                tmp1_str,
                tmp2_str);
    GNUNET_free (tmp1_str);
    GNUNET_free (tmp2_str);
    goto cleanup;
  }


  gcry_mpi_powm (tmp1, c1, r, elgamal_p);

  gcry_mpi_powm (tmp2, w, challenge, elgamal_p);
  gcry_mpi_mulm (tmp2, tmp2, commit2, elgamal_p);


  if (0 != gcry_mpi_cmp (tmp1, tmp2))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                "P%u: Received invalid partial decryption from P%u (eqn 2)\n",
                session->share->my_peer,
                (unsigned int) (info - session->info));
    goto cleanup;
  }


  GNUNET_CRYPTO_mpi_scan_unsigned (&info->partial_decryption,
                                   &d->partial_decryption,
                                   GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
cleanup:
  gcry_mpi_release (tmp1);
  gcry_mpi_release (tmp2);
  gcry_mpi_release (sigma);
  gcry_mpi_release (commit1);
  gcry_mpi_release (commit2);
  gcry_mpi_release (r);
  gcry_mpi_release (w);
  gcry_mpi_release (challenge);
  gcry_mpi_release (c1);
}


static void
insert_decrypt_element (struct DecryptSession *ds)
{
  struct GNUNET_SECRETSHARING_DecryptData d;
  struct GNUNET_SET_Element element;
  /* our share */
  gcry_mpi_t s;
  /* partial decryption with our share */
  gcry_mpi_t w;
  /* first component of the elgamal ciphertext */
  gcry_mpi_t c1;
  /* nonce for dlog zkp */
  gcry_mpi_t beta;
  gcry_mpi_t tmp;
  gcry_mpi_t challenge;
  gcry_mpi_t sigma;
  struct GNUNET_HashCode challenge_hash;

  /* make vagrind happy until we implement the real deal ... */
  memset (&d, 0, sizeof d);

  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "P%u: Inserting decrypt element\n",
              ds->share->my_peer);

  GNUNET_assert (ds->share->my_peer < ds->share->num_peers);

  GNUNET_CRYPTO_mpi_scan_unsigned (&c1, &ds->ciphertext.c1_bits,
                                   GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
  GNUNET_CRYPTO_mpi_scan_unsigned (&s, &ds->share->my_share,
                                   GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
  GNUNET_CRYPTO_mpi_scan_unsigned (&sigma,
                                   &ds->share->sigmas[ds->share->my_peer],
                                   GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);

  GNUNET_assert (NULL != (w = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (beta = gcry_mpi_new (0)));
  GNUNET_assert (NULL != (tmp = gcry_mpi_new (0)));

  // FIXME: unnecessary, remove once crypto works
  gcry_mpi_powm (tmp, elgamal_g, s, elgamal_p);
  if (0 != gcry_mpi_cmp (tmp, sigma))
  {
    char *sigma_str = mpi_to_str (sigma);
    char *tmp_str = mpi_to_str (tmp);
    char *s_str = mpi_to_str (s);
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "Share of P%u is invalid, ref sigma %s, "
                "computed sigma %s, s %s\n",
                ds->share->my_peer,
                sigma_str, tmp_str, s_str);
    GNUNET_free (sigma_str);
    GNUNET_free (tmp_str);
    GNUNET_free (s_str);
  }

  gcry_mpi_powm (w, c1, s, elgamal_p);

  element.data = (void *) &d;
  element.size = sizeof(struct GNUNET_SECRETSHARING_DecryptData);
  element.element_type = 0;

  d.ciphertext = ds->ciphertext;
  d.peer = my_peer;
  GNUNET_CRYPTO_mpi_print_unsigned (&d.partial_decryption,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8, w);

  // create the zero knowledge proof
  // randomly choose beta such that 0 < beta < q
  do
  {
    gcry_mpi_randomize (beta, GNUNET_SECRETSHARING_ELGAMAL_BITS - 1,
                        GCRY_WEAK_RANDOM);
  }
  while ((gcry_mpi_cmp_ui (beta, 0) == 0) || (gcry_mpi_cmp (beta, elgamal_q) >=
                                              0));
  // tmp = g^beta
  gcry_mpi_powm (tmp, elgamal_g, beta, elgamal_p);
  GNUNET_CRYPTO_mpi_print_unsigned (&d.nizk_commit1,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8, tmp);
  // tmp = (c_1)^beta
  gcry_mpi_powm (tmp, c1, beta, elgamal_p);
  GNUNET_CRYPTO_mpi_print_unsigned (&d.nizk_commit2,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8, tmp);

  // the challenge is the hash of everything up to the response
  GNUNET_CRYPTO_hash (offsetof (struct GNUNET_SECRETSHARING_DecryptData,
                                ciphertext) + (char *) &d,
                      offsetof (struct GNUNET_SECRETSHARING_DecryptData,
                                nizk_response)
                      - offsetof (struct GNUNET_SECRETSHARING_DecryptData,
                                  ciphertext),
                      &challenge_hash);

  GNUNET_CRYPTO_mpi_scan_unsigned (&challenge, &challenge_hash,
                                   sizeof(struct GNUNET_HashCode));

  // compute the response in tmp,
  // tmp = (c * s + beta) mod q
  gcry_mpi_mulm (tmp, challenge, s, elgamal_q);
  gcry_mpi_addm (tmp, tmp, beta, elgamal_q);

  GNUNET_CRYPTO_mpi_print_unsigned (&d.nizk_response,
                                    GNUNET_SECRETSHARING_ELGAMAL_BITS / 8, tmp);

  d.purpose.size = htonl (element.size - offsetof (struct
                                                   GNUNET_SECRETSHARING_DecryptData,
                                                   purpose));
  d.purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_SECRETSHARING_DECRYPTION);

  GNUNET_assert (GNUNET_OK ==
                 GNUNET_CRYPTO_eddsa_sign_ (my_peer_private_key,
                                            &d.purpose,
                                            &d.signature));

  GNUNET_CONSENSUS_insert (ds->consensus, &element, NULL, NULL);
  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
              "P%u: Inserting decrypt element done!\n",
              ds->share->my_peer);

  gcry_mpi_release (s);
  gcry_mpi_release (w);
  gcry_mpi_release (c1);
  gcry_mpi_release (beta);
  gcry_mpi_release (tmp);
  gcry_mpi_release (challenge);
  gcry_mpi_release (sigma);
}


/**
 * Check that @a msg is well-formed.
 *
 * @param cls identification of the client
 * @param msg the actual message
 * @return #GNUNET_OK (check deferred a bit)
 */
static int
check_client_decrypt (void *cls,
                      const struct
                      GNUNET_SECRETSHARING_DecryptRequestMessage *msg)
{
  /* we check later, it's complicated */
  return GNUNET_OK;
}


/**
 * Functions with this signature are called whenever a message is
 * received.
 *
 * @param cls identification of the client
 * @param msg the actual message
 */
static void
handle_client_decrypt (void *cls,
                       const struct
                       GNUNET_SECRETSHARING_DecryptRequestMessage *msg)
{
  struct ClientState *cs = cls;
  struct DecryptSession *ds;
  struct GNUNET_HashCode session_id;

  if (NULL != cs->decrypt_session)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (cs->client);
    return;
  }
  ds = GNUNET_new (struct DecryptSession);
  cs->decrypt_session = ds;
  ds->cs = cs;
  ds->start = GNUNET_TIME_absolute_ntoh (msg->start);
  ds->deadline = GNUNET_TIME_absolute_ntoh (msg->deadline);
  ds->ciphertext = msg->ciphertext;

  ds->share = GNUNET_SECRETSHARING_share_read (&msg[1],
                                               ntohs (msg->header.size)
                                               - sizeof(*msg),
                                               NULL);
  if (NULL == ds->share)
  {
    GNUNET_break (0);
    GNUNET_SERVICE_client_drop (cs->client);
    return;
  }

  /* FIXME: this is probably sufficient, but kdf/hash with all values would be nicer ... */
  GNUNET_CRYPTO_hash (&msg->ciphertext,
                      sizeof(struct GNUNET_SECRETSHARING_Ciphertext),
                      &session_id);
  ds->consensus = GNUNET_CONSENSUS_create (cfg,
                                           ds->share->num_peers,
                                           ds->share->peers,
                                           &session_id,
                                           ds->start,
                                           ds->deadline,
                                           &decrypt_new_element,
                                           ds);


  ds->info = GNUNET_new_array (ds->share->num_peers,
                               struct DecryptPeerInfo);
  for (unsigned int i = 0; i < ds->share->num_peers; i++)
  {
    ds->info[i].peer = ds->share->peers[i];
    ds->info[i].original_index = ds->share->original_indices[i];
  }
  insert_decrypt_element (ds);
  GNUNET_CONSENSUS_conclude (ds->consensus,
                             decrypt_conclude,
                             ds);
  GNUNET_SERVICE_client_continue (cs->client);
  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
              "decrypting with %u peers\n",
              ds->share->num_peers);
}


static void
init_crypto_constants (void)
{
  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));
}


/**
 * Initialize secretsharing service.
 *
 * @param cls closure
 * @param c configuration to use
 * @param service the initialized service
 */
static void
run (void *cls,
     const struct GNUNET_CONFIGURATION_Handle *c,
     struct GNUNET_SERVICE_Handle *service)
{
  cfg = c;
  my_peer_private_key = GNUNET_CRYPTO_eddsa_key_create_from_configuration (c);
  if (NULL == my_peer_private_key)
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "could not access host private key\n");
    GNUNET_break (0);
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
  init_crypto_constants ();
  if (GNUNET_OK !=
      GNUNET_CRYPTO_get_peer_identity (cfg,
                                       &my_peer))
  {
    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                "could not retrieve host identity\n");
    GNUNET_break (0);
    GNUNET_SCHEDULER_shutdown ();
    return;
  }
  GNUNET_SCHEDULER_add_shutdown (&cleanup_task,
                                 NULL);
}


/**
 * Callback called when a client connects to the service.
 *
 * @param cls closure for the service
 * @param c the new client that connected to the service
 * @param mq the message queue used to send messages to the client
 * @return @a c
 */
static void *
client_connect_cb (void *cls,
                   struct GNUNET_SERVICE_Client *c,
                   struct GNUNET_MQ_Handle *mq)
{
  struct ClientState *cs = GNUNET_new (struct ClientState);;

  cs->client = c;
  cs->mq = mq;
  return cs;
}


/**
 * Callback called when a client disconnected from the service
 *
 * @param cls closure for the service
 * @param c the client that disconnected
 * @param internal_cls should be equal to @a c
 */
static void
client_disconnect_cb (void *cls,
                      struct GNUNET_SERVICE_Client *c,
                      void *internal_cls)
{
  struct ClientState *cs = internal_cls;

  if (NULL != cs->keygen_session)
    keygen_session_destroy (cs->keygen_session);

  if (NULL != cs->decrypt_session)
    decrypt_session_destroy (cs->decrypt_session);
  GNUNET_free (cs);
}


/**
 * Define "main" method using service macro.
 */
GNUNET_SERVICE_MAIN
  ("secretsharing",
  GNUNET_SERVICE_OPTION_NONE,
  &run,
  &client_connect_cb,
  &client_disconnect_cb,
  NULL,
  GNUNET_MQ_hd_var_size (client_keygen,
                         GNUNET_MESSAGE_TYPE_SECRETSHARING_CLIENT_GENERATE,
                         struct GNUNET_SECRETSHARING_CreateMessage,
                         NULL),
  GNUNET_MQ_hd_var_size (client_decrypt,
                         GNUNET_MESSAGE_TYPE_SECRETSHARING_CLIENT_DECRYPT,
                         struct GNUNET_SECRETSHARING_DecryptRequestMessage,
                         NULL),
  GNUNET_MQ_handler_end ());
