add changelog
[oweals/gnunet.git] / src / secretsharing / gnunet-service-secretsharing.c
index f169a1655e811726b291a2c347f89e69129d0567..d2e47da8b889bc78f54f240526aa4518a5efba3b 100644 (file)
@@ -1,22 +1,22 @@
 /*
      This file is part of GNUnet.
-     (C) 2013 Christian Grothoff (and other contributing authors)
+     Copyright (C) 2013 GNUnet e.V.
 
-     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 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
-     General Public License for more details.
+     Affero 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.
-*/
+     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
@@ -53,7 +53,7 @@ struct KeygenPeerInfo
   struct GNUNET_CRYPTO_PaillierPublicKey paillier_public_key;
 
   /**
-   * The peer's commitment to his presecret.
+   * The peer's commitment to its presecret.
    */
   gcry_mpi_t presecret_commitment;
 
@@ -108,36 +108,26 @@ struct DecryptPeerInfo
 };
 
 
+/**
+ * State we keep per client.
+ */
+struct ClientState;
+
+
 /**
  * Session to establish a threshold-shared secret.
  */
 struct KeygenSession
 {
-  /**
-   * Keygen sessions are held in a linked list.
-   */
-  struct KeygenSession *next;
-
-  /**
-   * Keygen sessions are held in a linked list.
-   */
-  struct KeygenSession *prev;
-
   /**
    * Current consensus, used for both DKG rounds.
    */
   struct GNUNET_CONSENSUS_Handle *consensus;
 
   /**
-   * Client that is interested in the result
-   * of this key generation session.
-   */
-  struct GNUNET_SERVER_Client *client;
-
-  /**
-   * Message queue for 'client'
+   * Which client is this for?
    */
-  struct GNUNET_MQ_Handle *client_mq;
+  struct ClientState *cs;
 
   /**
    * Randomly generated coefficients of the polynomial for sharing our
@@ -223,30 +213,15 @@ struct KeygenSession
  */
 struct DecryptSession
 {
-  /**
-   * Decrypt sessions are stored in a linked list.
-   */
-  struct DecryptSession *next;
-
-  /**
-   * Decrypt sessions are stored in a linked list.
-   */
-  struct DecryptSession *prev;
-
   /**
    * Handle to the consensus over partial decryptions.
    */
   struct GNUNET_CONSENSUS_Handle *consensus;
 
   /**
-   * Client connected to us.
+   * Which client is this for?
    */
-  struct GNUNET_SERVER_Client *client;
-
-  /**
-   * Message queue for 'client'.
-   */
-  struct GNUNET_MQ_Handle *client_mq;
+  struct ClientState *cs;
 
   /**
    * When should we start communicating for decryption?
@@ -279,24 +254,31 @@ struct DecryptSession
 
 
 /**
- * Decrypt sessions are held in a linked list.
+ * State we keep per client.
  */
-static struct DecryptSession *decrypt_sessions_head;
+struct ClientState
+{
+  /**
+   * Decrypt session of the client, if any.
+   */
+  struct DecryptSession *decrypt_session;
 
-/**
* Decrypt sessions are held in a linked list.
- */
-static struct DecryptSession *decrypt_sessions_tail;
+  /**
  * Keygen session of the client, if any.
  */
+  struct KeygenSession *keygen_session;
 
-/**
- * Decrypt sessions are held in a linked list.
- */
-static struct KeygenSession *keygen_sessions_head;
+  /**
+   * Client this is about.
+   */
+  struct GNUNET_SERVICE_Client *client;
+
+  /**
+   * MQ to talk to @a client.
+   */
+  struct GNUNET_MQ_Handle *mq;
+};
 
-/**
- * Decrypt sessions are held in a linked list.
- */
-static struct KeygenSession *keygen_sessions_tail;
 
 /**
  * The ElGamal prime field order as libgcrypt mpi.
@@ -331,11 +313,6 @@ static struct GNUNET_CRYPTO_EddsaPrivateKey *my_peer_private_key;
  */
 static const struct GNUNET_CONFIGURATION_Handle *cfg;
 
-/**
- * Server for this service.
- */
-static struct GNUNET_SERVER_Handle *srv;
-
 
 /**
  * Get the peer info belonging to a peer identity in a keygen session.
@@ -349,8 +326,9 @@ 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 == memcmp (peer, &ks->info[i].peer, sizeof (struct GNUNET_PeerIdentity)))
+    if (0 == GNUNET_memcmp (peer, &ks->info[i].peer))
       return &ks->info[i];
   return NULL;
 }
@@ -368,8 +346,9 @@ 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 == memcmp (peer, &ds->info[i].peer, sizeof (struct GNUNET_PeerIdentity)))
+    if (0 == GNUNET_memcmp (peer, &ds->info[i].peer))
       return &ds->info[i];
   return NULL;
 }
@@ -409,7 +388,9 @@ time_between (struct GNUNET_TIME_Absolute start,
 static int
 peer_id_cmp (const void *p1, const void *p2)
 {
-  return memcmp (p1, p2, sizeof (struct GNUNET_PeerIdentity));
+  return memcmp (p1,
+                 p2,
+                 sizeof(struct GNUNET_PeerIdentity));
 }
 
 
@@ -427,8 +408,10 @@ 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 == memcmp (&haystack[i], needle, sizeof (struct GNUNET_PeerIdentity)))
+    if (0 == GNUNET_memcmp (&haystack[i],
+                            needle))
       return i;
   return -1;
 }
@@ -463,13 +446,19 @@ normalize_peers (struct GNUNET_PeerIdentity *listed,
     n += 1;
   }
 
-  normalized = GNUNET_new_array (n, struct GNUNET_PeerIdentity);
+  normalized = GNUNET_new_array (n,
+                                 struct GNUNET_PeerIdentity);
 
   if (GNUNET_NO == local_peer_in_list)
     normalized[n - 1] = my_peer;
 
-  memcpy (normalized, listed, num_listed * sizeof (struct GNUNET_PeerIdentity));
-  qsort (normalized, n, sizeof (struct GNUNET_PeerIdentity), &peer_id_cmp);
+  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);
@@ -548,32 +537,42 @@ compute_lagrange_coefficient (gcry_mpi_t coeff, unsigned int j,
 static void
 decrypt_session_destroy (struct DecryptSession *ds)
 {
-  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "destroying decrypt session\n");
-
-  GNUNET_CONTAINER_DLL_remove (decrypt_sessions_head, decrypt_sessions_tail, 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->client_mq)
+  if (NULL != ds->info)
   {
-    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "destroying decrypt MQ\n");
-    GNUNET_MQ_destroy (ds->client_mq);
-    ds->client_mq = NULL;
+    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->client)
+  if (NULL != ds->share)
   {
-    GNUNET_SERVER_client_disconnect (ds->client);
-    ds->client = NULL;
+    GNUNET_SECRETSHARING_share_destroy (ds->share);
+    ds->share = NULL;
   }
 
   GNUNET_free (ds);
 }
 
+
 static void
 keygen_info_destroy (struct KeygenPeerInfo *info)
 {
@@ -590,7 +589,7 @@ keygen_info_destroy (struct KeygenPeerInfo *info)
   if (NULL != info->preshare_commitment)
   {
     gcry_mpi_release (info->preshare_commitment);
-    info->presecret_commitment = NULL;
+    info->preshare_commitment = NULL;
   }
 }
 
@@ -598,14 +597,17 @@ keygen_info_destroy (struct KeygenPeerInfo *info)
 static void
 keygen_session_destroy (struct KeygenSession *ks)
 {
-  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "destroying keygen session\n");
-
-  GNUNET_CONTAINER_DLL_remove (keygen_sessions_head, keygen_sessions_tail, 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)
   {
-    unsigned int i;
-    for (i = 0; i < ks->num_peers; i++)
+    for (unsigned int i = 0; i < ks->num_peers; i++)
       keygen_info_destroy (&ks->info[i]);
     GNUNET_free (ks->info);
     ks->info = NULL;
@@ -619,8 +621,7 @@ keygen_session_destroy (struct KeygenSession *ks)
 
   if (NULL != ks->presecret_polynomial)
   {
-    unsigned int i;
-    for (i = 0; i < ks->threshold; i++)
+    for (unsigned int i = 0; i < ks->threshold; i++)
     {
       GNUNET_assert (NULL != ks->presecret_polynomial[i]);
       gcry_mpi_release (ks->presecret_polynomial[i]);
@@ -629,38 +630,21 @@ keygen_session_destroy (struct KeygenSession *ks)
     GNUNET_free (ks->presecret_polynomial);
     ks->presecret_polynomial = NULL;
   }
-
-  if (NULL != ks->client_mq)
-  {
-    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "destroying keygen MQ\n");
-    GNUNET_MQ_destroy (ks->client_mq);
-    ks->client_mq = 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;
   }
-
-  if (NULL != ks->client)
-  {
-    GNUNET_SERVER_client_disconnect (ks->client);
-    ks->client = NULL;
-  }
-
   GNUNET_free (ks);
 }
 
@@ -672,17 +656,12 @@ keygen_session_destroy (struct KeygenSession *ks)
  * @param tc unused
  */
 static void
-cleanup_task (void *cls, const struct GNUNET_SCHEDULER_TaskContext *tc)
+cleanup_task (void *cls)
 {
-  while (NULL != decrypt_sessions_head)
-    decrypt_session_destroy (decrypt_sessions_head);
-
-  while (NULL != keygen_sessions_head)
-    keygen_session_destroy (keygen_sessions_head);
+  /* Nothing to do! */
 }
 
 
-
 /**
  * Generate the random coefficients of our pre-secret polynomial
  *
@@ -695,17 +674,21 @@ generate_presecret_polynomial (struct KeygenSession *ks)
   gcry_mpi_t v;
 
   GNUNET_assert (NULL == ks->presecret_polynomial);
-  ks->presecret_polynomial = GNUNET_new_array (ks->threshold, gcry_mpi_t);
+  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);
+    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 
+    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));
+      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));
   }
 }
 
@@ -733,12 +716,13 @@ keygen_round1_new_element (void *cls,
   }
 
   /* elements have fixed size */
-  if (element->size != sizeof (struct GNUNET_SECRETSHARING_KeygenCommitData))
+  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",
-                element->size, sizeof (struct GNUNET_SECRETSHARING_KeygenCommitData));
+                "keygen commit data with wrong size (%u) in consensus, %u expected\n",
+                (unsigned int) element->size,
+                (unsigned int) sizeof(struct
+                                      GNUNET_SECRETSHARING_KeygenCommitData));
     return;
   }
 
@@ -749,27 +733,35 @@ keygen_round1_new_element (void *cls,
 
   if (NULL == info)
   {
-    GNUNET_log (GNUNET_ERROR_TYPE_WARNING, "keygen commit data with wrong peer identity (%s) in consensus\n",
+    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)))
+      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");
+    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))
+  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");
+    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);
+  GNUNET_CRYPTO_mpi_scan_unsigned (&info->presecret_commitment, &d->commitment,
+                                   512 / 8);
   info->round1_valid = GNUNET_YES;
 }
 
@@ -785,7 +777,8 @@ keygen_round1_new_element (void *cls,
  * @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)
+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;
 
@@ -823,10 +816,13 @@ keygen_round2_conclude (void *cls)
     if (GNUNET_YES == ks->info[i].round2_valid)
       share->num_peers++;
 
-  share->peers = GNUNET_new_array (share->num_peers, struct GNUNET_PeerIdentity);
+  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);
+    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;
@@ -841,7 +837,7 @@ keygen_round2_conclude (void *cls)
                                         GNUNET_SECRETSHARING_ELGAMAL_BITS / 8,
                                         ks->info[i].sigma);
       share->original_indices[i] = j;
-      if (0 == memcmp (&share->peers[i], &my_peer, sizeof (struct GNUNET_PeerIdentity)))
+      if (0 == GNUNET_memcmp (&share->peers[i], &my_peer))
         share->my_peer = j;
       j += 1;
     }
@@ -849,20 +845,25 @@ keygen_round2_conclude (void *cls)
 
   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_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,
+  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,
+  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);
+  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_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);
@@ -870,12 +871,377 @@ keygen_round2_conclude (void *cls)
   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_assert (GNUNET_OK == GNUNET_SECRETSHARING_share_write (share, &m[1],
+                                                                share_size,
+                                                                NULL));
 
   GNUNET_SECRETSHARING_share_destroy (share);
   share = NULL;
 
-  GNUNET_MQ_send (ks->client_mq, ev);
+  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);
 }
 
 
@@ -904,15 +1270,17 @@ insert_round2_element (struct KeygenSession *ks)
   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)));
+  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) +
-                  GNUNET_SECRETSHARING_ELGAMAL_BITS / 8 * ks->num_peers +
-                  sizeof (struct GNUNET_CRYPTO_PaillierCiphertext) * ks->num_peers +
-                  GNUNET_SECRETSHARING_ELGAMAL_BITS / 8 * ks->threshold);
+  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 = GNUNET_malloc (sizeof(struct GNUNET_SET_Element) + element_size);
   element->size = element_size;
   element->data = (void *) &element[1];
 
@@ -924,41 +1292,30 @@ insert_round2_element (struct KeygenSession *ks)
   pos = (void *) &d[1];
   last_pos = pos + element_size;
 
-  // exponentiated pre-shares
-  for (i = 0; i < ks->num_peers; i++)
-  {
-    ptrdiff_t remaining = last_pos - pos;
-    GNUNET_assert (remaining > 0);
-    gcry_mpi_set_ui (idx, i + 1);
-    // evaluate the polynomial
-    horner_eval (v, ks->presecret_polynomial, ks->threshold, idx, elgamal_q);
-    // take g to the result
-    gcry_mpi_powm (v, elgamal_g, v, 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 preshares\n",
               ks->local_peer_idx);
 
   // encrypted pre-shares
-  for (i = 0; i < ks->num_peers; i++)
+  // and fair encryption proof
   {
-    ptrdiff_t remaining = last_pos - pos;
-    struct GNUNET_CRYPTO_PaillierCiphertext *ciphertext;
-
-    GNUNET_assert (remaining > 0);
-    ciphertext = (void *) pos;
-    memset (ciphertext, 0, sizeof *ciphertext);
-    if (GNUNET_YES == ks->info[i].round1_valid)
+    for (i = 0; i < ks->num_peers; i++)
     {
-      gcry_mpi_set_ui (idx, i + 1);
-      // evaluate the polynomial
-      horner_eval (v, ks->presecret_polynomial, ks->threshold, idx, elgamal_q);
-      // encrypt the result
-      GNUNET_CRYPTO_paillier_encrypt (&ks->info[i].paillier_public_key, v, 0, ciphertext);
+      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;
     }
-    pos += sizeof *ciphertext;
   }
 
   GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "P%u: computed enc preshares\n",
@@ -970,45 +1327,36 @@ insert_round2_element (struct KeygenSession *ks)
     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);
+    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.size = htonl (element_size - offsetof (struct
+                                                    GNUNET_SECRETSHARING_KeygenRevealData,
+                                                    purpose));
   d->purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_SECRETSHARING_DKG2);
-  GNUNET_CRYPTO_eddsa_sign (my_peer_private_key, &d->purpose, &d->signature);
+  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? */
+  GNUNET_free (element);  /* FIXME: maybe stack-allocate instead? */
 
   gcry_mpi_release (v);
   gcry_mpi_release (idx);
 }
 
 
-static gcry_mpi_t
-keygen_reveal_get_exp_preshare (struct KeygenSession *ks,
-                                const struct GNUNET_SECRETSHARING_KeygenRevealData *d,
-                                unsigned int idx)
-{
-  unsigned char *pos;
-  gcry_mpi_t exp_preshare;
-
-  GNUNET_assert (idx < ks->num_peers);
-
-  pos = (void *) &d[1];
-  // skip exponentiated pre-shares we don't want
-  pos += GNUNET_SECRETSHARING_ELGAMAL_BITS / 8 * idx;
-  GNUNET_CRYPTO_mpi_scan_unsigned (&exp_preshare, pos, GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
-  return exp_preshare;
-}
-
 static gcry_mpi_t
 keygen_reveal_get_exp_coeff (struct KeygenSession *ks,
-                             const struct GNUNET_SECRETSHARING_KeygenRevealData *d,
+                             const struct
+                             GNUNET_SECRETSHARING_KeygenRevealData *d,
                              unsigned int idx)
 {
   unsigned char *pos;
@@ -1017,21 +1365,21 @@ keygen_reveal_get_exp_coeff (struct KeygenSession *ks,
   GNUNET_assert (idx < ks->threshold);
 
   pos = (void *) &d[1];
-  // skip exponentiated pre-shares
-  pos += GNUNET_SECRETSHARING_ELGAMAL_BITS / 8 * ks->num_peers;
   // skip encrypted pre-shares
-  pos += sizeof (struct GNUNET_CRYPTO_PaillierCiphertext) * ks->num_peers;
+  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);
+  GNUNET_CRYPTO_mpi_scan_unsigned (&exp_coeff, pos,
+                                   GNUNET_SECRETSHARING_ELGAMAL_BITS / 8);
   return exp_coeff;
 }
 
 
-static struct GNUNET_CRYPTO_PaillierCiphertext *
+static struct GNUNET_SECRETSHARING_FairEncryption *
 keygen_reveal_get_enc_preshare (struct KeygenSession *ks,
-                                const struct GNUNET_SECRETSHARING_KeygenRevealData *d,
+                                const struct
+                                GNUNET_SECRETSHARING_KeygenRevealData *d,
                                 unsigned int idx)
 {
   unsigned char *pos;
@@ -1039,11 +1387,26 @@ keygen_reveal_get_enc_preshare (struct KeygenSession *ks,
   GNUNET_assert (idx < ks->num_peers);
 
   pos = (void *) &d[1];
-  // skip exponentiated pre-shares
-  pos += GNUNET_SECRETSHARING_ELGAMAL_BITS / 8 * ks->num_peers;
   // skip encrypted pre-shares we're not interested in
-  pos += sizeof (struct GNUNET_CRYPTO_PaillierCiphertext) * idx;
-  return (struct GNUNET_CRYPTO_PaillierCiphertext *) pos;
+  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;
 }
 
 
@@ -1067,17 +1430,19 @@ keygen_round2_new_element (void *cls,
     return;
   }
 
-  expected_element_size = (sizeof (struct GNUNET_SECRETSHARING_KeygenRevealData) +
-                  GNUNET_SECRETSHARING_ELGAMAL_BITS / 8 * ks->num_peers +
-                  sizeof (struct GNUNET_CRYPTO_PaillierCiphertext) * ks->num_peers +
-                  GNUNET_SECRETSHARING_ELGAMAL_BITS / 8 * ks->threshold);
+  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",
-                element->size, expected_element_size);
+                "keygen round2 data with wrong size (%u) in consensus, %u expected\n",
+                (unsigned int) element->size,
+                (unsigned int) expected_element_size);
     return;
   }
 
@@ -1087,7 +1452,8 @@ keygen_round2_new_element (void *cls,
 
   if (NULL == info)
   {
-    GNUNET_log (GNUNET_ERROR_TYPE_WARNING, "keygen commit data with wrong peer identity (%s) in consensus\n",
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                "keygen commit data with wrong peer identity (%s) in consensus\n",
                 GNUNET_i2s (&d->peer));
     return;
   }
@@ -1111,21 +1477,27 @@ keygen_round2_new_element (void *cls,
   GNUNET_log (GNUNET_ERROR_TYPE_INFO, "got round2 element\n");
 
   if (ntohl (d->purpose.size) !=
-      element->size - offsetof (struct GNUNET_SECRETSHARING_KeygenRevealData, purpose))
+      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");
+    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))
+  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");
+    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);
+  info->preshare_commitment = keygen_reveal_get_exp_preshare (ks, d,
+                                                              ks->local_peer_idx);
 
   if (NULL == ks->public_key)
   {
@@ -1133,27 +1505,39 @@ keygen_round2_new_element (void *cls,
     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;
 
-  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,
-                                  keygen_reveal_get_enc_preshare (ks, d, ks->local_peer_idx),
-                                  preshare);
+  {
+    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);
 
-  // TODO: restore a valid secret from the decryption (the hard part, solving SVP with gauss)
   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));
+    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;
   }
 
@@ -1192,7 +1576,7 @@ keygen_round2_new_element (void *cls,
       // 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)); 
+      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);
@@ -1205,7 +1589,8 @@ keygen_round2_new_element (void *cls,
     exp_preshare = NULL;
     if (0 != cmp_result)
     {
-      GNUNET_log (GNUNET_ERROR_TYPE_WARNING, "P%u: reveal data from P%u incorrect\n",
+      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;
@@ -1213,7 +1598,19 @@ keygen_round2_new_element (void *cls,
   }
 
   // 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);
@@ -1235,8 +1632,10 @@ keygen_round1_conclude (void *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->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);
 
@@ -1275,15 +1674,22 @@ insert_round1_element (struct KeygenSession *ks)
 
   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_mpi_print_unsigned (v_data, GNUNET_SECRETSHARING_ELGAMAL_BITS
+                                    / 8, v);
 
-  GNUNET_CRYPTO_hash (v_data, GNUNET_SECRETSHARING_ELGAMAL_BITS / 8, &d->commitment);
+  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.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_assert (GNUNET_OK ==
+                 GNUNET_CRYPTO_eddsa_sign (my_peer_private_key,
+                                           &d->purpose,
+                                           &d->signature));
 
   GNUNET_CONSENSUS_insert (ks->consensus, element, NULL, NULL);
 
@@ -1292,74 +1698,106 @@ insert_round1_element (struct KeygenSession *ks)
 }
 
 
+/**
+ * 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 closure
- * @param client identification of the client
- * @param message the actual message
+ * @param cls identification of the client
+ * @param msg the actual message
  */
-static void handle_client_keygen (void *cls,
-                                  struct GNUNET_SERVER_Client *client,
-                                  const struct GNUNET_MessageHeader
-                                  *message)
+static void
+handle_client_keygen (void *cls,
+                      const struct GNUNET_SECRETSHARING_CreateMessage *msg)
 {
-  const struct GNUNET_SECRETSHARING_CreateMessage *msg =
-      (const struct GNUNET_SECRETSHARING_CreateMessage *) message;
+  struct ClientState *cs = cls;
   struct KeygenSession *ks;
-  unsigned int i;
-
-  GNUNET_log (GNUNET_ERROR_TYPE_INFO, "client requested key generation\n");
 
+  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);
-
-  /* FIXME: check if client already has some session */
-
-  GNUNET_CONTAINER_DLL_insert (keygen_sessions_head, keygen_sessions_tail, ks);
-
-  ks->client = client;
-  ks->client_mq = GNUNET_MQ_queue_for_server_client (client);
-
+  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 (i = 0; i < ks->num_peers; i++)
+  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);
+  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);
-
+  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_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_SERVER_receive_done (client, GNUNET_OK);
-
-  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "P%u: Waiting for round 1 elements ...\n", ks->local_peer_idx);
+  GNUNET_SERVICE_client_continue (cs->client);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "P%u: Waiting for round 1 elements ...\n",
+              ks->local_peer_idx);
 }
 
 
@@ -1395,40 +1833,56 @@ decrypt_conclude (void *cls)
     if (NULL != ds->info[i].partial_decryption)
       num++;
 
-  indices = GNUNET_malloc (num * sizeof (unsigned int));
+  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);
+  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",
+    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);
+    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_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);
+  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->client_mq, ev);
+  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?
 }
 
@@ -1491,62 +1945,78 @@ decrypt_new_element (void *cls,
 
   if (element->size != sizeof *d)
   {
-    GNUNET_log (GNUNET_ERROR_TYPE_ERROR, "element of wrong size in decrypt consensus\n");
+    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_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",
-                GNUNET_i2s (&d->peer));
+    GNUNET_log (GNUNET_ERROR_TYPE_ERROR, "decrypt element duplicate\n");
     return;
   }
 
-  if (0 != memcmp (&d->ciphertext, &session->ciphertext, sizeof (struct GNUNET_SECRETSHARING_Ciphertext)))
+  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));
+    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),
+  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));
+                                   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 (&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));
+                                   sizeof(struct
+                                          GNUNET_SECRETSHARING_FieldElement));
 
   GNUNET_CRYPTO_mpi_scan_unsigned (&commit1, &d->nizk_commit1,
-                                   sizeof (struct GNUNET_SECRETSHARING_FieldElement));
+                                   sizeof(struct
+                                          GNUNET_SECRETSHARING_FieldElement));
 
   GNUNET_CRYPTO_mpi_scan_unsigned (&commit2, &d->nizk_commit2,
-                                   sizeof (struct GNUNET_SECRETSHARING_FieldElement));
+                                   sizeof(struct
+                                          GNUNET_SECRETSHARING_FieldElement));
 
   GNUNET_CRYPTO_mpi_scan_unsigned (&r, &d->nizk_response,
-                                   sizeof (struct GNUNET_SECRETSHARING_FieldElement));
+                                   sizeof(struct
+                                          GNUNET_SECRETSHARING_FieldElement));
 
   GNUNET_CRYPTO_mpi_scan_unsigned (&w, &d->partial_decryption,
-                                   sizeof (struct GNUNET_SECRETSHARING_FieldElement));
+                                   sizeof(struct
+                                          GNUNET_SECRETSHARING_FieldElement));
 
   GNUNET_assert (NULL != (tmp1 = gcry_mpi_new (0)));
   GNUNET_assert (NULL != (tmp2 = gcry_mpi_new (0)));
@@ -1562,13 +2032,18 @@ decrypt_new_element (void *cls,
   {
     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, info - session->info, tmp1_str, tmp2_str);
+    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);
-    // return;
+    goto cleanup;
   }
 
 
@@ -1580,14 +2055,27 @@ decrypt_new_element (void *cls,
 
   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, info - session->info);
-    // return;
+    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_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);
 }
 
 
@@ -1621,7 +2109,8 @@ insert_decrypt_element (struct DecryptSession *ds)
                                    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_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)));
@@ -1635,7 +2124,8 @@ insert_decrypt_element (struct DecryptSession *ds)
     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, "
+    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);
@@ -1647,50 +2137,91 @@ insert_decrypt_element (struct DecryptSession *ds)
   gcry_mpi_powm (w, c1, s, elgamal_p);
 
   element.data = (void *) &d;
-  element.size = sizeof (struct GNUNET_SECRETSHARING_DecryptData);
-  element.type = 0;
+  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);
+  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 
+  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));
+    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);
+  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);
+  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),
+  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));
+                                   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);
+  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.size = htonl (element.size - offsetof (struct
+                                                   GNUNET_SECRETSHARING_DecryptData,
+                                                   purpose));
   d.purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_SECRETSHARING_DECRYPTION);
-  
-  GNUNET_CRYPTO_eddsa_sign (my_peer_private_key, &d.purpose, &d.signature);
+
+  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",
+  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;
 }
 
 
@@ -1698,37 +2229,46 @@ insert_decrypt_element (struct DecryptSession *ds)
  * Functions with this signature are called whenever a message is
  * received.
  *
- * @param cls closure
- * @param client identification of the client
- * @param message the actual message
+ * @param cls identification of the client
+ * @param msg the actual message
  */
-static void handle_client_decrypt (void *cls,
-                                   struct GNUNET_SERVER_Client *client,
-                                   const struct GNUNET_MessageHeader
-                                   *message)
+static void
+handle_client_decrypt (void *cls,
+                       const struct
+                       GNUNET_SECRETSHARING_DecryptRequestMessage *msg)
 {
-  const struct GNUNET_SECRETSHARING_DecryptRequestMessage *msg =
-      (const void *) message;
+  struct ClientState *cs = cls;
   struct DecryptSession *ds;
   struct GNUNET_HashCode session_id;
-  unsigned int i;
 
+  if (NULL != cs->decrypt_session)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (cs->client);
+    return;
+  }
   ds = GNUNET_new (struct DecryptSession);
-  // FIXME: check if session already exists
-  GNUNET_CONTAINER_DLL_insert (decrypt_sessions_head, decrypt_sessions_tail, ds);
-  ds->client = client;
-  ds->client_mq = GNUNET_MQ_queue_for_server_client (client);
+  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);
-  // FIXME: probably should be break rather than assert
-  GNUNET_assert (NULL != ds->share);
-
-  // 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->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,
@@ -1739,20 +2279,20 @@ static void handle_client_decrypt (void *cls,
                                            ds);
 
 
-  ds->info = GNUNET_new_array (ds->share->num_peers, struct DecryptPeerInfo);
-  for (i = 0; i < ds->share->num_peers; i++)
+  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_SERVER_receive_done (client, GNUNET_OK);
-
-  GNUNET_log (GNUNET_ERROR_TYPE_INFO, "decrypting with %u peers\n",
+  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);
 }
 
@@ -1761,112 +2301,115 @@ 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_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_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 struct KeygenSession *
-keygen_session_get (struct GNUNET_SERVER_Client *client)
-{
-  struct KeygenSession *ks;
-  for (ks = keygen_sessions_head; NULL != ks; ks = ks->next)
-    if (ks->client == client)
-      return ks;
-  return NULL;
-}
-
-static struct DecryptSession *
-decrypt_session_get (struct GNUNET_SERVER_Client *client)
-{
-  struct DecryptSession *ds;
-  for (ds = decrypt_sessions_head; NULL != ds; ds = ds->next)
-    if (ds->client == client)
-      return ds;
-  return NULL;
-}
-
-
-/**
- * Clean up after a client has disconnected
- *
- * @param cls closure, unused
- * @param client the client to clean up after
- */
-static void
-handle_client_disconnect (void *cls, struct GNUNET_SERVER_Client *client)
-{
-  struct KeygenSession *ks;
-  struct DecryptSession *ds;
-
-  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG, "handling client disconnect\n");
-
-  ks = keygen_session_get (client);
-  if (NULL != ks)
-    keygen_session_destroy (ks);
-
-  ds = decrypt_session_get (client);
-  if (NULL != ds)
-    decrypt_session_destroy (ds);
+                                     GNUNET_SECRETSHARING_ELGAMAL_G_HEX, 0,
+                                     NULL));
 }
 
 
 /**
- * Process template requests.
+ * Initialize secretsharing service.
  *
  * @param cls closure
- * @param server the initialized server
  * @param c configuration to use
+ * @param service the initialized service
  */
 static void
-run (void *cls, struct GNUNET_SERVER_Handle *server,
-     const struct GNUNET_CONFIGURATION_Handle *c)
+run (void *cls,
+     const struct GNUNET_CONFIGURATION_Handle *c,
+     struct GNUNET_SERVICE_Handle *service)
 {
-  static const struct GNUNET_SERVER_MessageHandler handlers[] = {
-    {handle_client_keygen, NULL, GNUNET_MESSAGE_TYPE_SECRETSHARING_CLIENT_GENERATE, 0},
-    {handle_client_decrypt, NULL, GNUNET_MESSAGE_TYPE_SECRETSHARING_CLIENT_DECRYPT, 0},
-    {NULL, NULL, 0, 0}
-  };
   cfg = c;
-  srv = server;
   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_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))
+  if (GNUNET_OK !=
+      GNUNET_CRYPTO_get_peer_identity (cfg,
+                                       &my_peer))
   {
-    GNUNET_log (GNUNET_ERROR_TYPE_ERROR, "could not retrieve host identity\n");
+    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                "could not retrieve host identity\n");
     GNUNET_break (0);
     GNUNET_SCHEDULER_shutdown ();
     return;
   }
-  GNUNET_SERVER_add_handlers (server, handlers);
-  GNUNET_SERVER_disconnect_notify (server, &handle_client_disconnect, NULL);
-  GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_UNIT_FOREVER_REL, &cleanup_task,
-                                NULL);
+  GNUNET_SCHEDULER_add_shutdown (&cleanup_task,
+                                 NULL);
 }
 
 
 /**
- * The main function for the template service.
+ * Callback called when a client connects to the service.
  *
- * @param argc number of arguments from the command line
- * @param argv command line arguments
- * @return 0 ok, 1 on error
+ * @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
  */
-int
-main (int argc, char *const *argv)
+static void *
+client_connect_cb (void *cls,
+                   struct GNUNET_SERVICE_Client *c,
+                   struct GNUNET_MQ_Handle *mq)
 {
-  return (GNUNET_OK ==
-          GNUNET_SERVICE_run (argc, argv, "secretsharing",
-                              GNUNET_SERVICE_OPTION_NONE, &run, NULL)) ? 0 : 1;
+  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 ());