refactor DHT for new service API

[oweals/gnunet.git] / src / cadet / gnunet-service-cadet_tunnel.c

/*
     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 General Public License as published
     by the Free Software Foundation; either version 3, or (at your
     option) any later version.

     GNUnet is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     General Public License for more details.

     You should have received a copy of the GNU General Public License
     along with GNUnet; see the file COPYING.  If not, write to the
     Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
     Boston, MA 02110-1301, USA.
*/

#include "platform.h"
#include "gnunet_util_lib.h"

#include "gnunet_signatures.h"
#include "gnunet_statistics_service.h"

#include "cadet_protocol.h"
#include "cadet_path.h"

#include "gnunet-service-cadet_tunnel.h"
#include "gnunet-service-cadet_connection.h"
#include "gnunet-service-cadet_channel.h"
#include "gnunet-service-cadet_peer.h"

#define LOG(level, ...) GNUNET_log_from(level,"cadet-tun",__VA_ARGS__)
#define LOG2(level, ...) GNUNET_log_from_nocheck(level,"cadet-tun",__VA_ARGS__)

#define REKEY_WAIT GNUNET_TIME_relative_multiply(GNUNET_TIME_UNIT_SECONDS, 5)

#if !defined(GNUNET_CULL_LOGGING)
#define DUMP_KEYS_TO_STDERR GNUNET_YES
#else
#define DUMP_KEYS_TO_STDERR GNUNET_YES
#endif

#define MIN_TUNNEL_BUFFER       8
#define MAX_TUNNEL_BUFFER       64
#define MAX_SKIPPED_KEYS        64
#define MAX_KEY_GAP             256
#define AX_HEADER_SIZE (sizeof (uint32_t) * 2\
                        + sizeof (struct GNUNET_CRYPTO_EcdhePublicKey))


/******************************************************************************/
/********************************   STRUCTS  **********************************/
/******************************************************************************/

struct CadetTChannel
{
  struct CadetTChannel *next;
  struct CadetTChannel *prev;
  struct CadetChannel *ch;
};


/**
 * Connection list and metadata.
 */
struct CadetTConnection
{
  /**
   * Next in DLL.
   */
  struct CadetTConnection *next;

  /**
   * Prev in DLL.
   */
  struct CadetTConnection *prev;

  /**
   * Connection handle.
   */
  struct CadetConnection *c;

  /**
   * Creation time, to keep oldest connection alive.
   */
  struct GNUNET_TIME_Absolute created;

  /**
   * Connection throughput, to keep fastest connection alive.
   */
  uint32_t throughput;
};


/**
 * Encryption systems possible.
 */
enum CadetTunnelEncryption
{
  /**
   * Default Axolotl system.
   */
  CADET_Axolotl

};

/**
 * Struct to old keys for skipped messages while advancing the Axolotl ratchet.
 */
struct CadetTunnelSkippedKey
{
  /**
   * DLL next.
   */
  struct CadetTunnelSkippedKey *next;

  /**
   * DLL prev.
   */
  struct CadetTunnelSkippedKey *prev;

  /**
   * When was this key stored (for timeout).
   */
  struct GNUNET_TIME_Absolute timestamp;

  /**
   * Header key.
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey HK;

  /**
   * Message key.
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey MK;

  /**
   * Key number for a given HK.
   */
  unsigned int Kn;
};


/**
 * Axolotl data, according to https://github.com/trevp/axolotl/wiki .
 */
struct CadetTunnelAxolotl
{
  /**
   * A (double linked) list of stored message keys and associated header keys
   * for "skipped" messages, i.e. messages that have not been
   * received despite the reception of more recent messages, (head).
   */
  struct CadetTunnelSkippedKey *skipped_head;

  /**
   * Skipped messages' keys DLL, tail.
   */
  struct CadetTunnelSkippedKey *skipped_tail;

  /**
   * Elements in @a skipped_head <-> @a skipped_tail.
   */
  unsigned int skipped;

  /**
   * 32-byte root key which gets updated by DH ratchet.
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey RK;

  /**
   * 32-byte header key (send).
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey HKs;

  /**
   * 32-byte header key (recv)
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey HKr;

  /**
   * 32-byte next header key (send).
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey NHKs;

  /**
   * 32-byte next header key (recv).
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey NHKr;

  /**
   * 32-byte chain keys (used for forward-secrecy updating, send).
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey CKs;

  /**
   * 32-byte chain keys (used for forward-secrecy updating, recv).
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey CKr;

  /**
   * ECDH for key exchange (A0 / B0).
   */
  struct GNUNET_CRYPTO_EcdhePrivateKey *kx_0;

  /**
   * ECDH Ratchet key (send).
   */
  struct GNUNET_CRYPTO_EcdhePrivateKey *DHRs;

  /**
   * ECDH Ratchet key (recv).
   */
  struct GNUNET_CRYPTO_EcdhePublicKey DHRr;

  /**
   * Message number (reset to 0 with each new ratchet, next message to send).
   */
  uint32_t Ns;

  /**
   * Message number (reset to 0 with each new ratchet, next message to recv).
   */
  uint32_t Nr;

  /**
   * Previous message numbers (# of msgs sent under prev ratchet)
   */
  uint32_t PNs;

  /**
   * True (#GNUNET_YES) if we have to send a new ratchet key in next msg.
   */
  int ratchet_flag;

  /**
   * Number of messages recieved since our last ratchet advance.
   * - If this counter = 0, we cannot send a new ratchet key in next msg.
   * - If this counter > 0, we can (but don't yet have to) send a new key.
   */
  unsigned int ratchet_allowed;

  /**
   * Number of messages recieved since our last ratchet advance.
   * - If this counter = 0, we cannot send a new ratchet key in next msg.
   * - If this counter > 0, we can (but don't yet have to) send a new key.
   */
  unsigned int ratchet_counter;

  /**
   * When does this ratchet expire and a new one is triggered.
   */
  struct GNUNET_TIME_Absolute ratchet_expiration;
};

/**
 * Struct containing all information regarding a tunnel to a peer.
 */
struct CadetTunnel
{
  /**
   * Endpoint of the tunnel.
   */
  struct CadetPeer *peer;

  /**
   * Type of encryption used in the tunnel.
   */
  enum CadetTunnelEncryption enc_type;

  /**
   * Axolotl info.
   */
  struct CadetTunnelAxolotl *ax;

  /**
   * State of the tunnel connectivity.
   */
  enum CadetTunnelCState cstate;

  /**
   * State of the tunnel encryption.
   */
  enum CadetTunnelEState estate;

  /**
   * Peer's ephemeral key, to recreate @c e_key and @c d_key when own ephemeral
   * key changes.
   */
  struct GNUNET_CRYPTO_EcdhePublicKey peers_ephemeral_key;

  /**
   * Encryption ("our") key. It is only "confirmed" if kx_ctx is NULL.
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey e_key;

  /**
   * Decryption ("their") key. It is only "confirmed" if kx_ctx is NULL.
   */
  struct GNUNET_CRYPTO_SymmetricSessionKey d_key;

  /**
   * Task to start the rekey process.
   */
  struct GNUNET_SCHEDULER_Task *rekey_task;

  /**
   * Paths that are actively used to reach the destination peer.
   */
  struct CadetTConnection *connection_head;
  struct CadetTConnection *connection_tail;

  /**
   * Next connection number.
   */
  uint32_t next_cid;

  /**
   * Channels inside this tunnel.
   */
  struct CadetTChannel *channel_head;
  struct CadetTChannel *channel_tail;

  /**
   * Channel ID for the next created channel.
   */
  CADET_ChannelNumber next_chid;

  /**
   * Destroy flag: if true, destroy on last message.
   */
  struct GNUNET_SCHEDULER_Task * destroy_task;

  /**
   * Queued messages, to transmit once tunnel gets connected.
   */
  struct CadetTunnelDelayed *tq_head;
  struct CadetTunnelDelayed *tq_tail;

  /**
   * Task to trim connections if too many are present.
   */
  struct GNUNET_SCHEDULER_Task * trim_connections_task;

  /**
   * Ephemeral message in the queue (to avoid queueing more than one).
   */
  struct CadetConnectionQueue *ephm_h;

  /**
   * Pong message in the queue.
   */
  struct CadetConnectionQueue *pong_h;
};


/**
 * Struct used to save messages in a non-ready tunnel to send once connected.
 */
struct CadetTunnelDelayed
{
  /**
   * DLL
   */
  struct CadetTunnelDelayed *next;
  struct CadetTunnelDelayed *prev;

  /**
   * Tunnel.
   */
  struct CadetTunnel *t;

  /**
   * Tunnel queue given to the channel to cancel request. Update on send_queued.
   */
  struct CadetTunnelQueue *tq;

  /**
   * Message to send.
   */
  /* struct GNUNET_MessageHeader *msg; */
};


/**
 * Handle for messages queued but not yet sent.
 */
struct CadetTunnelQueue
{
  /**
   * Connection queue handle, to cancel if necessary.
   */
  struct CadetConnectionQueue *cq;

  /**
   * Handle in case message hasn't been given to a connection yet.
   */
  struct CadetTunnelDelayed *tqd;

  /**
   * Continuation to call once sent.
   */
  GCT_sent cont;

  /**
   * Closure for @c cont.
   */
  void *cont_cls;
};


/******************************************************************************/
/*******************************   GLOBALS  ***********************************/
/******************************************************************************/

/**
 * Global handle to the statistics service.
 */
extern struct GNUNET_STATISTICS_Handle *stats;

/**
 * Local peer own ID (memory efficient handle).
 */
extern GNUNET_PEER_Id myid;

/**
 * Local peer own ID (full value).
 */
extern struct GNUNET_PeerIdentity my_full_id;


/**
 * Don't try to recover tunnels if shutting down.
 */
extern int shutting_down;


/**
 * Set of all tunnels, in order to trigger a new exchange on rekey.
 * Indexed by peer's ID.
 */
static struct GNUNET_CONTAINER_MultiPeerMap *tunnels;

/**
 * Own Peer ID private key.
 */
const static struct GNUNET_CRYPTO_EddsaPrivateKey *id_key;


/********************************  AXOLOTL ************************************/

/**
 * How many messages are needed to trigger a ratchet advance.
 */
static unsigned long long ratchet_messages;

/**
 * How long until we trigger a ratched advance.
 */
static struct GNUNET_TIME_Relative ratchet_time;


/******************************************************************************/
/********************************   STATIC  ***********************************/
/******************************************************************************/

/**
 * Get string description for tunnel connectivity state.
 *
 * @param cs Tunnel state.
 *
 * @return String representation.
 */
static const char *
cstate2s (enum CadetTunnelCState cs)
{
  static char buf[32];

  switch (cs)
  {
    case CADET_TUNNEL_NEW:
      return "CADET_TUNNEL_NEW";
    case CADET_TUNNEL_SEARCHING:
      return "CADET_TUNNEL_SEARCHING";
    case CADET_TUNNEL_WAITING:
      return "CADET_TUNNEL_WAITING";
    case CADET_TUNNEL_READY:
      return "CADET_TUNNEL_READY";
    case CADET_TUNNEL_SHUTDOWN:
      return "CADET_TUNNEL_SHUTDOWN";
    default:
      SPRINTF (buf, "%u (UNKNOWN STATE)", cs);
      return buf;
  }
  return "";
}


/**
 * Get string description for tunnel encryption state.
 *
 * @param es Tunnel state.
 *
 * @return String representation.
 */
static const char *
estate2s (enum CadetTunnelEState es)
{
  static char buf[32];

  switch (es)
  {
    case CADET_TUNNEL_KEY_UNINITIALIZED:
      return "CADET_TUNNEL_KEY_UNINITIALIZED";
    case CADET_TUNNEL_KEY_SENT:
      return "CADET_TUNNEL_KEY_SENT";
    case CADET_TUNNEL_KEY_PING:
      return "CADET_TUNNEL_KEY_PING";
    case CADET_TUNNEL_KEY_OK:
      return "CADET_TUNNEL_KEY_OK";
    case CADET_TUNNEL_KEY_REKEY:
      return "CADET_TUNNEL_KEY_REKEY";
    default:
      SPRINTF (buf, "%u (UNKNOWN STATE)", es);
      return buf;
  }
  return "";
}


/**
 * @brief Check if tunnel is ready to send traffic.
 *
 * Tunnel must be connected and with encryption correctly set up.
 *
 * @param t Tunnel to check.
 *
 * @return #GNUNET_YES if ready, #GNUNET_NO otherwise
 */
static int
is_ready (struct CadetTunnel *t)
{
  int ready;
  int conn_ok;
  int enc_ok;

  conn_ok = CADET_TUNNEL_READY == t->cstate;
  enc_ok = CADET_TUNNEL_KEY_OK == t->estate
           || CADET_TUNNEL_KEY_REKEY == t->estate
           || (CADET_TUNNEL_KEY_PING == t->estate
               && CADET_Axolotl == t->enc_type);
  ready = conn_ok && enc_ok;
  ready = ready || GCT_is_loopback (t);
  return ready;
}


/**
 * Get the channel's buffer. ONLY FOR NON-LOOPBACK CHANNELS!!
 *
 * @param tch Tunnel's channel handle.
 *
 * @return Amount of messages the channel can still buffer towards the client.
 */
static unsigned int
get_channel_buffer (const struct CadetTChannel *tch)
{
  int fwd;

  /* If channel is incoming, is terminal in the FWD direction and fwd is YES */
  fwd = GCCH_is_terminal (tch->ch, GNUNET_YES);

  return GCCH_get_buffer (tch->ch, fwd);
}


/**
 * Get the channel's allowance status.
 *
 * @param tch Tunnel's channel handle.
 *
 * @return #GNUNET_YES if we allowed the client to send data to us.
 */
static int
get_channel_allowed (const struct CadetTChannel *tch)
{
  int fwd;

  /* If channel is outgoing, is origin in the FWD direction and fwd is YES */
  fwd = GCCH_is_origin (tch->ch, GNUNET_YES);

  return GCCH_get_allowed (tch->ch, fwd);
}


/**
 * Get the connection's buffer.
 *
 * @param tc Tunnel's connection handle.
 *
 * @return Amount of messages the connection can still buffer.
 */
static unsigned int
get_connection_buffer (const struct CadetTConnection *tc)
{
  int fwd;

  /* If connection is outgoing, is origin in the FWD direction and fwd is YES */
  fwd = GCC_is_origin (tc->c, GNUNET_YES);

  return GCC_get_buffer (tc->c, fwd);
}


/**
 * Get the connection's allowance.
 *
 * @param tc Tunnel's connection handle.
 *
 * @return Amount of messages we have allowed the next peer to send us.
 */
static unsigned int
get_connection_allowed (const struct CadetTConnection *tc)
{
  int fwd;

  /* If connection is outgoing, is origin in the FWD direction and fwd is YES */
  fwd = GCC_is_origin (tc->c, GNUNET_YES);

  return GCC_get_allowed (tc->c, fwd);
}


/**
 * Create a new Axolotl ephemeral (ratchet) key.
 *
 * @param t Tunnel.
 */
static void
new_ephemeral (struct CadetTunnel *t)
{
  GNUNET_free_non_null (t->ax->DHRs);
  t->ax->DHRs = GNUNET_CRYPTO_ecdhe_key_create();
  #if DUMP_KEYS_TO_STDERR
  {
    struct GNUNET_CRYPTO_EcdhePublicKey pub;
    GNUNET_CRYPTO_ecdhe_key_get_public (t->ax->DHRs, &pub);
    LOG (GNUNET_ERROR_TYPE_DEBUG, "  new DHRs generated: pub  %s\n",
        GNUNET_i2s ((struct GNUNET_PeerIdentity *) &pub));
  }
  #endif
}


/**
 * Calculate HMAC.
 *
 * @param plaintext Content to HMAC.
 * @param size Size of @c plaintext.
 * @param iv Initialization vector for the message.
 * @param key Key to use.
 * @param hmac[out] Destination to store the HMAC.
 */
static void
t_hmac (const void *plaintext, size_t size,
        uint32_t iv, const struct GNUNET_CRYPTO_SymmetricSessionKey *key,
        struct GNUNET_CADET_Hash *hmac)
{
  static const char ctx[] = "cadet authentication key";
  struct GNUNET_CRYPTO_AuthKey auth_key;
  struct GNUNET_HashCode hash;

#if DUMP_KEYS_TO_STDERR
  LOG (GNUNET_ERROR_TYPE_INFO, "  HMAC %u bytes with key %s\n", size,
       GNUNET_i2s ((struct GNUNET_PeerIdentity *) key));
#endif
  GNUNET_CRYPTO_hmac_derive_key (&auth_key, key,
                                 &iv, sizeof (iv),
                                 key, sizeof (*key),
                                 ctx, sizeof (ctx),
                                 NULL);
  /* Two step: CADET_Hash is only 256 bits, HashCode is 512. */
  GNUNET_CRYPTO_hmac (&auth_key, plaintext, size, &hash);
  GNUNET_memcpy (hmac, &hash, sizeof (*hmac));
}


/**
 * Perform a HMAC.
 *
 * @param key Key to use.
 * @param hash[out] Resulting HMAC.
 * @param source Source key material (data to HMAC).
 * @param len Length of @a source.
 */
static void
t_ax_hmac_hash (struct GNUNET_CRYPTO_SymmetricSessionKey *key,
                struct GNUNET_HashCode *hash,
                void *source, unsigned int len)
{
  static const char ctx[] = "axolotl HMAC-HASH";
  struct GNUNET_CRYPTO_AuthKey auth_key;

  GNUNET_CRYPTO_hmac_derive_key (&auth_key, key,
                                 ctx, sizeof (ctx),
                                 NULL);
  GNUNET_CRYPTO_hmac (&auth_key, source, len, hash);
}


/**
 * Derive a key from a HMAC-HASH.
 *
 * @param key Key to use for the HMAC.
 * @param out Key to generate.
 * @param source Source key material (data to HMAC).
 * @param len Length of @a source.
 */
static void
t_hmac_derive_key (struct GNUNET_CRYPTO_SymmetricSessionKey *key,
                   struct GNUNET_CRYPTO_SymmetricSessionKey *out,
                   void *source, unsigned int len)
{
  static const char ctx[] = "axolotl derive key";
  struct GNUNET_HashCode h;

  t_ax_hmac_hash (key, &h, source, len);
  GNUNET_CRYPTO_kdf (out, sizeof (*out), ctx, sizeof (ctx),
                     &h, sizeof (h), NULL);
}


/**
 * Encrypt data with the axolotl tunnel key.
 *
 * @param t Tunnel whose key to use.
 * @param dst Destination for the encrypted data.
 * @param src Source of the plaintext. Can overlap with @c dst.
 * @param size Size of the plaintext.
 *
 * @return Size of the encrypted data.
 */
static int
t_ax_encrypt (struct CadetTunnel *t, void *dst, const void *src, size_t size)
{
  struct GNUNET_CRYPTO_SymmetricSessionKey MK;
  struct GNUNET_CRYPTO_SymmetricInitializationVector iv;
  struct CadetTunnelAxolotl *ax;
  size_t out_size;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "  t_ax_encrypt start\n");

  ax = t->ax;

  ax->ratchet_counter++;
  if (GNUNET_YES == ax->ratchet_allowed
      && (ratchet_messages <= ax->ratchet_counter
          || 0 == GNUNET_TIME_absolute_get_remaining (ax->ratchet_expiration).rel_value_us))
  {
    ax->ratchet_flag = GNUNET_YES;
  }

  if (GNUNET_YES == ax->ratchet_flag)
  {
    /* Advance ratchet */
    struct GNUNET_CRYPTO_SymmetricSessionKey keys[3];
    struct GNUNET_HashCode dh;
    struct GNUNET_HashCode hmac;
    static const char ctx[] = "axolotl ratchet";

    new_ephemeral (t);
    ax->HKs = ax->NHKs;

    /* RK, NHKs, CKs = KDF( HMAC-HASH(RK, DH(DHRs, DHRr)) ) */
    GNUNET_CRYPTO_ecc_ecdh (ax->DHRs, &ax->DHRr, &dh);
    t_ax_hmac_hash (&ax->RK, &hmac, &dh, sizeof (dh));
    GNUNET_CRYPTO_kdf (keys, sizeof (keys), ctx, sizeof (ctx),
                       &hmac, sizeof (hmac), NULL);
    ax->RK = keys[0];
    ax->NHKs = keys[1];
    ax->CKs = keys[2];

    ax->PNs = ax->Ns;
    ax->Ns = 0;
    ax->ratchet_flag = GNUNET_NO;
    ax->ratchet_allowed = GNUNET_NO;
    ax->ratchet_counter = 0;
    ax->ratchet_expiration =
      GNUNET_TIME_absolute_add (GNUNET_TIME_absolute_get(), ratchet_time);
  }

  t_hmac_derive_key (&ax->CKs, &MK, "0", 1);
  GNUNET_CRYPTO_symmetric_derive_iv (&iv, &MK, NULL, 0, NULL);

  #if DUMP_KEYS_TO_STDERR
  LOG (GNUNET_ERROR_TYPE_DEBUG, "  CKs: %s\n",
       GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->CKs));
  LOG (GNUNET_ERROR_TYPE_INFO, "  AX_ENC with key %u: %s\n", ax->Ns,
       GNUNET_i2s ((struct GNUNET_PeerIdentity *) &MK));
  #endif

  out_size = GNUNET_CRYPTO_symmetric_encrypt (src, size, &MK, &iv, dst);

  t_hmac_derive_key (&ax->CKs, &ax->CKs, "1", 1);

  LOG (GNUNET_ERROR_TYPE_DEBUG, "  t_ax_encrypt end\n");

  return out_size;
}


/**
 * Decrypt data with the axolotl tunnel key.
 *
 * @param t Tunnel whose key to use.
 * @param dst Destination for the decrypted data.
 * @param src Source of the ciphertext. Can overlap with @c dst.
 * @param size Size of the ciphertext.
 *
 * @return Size of the decrypted data.
 */
static int
t_ax_decrypt (struct CadetTunnel *t, void *dst, const void *src, size_t size)
{
  struct GNUNET_CRYPTO_SymmetricSessionKey MK;
  struct GNUNET_CRYPTO_SymmetricInitializationVector iv;
  struct CadetTunnelAxolotl *ax;
  size_t out_size;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "  t_ax_decrypt start\n");

  ax = t->ax;

  t_hmac_derive_key (&ax->CKr, &MK, "0", 1);
  GNUNET_CRYPTO_symmetric_derive_iv (&iv, &MK, NULL, 0, NULL);

  #if DUMP_KEYS_TO_STDERR
  LOG (GNUNET_ERROR_TYPE_DEBUG, "  CKr: %s\n",
       GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->CKr));
  LOG (GNUNET_ERROR_TYPE_INFO, "  AX_DEC with key %u: %s\n", ax->Nr,
       GNUNET_i2s ((struct GNUNET_PeerIdentity *) &MK));
  #endif

  GNUNET_assert (size >= sizeof (struct GNUNET_MessageHeader));
  out_size = GNUNET_CRYPTO_symmetric_decrypt (src, size, &MK, &iv, dst);
  GNUNET_assert (out_size == size);

  t_hmac_derive_key (&ax->CKr, &ax->CKr, "1", 1);

  LOG (GNUNET_ERROR_TYPE_DEBUG, "  t_ax_decrypt end\n");

  return out_size;
}


/**
 * Encrypt header with the axolotl header key.
 *
 * @param t Tunnel whose key to use.
 * @param msg Message whose header to encrypt.
 */
static void
t_h_encrypt (struct CadetTunnel *t, struct GNUNET_CADET_AX *msg)
{
  struct GNUNET_CRYPTO_SymmetricInitializationVector iv;
  struct CadetTunnelAxolotl *ax;
  size_t out_size;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "  t_h_encrypt start\n");

  ax = t->ax;
  GNUNET_CRYPTO_symmetric_derive_iv (&iv, &ax->HKs, NULL, 0, NULL);

  #if DUMP_KEYS_TO_STDERR
  LOG (GNUNET_ERROR_TYPE_INFO, "  AX_ENC_H with key %s\n",
       GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->HKs));
  #endif

  out_size = GNUNET_CRYPTO_symmetric_encrypt (&msg->Ns, AX_HEADER_SIZE,
                                              &ax->HKs, &iv, &msg->Ns);

  GNUNET_assert (AX_HEADER_SIZE == out_size);

  LOG (GNUNET_ERROR_TYPE_DEBUG, "  t_ax_encrypt end\n");
}


/**
 * Decrypt header with the current axolotl header key.
 *
 * @param t Tunnel whose current ax HK to use.
 * @param src Message whose header to decrypt.
 * @param dst Where to decrypt header to.
 */
static void
t_h_decrypt (struct CadetTunnel *t, const struct GNUNET_CADET_AX *src,
             struct GNUNET_CADET_AX *dst)
{
  struct GNUNET_CRYPTO_SymmetricInitializationVector iv;
  struct CadetTunnelAxolotl *ax;
  size_t out_size;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "  t_h_decrypt start\n");

  ax = t->ax;
  GNUNET_CRYPTO_symmetric_derive_iv (&iv, &ax->HKr, NULL, 0, NULL);

  #if DUMP_KEYS_TO_STDERR
  LOG (GNUNET_ERROR_TYPE_INFO, "  AX_DEC_H with key %s\n",
       GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->HKr));
  #endif

  out_size = GNUNET_CRYPTO_symmetric_decrypt (&src->Ns, AX_HEADER_SIZE,
                                              &ax->HKr, &iv, &dst->Ns);

  GNUNET_assert (AX_HEADER_SIZE == out_size);

  LOG (GNUNET_ERROR_TYPE_DEBUG, "  t_h_decrypt end\n");
}


/**
 * Decrypt and verify data with the appropriate tunnel key and verify that the
 * data has not been altered since it was sent by the remote peer.
 *
 * @param t Tunnel whose key to use.
 * @param dst Destination for the plaintext.
 * @param src Source of the message. Can overlap with @c dst.
 * @param size Size of the message.
 *
 * @return Size of the decrypted data, -1 if an error was encountered.
 */
static int
try_old_ax_keys (struct CadetTunnel *t, void *dst,
                 const struct GNUNET_CADET_AX *src, size_t size)
{
  struct CadetTunnelSkippedKey *key;
  struct GNUNET_CADET_Hash *hmac;
  struct GNUNET_CRYPTO_SymmetricInitializationVector iv;
  struct GNUNET_CADET_AX plaintext_header;
  struct GNUNET_CRYPTO_SymmetricSessionKey *valid_HK;
  size_t esize;
  size_t res;
  size_t len;
  unsigned int N;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "Trying old keys\n");
  hmac = &plaintext_header.hmac;
  esize = size - sizeof (struct GNUNET_CADET_AX);

  /* Find a correct Header Key */
  for (key = t->ax->skipped_head; NULL != key; key = key->next)
  {
    #if DUMP_KEYS_TO_STDERR
    LOG (GNUNET_ERROR_TYPE_DEBUG, "  Trying hmac with key %s\n",
         GNUNET_i2s ((struct GNUNET_PeerIdentity *) &key->HK));
    #endif
    t_hmac (&src->Ns, AX_HEADER_SIZE + esize, 0, &key->HK, hmac);
    if (0 == memcmp (hmac, &src->hmac, sizeof (*hmac)))
    {
      LOG (GNUNET_ERROR_TYPE_DEBUG, "  hmac correct\n");
      valid_HK = &key->HK;
      break;
    }
  }
  if (NULL == key)
    return -1;

  /* Should've been checked in -cadet_connection.c handle_cadet_encrypted. */
  GNUNET_assert (size > sizeof (struct GNUNET_CADET_AX));
  len = size - sizeof (struct GNUNET_CADET_AX);
  GNUNET_assert (len >= sizeof (struct GNUNET_MessageHeader));

  /* Decrypt header */
  GNUNET_CRYPTO_symmetric_derive_iv (&iv, &key->HK, NULL, 0, NULL);
  res = GNUNET_CRYPTO_symmetric_decrypt (&src->Ns, AX_HEADER_SIZE,
                                         &key->HK, &iv, &plaintext_header.Ns);
  GNUNET_assert (AX_HEADER_SIZE == res);
  LOG (GNUNET_ERROR_TYPE_DEBUG, "  Message %u, previous: %u\n",
       ntohl (plaintext_header.Ns), ntohl (plaintext_header.PNs));

  /* Find the correct Message Key */
  N = ntohl (plaintext_header.Ns);
  while (NULL != key && N != key->Kn)
    key = key->next;
  if (NULL == key || 0 != memcmp (&key->HK, valid_HK, sizeof (*valid_HK)))
    return -1;

  #if DUMP_KEYS_TO_STDERR
  LOG (GNUNET_ERROR_TYPE_INFO, "  AX_DEC_H with skipped key %s\n",
       GNUNET_i2s ((struct GNUNET_PeerIdentity *) &key->HK));
  LOG (GNUNET_ERROR_TYPE_INFO, "  AX_DEC with skipped key %u: %s\n",
       key->Kn, GNUNET_i2s ((struct GNUNET_PeerIdentity *) &key->MK));
  #endif

  /* Decrypt payload */
  GNUNET_CRYPTO_symmetric_derive_iv (&iv, &key->MK, NULL, 0, NULL);
  res = GNUNET_CRYPTO_symmetric_decrypt (&src[1], len, &key->MK, &iv, dst);

  /* Remove key */
  GNUNET_CONTAINER_DLL_remove (t->ax->skipped_head, t->ax->skipped_tail, key);
  t->ax->skipped--;
  GNUNET_free (key); /* GNUNET_free overwrites memory with 0xbaadf00d */

  return res;
}


/**
 * Delete a key from the list of skipped keys.
 *
 * @param t Tunnel to delete from.
 * @param HKr Header Key to use.
 */
static void
store_skipped_key (struct CadetTunnel *t,
                   const struct GNUNET_CRYPTO_SymmetricSessionKey *HKr)
{
  struct CadetTunnelSkippedKey *key;

  key = GNUNET_new (struct CadetTunnelSkippedKey);
  key->timestamp = GNUNET_TIME_absolute_get ();
  key->Kn = t->ax->Nr;
  key->HK = t->ax->HKr;
  t_hmac_derive_key (&t->ax->CKr, &key->MK, "0", 1);
  #if DUMP_KEYS_TO_STDERR
  LOG (GNUNET_ERROR_TYPE_DEBUG, "    storing MK for Nr %u: %s\n",
       key->Kn, GNUNET_i2s ((struct GNUNET_PeerIdentity *) &key->MK));
  LOG (GNUNET_ERROR_TYPE_DEBUG, "    for CKr: %s\n",
       GNUNET_i2s ((struct GNUNET_PeerIdentity *) &t->ax->CKr));
  #endif
  t_hmac_derive_key (&t->ax->CKr, &t->ax->CKr, "1", 1);
  GNUNET_CONTAINER_DLL_insert (t->ax->skipped_head, t->ax->skipped_tail, key);
  t->ax->Nr++;
  t->ax->skipped++;
}


/**
 * Delete a key from the list of skipped keys.
 *
 * @param t Tunnel to delete from.
 * @param key Key to delete.
 */
static void
delete_skipped_key (struct CadetTunnel *t, struct CadetTunnelSkippedKey *key)
{
  GNUNET_CONTAINER_DLL_remove (t->ax->skipped_head, t->ax->skipped_tail, key);
  GNUNET_free (key);
  t->ax->skipped--;
}


/**
 * Stage skipped AX keys and calculate the message key.
 *
 * Stores each HK and MK for skipped messages.
 *
 * @param t Tunnel where to stage the keys.
 * @param HKr Header key.
 * @param Np Received meesage number.
 *
 * @return GNUNET_OK if keys were stored.
 *         GNUNET_SYSERR if an error ocurred (Np not expected).
 */
static int
store_ax_keys (struct CadetTunnel *t,
               const struct GNUNET_CRYPTO_SymmetricSessionKey *HKr,
               uint32_t Np)
{
  int gap;


  gap = Np - t->ax->Nr;
  LOG (GNUNET_ERROR_TYPE_INFO, "Storing keys [%u, %u)\n", t->ax->Nr, Np);
  if (MAX_KEY_GAP < gap)
  {
    /* Avoid DoS (forcing peer to do 2*33 chain HMAC operations) */
    /* TODO: start new key exchange on return */
    GNUNET_break_op (0);
    LOG (GNUNET_ERROR_TYPE_WARNING, "Got message %u, expected %u+\n",
         Np, t->ax->Nr);
    return GNUNET_SYSERR;
  }
  if (0 > gap)
  {
    /* Delayed message: don't store keys, flag to try old keys. */
    return GNUNET_SYSERR;
  }

  while (t->ax->Nr < Np)
    store_skipped_key (t, HKr);

  while (t->ax->skipped > MAX_SKIPPED_KEYS)
    delete_skipped_key (t, t->ax->skipped_tail);

  return GNUNET_OK;
}


/**
 * Decrypt and verify data with the appropriate tunnel key and verify that the
 * data has not been altered since it was sent by the remote peer.
 *
 * @param t Tunnel whose key to use.
 * @param dst Destination for the plaintext.
 * @param src Source of the message. Can overlap with @c dst.
 * @param size Size of the message.
 *
 * @return Size of the decrypted data, -1 if an error was encountered.
 */
static int
t_ax_decrypt_and_validate (struct CadetTunnel *t, void *dst,
                           const struct GNUNET_CADET_AX *src, size_t size)
{
  struct CadetTunnelAxolotl *ax;
  struct GNUNET_CADET_Hash msg_hmac;
  struct GNUNET_HashCode hmac;
  struct GNUNET_CADET_AX plaintext_header;
  uint32_t Np;
  uint32_t PNp;
  size_t esize;
  size_t osize;

  ax = t->ax;
  esize = size - sizeof (struct GNUNET_CADET_AX);

  if (NULL == ax)
    return -1;

  /* Try current HK */
  t_hmac (&src->Ns, AX_HEADER_SIZE + esize, 0, &ax->HKr, &msg_hmac);
  if (0 != memcmp (&msg_hmac, &src->hmac, sizeof (msg_hmac)))
  {
    static const char ctx[] = "axolotl ratchet";
    struct GNUNET_CRYPTO_SymmetricSessionKey keys[3]; /* RKp, NHKp, CKp */
    struct GNUNET_CRYPTO_SymmetricSessionKey HK;
    struct GNUNET_HashCode dh;
    struct GNUNET_CRYPTO_EcdhePublicKey *DHRp;

    /* Try Next HK */
    LOG (GNUNET_ERROR_TYPE_DEBUG, "  trying next HK\n");
    t_hmac (&src->Ns, AX_HEADER_SIZE + esize, 0, &ax->NHKr, &msg_hmac);
    if (0 != memcmp (&msg_hmac, &src->hmac, sizeof (msg_hmac)))
    {
      /* Try the skipped keys, if that fails, we're out of luck. */
      return try_old_ax_keys (t, dst, src, size);
    }
    LOG (GNUNET_ERROR_TYPE_INFO, "next HK worked\n");

    HK = ax->HKr;
    ax->HKr = ax->NHKr;
    t_h_decrypt (t, src, &plaintext_header);
    Np = ntohl (plaintext_header.Ns);
    PNp = ntohl (plaintext_header.PNs);
    DHRp = &plaintext_header.DHRs;
    store_ax_keys (t, &HK, PNp);

    /* RKp, NHKp, CKp = KDF (HMAC-HASH (RK, DH (DHRp, DHRs))) */
    GNUNET_CRYPTO_ecc_ecdh (ax->DHRs, DHRp, &dh);
    t_ax_hmac_hash (&ax->RK, &hmac, &dh, sizeof (dh));
    GNUNET_CRYPTO_kdf (keys, sizeof (keys), ctx, sizeof (ctx),
                       &hmac, sizeof (hmac), NULL);

    /* Commit "purported" keys */
    ax->RK = keys[0];
    ax->NHKr = keys[1];
    ax->CKr = keys[2];
    ax->DHRr = *DHRp;
    ax->Nr = 0;
    ax->ratchet_allowed = GNUNET_YES;
  }
  else
  {
    LOG (GNUNET_ERROR_TYPE_DEBUG, "current HK\n");
    t_h_decrypt (t, src, &plaintext_header);
    Np = ntohl (plaintext_header.Ns);
    PNp = ntohl (plaintext_header.PNs);
  }
  LOG (GNUNET_ERROR_TYPE_INFO, "  got AX Nr %u\n", Np);
  if (Np != ax->Nr)
    if (GNUNET_OK != store_ax_keys (t, &ax->HKr, Np))
      /* Try the skipped keys, if that fails, we're out of luck. */
      return try_old_ax_keys (t, dst, src, size);

  osize = t_ax_decrypt (t, dst, &src[1], esize);
  ax->Nr = Np + 1;

  if (osize != esize)
  {
    GNUNET_break_op (0);
    return -1;
  }

  return osize;
}


/**
 * Pick a connection on which send the next data message.
 *
 * @param t Tunnel on which to send the message.
 *
 * @return The connection on which to send the next message.
 */
static struct CadetConnection *
tunnel_get_connection (struct CadetTunnel *t)
{
  struct CadetTConnection *iter;
  struct CadetConnection *best;
  unsigned int qn;
  unsigned int lowest_q;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "tunnel_get_connection %s\n", GCT_2s (t));
  best = NULL;
  lowest_q = UINT_MAX;
  for (iter = t->connection_head; NULL != iter; iter = iter->next)
  {
    LOG (GNUNET_ERROR_TYPE_DEBUG, "  connection %s: %u\n",
         GCC_2s (iter->c), GCC_get_state (iter->c));
    if (CADET_CONNECTION_READY == GCC_get_state (iter->c))
    {
      qn = GCC_get_qn (iter->c, GCC_is_origin (iter->c, GNUNET_YES));
      LOG (GNUNET_ERROR_TYPE_DEBUG, "    q_n %u, \n", qn);
      if (qn < lowest_q)
      {
        best = iter->c;
        lowest_q = qn;
      }
    }
  }
  LOG (GNUNET_ERROR_TYPE_DEBUG, " selected: connection %s\n", GCC_2s (best));
  return best;
}


/**
 * Callback called when a queued message is sent.
 *
 * Calculates the average time and connection packet tracking.
 *
 * @param cls Closure (TunnelQueue handle).
 * @param c Connection this message was on.
 * @param q Connection queue handle (unused).
 * @param type Type of message sent.
 * @param fwd Was this a FWD going message?
 * @param size Size of the message.
 */
static void
tun_message_sent (void *cls,
              struct CadetConnection *c,
              struct CadetConnectionQueue *q,
              uint16_t type, int fwd, size_t size)
{
  struct CadetTunnelQueue *qt = cls;
  struct CadetTunnel *t;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "tun_message_sent\n");

  GNUNET_assert (NULL != qt->cont);
  t = NULL == c ? NULL : GCC_get_tunnel (c);
  qt->cont (qt->cont_cls, t, qt, type, size);
  GNUNET_free (qt);
}


static unsigned int
count_queued_data (const struct CadetTunnel *t)
{
  struct CadetTunnelDelayed *iter;
  unsigned int count;

  for (count = 0, iter = t->tq_head; iter != NULL; iter = iter->next)
    count++;

  return count;
}

/**
 * Delete a queued message: either was sent or the channel was destroyed
 * before the tunnel's key exchange had a chance to finish.
 *
 * @param tqd Delayed queue handle.
 */
static void
unqueue_data (struct CadetTunnelDelayed *tqd)
{
  GNUNET_CONTAINER_DLL_remove (tqd->t->tq_head, tqd->t->tq_tail, tqd);
  GNUNET_free (tqd);
}


/**
 * Cache a message to be sent once tunnel is online.
 *
 * @param t Tunnel to hold the message.
 * @param msg Message itself (copy will be made).
 */
static struct CadetTunnelDelayed *
queue_data (struct CadetTunnel *t, const struct GNUNET_MessageHeader *msg)
{
  struct CadetTunnelDelayed *tqd;
  uint16_t size = ntohs (msg->size);

  LOG (GNUNET_ERROR_TYPE_DEBUG, "queue data on Tunnel %s\n", GCT_2s (t));

  GNUNET_assert (GNUNET_NO == is_ready (t));

  tqd = GNUNET_malloc (sizeof (struct CadetTunnelDelayed) + size);

  tqd->t = t;
  GNUNET_memcpy (&tqd[1], msg, size);
  GNUNET_CONTAINER_DLL_insert_tail (t->tq_head, t->tq_tail, tqd);
  return tqd;
}


/**
 * Sends an already built message on a tunnel, encrypting it and
 * choosing the best connection.
 *
 * @param message Message to send. Function modifies it.
 * @param t Tunnel on which this message is transmitted.
 * @param c Connection to use (autoselect if NULL).
 * @param force Force the tunnel to take the message (buffer overfill).
 * @param cont Continuation to call once message is really sent.
 * @param cont_cls Closure for @c cont.
 * @param existing_q In case this a transmission of previously queued data,
 *                   this should be TunnelQueue given to the client.
 *                   Otherwise, NULL.
 * @return Handle to cancel message.
 *         NULL if @c cont is NULL or an error happens and message is dropped.
 */
static struct CadetTunnelQueue *
send_prebuilt_message (const struct GNUNET_MessageHeader *message,
                       struct CadetTunnel *t,
                       struct CadetConnection *c,
                       int force,
                       GCT_sent cont,
                       void *cont_cls,
                       struct CadetTunnelQueue *existing_q)
{
  struct GNUNET_MessageHeader *msg;
  struct GNUNET_CADET_AX *ax_msg;
  struct CadetTunnelQueue *tq;
  size_t size = ntohs (message->size);
  char cbuf[sizeof (struct GNUNET_CADET_AX) + size] GNUNET_ALIGN;
  size_t esize;
  uint32_t mid;
  uint16_t type;
  int fwd;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "GMT Send on Tunnel %s\n", GCT_2s (t));

  if (GNUNET_NO == is_ready (t))
  {
    struct CadetTunnelDelayed *tqd;
    /* A non null existing_q indicates sending of queued data.
     * Should only happen after tunnel becomes ready.
     */
    GNUNET_assert (NULL == existing_q);
    tqd = queue_data (t, message);
    if (NULL == cont)
      return NULL;
    tq = GNUNET_new (struct CadetTunnelQueue);
    tq->tqd = tqd;
    tqd->tq = tq;
    tq->cont = cont;
    tq->cont_cls = cont_cls;
    return tq;
  }

  GNUNET_assert (GNUNET_NO == GCT_is_loopback (t));

  GNUNET_assert (CADET_Axolotl == t->enc_type);
  ax_msg = (struct GNUNET_CADET_AX *) cbuf;
  msg = &ax_msg->header;
  msg->size = htons (sizeof (struct GNUNET_CADET_AX) + size);
  msg->type = htons (GNUNET_MESSAGE_TYPE_CADET_AX);
  esize = t_ax_encrypt (t, &ax_msg[1], message, size);
  ax_msg->Ns = htonl (t->ax->Ns++);
  ax_msg->PNs = htonl (t->ax->PNs);
  GNUNET_CRYPTO_ecdhe_key_get_public (t->ax->DHRs, &ax_msg->DHRs);
  t_h_encrypt (t, ax_msg);
  t_hmac (&ax_msg->Ns, AX_HEADER_SIZE + esize, 0, &t->ax->HKs, &ax_msg->hmac);
  GNUNET_assert (esize == size);

  if (NULL == c)
    c = tunnel_get_connection (t);
  if (NULL == c)
  {
    /* Why is tunnel 'ready'? Should have been queued! */
    if (NULL != t->destroy_task)
    {
      GNUNET_break (0);
      GCT_debug (t, GNUNET_ERROR_TYPE_WARNING);
    }
    return NULL; /* Drop... */
  }

  mid = 0;
  type = ntohs (message->type);
  switch (type)
  {
    case GNUNET_MESSAGE_TYPE_CADET_DATA:
    case GNUNET_MESSAGE_TYPE_CADET_DATA_ACK:
      if (GNUNET_MESSAGE_TYPE_CADET_DATA == type)
        mid = ntohl (((struct GNUNET_CADET_Data *) message)->mid);
      else
        mid = ntohl (((struct GNUNET_CADET_DataACK *) message)->mid);
      /* Fall thru */
    case GNUNET_MESSAGE_TYPE_CADET_KEEPALIVE:
    case GNUNET_MESSAGE_TYPE_CADET_CHANNEL_CREATE:
    case GNUNET_MESSAGE_TYPE_CADET_CHANNEL_DESTROY:
    case GNUNET_MESSAGE_TYPE_CADET_CHANNEL_ACK:
    case GNUNET_MESSAGE_TYPE_CADET_CHANNEL_NACK:
      break;
    default:
      GNUNET_break (0);
      LOG (GNUNET_ERROR_TYPE_ERROR, "type %s not valid\n", GC_m2s (type));
  }
  LOG (GNUNET_ERROR_TYPE_DEBUG, "type %s\n", GC_m2s (type));

  fwd = GCC_is_origin (c, GNUNET_YES);

  if (NULL == cont)
  {
    GNUNET_break (NULL == GCC_send_prebuilt_message (msg, type, mid, c, fwd,
                                                     force, NULL, NULL));
    return NULL;
  }
  if (NULL == existing_q)
  {
    tq = GNUNET_new (struct CadetTunnelQueue); /* FIXME valgrind: leak*/
  }
  else
  {
    tq = existing_q;
    tq->tqd = NULL;
  }
  tq->cq = GCC_send_prebuilt_message (msg, type, mid, c, fwd, force,
                                      &tun_message_sent, tq);
  GNUNET_assert (NULL != tq->cq);
  tq->cont = cont;
  tq->cont_cls = cont_cls;

  return tq;
}


/**
 * Send all cached messages that we can, tunnel is online.
 *
 * @param t Tunnel that holds the messages. Cannot be loopback.
 */
static void
send_queued_data (struct CadetTunnel *t)
{
  struct CadetTunnelDelayed *tqd;
  struct CadetTunnelDelayed *next;
  unsigned int room;

  LOG (GNUNET_ERROR_TYPE_INFO, "Send queued data, tunnel %s\n", GCT_2s (t));

  if (GCT_is_loopback (t))
  {
    GNUNET_break (0);
    return;
  }

  if (GNUNET_NO == is_ready (t))
  {
    LOG (GNUNET_ERROR_TYPE_WARNING, "  not ready yet: %s/%s\n",
         estate2s (t->estate), cstate2s (t->cstate));
    return;
  }

  room = GCT_get_connections_buffer (t);
  LOG (GNUNET_ERROR_TYPE_DEBUG, "  buffer space: %u\n", room);
  LOG (GNUNET_ERROR_TYPE_DEBUG, "  tq head: %p\n", t->tq_head);
  for (tqd = t->tq_head; NULL != tqd && room > 0; tqd = next)
  {
    LOG (GNUNET_ERROR_TYPE_DEBUG, " sending queued data\n");
    next = tqd->next;
    room--;
    send_prebuilt_message ((struct GNUNET_MessageHeader *) &tqd[1],
                           tqd->t, NULL, GNUNET_YES,
                           NULL != tqd->tq ? tqd->tq->cont : NULL,
                           NULL != tqd->tq ? tqd->tq->cont_cls : NULL,
                           tqd->tq);
    unqueue_data (tqd);
  }
  LOG (GNUNET_ERROR_TYPE_DEBUG, "GCT_send_queued_data end\n", GCP_2s (t->peer));
}


/**
 * @brief Resend the AX KX until we complete the handshake.
 *
 * @param cls Closure (tunnel).
 */
static void
ax_kx_resend (void *cls)
{
  struct CadetTunnel *t = cls;

  t->rekey_task = NULL;
  if (CADET_TUNNEL_KEY_OK == t->estate)
  {
    /* Should have been canceled on estate change */
    GNUNET_break (0);
    return;
  }

  GCT_send_ax_kx (t, CADET_TUNNEL_KEY_SENT >= t->estate);
}


/**
 * Callback called when a queued message is sent.
 *
 * @param cls Closure.
 * @param c Connection this message was on.
 * @param type Type of message sent.
 * @param fwd Was this a FWD going message?
 * @param size Size of the message.
 */
static void
ephm_sent (void *cls,
           struct CadetConnection *c,
           struct CadetConnectionQueue *q,
           uint16_t type, int fwd, size_t size)
{
  struct CadetTunnel *t = cls;
  LOG (GNUNET_ERROR_TYPE_DEBUG, "ephemeral sent %s\n", GC_m2s (type));

  t->ephm_h = NULL;

  if (CADET_TUNNEL_KEY_OK == t->estate)
    return;

  if (CADET_Axolotl == t->enc_type)
  {
    if (NULL != t->rekey_task)
    {
      GNUNET_break (0);
      GCT_debug (t, GNUNET_ERROR_TYPE_WARNING);
      GNUNET_SCHEDULER_cancel (t->rekey_task);
    }
    t->rekey_task = GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_UNIT_SECONDS,
                                                  &ax_kx_resend, t);
  }
}


/**
 * Sends key exchange message on a tunnel, choosing the best connection.
 * Should not be called on loopback tunnels.
 *
 * @param t Tunnel on which this message is transmitted.
 * @param message Message to send. Function modifies it.
 *
 * @return Handle to the message in the connection queue.
 */
static struct CadetConnectionQueue *
send_kx (struct CadetTunnel *t,
         const struct GNUNET_MessageHeader *message)
{
  struct CadetConnection *c;
  struct GNUNET_CADET_KX *msg;
  size_t size = ntohs (message->size);
  char cbuf[sizeof (struct GNUNET_CADET_KX) + size];
  uint16_t type;
  int fwd;
  GCC_sent cont;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "GMT KX on Tunnel %s\n", GCT_2s (t));

  /* Avoid loopback. */
  if (GCT_is_loopback (t))
  {
    GNUNET_break (0);
    return NULL;
  }
  type = ntohs (message->type);

  /* Even if tunnel is "being destroyed", send anyway.
   * Could be a response to a rekey initiated by remote peer,
   * who is trying to create a new channel!
   */

  /* Must have a connection, or be looking for one. */
  if (NULL == t->connection_head)
  {
    LOG (GNUNET_ERROR_TYPE_DEBUG, "%s with no connection\n", GC_m2s (type));
    if (CADET_TUNNEL_SEARCHING != t->cstate)
    {
      GNUNET_break (0);
      GCT_debug (t, GNUNET_ERROR_TYPE_ERROR);
    }
    return NULL;
  }

  msg = (struct GNUNET_CADET_KX *) cbuf;
  msg->header.type = htons (GNUNET_MESSAGE_TYPE_CADET_KX);
  msg->header.size = htons (sizeof (struct GNUNET_CADET_KX) + size);
  c = tunnel_get_connection (t);
  if (NULL == c)
  {
    if (NULL == t->destroy_task && CADET_TUNNEL_READY == t->cstate)
    {
      GNUNET_break (0);
      GCT_debug (t, GNUNET_ERROR_TYPE_ERROR);
    }
    return NULL;
  }
  switch (type)
  {
    case GNUNET_MESSAGE_TYPE_CADET_AX_KX:
      GNUNET_assert (NULL == t->ephm_h);
      cont = &ephm_sent;
      break;
    default:
      LOG (GNUNET_ERROR_TYPE_DEBUG, "unkown type %s\n", GC_m2s (type));
      GNUNET_assert (0);
  }
  GNUNET_memcpy (&msg[1], message, size);

  fwd = GCC_is_origin (c, GNUNET_YES);

  return GCC_send_prebuilt_message (&msg->header, type, 0, c,
                                    fwd, GNUNET_YES,
                                    cont, t);
}


/**
 * Called only on shutdown, destroy every tunnel.
 *
 * @param cls Closure (unused).
 * @param key Current public key.
 * @param value Value in the hash map (tunnel).
 *
 * @return #GNUNET_YES, so we should continue to iterate,
 */
static int
destroy_iterator (void *cls,
                const struct GNUNET_PeerIdentity *key,
                void *value)
{
  struct CadetTunnel *t = value;

  LOG (GNUNET_ERROR_TYPE_DEBUG,
       "GCT_shutdown destroying tunnel at %p\n", t);
  GCT_destroy (t);
  return GNUNET_YES;
}


/**
 * Notify remote peer that we don't know a channel he is talking about,
 * probably CHANNEL_DESTROY was missed.
 *
 * @param t Tunnel on which to notify.
 * @param gid ID of the channel.
 */
static void
send_channel_destroy (struct CadetTunnel *t, unsigned int gid)
{
  struct GNUNET_CADET_ChannelManage msg;

  msg.header.type = htons (GNUNET_MESSAGE_TYPE_CADET_CHANNEL_DESTROY);
  msg.header.size = htons (sizeof (msg));
  msg.chid = htonl (gid);

  LOG (GNUNET_ERROR_TYPE_DEBUG,
       "WARNING destroying unknown channel %u on tunnel %s\n",
       gid, GCT_2s (t));
  send_prebuilt_message (&msg.header, t, NULL, GNUNET_YES, NULL, NULL, NULL);
}


/**
 * Demultiplex data per channel and call appropriate channel handler.
 *
 * @param t Tunnel on which the data came.
 * @param msg Data message.
 * @param fwd Is this message fwd? This only is meaningful in loopback channels.
 *            #GNUNET_YES if message is FWD on the respective channel (loopback)
 *            #GNUNET_NO if message is BCK on the respective channel (loopback)
 *            #GNUNET_SYSERR if message on a one-ended channel (remote)
 */
static void
handle_data (struct CadetTunnel *t,
             const struct GNUNET_CADET_Data *msg,
             int fwd)
{
  struct CadetChannel *ch;
  char buf[128];
  size_t size;
  uint16_t type;

  /* Check size */
  size = ntohs (msg->header.size);
  if (size <
      sizeof (struct GNUNET_CADET_Data) +
      sizeof (struct GNUNET_MessageHeader))
  {
    GNUNET_break (0);
    return;
  }
  type = ntohs (msg[1].header.type);
  LOG (GNUNET_ERROR_TYPE_DEBUG, " payload of type %s\n", GC_m2s (type));
  SPRINTF (buf, "# received payload of type %hu", type);
  GNUNET_STATISTICS_update (stats, buf, 1, GNUNET_NO);


  /* Check channel */
  ch = GCT_get_channel (t, ntohl (msg->chid));
  if (NULL == ch)
  {
    GNUNET_STATISTICS_update (stats, "# data on unknown channel",
                              1, GNUNET_NO);
    LOG (GNUNET_ERROR_TYPE_DEBUG, "channel 0x%X unknown\n", ntohl (msg->chid));
    send_channel_destroy (t, ntohl (msg->chid));
    return;
  }

  GCCH_handle_data (ch, msg, fwd);
}


/**
 * Demultiplex data ACKs per channel and update appropriate channel buffer info.
 *
 * @param t Tunnel on which the DATA ACK came.
 * @param msg DATA ACK message.
 * @param fwd Is this message fwd? This only is meaningful in loopback channels.
 *            #GNUNET_YES if message is FWD on the respective channel (loopback)
 *            #GNUNET_NO if message is BCK on the respective channel (loopback)
 *            #GNUNET_SYSERR if message on a one-ended channel (remote)
 */
static void
handle_data_ack (struct CadetTunnel *t,
                 const struct GNUNET_CADET_DataACK *msg,
                 int fwd)
{
  struct CadetChannel *ch;
  size_t size;

  /* Check size */
  size = ntohs (msg->header.size);
  if (size != sizeof (struct GNUNET_CADET_DataACK))
  {
    GNUNET_break (0);
    return;
  }

  /* Check channel */
  ch = GCT_get_channel (t, ntohl (msg->chid));
  if (NULL == ch)
  {
    GNUNET_STATISTICS_update (stats, "# data ack on unknown channel",
                              1, GNUNET_NO);
    LOG (GNUNET_ERROR_TYPE_DEBUG, "WARNING channel %u unknown\n",
         ntohl (msg->chid));
    return;
  }

  GCCH_handle_data_ack (ch, msg, fwd);
}


/**
 * Handle channel create.
 *
 * @param t Tunnel on which the message came.
 * @param msg ChannelCreate message.
 */
static void
handle_ch_create (struct CadetTunnel *t,
                  const struct GNUNET_CADET_ChannelCreate *msg)
{
  struct CadetChannel *ch;
  size_t size;

  /* Check size */
  size = ntohs (msg->header.size);
  if (size != sizeof (struct GNUNET_CADET_ChannelCreate))
  {
    GNUNET_break_op (0);
    return;
  }

  /* Check channel */
  ch = GCT_get_channel (t, ntohl (msg->chid));
  if (NULL != ch && ! GCT_is_loopback (t))
  {
    /* Probably a retransmission, safe to ignore */
    LOG (GNUNET_ERROR_TYPE_DEBUG, "   already exists...\n");
  }
  ch = GCCH_handle_create (t, msg);
  if (NULL != ch)
    GCT_add_channel (t, ch);
}



/**
 * Handle channel NACK: check correctness and call channel handler for NACKs.
 *
 * @param t Tunnel on which the NACK came.
 * @param msg NACK message.
 */
static void
handle_ch_nack (struct CadetTunnel *t,
                const struct GNUNET_CADET_ChannelManage *msg)
{
  struct CadetChannel *ch;
  size_t size;

  /* Check size */
  size = ntohs (msg->header.size);
  if (size != sizeof (struct GNUNET_CADET_ChannelManage))
  {
    GNUNET_break (0);
    return;
  }

  /* Check channel */
  ch = GCT_get_channel (t, ntohl (msg->chid));
  if (NULL == ch)
  {
    GNUNET_STATISTICS_update (stats, "# channel NACK on unknown channel",
                              1, GNUNET_NO);
    LOG (GNUNET_ERROR_TYPE_DEBUG, "WARNING channel %u unknown\n",
         ntohl (msg->chid));
    return;
  }

  GCCH_handle_nack (ch);
}


/**
 * Handle a CHANNEL ACK (SYNACK/ACK).
 *
 * @param t Tunnel on which the CHANNEL ACK came.
 * @param msg CHANNEL ACK message.
 * @param fwd Is this message fwd? This only is meaningful in loopback channels.
 *            #GNUNET_YES if message is FWD on the respective channel (loopback)
 *            #GNUNET_NO if message is BCK on the respective channel (loopback)
 *            #GNUNET_SYSERR if message on a one-ended channel (remote)
 */
static void
handle_ch_ack (struct CadetTunnel *t,
               const struct GNUNET_CADET_ChannelManage *msg,
               int fwd)
{
  struct CadetChannel *ch;
  size_t size;

  /* Check size */
  size = ntohs (msg->header.size);
  if (size != sizeof (struct GNUNET_CADET_ChannelManage))
  {
    GNUNET_break (0);
    return;
  }

  /* Check channel */
  ch = GCT_get_channel (t, ntohl (msg->chid));
  if (NULL == ch)
  {
    GNUNET_STATISTICS_update (stats, "# channel ack on unknown channel",
                              1, GNUNET_NO);
    LOG (GNUNET_ERROR_TYPE_DEBUG, "WARNING channel %u unknown\n",
         ntohl (msg->chid));
    return;
  }

  GCCH_handle_ack (ch, msg, fwd);
}


/**
 * Handle a channel destruction message.
 *
 * @param t Tunnel on which the message came.
 * @param msg Channel destroy message.
 * @param fwd Is this message fwd? This only is meaningful in loopback channels.
 *            #GNUNET_YES if message is FWD on the respective channel (loopback)
 *            #GNUNET_NO if message is BCK on the respective channel (loopback)
 *            #GNUNET_SYSERR if message on a one-ended channel (remote)
 */
static void
handle_ch_destroy (struct CadetTunnel *t,
                   const struct GNUNET_CADET_ChannelManage *msg,
                   int fwd)
{
  struct CadetChannel *ch;
  size_t size;

  /* Check size */
  size = ntohs (msg->header.size);
  if (size != sizeof (struct GNUNET_CADET_ChannelManage))
  {
    GNUNET_break (0);
    return;
  }

  /* Check channel */
  ch = GCT_get_channel (t, ntohl (msg->chid));
  if (NULL == ch)
  {
    /* Probably a retransmission, safe to ignore */
    return;
  }

  GCCH_handle_destroy (ch, msg, fwd);
}


/**
 * Free Axolotl data.
 *
 * @param t Tunnel.
 */
static void
destroy_ax (struct CadetTunnel *t)
{
  if (NULL == t->ax)
    return;

  GNUNET_free_non_null (t->ax->DHRs);
  GNUNET_free_non_null (t->ax->kx_0);
  while (NULL != t->ax->skipped_head)
    delete_skipped_key (t, t->ax->skipped_head);
  GNUNET_assert (0 == t->ax->skipped);

  GNUNET_free (t->ax);
  t->ax = NULL;

  if (NULL != t->rekey_task)
  {
    GNUNET_SCHEDULER_cancel (t->rekey_task);
    t->rekey_task = NULL;
  }
  if (NULL != t->ephm_h)
  {
    GCC_cancel (t->ephm_h);
    t->ephm_h = NULL;
  }
}


/**
 * Handle Axolotl handshake.
 *
 * @param t Tunnel this message came on.
 * @param msg Key eXchange Pong message.
 */
static void
handle_kx_ax (struct CadetTunnel *t, const struct GNUNET_CADET_AX_KX *msg)
{
  struct CadetTunnelAxolotl *ax;
  struct GNUNET_HashCode key_material[3];
  struct GNUNET_CRYPTO_SymmetricSessionKey keys[5];
  const char salt[] = "CADET Axolotl salt";
  const struct GNUNET_PeerIdentity *pid;
  int am_I_alice;

  LOG (GNUNET_ERROR_TYPE_INFO, "<== {     AX_KX} on %s\n", GCT_2s (t));

  if (NULL == t->ax)
  {
    /* Something is wrong if ax is NULL. Whose fault it is? */
    GNUNET_break (CADET_Axolotl == t->enc_type);
    return;
  }
  ax = t->ax;

  pid = GCT_get_destination (t);
  if (0 > GNUNET_CRYPTO_cmp_peer_identity (&my_full_id, pid))
    am_I_alice = GNUNET_YES;
  else if (0 < GNUNET_CRYPTO_cmp_peer_identity (&my_full_id, pid))
    am_I_alice = GNUNET_NO;
  else
  {
    GNUNET_break_op (0);
    return;
  }

  if (0 != (GNUNET_CADET_AX_KX_FLAG_FORCE_REPLY & ntohl (msg->flags)))
  {
    if (NULL != t->rekey_task)
    {
      GNUNET_SCHEDULER_cancel (t->rekey_task);
      t->rekey_task = NULL;
    }
    GCT_send_ax_kx (t, GNUNET_NO);
  }

  if (0 == memcmp (&ax->DHRr, &msg->ratchet_key, sizeof(msg->ratchet_key)))
  {
    LOG (GNUNET_ERROR_TYPE_INFO, " known ratchet key, exit\n");
    return;
  }

  LOG (GNUNET_ERROR_TYPE_INFO, " is Alice? %s\n", am_I_alice ? "YES" : "NO");

  ax->DHRr = msg->ratchet_key;

  /* ECDH A B0 */
  if (GNUNET_YES == am_I_alice)
  {
    GNUNET_CRYPTO_eddsa_ecdh (id_key,              /* A */
                              &msg->ephemeral_key, /* B0 */
                              &key_material[0]);
  }
  else
  {
    GNUNET_CRYPTO_ecdh_eddsa (ax->kx_0,            /* B0 */
                              &pid->public_key,    /* A */
                              &key_material[0]);
  }

  /* ECDH A0 B */
  if (GNUNET_YES == am_I_alice)
  {
    GNUNET_CRYPTO_ecdh_eddsa (ax->kx_0,            /* A0 */
                              &pid->public_key,    /* B */
                              &key_material[1]);
  }
  else
  {
    GNUNET_CRYPTO_eddsa_ecdh (id_key,              /* A */
                              &msg->ephemeral_key, /* B0 */
                              &key_material[1]);


  }

  /* ECDH A0 B0 */
  /* (This is the triple-DH, we could probably safely skip this,
     as A0/B0 are already in the key material.) */
  GNUNET_CRYPTO_ecc_ecdh (ax->kx_0,             /* A0 or B0 */
                          &msg->ephemeral_key,  /* B0 or A0 */
                          &key_material[2]);

  #if DUMP_KEYS_TO_STDERR
  {
    unsigned int i;
    for (i = 0; i < 3; i++)
      LOG (GNUNET_ERROR_TYPE_INFO, "km[%u]: %s\n",
           i, GNUNET_h2s (&key_material[i]));
  }
  #endif

  /* KDF */
  GNUNET_CRYPTO_kdf (keys, sizeof (keys),
                     salt, sizeof (salt),
                     &key_material, sizeof (key_material), NULL);

  if (0 == memcmp (&ax->RK, &keys[0], sizeof(ax->RK)))
  {
    LOG (GNUNET_ERROR_TYPE_INFO, " known handshake key, exit\n");
    return;
  }
  ax->RK = keys[0];
  if (GNUNET_YES == am_I_alice)
  {
    ax->HKr = keys[1];
    ax->NHKs = keys[2];
    ax->NHKr = keys[3];
    ax->CKr = keys[4];
    ax->ratchet_flag = GNUNET_YES;
  }
  else
  {
    ax->HKs = keys[1];
    ax->NHKr = keys[2];
    ax->NHKs = keys[3];
    ax->CKs = keys[4];
    ax->ratchet_flag = GNUNET_NO;
    ax->ratchet_allowed = GNUNET_NO;
    ax->ratchet_counter = 0;
    ax->ratchet_expiration =
      GNUNET_TIME_absolute_add (GNUNET_TIME_absolute_get(), ratchet_time);
  }
  ax->PNs = 0;
  ax->Nr = 0;
  ax->Ns = 0;
  GCT_change_estate (t, CADET_TUNNEL_KEY_PING);
  send_queued_data (t);
}


/**
 * Demultiplex by message type and call appropriate handler for a message
 * towards a channel of a local tunnel.
 *
 * @param t Tunnel this message came on.
 * @param msgh Message header.
 * @param fwd Is this message fwd? This only is meaningful in loopback channels.
 *            #GNUNET_YES if message is FWD on the respective channel (loopback)
 *            #GNUNET_NO if message is BCK on the respective channel (loopback)
 *            #GNUNET_SYSERR if message on a one-ended channel (remote)
 */
static void
handle_decrypted (struct CadetTunnel *t,
                  const struct GNUNET_MessageHeader *msgh,
                  int fwd)
{
  uint16_t type;
  char buf[256];

  type = ntohs (msgh->type);
  LOG (GNUNET_ERROR_TYPE_DEBUG, "<-- %s on %s\n", GC_m2s (type), GCT_2s (t));
  SPRINTF (buf, "# received encrypted of type %hu (%s)", type, GC_m2s (type));
  GNUNET_STATISTICS_update (stats, buf, 1, GNUNET_NO);

  switch (type)
  {
    case GNUNET_MESSAGE_TYPE_CADET_KEEPALIVE:
      /* Do nothing, connection aleady got updated. */
      GNUNET_STATISTICS_update (stats, "# keepalives received", 1, GNUNET_NO);
      break;

    case GNUNET_MESSAGE_TYPE_CADET_DATA:
      /* Don't send hop ACK, wait for client to ACK */
      handle_data (t, (struct GNUNET_CADET_Data *) msgh, fwd);
      break;

    case GNUNET_MESSAGE_TYPE_CADET_DATA_ACK:
      handle_data_ack (t, (struct GNUNET_CADET_DataACK *) msgh, fwd);
      break;

    case GNUNET_MESSAGE_TYPE_CADET_CHANNEL_CREATE:
      handle_ch_create (t, (struct GNUNET_CADET_ChannelCreate *) msgh);
      break;

    case GNUNET_MESSAGE_TYPE_CADET_CHANNEL_NACK:
      handle_ch_nack (t, (struct GNUNET_CADET_ChannelManage *) msgh);
      break;

    case GNUNET_MESSAGE_TYPE_CADET_CHANNEL_ACK:
      handle_ch_ack (t, (struct GNUNET_CADET_ChannelManage *) msgh, fwd);
      break;

    case GNUNET_MESSAGE_TYPE_CADET_CHANNEL_DESTROY:
      handle_ch_destroy (t, (struct GNUNET_CADET_ChannelManage *) msgh, fwd);
      break;

    default:
      GNUNET_break_op (0);
      LOG (GNUNET_ERROR_TYPE_WARNING,
           "end-to-end message not known (%u)\n",
           ntohs (msgh->type));
      GCT_debug (t, GNUNET_ERROR_TYPE_WARNING);
  }
}


/******************************************************************************/
/********************************    API    ***********************************/
/******************************************************************************/
/**
 * Decrypt old format and demultiplex by message type. Call appropriate handler
 * for a message towards a channel of a local tunnel.
 *
 * @param t Tunnel this message came on.
 * @param msg Message header.
 */
void
GCT_handle_encrypted (struct CadetTunnel *t,
                      const struct GNUNET_MessageHeader *msg)
{
  uint16_t size = ntohs (msg->size);
  char cbuf [size];
  int decrypted_size;
  uint16_t type;
  const struct GNUNET_MessageHeader *msgh;
  unsigned int off;

  type = ntohs (msg->type);
  switch (type)
  {
  case GNUNET_MESSAGE_TYPE_CADET_AX:
    {
      const struct GNUNET_CADET_AX *emsg;

      GNUNET_STATISTICS_update (stats, "# received Axolotl", 1, GNUNET_NO);
      emsg = (const struct GNUNET_CADET_AX *) msg;
      decrypted_size = t_ax_decrypt_and_validate (t, cbuf, emsg, size);
    }
    break;
  default:
    GNUNET_break_op (0);
    return;
  }

  if (-1 == decrypted_size)
  {
    GNUNET_break_op (0);
    GNUNET_STATISTICS_update (stats, "# unable to decrypt", 1, GNUNET_NO);
    LOG (GNUNET_ERROR_TYPE_WARNING, "Wrong crypto on tunnel %s\n", GCT_2s (t));
    GCT_debug (t, GNUNET_ERROR_TYPE_WARNING);
    return;
  }
  GCT_change_estate (t, CADET_TUNNEL_KEY_OK);

  /* FIXME: this is bad, as the structs returned from
     this loop may be unaligned, see util's MST for
     how to do this right. */
  off = 0;
  while (off + sizeof (struct GNUNET_MessageHeader) <= decrypted_size)
  {
    uint16_t msize;

    msgh = (const struct GNUNET_MessageHeader *) &cbuf[off];
    msize = ntohs (msgh->size);
    if (msize < sizeof (struct GNUNET_MessageHeader))
    {
      GNUNET_break_op (0);
      return;
    }
    if (off + msize < decrypted_size)
    {
      GNUNET_break_op (0);
      return;
    }
    handle_decrypted (t, msgh, GNUNET_SYSERR);
    off += msize;
  }
}


/**
 * Demultiplex an encapsulated KX message by message type.
 *
 * @param t Tunnel on which the message came.
 * @param message Payload of KX message.
 *
 * FIXME: not needed anymore
 *  - substitute with call to kx_ax
 *  - eliminate encapsulation
 */
void
GCT_handle_kx (struct CadetTunnel *t,
               const struct GNUNET_MessageHeader *message)
{
  uint16_t type;
  char buf[256];

  type = ntohs (message->type);
  LOG (GNUNET_ERROR_TYPE_DEBUG, "kx message received: %s\n", GC_m2s (type));
  sprintf (buf, "# received KX of type %hu (%s)", type, GC_m2s (type));
  GNUNET_STATISTICS_update (stats, buf, 1, GNUNET_NO);
  switch (type)
  {
    case GNUNET_MESSAGE_TYPE_CADET_AX_KX:
      handle_kx_ax (t, (const struct GNUNET_CADET_AX_KX *) message);
      break;
    default:
      GNUNET_break_op (0);
      LOG (GNUNET_ERROR_TYPE_WARNING, "kx message %s unknown\n", GC_m2s (type));
  }
}

/**
 * Initialize the tunnel subsystem.
 *
 * @param c Configuration handle.
 * @param key ECC private key, to derive all other keys and do crypto.
 */
void
GCT_init (const struct GNUNET_CONFIGURATION_Handle *c,
          const struct GNUNET_CRYPTO_EddsaPrivateKey *key)
{
  unsigned int expected_overhead;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "init\n");

  expected_overhead = 0;
  expected_overhead += sizeof (struct GNUNET_CADET_AX);
  expected_overhead += sizeof (struct GNUNET_CADET_Data);
  expected_overhead += sizeof (struct GNUNET_CADET_ACK);
  GNUNET_assert (GNUNET_CONSTANTS_CADET_P2P_OVERHEAD == expected_overhead);

  if (GNUNET_OK !=
      GNUNET_CONFIGURATION_get_value_number (c,
                                             "CADET",
                                             "RATCHET_MESSAGES",
                                             &ratchet_messages))
  {
    GNUNET_log_config_invalid (GNUNET_ERROR_TYPE_WARNING,
                               "CADET",
                               "RATCHET_MESSAGES",
                               "USING DEFAULT");
    ratchet_messages = 64;
  }
  if (GNUNET_OK !=
      GNUNET_CONFIGURATION_get_value_time (c,
                                           "CADET",
                                           "RATCHET_TIME",
                                           &ratchet_time))
  {
    GNUNET_log_config_invalid (GNUNET_ERROR_TYPE_WARNING,
                               "CADET", "RATCHET_TIME", "USING DEFAULT");
    ratchet_time = GNUNET_TIME_UNIT_HOURS;
  }


  id_key = key;
  tunnels = GNUNET_CONTAINER_multipeermap_create (128, GNUNET_YES);
}


/**
 * Shut down the tunnel subsystem.
 */
void
GCT_shutdown (void)
{
  LOG (GNUNET_ERROR_TYPE_DEBUG, "Shutting down tunnels\n");
  GNUNET_CONTAINER_multipeermap_iterate (tunnels, &destroy_iterator, NULL);
  GNUNET_CONTAINER_multipeermap_destroy (tunnels);
}


/**
 * Create a tunnel.
 *
 * @param destination Peer this tunnel is towards.
 */
struct CadetTunnel *
GCT_new (struct CadetPeer *destination)
{
  struct CadetTunnel *t;

  t = GNUNET_new (struct CadetTunnel);
  t->next_chid = 0;
  t->peer = destination;

  if (GNUNET_OK !=
      GNUNET_CONTAINER_multipeermap_put (tunnels, GCP_get_id (destination), t,
                                         GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_FAST))
  {
    GNUNET_break (0);
    GNUNET_free (t);
    return NULL;
  }
  t->ax = GNUNET_new (struct CadetTunnelAxolotl);
  new_ephemeral (t);
  t->ax->kx_0 = GNUNET_CRYPTO_ecdhe_key_create ();
  return t;
}


/**
 * Change the tunnel's connection state.
 *
 * @param t Tunnel whose connection state to change.
 * @param cstate New connection state.
 */
void
GCT_change_cstate (struct CadetTunnel* t, enum CadetTunnelCState cstate)
{
  if (NULL == t)
    return;
  LOG (GNUNET_ERROR_TYPE_DEBUG, "Tunnel %s cstate %s => %s\n",
       GCP_2s (t->peer), cstate2s (t->cstate), cstate2s (cstate));
  if (myid != GCP_get_short_id (t->peer) &&
      CADET_TUNNEL_READY != t->cstate &&
      CADET_TUNNEL_READY == cstate)
  {
    t->cstate = cstate;
    if (CADET_TUNNEL_KEY_OK == t->estate)
    {
      LOG (GNUNET_ERROR_TYPE_DEBUG, "  cstate triggered send queued data\n");
      send_queued_data (t);
    }
    else if (CADET_TUNNEL_KEY_UNINITIALIZED == t->estate)
    {
      LOG (GNUNET_ERROR_TYPE_DEBUG, "  cstate triggered kx\n");
      GCT_send_ax_kx (t, GNUNET_NO);
    }
    else
    {
      LOG (GNUNET_ERROR_TYPE_DEBUG, "estate %s\n", estate2s (t->estate));
    }
  }
  t->cstate = cstate;

  if (CADET_TUNNEL_READY == cstate
      && CONNECTIONS_PER_TUNNEL <= GCT_count_connections (t))
  {
    LOG (GNUNET_ERROR_TYPE_DEBUG, "  cstate triggered stop dht\n");
    GCP_stop_search (t->peer);
  }
}


/**
 * Change the tunnel encryption state.
 *
 * If the encryption state changes to OK, stop the rekey task.
 *
 * @param t Tunnel whose encryption state to change, or NULL.
 * @param state New encryption state.
 */
void
GCT_change_estate (struct CadetTunnel* t, enum CadetTunnelEState state)
{
  enum CadetTunnelEState old;

  if (NULL == t)
    return;

  old = t->estate;
  t->estate = state;
  LOG (GNUNET_ERROR_TYPE_DEBUG, "Tunnel %s estate was %s\n",
       GCP_2s (t->peer), estate2s (old));
  LOG (GNUNET_ERROR_TYPE_DEBUG, "Tunnel %s estate is now %s\n",
       GCP_2s (t->peer), estate2s (t->estate));

  if (CADET_TUNNEL_KEY_OK != old && CADET_TUNNEL_KEY_OK == t->estate)
  {
    if (NULL != t->rekey_task)
    {
      GNUNET_SCHEDULER_cancel (t->rekey_task);
      t->rekey_task = NULL;
    }
    /* Send queued data if tunnel is not loopback */
    if (myid != GCP_get_short_id (t->peer))
      send_queued_data (t);
  }
}


/**
 * @brief Check if tunnel has too many connections, and remove one if necessary.
 *
 * Currently this means the newest connection, unless it is a direct one.
 * Implemented as a task to avoid freeing a connection that is in the middle
 * of being created/processed.
 *
 * @param cls Closure (Tunnel to check).
 */
static void
trim_connections (void *cls)
{
  struct CadetTunnel *t = cls;

  t->trim_connections_task = NULL;
  if (GCT_count_connections (t) > 2 * CONNECTIONS_PER_TUNNEL)
  {
    struct CadetTConnection *iter;
    struct CadetTConnection *c;

    for (c = iter = t->connection_head; NULL != iter; iter = iter->next)
    {
      if ((iter->created.abs_value_us > c->created.abs_value_us)
          && GNUNET_NO == GCC_is_direct (iter->c))
      {
        c = iter;
      }
    }
    if (NULL != c)
    {
      LOG (GNUNET_ERROR_TYPE_DEBUG, "Too many connections on tunnel %s\n",
           GCT_2s (t));
      LOG (GNUNET_ERROR_TYPE_DEBUG, "Destroying connection %s\n",
           GCC_2s (c->c));
      GCC_destroy (c->c);
    }
    else
    {
      GNUNET_break (0);
    }
  }
}


/**
 * Add a connection to a tunnel.
 *
 * @param t Tunnel.
 * @param c Connection.
 */
void
GCT_add_connection (struct CadetTunnel *t, struct CadetConnection *c)
{
  struct CadetTConnection *aux;

  GNUNET_assert (NULL != c);

  LOG (GNUNET_ERROR_TYPE_DEBUG, "add connection %s\n", GCC_2s (c));
  LOG (GNUNET_ERROR_TYPE_DEBUG, " to tunnel %s\n", GCT_2s (t));
  for (aux = t->connection_head; aux != NULL; aux = aux->next)
    if (aux->c == c)
      return;

  aux = GNUNET_new (struct CadetTConnection);
  aux->c = c;
  aux->created = GNUNET_TIME_absolute_get ();

  GNUNET_CONTAINER_DLL_insert (t->connection_head, t->connection_tail, aux);

  if (CADET_TUNNEL_SEARCHING == t->cstate)
    GCT_change_cstate (t, CADET_TUNNEL_WAITING);

  if (NULL != t->trim_connections_task)
    t->trim_connections_task = GNUNET_SCHEDULER_add_now (&trim_connections, t);
}


/**
 * Remove a connection from a tunnel.
 *
 * @param t Tunnel.
 * @param c Connection.
 */
void
GCT_remove_connection (struct CadetTunnel *t,
                       struct CadetConnection *c)
{
  struct CadetTConnection *aux;
  struct CadetTConnection *next;
  unsigned int conns;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "Removing connection %s from tunnel %s\n",
       GCC_2s (c), GCT_2s (t));
  for (aux = t->connection_head; aux != NULL; aux = next)
  {
    next = aux->next;
    if (aux->c == c)
    {
      GNUNET_CONTAINER_DLL_remove (t->connection_head, t->connection_tail, aux);
      GNUNET_free (aux);
    }
  }

  conns = GCT_count_connections (t);
  if (0 == conns
      && NULL == t->destroy_task
      && CADET_TUNNEL_SHUTDOWN != t->cstate
      && GNUNET_NO == shutting_down)
  {
    if (0 == GCT_count_any_connections (t))
      GCT_change_cstate (t, CADET_TUNNEL_SEARCHING);
    else
      GCT_change_cstate (t, CADET_TUNNEL_WAITING);
  }

  /* Start new connections if needed */
  if (CONNECTIONS_PER_TUNNEL > conns
      && CADET_TUNNEL_SHUTDOWN != t->cstate
      && GNUNET_NO == shutting_down)
  {
    LOG (GNUNET_ERROR_TYPE_DEBUG, "  too few connections, getting new ones\n");
    GCP_connect (t->peer); /* Will change cstate to WAITING when possible */
    return;
  }

  /* If not marked as ready, no change is needed */
  if (CADET_TUNNEL_READY != t->cstate)
    return;

  /* Check if any connection is ready to maintain cstate */
  for (aux = t->connection_head; aux != NULL; aux = aux->next)
    if (CADET_CONNECTION_READY == GCC_get_state (aux->c))
      return;
}


/**
 * Add a channel to a tunnel.
 *
 * @param t Tunnel.
 * @param ch Channel.
 */
void
GCT_add_channel (struct CadetTunnel *t, struct CadetChannel *ch)
{
  struct CadetTChannel *aux;

  GNUNET_assert (NULL != ch);

  LOG (GNUNET_ERROR_TYPE_DEBUG, "Adding channel %p to tunnel %p\n", ch, t);

  for (aux = t->channel_head; aux != NULL; aux = aux->next)
  {
    LOG (GNUNET_ERROR_TYPE_DEBUG, "  already there %p\n", aux->ch);
    if (aux->ch == ch)
      return;
  }

  aux = GNUNET_new (struct CadetTChannel);
  aux->ch = ch;
  LOG (GNUNET_ERROR_TYPE_DEBUG,
       " adding %p to %p\n", aux, t->channel_head);
  GNUNET_CONTAINER_DLL_insert_tail (t->channel_head,
                                    t->channel_tail,
                                    aux);

  if (NULL != t->destroy_task)
  {
    GNUNET_SCHEDULER_cancel (t->destroy_task);
    t->destroy_task = NULL;
    LOG (GNUNET_ERROR_TYPE_DEBUG, " undo destroy!\n");
  }
}


/**
 * Remove a channel from a tunnel.
 *
 * @param t Tunnel.
 * @param ch Channel.
 */
void
GCT_remove_channel (struct CadetTunnel *t, struct CadetChannel *ch)
{
  struct CadetTChannel *aux;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "Removing channel %p from tunnel %p\n", ch, t);
  for (aux = t->channel_head; aux != NULL; aux = aux->next)
  {
    if (aux->ch == ch)
    {
      LOG (GNUNET_ERROR_TYPE_DEBUG, " found! %s\n", GCCH_2s (ch));
      GNUNET_CONTAINER_DLL_remove (t->channel_head,
                                   t->channel_tail,
                                   aux);
      GNUNET_free (aux);
      return;
    }
  }
}


/**
 * Search for a channel by global ID.
 *
 * @param t Tunnel containing the channel.
 * @param chid Public channel number.
 *
 * @return channel handler, NULL if doesn't exist
 */
struct CadetChannel *
GCT_get_channel (struct CadetTunnel *t, CADET_ChannelNumber chid)
{
  struct CadetTChannel *iter;

  if (NULL == t)
    return NULL;

  for (iter = t->channel_head; NULL != iter; iter = iter->next)
  {
    if (GCCH_get_id (iter->ch) == chid)
      break;
  }

  return NULL == iter ? NULL : iter->ch;
}


/**
 * @brief Destroy a tunnel and free all resources.
 *
 * Should only be called a while after the tunnel has been marked as destroyed,
 * in case there is a new channel added to the same peer shortly after marking
 * the tunnel. This way we avoid a new public key handshake.
 *
 * @param cls Closure (tunnel to destroy).
 */
static void
delayed_destroy (void *cls)
{
  struct CadetTunnel *t = cls;
  struct CadetTConnection *iter;

  t->destroy_task = NULL;
  LOG (GNUNET_ERROR_TYPE_DEBUG,
       "delayed destroying tunnel %p\n",
       t);
  t->cstate = CADET_TUNNEL_SHUTDOWN;
  for (iter = t->connection_head; NULL != iter; iter = iter->next)
  {
    GCC_send_destroy (iter->c);
  }
  GCT_destroy (t);
}


/**
 * Tunnel is empty: destroy it.
 *
 * Notifies all connections about the destruction.
 *
 * @param t Tunnel to destroy.
 */
void
GCT_destroy_empty (struct CadetTunnel *t)
{
  if (GNUNET_YES == shutting_down)
    return; /* Will be destroyed immediately anyway */

  if (NULL != t->destroy_task)
  {
    LOG (GNUNET_ERROR_TYPE_WARNING,
         "Tunnel %s is already scheduled for destruction. Tunnel debug dump:\n",
         GCT_2s (t));
    GCT_debug (t, GNUNET_ERROR_TYPE_WARNING);
    GNUNET_break (0);
    /* should never happen, tunnel can only become empty once, and the
     * task identifier should be NO_TASK (cleaned when the tunnel was created
     * or became un-empty)
     */
    return;
  }

  LOG (GNUNET_ERROR_TYPE_DEBUG, "Tunnel %s empty: scheduling destruction\n",
       GCT_2s (t));

  // FIXME make delay a config option
  t->destroy_task = GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_UNIT_MINUTES,
                                                  &delayed_destroy, t);
  LOG (GNUNET_ERROR_TYPE_DEBUG, "Scheduled destroy of %p as %p\n",
       t, t->destroy_task);
}


/**
 * Destroy tunnel if empty (no more channels).
 *
 * @param t Tunnel to destroy if empty.
 */
void
GCT_destroy_if_empty (struct CadetTunnel *t)
{
  LOG (GNUNET_ERROR_TYPE_DEBUG, "Tunnel %s destroy if empty\n", GCT_2s (t));
  if (0 < GCT_count_channels (t))
    return;

  GCT_destroy_empty (t);
}


/**
 * Destroy the tunnel.
 *
 * This function does not generate any warning traffic to clients or peers.
 *
 * Tasks:
 * Cancel messages belonging to this tunnel queued to neighbors.
 * Free any allocated resources linked to the tunnel.
 *
 * @param t The tunnel to destroy.
 */
void
GCT_destroy (struct CadetTunnel *t)
{
  struct CadetTConnection *iter_c;
  struct CadetTConnection *next_c;
  struct CadetTChannel *iter_ch;
  struct CadetTChannel *next_ch;
  unsigned int keepalives_queued;

  if (NULL == t)
    return;

  LOG (GNUNET_ERROR_TYPE_DEBUG,
       "destroying tunnel %s\n",
       GCP_2s (t->peer));
  GNUNET_break (GNUNET_YES ==
                GNUNET_CONTAINER_multipeermap_remove (tunnels,
                                                      GCP_get_id (t->peer), t));

  for (iter_c = t->connection_head; NULL != iter_c; iter_c = next_c)
  {
    next_c = iter_c->next;
    GCC_destroy (iter_c->c);
  }
  for (iter_ch = t->channel_head; NULL != iter_ch; iter_ch = next_ch)
  {
    next_ch = iter_ch->next;
    GCCH_destroy (iter_ch->ch);
    /* Should only happen on shutdown, but it's ok. */
  }
  keepalives_queued = 0;
  while (NULL != t->tq_head)
  {
    /* Should have been cleaned by destuction of channel. */
    struct GNUNET_MessageHeader *mh;
    uint16_t type;

    mh = (struct GNUNET_MessageHeader *) &t->tq_head[1];
    type = ntohs (mh->type);
    if (0 == keepalives_queued && GNUNET_MESSAGE_TYPE_CADET_KEEPALIVE == type)
    {
      keepalives_queued = 1;
      LOG (GNUNET_ERROR_TYPE_DEBUG,
           "one keepalive left behind on tunnel shutdown\n");
    }
    else if (GNUNET_MESSAGE_TYPE_CADET_CHANNEL_DESTROY == type)
    {
      LOG (GNUNET_ERROR_TYPE_WARNING,
           "tunnel destroyed before a CHANNEL_DESTROY was sent to peer\n");
    }
    else
    {
      GNUNET_break (0);
      LOG (GNUNET_ERROR_TYPE_ERROR,
           "message left behind on tunnel shutdown: %s\n",
           GC_m2s (type));
    }
    unqueue_data (t->tq_head);
  }


  if (NULL != t->destroy_task)
  {
    LOG (GNUNET_ERROR_TYPE_DEBUG,
         "cancelling dest: %p\n",
         t->destroy_task);
    GNUNET_SCHEDULER_cancel (t->destroy_task);
    t->destroy_task = NULL;
  }

  if (NULL != t->trim_connections_task)
  {
    LOG (GNUNET_ERROR_TYPE_DEBUG, "cancelling trim: %p\n",
         t->trim_connections_task);
    GNUNET_SCHEDULER_cancel (t->trim_connections_task);
    t->trim_connections_task = NULL;
  }

  GNUNET_STATISTICS_update (stats, "# tunnels", -1, GNUNET_NO);
  GCP_set_tunnel (t->peer, NULL);

  if (NULL != t->rekey_task)
  {
    GNUNET_SCHEDULER_cancel (t->rekey_task);
    t->rekey_task = NULL;
  }
  if (NULL != t->ax)
    destroy_ax (t);

  GNUNET_free (t);
}


/**
 * @brief Use the given path for the tunnel.
 * Update the next and prev hops (and RCs).
 * (Re)start the path refresh in case the tunnel is locally owned.
 *
 * @param t Tunnel to update.
 * @param p Path to use.
 *
 * @return Connection created.
 */
struct CadetConnection *
GCT_use_path (struct CadetTunnel *t, struct CadetPeerPath *path)
{
  struct CadetConnection *c;
  struct GNUNET_CADET_Hash cid;
  unsigned int own_pos;

  if (NULL == t || NULL == path)
  {
    GNUNET_break (0);
    return NULL;
  }

  if (CADET_TUNNEL_SHUTDOWN == t->cstate)
  {
    GNUNET_break (0);
    return NULL;
  }

  for (own_pos = 0; own_pos < path->length; own_pos++)
  {
    if (path->peers[own_pos] == myid)
      break;
  }
  if (own_pos >= path->length)
  {
    GNUNET_break_op (0);
    return NULL;
  }

  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE, &cid, sizeof (cid));
  c = GCC_new (&cid, t, path, own_pos);
  if (NULL == c)
  {
    /* Path was flawed */
    return NULL;
  }
  GCT_add_connection (t, c);
  return c;
}


/**
 * Count all created connections of a tunnel. Not necessarily ready connections!
 *
 * @param t Tunnel on which to count.
 *
 * @return Number of connections created, either being established or ready.
 */
unsigned int
GCT_count_any_connections (struct CadetTunnel *t)
{
  struct CadetTConnection *iter;
  unsigned int count;

  if (NULL == t)
    return 0;

  for (count = 0, iter = t->connection_head; NULL != iter; iter = iter->next)
    count++;

  return count;
}


/**
 * Count established (ready) connections of a tunnel.
 *
 * @param t Tunnel on which to count.
 *
 * @return Number of connections.
 */
unsigned int
GCT_count_connections (struct CadetTunnel *t)
{
  struct CadetTConnection *iter;
  unsigned int count;

  if (NULL == t)
    return 0;

  for (count = 0, iter = t->connection_head; NULL != iter; iter = iter->next)
    if (CADET_CONNECTION_READY == GCC_get_state (iter->c))
      count++;

  return count;
}


/**
 * Count channels of a tunnel.
 *
 * @param t Tunnel on which to count.
 *
 * @return Number of channels.
 */
unsigned int
GCT_count_channels (struct CadetTunnel *t)
{
  struct CadetTChannel *iter;
  unsigned int count;

  for (count = 0, iter = t->channel_head;
       NULL != iter;
       iter = iter->next, count++) /* skip */;

  return count;
}


/**
 * Get the connectivity state of a tunnel.
 *
 * @param t Tunnel.
 *
 * @return Tunnel's connectivity state.
 */
enum CadetTunnelCState
GCT_get_cstate (struct CadetTunnel *t)
{
  if (NULL == t)
  {
    GNUNET_assert (0);
    return (enum CadetTunnelCState) -1;
  }
  return t->cstate;
}


/**
 * Get the encryption state of a tunnel.
 *
 * @param t Tunnel.
 *
 * @return Tunnel's encryption state.
 */
enum CadetTunnelEState
GCT_get_estate (struct CadetTunnel *t)
{
  if (NULL == t)
  {
    GNUNET_break (0);
    return (enum CadetTunnelEState) -1;
  }
  return t->estate;
}

/**
 * Get the maximum buffer space for a tunnel towards a local client.
 *
 * @param t Tunnel.
 *
 * @return Biggest buffer space offered by any channel in the tunnel.
 */
unsigned int
GCT_get_channels_buffer (struct CadetTunnel *t)
{
  struct CadetTChannel *iter;
  unsigned int buffer;
  unsigned int ch_buf;

  if (NULL == t->channel_head)
  {
    /* Probably getting buffer for a channel create/handshake. */
    LOG (GNUNET_ERROR_TYPE_DEBUG, "  no channels, allow max\n");
    return MIN_TUNNEL_BUFFER;
  }

  buffer = 0;
  for (iter = t->channel_head; NULL != iter; iter = iter->next)
  {
    ch_buf = get_channel_buffer (iter);
    if (ch_buf > buffer)
      buffer = ch_buf;
  }
  if (MIN_TUNNEL_BUFFER > buffer)
    return MIN_TUNNEL_BUFFER;

  if (MAX_TUNNEL_BUFFER < buffer)
  {
    GNUNET_break (0);
    return MAX_TUNNEL_BUFFER;
  }
  return buffer;
}


/**
 * Get the total buffer space for a tunnel for P2P traffic.
 *
 * @param t Tunnel.
 *
 * @return Buffer space offered by all connections in the tunnel.
 */
unsigned int
GCT_get_connections_buffer (struct CadetTunnel *t)
{
  struct CadetTConnection *iter;
  unsigned int buffer;

  if (GNUNET_NO == is_ready (t))
  {
    if (count_queued_data (t) >= 3)
      return 0;
    else
      return 1;
  }

  buffer = 0;
  for (iter = t->connection_head; NULL != iter; iter = iter->next)
  {
    if (GCC_get_state (iter->c) != CADET_CONNECTION_READY)
    {
      continue;
    }
    buffer += get_connection_buffer (iter);
  }

  return buffer;
}


/**
 * Get the tunnel's destination.
 *
 * @param t Tunnel.
 *
 * @return ID of the destination peer.
 */
const struct GNUNET_PeerIdentity *
GCT_get_destination (struct CadetTunnel *t)
{
  return GCP_get_id (t->peer);
}


/**
 * Get the tunnel's next free global channel ID.
 *
 * @param t Tunnel.
 *
 * @return GID of a channel free to use.
 */
CADET_ChannelNumber
GCT_get_next_chid (struct CadetTunnel *t)
{
  CADET_ChannelNumber chid;
  CADET_ChannelNumber mask;
  int result;

  /* Set bit 30 depending on the ID relationship. Bit 31 is always 0 for GID.
   * If our ID is bigger or loopback tunnel, start at 0, bit 30 = 0
   * If peer's ID is bigger, start at 0x4... bit 30 = 1
   */
  result = GNUNET_CRYPTO_cmp_peer_identity (&my_full_id, GCP_get_id (t->peer));
  if (0 > result)
    mask = 0x40000000;
  else
    mask = 0x0;
  t->next_chid |= mask;

  while (NULL != GCT_get_channel (t, t->next_chid))
  {
    LOG (GNUNET_ERROR_TYPE_DEBUG, "Channel %u exists...\n", t->next_chid);
    t->next_chid = (t->next_chid + 1) & ~GNUNET_CADET_LOCAL_CHANNEL_ID_CLI;
    t->next_chid |= mask;
  }
  chid = t->next_chid;
  t->next_chid = (t->next_chid + 1) & ~GNUNET_CADET_LOCAL_CHANNEL_ID_CLI;
  t->next_chid |= mask;

  return chid;
}


/**
 * Send ACK on one or more channels due to buffer in connections.
 *
 * @param t Channel which has some free buffer space.
 */
void
GCT_unchoke_channels (struct CadetTunnel *t)
{
  struct CadetTChannel *iter;
  unsigned int buffer;
  unsigned int channels = GCT_count_channels (t);
  unsigned int choked_n;
  struct CadetChannel *choked[channels];

  LOG (GNUNET_ERROR_TYPE_DEBUG, "GCT_unchoke_channels on %s\n", GCT_2s (t));
  LOG (GNUNET_ERROR_TYPE_DEBUG, " head: %p\n", t->channel_head);
  if (NULL != t->channel_head)
    LOG (GNUNET_ERROR_TYPE_DEBUG, " head ch: %p\n", t->channel_head->ch);

  if (NULL != t->tq_head)
    send_queued_data (t);

  /* Get buffer space */
  buffer = GCT_get_connections_buffer (t);
  if (0 == buffer)
  {
    return;
  }

  /* Count and remember choked channels */
  choked_n = 0;
  for (iter = t->channel_head; NULL != iter; iter = iter->next)
  {
    if (GNUNET_NO == get_channel_allowed (iter))
    {
      choked[choked_n++] = iter->ch;
    }
  }

  /* Unchoke random channels */
  while (0 < buffer && 0 < choked_n)
  {
    unsigned int r = GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK,
                                               choked_n);
    GCCH_allow_client (choked[r], GCCH_is_origin (choked[r], GNUNET_YES));
    choked_n--;
    buffer--;
    choked[r] = choked[choked_n];
  }
}


/**
 * Send ACK on one or more connections due to buffer space to the client.
 *
 * Iterates all connections of the tunnel and sends ACKs appropriately.
 *
 * @param t Tunnel.
 */
void
GCT_send_connection_acks (struct CadetTunnel *t)
{
  struct CadetTConnection *iter;
  uint32_t allowed;
  uint32_t to_allow;
  uint32_t allow_per_connection;
  unsigned int cs;
  unsigned int buffer;

  LOG (GNUNET_ERROR_TYPE_DEBUG, "Tunnel send connection ACKs on %s\n",
       GCT_2s (t));

  if (NULL == t)
  {
    GNUNET_break (0);
    return;
  }

  if (CADET_TUNNEL_READY != t->cstate)
    return;

  buffer = GCT_get_channels_buffer (t);
  LOG (GNUNET_ERROR_TYPE_DEBUG, "  buffer %u\n", buffer);

  /* Count connections, how many messages are already allowed */
  cs = GCT_count_connections (t);
  for (allowed = 0, iter = t->connection_head; NULL != iter; iter = iter->next)
  {
    allowed += get_connection_allowed (iter);
  }
  LOG (GNUNET_ERROR_TYPE_DEBUG, "  allowed %u\n", allowed);

  /* Make sure there is no overflow */
  if (allowed > buffer)
    return;

  /* Authorize connections to send more data */
  to_allow = buffer - allowed;

  for (iter = t->connection_head;
       NULL != iter && to_allow > 0;
       iter = iter->next)
  {
    if (CADET_CONNECTION_READY != GCC_get_state (iter->c)
        || get_connection_allowed (iter) > 64 / 3)
    {
      continue;
    }
    GNUNET_assert(cs != 0);
    allow_per_connection = to_allow/cs;
    to_allow -= allow_per_connection;
    cs--;
    GCC_allow (iter->c, allow_per_connection,
               GCC_is_origin (iter->c, GNUNET_NO));
  }

  if (0 != to_allow)
  {
    /* Since we don't allow if it's allowed to send 64/3, this can happen. */
    LOG (GNUNET_ERROR_TYPE_DEBUG, "  reminding to_allow: %u\n", to_allow);
  }
}


/**
 * Cancel a previously sent message while it's in the queue.
 *
 * ONLY can be called before the continuation given to the send function
 * is called. Once the continuation is called, the message is no longer in the
 * queue.
 *
 * @param q Handle to the queue.
 */
void
GCT_cancel (struct CadetTunnelQueue *q)
{
  if (NULL != q->cq)
  {
    GNUNET_assert (NULL == q->tqd);
    GCC_cancel (q->cq);
    /* tun_message_sent() will be called and free q */
  }
  else if (NULL != q->tqd)
  {
    unqueue_data (q->tqd);
    q->tqd = NULL;
    if (NULL != q->cont)
      q->cont (q->cont_cls, NULL, q, 0, 0);
    GNUNET_free (q);
  }
  else
  {
    GNUNET_break (0);
  }
}


/**
 * Check if the tunnel has queued traffic.
 *
 * @param t Tunnel to check.
 *
 * @return #GNUNET_YES if there is queued traffic
 *         #GNUNET_NO otherwise
 */
int
GCT_has_queued_traffic (struct CadetTunnel *t)
{
  return (NULL != t->tq_head) ? GNUNET_YES : GNUNET_NO;
}


/**
 * Sends an already built message on a tunnel, encrypting it and
 * choosing the best connection if not provided.
 *
 * @param message Message to send. Function modifies it.
 * @param t Tunnel on which this message is transmitted.
 * @param c Connection to use (autoselect if NULL).
 * @param force Force the tunnel to take the message (buffer overfill).
 * @param cont Continuation to call once message is really sent.
 * @param cont_cls Closure for @c cont.
 *
 * @return Handle to cancel message. NULL if @c cont is NULL.
 */
struct CadetTunnelQueue *
GCT_send_prebuilt_message (const struct GNUNET_MessageHeader *message,
                           struct CadetTunnel *t, struct CadetConnection *c,
                           int force, GCT_sent cont, void *cont_cls)
{
  return send_prebuilt_message (message, t, c, force, cont, cont_cls, NULL);
}


/**
 * Send an Axolotl KX message.
 *
 * @param t Tunnel on which to send it.
 * @param force_reply Force the other peer to reply with a KX message.
 */
void
GCT_send_ax_kx (struct CadetTunnel *t, int force_reply)
{
  struct GNUNET_CADET_AX_KX msg;
  enum GNUNET_CADET_AX_KX_Flags flags;

  LOG (GNUNET_ERROR_TYPE_INFO, "==> {     AX_KX} on %s\n", GCT_2s (t));
  if (NULL != t->ephm_h)
  {
    LOG (GNUNET_ERROR_TYPE_INFO, "     already queued\n");
    return;
  }

  msg.header.size = htons (sizeof (msg));
  msg.header.type = htons (GNUNET_MESSAGE_TYPE_CADET_AX_KX);
  flags = GNUNET_CADET_AX_KX_FLAG_NONE;
  if (force_reply)
    flags |= GNUNET_CADET_AX_KX_FLAG_FORCE_REPLY;
  msg.flags = htonl (flags);
  GNUNET_CRYPTO_ecdhe_key_get_public (t->ax->kx_0, &msg.ephemeral_key);
  GNUNET_CRYPTO_ecdhe_key_get_public (t->ax->DHRs, &msg.ratchet_key);

  t->ephm_h = send_kx (t, &msg.header);
  if (CADET_TUNNEL_KEY_UNINITIALIZED == t->estate)
    GCT_change_estate (t, CADET_TUNNEL_KEY_SENT);
}


/**
 * Is the tunnel directed towards the local peer?
 *
 * @param t Tunnel.
 *
 * @return #GNUNET_YES if it is loopback.
 */
int
GCT_is_loopback (const struct CadetTunnel *t)
{
  return (myid == GCP_get_short_id (t->peer));
}


/**
 * Is the tunnel this path already?
 *
 * @param t Tunnel.
 * @param p Path.
 *
 * @return #GNUNET_YES a connection uses this path.
 */
int
GCT_is_path_used (const struct CadetTunnel *t, const struct CadetPeerPath *p)
{
  struct CadetTConnection *iter;

  for (iter = t->connection_head; NULL != iter; iter = iter->next)
    if (path_equivalent (GCC_get_path (iter->c), p))
      return GNUNET_YES;

  return GNUNET_NO;
}


/**
 * Get a cost of a path for a tunnel considering existing connections.
 *
 * @param t Tunnel.
 * @param path Candidate path.
 *
 * @return Cost of the path (path length + number of overlapping nodes)
 */
unsigned int
GCT_get_path_cost (const struct CadetTunnel *t,
                   const struct CadetPeerPath *path)
{
  struct CadetTConnection *iter;
  const struct CadetPeerPath *aux;
  unsigned int overlap;
  unsigned int i;
  unsigned int j;

  if (NULL == path)
    return 0;

  overlap = 0;
  GNUNET_assert (NULL != t);

  for (i = 0; i < path->length; i++)
  {
    for (iter = t->connection_head; NULL != iter; iter = iter->next)
    {
      aux = GCC_get_path (iter->c);
      if (NULL == aux)
        continue;

      for (j = 0; j < aux->length; j++)
      {
        if (path->peers[i] == aux->peers[j])
        {
          overlap++;
          break;
        }
      }
    }
  }
  return path->length + overlap;
}


/**
 * Get the static string for the peer this tunnel is directed.
 *
 * @param t Tunnel.
 *
 * @return Static string the destination peer's ID.
 */
const char *
GCT_2s (const struct CadetTunnel *t)
{
  if (NULL == t)
    return "(NULL)";

  return GCP_2s (t->peer);
}


/******************************************************************************/
/*****************************    INFO/DEBUG    *******************************/
/******************************************************************************/

static void
ax_debug (const struct CadetTunnelAxolotl *ax, enum GNUNET_ErrorType level)
{
  struct GNUNET_CRYPTO_EcdhePublicKey pub;
  struct CadetTunnelSkippedKey *iter;

  LOG2 (level, "TTT  RK  \t %s\n",
        GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->RK));

  LOG2 (level, "TTT  HKs \t %s\n",
        GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->HKs));
  LOG2 (level, "TTT  HKr \t %s\n",
        GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->HKr));
  LOG2 (level, "TTT  NHKs\t %s\n",
        GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->NHKs));
  LOG2 (level, "TTT  NHKr\t %s\n",
        GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->NHKr));

  LOG2 (level, "TTT  CKs \t %s\n",
        GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->CKs));
  LOG2 (level, "TTT  CKr \t %s\n",
        GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->CKr));

  GNUNET_CRYPTO_ecdhe_key_get_public (ax->DHRs, &pub);
  LOG2 (level, "TTT  DHRs\t %s\n",
        GNUNET_i2s ((struct GNUNET_PeerIdentity *) &pub));
  LOG2 (level, "TTT  DHRr\t %s\n",
        GNUNET_i2s ((struct GNUNET_PeerIdentity *) &ax->DHRr));

  LOG2 (level, "TTT  Nr\t %u\tNs\t%u\n", ax->Nr, ax->Ns);
  LOG2 (level, "TTT  PNs\t %u\tSkipped\t%u\n", ax->PNs, ax->skipped);
  LOG2 (level, "TTT  Ratchet\t%u\n", ax->ratchet_flag);

  for (iter = ax->skipped_head; NULL != iter; iter = iter->next)
  {
    LOG2 (level, "TTT    HK\t %s\n",
          GNUNET_i2s ((struct GNUNET_PeerIdentity *) &iter->HK));
    LOG2 (level, "TTT    MK\t %s\n",
          GNUNET_i2s ((struct GNUNET_PeerIdentity *) &iter->MK));
  }
}

/**
 * Log all possible info about the tunnel state.
 *
 * @param t Tunnel to debug.
 * @param level Debug level to use.
 */
void
GCT_debug (const struct CadetTunnel *t, enum GNUNET_ErrorType level)
{
  struct CadetTChannel *iterch;
  struct CadetTConnection *iterc;
  int do_log;

  do_log = GNUNET_get_log_call_status (level & (~GNUNET_ERROR_TYPE_BULK),
                                       "cadet-tun",
                                       __FILE__, __FUNCTION__, __LINE__);
  if (0 == do_log)
    return;

  LOG2 (level, "TTT DEBUG TUNNEL TOWARDS %s\n", GCT_2s (t));
  LOG2 (level, "TTT  cstate %s, estate %s\n",
       cstate2s (t->cstate), estate2s (t->estate));
#if DUMP_KEYS_TO_STDERR
  if (CADET_Axolotl == t->enc_type)
  {
    ax_debug (t->ax, level);
  }
  else
  {
    LOG2 (level, "TTT  peers EPHM:\t %s\n",
          GNUNET_i2s ((struct GNUNET_PeerIdentity *) &t->peers_ephemeral_key));
    LOG2 (level, "TTT  ENC key:\t %s\n",
          GNUNET_i2s ((struct GNUNET_PeerIdentity *) &t->e_key));
    LOG2 (level, "TTT  DEC key:\t %s\n",
          GNUNET_i2s ((struct GNUNET_PeerIdentity *) &t->d_key));
  }
#endif
  LOG2 (level, "TTT  tq_head %p, tq_tail %p\n", t->tq_head, t->tq_tail);
  LOG2 (level, "TTT  destroy %p\n", t->destroy_task);

  LOG2 (level, "TTT  channels:\n");
  for (iterch = t->channel_head; NULL != iterch; iterch = iterch->next)
  {
    GCCH_debug (iterch->ch, level);
  }

  LOG2 (level, "TTT  connections:\n");
  for (iterc = t->connection_head; NULL != iterc; iterc = iterc->next)
  {
    GCC_debug (iterc->c, level);
  }

  LOG2 (level, "TTT DEBUG TUNNEL END\n");
}


/**
 * Iterate all tunnels.
 *
 * @param iter Iterator.
 * @param cls Closure for @c iter.
 */
void
GCT_iterate_all (GNUNET_CONTAINER_PeerMapIterator iter, void *cls)
{
  GNUNET_CONTAINER_multipeermap_iterate (tunnels, iter, cls);
}


/**
 * Count all tunnels.
 *
 * @return Number of tunnels to remote peers kept by this peer.
 */
unsigned int
GCT_count_all (void)
{
  return GNUNET_CONTAINER_multipeermap_size (tunnels);
}


/**
 * Iterate all connections of a tunnel.
 *
 * @param t Tunnel whose connections to iterate.
 * @param iter Iterator.
 * @param cls Closure for @c iter.
 */
void
GCT_iterate_connections (struct CadetTunnel *t, GCT_conn_iter iter, void *cls)
{
  struct CadetTConnection *ct;

  for (ct = t->connection_head; NULL != ct; ct = ct->next)
    iter (cls, ct->c);
}


/**
 * Iterate all channels of a tunnel.
 *
 * @param t Tunnel whose channels to iterate.
 * @param iter Iterator.
 * @param cls Closure for @c iter.
 */
void
GCT_iterate_channels (struct CadetTunnel *t, GCT_chan_iter iter, void *cls)
{
  struct CadetTChannel *cht;

  for (cht = t->channel_head; NULL != cht; cht = cht->next)
    iter (cls, cht->ch);
}