tor/src/or/command.c

/* Copyright (c) 2001 Matej Pfajfar.
 * Copyright (c) 2001-2004, Roger Dingledine.
 * Copyright (c) 2004-2007, Roger Dingledine, Nick Mathewson. */
/* See LICENSE for licensing information */
/* $Id$ */
const char command_c_id[] =
  "$Id$";

/**
 * \file command.c
 * \brief Functions for processing incoming cells.
 **/

/* In-points to command.c:
 *
 * - command_process_cell(), called from
 *   connection_or_process_cells_from_inbuf() in connection_or.c
 */

#include "or.h"

/** Keep statistics about how many of each type of cell we've received. */
uint64_t stats_n_padding_cells_processed = 0;
uint64_t stats_n_create_cells_processed = 0;
uint64_t stats_n_created_cells_processed = 0;
uint64_t stats_n_relay_cells_processed = 0;
uint64_t stats_n_destroy_cells_processed = 0;
uint64_t stats_n_versions_cells_processed = 0;
uint64_t stats_n_netinfo_cells_processed = 0;
uint64_t stats_n_cert_cells_processed = 0;
uint64_t stats_n_link_auth_cells_processed = 0;

/* These are the main functions for processing cells */
static void command_process_create_cell(cell_t *cell, or_connection_t *conn);
static void command_process_created_cell(cell_t *cell, or_connection_t *conn);
static void command_process_relay_cell(cell_t *cell, or_connection_t *conn);
static void command_process_destroy_cell(cell_t *cell, or_connection_t *conn);
static void command_process_versions_cell(var_cell_t *cell,
                                          or_connection_t *conn);
static void command_process_netinfo_cell(cell_t *cell, or_connection_t *conn);
static void command_process_cert_cell(var_cell_t *cell, or_connection_t *conn);
static void command_process_link_auth_cell(cell_t *cell,or_connection_t *conn);

#ifdef KEEP_TIMING_STATS
/** This is a wrapper function around the actual function that processes the
 * <b>cell</b> that just arrived on <b>conn</b>. Increment <b>*time</b>
 * by the number of microseconds used by the call to <b>*func(cell, conn)</b>.
 */
static void
command_time_process_cell(cell_t *cell, or_connection_t *conn, int *time,
                               void (*func)(cell_t *, or_connection_t *))
{
  struct timeval start, end;
  long time_passed;

  tor_gettimeofday(&start);

  (*func)(cell, conn);

  tor_gettimeofday(&end);
  time_passed = tv_udiff(&start, &end) ;

  if (time_passed > 10000) { /* more than 10ms */
    log_debug(LD_OR,"That call just took %ld ms.",time_passed/1000);
  }
  if (time_passed < 0) {
    log_info(LD_GENERAL,"That call took us back in time!");
    time_passed = 0;
  }
  *time += time_passed;
}
#endif

/** Process a <b>cell</b> that was just received on <b>conn</b>. Keep internal
 * statistics about how many of each cell we've processed so far
 * this second, and the total number of microseconds it took to
 * process each type of cell.
 */
void
command_process_cell(cell_t *cell, or_connection_t *conn)
{
#ifdef KEEP_TIMING_STATS
  /* how many of each cell have we seen so far this second? needs better
   * name. */
  static int num_create=0, num_created=0, num_relay=0, num_destroy=0;
  /* how long has it taken to process each type of cell? */
  static int create_time=0, created_time=0, relay_time=0, destroy_time=0;
  static time_t current_second = 0; /* from previous calls to time */

  time_t now = time(NULL);

  if (now > current_second) { /* the second has rolled over */
    /* print stats */
    log_info(LD_OR,
         "At end of second: %d creates (%d ms), %d createds (%d ms), "
         "%d relays (%d ms), %d destroys (%d ms)",
         num_create, create_time/1000,
         num_created, created_time/1000,
         num_relay, relay_time/1000,
         num_destroy, destroy_time/1000);

    /* zero out stats */
    num_create = num_created = num_relay = num_destroy = 0;
    create_time = created_time = relay_time = destroy_time = 0;

    /* remember which second it is, for next time */
    current_second = now;
  }
#endif

#ifdef KEEP_TIMING_STATS
#define PROCESS_CELL(tp, cl, cn) STMT_BEGIN {                   \
    ++num ## tp;                                                \
    command_time_process_cell(cl, cn, & tp ## time ,            \
                              command_process_ ## tp ## _cell);  \
  } STMT_END
#else
#define PROCESS_CELL(tp, cl, cn) command_process_ ## tp ## _cell(cl, cn)
#endif

  /*XXXX020 reject all but VERSIONS, NETINFO, CERT, LINK_AUTH when
   * handshaking. */
  switch (cell->command) {
    case CELL_PADDING:
      ++stats_n_padding_cells_processed;
      /* do nothing */
      break;
    case CELL_CREATE:
    case CELL_CREATE_FAST:
      ++stats_n_create_cells_processed;
      PROCESS_CELL(create, cell, conn);
      break;
    case CELL_CREATED:
    case CELL_CREATED_FAST:
      ++stats_n_created_cells_processed;
      PROCESS_CELL(created, cell, conn);
      break;
    case CELL_RELAY:
      ++stats_n_relay_cells_processed;
      PROCESS_CELL(relay, cell, conn);
      break;
    case CELL_DESTROY:
      ++stats_n_destroy_cells_processed;
      PROCESS_CELL(destroy, cell, conn);
      break;
    case CELL_VERSIONS:
      tor_fragile_assert();
      break;
    case CELL_NETINFO:
      ++stats_n_netinfo_cells_processed;
      PROCESS_CELL(netinfo, cell, conn);
      break;
    case CELL_CERT:
      tor_fragile_assert();
      break;
    case CELL_LINK_AUTH:
      ++stats_n_link_auth_cells_processed;
      PROCESS_CELL(link_auth, cell, conn);
      break;
    default:
      log_fn(LOG_INFO, LD_PROTOCOL,
             "Cell of unknown type (%d) received. Dropping.", cell->command);
      break;
  }
}

/** Process a <b>cell</b> that was just received on <b>conn</b>. Keep internal
 * statistics about how many of each cell we've processed so far
 * this second, and the total number of microseconds it took to
 * process each type of cell.
 */
void
command_process_var_cell(var_cell_t *cell, or_connection_t *conn)
{
#ifdef KEEP_TIMING_STATS
  /* how many of each cell have we seen so far this second? needs better
   * name. */
  static int num_versions=0, num_cert=0;

  time_t now = time(NULL);

  if (now > current_second) { /* the second has rolled over */
    /* print stats */
    log_info(LD_OR,
             "At end of second: %d versions (%d ms), %d cert (%d ms)",
             num_versions, versions_time/1000,
             cert, cert_time/1000);

    num_versions = num_cert = 0;
    versions_time = cert_time = 0;

    /* remember which second it is, for next time */
    current_second = now;
  }
#endif

  /*XXXX020 reject all when not handshaking. */
  switch (cell->command) {
    case CELL_VERSIONS:
      ++stats_n_versions_cells_processed;
      PROCESS_CELL(versions, cell, conn);
      break;
    case CELL_CERT:
      ++stats_n_cert_cells_processed;
      PROCESS_CELL(cert, cell, conn);
      break;
    default:
      log_warn(LD_BUG,
               "Variable-length cell of unknown type (%d) received.",
               cell->command);
      tor_fragile_assert();
      break;
  }
}

/** Process a 'create' <b>cell</b> that just arrived from <b>conn</b>. Make a
 * new circuit with the p_circ_id specified in cell. Put the circuit in state
 * onionskin_pending, and pass the onionskin to the cpuworker. Circ will get
 * picked up again when the cpuworker finishes decrypting it.
 */
static void
command_process_create_cell(cell_t *cell, or_connection_t *conn)
{
  or_circuit_t *circ;
  int id_is_high;

  if (we_are_hibernating()) {
    log_info(LD_OR,
             "Received create cell but we're shutting down. Sending back "
             "destroy.");
    connection_or_send_destroy(cell->circ_id, conn,
                               END_CIRC_REASON_HIBERNATING);
    return;
  }

  if (!server_mode(get_options())) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Received create cell (type %d) from %s:%d, but we're a client. "
           "Sending back a destroy.",
           (int)cell->command, conn->_base.address, conn->_base.port);
    connection_or_send_destroy(cell->circ_id, conn,
                               END_CIRC_REASON_TORPROTOCOL);
    return;
  }

  /* If the high bit of the circuit ID is not as expected, close the
   * circ. */
  id_is_high = cell->circ_id & (1<<15);
  if ((id_is_high && conn->circ_id_type == CIRC_ID_TYPE_HIGHER) ||
      (!id_is_high && conn->circ_id_type == CIRC_ID_TYPE_LOWER)) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Received create cell with unexpected circ_id %d. Closing.",
           cell->circ_id);
    connection_or_send_destroy(cell->circ_id, conn,
                               END_CIRC_REASON_TORPROTOCOL);
    return;
  }

  if (circuit_get_by_circid_orconn(cell->circ_id, conn)) {
    routerinfo_t *router = router_get_by_digest(conn->identity_digest);
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Received CREATE cell (circID %d) for known circ. "
           "Dropping (age %d).",
           cell->circ_id, (int)(time(NULL) - conn->_base.timestamp_created));
    if (router)
      log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
             "Details: nickname \"%s\", platform %s.",
             router->nickname, escaped(router->platform));
    return;
  }

  circ = or_circuit_new(cell->circ_id, conn);
  circ->_base.purpose = CIRCUIT_PURPOSE_OR;
  circuit_set_state(TO_CIRCUIT(circ), CIRCUIT_STATE_ONIONSKIN_PENDING);
  if (cell->command == CELL_CREATE) {
    circ->_base.onionskin = tor_malloc(ONIONSKIN_CHALLENGE_LEN);
    memcpy(circ->_base.onionskin, cell->payload, ONIONSKIN_CHALLENGE_LEN);

    /* hand it off to the cpuworkers, and then return. */
    if (assign_to_cpuworker(NULL, CPUWORKER_TASK_ONION, circ) < 0) {
      log_warn(LD_GENERAL,"Failed to hand off onionskin. Closing.");
      circuit_mark_for_close(TO_CIRCUIT(circ), END_CIRC_REASON_INTERNAL);
      return;
    }
    log_debug(LD_OR,"success: handed off onionskin.");
  } else {
    /* This is a CREATE_FAST cell; we can handle it immediately without using
     * a CPU worker. */
    char keys[CPATH_KEY_MATERIAL_LEN];
    char reply[DIGEST_LEN*2];
    tor_assert(cell->command == CELL_CREATE_FAST);
    if (fast_server_handshake(cell->payload, reply, keys, sizeof(keys))<0) {
      log_warn(LD_OR,"Failed to generate key material. Closing.");
      circuit_mark_for_close(TO_CIRCUIT(circ), END_CIRC_REASON_INTERNAL);
      return;
    }
    if (onionskin_answer(circ, CELL_CREATED_FAST, reply, keys)<0) {
      log_warn(LD_OR,"Failed to reply to CREATE_FAST cell. Closing.");
      circuit_mark_for_close(TO_CIRCUIT(circ), END_CIRC_REASON_INTERNAL);
      return;
    }
  }
}

/** Process a 'created' <b>cell</b> that just arrived from <b>conn</b>.
 * Find the circuit
 * that it's intended for. If we're not the origin of the circuit, package
 * the 'created' cell in an 'extended' relay cell and pass it back. If we
 * are the origin of the circuit, send it to circuit_finish_handshake() to
 * finish processing keys, and then call circuit_send_next_onion_skin() to
 * extend to the next hop in the circuit if necessary.
 */
static void
command_process_created_cell(cell_t *cell, or_connection_t *conn)
{
  circuit_t *circ;

  circ = circuit_get_by_circid_orconn(cell->circ_id, conn);

  if (!circ) {
    log_info(LD_OR,
             "(circID %d) unknown circ (probably got a destroy earlier). "
             "Dropping.", cell->circ_id);
    return;
  }

  if (circ->n_circ_id != cell->circ_id) {
    log_fn(LOG_PROTOCOL_WARN,LD_PROTOCOL,
           "got created cell from Tor client? Closing.");
    circuit_mark_for_close(circ, END_CIRC_REASON_TORPROTOCOL);
    return;
  }

  if (CIRCUIT_IS_ORIGIN(circ)) { /* we're the OP. Handshake this. */
    origin_circuit_t *origin_circ = TO_ORIGIN_CIRCUIT(circ);
    int err_reason = 0;
    log_debug(LD_OR,"at OP. Finishing handshake.");
    if ((err_reason = circuit_finish_handshake(origin_circ, cell->command,
                                 cell->payload)) < 0) {
      log_warn(LD_OR,"circuit_finish_handshake failed.");
      circuit_mark_for_close(circ, -err_reason);
      return;
    }
    log_debug(LD_OR,"Moving to next skin.");
    if ((err_reason = circuit_send_next_onion_skin(origin_circ)) < 0) {
      log_info(LD_OR,"circuit_send_next_onion_skin failed.");
      /* XXX push this circuit_close lower */
      circuit_mark_for_close(circ, -err_reason);
      return;
    }
  } else { /* pack it into an extended relay cell, and send it. */
    log_debug(LD_OR,
              "Converting created cell to extended relay cell, sending.");
    relay_send_command_from_edge(0, circ, RELAY_COMMAND_EXTENDED,
                                 cell->payload, ONIONSKIN_REPLY_LEN,
                                 NULL);
  }
}

/** Process a 'relay' <b>cell</b> that just arrived from <b>conn</b>. Make sure
 * it came in with a recognized circ_id. Pass it on to
 * circuit_receive_relay_cell() for actual processing.
 */
static void
command_process_relay_cell(cell_t *cell, or_connection_t *conn)
{
  circuit_t *circ;
  int reason, direction;

  circ = circuit_get_by_circid_orconn(cell->circ_id, conn);

  if (!circ) {
    log_debug(LD_OR,
              "unknown circuit %d on connection from %s:%d. Dropping.",
              cell->circ_id, conn->_base.address, conn->_base.port);
    return;
  }

  if (circ->state == CIRCUIT_STATE_ONIONSKIN_PENDING) {
    log_fn(LOG_PROTOCOL_WARN,LD_PROTOCOL,"circuit in create_wait. Closing.");
    circuit_mark_for_close(circ, END_CIRC_REASON_TORPROTOCOL);
    return;
  }

  if (CIRCUIT_IS_ORIGIN(circ)) {
    /* if we're a server and treating connections with recent local
     * traffic better, then this is one of them. */
    conn->client_used = time(NULL);
  }

  if (!CIRCUIT_IS_ORIGIN(circ) &&
      cell->circ_id == TO_OR_CIRCUIT(circ)->p_circ_id)
    direction = CELL_DIRECTION_OUT;
  else
    direction = CELL_DIRECTION_IN;

  if ((reason = circuit_receive_relay_cell(cell, circ, direction)) < 0) {
    log_fn(LOG_PROTOCOL_WARN,LD_PROTOCOL,"circuit_receive_relay_cell "
           "(%s) failed. Closing.",
           direction==CELL_DIRECTION_OUT?"forward":"backward");
    circuit_mark_for_close(circ, -reason);
  }
}

/** Process a 'destroy' <b>cell</b> that just arrived from
 * <b>conn</b>. Find the circ that it refers to (if any).
 *
 * If the circ is in state
 * onionskin_pending, then call onion_pending_remove() to remove it
 * from the pending onion list (note that if it's already being
 * processed by the cpuworker, it won't be in the list anymore; but
 * when the cpuworker returns it, the circuit will be gone, and the
 * cpuworker response will be dropped).
 *
 * Then mark the circuit for close (which marks all edges for close,
 * and passes the destroy cell onward if necessary).
 */
static void
command_process_destroy_cell(cell_t *cell, or_connection_t *conn)
{
  circuit_t *circ;
  int reason;

  circ = circuit_get_by_circid_orconn(cell->circ_id, conn);
  reason = (uint8_t)cell->payload[0];
  if (!circ) {
    log_info(LD_OR,"unknown circuit %d on connection from %s:%d. Dropping.",
             cell->circ_id, conn->_base.address, conn->_base.port);
    return;
  }
  log_debug(LD_OR,"Received for circID %d.",cell->circ_id);

  if (!CIRCUIT_IS_ORIGIN(circ) &&
      cell->circ_id == TO_OR_CIRCUIT(circ)->p_circ_id) {
    /* the destroy came from behind */
    circuit_set_p_circid_orconn(TO_OR_CIRCUIT(circ), 0, NULL);
    circuit_mark_for_close(circ, reason|END_CIRC_REASON_FLAG_REMOTE);
  } else { /* the destroy came from ahead */
    circuit_set_n_circid_orconn(circ, 0, NULL);
    if (CIRCUIT_IS_ORIGIN(circ)) {
      circuit_mark_for_close(circ, reason|END_CIRC_REASON_FLAG_REMOTE);
    } else {
      char payload[1];
      log_debug(LD_OR, "Delivering 'truncated' back.");
      payload[0] = (char)reason;
      relay_send_command_from_edge(0, circ, RELAY_COMMAND_TRUNCATED,
                                   payload, sizeof(payload), NULL);
    }
  }
}

/** Process a 'versions' cell.  The current link protocol version must be 0
 * to indicate that no version has yet been negotiated. DOCDOC say more. */
static void
command_process_versions_cell(var_cell_t *cell, or_connection_t *conn)
{
  int highest_supported_version = 0;
  const char *cp, *end;
  if (conn->link_proto != 0 ||
      conn->_base.state != OR_CONN_STATE_OR_HANDSHAKING ||
      (conn->handshake_state && conn->handshake_state->received_versions)) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR,
           "Received a VERSIONS cell on a connection with its version "
           "already set to %d; dropping", (int) conn->link_proto);
    return;
  }
  tor_assert(conn->handshake_state);
  end = cell->payload + cell->payload_len;
  for (cp = cell->payload; cp+1 < end; ++cp) {
    uint16_t v = ntohs(get_uint16(cp));
    if (v == 1 || v == 2) {
      if (v > highest_supported_version)
        highest_supported_version = v;
    }
  }
  if (!highest_supported_version) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR,
           "Couldn't find a version in common; defaulting to v1.");
    /*XXXX020 just break the connection! */
    conn->link_proto = 1;
    return;
  }
  conn->link_proto = highest_supported_version;
  conn->handshake_state->received_versions = 1;

  if (highest_supported_version >= 2) {
    /*XXXX020 check return values. */
    connection_or_send_netinfo(conn);
    connection_or_send_cert(conn);
    if (conn->handshake_state->started_here)
      connection_or_send_link_auth(conn);
  } else {
    /* XXXX020 finish v1 verification. */
  }
}

/** Process a 'netinfo' cell. DOCDOC say more. */
static void
command_process_netinfo_cell(cell_t *cell, or_connection_t *conn)
{
  time_t timestamp;
  uint8_t my_addr_type;
  uint8_t my_addr_len;
  const char *my_addr_ptr;
  const char *cp, *end;
  uint8_t n_other_addrs;
  time_t now = time(NULL);
  if (conn->link_proto < 2) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR,
           "Received a NETINFO cell on %s connection; dropping.",
           conn->link_proto == 0 ? "non-versioned" : "a v1");
    return;
  }
  if (conn->_base.state != OR_CONN_STATE_OR_HANDSHAKING) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR,
           "Received a NETINFO cell on a non-handshaking; dropping.");
    return;
  }
  tor_assert(conn->handshake_state &&
             conn->handshake_state->received_versions);
  if (conn->handshake_state->received_netinfo) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR,
           "Received a duplicate NETINFO cell; dropping.");
    return;
  }
  /* Decode the cell. */
  timestamp = ntohl(get_uint32(cell->payload));
  if (abs(now - conn->handshake_state->sent_versions_at) < 180) {
    conn->handshake_state->apparent_skew = now - timestamp;
  }

  my_addr_type = (uint8_t) cell->payload[4];
  my_addr_len = (uint8_t) cell->payload[5];
  my_addr_ptr = cell->payload + 6;
  end = cell->payload + CELL_PAYLOAD_SIZE;
  cp = cell->payload + 6 + my_addr_len;
  if (cp >= end) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR,
           "Address too long in netinfo cell; dropping.");
    /*XXXX020 reject and break OR conn! */
    return;
  } else if (my_addr_type == RESOLVED_TYPE_IPV4 && my_addr_len == 4) {
    conn->handshake_state->my_apparent_addr = ntohl(get_uint32(my_addr_ptr));
  }

  n_other_addrs = (uint8_t) *cp++;
  while (n_other_addrs && cp < end-2) {
    /* Consider all the other addresses; if any matches, this connection is
     * "canonical." */
    uint8_t other_addr_type = (uint8_t) *cp++;
    uint8_t other_addr_len = (uint8_t) *cp++;
    if (cp + other_addr_len >= end)
      break; /*XXXX020 protocol warn. */
    if (other_addr_type == RESOLVED_TYPE_IPV4 && other_addr_len == 4) {
      uint32_t addr = ntohl(get_uint32(cp));
      if (addr == conn->real_addr) {
        conn->handshake_state->apparently_canonical = 1;
        break;
      }
    }
    cp += other_addr_len;
    --n_other_addrs;
  }

  conn->handshake_state->received_netinfo = 1;
}

/*XXXX020 move to connection_or.c */
/** DOCDOC Called when we're done authenticating; act on stuff we
 * learned in netinfo. */
void
connection_or_act_on_netinfo(or_connection_t *conn)
{
  long delta;
  if (!conn->handshake_state)
    return;

  tor_assert(conn->handshake_state->authenticated != 0);

  delta = conn->handshake_state->apparent_skew;
  /*XXXX020 magic number 3600 */
  if (abs(delta) > 3600 &&
      router_get_by_digest(conn->identity_digest)) {
    char dbuf[64];
    /*XXXX020 not always warn!*/
    format_time_interval(dbuf, sizeof(dbuf), delta);
    log_fn(LOG_WARN, LD_HTTP, "Received NETINFO cell with skewed time from "
           "server at %s:%d.  It seems that our clock is %s by %s, or "
           "that theirs is %s. Tor requires an accurate clock to work: "
           "please check your time and date settings.",
           conn->_base.address, (int)conn->_base.port,
           delta>0 ? "ahead" : "behind", dbuf,
           delta>0 ? "behind" : "ahead");
    control_event_general_status(LOG_WARN,
                                 "CLOCK_SKEW SKEW=%ld SOURCE=OR:%s:%d",
                                 delta, conn->_base.address, conn->_base.port);
  }

  /* XXX020 possibly, learn my address from my_apparent_addr */

  if (conn->handshake_state->apparently_canonical) {
    conn->is_canonical = 1;
  }
}

static void
command_process_cert_cell(var_cell_t *cell, or_connection_t *conn)
{
  (void) cell;
  (void) conn;

  /* Parse certs. */
  /* Verify that identity cert has signed peer cert in SSL, or
   * peer cert in the cell. */
  /* Verify that identity cert is self-signed. */
  /* Learn ID digest. */
  /* Learn cert digests. */
  /* Remember peer cert public key. */
  /* set received_certs. */
}

#define LINK_AUTH_STRING "Tor initiator certificate verification"

/** DOCDOC */
static void
command_process_link_auth_cell(cell_t *cell, or_connection_t *conn)
{
  or_handshake_state_t *s;
  char hmac[DIGEST_LEN];
  uint16_t len;
  size_t sig_len;
  const char *sig;
  char *checked = NULL;
  int checked_len;
  tor_assert(conn);
  if (conn->_base.state != OR_CONN_STATE_OR_HANDSHAKING) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR,
           "Received a LINK_AUTH cell on connection in the wrong state; "
           "dropping.");
    return;
  }
  s = conn->handshake_state;
  tor_assert(s);
  if (s->started_here) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR,
           "Got a LINK_AUTH cell from a server; closing the connection.");
    goto err;
  }
  if (!s->received_netinfo || !s->received_versions || !s->received_certs) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR, "Got a LINK_AUTH cell too early; "
           "closing the connection");
    goto err;
  }
  len = ntohs(get_uint16(cell->payload));
  if (len < 2 || 2+len > CELL_PAYLOAD_SIZE) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR, "Bad length field (%d) on LINK_AUTH cell;"
           " closing the connection", (int)len);
    goto err;
  }
  if (cell->payload[2] != 0x00) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR, "Unrecognized LINK_AUTH signature "
           "version; closing the connection");
    goto err;
  }
  connection_or_compute_link_auth_hmac(conn, hmac);

  tor_assert(s->signing_key);

  sig = cell->payload+3;
  sig_len = len-1;
  checked = tor_malloc(crypto_pk_keysize(s->signing_key));
  checked_len = crypto_pk_public_checksig(s->signing_key,checked,sig,sig_len);
  if (checked_len < 0) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR, "Bad signature on LINK_AUTH cell; "
           "closing the connection");
    goto err;
  }
  if (checked_len != DIGEST_LEN) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR, "Bad length (%d) of signed material in "
           "LINK_AUTH cell; closing the connection", checked_len);
    goto err;
  }
  if (memcmp(checked, hmac, DIGEST_LEN) != 0) {
    log_fn(LOG_PROTOCOL_WARN, LD_OR, "Bad signed data in LINK_AUTH cell; "
           "closing the connection.");
    goto err;
  }

  /* Okay, we're authenticated. */
  s->authenticated = 1;

  /* XXXX020 act on being authenticated: */

  return;
 err:
  tor_free(checked);
  connection_mark_for_close(TO_CONN(conn));
}