def VerifyMessageSignature(self, response_comms, packed_message_list, cipher,
                             cipher_verified, api_version, remote_public_key):
    """Verifies the message list signature.

    In the server we check that the timestamp is later than the ping timestamp
    stored with the client. This ensures that client responses can not be
    replayed.

    Args:
      response_comms: The raw response_comms rdfvalue.
      packed_message_list: The PackedMessageList rdfvalue from the server.
      cipher: The cipher object that should be used to verify the message.
      cipher_verified: If True, the cipher's signature is not verified again.
      api_version: The api version we should use.
      remote_public_key: The public key of the source.

    Returns:
      An rdf_flows.GrrMessage.AuthorizationState.
    """
    if (not cipher_verified and
        not cipher.VerifyCipherSignature(remote_public_key)):
      stats_collector_instance.Get().IncrementCounter(
          "grr_unauthenticated_messages")
      return rdf_flows.GrrMessage.AuthorizationState.UNAUTHENTICATED

    try:
      client_id = cipher.cipher_metadata.source
      try:
        client = self.client_cache.Get(client_id)
      except KeyError:
        client = aff4.FACTORY.Create(
            client_id,
            aff4.AFF4Object.classes["VFSGRRClient"],
            mode="rw",
            token=self.token)
        self.client_cache.Put(client_id, client)
        stats_collector_instance.Get().SetGaugeValue(
            "grr_frontendserver_client_cache_size", len(self.client_cache))

      ip = response_comms.orig_request.source_ip
      client.Set(client.Schema.CLIENT_IP(ip))

      # The very first packet we see from the client we do not have its clock
      remote_time = client.Get(client.Schema.CLOCK) or rdfvalue.RDFDatetime(0)
      client_time = packed_message_list.timestamp or rdfvalue.RDFDatetime(0)

      # This used to be a strict check here so absolutely no out of
      # order messages would be accepted ever. Turns out that some
      # proxies can send your request with some delay even if the
      # client has already timed out (and sent another request in
      # the meantime, making the first one out of order). In that
      # case we would just kill the whole flow as a
      # precaution. Given the behavior of those proxies, this seems
      # now excessive and we have changed the replay protection to
      # only trigger on messages that are more than one hour old.

      if client_time < remote_time - rdfvalue.Duration("1h"):
        logging.warning("Message desynchronized for %s: %s >= %s", client_id,
                        remote_time, client_time)
        # This is likely an old message
        return rdf_flows.GrrMessage.AuthorizationState.DESYNCHRONIZED

      stats_collector_instance.Get().IncrementCounter(
          "grr_authenticated_messages")

      # Update the client and server timestamps only if the client
      # time moves forward.
      if client_time > remote_time:
        client.Set(client.Schema.CLOCK, client_time)
        client.Set(client.Schema.PING, rdfvalue.RDFDatetime.Now())

        clock = client_time
        ping = rdfvalue.RDFDatetime.Now()

        for label in client.Get(client.Schema.LABELS, []):
          stats_collector_instance.Get().IncrementCounter(
              "client_pings_by_label", fields=[label.name])
      else:
        clock = None
        ping = None
        logging.warning("Out of order message for %s: %s >= %s", client_id,
                        remote_time, client_time)

      client.Flush()
      if data_store.RelationalDBEnabled():
        source_ip = response_comms.orig_request.source_ip
        if source_ip:
          last_ip = rdf_client_network.NetworkAddress(
              human_readable_address=response_comms.orig_request.source_ip)
        else:
          last_ip = None

        if ping or clock or last_ip:
          try:
            data_store.REL_DB.WriteClientMetadata(
                client_id.Basename(),
                last_ip=last_ip,
                last_clock=clock,
                last_ping=ping,
                fleetspeak_enabled=False)
          except db.UnknownClientError:
            pass

    except communicator.UnknownClientCertError:
      pass

    return rdf_flows.GrrMessage.AuthorizationState.AUTHENTICATED
Verifies the message list signature.

    In the server we check that the timestamp is later than the ping timestamp
    stored with the client. This ensures that client responses can not be
    replayed.

    Args:
      response_comms: The raw response_comms rdfvalue.
      packed_message_list: The PackedMessageList rdfvalue from the server.
      cipher: The cipher object that should be used to verify the message.
      cipher_verified: If True, the cipher's signature is not verified again.
      api_version: The api version we should use.
      remote_public_key: The public key of the source.

    Returns:
      An rdf_flows.GrrMessage.AuthorizationState.