Extract and verify server message.

    Args:
        response_comms: A ClientCommunication rdfvalue

    Returns:
       list of messages and the CN where they came from.

    Raises:
       DecryptionError: If the message failed to decrypt properly.

def DecodeMessages(self, response_comms):
    """Extract and verify server message.

    Args:
        response_comms: A ClientCommunication rdfvalue

    Returns:
       list of messages and the CN where they came from.

    Raises:
       DecryptionError: If the message failed to decrypt properly.
    """
    # Have we seen this cipher before?
    cipher_verified = False
    try:
      cipher = self.encrypted_cipher_cache.Get(response_comms.encrypted_cipher)
      stats_collector_instance.Get().IncrementCounter(
          "grr_encrypted_cipher_cache", fields=["hits"])

      # Even though we have seen this encrypted cipher already, we should still
      # make sure that all the other fields are sane and verify the HMAC.
      cipher.VerifyReceivedHMAC(response_comms)
      cipher_verified = True

      # If we have the cipher in the cache, we know the source and
      # should have a corresponding public key.
      source = cipher.GetSource()
      remote_public_key = self._GetRemotePublicKey(source)
    except KeyError:
      stats_collector_instance.Get().IncrementCounter(
          "grr_encrypted_cipher_cache", fields=["misses"])
      cipher = ReceivedCipher(response_comms, self.private_key)

      source = cipher.GetSource()
      try:
        remote_public_key = self._GetRemotePublicKey(source)
        if cipher.VerifyCipherSignature(remote_public_key):
          # At this point we know this cipher is legit, we can cache it.
          self.encrypted_cipher_cache.Put(response_comms.encrypted_cipher,
                                          cipher)
          cipher_verified = True

      except UnknownClientCertError:
        # We don't know who we are talking to.
        remote_public_key = None

    # Decrypt the message with the per packet IV.
    plain = cipher.Decrypt(response_comms.encrypted, response_comms.packet_iv)
    try:
      packed_message_list = rdf_flows.PackedMessageList.FromSerializedString(
          plain)
    except rdfvalue.DecodeError as e:
      raise DecryptionError(e)

    message_list = self.DecompressMessageList(packed_message_list)

    # Are these messages authenticated?
    # pyformat: disable
    auth_state = self.VerifyMessageSignature(
        response_comms,
        packed_message_list,
        cipher,
        cipher_verified,
        response_comms.api_version,
        remote_public_key)
    # pyformat: enable

    # Mark messages as authenticated and where they came from.
    for msg in message_list.job:
      msg.auth_state = auth_state
      msg.source = cipher.cipher_metadata.source

    return (message_list.job, cipher.cipher_metadata.source,
            packed_message_list.timestamp)