def read_message(self):
        """
        Only one thread should ever be in this function (no other locking is
        done).

        @raise SSHException: if the packet is mangled
        @raise NeedRekeyException: if the transport should rekey
        """
        header = self.read_all(self.__block_size_in, check_rekey=True)
        if self.__block_engine_in != None:
            header = self.__block_engine_in.decrypt(header)
        if self.__dump_packets:
            self._log(DEBUG, util.format_binary(header, 'IN: '));
        packet_size = struct.unpack('>I', header[:4])[0]
        # leftover contains decrypted bytes from the first block (after the length field)
        leftover = header[4:]
        if (packet_size - len(leftover)) % self.__block_size_in != 0:
            raise SSHException('Invalid packet blocking')
        buf = self.read_all(packet_size + self.__mac_size_in - len(leftover))
        packet = buf[:packet_size - len(leftover)]
        post_packet = buf[packet_size - len(leftover):]
        if self.__block_engine_in != None:
            packet = self.__block_engine_in.decrypt(packet)
        if self.__dump_packets:
            self._log(DEBUG, util.format_binary(packet, 'IN: '));
        packet = leftover + packet

        if self.__mac_size_in > 0:
            mac = post_packet[:self.__mac_size_in]
            mac_payload = struct.pack('>II', self.__sequence_number_in, packet_size) + packet
            my_mac = compute_hmac(self.__mac_key_in, mac_payload, self.__mac_engine_in)[:self.__mac_size_in]
            if my_mac != mac:
                raise SSHException('Mismatched MAC')
        padding = ord(packet[0])
        payload = packet[1:packet_size - padding]
        
        if self.__dump_packets:
            self._log(DEBUG, 'Got payload (%d bytes, %d padding)' % (packet_size, padding))

        if self.__compress_engine_in is not None:
            payload = self.__compress_engine_in(payload)

        msg = Message(payload[1:])
        msg.seqno = self.__sequence_number_in
        self.__sequence_number_in = (self.__sequence_number_in + 1) & 0xffffffffL

        # check for rekey
        raw_packet_size = packet_size + self.__mac_size_in + 4
        self.__received_bytes += raw_packet_size
        self.__received_packets += 1
        if self.__need_rekey:
            # we've asked to rekey -- give them some packets to comply before
            # dropping the connection
            self.__received_bytes_overflow += raw_packet_size
            self.__received_packets_overflow += 1
            if (self.__received_packets_overflow >= self.REKEY_PACKETS_OVERFLOW_MAX) or \
               (self.__received_bytes_overflow >= self.REKEY_BYTES_OVERFLOW_MAX):
                raise SSHException('Remote transport is ignoring rekey requests')
        elif (self.__received_packets >= self.REKEY_PACKETS) or \
             (self.__received_bytes >= self.REKEY_BYTES):
            # only ask once for rekeying
            self._log(DEBUG, 'Rekeying (hit %d packets, %d bytes received)' %
                      (self.__received_packets, self.__received_bytes))
            self.__received_bytes_overflow = 0
            self.__received_packets_overflow = 0
            self._trigger_rekey()

        cmd = ord(payload[0])
        if cmd in MSG_NAMES:
            cmd_name = MSG_NAMES[cmd]
        else:
            cmd_name = '$%x' % cmd
        if self.__dump_packets:
            self._log(DEBUG, 'Read packet <%s>, length %d' % (cmd_name, len(payload)))
        return cmd, msg