module Net::SSH::Transport::PacketStream

def poll_next_packet
  if @packet.nil?
    minimum = server.block_size < 4 ? 4 : server.block_size
    return nil if available < minimum
    data = read_available(minimum)
    # decipher it
    @packet = Net::SSH::Buffer.new(server.update_cipher(data))
    @packet_length = @packet.read_long
  end
  need = @packet_length + 4 - server.block_size
  raise Net::SSH::Exception, "padding error, need #{need} block #{server.block_size}" if need % server.block_size != 0
  return nil if available < need + server.hmac.mac_length
  if need > 0
    # read the remainder of the packet and decrypt it.
    data = read_available(need)
    @packet.append(server.update_cipher(data))
  end
  # get the hmac from the tail of the packet (if one exists), and
  # then validate it.
  real_hmac = read_available(server.hmac.mac_length) || ""
  @packet.append(server.final_cipher)
  padding_length = @packet.read_byte
  payload = @packet.read(@packet_length - padding_length - 1)
  my_computed_hmac = server.hmac.digest([server.sequence_number, @packet.content].pack("NA*"))
  raise Net::SSH::Exception, "corrupted mac detected" if real_hmac != my_computed_hmac
  # try to decompress the payload, in case compression is active
  payload = server.decompress(payload)
  debug { "received packet nr #{server.sequence_number} type #{payload.getbyte(0)} len #{@packet_length}" }
  server.increment(@packet_length)
  @packet = nil
  return Packet.new(payload)
end