===Structure===

The Endless Online packet stream consists of a two byte length followed by possibly encoded data:

:<tt>(Length:2) (Data:''n'')</tt> where ''n'' is the number specified by Length.

The same format is followed for both client-to-server and server-to-client communication.

The first two bytes of the (unencrypted) data are are the packet action and packet family. The remaining data is packet dependent. See [Packet_List] for a list of packet IDs.

:<tt>(PacketAction:1) (PacketFamily:1) (Data:''n'')</tt> where ''n'' is the length, minus 2.

There is commonly a third byte included with packets sent client-to-server which is an additional security mechanism. This is discussed in the Encryption section.

====Number Encoding====

Numbers are not sent in plain Two's compliment, as the stream is binary safe (contains no zero bytes) and consequently they have a slightly smaller range of possible values they can represent. See [EncodeNumber] for an algorithm to encode/decode numbers.

===Encryption===

Communication between the Endless Online client and server is very lightly encrypted and fairly easy to encode/decode. It is a stateful process and not all packets are encoded.

There are three stages: Flipping, interleaving and "dickwinding". These only apply to the actual packet data (including the ID) and ''not'' the length bytes. For most client-to-server packets there is also a byte inserted immediately following the packet ID which we will refer to as the "counter".