What is the cookie
==================
The cookie is used to prevent address spoofing, without the firewall having to remember the ip-address of the clients.
It does this by forcing the client to send a cookie, it can only get from the server. The cookie is generated from the current time, the source and destination address and port, and a secret that only the server has.
It works on the same principle that authenticators do. This method forces the client to reply with the same ip/port to the same server ip/port in order to continue.

How is the cookie generated
===========================
The cookie is the hashed `ClientIp`, `ServerIp`, `ClientPort` and `ServerPort` using `siphash24` and a key that is one quarter of a `cookie_seed`. Every second another quarter of the `cookie_seed` is used as key.
The server generates a new `cookie_seed` every 4 seconds, and always keeps 2 `cookie_seeds`. That means a client has atleast 4 seconds, and atmost 8 seconds to reply before the cookie becomes invalid. ClientIp, ServerIp, ClientPort and ServerPort are in network order.

```
cookie_seed = sha512(random_seed << 32 | (unix_time & 0xffffffff))
cookie = siphash24(cookie_seed >> ((time & 3) * 128), Concat(ClientIp, ServerIp, ClientPort, ServerPort))
```

What is the `Random-Seed`
========================
The server keeps a secret called  `random-seed`. Should a attacker ever get hold of the `random-seed` a new `random-seed` must be used. Otherwise any protection that the cookie offers would be compromised. Since a cookie is only valid for atmost eight seconds, changing the `random-seed` would at the worst prevent users from logging into a Teamspeak-Server for atmost eight seconds, but the most common case would be no outage what so ever.
 
How to generate a `Random-Seed`
-------------------------------
```
xxd -l 60 -c 60 -p /dev/urandom > random_seed
```

Format of `COMMAND_GET_COOKIE`
=============================

```
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      'T'      |      'S'      |       '3'     |      'I'      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      'N'      |      'I'      |       'T'     |      '1'      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           PacketId            |           ClientId            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Type + Flags |                Client Version                ->
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                |    Command    |           Timestamp          ->
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                |       Random Sequence        ->
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                |            RESERVERD          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            RESERVED                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            RESERVERD          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
```
* All fields are encoded in big endian, unless otherwise specified.
* `PacketId` is always `101`.
* `ClientId` is always `0`.
* `Type + Flags` is always `0x88`.
* `Client Version` is the build number of the client.
* `Command` is always `0`.
* `Timestamp` is the unixtime of the client maschine, encoded in big endian.
* `Random Sequence` is a random value generated by the client.
* Every `RESERVED` field must be zero.