Added first version of cookie.md

cookie.md

@@ -0,0 +1,61 @@
+What is the cookie
+==================
+The cookie is used to prevent address spoffing, 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. And force 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.
+
+```
+cookie_seed = sha512(random_seed << 4 | (time & ~3))
+cookie = siphash24(cookie_seed >> ((time & 3) * 16), 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 lower encodian, 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.