I breifly covered the subject of IGZ's encryption in a forum post earlier. Now I figure maybe I should explain how to apply it. I'm not going to post code for the most part, and to follow along you may need a pretty decent understanding of datatypes.

Average Packet Header
[dword header CRC] [dword payload length] [dword payload CRC] [dword command] [dword sequence]

* note: the header is used differently during the handshake phase

Each CRC is a simple (depending on how you look at it) CRC32 value. You should be able to find publicly available crc32 modules for whatever language you are coding in.

Secure Handshake
CLIENT:  00 01 00 00 00 00 00 00 01 00 00 00 00 00 00 00 9A 4E 25 69 00 00 00 00
SERVER: 00 01 00 00 DC D9 62 77 01 00 00 00 8A 59 B5 5B DC 40 26 A9 00 00 00 00
CLIENT:  66 67 66 67 66 67 66 67 00 00 00 00 00 00 00 00 ?? ?? ?? ?? 01 00 00 00
* underlined bytes are the client supplied key (supposed to be generated at random), ?? bytes are the CRC which we need to compute
SERVER: CF 1C 12 6E CC 6B 97 8C 3C 2B DF DD EA 67 FC 3B 78 AC 60 39 01 00 00 00
* underlined is the server supplied key. Our new ice key is now determined by a byte to byte xor of the client and server keys.

 More info to come as I feel like posting it ...