Calculating the checksum

Algorithm

The game’s checksum calculation is just a generic CRC32 checksum algorithm. It’s the fast implementation because it uses a pre-calculated table.

Usually, the table is generated from a fixed poly. In our case, the poly is selected by the server from a list of 256 different polys. The selected index of the poly in the list is transmitted in the first handshake message.

Operation

• The checksum is generated before encryption.
• The checksum includes all bytes of the message.
• Since the checksum byte is part of the calculation, it’s treated as 0 for the calculation.

Trivia

• Properitary multi-connection endpoints will not reuse already generated tables
• Some third-party implementations just ship all 256 tables (65.536 bytes) pre-generated as code.