If you want to know what method they used to compress the game, you just have to open it with arc.
If the files have pass, you can use WinHEX to know what the pass is.
Example: The cod4 mw v2 uses the following pass for its files:
data1.cpak
corepack.4156
data2.cpak
corepack.4156
data 3.cpak
m,.wsltr,il....445
