in the source code go to the header folder and AESFile.h
"[CC]Header\AESFile.h"
you can change it or add your own version and key in that file, a couple of things to be aware of, if you change the existing key, or remove the existing key, you will HAVE to unpack all the files in "Data\Script\Direct" and repack them with your new key, and they are bin files, but with beff or befl extensions and you will have to have to compile the tool as well to repack them with the new key, if you change/remove the key from the aesfile.h any compiled packing tool after will not be able to read the old aes encrypted bin files, and the tool can only unpack and repack 83 ( don't ask me why 83 ) files at a time, and there are 4200 files in the "Data\Script\Direct" so it will take a while, another thing to be aware of, without a obfuscation system in the source code, the key can easily be obtained by hex editing the compiled client since it will be in plain text in a hex editor. However I gave you a decent amount of information the rest is up to you to figure out yourself.