I wrote up a tool today called guessfsb that guesses a decryption key for FSB4.
It's on my Stream ripping tools page:
http://hcs64.com/vgm_ripping.htmlI know it isn't really needed as you can find the key right in the file, but I thought it'd be nice to automate. Only works with FSB4 and keys < 16 bytes at the moment, only tested it with Brutal Legend thus far.