What part of the XBox game is signed?

[SCVirus]

I am wondering the reason why you cannot simply make a 1:1 backup of an xbox disc using either, An Xbox with linux on it, and simply ripping the disc like you would any disc on a computer, or use a modded pc dvd-rom drive to rip the game from your pc? I though you couldn't use backups because 'they arn't signed' in which case why can you not copy it 1:1 and copy the 'signing' with it. This has been bothering me whats the reason this can't be done? Have i got something completely wrong about the mechanics of this system?