I'm not quite sure on this, but I do believe the CPU Key decrypts the Nand.
That's not an accurate piece of information, don't go around telling people
Just a theory, I'm somewhat new to JTaging, but i repair xbox's, so i know my way around them.
There are other ways to get your cpu key although.
I believe a method would be to install XBR, i don't have any links but I'm sure someone around the fourm will post something, or just look around!