It's a basic substitution cipher, usually one of the first that you learn if you're interested in codes and codebreaking. It's the one where you write the alphabet into a tic-tac-toe board, then a tic-tac-toe board with dots, then an x, then an x with dots.
Anyway, although I know there was no way that I was one of the first 33, I still put in an entry on the cover puzzle. I am such a geek.