This tutorial is for the experienced reverse engineer and not a beginners help. I'm not going deep into everything, but you should be able to follow my ideas. It took me some time to understand how this protection works and I had to invent some tricks to solve it. You might have to Google some topics like RSA or modular inverse to understand the math behind it. I'm also assuming, that you know how to use the tools that I'm going to use.
I hope you learn something from this tutorial and who knows, I can get you interested in iPhone reversing.