I'm cited on the first page of this paper (reference 20) for my work on the Android Master Key vulnerability (which I didn't find, to be clear, but I did most of the exploitation people saw), and, while this paper looks AWESOME (and I'm very excited to read it in detail), if you are interested in this concept but feel you need something a bit more concrete--maybe with diagrams and some hand-holding--to understand what is going on, I will recommend my series of articles on Master Key as an introduction.

https://www.saurik.com/masterkey1.html

https://www.saurik.com/masterkey2.html

https://www.saurik.com/masterkey3.html