For now, let’s use "Pontifex" as the working title of this cryptosystem. It is a post-war system. What I mean by that is that, after seeing what Turing and company did to Enigma, I came to the (now obvious) conclusion that any modern system had better be resistant to machine cryptanalysis. Pontifex uses a 54-element permutation as its key—one key per message, mind you!—and it uses that permutation (which we will denote as T) to generate a keystream which is added, modulo 26, to the plaintext (P), as in a one-time pad. The process of generating each character in the keystream alters T in a reversible but more or less "random" fashion.