a secret veiled by a secret
It's the simplest possible system. Consider only the outer circle. To code something, you replace each letter of your original message with the letter that precedes it: for A you write Z, for B you write A and so on. Child's play for a secret agent nowadays, but back then it was considered witchcraft.
For all its overwhelming scholarly depth, clever code play and its appeal to the secret conspiracy theorist that's in all of us, the story of Foucault's Pendulum by Umberto Eco is ultimately disappointing, and (at 600+ pages) easily outwitted by any old short story by Borges.
It's kind of ironic that Eco, a professor of semiotics (the study of signs and symbols), should lose sight of the forest for the trees - but then, that's probably the postmodernist point he wants to make.
To cite two of the book's many (and mostly very obscure) quotations:
Our cause is a secret within a secret, a secret that only another secret can explain; it is a secret about a secret that is veiled by a secret.
- Ja'far as-Sadiq, sixth Imam
The conspiracy theory of society ... comes from abandoning God and then asking: "Who is in his place?"
- Karl Popper, Conjectures and Refutations, 1969