one time pads.

From: Eric Hughes <hughes@soda.berkeley.edu>

Keith's CD-for-a-pad idea is a variant of a book code. In a book code, parts of the key are in various standard books, often the bible.

Advantages: easier key distribution.

Disadvantages: key material is public. Should an internal spy learn the few bits of addressing information (which CD, where), the cipher is compromised.

Actually, in practice internal spies were almost never needed to break book cyphers. They in fact provide only laughable security. Traditional ones didn't even require that the cryptanalyst ever determine what book was being used! Perry