This site makes a transition from the modular arithmetic (of the last section) to writing secret codes (of this section). A Caesar cipher is a secret code in which each letter in the plain text is replaced by a letter some fixed number of positions down the alphabet. For example, a shift of 4, A is replaced by D, B is replaced by E, and so on. It is named after Julius Caesar, who used it to communicate with his generals.
This (slightly more advanced) site discusses cryptography and PGP:
International Association for Cryptologic Research:
It is even interesting to check out the National Security Agency’s site: