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.

http://www.math.sunysb.edu/~scott/papers/MSTP/crypto/3Caesar_Cipher.html

http://distance-ed.math.tamu.edu/techtools/flash/crypto/crypto.html

This (slightly more advanced) site discusses cryptography and PGP:

https://ssd.eff.org/en/module/introduction-public-key-cryptography-and-pgp

International Association for Cryptologic Research:

http://www.iacr.org

It is even interesting to check out the National Security Agency’s site:

http://www.nsa.gov/