Cryptography

In the book Cryptonomicon an encryption method called "pontifex" is described. Here is an explanation of the encryption method also referred to as Solitare due to the fact that it can be performed by hand with a deck of playing cards: Solitare Encryption

Here is a working example of pontifex encryption:
Pontifex Encryption

Other great cryptography links:
Open Source Encryption
Public Key Encryption Basics
Public Key Encryption Details
Matt Blaze's Cryptography Page