Something you might see in the realm of security is one-time pads. These are used alot in military operations and high-security outfits. Its basically a pad with a series of alphanumerical text chunks on it, each of which you will use to authenticate once then mark out. Once a block has been used once, it is not valid again. Ever. This page features a customizable one-time pad so you can play around with it and learn about the concept. It also has some good information on the process as well.