Bruce Schneier's Solitaire - Secure Encrypted Communications Using a Deck of Cards

I just finished reading Neal Stephenson's Novel Cryptonomicon.  In the book, two characters in prison use a deck of cards to encrypt their written communications.  Well, it turns out that it's actually a real encryption technique invented by Cryptologist Bruce Schneier for the book, but it is also intended to provide serious security in actual field use, check it out, it's a pretty cool concept.  Here is the overview from Scneier's Website: http://www.schneier.com/solitaire.html

"In Neal Stephenson's novel Cryptonomicon, the character Enoch Root describes a cryptosystem code-named "Pontifex" to another character named Randy Waterhouse, and later reveals that the steps of the algorithm are intended to be carried out using a deck of playing cards. These two characters go on to exchange several encrypted messages using this system. The system is called "Solitaire" (in the novel, "Pontifex" is a code name intended to temporarily conceal the fact that it employs a deck of cards) and I designed it to allow field agents to communicate securely without having to rely on electronics or having to carry incriminating tools. An agent might be in a situation where he just does not have access to a computer, or may be prosecuted if he has tools for secret communication. But a deck of cards...what harm is that?

Solitaire gets its security from the inherent randomness in a shuffled deck of cards. By manipulating this deck, a communicant can create a string of "random" letters that he then combines with his message. Of course Solitaire can be simulated on a computer, but it is designed to be implemented by hand.

Solitaire may be low-tech, but its security is intended to be high-tech. I designed Solitaire to be secure even against the most well-funded military adversaries with the biggest computers and the smartest cryptanalysts. Of course, there is no guarantee that someone won't find a clever attack against Solitaire (watch this space for updates), but the algorithm is certainly better than any other pencil-and-paper cipher I've ever seen.

It's not fast, though. It can take an evening to encrypt or decrypt a reasonably long message. In David Kahn's book Kahn on Codes, he describes a real pencil-and-paper cipher used by a Soviet spy. Both the Soviet algorithm and Solitaire take about the same amount of time to encrypt a message: most of an evening"