In Neal Stephenson’s book Cryptonomicon, the character Enoch Root describes a cryptosystem with the code name “Pontifex” to another character named Randy Waterhouse, and later reveals that the steps of the algorithm are meant to be performed by means of a simple card game. The system is called “Solitaire”.
Solitaire, named after the card game Solitaire, is a symmetric stream cipher developed by Bruce Schneier.
This algorithm uses a deck of 52 cards plus two jokers to generate the key. The peculiarity should be that the algorithm, while relatively secure, can still be used “by hand” (without computers or other technical computing aids).
Solitaire generates the key stream using a deck of cards. I.e. each letter of the text is encrypted or decrypted with a letter generated from the deck of cards.
The bridge order is used. From the lowest to the highest suit we have clubs, diamonds, hearts and spades. This assigns Ace of Clubs to King of Clubs from 1 to 13, Ace of Diamonds to King of Diamonds from 14 to 26, Ace of Hearts to King of Hearts from 1 to 13, and Ace of Spades to King of Spades from 14 to 26. Numbers from 1 to 26 are required, not 1 to 52, so that letters can be generated.
However, a flaw in the algorithm was found and published shortly after its release.