Solitaire (cipher)

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The Solitaire cryptographic algorithm was designed by Bruce Schneier at the request of Neal Stephenson for use in his novel Cryptonomicon, in which field agents use it to communicate securely without having to rely on electronics or having to carry incriminating tools.^[1] It was designed to be a manual cryptosystem calculated with an ordinary deck of playing cards. In Cryptonomicon, this algorithm was originally called Pontifex to hide the fact that it involved playing cards.

One of the motivations behind Solitaire's creation is that in totalitarian environments, a deck of cards is far more affordable (and less incriminating) than a personal computer with an array of cryptological utilities. However, as Schneier warns in the appendix of Cryptonomicon, just about everyone with an interest in cryptanalysis will now know about this algorithm, so carrying a deck of cards may also be considered incriminating. Furthermore, analysis has revealed flaws in the cipher such that it is now considered insecure.