Solitaire is a stream cipher designed to be implemented using a deck of cards. It was invented by Bruce Schneier for use in the novel Cryptonomicon, by Neal Stephenson [1], where it was called Pontifex. 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 which he then combines with his message. Solitaire can be simulated on a computer, but it is designed to be used by hand.
Manual ciphers are intended to be used by spies in the field who do not want to be caught carrying evidence that they send and receive encrypted messages. In David Kahn's book Kahn on Codes [2], 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.
Solitaire, as described in the appendix to Cryptonomicon, has a cryptographic weakness. While this weakness does not affect the...
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References
Stephenson, Neal (2002). Cryptonomicom. Avon Eos Books, Avon.
Kahn, David (1984). Kahn on Codes: Secrets of the New Cryptology. Macmillan Publishing Co., London.
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Schneier, B. (2005). Solitaire. In: van Tilborg, H.C.A. (eds) Encyclopedia of Cryptography and Security. Springer, Boston, MA . https://doi.org/10.1007/0-387-23483-7_408
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