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.
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.
To encrypt a message: To decrypt a ciphertext: This algorithm generates keystream values by moving cards within the deck.
[2] In 2019 Daniel Shiu proposed modifications to the algorithm which would increase its security, at the cost of making it more difficult for the user to implement manually.