[gibsjose]

It is quite fascinating to imagine inmates passing their handwritten Vigenère encrypted secrets using Diffie-Hellman key exchange.

[wyager]

As I recall, Neil Stephenson commissioned Bruce Schneier to come up with a reasonably secure cryptosystem that could be computed using a deck of cards, so that a character in Cryptonomicon could do something similar while imprisoned.

[grogenaut]

https://www.schneier.com/academic/solitaire/

[userbinator]

The two indices and the permutation operations are very reminiscent of RC4.