BruceSchneier's SymmetricCipher that uses matched decks of PlayingCards to perform encryption and decryption. Popularised in NealStephenson's "Cryptonomicon". The idea is that the order of the cards in the deck is the key; you encrypt by performing a sequence of cuts on the deck for each letter of the message and doing a few additions in your head. The ciphertext is a list of cards and suits. The person at the other end has a deck in the same order as yours, and decrypts by performing a sequence of cuts and doing additions in their head. If the police come to take you away, you just give your deck a quick shuffle; the key is forever lost, and no-one can prove you were doing anything other than obsessively writing down bridge hands.
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