The Solitaire cipher is an encryption scheme devised by Bruce Schneier which relies only on a pack of cards for encryption and decryption. The idea is that secret agents are less likely to be placed under suspicion for carrying a deck of cards than if they're hiding the Enigma machine under their shirt. The encryption and decryption algorithms are fairly straightforward; however, the system is rather slow, so I wrote a script in Python to simulate decryption given a starting deck.
This algorithm is used in Puzzle 4 of the Argonne National Laboratory's 2017 Cyber Defense Competition, so I won't release the code until the solutions have been published. Good luck to everyone competing!
Update 3/10/17: the solutions are up! Check out the Python code.