PISA: Improving the pseudo-randomness of the pen-and-paper cipher based on the Solitaire algorithm

Abstract

Solitaire Cipher was designed by Bruce Schneier as a Pen and Paper Cipher in 1999. It is designed for field agents to communicate secret messages without having electricity or sophisticated electronics equipment. Its main advantage is that the natural randomness of a shuffled deck of cards contributes to its strength and security. However, the main disadvantage of solitaire is that its output keystream is not truly random, its bias i.e. the probability of getting two keystream characters twice in a row is about 1/22.5 instead of 1/26 as identified by Mirdek. This is due to the fact that Solitaire is a non-reversible cipher and it exhibits bias during key generation. To overcome this weakness, we have proposed a revised Solitaire algorithm to improve the randomness in the output keystream and also to reduce the element of biasness while the cipher remains non-reversible. We have also introduced a new and easy mechanism to generate initial deck setting (key) from a passphrase. The proposed cipher is implemented in MATLAB and the generated keystream is tested for randomness.