How we set new world records in breaking Playfair ciphertexts

Abstract The Playfair cipher is a well-known manual encryption method developed in the 19th century. Until 2018, known cryptanalysis techniques, with computer assistance, could solve non-keyword-based Playfair ciphertexts if they had at least 60 letters to work with. Shorter ciphertexts were effectively impossible to solve in the absence of a crib. In this article, we show how we introduced several improvements in these cryptanalysis methods, which made it possible to do much better. This resulted in the (unofficial) world record for the shortest Playfair message broken going down from 60 via 50, 40, 32, and 28 to 26 letters. The cryptanalysis techniques used include hill climbing, simulated annealing, tabu search, and plaintext-based dictionary attacks. For readers interested in improving the current record, we also provide unsolved Playfair challenges consisting of 24 and 22 letters.