The Playfair cipher, invented by Charles Wheatstone in 1854 and promoted by Lyon Playfair, encrypts pairs of letters through a 5×5 key square. It was used operationally by British forces through both World Wars. A long-standing cryptanalytic question is: what is the shortest Playfair ciphertext that can be uniquely solved? With Louie Helm, I published new world records for short Playfair solutions at HistoCrypt 2025, reducing the previously known minimum by a meaningful margin and supplying the methods and corpora used.
The attack combines stochastic search over the 25-letter key square with an n-gram language-model fitness function. The paper below reports the shortest uniquely solved ciphertexts that we were able to find and sets out the parameter choices that made the attack tractable at such short lengths.
Bean, R. and Helm, L. (2025). New records for Playfair solutions. In Proceedings of HistoCrypt 2025.
Conference talk: HistoCrypt 2025, Poznań, 17 June 2025 (L. Helm, presenter).
Related prior work: Bean, R. (2020). The use of Project Gutenberg and hexagram statistics to help solve famous unsolved ciphers. In Proceedings of HistoCrypt 2020.