Cryptanalysis of Typex

Rotor cipher machines played a large role in World War II: Germany used Enigma; America created Sigaba; Britain developed Typex. The breaking of Enigma by Polish and (later) British cryptanalysts had an enormous impact on the war. However, despite being based on the commercial version of the Enigma, there is no documented successful attack on Typex during its time in service. This paper covers the Typex machine. We consider the development of Typex, we discuss how Typex works, and we present and analyze two cryptanalytic attacks on the cipher. The first attack assumes the rotor wirings are known and uses Turing’s crib attack—originally developed for Enigma—to recover the settings of the stepping rotors. The second attack assumes that the rotor wirings are unknown. This ciphertext-only attack uses a hill-climb to determine the wirings of the stepping rotors. Finally, we briefly consider an attack developed by Polish cryptanalysts to recover the Enigma rotor wirings and we argue that Typex was significantly more resistant to this particular attack.