Cryptanalysis of Typex

• Published in: Cryptologia Vol. 38; no. 2; pp. 116 - 132
• Main Authors: Chang, Kelly, Low, Richard M., Stamp, Mark
• Format: Journal Article
• Language: English
• Published: Philadelphia, PA Taylor & Francis Group 03-04-2014; Taylor & Francis; Taylor & Francis Inc
• Subjects: Americas; Applied sciences; Breaking; Britain; British; cryptanalysis; Cryptography; Data analysis; Enigma; Exact sciences and technology; hill climb; Information, signal and communications theory; Machinery; rotor machine; Rotors; Signal and communications theory; Telecommunications and information theory; Typex; Wiring; World War II; Enigma; Typex; Turing machine; Hill climbing method; War; rotor machine; History; Rotor; Military application; Wiring; Service time; Shaft; Cryptanalysis; Safety; Cryptography; Ciphertext attack; hill climb
• ISSN: 0161-1194; 1558-1586
• DOI: 10.1080/01611194.2013.797051

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 article covers the Typex machine. Researchers consider the development of Typex, discuss how Typex works, and 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, researchers briefly consider an attack developed by Polish cryptanalysts to recover the Enigma rotor wirings, and they argue that Typex was significantly more resistant to this particular attack.