Conference article

Solving a Tunny Challenge with Computerized “Testery” Methods

George Lasry
The DECRYPT Project

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Published in: Proceedings of the 3rd International Conference on Historical Cryptology HistoCrypt 2020

Linköping Electronic Conference Proceedings 171:13, p. 96-105

NEALT Proceedings Series 44:13, p. 96-105

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Published: 2020-05-19

ISBN: 978-91-7929-827-2

ISSN: 1650-3686 (print), 1650-3740 (online)


The Lorenz SZ42, codenamed Tunny, was a teleprinter encryption device used by Germany during WW2 for strategic communications. Its successful cryptanalysis at Bletchley Park (BP) provided the Allies with high-grade intelligence about several fronts, as well as for the preparations for the D-Day landings. The story of Tunny’s code-breaking and Colossus is well known, following the declassification of the General Report on Tunny in 2000 (Good et al., 1945), and the publication of several books (Reeds et al., 2015; Gannon, 2014; Copeland, 2010; Roberts, 2017; Mayo-Smith, 2014). The work on Colossus and other machines was carried out in the Newmanry, under the leadership of the mathematician Max Newman. The work of the Testery, the other Tunny section at BP, is less known. Named after his commander, Major Ralph Tester, the Testery was responsible for the development and application of hand methods, that complemented the work of machines like Colossus. For some reason, the report on the Testery was not declassified until 2018. Following its recent release, it is possible to fully assess the achievements of the Testery cryptanalysts and their key contribution to BP’s success against Tunny (Testery, 1945). The work described in this article is an t-tempt to determine whether the Testery manual methods can be mechanized with modern computing. The author was able to automate some of the techniques and partially automate some others. With these techniques, the author also succeeded in recovering the key settings and the plain-text of two Tunny challenge messages.


Lorenz SZ42; Tunny; Teleprinter; Testery; Cryptanalysis; Cipher Challenge


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