The Secrets of the Lorenz Schlüsselzusatz SZ42

The Secrets of the Lorenz Schlüsselzusatz SZ42 is a Web page that I have dedicated for information about the German teleprinter cipher machine SZ42. The Lorenz Schlüsselzusatz SZ42 belonged, together with the Siemens Geheimschreiber T52, to Nazi Germany’s most secret cryptographic systems. Both machines were designed to be high security systems and they carried high level strategic communications directly from the Supreme Command of the German Armed Forces (Wehrmacht) and the High Commands of the Army, Navy and Air Force (OKH, OKM and OKL). The Lorenz SZ42 was an Army machine and it was mainly used by OKW and OKH. The Siemens T52 machines were mainly used by the Navy (OKM) and Air Force (OKL), however it also had more civilian users such as the German Foreign Office.

Bletchley Park's (BP) success in breaking the Lorenz SZ42, called Tunny at BP, was in itself a considerable achievement, but the intelligence value was enormous. Where the Enigma messages would reveal information about an ongoing operation or a field commander's next move the Lorenz SZ42 messages would reveal the thinking, deliberations and plans of the Wehrmacht and those of Hitler himself. The planning and success of the allied D-Day operations were in large measures dependent on this source of extraordinary intelligence. While Colossus, the machine built by Tommy Flowers and his team at the Post Office Research Station for breaking Tunny at BP, is well known Tunny itself is not; and Colossus is therefore often erroneously mentioned as being used to break the Enigma. Similarly the Enigma machine is well known and its ciphering operations generally well understood but the same can not be said for the Lorenz SZ42. The machine is much more complex than the Enigma and all the technical details of the machine has until now not been fully revealed. Very few of the machines are publicly available and only four more or less complete machines are known to exist. The lack of a good simulator for this machine as well as authentic ciphertexts means that it is difficult to get a good feeling for how the machine works and its basic security.

I hope that the information I am publishing on this Web page will help to change this situation. The plan is to slowly release information that to my knowledge is largely unknown and that has not previously been published. I will then try to look into the pre-history of the SZ42 and also reveal the development of its successor the SZ42c. I also hope to show some of the work the Germans did to analyse the security of the machine and what they did to improve the security of it. For the time being the page should be seen as work in progress because it will take some time to prepare all the material for publication.

Lorenz SZ42 on display at Bletchley Park, Photo: F. Weierud © 2009

Perhaps 2015 and 2016 will be the years of Tunny and Colossus. Last year we saw the publication in book form of the General Report on Tunny With Emphasis on Statistical Methods written by I. J. Good, D. Michie, and G. Timms (Eds.) in 1945. The title of the book is Breaking Teleprinter Ciphers at Bletchley Park and it has been edited by James A. Reeds, Whitfield Diffie, and J. V. Field. I feel the editors have done an excellent job and their introductory material and notes is a gold mine of information about Colossus and the FISH ciphers. If you really want to know about Tunny and how the machine was broken in all its details then this is the book for you.

Tunny is also in the news these days. The Norwegian Armed Forces Museum (FMU) in Oslo has generously offered their one and only Lorenz SZ42 machine on a long-term loan to The National Museum of Computing (TNMOC) at Bletchley Park (BP). BP already has its own Lorenz SZ42 machine that they received from GCHQ many years ago, see the photo, but unfortunately friendly neighbours are in short supply at BP these days. Since the Bletchley Park Trust so to speak locked the gate on TNMOC they are no longer on speaking terms and TNMOC had to go abroad to find the sorely needed Tunny machine for their museum. Now thanks to FMU in Oslo TNMOC has closed the circle and they can now show the whole chain from the Germans enciphering their Top Secret messages on the Lorenz SZ42, the messages being intercepted at Knockholt and elsewhere, to the Wrens setting up and running the Colossus and finally decoding the messages on the Tunny.

With FMU showing the way with their generous gift I felt it was time for me to make a small contribution in the form of offering copies of the original instruction and operating manuals for the Lorenz SZ42 cipher machine. To my knowledge these documents are not so well known and they have never been made public before.