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. Previously the lack of a good simulator for this machine as well as authentic ciphertexts meant that it was difficult to get a good feeling for how the machine works and to assess its basic security.

Authentic Lorenz SZ42 ciphertexts are still a scarce commodity, however now we have a really nice simulator for the Lorenz SZ40 and SZ42 machines written by Martin Gillow and aptly named the Virtual Lorenz. The Virtual Lorenz is able to simulate the SZ40, SZ42a and SZ42b as well as the new model, developed at very end of the war, the SZ42C. On top of that the simulator is extremely easy to use as it runs in your Web browser. There is no installation to worry about and it runs on all platforms and all operating systems. Just the thing you need to learn more about the Lorenz Schlüsselzusatz.

I hope that the information I am planning to publish here will help to increase the knowledge about this machine. 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.

The majority of the TICOM documents released here are available in the TICOM collection of the German Foreign Office’s Political Archive.
Source: Bestand Rückgabe TICOM, Politisches Archiv des Auswärtigen Amts, Berlin.

Last update: 25 May 2017.


Lorenz SZ42 Publications

Here is a list of some papers and articles that describes the Lorenz SZ42, the Colossus and the work at Bletchley Park to break the machine.

Historical Documents

Here are some more historical documents related to the Lorenz SZ42.

German Manuals and Operating Instructions

Here are the instruction and operating manuals for the Lorenz SZ42 as well as operating manuals for the ancillary equipment.

Related Information

Lorenz Messages and Keys

When the SZ42 came into service in 1942 the SZ40 did not cease to be used. As shown in the document below,
listing the keys in use at the German Foreign Office’s teleprinter office in October‐November 1943, the SZ40
continued to be used on links connected over cable connections. Even in April‐May 1945 keys were issued for
the SZ40.

Lorenz Related Documents and Papers

Lorenz Simulators and Software

Lorenz SZ42 Links

Here are a few links to other sources of Lorenz SZ42 information.



Creative Commons License
This work is licensed under a
Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
The material described on these pages is created, collected, edited and published
by Frode Weierud, © April 2016

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