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Lorenz SZ-40/42   TUNNY
Teleprinter cipher attachment - wanted item

The SZ-40 was an electro-mechanical wheel-based cipher machine for teleprinter signals (telex). It was developed by Lorenz and used during WWII by the German Army for communication at the highest level. The machine was improved twice (SZ-42a and SZ-42b) and was broken during WWII by the codebreakers at Bletchley Park (UK), with the aid of Colossus, the first electronic digital computer. The SZ-40/42 was codenamed TUNNY by the codebreakers at Bletchley Park (BP).

During WWII, the German Army used a variety of cipher machines, of which the Enigma machine is probably known best. For secure teleprinter communication (telex) they used the Siemens T-52 Geheimschreiber, the Lorenz SZ-40, and later also the Siemens T-43 one-time pad machine.

The Lorenz SZ-40/42 was used by the German Army High Command (Oberst-Kommando der Wehrmacht, or OKW) for communication at the highest level, between Hitler and his Generals. The machine was called Schlüsselzusatz (SZ) which means Encryption Add-on. It was connected between a teleprinter and the line, and was suitable for both online and offline use.

Only a small number of SZ-40 and SZ-42 units were ever built. The image on the right shows one of the very few machines that have survived. It was found in Germany and is now on public display in the museum at Bletchley Park.

Another SZ-42 will be on display at the National Museum of Computing (TNMOC), also located at Bletchley Park. Please note that the Lorenz SZ-40/42 is often mistakenly called Geheimschreiber, for example in the 2012 BBC Documentary The Lost Heroes of Bletchley Park and on several web­sites. Although being a general description, the name Geheimschreiber (secret writer) is usually associated with the Siemens T-52 and not for the SZ-40/42. Although the two machines use a similar principle, they are not identical.

SZ040/42 in a showcase at Bletchley Park

Breaking Tunny
The majority of messages sent by the German army during WWII, were encrypted with the well-known Enigma machine. Such messages were always send via radio by means of morse code. Generally speaking, these were messages of a tactical nature. For the higest level in the command chain however, the Germans used a variety of far more complex cipher machines, all of which operated on teleprinter signals (telex). Three different cipher machines are known to have been used by the German High Command: the Siemens T-52, the Siemens T-43 and the Lorenz.

The Siemens T-52, also known as the Geheimschreiber, was mainly operated via landlines rather than via radio, which is why it was hard to intercept its messages. It was occasionally broken by BP and also by the Swedish Intelligence Agency during WWII. The Siemens T-43 was a so-called mixer machine that operated as a One-Time Pad and was therefore never broken.

The first non-morse transmissions by the Germans were intercepted in the second half of 1940. Such transmissions were generally called FISH by BP, and fish-names were used for the various networks. In June 1941 the Germans started the first expirimental link with a Lorenz SZ-40 machine, which was called TUNNY by BP.

In August 1941, the first breakthrough was achieved as two long messages were received in depth. In the event the receiving end notified the initiator of the message that it had not been received and requested it to be sent again. The operator at the sending end retransmitted the message using the same message key (i.e. the same start positions of the wheels). As he was typing the message by hand however, he started making mistakes and occasionally used short cuts. This led to small variations in the cipher text that enabled the codebreakers, lead by mathematician Bill Tutte, to recover nearly 4000 characters of the key stream.

By January 1942, the codebreakers had deduced the working principle of the Lorenz machine, unsing nothing but the recovered key stream from the August 1941 message.

The British Tunny
Once the codebreakers at Bletchley Park had figured out how the TUNNY worked by January 1942, it was possible to build a functional replica. The replica machine was called the British Tunny and was developed at Tommy Flowers' laboratory at Dollis Hill by Gil Hayward, Allen (Doc) Coombs, Bill Chandler and Sid Broadhurst, without ever having seen a real Lorenz SZ-40/42 machine.

It is only after the D-Day landings in 1944, that the first Lorenz machine was captured and was shown to the codebreakers. They were amazed to see the relatively small mechanical machine of which they had created the electronic equivalent.

A fully functional replica of the British Tunny is on public display at The National Museum of Computing (TNMOC) in Block H at Bletchley Park. The image on the right shows the British Tunny being explained by volunteer Kevin Coleman.

Heath Robinson
The first machine to assist in decoding the Tunny messages, was the Heath Robinson. It was developed by a Tommy Flowers and Frank Morell of the General Post Office (GPO) research station at Dollis Hill (London). Flowers had been introduced to Max Newman by Alan Turing who admired Flowers for his insight in electronics and the use of electronic thermionic valves (vacuum tubes).

Heath Robinson had two paper tape readers, one for the cipher text and one for the Chi-wheel starting positions that were tested at 1000 characters per second. Stretching of the tape at this high speed however, made it difficult to maintain synchronisation between the two tapes.

Heat Robinson was recreated by Tony Sale in 2001 while he was temporarily unable to work on his Colossus Rebuild (see below). It is now on display at the computer museum (TNMOC).

 More about Heath Robinson


Once the first Heath Robinson machines were up and running, developer Tommy Flowers started to work out his plans for a far more advanced machine that would not only be faster than Heat Robinson, but also more flexible. It turned out to be the first programmable electronic computer.

Because the machine contained more than 1700 thermionic valves (vacuum tubes) and its enormous size, Flowers called it Colossus. It barely fitted a room and produced a lot of heat, but it was five times faster than Heath Robinson and could be programmed for specific tasks.

Because of this, Colossus is now generally accepted as the first programmable electronic computer. In any case, it was conceived before the American ENIAC, which for a long time was thought to be the first. This was not known at the time as Colossus was kept secret until 1974.

The first Colossus was delivered at Bletchley Park on 18 January 1944 and broke its first message on 5 February of that year. It was succeeded by Colossus Mark II, which consisted of no less than 2400 valves. The first Colossus Mark II was delivered on 1 June 1944 — just five days before the D-Day landings on the Normandy coast — and immediately produced good intelligence. In total 10 Colossi were installed before the end of the war. After the war all Colossi were dismantled.

In 1993, a team led by Tony Sale, inspired by Professor Brian Randell, started the reconstruction of Colossus, starting with nothing more than 8 wartime photographs. It took him 16 years and a lot of detective work, but in the end he got Colossus running again. It is now on display at The National Museum of Computing (TNMOC) in Block H at Bletchley Park.

 More about Colossus

Known locations
  • SZ-40
  • SZ-42a
  • SZ-42b
  • SZ-42c
SZ-42 Simulator
Release 23 January 2018

Jerry McCarthy in the UK, has made a fully operational SZ-42 (Tunny) simulator that runs on nearly any Windows® computer. It even has a Colossus simulation function, and comes with full documentation in PDF format.

Although developed as a 32-bit application, the simulator runs happily on 64-bit Windows 7 and also on Windows 10. Nevertheless, no warranties are made and the software is supplied as-is. Use it at your own risk (read the disclaimer below).

 Download the simulator


DISCLAIMER — Although the software presented here was thoroughly tested and every effort was made to ensure that it is free from errors, such can not be guaranteed. Neither the author – Jerry McCarthy – nor Crypto Museum can be held responsible for any loss or damage, direct or indirect, arising from the use or misuse of the software. Furthermore, we can not guarantee the fitness of the software for any purpose.

Wanted item
As only a few Lorenz SZ-40/42 machines have survived the war, the chances are very slim that we will ever find one on the surplus market. Nevertheless, we have listed it here as a wanted item, as we think it is needed to tell the other half of the story of Bletchley Park. Until that time, this page will be used as a placeholder for information about the Lorenz SZ-40/42. If you have any information that you want to share with us, please contact us.

  1. Der Schlüsselzusatz 42 zum Fernschreiber, Heft I, Vorläufige Gerätebeschreibung
    The Cipher Attachment 42 for Teleprinter, Volume I - Preliminary Device Description.
    15 May 1944. TICOM document No. 663. 1
     Heft I, Zeichnungen (Volume 1, Drawings)
     English translation: Volume I, Provisional Descriptions

  2. Der Schlüsselzusatz 42 zum Fernschreiber, Heft II - Vorläufige Betriebsvorschrift
    The Cipher Attachment 42 for Teleprinter, Volume II - Preliminatry Operating Instructions.
    15 May 1944. TICOM document No. 664. 1
     English translation: Volume II, Tentative Rules for Cipher Attachment 42

  3. SZ-42, Heft III - Gebrauchsanleitung für die Schlüsseleinrichtungen und -unterlagen
    The Cipher Attachment 42 for Teleprinter, Volume III - User Instructions for the Cipher Key and Key Material. 20 May 1944. TICOM document No. 665. 1
     English translation: Volume III, Instructions for Use of Keys and Key Material
  1. Common title for these documents: Der Schlüsselzusatz 42 zum Fernschreiber.
  2. Document kindly provided by Frode Weierud's CryptoCellar.

  1. B. Jack Copeland, Colossus, Breaking the German Tunny Code at Bletchley Park
    An illustrated history. The Rutherford Journal, Volume 3, 2010.

  2. Tony Sale, The Lorenz Cipher - and how Bletchley Park broke it
    Retrieved June 2011.

  3. Tony Sale, Colossus Rebuild Project
    Retrieved June 2011.

  4. Tony Sale, Colossus 1943-1996,
    And How it Helped to Break the German Lorenz Cipher in WWII.
    ISBN 978-0947712365.

  5. Wikipedia, Lorenz cipher
    Retrieved January 2014.

  6. Wikipedia, Cryptanalysis of the Lorenz cipher
    Retrieved January 2014.

  7. Frode Weierud, The Secrets of the Lorenz Schlüsselzusatz SZ42
    Website. 30 April 2016.
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Crypto Museum. Created: Tuesday 10 January 2012. Last changed: Thursday, 11 January 2024 - 10:50 CET.
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