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Siemens T-43   Sägefisch
The first mixer machine - wanted item

The T-43 was a One-Time Tape (OTT) cipher machine for teleprinter signals (telex), developed by Siemens & Halske in 1943 and introduced in 1944, close towards the end of WWII. Machines like the T-43 are commonly called mixers, as they mix the clear text with data from a key tape. The key tape is used as a One-Time Pad (OTP). The T-43 was arguably the first machine in this class.

The official name for the machine is Schlüssel-Fernschreibmaschine SFM T-43. It is based on the earlier T-37 teleprinter, and has a paper-tape reader/puncher to the left of the keyboard.

For each character typed on the keyboard, a random character is read from the key tape and mixed with the original character by means of an XOR-operation. Once the key character is read from the paper tape, it is destroyed by the built-in puncher, so that it can not be used again.

Images of the T-43 are extremely rare and they all seem to originate from the same -single- source. The image on the right is currently the best one we have been able to locate.
  

The T-43 was codenamed Sägefisch (Sawfish) by the Germans, because of the typical sound of its modulated signal. The codebreakers at Bletchley Park called it 'Trasher'. It is currently unknown how the key tapes were produced and distributed, but according to eye witness Georg Glünder, the designers said that the tapes were created by Random Number Generators (RNG) and that they did not have a cryptographic period [1]. This is contradicted by [4] however.

The principle of the T-43 mixer machine is based on the so-called Vernam Cipher invented by Gilbert Vernam in the US in 1917. Gilbert Vernam was also the inventor of the stream cipher and the co-inventor of the One-Time Pad (OTP). According the NSA, it is probably one of the most important inventions in the history of cryptography. Nevertheless the Germans were probably the first to develop a practical implementation of it for the encryption of telegraph signals: the T-43.

Captured machines
The T-43 entered service relatively late in the war and only a modest number of machines was built. It is believed that between 30 and 50 machines were produced [3]. At the end of the war, some T-43s were found and confiscated by the Allies. It is known that the American TICOM-commission shipped six T-43 machines to the US and an equal number of German crypto-experts. The machines that were captured in Norway, were shipped to Bletchley Park (UK) [3].

It will come as no surprise that the T-43 was kept secret by the Allies for many years. Strangely enough though, the history of the machine is still a mystery today. At this time (2013) no surviving machines are known to the public and further information from British and American archives has not yet been released. Shortly after WWII, the US, and later also the new Allied organisation NATO, started using ETCRRM mixer machines that were based on the T-43 principle. These machines were made by the Norwegian manufacturer STK and sold to the US and to NATO.

Weaknesses
If the key tapes that were used with the T-43, consisted of a truely random sequence of characters, the machine would theoretically have been unbreakable. According to [4] however, the key sequence was pseudo-random as it was generated by two Siemens T-52e machines.

Another weakness was caused by TEMPEST problems. The delay of the cipher relay caused a phase-shift between the clear text and the cipher text, which was visible on an oscilloscope. This unwanted side-effect was discovered by the German crypto-experts of OKW-Chi. As this allowed the clear text to be reconstructed from the transmitted signal, extra filters were later added to the existing T-43 machines. Meanwhile, Siemens developed an improved machine with a different construction that would not exhibit the TEMPEST problem, but the war had ended before that machine could be released.

It is not known whether or not the above weaknesses were exploited by Bletchley Park, but no evidence to this effect has been found so far. Cryptanalysts at BP initially thought that the key tapes were produced by a Lorenz SZ-42 as they found regularities in the key sequence. This is probably the reason why the codebreakers chose a fish-name for this machine (Trasher).

Patents
Over the years, many have claimed to have invented the principle of the mixer machine. In the late 1950s, the Dutch PTT filed a design for such a machine that later became known as the Ecolex cipher machine. Although a patent was granted, they were by no means the first ones to have 'invented' it. Several years earlier, in 1952, a similar patent was filed by Bjørn Røhrholdt and Kåre Meisingset of STK in Norway. That machine became known as the ETCRRM and was soon one of the favorites of the Americans. It was later used on the Washington-Moscow hotline.

Snapshot from German Patent DE371087. Click to read the complete document.

All these patents can be declared prior art as Siemens developed the T-43 in 1943 and it was based on a (Siemens) patent filed back in 1921. The image above shows part of the main drawing of German Patent DE371087 in which the two tape readers are clearly visible side-by-side.

But even the German patent would probably not have survived a potential lawsuit. It was the American Gilbert Vernam who filed his design for the same concept first, on 13 September 1918. Although his drawings are not identical to the drawing of the German patent, there are some remarkable similarities. In the patent, Siemens recognizes earlier developments in this field and adds online use to it. The T-43 might therefore be the first to use it for online encryption.

First
Although Gilbert Vernam's invention dates back to 1918, in his paper of 1926 [7] he describes the keystream as a mixture of two looped punched-paper tapes of a different length. This means that the cipher period is finite. The Telekrypton machine of 1933 was based on this principle.

In 1942, at the request of British Intelligence, Canadian communications expert Benjamin deForest Bayly converted the American Telekrypton into a real One-Time Tape (OTT) cipher machine. The machine is ready in 1943 and becomes known as Rockex. Around the same time, another machine, known as the 5-UCO enters service in the UK. For the remainder of the war it is used for the distribution of Ultra Intelligence. As all three systems are introduced around the same time, it seems fair to recognise the T-43, the Rockex and the 5-UCO as the first mixers.

Help required
Please note that this page is still under construction and that it currently only acts as a placeholder for information about the Siemens T-43. If you have any information about this machine that you can share, please contact us.

References
  1. Georg Glünder, Als Funker und 'Geheimschreiber' im Krieg 1941-1945
    München (Germany). PIONIER 11/12-1989, 1/2-1990. First publication.
    Reproduction 27 March 1990. Later reproduced in [2].

  2. Georg Glünder, Als Funker un 'Geheimschreiber' im Krieg 1941-1945
    The Enigma Bulletin. No. 1, December 1990. German.

  3. Klaus Schmeh, Hitlers letzte Maschinen
    Hitler's last machines (German). 2 August 2004. Retrieved May 2012.

  4. Michael Pröse, Chiffriermaschinen und Entzifferungsgeräte im Zweiten Weltkrieg:
    Technikgeschichte und informatikhistorische Aspekte.
    Dissertation, Leipzig, December 2004. German.

  5. German Patent DE371087
    Patent describing the principle of a mixer machines. Filed: 10 July 1921.

  6. Gilbert Vernam, US Patent US1310719
    First patent describing a machine for 'mixing' telegraph signals.
    Filed: 13 September 1918.
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Crypto Museum. Created: Wednesday 15 February 2012. Last changed: Saturday, 14 May 2016 - 09:21 CET.
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