The 'Lorenz' Machine, 'Tunny', 'Robinson' and 'Colossus'

LINKS to other pages in the Bletchley Park site and to the Colin Day Travelling Days series:

1 : Bletchley Park Estate to 1939
2 : Enigma and GC&CS.
3 : The Poles and Enigma
4 : Turing and the Bombes
5 : The Huts: An overview
6 : German Naval Codes
7 : Huts 3,4,6 and 8
8 : Blocks A,B,C,D,E and F
9 : Views of the Estate (1)
10 : Views of the Estate (2)
11 : Lorenz and Colossus
12 : Finale, Links, Bibliography
Bletchley Park Guest Book:
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THE HIGH COMMAND of the German Army, having decided that a very high security device was required to enable it to communicate by radio, approached the Lorenz company to design and produce a suitable teleprinter machine. A method for enciphering teleprinter messages was invented by the American, Gilbert Vernam, in 1918 and employed a 32-symbol Baudot code which comprised five channels, each of which contained a stream of bits represented as ‘no-hole’ or ‘hole’, ‘0’ or ‘1’, and ‘dot or ‘cross’. A message was encoded by combining the original with a sequence of obscuring characters using modulo 2 addition (exclusive NOR in boolean terms). At the receiving end the message was decoded by once again adding the same sequence of obscuring characters. (Incidentally, the Enigma machine used the normal 26-letter alphabet and Morse code.)

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The Lorenz machine was multi rotor machine, a great advance on the Enigma three and four rotor machines. Teleprinter signals transmitted by the Germans using a Lorenz machine were first heard by the British in early 1940. At Bletchley Park a section, headed by Major Ralph Tester, was set up in B Block at Bletchley Park to examine Lorenz encoded messages. The section later moved to F Block which had been built close to Hut 11 in early 1943 (but has, since the war, been demolished). The Lorenz messages were given the general classification, ‘Fish’, and individual codes were provided with such names as ‘Tunny’ and ‘Jellyfish’.

Brigadier John Tiltman, who was working in what had then become known as 'The Testery, knew of the Vernam system and was able to identify messages that had been encoded using this method. Tiltman and Bill Tutte exploited a number of errors made by German radio operators and began to work out how the Lorenz encoding machine functioned. Eventually this led to the successful decoding of Lorenz messages.

   One of the problems, however, was the inordinate time taken to work out the correct settings; by the time the message was decoded the information was useless. Max Newman, a mathematician working at Bletchley, believed that it would be possible to automate some parts of the process for finding the settings. This would involve using two loops of paper tape, the first containing the original message to be decoded and the second containing a repeating pseudo random sequence. Each possible setting was to be tested and, hopefully, the machine would eventually produce the correct one.

A suitable machine, known as ‘Heath Robinson’, after the quirky cartoonist/designer, and also as a ‘Tunny Rack’, was designed at Newman’s request by TRE in Malvern and built by the Post Office Research Laboratories at Dollis Hill, London. ‘Heath Robinson’ was delivered to Bletchley Park and installed in Hut 11 which had earlier held the original Turing Bombes.

   Robinson consisted of three parts, the frame on which the two teleprinter paper tapes were mounted and read optically, known as the Bedstead, a rack containing the counters, another rack the valved logic circuits and a plug-jack panel. for plugging up the algorithms. The short counters rack was produced at TRE and the Bedstead and valve rack at the GPO research labs at Dollis Hill.

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However, although Robinson provided results it also proved to be unreliable because of the difficulties arising in synchronising the two tapes in the mechanical tape reader.

   Tommy Flowers, an electronics engineer at the Post Office Research Laboratories, managed to dispense with the paper tapes, replicating them electronically by using a large number of valves (vacuum tubes). Modified digital switching, using valves instead of mechanical relays, also increased the overall speed of the unit which was given the name,‘Colossus’.

A picture of the tenth and last Colossus machine to be installed at Bletchley Park. (left)

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Work on Colossus had begun in March 1943; in January 1944 the first unit was delivered to Bletchley Park. Combined with the ‘Tunny’ frame, Colossus was able to decode the Lorenz messages within hours. (N.B. Colossus was not used to decode the Enigma ciphers.) An improved version, Mark 2, was put into service in June 1944 in time to assist the Normandy campaign. By the end of the war a total of ten machines had been installed at Bletchley Park, five in the annexe of the now demolished F Block and five in H Block which is now the site of the current Colossus rebuild (and also houses most of the exhibits shown in this website). After the war eight of the machines were dismantled. The other two were removed to Easthope. It should be realised that, in spite of many statements to the contrary, Alan Turing did not design Colossus. Its mechanism, however, did rely upon the theories that Turing had used to solve the German Naval Enigma codes. And Turing's subsequent understanding that Colossus had functioned correctly confirmed his resolve to go ahead with his ideas for a more advanced computer.

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In the mid-1990's plans were put into place to rebuild a working replica of Colossus. On 6 June 1996 the Duke of Kent formally switched on the rebuild that had taken over two years to complete.

The rebuilt Colossus now on show at Bletchley Park. (left and above)

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