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Operation
The teleprinters of the day output each character as five parallel bits on five lines, typically encoded in the Baudot code or something similar. The Lorenz machine output groups of five pseudorandom bits to be XORed with the plaintext. The pseudorandom bits were generated by ten pinwheels, five of which stepped regularly, termed the χ ("chi") wheels, and five of which were stepped irregularly, termed the ψ ("psi") wheels. The stepping of the ψ wheels was determined by two more pinwheels, termed the "motor wheels". Apart from the stepping of the five irregular pinwheels (which either all stepped together, or all stayed together), the Lorenz machine is actually five parallel pseudorandom generators; there is no other interaction between the five lines. The numbers of pins on all the wheels were relatively prime.
Colonel Parker Hitt of the US Army first proposed a very similar device in 1914. [1]. He explained its use in his Manual for Military Ciphers published in 1916. But Col. Hitt's design was without the feature that allowed the stepping of five wheels to be irregular. [2]
Cryptanalysis
British cryptographers at Bletchley Park had deduced the operation of the machine by January 1942 without ever having seen a Lorenz machine. This was made possible because of a mistake made by a German operator. On 30 August 1941, a 4,500 character message was transmitted; however, the message was not received correctly at the other end, so (after the recipient sent an unencoded request for retransmission, which let the codebreakers know what was happening) the message was retransmitted with the same key settings (HQIBPEXEZMUG); a forbidden practice. Moreover, the second time the operator made a number of small alterations to the message, such as using abbreviations (The second message was only 4000 characters long). From these two related ciphertexts, John Tiltman and his team were able to recover both the plaintext and the keystream. From the keystream, the entire structure of the machine was reconstructed by W. T. Tutte.
Originally the Tunny traffic was intercepted at the Foreign Office Y Station operated by the Metropolitan Police at Denmark Hill in Camberwell, London but due to lack of resource at this time (~1941) it was given a low priority. A new station, Knockholt in Kent, was later constructed specifically to receive Tunny traffic so that the messages could be efficiently recorded and sent to Bletchley Park. The head of Y station, Harold Kentwothy, moved to head up Knockholt. He was later promoted to head the Foreign Office Research and Development Establishment (F.O.R.D.E).
Several complex machines were built by the British to attack Tunny. The first was a family of machines known as "Heath Robinsons", which used several high-speed paper tapes, along with electronic logic circuitry, to find the pin wheel settings of the Lorenz machine.
The next was the Colossus, the world's first electronic digital computer. This was developed by the British G.P.O engineer Tommy Flowers at Dollis Hill in London (the Post Office research station). Like ENIAC, it did not have a stored program, and was programmed through plugboards and jumper cables. It was both faster and more reliable than the Heath Robinsons; using it, the British were able to speed up the process of finding the Lorenz pin wheel settings.
The third machine was the Tunny Emulator, the machine designed by Blechley Park based on the reverse engineering work done by Tiltman's team in the Testery to emulate the Lorenz Cypher Machine. When the pin wheel settings were found by colossus the tunny machine was set up and run so the messages could be read.
The Swedish cryptanalytic service, the FRA (Försvarets Radioanstalt), also broke into an early version of the Lorenz system; their break occurred in April, 1943. They tapped cables carrying traffic between Germany and Norway. The work was led by mathematician Arne Beurling.
See also
Further reading
- Stephen Budiansky, Battle of Wits (Free Press, New York, 2000) Contains a short but informative section (pages 312-315) describing the operation of Tunny, and how it was attacked.
- Paul Gannon "Colossus: Bletchley Park's Greatest Secret" (Atlantic Books, 2006) Using recently declassified material and dealing exclusively with the efforts to break into Tunny. Clears up many previous misconceptions about Fish traffic, the Lorenz cipher machine and Colossus.
- F. H. Hinsley, Alan Stripp, Codebreakers: The Inside Story of Bletchley Park (Oxford University, 1993) Contains a lengthy section (pages 139-192) about Tunny, the British attack on it, and the British replicas of the Lorenz machine.
- Michael Smith, Station X: Decoding Nazi Secrets (TV Books, New York, 2001) Contains a lengthy section (pages 183-202) about Tunny and the British attack on it.
References
- Jack Good, Donald Michie, and Geoffrey Timms, General Report on Tunny, 1945, HW 25/4 and HW 25/5
- W. T. Tutte, FISH and I Transcript of a lecture by Prof. Tutte explaining how he broke into Tunny.
- Donald W. Davies, The Lorenz Cipher Machine SZ42, (reprinted in Selections from Cryptologia: History, People, and Technology, Artech House, Norwood, 1998)
- Entry for "Tunny" in the GC&CS Cryptographic Dictionary
- The Lorenz Cipher and how Bletchley Park broke it by Tony Sale
External links
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