- For the fish, see Tuna.
The Lorenz SZ 40 and SZ 42 (Schlüsselzusatz, meaning "cipher attachment") were German cipher machines used during World War II for teleprinter circuits. British codebreakers, who referred to encrypted German teleprinter traffic as "Fish", termed the machine and its traffic "Tunny". While the well-known Enigma machine was generally used by field units, the Lorenz machine was used for high-level communications which could support the heavy machine, teletypewriter and attendant fixed circuits. The machine itself measured 20in × 18in × 18in (51cm × 46cm × 46cm), and served as an attachment to a standard Lorenz teleprinter. The machines implemented a stream cipher.
Operation  The Lorenz machine had 12 wheels containing 501 pins. 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 wheels, and five of which were stepped irregularly, termed the ψ wheels. The stepping of the ψ wheels was determined by two more pinwheels. 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.
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 4,000 character message was transmitted; however, the message was not received correctly at the other end, so the message was retransmitted with the same key settings — a practice forbidden by procedure. Moreover, the second time the operator made a number of small alterations to the message, such as using abbreviations. From these two related ciphertexts, John Tiltman was able to recover both the plaintext and the keystream. From the keystream, the entire structure of the machine was reconstructed by W. T. Tutte.
Tunny traffic was intercepted at Knockholt in Kent, before being sent to Bletchley Park.
Several complex machines were eventually 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 help break into Tunny.
The next was the Colossus, the world's first electronic digital computer (although, 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 read Tunny traffic essentially at will.
See also
Further reading - Brian Johnson, The Secret War (BBC, London, Methuen, New York, 1978) Contains a mostly accurate section (pages 338-347) on Tunny, and the creation of Colossus to break into it.
- 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.
References - Jack Good, Donald Michie, and Geoffrey Timms, General Report on Tunny, 1945, HW 25/4 and HW 25/5 [1] (http://www.alanturing.net/turing_archive/archive/index/tunnyreportindex.html).
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