Lorenz SZ 40/42
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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 termed the machine and its traffic "Tunny". While the well-known Enigma machine was generally used by field units, Tunny was used for high-level communications which could support the heavy machine, teletypewriter and attendant fixed circuits. The machine itself measured 20in × 18in × 18in, and served as an attachment to a standard Lorenz teleprinter. The machines implemented a stream cipher.
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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 <math>\chi<math> wheels, and five of which were stepped irregularly, termed the <math>\psi<math> wheels. The stepping of the <math>\psi<math> 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.
Colossus, the world's first electronic digital programmable computer, was built to attack Tunny.
See also
References
- Jack Good, Donald Michie, and Geoffrey Timms, General Report on Tunny, 1945, HW 25/4 and HW 25/5 [1].
External links
de:Lorenz-Chiffre
