Tunny (cryptography)

The Lorenz SZ40, SZ42, SZ42A and SZ42B were German rotor stream cipher machines used by the German Army during World War II. They were developed by C. Lorenz AG in Berlin. The model name SZ was derived from Schlüssel-Zusatz, meaning cipher attachment. The instruments implemented a Vernam stream cipher.

British cryptographers, who referred to encrypted German teleprinter traffic as Fish, dubbed the machine and its traffic Tunny (meaning tunafish) and deduced its logical structure three years before they saw a machine.

The SZ machines were in-line attachments to standard teleprinters. An experimental link using SZ40 machines was started in June 1941. The enhanced SZ42 machines were brought into substantial use from mid-1942 onwards for high-level communications between the German High Command in Wünsdorf close to Berlin, and Army Commands throughout occupied Europe. The more advanced SZ42A came into routine use in February 1943 and the SZ42B in June 1944.

Wireless telegraphy (WT) rather than land-line circuits was used for this traffic. These non-Morse (NoMo) messages were picked up by Britain's Y-stations at Knockholt and Denmark Hill and sent to Government Code and Cypher School at Bletchley Park (BP). Some were deciphered using hand methods before the process was partially automated, first with Robinson machines and then with the Colossus computers. The deciphered Lorenz messages made one of the most significant contributions to British Ultra military intelligence and to Allied victory in Europe, due to the high-level strategic nature of the information that was gained from Lorenz decrypts.

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