Cryptanalysis of the Lorenz cipher

Cryptanalysis of the Lorenz cipher was the process that enabled the British to read high-level German army messages during World War II. The British Government Code and Cypher School (GC&CS) at Bletchley Park decrypted many communications between the Oberkommando der Wehrmacht (OKW, German High Command) in Berlin and their army commands throughout occupied Europe, some of which were signed "Adolf Hitler, Führer". These were intercepted non-Morse radio transmissions that had been enciphered by the Lorenz SZ teleprinter rotor stream cipher attachments. Decrypts of this traffic became an important source of "Ultra" intelligence, which contributed significantly to Allied victory.

For its high-level secret messages, the German armed services enciphered each character using various online Geheimschreiber (secret writer) stream cipher machines at both ends of a telegraph link using the 5-bit International Telegraphy Alphabet No. 2 (ITA2). These machines were the Lorenz SZ (SZ for Schlüssel-Zusatz, meaning "cipher attachment") for the army, the Siemens and Halske T52 for the air force and the Siemens T43, which was little used and never broken by the Allies.

Bletchley Park decrypts of messages enciphered with the Enigma machines revealed that the Germans called one of their wireless teleprinter transmission systems "Sägefisch" (sawfish), which led British cryptographers to refer to encrypted German radiotelegraphic traffic as "Fish". "Tunny" (tunafish) was the name given to the first non-Morse link, and it was subsequently used for the Lorenz SZ machines and the traffic enciphered by them.

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