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, Fuhrer". These were intercepted non-Morse radio transmissions that had been enciphered by the Lorenz SZ teletypewriter rotor stream cipher attachments. Decrypts of this traffic became an important source of "Ultra" intelligence.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 Schlüsselzusatz, meaning "cipher attachment") machine 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 teleprinter traffic as "Fish". "Tunny" 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.As with the entirely separate Cryptanalysis of the Enigma, it was German operational shortcomings that allowed the initial diagnosis of the system, and a way into decryption. Unlike Enigma, no physical machine reached allied hands until the very end of the war in Europe, long after wholesale decryption had been established. Initially, operator errors produced a number of pairs of transmissions sent with the same keys, giving a "depth", which often allowed manual decryption to be achieved. One long depth also allowed the complete logical structure of the machine to be worked out, a quite remarkable cryptanalytical feat on which the subsequent comprehensive decrypting of Tunny messages relied.When depths became less frequent, decryption was achieved by a combination of manual and automated methods. The first machine to automate part of the decrypting process was called "Heath Robinson" and it was followed by several other "Robinsons". These were, however, slow and unreliable, and were supplemented by the much faster and flexible "Colossus" the world's first electronic, programmable digital computer, ten of which were in use by the end of the war.Albert W. Small, an American cryptographer from the US Signal Corps who was seconded to Bletchley Park and worked on Tunny, said in his December 1944 report back to Arlington Hall that:Daily solutions of Fish messages at GC&CS reflect a background of British mathematical genius, superb engineering ability, and solid common sense. Each of these has been a necessary factor. Each could have been overemphasised or underemphasised to the detriment of the solutions; a remarkable fact is that the fusion of the elements has been apparently in perfect proportion. The result is an outstanding contribution to cryptanalytic science.
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