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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 teletypewriter 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 ''Schluesselzusatzgeraet'', 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. The cryptanalysts did not see one of these machines until after the end of the war in Europe. 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 wholesale 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 more flexible "Colossus", the world's first electronic, programmable digital computer, ten of which were in use by the end of the war, by which time some 90% of Tunny messages were being decrypted at Bletchley Park. 〔All but two of the Colossus computers, which were taken to GCHQ, were destroyed in 1945, and the whole project was kept strictly secret until the 1970s. Thus Colossus did not feature in many early descriptions of the development of electronic computers. 〕 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: ==The German Tunny machines== The Lorenz SZ cipher attachments implemented a Vernam stream cipher, using a complex array of twelve wheels that delivered what should have been a cryptographically secure pseudorandom number as a key stream. The key stream was combined with the plaintext to produce the ciphertext at the transmitting end using the exclusive or (XOR) function. At the receiving end, an identically configured machine produced the same key stream which was combined with the ciphertext to produce the plaintext, i. e. the system implemented a symmetric-key algorithm. The right hand five wheels, the ''chi'' () wheels, changed the five impulses (bits) of the incoming character, advancing one position every time. The left hand five, the ''psi'' () wheels, further changed the result of the ''chi'' transform, but they did not always move on with each new character. The central two ''mu'' () or "motor" wheels determined whether or not the ''psi'' wheels rotated with a new character. The enhanced SZ42A and SZ42B machines had a more complex arrangement for advancing the ''psi'' wheels than the original SZ40. Each wheel had a number of cams that could be set in one of two positions. The numbers of cams on the set of wheels were co-prime with each other giving an extremely long period before the key sequence repeated. The process of working out which of the 501 cams were in the raised position was called "wheel breaking" at Bletchley Park. Deriving the start positions of the wheels for a particular transmission was termed "wheel setting" or simply "setting". The fact that the ''psi'' wheels all moved together, but not with every input character, was a major weakness of the machines that contributed to cryptanalytical success. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cryptanalysis of the Lorenz cipher」の詳細全文を読む スポンサード リンク
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