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Colossus was the name of a series of computers developed by British codebreakers in 1943-1945 to help in the cryptanalysis of the Lorenz cipher. Colossus used thermionic valves (vacuum tubes) and thyratrons to perform Boolean and counting operations. Colossus is thus regarded as the world's first programmable, electronic, digital computer, although it was programmed by plugs and switches and not by a stored program. Colossus was designed by the engineer Tommy Flowers to solve a problem posed by mathematician Max Newman at the Government Code and Cypher School (GC&CS) at Bletchley Park. Alan Turing's use of probability in cryptanalysis〔See Banburismus〕 contributed to its design. It has sometimes been erroneously stated that Turing designed Colossus to aid the cryptanalysis of the Enigma. Turing's machine that helped decode Enigma was the electromechanical Bombe, not Colossus. The prototype, Colossus Mark 1, was shown to be working in December 1943 and was operational at Bletchley Park on 5 February 1944. An improved Colossus Mark 2 that used shift registers to quintuple the processing speed, first worked on 1 June 1944, just in time for the Normandy Landings on D-Day. Ten Colossi were in use by the end of the war and an eleventh was being commissioned. Bletchley Park's use of these machines allowed the Allies to obtain a vast amount of high-level military intelligence from radiotelegraphy messages between the German High Command (OKW) and their army commands throughout occupied Europe. The destruction of the Colossus hardware and blueprints, as part of the effort to maintain a project secrecy that was kept up into the 1970s, deprived most of those involved with Colossus of credit for their pioneering advancements in electronic digital computing during their lifetimes. A functioning replica of a Colossus computer was completed in 2007 and is on display at The National Museum of Computing at Bletchley Park. ==Purpose and origins== The Colossus computers were used to help decipher radio teleprinter messages that used had been encrypted using an unknown device. The British called encrypted German teleprinter traffic "Fish", and the unknown machine and its intercepted messages "Tunny". Before the Germans increased the security of their operating procedures, British cryptanalysts diagnosed how the machine functioned and built a machine that emulated it, (British Tunny). It was deduced that the machine had twelve wheels and used a Vernam ciphering technique on message characters in the standard 5-bit ITA2 code. It did this by combining the plaintext characters with a stream of key characters using the XOR Boolean function to produce the ciphertext. In August 1941, an operating blunder led to the transmission of two versions of the same message with identical machine settings. These were intercepted and worked on at Bletchley Park. First, John Tiltman, a very talented GC&CS cryptanalyst derived a key stream of almost 4000 characters. Then Bill Tutte, a newly arrived member of the Research Section, used this key stream to work out the logical structure of the Lorenz machine. He deduced that the twelve wheels consisted of two groups of five, which he named the χ (''chi'') and ψ (''psi'') wheels, the remaining two he called μ (''mu'') or "motor" wheels. The ''chi'' wheels stepped regularly with each letter that was encrypted, while the ''psi'' wheels stepped irregularly, under the control of the motor wheels. With a truly random key stream, a Vernam cipher removes the natural language property of a plaintext message of having an uneven frequency distribution of the different characters, to produce a uniform distribution in the ciphertext. The Tunny machine worked well in this respect, but the cryptanalysts worked out that examining the character-to-character changes of character streams, instead of the frequency distribution of ciphertext characters, showed a departure from uniformity which provided a way into the system. This was achieved by "differencing" in which each bit or character was XOR-ed with its successor. After Germany surrendered, allied forces captured a Tunny machine and discovered that it was the electromechanical Lorenz SZ (''Schluesselzusatzgeraet'') in-line cipher machine. In order to decrypt the transmitted messages, two tasks that had to be performed. The first was "wheel breaking", which was the discovery of the cam patterns for all the wheels. These patterns were set up on the Lorenz machine and then used for a fixed period of time for a succession of different messages. Each message was enciphered with a different start position of the wheels. Alan Turing invented a method of wheel-breaking that became known as Turingery. Turing's technique was further developed into "Rectangling" for which Colossus could produce tables for manual analysis. Colossi 2, 4, 6, 7 and 9 had a "gadget" to aid this process. The second task was "wheel setting", which worked out the start positions of the wheels for a particular message, and could only be attempted once the cam patterns were known. Colossus was initially designed to discover the start position of the ''chi'' wheels for a message, not for completely decrypting it. To do this it compared two character streams, counting statistics based on a succession of programmable Boolean functions. The ciphertext was read at high speed from a paper tape. The key stream was generated internally, in a simulation of the Lorenz machine. Colossus also produced frequency counts of the characters in processed ciphertext, which were essential in checking the settings that had been found. 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「Colossus computer」の詳細全文を読む スポンサード リンク
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