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FISH and I By William T. Tutte. Professor William T. Tutte worked from 1941 to 1945 in the British cryptanalytic headquarters at Bletchley Park, the most succesfull intelligence agency in world history. His work, which combined elements of statistics and combinatorics, was instrumental in the breaking of FISH, a series of codes that were used by the German command for encrypting communications between the highest authorities ( FISH was the nick used by the Bletchley Park's team to name all intercepted traffic generated by the german Lorenz SZ40/SZ42, or TUNNY, machines ). Subsequent cryptanalytic work on FISH included the development of Colossus, the world's first electronic computer. Part 3 of this book tells the story of FISH. It tells that the first FISH traffic to be intercepted was on a German Army radio link between Athens and Vienna from the middle of 1941. The letters used were those of the International Teleprinter Code. There were two basic symbols, called at Bletchley Park: Dot and Cross. They would equally well have been called: Zero and One. Or, with electrical switches in mid: On and Off. Each letter was a sequence of 5 basic symbols, so there were 32 letters in all. Table 1 sets out this International code. In an additive cipher we convert the clear message ( C ) into the cipher message ( Z ) by adding to it, letter by letter, a sequence of letters called the Key ( K ).
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