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General Report on Tunny
The 505-page typescript document General report on Tunny: With Emphasis on Statistical Methods was written at the Government Code and Cypher School (GC&CS) at Bletchley Park in 1945 but remained classified until the year 2000. It is, in its own words, "...an account of machine and statistical methods for breaking Tunny ciphers." Tunny was the codename given by the British to the cipher that was a product of the Lorenz SZ40, SZ42a and SZ42b machines, which were not seen by anyone on the allied side in World War II until Field Marshall Albert Kesselring surrendered in early May 1945. Its titular authors are I.J. (Jack) Good, Donald Michie and Geoffrey Timms, but probably contains extracts from the work of others, as the authors express thanks in particular to the reports of the Research Section, the Testery and the section dealing with Signals Intelligence and Traffic Analysis (Sixta) "which have in many places been quoted verbatim". The subtitle relates to a complementary documentent entitled Solution of German Teleprinter Cyphers (Testery) Linguistic Methods which is listed in the National Archives (Ref. HW 25/28) but which, at the time of writing in 2021, has not been declassified.