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ABSTRACT: Lists what the Sinkov Mission received during their visit to Bletchley Park in early 1941, and the information subsequently given by Bletchley to US Navy codebreakers in OP-20-G about naval Enigma. Also considers how far OP-20-G progressed in its attack on Enigma using that information.
KEYWORDS: Bletchley Park, bombes, Currier, Denniston, Driscoll, Enigma, OP-20-G, Purple, Rosen, Sinkov, Weeks, Tiltman, Wenger.
The Sinkov Mission was a joint US Army and Navy team headed by Captain Abraham Sinkov from the Army's Signal Intelligence Service (SIS). Sinkov's Army colleague was Lieutenant Leo Rosen, a mathematician, while the Navy members were Lieutenant (jg) Robert Weeks, a Japanese linguist, and Ensign Prescott Currier, both from the US Navy's codebreaking unit, OP-20-G.1 The Mission arrived at the British Government Code and Cypher School (the codebreaking center at Bletchley Park (BP), Buckinghamshire) around 7 February 1941, and delivered an American clone of the Japanese diplomatic service's cipher machine (codenamed "Purple" by the SIS), to the British codebreakers, together with a significant amount of other codebreaking material.2
At least one author has claimed that Britain double-crossed America by reneging on a deal to give the Mission an Enigma machine.3 The principal work on Anglo-American cryptanalytical co-operation more soberly concluded that the Sinkov Mission "arrived back in the United States with little to show for their strenuous efforts except general impressions of Bletchley and British intelligence practice."4 Although a recent paper has shown that, despite an apparent claim to the contrary by Sinkov, his group was shown the bombes used to break Enigma, with the express approval of Prime Minister Winston Churchill, when that paper was published relatively little was known about what material the Mission had acquired from BP.5 Indeed, few details are still available about the documents received by the Army members, although it is known that Sinkov was given a substantial amount of information about German, Italian and Russian codes and ciphers, and other matters, including the use of punched card machines for preparing difference tables for super-enciphered codes.6
Following Churchill's approval to Sinkov's group being shown the bombes, Commander Alastair Denniston,7 the operational head of BP, informed Brigadier Stewart Menzies, BP's Director (and head of the British Secret Service, MI 6), that "Our American colleagues have been...