Memorandum to OP-20-G on Naval Enigma (c.1941)

Abstract

The document that forms this chapter was written by Alan Turing and sent to the US Navy codebreaking unit, OP-20-G, in Washington, DC, probably to the celebrated cryptanalyst Mrs Agnes Driscoll. It is undated, but was probably dispatched in the autumn of 1941. Turing solved the indicating system of the principal Naval Enigma cipher, Heimisch (codenamed ‘Dolphin’ by the Government Code and Cypher School (GC & CS)), at Bletchley Park by the end of 1939. Typically, he thought Dolphin ‘could be broken because it would be so interesting to break it’ (see ‘Enigma’, p. 257).Hut 8 at Bletchley Park solved some wartime Naval Enigma signals in May and June 1940. Internal evidence shows that Turing wrote his outstanding Treatise on the Enigma around autumn 1940.GC&CS read Dolphin traffic currently, using captured keys, in June and July 1941. The resulting data provided enough cribs and other information to break Dolphin signals cryptanalytically from August onwards, within 24 to 36 hours of their transmission. Mrs Driscoll was assigned to attack Naval Enigma, with two assistants, around October 1940. However, the US Navy was then intercepting only a small proportion of the Naval Enigma signals being transmitted, and was unable to make any progress against Dolphin, especially since it could not reconstruct the wiring of Enigma’s wheels. It did not even fully understand the wheels’ noncyclometric motion, which considerably complicated any solution of Naval Enigma, in particular, since each of the special Kriegsmarine wheels VI to VIII had two notches. Notching made the wheel motion irregular, especially when a doubly notched wheel was in the middle or right-hand position. Using two doubly notched wheels could reduce Enigma’s period from its normal 16,900 (26´25´26) to 4,056 (24´13´13). In February 1941, following lengthy negotiations between the US Army and Navy, a four-man team led by Abraham Sinkov visited GC & CS. (Sinkov was accompanied by Leo Rosen, also from the US Army’s Signal Intelligence Service, and Lt. Robert Weeks and Lt. Prescott Currier, both from OP-20-G.)