Alan M. Turing’s Contributions to Co-operation Between the UK and the US

Summary

Alan Turing’s visit to the US Navy Cryptanalytic Section (Op-20-G) and the US Army’s Signal Security Agency during the winter of 1942–1943 was a significant milestone in the collaboration between the British Government Code and Cypher School (GC & CS) and its US counterparts. As technical expert for the GC & CS, Turing viewed the progress of Op-20-G as it designed and developed its own Bombe and other machine aids for defeating German Enigma ciphers. Some of these machines were requested by Turing and John Tiltman for use at Bletchley Park. Not merely an observer, Turing consulted with and advised Op-20-G on Enigma-related matters before and during his visit. Obtaining clearance for Turing to view the X-system, a voice scrambler being developed at Bell Telephone Labs, required the intercession of Field Marshal Sir John Dill and a personal appeal to General George C. Marshall. It may have inadvertently contributed to the later signing of the British-United States Agreement (BRUSA) signed in May 1943. Turing’s report on the X-system was key to its acceptance and installation in London. His activities in the United States reveal him to have been an expert in all aspects of machine-based cryptanalysis who influenced the development of the US Navy’s Bombe program and possibly speech encipherment at Bell Labs.