Turing and Cryptanalysis: Mechanization and Probabilities

The contribution of Alan M. Turing (1912-1954) to cryptanalysis is often overlooked or little known. There is also an unfortunate confusion between Turing's abstract conception of mechanical calculation, his conception of the Bombes which were used to break Enigma’s code, and the production of Colossus, the first electronic calculator designed and produced at Bletchley Park. This confusion can be explained both by an all too often hagiographic vision of the events attached to the life and work of Turing, and by the secret which has long reigned over the productions of Bletchley Park. This paper aims to integrate Turing's work in the context of the vast cryptanalysis enterprise that Bletchley Park constituted during the Second World War, and to analyze the conditions of the irreversible relationship thus engaged between cryptanalysis, mechanization and mathematics. If Turing did not directly contribute to the achievement of the Colossus, he developed the probabilistic approach to cryptanalysis, which Irving John Good (1916-2009) revealed several decades later, and which is today at the foundation of sequential analysis.