Bletchley Park - WWII

Bletchley Park (BP), an Edwardian mansion built in an idiosyncratic style, was bought in 1938 as a site to which GC&CS could be evacuated when war came. The estate had happened to come on the market when the London-based GC&CS management realized that war was both inevitable and fairly imminent, and that it would probably involve bombing of London. During the Munich Crisis in September that year the Service sections were moved to BP. They returned when the crisis passed, and BP was fitted out over the next few months with communications and power and some wooden huts were erected.

GC&CS staff digging bomb shelters during the Munich Crisis evacuation, September 1938

On August 1939 about 180 GC&CS people moved from London to BP while about 20, who produced COMSEC (Communications Security) materials (keys, code books etc), moved to Mansfield College Oxford (to be nearer their main printers, the Oxford University Press).

BP had great successes decrypting messages of many nations enciphered on cipher machines (notably, but far from only, the German ENIGMA machine), but also against many manual ciphers. One of the most significant wartime effects of the BP successes against ENIGMA was U-boat against allied merchant shipping losses, correlated with readability of the U-boat ENIGMA ciphers. The restoration of readability late in 1942 probably prevented the collapse of British resistance through starvation of the civilian population through Atlantic convoy losses. At one point in 1942 food supplies were down to about 6 weeks' holdings. ENIGMA decrypts from German Army and Air Force were also of immense assistance to Allied commanders in all Western theatres.

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Among BP's major wartime achievements were the design, installation and operation of the world's first electronic computer, COLOSSUS. The COLOSSUS machines, at BP and later also in the US, were used to set to recover the set-ups of messages sent by the German 'Lorenz' ('TUNNY') online enciphered teleprinter devices.

They were designed by a team which included the young mathematician Alan Turing, who had postulated some still fundamental principles of a computer-like device before the War. Another was the later Professor of Mathematics, Max Newman. When Newman and Turing had diagnosed how TUNNY worked (from intercepted cipher alone - no TUNNY machine was acquired until after the German defeat), they devised a mathematical attack on it. Newman asked Tommy Flowers at the GPO Research Centre at Dollis Hill to build a machine to implement such an attack, based on electro-mechanical switches like those used in the 'Bombe's which attacked Enigma messages.

Within a month Flowers had replied that such a machine was not practical, but he could build one with thermionic valves. The resulting decrypts gave thorough insight into German strategic thinking (whereas ENIGMA decrypts were mostly from tactical and administrative links), and greatly helped the planning for D-Day. In particular, they confirmed that Allied deception plans about the likely location and timing of the opening of the Second Front were working.

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