Bletchley Park, a country house in Buckinghamshire, was bought by SIS in 1938 as a site to which GC&CS and SIS could be evacuated when war came: it was widely expected that London would be the target of a massive aerial assault at the very start of any war. During the Munich Crisis in the autumn of 1938 the Service sections were moved to BP. They returned to London after the crisis had passed, but BP was fitted out over the next few months with communications and power and the first wooden huts were erected in its grounds to cope with the size of the ever expanding GC&CS.
GC&CS staff digging bomb shelters during the Munich Crisis evacuation, September 1938
On 15 August 1939 about 180 GC&CS people moved from London to BP while about 20, who produced Communications Security materials (cipher keys, code books, one time pads etc), moved to Mansfield College Oxford to be nearer their main printers, the Oxford University Press. By the end of 1944, some 10,000 people were employed at BP itself, with a larger number engaged on Sigint collection and dissemination tasks around the world.
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BP’s great success was due to the mechanisation of the decryption process keeping pace with the mechanisation of encryption. Although the decryption of Enigma is the best known of BP’s exploits, other successes, such as the decryption of Luftwaffe hand ciphers, and the development of COLOSSUS, the world’s first computer, to solve enciphered German telecipher, made a significant contribution to allied victory.
In parallel with the growth of decryption, WWII saw the development of a handling system for Sigint reports which was designed to protect the source of the intelligence and restrict knowledge of it. This meant that the reporting could continue to provide uniquely valuable intelligence to allied commanders throughout the war.