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MAKING THE MODERN WORLD
Stories about the lives we've made

story:Science in war

scene:Vital step to victory: Breaking codes


Resource Descriptions

General Guderian with an Enigma operator, France, June 1940.
Alan Turing’s ideas grew into computers called 'bombes'.
A Bletchley Park 'bombe' in action.
Women from a range of backgrounds came to Bletchley Park.
Jean Hazlerigg became a successful code-breaker.
Map of the Battle of Cape Matapan.
Italian ships under attack during the Battle of Cape Matapan.
The main house at Bletchley Park.
Interior of a hut at Bletchley Park.
Interior of a hut at Bletchley park.
Memorandum noting the invaluable contribution of female assistants.
Mathematician and computer pioneer Alan Turing.
Mathematician and computer pioneer Alan Turing.
Gordon Welchman, a former Cambridge mathematics don.
T. H. Flowers, part of the team from Dollis Hill who built the Colossus.
Colossus was mainly developed to break 'Tunny', a German army code.
Max Newman was the leader of the group that developed Colossus.

Even before Beryl Power's Central Register got going, Commander Alistair Denniston listed 'men of the Professor for the top-secret Government Code and Cypher School at Bletchley Park in Buckinghamshire. He also started to plan for 'a small reserve of young women with good language qualifications'. On 3 September 1939, the very day on which war was declared, 16 high-grade academics, including the mathematician and computer pioneer Alan Turing, were recruited for the British code-breaking effort.

Much secret German communication was encrypted by the Enigma machine. This was originally developed for business use, but the high degree of security it offered and its portability led to its adoption both for high-level official German government communications and for military operations. Bletchley Park focused on breaking the constantly shifting Enigma cipher.

The first success was achieved in Poland under threat of German invasion. Cryptanalyst Marian Rejewski and colleagues first had to work out the wiring pattern of the Enigma machines and construct replicas. Breaking the codes relied on a combination of techniques. Repetitions within messages or regularly used forms of text (for example, the introductions to weather reports) could give a key. However, it was normally necessary to try out a huge number of possible rotor combinations. To do this they devised an electro-mechanical machine that came to be known as a 'bombe'. The bombe was, in effect, a set of Enigma machines working in reverse to try out a large number of possible settings. Like a combination lock, hitting on the correct initial setting would 'open' the coded message. Months before the German invasion of Poland, information on this work was passed to British intelligence.

As the war went on, progressive improvements to Enigma machines and to German operating procedures repeatedly robbed Bletchley of the ability to read Enigma. For example, in 1940 no German naval codes could be read for nearly seven months. Turing and other mathematicians countered these improvements with new analytical procedures and worked to improve the bombes, collaborating with Post Office electronic engineers to develop the more complex 'Robinsons' and eventually the Colossus design, one of the precursors of the Computer Age. Nevertheless, mathematical methods were often slow. Captured U-boats and other German ships were combed for documents that could provide shortcuts through the code-breaking process and raids were deliberately mounted against German weather ships so that codebooks could be seized.

Bletchley Park started out more like a Cambridge college than a military site. It ended the war as a vast code-breaking 'factory' that employed, at its peak, about 10,000 people. 'Ultra' (the intelligence information Bletchley Park provided) made a vital contribution to the Allied victory.

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