Bletchley Park

In the summer of 1939, a small team of codebreakers arrived at the Government Code and Cipher School's (GC&CS) new home at Bletchley Park, Buckinghamshire. Their mission was to crack the backbone of German military and intelligence communications, the Enigma cipher.

Enigma

The Germans thought Enigma was unbreakable. The combination of rotating wheels, electrical contacts and wires meant that the odds against anyone who did not know the machine's settings being able to break Enigma were 150 million million million to one!

But Bletchley Park achieved a breakthrough when the Poles passed on their knowledge of how the machine worked. This helped the codebreakers exploit a design weakness in Enigma - that no letter could ever be encrypted as itself.

At the same time, Bletchley Park mathematician Alan Turing realised that 'cribs' offered a way of cracking Enigma. A 'crib' is a piece of encrypted text whose true meaning is known or can be guessed. German messages were formulaic in places and the first line often contained standard information, for example weather conditions. Once a crib was known, it was still necessary to check thousands of potential Enigma settings to read a message, and to do this quickly Turing designed a electro-mechanical codebreaking machine called a Bombe. Each Bombe simulated the actions of 10 Enigma machines and was able to check all potential settings at high speed.

Cracking the 'impenetrable' Enigma code enabled Britain to foil Luftwaffe bombing raids, minimise U-Boat attacks and secure sea-based supply routes

Colossus

Further codebreaking success enabled Bletchley Park to exploit Lorenz, a highly sophisticated cipher used personally by Hitler and his High Command. But many of the messages still took several weeks to decipher - a computing machine was needed. The result was Colossus, the world's first programmable electronic computer, designed by Max Newman.

Colossus was the size of a living room and weighed about one tonne. Its 2,400 valves replicated the pattern of an encrypted Lorenz message as electrical signals. This breakthrough in computing remained a secret for many years, to the extent that two Americans took the credit for inventing the computer in 1945. But the creation of Colossus proved to be a key contributor to the success on D-Day.

The end

It is estimated that over 10,000 people worked at Bletchley Park at the height of its wartime activity. Their work affected the fate of nations and helped shorten the war by at least two years. But by March 1946, the people were gone and every scrap of evidence of their codebreaking exploits had been removed from Bletchley Park.

Nevertheless, the codebreaking effort continued when the GC&CS was re-named GCHQ and moved to London. It re-located to Cheltenham in 1952.

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