Bletchley Park

Bletchley Park is in Bletchley (no surprise), which is near Milton Keynes. It was purchased by the government in 1939 to house the top-secret Code and Cipher unit. Near a train station which runs to London (Euston) and what amounted to main roads at the time, it was deemed an ideal location. The first group of mathematicians and top code breakers came up here ostensibly on a "shooting party" complete with a cook from the Savoy Hotel!

During the war, over 8,000 people worked here in one capacity or another. The big challenge was to break the codes sent by the Germans using "Enigma" machines. These were commercially available before the war, so the Allies knew how they worked but cracking the code was a challenge. The machines worked on the simple process of substitution e.g., the letter A changes to the letter D in the cipher. The Enigma basically complicates this process by

continuing the substitutions by means of a 3 rotors each with 26 settings. In the end there are 15 billion billion possible settings. So, these ciphers are not the sort that a person can work by hand!

Two things helped the English (aided by the Polish, who had been busily working on the project before the war, not liking how things looked in Germany). 1. the Engima can never use the actual letter as a cipher for itself (A can never be represented as A) and 2. The Germans tended to use certain stock phrases.

At Bletchley Park, mathematicians and other scientists worked out how to test what possible combination of rotors and substitutions would lead to solving particular messages using this machine (not a computer), called a Bombe (after the Polish prototype, the Bomba).

By the end of the war, there was a whole room full of these machines running 24 hours a day operated by Wrens (Women's Royal Navel Service). One of the great minds behind the Bombe was Cambridge mathematician Alan Turing. It is reckoned that the war was shortened by about 2 years thanks to the work of the code-breakers.

The Germans used a different system, the Lorenz cipher, for their top-level communications and longer term planning. This uses a radio teleprinter system, which sounds like (but is different from) morse code only sped up considerably. The Allies had never seen a Lorenz cipher, so they had no idea how it worked yet they had to find a way to decode the messages. They were aided in this by a mistake made by the Germans. When a message was not received properly, the operator sent the same message again using the same settings on his Lorenz machine (a no-no), except that he shortened the 4000 character message by about 500 characters (using short forms of words); these transmissions helped the mathematicians work out the code.

They developed this machine:

the first semi-programmable electronic computer to help decode the messages.

The versions of the Bombe and the Colossus seen in these photos from Bletchley Park are rebuilt versions according to plans and photos only released in recent decades. The war-time versions of these machines have (supposedly) been destroyed.