Bletchley Park

The first Monday of May is a Bank Holiday in the UK, celebrating May Day. We took the opportunity to visit Bletchley Park, where the secret codebreakers of World War II did their stuff.

Enigma and the Bombe

I’d always thought that the Nazis invented the Enigma machine, but there was a commercial model available for use before the war. The Nazis modified it make it more secure. But not secure enough.

Enigma machine (three rotors)The Enigma machine has various settings that can be changed at will to make deciphering its output that bit more tricky. Alan Turing and Gordon Welchman, inspired by the Polish Bomba, developed the Bombe, which was a data processing machine (not a computer), which made deducing the settings that much quicker. German transmissions often used the same phrases, such as “weather forecast” (but in German, of course) or “to the group”. These were called cribs and were used to create a menu indicating how the plugs at the back of the Bombe should be connected. If this was correct, it would speed up the decoding process even more.

The front of the Bombe was filled with 108 drums, arranged across three rows. Each column of three drums represented the three rotors of the Enigma machine. They’d go through every single possible combination of letters (17,576), which would take 12 minutes. If they found one that might be right, the machine would stop, and the potential solution tested on a checking machine. If it checked out, it meant they had the correct Enigma settings for that day. The settings changed every day at midnight, so Bombes were running daily.

The British used a Typex machine, not an Enigma machine, to encode their messages. This was more secure than an Enigma machine and was never cracked by the Axis (that we know of, anyway). At Bletchley, they had a Typex machine modified to act like a Germna Enigma machine. Once they had the key, they typed the letters they’d obtained from the German morse signals into the Typex machine. If all was well, plain text German would come out the other end. Then all they had to do was translate it. It was all pretty clever stuff.

model of a German U-boatWe also managed to crack the special four-rotor version that the German U-boats used. We had to build Bombes with a fourth drum to correspond with the fourth rotor.

Two Royal Navy sailors lost their lives pinching a four-rotor Enigma code book from a sinking U-boat; because of their bravery, and the bravery of the other sailor with them, it became much easier for the British to crack U-boat messages, saving many more lives.

And it was all ultra top secret. Under no circumstances could anything code-named Ultra be acted on unless it could be made to look like the intelligence obtained from Enigma transmissions was obtained by some other means, often spies.

They say cracking Enigma shortened the war by two years.

Lorenz and Colossus

Lorenz was a machine that was similar to Enigma, but much more complex. Hitler himself used it, along with the High Command and the German Army Field Marshals. The Bletchley Park team didn’t get anywhere with it till a German operator made a colossal mistake. A long transmission was reported as unreceived, so the operator sent it again without changing the settings. This allowed the chief cryptanalyst, John Tiltman, to decipher it in ten days. Bill Tutte, a Cambridge chemistry graduate, mathematically deduced how the Lorenz machine worked. This worked well until the Germans made it more complicated. The number-crunching required was phenomenal, so Tommy Flowers, who worked for the Post Office, designed Colossus.

Colossus is the predecessor of the modern computer. It was the world’s first practical electronic digital and information processing machine. The Colossi allowed the Allies to deceive Hitler about where the D-Day landings were going to be.

Other machines

SG-41 cypher machineOther coding machines existed, and were used by the Italians and the Japanese. Bletchley Park codebreakers broke these codes, too, along with non-machine-based codes, such as Barbara.

There was one machine that we didn’t crack: the SG-41. It was intended to replace Enigma, but it was never successfully deployed because of production problems and the end of the war.

I’m pretty glad we did crack all these codes. I don’t like to think what would have happened if Hitler had won the war.


We bought a copy of Bletchley Park: demystifying the Bombe by Dermot Turing, Alan Turing’s nephew. It explains how the Bombe works. Colin suggested I write a computer program to model it. So that’s what I’m going to do.

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