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How Bletchley Park broke the German Enigma code

David Ludlow
20 Jan 2011
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In part 2 of our series we look at how Enigma was broken and how the first computer was created in the process.

As we found out in part one of our look at Bletchley Park the Polish had given the British a head start on breaking the Enigma code. Unfortunately, the Germans had changed their system for sending the message indicators, making the code harder to break, until Alan Turing had a breakthrough. His idea was to use message cribs, which relied on a flaw in humanity rather than a flaw in the encryption system.

From previous messages, Turing and his team knew that certain stations would always send a broadcast starting with the same phrase, such as the 'The weather report for today is'. Knowing this, they could take the coded text and know how the first characters would decode. With the knowledge that no letter encrypted could be the same letter decrypted, the number of combinations possible was massively reduced.

Of course, this isn't as easy as it sounds and finding the initial message was incredibly hard. So hard, in fact, that Bletchley Park had an entire team of people dedicated to finding cribs. Once a crib had been found there was still a need to brute force attack the possible combinations of rotor settings and plugboard.

The Bombe

To do this, Turing devised the Bombe. This electromechanical device acted like 36 Enigma machines, automatically stepping through every single possible rotor setting on a traditional three-rotor Enigma machine. The key point here was that the Bombe was rewired based on the information retrieved from the crib, using a 'menu' provided by the cryptographic team.

As the Bombe worked its way through every permutation of rotor settings, electrical current would either flow or not flow through the system, which was checked by the Bombe's comparator unit. Using this method it was possible to check for a logical contradiction, ruling out particular rotor settings; if there was no contradiction the machine would stop and the rotor settings could be noted down. These could then be tested by hand on a Typex machine modified to work like Enigma. Meanwhile the Bombe could be started again, looking for the next possible solution, until the code had been broken.

Bombe being operated

Crucially Gordon Welchman, another cryptoanalyst, made an adjustment to the bombe, adding a diagonal board at the rear, which rendered it more efficient in its attack on Engima ciphers. The result was that codes were being broken faster than ever, and the information retrieved was more pertinent.

Admittedly, this is an over simplification of how hard Enigma proved to crack, and it took a lot of hard work and a great deal of genius.

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