Bombes For Dummies

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Today I spent a few hours in the company of John Harper and his team leanring about their work of the last 14 years; The Turing Bombe Rebuild Project. Bombes are large electro-mechnical machines (*not* computers) primarily designed by Alan Turing that attempt a kind of ‘brute force’ attack on Enigma cyphers. What you can see is 36 Enigmas arranged in vertical columns. The operation of the machine takes a book, rather than a small post here, to explain. Missing about 99.9% of the story out, you give it a ‘guess’ on how one letter may have been encypered as another. It’ll then look for Enigma settings that would match that guess. In a little over 10 minutes it can test all of the possible rotor settings. Once a possibility (an ‘open’ circuit) is found, the machine stops and indicates possible settings. The operator would then test these settings on a ‘checking machine’ to see if the plaintext appeared. If it did, “Job’s Up!” and on to the next one.

Over 200 Bombes were built in the UK although none survive. The Bombe in this video was painstakingly rebuilt and is now fully operational. You can see it break real messages on most weekends at Bletchley Park. Regardless of it’s complexities it’s a wonderful piece of machinery and quite beautiful to watch.