How was the Enigma code broken?

One of the world's most famous codes, and how it was broken...
17 September 2019

Interview with 

James Grime; Elizabeth Bruton, London Science Museum


An enigma machine, used by German forces in the Second World War to send coded messages


First up, we’re winding back the clock to look at one of the most famous code-breaking endeavours: the effort to crack the Enigma code. Adam Murphy went to the Science Museum in London to speak with Elizabeth Bruton, who’s the curator of a new exhibition all about cryptography and the Enigma Code...

Adam - Cryptography, or hiding information so no-one can see it, has a long and storied history. From ancient times right up to today, people have wanted to hide their sensitive data. One of the most famous cases of this was of World War II and the German Enigma code.

Elizabeth - So we're in an area that looks a little bit like a Bletchley Park hut during the Second World War; that's the centre for British codebreaking in the lead up to - and indeed during - the Second World War. And the initial display relates to the Polish contribution to cryptography, and indeed to breaking the Enigma cipher system used by the German military before - and indeed during - the Second World War. So one of the rarest objects we have on display - and one we’re truly honoured to have here - is an Enigma copy, or an Enigma double, loaned to the Science Museum for the duration of the exhibition by the Pilsudski Institute. Now from the early 1930s onwards, Polish cryptographers and mathematicians were able to intercept field messages sent by the German military using the Enigma cipher system. They'd also had a copy of the civilian version, and through this and some... I suppose the best way to describe it is some skullduggery in terms of describing the parts, they were able to essentially reverse engineer a German military Enigma cipher machine, having never actually seen one. They produced a design, which they built from sort of the late 1930s through to the 1940s when they transferred themselves from Poland to France after the fall of Poland in late 1939. And this is the very machine we have on display. Fewer than 100 were made before and during the Second World War, and as far as we know only two have survived. And we have one on display here.

Adam - So the British weren't the first to take a shot at breaking Enigma. But what was Enigma and how did it work?

Elizabeth - And what some people may not know is that Enigma was first and foremost a civilian system designed to keep radio traffic of banks and post offices and other commercial ventures secure. It was purchased by the German military in the late 1920s, and from there on the civilian version was no longer sold. And the more complex military version which had a plugboard, which significantly increased the number of options and therefore complexities, possible settings, was used as the main field communication systems across all three branches of the German military from the late 1920s through to the end of the Second World War. Those who worked with it believed that it was unbreakable, that there was millions of billions of possible combinations, and in order to try out every single possible setting - bearing in mind the settings changed daily, so it wasn't that you had forever - it would have taken more than the lifetime of the universe if you did it manually. And indeed that is the case, but there was a number of weaknesses of the Enigma cipher system. For example, it couldn't encrypt a letter as itself, so A would never be encrypted as A. You could produce these cribs, which is where you made a very educated guess as to some feature of the message: whether it was the opening thing, for example, maybe someone opened or finished a message with ‘Heil Hitler’; or it could be a weather report, which had a very standard form, so if you knew the form of the weather report and the weather as it was in that particular location, then you could make a very educated guess as to the content. And if you know what the decrypted version of it's going to be, you can essentially map the encrypted onto the decrypted to try and make an educated guess as to the key, the different sections of the rotors and plugboards and so on that it was set to. But it was a race against time, because you had essentially 24 hours before the key setting changed and you're almost back to square one.

Adam - What was the answer then? What was the Enigma antidote?

Elizabeth - The British used a system called bombe based on a manual version developed by the Poles. And essentially it tried a number of possible key settings until it found a possible version that didn't clash with their thought of what an encrypted-to-decrypted letter was. Then they would stop; they would check that with an adapted British Typex machine, that is the British cipher machine; see if a message was produced in comprehensible German. If it was then they'd found the key setting for that particular network for that particular day; if it wasn't, then they would go back again and try again until they found the key settings.



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