Breaking the Nazi codes

The road to breaking the enigma cipher.
12 May 2020

Interview with 

James Grime


An enigma codebreaking machine behind glass


The efforts that would put an end to World War Two were underway. In Britain, one of those avenues was breaking the German ciphers, the code they used, called Enigma. Enigma was a tough nut to crack. Complex and constantly changing. Not only did it change every day, it changed every time you typed a key. So "e" would become "q" with one keystroke, then "s" the next. So how did it work, and how did we break it. Adam Murphy spoke to mathematician, and enigma expert James Grime…

James - The Enigma machine was actually invented by a German engineer called Arthur Scherbius. He invented it in 1918 a little bit too late for world war one. And he would sell these machines to businesses who wanted to send secret information. And then the military started to use them as well. It's wood, it's steel, but it's about the size of a typewriter. It has a keyboard and when you type on the keyboard, your code letters actually light up. There's a second set of letters and so when I press a letter like T, then it's going to light up a code letter. Maybe W.

Adam - The Enigma machine was not dissimilar to a typewriter. The code was contained in three wheels, which you chose out of a possible five with 26 starting positions and some wires on the front and a plug board just to add some little extra complexity. And you had to sit down and set that up each morning.

James - For the Germans, they would have a code book and every day it told you how to set up the machine for that day. Without that key sheet, you won't know how to use the machine. So altogether we have three wheels to put into the machine, their position, these wires at the front and the total number of ways you can set up the Enigma machine is a large number. It's 159 million million million possibilities, which is far too many for a codebreaker to check. And it changes every day, which is the hard thing about it. The British code breakers and the Polish code breakers, they were very aware of how it worked, but not maybe the details of the wiring inside the machine. And that's when the Polish were actually able to work out the wiring inside machine without ever seeing the machine itself. Just from the codes, they were able to deduce all the wiring inside the machine and then build their own.

Adam - So how did the British go about cracking it?

James - So the British - thankfully, they had a kind of a headstart because they saw the Polish methods, the Polish passed their information to the British, but the Polish method was based on a flaw in the German procedure. If the Germans changed their procedures, that method won't work anymore. So that's where Alan Turing comes in. Alan Turing had a different method for breaking the Enigma code, which was based on the flaw in the machine itself. When you're using the Enigma machine, if you press a letter like E and if you kept pressing the letter E repeatedly, it keeps changing the code. This is one of the reasons why Enigma is so hard to break. E might not be the same if I keep pressing it over and over again. But, there's one letter that it will never become and it will never become itself. It's not much of a clue, it is a flaw in the machine. So what they can do now is they can try and guess a word that might be in your message. So what the Germans would do every morning is they would send a weather report. So you can use a phrase in that report, let's use the word "weather", maybe "Wetter" in German. So if I can find where that word fits in the code, I can now start to work out the correct position of the wheels that makes that bit of code, say the word, "weather". It's a guess, but using that guess, we might be able to break the code for that day. They could speed it up by building these very large Bombe machines, Bombe machines and these were like simultaneous Enigma machines, like 12 simultaneous Enigma machines, and it was actually a process of elimination. It was faster to reject the incorrect settings then to go looking for the correct. And you could find the settings on a good day in under 20 minutes.

Adam - There was something else at the facility where Enigma was cracked at Bletchley Park. An early computer called Colossus, which you could call a forerunner of the computers that we have today.

James - Colossus, people may have heard of Colossus, and it's another code breaking machine that was at Bletchley park. Colossus was built to break a different code machine that the Germans were using. It was the machine called Lorenz and this machine, even more difficult than Enigma, was used by the top generals of the Nazi party and we broke that code as well. Enigma has three wheels inside and they turn as it goes along and it changes the code. Lorenz had 12 wheels inside, so if you're talking about how many possibilities, well Enigma has a number that's something like 20 digits long. The number of possibilities for setting up Lorenz was a number that was 170 digits long, which is a ridiculous number. It's more than are atoms in the universe. If each atom in the universe was itself a universe, there would be more Lorenz settings than there are atoms in a universe of universes of atoms, and it's too many to check. And Colossus is arguably the world's first digital computer. If that's true, if the Colossus machine can be considered the world's first computer, then it was built in secret at Bletchley Park to break this top secret German code.

Adam - And what happened to Colossus, and all of this work, once the war was won?

James - After the war, there was an order from Winston Churchill to say, destroy all the material. So they had a big bonfire. They burnt all their work. The machines themselves were destroyed. This story was secret under the Official Secrets Act for 30 years, many of these code breakers were not allowed to tell their friends and family. Some died without their family ever knowing what they did during the war. And then in 1974 one of the people who worked at Bletchley Park, who wasn't a code breaker, but he was in charge of sending the information out to the generals in the field. He wrote a book and the secret came out, and it was a bit of a controversy at the time because they weren't supposed to be telling these secrets. Even though this is 1974 we're talking about.


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