The Enigma Machine: How Alan Turing Helped Break the Unbreakable Nazi Code

In 2001, none other than Sir Mick Jagger bought the rights to a novel by Robert Harris called Enigma. The novel, a fictionalized account of WWII British codebreakers, then became a feature film, written by Tom Stoppard, produced by Sir Mick, and starring Mr. Dougray Scott and Ms. Kate Winslett as derring-do Bletchley Park mathematicians and cryptanalysts employed in a race against time and the Nazis to break the fabled Enigma code before all hell breaks loose. It all sounds very dramatic (and I’ve heard the film is entertaining), but things didn’t happen quite like that. Reality is never so formulaic or so good-looking. But the Enigma code was broken, and the story of the code machine and its eventual decryption is fascinating on its own terms. As University of Cambridge “Enigma Project Officer” Dr. James Grime says–in the series of videos above and below–it’s a story of “how mathematicians can save lives.” Still with me?

Okay, so in the first video above, Dr. Grime gives us a thorough tour of the Enigma machine (Sir Mick owns one, by the way… but back to the history…). Developed by the Germans, it’s a marvelous encryption method set into a small box that when opened resembles little more than a fancy WWII-era typewriter. Oh, but it’s clever, you see, because the Enigma machine (the one above belongs to science writer Simon Singh) translates ordinary messages into code through an ingenious method by which no letter in the code ever repeats, making it almost impossible to decode in the ordinary ways. The machine was quite complicated for its time; it works by sending the characters typed by the keys through a series of circuits—first through three rotors like those on a combination bike lock, but each with 26 places instead of ten.

Now at this point, the machine was nothing more than what was available to any bank or business wishing to transmit trade secrets. But the German military machines had an extra layer of encoding: at the front of their machines was a “plugboard,” something like a small switchboard. This allowed the coding coming through the rotors to be resequenced for an extra level of scrambling. In the German military machines, the total number of possible combinations for message encryptions comes to a staggering figure in the quadrillions. (The exact number? 158,962,555,217,826,360,000). There’s a little more to the machine than that, but Dr. Grime can explain it much better than I.

Of course, the Enigma Machine had to have a fatal flaw. Otherwise, no novel, no movie, no drama (and maybe no victory?). What was it, you ask? Amazingly, as you will learn above, the very thing that made the Enigma nearly impossible to break, its ability to encode messages without ever repeating a letter, also made the code decipherable. But first, Alan Turing had to step in. Sadly, Turing is missing from Enigma the film. (More sadly, he was disgraced by the country he served, which put him on trial for his sexuality and humiliated him to the point of suicide). But as Grime shows above, Turing is one of the real heroes of the Enigma code story. Cryptanalysts initially discovered that they could decipher ordinary words and phrases (like “Heil Hitler”) in the Enigma messages by matching them up with strings of random letters that never repeated.

But this was not enough. In order for the Enigma code to work for the Germans, each operator—sender and receiver—had to have exactly the same settings on their rotors and plugboards. (The messages were transmitted over radio via Morse code). Each month had its own settings, printed on code sheets in soluble ink that easily dissolved in water. If the Allied codebreakers deciphered the settings, their decryption would be useless weeks later. Furthermore, the German navy had a more complicated method of encoding than either the army or air force. The Polish had developed a machine called the Bombe, which could decipher army and air force codes, but not navy. What Turing did, along with Gordon Welchman, was develop his own version of the Bombe machine, which allowed him to break any version of the Enigma code in under 20 minutes since it bypassed most of the tedious guesswork and trial and error involved in earlier by-hand methods.

This is all very dramatic stuff, and we haven’t had one celebrity step in to dress it up. While I’m certain that Enigma the film is a treat, I’m grateful to Dr. Grime for his engagement with the actual codebreaking methods and real personalities involved.

A third video of extra footage and outtakes is available here if you’re still hungry for more WWII codebreaking secrets.

via Science Dump

Josh Jones is a writer and musician. He recently completed a dissertation on land, literature, and labor.