Alan Turing and the Enigma Machine: How the Nazi Code Was Broken

How the Enigma Machine Encrypted Text

According to the Bletchley Park Trust, the German military Enigma, in its Enigma I version, enciphered letters by routing them through three removable rotors, which could be reordered to change the cipher for each session. An adjustable reflector then sent the signal back through those same rotors, pushing it along the same path twice.

The Enigma I was also the first model to add a plugboard: twenty-six sockets, wired together in pairs, that rerouted two letters in the gap between a key press and the signal’s arrival at the rotors. No civilian Enigma carried one. From 1938, two more rotors joined the original three: the operator now picked three out of five for each session, and every different combination of rotors, rings and plugboard settings produced a different cipher.

The Crib and the Bombe: Bletchley Park’s Method

Enigma’s keys changed every day and, according to The National Museum of Computing, had to be found in time to read between 3,000 and 5,000 intercepted messages daily. Turing and the Cambridge mathematician Gordon Welchman built the Bombe toward the end of 1939, an electromechanical device meant to speed up the search for those settings.

Practical example: a cryptanalyst would spot, in an intercepted message, a pattern that matched a crib — a fragment of plaintext they suspected was hiding in there. The Bombe, which held several drums wired together like multiple Enigma machines, then mechanically tested many setting combinations, discarding whichever ones clashed with the crib until only the daily key was left standing.

Some daily keys fell within 2 to 4 hours, The National Museum of Computing notes; others were never broken at all. Speed mattered, because the key would change again the next day.

Bletchley Park: The Team Behind the Machine

The work went on in wooden huts, built across the grounds of the Bletchley Park estate. Turing ran Hut Eight, given over to cryptanalysis of German naval signals. Gordon Welchman’s contribution, the diagonal board, sharply cut the number of false stops the Bombe produced — the dead ends in the hunt for the key. Victory, the first Bombe, running from 14 March 1940, was still Turing’s original design, without that upgrade: the first machine with Welchman’s diagonal board, nicknamed Agnes, entered service in August 1940. Hundreds of Bombes were kept running by the Wrens, the Royal Navy’s servicewomen: by the end of the war, almost 1,676 women of the WRNS and 263 men of the RAF were operating 211 of them.

The first dedicated building, Hut 11, held up to six machines from March 1941. Hut 11A replaced it in February 1942, with room for nine.

A few years after the war, in 1952, Turing himself was convicted over his homosexuality: the Recap Was Alan Turing Gay? The 1952 Conviction, Chemical Castration and Pardon tells what happened to him.

From the Air Force to the U-boats: The War by Radio

By May 1940, Victory was already breaking German air force messages, the Luftwaffe’s traffic. Naval ciphers held out longer: the breakthrough came in February 1941, thanks to documents captured from the German trawler Krebs, off the coast of Norway. From that point on, helped by further captures, German submarine communications fell largely under the control of the Hut Eight codebreakers, directed by Turing.

On 1 February 1942, the U-boats switched to a four-rotor version of Enigma, more complex than anything used before, and their communications went effectively unreadable until December of that year.

How Much It Really Mattered

Those who had worked on the decryption at Bletchley Park kept total silence about it for thirty years after the war ended, until the mid-1970s. Harry Hinsley, the official historian of British intelligence, judged in a 1993 address that without the Enigma breakthrough the Allied invasion of Europe would have slipped to 1946 or 1947, with the war dragging on past 1945. Hinsley suggested that scenario might have brought biological weapons such as anthrax and American atomic bombing raids roughly once a month after July 1945.

Two Machines, One Surname

The Enigma machine was the physical device the Germans used to encrypt their messages, and the Bombe is what Bletchley Park built to break it. The Turing machine is something else, despite the near-identical name: a theoretical concept, built from a tape, a head and a set of states, which defined what a computer can or cannot calculate. The author and the surname in the title are all the two machines have in common.

For the wider context of the conflict, the Recap on World War II is worth a look. The step-by-step method Turing and Welchman built, a sequence of mechanical steps narrowing down the possibilities to a single key, points toward the modern idea of an algorithm; the Bombes themselves, large-scale electromechanical calculating machines, open onto the broader history of computers.