How Alan Turing's Codebreaking Machine Actually Worked
The Enigma machine could scramble a message billions of different ways. Turing's Bombe did not guess the answer — it eliminated every setting that could not possibly be right.
The German military's Enigma machine looked like an oversized typewriter, but inside it was an electromechanical puzzle box: a keyboard wired through a set of rotors and a plugboard to a panel of lamps. Type a letter, and the electrical signal passed through the rotors' current wiring and lit a different letter on the lamp board — that was the coded output. Because the rotors themselves shifted position with every keystroke, the same letter typed twice in a row could light up two completely different lamps. The number of possible starting configurations ran into the billions, and German operators changed the settings daily.
Breaking Enigma was not first a British achievement. In the 1920s and 1930s, Polish mathematicians — chiefly Marian Rejewski — worked out how to exploit weaknesses in an earlier, simpler version of the cipher, and by 1938 they had built a machine called the "Bomba" to speed up the search for daily settings. In July 1939, with war weeks away, Polish intelligence handed Britain and France everything they had: reconstructed Enigma machines and detailed notes on their methods. It was a head start, not a finished solution — the Germans had since added extra rotors and tightened their procedures, which made the Polish Bomba obsolete almost as soon as Britain received it.
What the Bombe Actually Did
At Bletchley Park, Alan Turing and fellow mathematician Gordon Welchman designed a new machine, built by the British Tabulating Machine Company and known as the Bombe. It did not decode a message directly. Instead, codebreakers relied on a "crib" — a guess at a word or phrase likely to appear somewhere in a message, such as a routine weather report or a standard sign-off. Because of one quirk in Enigma's design, a letter could never encode as itself, so a crib that lined up letter-for-letter with the ciphertext could be tested against every possible rotor setting. The Bombe worked through those settings mechanically and electrically, and the moment a setting produced a logical contradiction — the crib simply could not be true under that configuration — the machine eliminated it and moved on. What was left, after running through the possibilities, was a small number of settings worth checking by hand.
Manufacturing the Clues
Good cribs were not always available, so codebreakers sometimes engineered them. In one documented tactic, the Royal Air Force deliberately mined a stretch of water the Germans had already swept clear, expecting the German navy to report the new mines in a message. Because codebreakers already knew roughly what a fresh minefield report would say, that expected phrase became the crib. It is a reminder that Bletchley Park's work was not purely mathematical — it combined logical deduction with a working knowledge of German military routine. By mid-1940, this combination let Bletchley read German air force signals, and Turing personally led Hut 8, the section that broke the far harder German naval Enigma, which used a stricter set of procedures than the other branches.
Told as an illustrated story
Frequently Asked Questions
Did Alan Turing invent the Bombe entirely on his own?
No. Turing designed it together with fellow Bletchley Park mathematician Gordon Welchman, and the whole effort built on earlier work by Polish cryptographers, who had broken a simpler version of Enigma in the 1930s and shared their methods with Britain in July 1939, months before war broke out.
Did the Bombe decode messages by itself?
Not directly. It ruled out impossible rotor settings by testing them against a known or guessed piece of plain text called a crib, using logical contradictions to eliminate wrong answers. Once the Bombe narrowed the possibilities down, codebreakers still confirmed and decoded the message by hand.
Why did nobody know about this work for so long after the war?
The British government kept the Enigma codebreaking effort classified for nearly thirty years after 1945, partly because some captured Enigma machines were still being used, unknowingly, by other governments Britain was monitoring. Turing and his Bletchley Park colleagues received no public recognition for their wartime work during that period.