Bletchley Park

Bletchley Park is an unremarkable Victorian mansion in Buckinghamshire, a building whose architecture has been described by unkind visitors as several styles arguing with one another. It was bought in the late 1930s for the Government Code and Cypher School, and part of its appeal was pure geography: it stood beside the railway junction at Bletchley, on the line that joins Oxford and Cambridge, the two universities from which a great many of its future staff would be quietly recruited. When war came in 1939, the codebreakers moved in. Within a few years the lawns had disappeared under wooden huts and brick blocks, and the population had swollen from a few dozen people to something approaching ten thousand.

Their problem had a name: Enigma. It looked like a heavy typewriter in a wooden box, and it was used across the German armed forces to turn plain German into a stream of nonsense letters. Inside sat a set of rotating wheels, or rotors, which shifted position with every single key stroke, so that pressing the same letter twice produced two different letters. In front of them was a plugboard, on which pairs of letters were swapped by cables before and after they passed through the rotors. The wiring, the order of the rotors, their starting positions and the plugboard pairs made up the day's settings — and the settings were changed daily, at midnight.

The result was a number of possible configurations so vast that it is usually written as a power of ten rather than spoken aloud. German cryptographers ran the arithmetic, looked at the answer, and concluded that no enemy could ever work through so many combinations in the few hours before the settings changed again. They were, on the arithmetic alone, correct. What they had not allowed for was that a cipher is not only a machine; it is also the people who operate it. Enigma had one structural weakness — a letter could never be enciphered as itself — and its users had habits. Those two facts, and an extraordinary amount of ingenuity, were the crack in the wall.

The first breaks into Enigma were not British at all. Through the 1930s a small team of Polish mathematicians — above all Marian Rejewski, with Jerzy Różycki and Henryk Zygalski — attacked the machine with pure mathematics and built devices of their own to hunt for its settings. In July 1939, weeks before the invasion of their country, they handed their work to British and French intelligence at a meeting near Warsaw. Bletchley therefore did not begin from nothing; it began from a Polish head start, a debt that went unacknowledged for decades and is now, rightly, part of every honest telling. On that foundation Alan Turing and Gordon Welchman designed the Bombe: a large electromechanical machine, clattering and hot, which raced through possible settings and did not identify the right one so much as eliminate the impossible ones, leaving a handful of candidates for humans to test.

The Bombe needed a way in, and the way in was German routine. Cryptanalysts called it a crib — a piece of text they could guess was present. Weather stations sent reports at the same hour in the same words; units signed off in the same formula; one station reportedly transmitted "nothing to report" so faithfully that it became a gift. Every anomaly in the traffic, every operator who reused a setting or sent the same message twice on two networks, opened a door. Around this sat an immense, painstaking machine of human labour: signals were intercepted at listening stations along the coast, motorcycled to Buckinghamshire, logged, sorted, and indexed on hand-written cards, so that what one hut deciphered could be checked against everything already known.

Roughly three-quarters of the staff were women, many of them Wrens of the Women's Royal Naval Service, who ran the Bombes through night shifts of noise, oil and tedium, most of whom were never told what the machines were for. Dilly Knox led a section of formidable linguists; Joan Clarke worked on naval Enigma; Mavis Batey broke into the Italian naval traffic, work which fed the British victory off Cape Matapan in 1941. Meanwhile a second and harder target, the Lorenz cipher used for Hitler's highest-level communications, was unpicked by Bill Tutte and John Tiltman — Tutte reconstructing a machine he had never seen. To break it fast enough, Tommy Flowers, a Post Office engineer, built Colossus out of thousands of valves: an electronic machine, programmable by switches and plugs, and now widely regarded as a landmark in the history of computing.

Intelligence from the broken traffic was codenamed Ultra, and it was handled with a discretion that bordered on the ruthless. Nothing could be acted upon in a way that might let the Germans deduce that their cipher was broken, since a single change of procedure would have closed the door. Commanders were given the substance but rarely the source; cover stories were invented — a reconnaissance aircraft conveniently sent over a convoy, an imaginary agent — so that any German enquiry would find an innocent explanation. Ultra was classified at a level above top secret, its circulation restricted to a small number of officers, most of whom knew only what their own operations required. In the Battle of the Atlantic, where the reading of naval Enigma helped convoys steer around the U-boat lines, the effect was measured in ships and lives.

Everyone who worked there signed the Official Secrets Act, and for about thirty years the silence held — an astonishing fact when you consider the numbers involved. Churchill is said to have called them the geese that laid the golden eggs and never cackled; the remark is reported rather than documented, but it captures the arrangement exactly. Men and women went home in 1945 to families who assumed they had spent the war doing dull clerical work, and said nothing. Some let their spouses believe it for the rest of their lives. Only in the 1970s, with the first published accounts, did the story begin to come out — by which time many of those who had done the work were dead, and some of the survivors found that no one quite believed them.

The echoes are long. Colossus and the thinking behind it fed directly into the birth of modern computing, though the machines themselves were broken up and the details stayed classified for decades, so that the pioneers went publicly uncredited. The organisation continued as GCHQ, and the questions Bletchley raised — how much of everyone's communication a state may read, and who watches the watchers — have only grown louder. Alan Turing was prosecuted in 1952 for homosexual acts, then a crime in Britain, and died in 1954; he received a posthumous royal pardon in 2013, and a later law extended pardons to thousands of others. The site itself was nearly demolished before being saved as a museum. Historians commonly estimate that the work shortened the war by around two years — an estimate, not a measurement, and one that is argued over, but almost nobody argues it was worth nothing.