Bletchley Park

• 1. Introduction

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  Bletchley Park sits in Bletchley, Milton Keynes, an English country estate that once held a secret so consequential that the men and women who kept it were forbidden from speaking of it for thirty years after the war ended. At its peak, in January 1945, nearly nine thousand people worked there around the clock. Three-quarters of them were women. The institution they served was the Government Code and Cypher School, known as GC&CS, and their task was to break the secret communications of Nazi Germany and its allies. What they built to do it would become the world's first programmable digital electronic computer. What they learned would shape the course of the Second World War. And for a generation, no one outside those walls was allowed to know any of it. This is the story of how an unlikely country estate became the nerve centre of Allied intelligence, who worked there, what they broke, and why the secret held as long as it did.

• 2. A House With Hidden Advantages

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  In May 1938, Admiral Sir Hugh Sinclair, head of the Secret Intelligence Service, bought a mansion and fifty-eight acres of land using six thousand pounds of his own money. The British Government had told him they lacked the budget. The property had passed through several owners since the architect Samuel Lipscomb Seckham first built there after purchasing it in 1877. A later owner, Sir Herbert Samuel Leon, had expanded the house considerably, and the architect Landis Gores would later describe the result as a "maudlin and monstrous pile" combining Victorian Gothic, Tudor, and Dutch Baroque styles. After Leon's death in 1926, his widow Fanny Leon occupied it until her own death in 1937, and by 1938 a builder had bought it for a housing development. Sinclair and his colleagues had inspected it while posing as a shooting party under the cover name "Captain Ridley's shooting party." What they found persuaded them. Bletchley station, where the Oxford-Cambridge railway met the main West Coast line connecting London to Birmingham, Manchester, Liverpool, Glasgow, and Edinburgh, sat almost immediately adjacent to the estate. Watling Street, later designated the A5, ran close by. A telegraph and telephone repeater station operated in nearby Fenny Stratford. For an organisation that would need to receive intercepted signals and distribute intelligence to commanders across the country, it was a geographically central and well-connected location, and that combination was the deciding factor.

• 3. The People Inside the Gates

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  Before the first GC&CS personnel moved to Bletchley Park on the 15th of August 1939, Admiral Denniston, the operational head of the organisation from 1919 to 1942, wrote to the Foreign Office about recruiting "men of the professor type." The British War Office went further, approaching top solvers of cryptic crossword puzzles on the grounds that lateral thinking was the skill needed. In one 1941 stratagem, The Daily Telegraph was asked to run a crossword competition; promising contestants were discreetly approached afterward about "a particular type of work as a contribution to the war effort." The staff that assembled was genuinely eclectic. Dillwyn Knox's background was papyrology. Hugh Alexander and Stuart Milner-Barry were chess champions. Alan Turing and Gordon Welchman had begun training in 1938 and reported to Bletchley the day after war was declared. Joan Clarke was one of very few women employed there as a full-fledged cryptanalyst. The institution earned the nickname the "Golf, Cheese and Chess Society," and its mix of scientists and young women from high society was sometimes described as "Boffins and Debs." Among those who later became famous in other fields were historian Asa Briggs, politician Roy Jenkins, and novelist Angus Wilson. Staff worked a six-day week rotating through three shifts, the midnight-to-eight shift being the most disliked. At the end of the third week, a worker finishing at eight in the morning returned at four in the afternoon, logging sixteen hours that day. Some staff, described in records as "girls," collapsed and required extended rest. Leave came once a quarter, a single week at a time.

• 4. Breaking Enigma

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  Five weeks before the outbreak of war, Warsaw's Cipher Bureau revealed its achievements in breaking Enigma to astonished French and British representatives. The Poles also sent an Enigma clone to Britain in August 1939. That technical foundation, combined with the British team's own methods, finally made attacks on the cipher barely feasible. Properly used, the German Enigma should have been virtually unbreakable; it was flaws in German cryptographic discipline that created the openings GC&CS needed. The bombe, an electromechanical device designed to uncover the daily settings of the Enigma machines, was developed by Alan Turing, with an important contribution from Gordon Welchman, and engineered by Harold "Doc" Keen of the British Tabulating Machine Company. Each machine stood about seven feet high and wide, two feet deep, and weighed approximately a ton. Luftwaffe messages were the first to be read in quantity. The German navy proved far harder; the capture of code books was needed before naval Enigma could be broken. When the German navy introduced a four-rotor Enigma for its Atlantic U-boat communications in February 1942, that traffic became unreadable for ten months. It was ultimately the success of the US Navy Bombe, not the British version, that became the main source of reading those messages for the remainder of the war. At its peak, GC&CS was reading approximately four thousand messages per day. Ultra decrypts played a role in the Battle of Cape Matapan, the Battle of North Cape, and the Allied effort against German forces in North Africa. General Sir Claude Auchinleck wrote that without Ultra, "Rommel would have certainly got through to Cairo." Prior to the Normandy landings in June 1944, the Allies knew the locations of all but two of Germany's fifty-eight Western-front divisions.

• 5. Women Who Cracked the Codes

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  Mavis Lever solved the signals that revealed the Italian Navy's operational plans before the Battle of Cape Matapan in 1941. Admiral Cunningham visited Bletchley in person a few weeks after the British naval victory to congratulate the team, which was rare; most staff never learned the results of their work. Lever worked under Dillwyn Knox, whose all-female section in Hut 16 was sometimes called "Dilly's Fillies." Margaret Rock worked alongside her, and together the two women solved the Abwehr cipher. Knox's section also included Jean Perrin, Clare Harding, Rachel Ronald, and Elisabeth Granger. In Hut 8, Cicely Mayhew, recruited straight from university, translated decoded German Navy signals. Jane Fawcett, who after the war became an opera singer and buildings conservationist, decrypted a vital message concerning the German battleship Bismarck. Eleanor Ireland worked on the Colossus computers. Ruth Briggs, a German scholar, worked within the Naval Section. The Wrens, members of the Women's Royal Naval Service formally posted to HMS Pembroke V, operated the Colossus machines in the section named the Newmanry after its head Max Newman. In a detail that only came to light decades later, the 2014 museum restoration was marked by a visit from Catherine, Duchess of Cambridge, whose paternal grandmother Valerie Middleton and Valerie's twin sister Mary worked in Hut 6, managing the interception of enemy and neutral diplomatic signals. A memorial at Bletchley Park commemorates their service.

• 6. Colossus and the Birth of Computing

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  The Lorenz SZ42 was a twelve-rotor on-line teleprinter cipher machine used for Germany's highest-level command messages, and it was far more complex than Enigma. Bletchley codenamed the traffic it carried Tunny. The cryptanalysts in the Testery, named after its head Ralph Tester, worked out the logical structure of the Lorenz machine without ever seeing it physically, relying on German operator errors to illuminate its mechanics. What they needed to break Tunny reliably was machinery that could work through possible wheel settings at a speed no human team could match. That machinery was Colossus, designed and built by Tommy Flowers and his team at the Post Office Research Station at Dollis Hill. The prototype first worked in December 1943, was delivered to Bletchley Park in January 1944, and operated on live traffic from the 5th of February 1944. Enhancements produced the Mark 2 Colossus, the first of which was running at Bletchley Park on the morning of the 1st of June 1944, in time for the Normandy landings. Flowers then produced one Colossus per month for the rest of the war, reaching a total of ten with an eleventh partially built. The machines were housed in Block H, which today is home to the National Museum of Computing, where a rebuilt Colossus is on display. After the war ended, the machines were destroyed and the work classified; the computing history they represented remained unknown to the public for decades.

• 7. Secrecy, Leaks, and Silence

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  A 1942 security warning distributed within Bletchley read: "Do not talk at meals. Do not talk in the transport. Do not talk travelling. Do not talk in the billet. Do not talk by your own fireside. Be careful even in your Hut..." The intelligence produced at Bletchley was classified above even the normally highest level, described as Britain's "Ultra secret." All staff signed the Official Secrets Act. Churchill referred to them as "the geese that laid the golden eggs and never cackled." The secrecy held remarkably well, but not perfectly. Jock Colville, assistant private secretary to Churchill, recorded in his diary on the 31st of July 1941 that the newspaper proprietor Lord Camrose had discovered Ultra and that security leaks were growing in number and seriousness. The Soviet Union was never officially informed of Bletchley's existence, a deliberate choice reflecting Churchill's distrust even within the wartime alliance. Yet Bletchley was infiltrated by John Cairncross, a member of the Cambridge Spy Ring, who leaked Ultra material to Moscow. In 2020, unique amateur film footage of the outstation at nearby Whaddon Hall came to light after being anonymously donated to the Bletchley Park Trust; a Trust spokesman noted it was "very, very rare even to have still photographs" of the park and its associated sites. The thirty-year rule meant there was no official mention of Bletchley's work until the mid-1970s. Many relatives of those who worked there knew only that a child, spouse, or parent had done some kind of secret war work. Public discussion in the English-speaking world finally opened with the publication of F. W. Winterbotham's The Ultra Secret in 1974. Not until July 2009 did the British government fully acknowledge the contribution of those who served there, when a commemorative medal bearing the inscription GC&CS 1939-1945 Bletchley Park and its Outstations was struck.

• 8. Preservation and Memory

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  Block F, which had housed the Newmanry, the Testery, and the Japanese Military Air Section, was demolished in 1987 as the site was being run down with tenants leaving. The Bletchley Park Trust was established in 1991 by a group who recognised that the site was at risk of being sold for housing. Milton Keynes Borough Council declared most of the park a conservation area in February 1992. The site opened to visitors in 1993 and was formally inaugurated by the Duke of Kent in July 1994. In October 2005, American billionaire Sidney Frank donated half a million pounds to fund a science centre dedicated to Alan Turing. A letter to The Times in July 2008, signed by more than a hundred academics, condemned the neglect of the site. English Heritage announced a donation of three hundred thousand pounds in November 2008. A Heritage Lottery Fund grant of four-point-six million pounds followed in October 2011, conditional on the Trust raising one-point-seven million in match funding; Google contributed five hundred and fifty thousand pounds and by June 2012 the Trust had raised two-point-four million. The completed eight-million-pound restoration project was marked in June 2014 by a visit from Catherine, Duchess of Cambridge. During the COVID-19 pandemic the Trust expected to lose more than two million pounds in 2020 and face cuts of a third of its workforce. Former MP John Leech, who had led the successful campaign to pardon Alan Turing and implement what became known as Turing's Law, asked major technology companies to donate four hundred thousand pounds each to secure the Trust's future. Block H, where the Colossus machines once ran, now houses the National Museum of Computing, which opened in 2007 and maintains a working reconstruction of both a Bombe and a Colossus.