Engineering places: Bletchley Park

The introduction of electro-mechanical encoding machines with billions of permutations at the end of the First World War brought a new challenge to code-breakers. Perhaps the greatest threat was the German Enigma machine and its far more complex big brother, the Lorenz machine.

Not that this seemed to bother the British government, who were aware Polish intelligence had managed to ‘break’ an Enigma during its testing with the German army. It did, however, bother Admiral Sir Hugh Sinclair, head of the Secret Intelligence Service (SIS/MI6) and all cryptanalysts who realised that the Polish Enigma was only ‘broken’ until the Germans changed the machine’s rotor settings, which at the time they rarely did.

With some prescience, Admiral Sinclair decided to set up a joint physical base for SIS and the Government Code and Cipher School (GC&CS), settling on Bletchley Park as their new home. With no government money available to buy the site, he bought it with his own money.

The first 150 staff moved in on 18 September 1938 under cover of being ‘Captain Ridley’s Shooting Party’. Less than a year later, in the run-up to the German invasion of Poland and the outbreak of the Second World War, the Enigma rotor settings were being changed at least once a day and the Poles had told Britain that they could no longer read Enigma traffic.

By mid-August 1939 it was decided to recruit “men of the professor type” to deal with new challenges. On 4 September, Alan Turing arrived at Bletchley along with a host of the country’s finest mathematicians and cryptographers.

To accommodate their work a series of temporary buildings – the famous ‘huts’ – were erected, and by January 1940 these eccentric teams had broken the German Army administrative key that became known as ‘the Green’. Inspired by their success, the cracking of ‘Red’ Enigma used by Luftwaffe army liaison officers quickly followed.

Breaking codes created a problem for Bletchley. If all the ‘Ultra’ information they had was now acted on, the Germans might reasonably deduce that their ‘unbreakable’ code had been broken. A cover story was needed, and it was leaked that the information was coming from an MI6 officer codenamed ‘Boniface’ using his network of (fictitious) spies inside Germany.

For many working at Bletchley the problem was not the breaking of the codes, but the time it took to discover daily rotor settings. For this, Turing began working on his electro-mechanical ‘bombes’, which could quickly grind through thousands of possible settings. Meanwhile the cryptographers worked on getting ‘depth’ on transmissions, looking for sloppy encoding, repeated messages, failures to change the rotor settings, and repetitive phrases such as U-Boat weather reports and the inevitable ‘Heil Hitlers!’.

In April 1940, the system was put to the test with the German invasion of Norway and Denmark. The brilliant work of John Herivel identified lazy operator settings in the first message of each day, leading to the breaking of the ‘Yellow’ cipher. By early September, these ‘tips’ could be combined with the use of the first working ‘bombe’.

Alongside all the technical development with bombes, many breakthroughs still came from deduction and espionage. Traffic analysis in Hut 4 identified vital setting data in the daily repetitions and idle chatter of operators, all of which proved vital in Jane Fawcett’s decrypt that led to the tracking down and sinking of the Bismarck.

So impressed was Churchill that he visited the site in September 1941 and met Turing. Just a month later, Turing wrote directly to Churchill asking for more resources. A personal reply ordered they be given whatever they need. From this point on, the floodgates truly opened.

While Enigma codes were now regularly being broken, the 12-rotor Lorenz machines used by German High Command and their top-level ‘Tunny’ codes remained out of reach until spring 1942, when Bill Tutte created a framework to crack them. In July of that year the ‘FISH Subsection’ was tasked with decrypting Tunny and began looking for ‘depth’ in the messages. Yet Tunny would require more than a bombe. It would need a computer.

In 1943, Turing’s ideas for a machine to crack Tunny came to fruition with the prototype of Colossus, built at the Post Office Research Station at Dollis Hill by PO engineer Tommy Flowers. This – the first programmable electronic digital computer – was so secret that its existence was denied by government until 1970. Just a month after prototyping, the first Colossus was installed at Bletchley. It was just in time, and Flowers’ Mark II machine was operational to decode High Command communiques for D-Day in June 1944. From then on, Flowers built a new Colossus every month until the end of the war. It was said that Eisenhower had an Ultra decrypt in his pocket when he authorised the landings in Normandy.

By the end of June 1945, the work at Bletchley had paid off and the British Chiefs of Staff could formally visit to thank the 10,000 people who now worked there. After the war, everything at Bletchley was classified. The bombes were dismantled, the Colossus machines destroyed (all except one) and the staff sent back to their peacetime jobs, with a stern reminder they had signed the Official Secrets Act. It would be 1974 before a book on Bletchley’s vital war work appeared.

Bletchley remained associated with British Intelligence until 1987, but was saved for the nation in 1991 by the Bletchley Park Trust. Today it remains as a tribute to the eccentric collection of “boffins and debs” who shortened the war and, perhaps unwittingly, in creating the computer, started building the world that would follow.

Timeline: Bletchley Park

Years following 1883: Bletchley Park is built for financier Sir Herbert Leon.

1919: Government Code and Cipher School (GC&CS) created.

May 1938: Admiral Sir Hugh Sinclair buys Bletchley Park as a joint base for GC&CS and SIS.

18 September 1938: 150 recruits arrive as ‘Captain Ridley’s Shooting Party’.

1939: Staff expanded in run-up to war.

14 January 1940: ‘Green’ Enigma key deciphered, soon followed by ‘Red’.

14 Mar 1940: First ‘Bombe’ decipher machine.

21 November 1940: Bombing damages site.

25 April 1941: Discovery that Crete may be a target for major assault.

10 May 1941: Captured cipher settings and traffic analysis contribute to sinking of Bismarck battleship.

June 1941: German attack on Soviet Union brings Bletchley details of the massacre of Jews. Churchill orders saving of evidence.

Second half of 1941: Hut 8, led by Turing, cracks the Dolphin key used by U-Boats attacking Atlantic convoys.

Spring 1942: Bill Tutte creates a framework to break 12-wheel Tunny codes.

2 October 1942: Bletchley and US Navy code team agree “full collaboration”.

December 1943: Tommy Flowers demonstrates Colossus computer; first machine delivered a month later to assist with Tunny decrypts.

1 June 1944: Mk 2 Colossus first works at Bletchley – in time for D-Day.

28 June 1945: Staff thanked for their war effort. All records are to be destroyed.

1974: Publication of ‘The Ultra Secret’ by FW Winterbotham.

1987: Bletchley closes, after 50 years of use by British Intelligence.

1991: Bletchley Park Trust set up to save the site for the nation.

July 2009: Bletchley personnel recognised with a commemorative badge.

June 2014: £8m restoration completed.