History Extra logo
The official website for BBC History Magazine and BBC History Revealed

Your guide to Bletchley Park, the key WW2 decryption centre

Bletchley Park housed Britain's key decryption centre during the Second World War and is credited with shortening the conflict by at least two years. Sinclair McKay tells you everything you need to know about the famous decryption centre...

Wrens operating the Colossus computer at Bletchley Park, 1943
Published: December 9, 2020 at 7:57 am
Try 6 issues for only £9.99 when you subscribe to BBC History Magazine or BBC History Revealed

Q: What role did Bletchley Park play during WW2?

A: Bletchley Park was the Buckinghamshire house and estate that played host to Britain’s code-breaking triumphs – from the success in breaking the ‘insoluble’ Nazi Enigma codes to its incredible work on Japanese cryptology.

Advertisement

Its role was so secret – everyone who worked there signed the Official Secrets Act for life – that we only really started to learn about its achievements decades afterwards. And there is still much to emerge.

Q: When and how was the code-breaking centre established?

A: Before it was evacuated to Bletchley, the Government Code and Cypher School – as it was then known – operated from a building near Westminster. There had been a special code-breaking department since the First World War – then, it was referred to as ‘Room 40’.

In the interwar years, they analysed Soviet traffic, among other things. When it seemed obvious that war was coming once more, many of the expert cryptanalysts of Room 40 set up the new establishment at Bletchley in 1938. They recruited crucial new blood, including brilliant young mathematician Alan Turing, as well as lecturers and undergraduates from universities all over the country.

Q: How many code-breakers worked there? What did an average day consist of?

A: Numbers rose as the war went on; from a relatively small team in 1938 to something like 10,000 people – code-breakers, Wrens, WAAFs, posh debutantes working on the cross-indexing system etc.

Obviously, they didn’t live at the house. All these people, mainly young, were billeted around the town and nearby villages. Bletchley locals were disconcerted by some of the eccentric code-breakers, and they formed a theory that the secret establishment was in fact a special lunatic asylum.

There was no average day. Code-breaking went on round the clock, in a three-shift system, and in different huts – very often the work was done in plain wooden huts. There could either be months of agonising paralysis (as there was in the battle to crack the nightmarishly complex Naval Enigma) or nights of giddying triumph, as when 20-year-old Mavis Batey broke the code that led to British victory in the Battle of Cape Matapan.

Bletchley Park pictured in 1926
Bletchley Park pictured in 1926. (Photo by Evening Standard/Getty Images)

Q: Roughly how many ciphers and codes were decrypted in total?

A: It is simply impossible to know – largely because Bletchley was so mind-bogglingly successful. They read messages from the German army, navy, air force, secret service… even messages from the desk of Hitler himself. Countless thousands upon thousands of communications.

They cracked Italian and Japanese cyphers, and the operation was spread across the whole wide world. In posts from Cairo to Murmansk, dedicated secret listeners for the Y Service intercepted all the secret coded radio messages – and relayed them back to England, and ultimately back to this incredible code-breaking factory.

The other miracle was the secrecy; the fact that the Germans never really guessed adds to this astonishing success.

Which of Bletchley Park's successful decodings of the German military's intelligence was of most benefit to the Allies?

Various branches of the German military – the Abwehr, Luftwaffe, Wehrmacht, navy and so on – all used variations of the Enigma machine with their own specific encryption settings. Each of these had to be deciphered independently at Bletchley Park.

There is little doubt that the resulting intelligence – named ‘Ultra’ by the British – made a very considerable, even decisive contribution to the Allied war effort. Its success stories are legion. It was through Ultra, for example, that Rommel’s plans were divined on the eve of Alam Halfa, a battle that spelt the beginning of the end of the Afrika Korps. It was Ultra intelligence that forewarned the Soviets of German intentions at the battle of Kursk, defeat in which robbed the Wehrmacht of its offensive capacity. The cracking of the Italian naval code, meanwhile, facilitated the Allied victory at the battle of Cape Matapan in 1941, which checked Italian ambitions in the Mediterranean.

D-Day itself would scarcely have taken place, never mind succeeded, without the crucial intelligence provided by Bletchley Park. At every turn, it seems, the Allies drew enormous benefit from their ability to read enemy signals, and thereby learn what their opponents had in mind. Taken together, therefore, the various strands of ‘Ultra’ intelligence were clearly of vital importance to the Allied cause, yet any definitive assessment of their comparative merits is very difficult to make.

For argument’s sake, therefore, I will plump for the cracking of the German naval codes in 1941–2, by which the balance in the battle of the Atlantic was decisively shifted in favour of the Allies. Were it not for that achievement, Britain might well have been rendered lame militarily or starved into submission long before D-Day, thereby rendering Ultra’s other successes rather superfluous.

Q&A answered by historian Roger Moorhouse

Q: Is it true that the 'Ultra' intelligence produced at Bletchley Park shortened the war by two to four years?

A: President Eisenhower credited the work of ‘BP’, as it was called, with having shortened the war by two years. Think for a moment of how many lives those two years might represent; the countless people saved simply by the ending of the conflict.

Bletchley veteran and distinguished historian, Professor Sir Harry Hinsley, reckoned it was three years. Apart from anything else, without Bletchley’s absolutely crucial intelligence, the D-Day landings in 1944 might never have worked, and in the extra time it would have taken the Allies to get a foothold in Europe, who knows how many more victims the Nazis would have murdered? And what sort of new and terrible weapons the Nazis could have developed?

Bear in mind the other great element of the Bletchley story: thanks to a brilliant engineer called Tommy Flowers, the computer age was brought into being there. He developed a code-breaking machine – the Colossus – that was in effect a proto-computer. Engineers at Google and Apple recognise and pay tribute to the Park’s importance.

Did you know that Bletchley employed women to work for the war effort?

Bletchley drew together a wide mixture of civilian and service personnel in what was effectively a ‘green field’ organisation. It moved from being simply a codebreaking operation to a more integrated signals intelligence entity, linking interception, cryptanalysis, translation, intelligence analysis and intelligence dissemination. This worked on a factory-like basis to produce a continuous flow of useable intelligence.

At its height in 1944, Bletchley Park employed close to 10,000 people, up to three-quarters of whom were women, performing a wide array of tasks.

Read more about the female codebreakers of Bletchley Park

Listen: Tessa Dunlop shares the stories of women who worked at Britain’s codebreaking centre in World War Two, on this episode of the HistoryExtra podcast:


Q: When was Bletchley Park decommissioned?

A: It was never decommissioned. Its work – and in some ways, its spirit and ethos – still goes on today. After the war, the work of the Park was packed up (all decrypts carefully destroyed) and first moved to Eastcote, in north-west London.

Some of the code-breakers stayed on. The need for their brilliance was still there – the war had simply turned Cold. After several years, new premises for the increasingly sophisticated technological operation were found, in the pleasant town of Cheltenham. They are still there now, and the organisation is better known as GCHQ.

Bletchley's most famous codebreaker is Alan Turing…

Born in 1912, Turing studied mathematics at King’s College and afterwards he completed his PhD at Princeton in the US. His thesis was ‘Systems of logic based on ordinals’. Turing’s most important theoretical work ‘On computable numbers’ was written in 1936. This essentially founded modern computer science.

Turing arrived at Bletchley in 1939 and soon became the head of the Naval Enigma Team. He played a vital role in breaking German codes during the Second World War, working with a team of colleagues including Dilly Knox, who had broken an Italian naval enigma cipher as early as 1937. In 1945, Turing was awarded an OBE for his wartime services. But, Christopher Grey stresses, "it certainly wasn’t the case that Turing alone cracked Enigma, any more than there was a single Enigma to be cracked".

Read more about the life of codebreaker Alan Turing

Sinclair McKay is the author of The Lost World Of Bletchley Park (Aurum Press, 2013) and The Secret Listeners: How the Wartime Y Service Intercepted the Secret German Codes for Bletchley Park (Aurum Press, 2012)

Advertisement

This article was originally published by HistoryExtra in 2014

Advertisement
Advertisement
Advertisement

Sponsored content