It looks as if Bletchley Park is the single greatest achievement of Britain during 1939-45, perhaps during the [20th] century as a whole.

GEORGE STEINER

Enigma

Enigma's Origins


The Enigma machine wasn’t first broken by the ‘Ultra’ team at Bletchley Park. The initial solution was made by a young Polish mathematician called Marian Rejewski in 1932. Even in 1939 the British hadn’t made much headway with it, until Polish intelligence passed the secret to the British just five weeks before the outbreak of war. This gave the codebreakers at Bletchley a fighting chance to decode the Germans’ military messages.

 
The first Enigma machine was built at the end of the First World War and in the 1920s they were available to buy commercially. They were used by banks and other companies to conceal sensitive information from competitors, but the German army soon realised their possible military applications. Polish Intelligence received the operating instructions from a disgruntled German in the cipher bureau, and the Polish mathematician Marian Rejewski’s clever guess was that the wiring connections between the machine's keyboard and encoding mechanism were simply in alphabetical order. As a result the Poles were already deciphering Enigma messages in 1933 – by January 1938 they were able to read about 75% of the Wehrmacht’s messages.
 
In 1938, however, German security improved the machines, making it much more difficult to crack the codes, and this was the real achievement of British mathematician Alan Turing and the other code-breakers at Bletchley Park. The 'key for the day' could be any one of about 364 billion possible settings, and the German codes changed every day. Turing built the machine called the 'Bombe' at Bletchley Park to decode messages, based on recurring patterns in German messages. Over 200 ‘Bombe’ machines were built, and after the war they were all destroyed to preserve the secret.
 
At the height of war there were 9,000 people working at Bletchley – the first code breakers had been summoned under the codename ‘Captain Ridley’s shooting party’, before war broke out. The Germans never realized their codes had been broken, even though at good times messages were being read almost as fast as they were received by their German recipients. Eisenhower said the work at Bletchley Park shortened the war by two years. 

Enigma Mark 2: Lorenz


One of the most impressive achievements of the Bletchley Park team was not cracking Enigma but in beating the Lorenz machine, an even more fiendishly complicated cipher machine than Enigma. It was used for the most important messages passed between German Field Marshals and the central command in Berlin. Cracking it required great intelligence, and when it became virtually impossible to break by hand, machines were built to break the codes.

 
Breaking the codes also relied on errors in transmission – if the machines had all been used with no human error by the German operators it would have been much harder to spot differences and break the code. The ‘Colossus’ machine could read paper tape at 5,000 characters a second, and the tape inside its wheels travelled at 30 miles per hour, speeding up the process greatly. Thanks to Colossus the Allies learned that Hitler had no clue about the truth behind the D-Day deception campaigns.
 
The Daily Telegraph held its first crossword competition during the Second World War. Hopefuls had to complete it under exam conditions. MI5 invited the best entrants to work as code-breakers at Bletchley Park. The film of Enigma rather underestimates the Polish contribution – and the treacherous Polish spy never existed. In fact, the only spy at Bletchley was the British John Cairncross, who was passing information to the Soviet Union.
 
The military version of the Enigma cipher was actually easier to break than the civilian version which had existed since 1923. For military use, an extra level of encryption was added, which ensured that no letter could ever map to itself. The Germans apparently saw this as a good thing, but it made breaking the cipher easier, because any attempted solution in which any letter mapped to itself could at once be discarded. 

To be invited to Bletchley, potential code breakers had to complete the Telegraph crossword in under 12 minutes.

SIR HARRY HINSLEY (1918-98)

Ultra shortened the war 'by not less than two years and probably by four years'; moreover, in the absence of Ultra, it is uncertain how the war would have ended.

An Enigma machine worth £100,000 was stolen from Bletchley Park in 2000 and posted to Jeremy Paxman 6 months later.

PROFESSOR RICHARD HOLMES (1946-2011)

This [Bletchley Park] is sacred ground. If this isn't worth preserving, what is?

The chance of cracking the Enigma machine was about the same as winning the lottery 11,000,000,000,000 times.

In 1943 Turing's machines were cracking total of 84,000 Enigma messages a month - two messages a minute.

Bletchley Park was the world's first electronic computing facility.