Fifty miles (80km) north-west of London lies Bletchley Park. In 1883, it became home to the Leon family, whose patriach was a wealthy City of London financier. Herbert Samuel Leon bought over 300 acres of land beside the London and North-Western Railway line that passed through Bletchley, Buckinghamshire, developing sixty of those acres into his country estate. At the heart of the estate, he built a mansion in a curious mixture of architectural styles. One of Bletchley's greatest benefactors, he was much loved by the local people. He was awarded a baronetcy in 1911.
Following the deaths of Sir Herbert and Lady Fanny Leon, the Park fell into the hands of property developer Captain Hubert Faulkner, who intended to demolish the buildings and sell the land as a housing site.
But the Government was about to intervene. It was 1938 and the threat of war loomed as Hitler invaded first Austria and then Czechoslovakia. The Government Code and Cypher School, then based in London, needed a safer home where its intelligence work could carry on unhindered by enemy air attacks. At a junction of major road, rail and teleprinter connections to all parts of the country, Bletchley Park was eminently suitable.
Commanded by Alastair Denniston, the Park was given the cover name Station X, being the tenth of a large number of sites acquired by MI6 for its wartime operations.
After meticulous preparation and a series of trial runs, the codebreakers arrived in earnest in August 1939. They masqueraded as ‘Captain Ridley's Shooting Party’ to disguise their true identity. It was to be the first instalment in one of the most remarkable stories of the Second World War.
The Enigma cypher was the backbone of German military and intelligence communications. Invented in 1918, it was initially designed to secure banking communications, but achieved little success in that sphere. The German military, however, were quick to see its potential.
They thought it to be unbreakable, and not without good reason. Enigma's complexity was bewildering. Typing in a letter of plain German into the machine sent electrical impulses through a series of rotating wheels, electrical contacts and wires to produce the encyphered letter, which lit up on a panel above the keyboard. By typing the resulting code into his own machine, the recipient saw the decyphered message light up letter by letter. The rotors and wires of the machine could be configured in many, many different ways. The odds against anyone who did not know the settings being able to break Enigma were a staggering 150 million million million to one.
The Poles had broken Enigma in 1932, when the encoding machine was undergoing trials with the German Army. They even managing to reconstruct a machine. At that time, the cypher altered only once every few months. With the advent of war, it changed at least once a day, effectively locking the Poles out. But in July 1939, they had passed on their knowledge to the British and the French. This enabled the codebreakers to make critical progress in working out the order in which the keys were attached to the electrical circuits, a task that had been impossible without an Enigma machine in front of them.
Armed with this knowledge, the codebreakers were then able to exploit a chink in Enigma's armour. A fundamental design flaw meant that no letter could ever be encrypted as itself; an A in the original message, for example, could never appear as an A in the code. This gave the codebreakers a toehold. Errors in messages sent by tired, stressed or lazy German operators also gave clues. In January 1940 came the first break into Enigma.
It was in Huts 3,6,4 and 8 that the highly effective Enigma decrypt teams worked. The huts operated in pairs and, for security reasons, were known only by their numbers. The codebreakers concentrating on the Army and Air Force cyphers were based in Hut 6, supported by a team in the neighbouring Hut 3 who turned the decyphered messages into intelligence reports. Hut 8 decoded messages from the German Navy, with Hut 4 the associated naval intelligence hut. Their raw material came from the 'Y' Stations: a web of wireless intercept stations dotted around Britain and in a number of countries overseas. These stations listened in to the enemy's radio messages and sent them to Bletchley Park to be decoded and analysed.
To speed up the codebreaking process, the brilliant mathematician Alan Turing developed an idea originally proposed by Polish cryptanalysts. The result was the Bombe: an electro-mechanical machine that greatly reduced the odds, and thereby the time required, to break the daily-changing Enigma keys.
With the declaration of peace, the frenzy of codebreaking activity ceased.
On Churchill's orders, every scrap of 'incriminating' evidence was destroyed. As the Second World War gave way to the Cold War, it was vital that Britain's former ally, the USSR, should learn nothing of Bletchley Park's wartime achievements.
The thousands who had worked there departed. Some continued to use their remarkable expertise to break other countries' cyphers, working under a new name: the Government Communications Headquarters (GCHQ).
The site became home to a variety of training schools: for teachers, Post Office workers, air traffic control system engineers, and members of GCHQ. In 1987, after a fifty-year association with British Intelligence, Bletchley Park was finally decommissioned.
For decades, the codebreakers would remain silent about their achievements. It was not until the wartime information was declassified in the mid-1970s that the truth would begin to emerge. And the impact of those achievements on the outcome of the war and subsequent developments in communications still has not been recognised fully.
Sources: Bletchley Park National Codes Center