Bletchley Park in WWII
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. 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 managed 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.