Bletchley Park Tour
Enigma - breaking the unbreakable code
The Enigma cypher machine 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.