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In the summer of 1939, a small team of code breakers arrived at the Government Code and Cipher School's (GC&CS) new home at Bletchley Park, Buckinghamshire. Their mission was to crack the backbone of German military and intelligence communications, the Enigma cipher.
The Germans thought Enigma was unbreakable. The combination of rotating wheels, electrical contacts and wires meant that the odds against anyone who did not know the machine's settings being able to break Enigma were 150 million million million to one!
But Bletchley Park achieved a breakthrough when the Poles passed on their knowledge of how the machine worked. This helped the code breakers exploit a design weakness in Enigma - that no letter could ever be encrypted as itself.
At the same time, Bletchley Park mathematician Alan Turing realised that 'cribs' offered a way of cracking Enigma. A 'crib' is a piece of encrypted text whose true meaning is known or can be guessed. German messages were formulaic in places and the first line often contained standard information, for example weather conditions. Once a crib was known, it was still necessary to check thousands of potential Enigma settings to read a message, and to do this quickly Turing designed a electromechanical code breaking machine called a Bombe. Each Bombe simulated the actions of 10 Enigma machines and was able to check all potential settings at high speed.
Cracking the 'impenetrable' Enigma code enabled Britain to foil Luftwaffe bombing raids, minimise U-boat attacks and secure sea-based supply routes
Simple mono-alphabetic substitution ciphers have been around for a long time but they suffer the severe disadvantage that the same plain text message is always encoded as the same enciphered characters. Breaking such a code is quite trivial. The Enigma cipher machine used a series of rotors which continually changed the wiring for each character to be encoded. Encoding a string of 'A''s will not produce the same enciphered output character but rather a string of garbled text.
The actual machine was developed in 1918 by Arthur Scherbius as a machine to allow businesses to communicate confidential documents. It was later (much later) adopted by the German Navy. The machine had a keyboard which allowed the user to enter a character. The electrical signal from that key was swapped for another letter by a rotor (for example, the letter 'A' might emerge from the first rotor as 'E'). The signal emerging from the first rotor then went through two other rotors to be further scrambled.
Now for the real trick: the rotor advanced one position for each letter enciphered. Each rotor had 26 positions (one for each letter of the alphabet) and once a rotor had advanced 26 positions it would start over - after advancing the next rotor to the left much like a car odometer does. If only one rotor were used, transposition would be repeated after 26 letters. With two rotors it occurred after 26 x 26 = 676 letters, with three 26 x 26 x 26 = 17,576. This made the enciphered code very difficult to decode.
The Enigma machine had a fixed umkehrwalze (or reflecting rotor) on it's left side after the third rotor which "reflected" the electrical pulses back through the string of 3 rotors in the opposite path. This resulted in reciprocity between encipherment and decipherment; if "a" was enciphered as "X" then "x" would be enciphered as "A" so that this machine could either encipher or decipher at any given rotor setting.
The disadvantage was that if somehow one letter of a message was decoded, a second one would also be known. As well, no letter could ever represent itself.
Another complication added was the Steckerboard, a a plug board with 26 jacks and a number of patch cords (usually 6) which could cross-connect any two letters between the keyboard and the rotors. This raised the number of possible encipherments to a staggering 10 quadrillion. You can understand why the Germans were convinced that their codes were quite unbreakable!
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