How the Bletchley Park code-breaking station came into existence
In part one of our feature, we look at how the Polish started to break the German Enigma code before passing the research onto the British.
THE BRITISH TAKE OVER
Time was running out for the Polish. On 15th March 1939 the Germans took Bohemia and Moravia, and then withdrew from the German-Polish Non-Aggression Pact on 27th April 1939. With the huge threat on their doorstep, a conference was held in Warsaw on the 26th July, where the Polish revealed to the British and French that they’d broken Enigma and pledged to give each country a working reconstruction of the military Engima machine, along with details of their code-breaking techniques.
This was vital information, as the British had not been able to find a way into Enigma and were struggling to decode messages. Armed with the Polish information on code breaking and the internal wiring of the Enigma, the British were ready to continue the work.
On 15th August 1939 the Government Code and Cypher School (GC&CS) moved into Bletchley Park secretly, with the mission to decode Enigma signals. The park was chosen for several reasons, including its proximity to Oxford and Cambridge (many of the code-breakers were recruited from here), it was on the main railway line between London and Birmingham, Manchester and Glasgow, and the nearby repeater station at Fenny Stratford made it easy to lay dedicated cables for telephone and telegraph circuits.
An amazing team of mathematicians was put together, with the early work performed by Dilly Knox, John Jeffries (both who tragically died of separate illnesses before the war’s end) and, most famously, Alan Turing.
He was to play an very influential part in breaking Enigma, as since the Poles had found the flaw in the way that the message indicator was being transmitted, the Germans had changed tactics and started printing its code books without the ground setting. Instead, the cryptographic key for a day would constitute only of the wheel order (which rotors to use and their order), the ring settings (where the alphabetic ring on each rotor should be placed) and the plug settings of the plugboard.
The operator of a machine would pick a random rotor order (say, ‘XVF’) and used it to encode a second rotor order (say, ‘LPR’). The second rotor order was used to encrypt the final message. To transmit a message, first, in plaintext, the first rotor order was transmitted (‘XVF’), then the encoded second rotor order was sent. Finally, the ciphertext of the message was sent. The result was that every single message was encoded with a different rotor setting.
Turing knew from the Polish system how the Enigma was wired, but as the message indicator system had been replaced he had to find a different way of getting into the code. It’s how he did this that helped change the course of the war.