The ‘Shire – 12 Sept: Bletchley Park

I have to apologise that this entry is out of sequence. I just found it in the ‘drafts’ pile!

Monday was Bletchley Park day. This place had remained obscure until the mid 70s when it was finally revealed that it had been a top secret base for decrypting German radio traffic during the war. Somehow, about 10,000 people that had worked at Bletchley had kept their secret for over 30 years!

During the war, the British had collected a crew of linguists, mathematicians and various other disciplines and set to work trying to decipher the German Enigma ciphers. The Enigma was an off-the-shelf encryption machine that was available before the war (from Harrod’s if you like), which was then modified by the Germans. Each German unit carried one and was required to report daily on their whereabouts and activities.

The machine itself simply translated a button press on the typewriter-like keyboard into a different letter, which lit up on a display above the keyboard. The encryption was performed by the wiring between the keyboard letter and the lights. This was changed for each transmission by setting three rotors to a certain combination per the daily encryption sheet. The wiring from the keyboard passed through the rotors, so by setting the rotors to a different position you would completely change the way that the message was encrypted. There was another variable in the encryption that they called the ‘starting position’, but I can’t remember how this worked.

With a few clues and some mistakes by the Germans, the Brits were eventually able to decrypt the Enigma messages. The first clue was given in the way the wiring was laid out: the letter pressed on the keyboard could never be encrypted into the same letter. This meant that for a known plaintext, you could compare the ciphertext (the encrypted version) and rule out all possibilities where a letter would be translated into itself.

Second, not all units had anything interested to say. Some, posted far behind the front and not doing much at all would understandably get a bit lazy and fall into routine. There was a particular German phrase used which meant ‘no special events’, and this would be broadcast almost daily. The listening Brits, then, could predict these units, narrow down the potential rotor positions based on the non-matching letters, and by doing so figure out the rotor positions for the entire German navy for that day. Once done, it was trivial to decrypt every message of the day – you simply set the same rotor combination and typed the ciphertext into your own Enigma.

However, there is one more problem. If you consistently predict the enemy movements, it would only be a matter of time until they caught on and changed their encryption methods. So, a great charade began. For example, before intercepting a fleet or attacking a U-boat, one had to accidentally stumble across it. And those doing the stumbling must also be made to think it was an accident.

And so the war of the seas was turned around.

Later in the war, a new signal was intercepted. This was not the morse code over the radio that the operators were used to. This was encrypted teletype. The encryption was too much for the previous (mechanical computing) methods to handle, and so the world’s first computer was created (by the Post Office!) to solve the problem. It was called the Colossus. It is a ‘real’ computer: blinking lights, rows and rows of huge vacuum valves, spinning loops of punched paper tape and ancient mad professor to explain it all. Awesome!

Bletchley’s other claims to fame: the brilliant Alan Turing (one of the fathers of computer science) ran the code breakers unit and Ian Flemming of James Bond fame was closely associated with the naval intelligence coming out of Bletchley.