The Blog of Harald

Here’s an entry covering two of my favourite topics!

Today Sensity posted Coventry Cathedral. I love his photpgraphy! I don’t remember how I tripped over his photoblog; if I recall correctly, it was right around the time he built a new studio in the attic. Anyway, I’ve been reading (viewing?) it ever since.

“Coventry” is one of the classic stories of information warfare. To maintain secrecy, the Germans used a complicated machine called Enigma to encrypt their radio communications. They believed (with good reason) that Enigma was unbreakable. By the later part of 1940, the Allies _had_ cracked the code, thanks to the work of brilliant cryptologists at Bletchley Park. It is easy to argue that this project (codename Ultra) won us the war; it’s amazing what you can do when you know the enemy’s plans in advance.

Back to Coventry. On the night of November 14/15, 1940, German bombers substantially destroyed the city center of Coventry, including the 14th century cathedral. 545 civilians were killed; 4,865 were injured. The city’s infrastructure (buildings, gas mains, transit) was destroyed.

Thanks to Ultra, Churchill knew that the raid was coming, some say as early as November 12th. However, if the Allies had _acted_ on this knowledge, the Germans would have known that Enigma was broken, and changed their codes. In order to protect the secret of Ultra, Command chose not to defend Coventry. Many lives were lost, and a city destroyed.

It’s a good story, but it’s not true. The reality is both mundane and more plausible. The Allies knew a raid was coming, but they didn’t know exactly when, and had four different potential targets. The Germans used radio beams to guide the bombers to their targets; it was only shortly before the raid that the RAF determined that the beams intersected over Coventry. Jammers were sent out to disrupt the radio signals bit their equipment was incorrectly set. RAF fighters sent to intercept the bombers downed only _one_ plane (out of over 500). Some believe that this last point is what led to the myth; claiming that Coventry was destroyed to protect Ultra is better than admitting that the RAF completely failed to stop (or even slow down) the attack.

Still, the lesson is a valuable one. Intercepting enemy communcation (encrypted or not) is only part of the problem; the other part is hiding your interceptions from the enemy. If your opponent discovers that an important plan has been intercepted, he’s goign to change that plan (or worse, start deliberately feeding you false information).