As long time readers of my blog might remember, I’ve been fascinated by old cryptographic machines. I spent quite a bit of time tinkering around with them back when I was working on Simon Singh’s cipher challenge in his book. In particular, I spent a considerable amount of time reading up on the German Enigma machine, and eventually managed to break Stage 8 of that challenge using an Enigma machine simulator that I coded up. I also have a fair number of books on Enigma.
For all that, I didn’t actually know much about the other great code breaking effort at Bletchley Park: the break of the German “Tunny” code using Colossus, an even more impressive machine than the “Bombe” which allowed the breaking of Enigma. My lovely wife scanned my Amazon wishlist at Christmas, and picked up this book for me for my Kindle.
If your tastes in reading are sufficiently refined to the point where reading about sixty year old code machines is interesting, I think you’ll enjoy the book. It isn’t too technical/nuts-n-bolts, but it does give a good basic idea of how the Tunny operated, and how the British developed an advanced code-breaking bureau that saved thousands of British lives and allowed the preservation of important supply lines in the face of German U-boats. If you are more interested in the history, you might do well to also pick up a copy of Codebreakers: The Inside Story of Bletchley Park, but Colossus includes more technical details on the Tunny.
After the war, the British destroyed Colossus, and most of the records of its function were lost or classified. But in 2000, the British released the “General Report on Tunny with Emphasis on Statistical Methods” which was written in 1945, and details much of the techniques they used to attack the Tunny. It’s online at alanturing.net and also makes for some interesting (and free!) reading.