4. October 2014

Bletchley Park and the rebuilt bombe

A visit to Milton Keynes gave me a brief look into former codebreakers’ life at Bletchley Park during World War II. The exhibition also featured a rebuilt exemplar of the bombe, an electromechanical machine that played a role in breaking the code of the Enigma. I got fascinated for several reasons.

North of London. An hour by train. There is Bletchley Park, the famous site where the codebreakers worked in the World War II in order to decrypt, edit, and translate messages intercepted from radio communication between German military forces. For someone who is interested in history, in computer science, and in extraordinary achievements, this is definitely a must see. It is one of these museums I can walk into, stay for four hours and totally forget about lunch – which otherwise rarely happens.

Whenever I learn about something new, I need to make a decision where to start. At Bletchley Park, I concentrated on a device oddly named the bombe. Trying to understand how it works, trying to understand its evolution, trying to understand who built it, who operated it, and how well it works appeared as a good starting point for me to discover the other former secrets of Bletchley Park.

This is a picture of a rebuilt bombe. Building the replica took more than 10 years of effort! But with the amazing result that we can now see it in motion. And according to the guide the volunteers who built it actually accept a codebreaking challenge from time to time. So, if you ever make it to Bletchley Park, don’t miss out on the live demonstration.

The rebuilding efforts of a bombe reminded me instantly of the difference engine by Charles Babbage. The difference engine was designed by Charles Babbage but he sadly could not complete it. Today, there are two difference engines built, one is in the London Science Museum and the other one in the Computer History Museum in Mountain View. The latter is operated by volunteers and is the most impressive way of evaluating polynomials I have ever seen.

Back to the bombe, there is a guide book available, which explains the operations of both the Enigma and the bombe. In short, the bombe was a brute-force attack designed to find relative rotor positions (the Enigma uses rotors to encrypt letters that satisfied the constraints given by a so-called crib. A crib is known plain-text that serves as input for the bombe. Finding a crib in en encrypted message was an art by itself, but assuming we know the plain text for a sequence of letters in the encrypted message, the bombe can be wired to search through the set of possible rotor positions by brute force, yielding a piece in the daily puzzles of breaking the code.

It is a long story and much more can be said about Bletchley Park and the bombe. And Colossus. And Alan Turing. And about the thousands of employees running the show during World War Two. I will certainly be back.