Following our visit to the National Museum of Computing (see Part 1), we returned the next day for Bletchley Park proper. It’s a deceptively huge visitor facility which begins with two brief exhibition galleries designed as an introduction to the context of the second world war and the general codebreaking process. Useful as these were, they were somewhat overcrowded and didn’t quite fill the gap of the one thing I felt was really missing from the entire experience, namely a single, simple overview of exactly what happened at Bletchley Park and the way the various machines were developed and used.

That said, you certainly can’t criticize the park for lacking on content or detail. After leaving the initial visitor center you have the freedom to walk around the entire grounds, with options to take a guided tour, use a multimedia guide (a converted iPod touch) or simply rely on the various information boards. You can also visit almost a dozen of the buildings containing various exhibitions.

We started in Block B, which serves as the main “museum” of the facility. It’s absolutely packed with galleries on a host of subjects including Alan Turing’s life and work including a collection of his mathematical papers and a transcript of the apology made in 2009 by the then-Prime Minister for the way he was treated, being convicted and chemically castrated for the “crime” of being gay.

Other galleries covered the working of the Enigma machine; life working at the park (and the way recruitment centered on the ability to do the work rather than social background or demographics); and a timeline of Bletchley Park’s achievements and how they related to the course of the war. There was also a fascinating section on breaking Japanese ciphers, something that was in some ways harder thanks to the use of a different alphabet to that shared by English and German. One chilling highlight was a Japanese dictionary used by a codebreaker in which she had added a piece of paper with a phrase that she’d intercepted and decoded but did not appear in the dictionary and which she could not comprehend: “atomic bomb.”

Another exhibit covered the way the work of spies fitted in with the codebreaking efforts. It turned out that contrary to the impression given in the Imitation Game movie, the codebreakers did not decide what was done with the information they gathered. Instead it was senior military staff who figured out ways to use the information without risking Germany finding out the ciphers had been broken. Sometimes this simply involved physically confirming details by encouraging reconnaissance planes to look in a specific area, but often it involved spies pretending to have acquired the information.

The highlight of the block was undoubtedly the section of the Bombe (pictured above), a machine developed by Turing and Gordon Welchman (and based on an earlier Polish machine) that was designed to discover the daily settings used by German operators on Enigma. Each Bombe was made up of rotating drums that combined to simulate 36 different Enigma machines. In very simplified manner it worked on the following process:

• Staff would use linguistic knowledge and skills to try to find a ‘crib’, which was a string of letters in the encrypted message that might match a particular phrase (such as “Heil Hitler”) expected to appear in the message.
• The Bombe could test every potential combination of letter pairs, trying to find the 13 pairs that were transposed in that day’s encryption. The “crib” was then used to come up with initial starting points to increase the chances of confirming a match quickly.
• The drums on the machine were all connected electronically. As each drum rotated (corresponding to a different letter each time) the circuit changed. Each set of rotors had 26 wires (one in either direction for the 13 letter pairs) and was set up such that as long as each tested combination was found to be impossible based on the available information, the electricity continued to flow. If the machine couldn’t rule out particular combinations, the circuit was broken and the machine came to a stop.
• An operator would then take the potential set of 13 pairs and use a replica Enigma machine to decode a message and see if the result was plain text German.

In effect, the machine simply used the same process as a brute-force attack on encrypted data today. However, technically it’s usually classed not as a computer but an electro-magnetic machine. That’s because, while the data input could change, it always carried out the same function.

Those with a serious love of engineering will likely have been satisfied with an extensive display of seemingly every component ever used in a Bombe. For the rest of us the highlight was a demonstration of a rebuilt Bombe by a woman who operated it at Bletchley during the war (and is presumably now in her early 90s.) After describing the sheer noise and heat involved in its operation, she explained how at the end of the war she helped dismantle it down to individual components to avoid Russia discovering its secrets. She was told that if questioned about her wartime activity she should simply say she had been a shorthand typist. She then shared an ironic story: after the war she and her husband ran a laundrette (laundromat) in which “every day I’d sit watching a dozen drums spinning round, looking at everyone’s secrets!”

(In part 3, coming on Monday, I’ll cover the rest of Bletchley Park’s exhibits.)