You never know what to expect next from Bletchley Park. Every time I turn up there something comes out of the blue that reminds me why it is one of the most fascinating places on Earth and this week was no exception. I knew the Bombe team were busy and ‘up to something’ and that GCHQ were involved, but not much more than that.
It turns out that GCHQ have challenged the Bombe rebuild team to a spot of codebreaking. As part of The Times Cheltenham Science Festival, GCHQ have teamed up with Bletchley Park to do something that hasn’t been done in a long time; live codebreaking using WWII technology. Best of all, the public can take part. They’ve been working away at this all week and this weekend it gets very special indeed.
GCHQ have set up an original Enigma machine on their stand at the festival. Every day the Enigma is reconfigured with new settings. As the public visit, they are offered the chance to operate the machine and encrypt a message of their choosing. The resulting ciphertext has been sent over Twitter to the Bombe team at Bletchley Park and they have to crack it. This is particularly difficult for our modern-day codebreakers as cribbing (guessing the partial content of the message) is going to be very difficult indeed. However, after about three hours on each day, the Enigma machine’s rotors, order, ring settings and plugboard (‘stecker’) configurations have been successfully identified.
On Saturday, the teams will be joined by the Cheltenham and Bletchley Amateur Radio Societies. They will add a further sense of authenticity to proceedings by relaying the messages from Cheltenham to Bletchley Park using morse code. The plaintext will then be Tweeted back in a nod to the progress of technology.
When a piece of ciphertext is handed to the codebreakers, they will first crib it. This is the process of identifying the position of known (or guessed) piece of text in the ciphertext. This is made easier as Enigma could never encipher a letter as itself, so bad positioning would often be given away by two letters matching between the ciphertext and plaintext. When the position of a piece of text has been established, a menu is then drawn up. This is a diagram showing which key-presses resulting in what ciphertext and potential links between them (the operator pressed ‘C’ and ‘J’ lit up, so they are connected). The rear of the Bombe contains a complex wiring system called the Diagonal Board. This can be ‘plugged up’ to match the menu. The Bombe now checks a range of possible settings using it’s array of 36 ‘Letchworth Enigmas’, analogues for the German machines. When a setting is found that could produce the menu, the machine stops and provides information on the rotor positions and the plugboard. Once a good ‘stop’ has been found, our codebreakers will then refine this information, filling in the gaps, until the full set up is known.
During WWII, the discovered settings would then be transferred to Typex machines, the British Enigma equivalent. They were of very similar design and could be modified to operate identically. Wrens (WRNS – Women’s Royal Naval Service) would operate these machines, converting the day’s ciphertext into plaintext.
This is one of the very few times Phoenix, the ‘rebuild’ Bombe, will have been used in anger, as the operators do not know whether the information output by the machine is correct until they verify it. Everything is being undertaken as authentically as possible. Messages are cribbed, menus drawn up, the Bombe is then ‘plugged up’ and ran. Each stop is tested on a rebuilt Checking Machine and when settings are found, plaintext is generated using an original modified Typex cipher machine. The plaintext is then tweeted back to the sender to prove their message could be read.
Here’s the decision you need to make. If you come to Bletchley Park this weekend, you can see real WWII-era codebreaking at work, including seeing the Bombe hunting for the settings. If you go Cheltenham, you have the very rare opportunity to operate a real Enigma machine and to have your code broken. A constant video linkup between the two sites will allow you to view progress. They’re running Saturday and Sunday, so why not do both?