...will be to Bletchley Park:
The National Museum of Computing is a must-see if you are ever in the UK. It was a short 30ish minute train ride up from London. We spent the whole afternoon there.
There is a rebuild of the Colossus, the the world's first electronic computer. It had a single purpose: to help decipher the Lorenz-encrypted (Tunny) messages between Hitler and his generals during World War II. The Colossus Gallery housing the rebuild of Colossus tells that remarkable story.
We saw the Turing-Welchman Bombe machine, an electro-mechanical device used to break Enigma-enciphered messages about enemy military operations during the Second World War. They offer guided tours (recommended as the volunteers have encyclopedic knowledge) and we were able to encrypt a message with the German Enigma (there's a 90 second video I made, here) and decrypt it with the Bombe, which is effectively 12 Engimas working in parallel, backwards.
I wanted to understand the computing power these systems had then, and now. Check out the website where you can learn about the OctaPi - a Raspberry Pi array of eight Pis working together to brute-force Engima. You can make your own here!
Yes, there's a Raspberry Pi Enigma-cracker. If only we'd had one in 1940...