I did not know anything about Bletchley Park or National Museum of Computing just 1H north of London until I was in London and had dinner with my friend and EFREI schoolmate, Jerome Abela, who told me about it. It is purposely 1H outside of London just next to a train station, because it was a very secret encrypted message breaking and decoding base during WWII and in case London ever were to be bombed, this place that didn't look like anything, would be spared.
I didn't quite know that Bletchley Park is actually separate from the National Museum of Computing (they are adjacent, but separated by a fence, needing an annoying 10-15mn walk all the way around), and Bletchey Park actually opens earlier, so it's probably best to visit first (also check the National Museum of Computing's website for which dsays they have guided tours and guides showing the hardware (well worth it).
I'll start with the National Museum of Computing as it was the most exciting to me with its fully functional rebuild bombe and colossus decryption machines for Enigma and Lorenz (the much more secure encryption system German Command used):
One big mistake the germans did was to send a weather report starting with the same german word (known plaintext) every morning. This allowed building a computer (bombe) that tried all rotor combinations to turn the crypted message into the known platintext:
The even more impressive machine (by a lot) was the aptly named "Colossus" which was build from scratch from a reverse engineered design (that part is so impressive), to decrypt the much more secure Lorenz encryption:
the computer reads the encrypted message from this paper tape (5 bit ASCII)
this machine did the decryption once the other machine had output the decryption parameters
How lorenz was reverse engineered and cracked is complicated and super impressive, but basically all came down to the almost same message being sent twice with the same key which allowed for a known plaintext attack:
This video shows the different machines in action Bombe rebuild (to break enigma), Colossus rebuild (to break Lorenz), plus the oldest still working half-mechanical computer (Harwell Dekatron):
And the museum had lot of other computers, a collection that is close to being as good as the one at the computer history museum in Mountain View, CA:
they have a whole collection of tubes (pre-transistors) to replace the ones that break on their machines
impressive they had so many of those machines, still working
I had one of those
and this too (Asmtrad CPC464+memory upgrade+floppy)
didn't have this one at home, but worked with them at SGI
those I never had, but I wish I did. Archimedes was awesome and way ahead of its time with Arm RISC CPUs
The Harwell Dekatron was also a very cool (and still working) computer I had never seen:
Watch it in action:
Bletchley Park had different buildings each with their story and what they were used for:
Lots of info on the machines, an earlier crack of older enigma machines was to use EINS for plaintext attacks:
They had many displays on the brilliant mathematicians that broke the codes and built the machines. Alan Turing was one of them, but they were multiple others:
Despite some inefficiencies in having to go back and forth between the 2 museums to join a timed tour at Bletchley (which actually is skipable if you are short on time), I spent the entire day there (open to close) and it was very well worth it.