Bombe Machine, Bletchley Park
Remember when computers were actually used for computing? Me neither.
This is the Bombe Machine, a device used to help decipher the German Engima Machine signals - it was akin to a brute force password cracker today.
This is from a trip to Bletchley Park - the UK's main decryption center during the war - and the most interesting thing was the revelation as to how a lot of the codes were cracked. In the end, it came down to some very intelligent guesswork and a lot of luck.
For example, a lot of the message senders would use names of towns and girlfriend names as the cipher.
In one instance, a guy in Greece had been sending a message to another guy in Germany. It was a 4000 character long message and would have taken a long time for him to type. He sent it through, but the guy in Germany messaged back asking him to re-send the message as they didn't receive the signal properly; curiously, the UK interception center DID receive it properly. But this would have frustrated the Greek message sender, and he re-sent the message, but this time by using a lot of abbreviations and he didn't reset the Engima machine before sending it in his ire, even though he should have done this. The two messages combined, with the knowledge that abbreviations were being used and they were using the same cipher, helped decipher it and did warn the Allies of an impending attack. I would also say it ended the war but this isn't a goddamn fairy tale.