Encryption is a funny thing. Since the dawn of communication, whenever people have wanted to keep their secrets secret, they’ve used some sort of encryption. I’m sure parents started spelling things so their kids wouldn’t understand as soon as there was spelling. Using words their kids wouldn’t understand, while the kid sits there thinking “Oh, Dad, you’re so embarrassing, thinking I don’t know what that means…”. Encryption is all about keeping information away from people, particularly those who could use it against you. Ask the folk from Bletchley Park if you don’t realise how this can impact world events.
Rob links to Enigma and Bomba (the British system for decrypting Enigma messages) but there’s another interesting story out of Bletchley Park as well: the Lorenz cipher, which was cracked by a Polish mathematician early on, but decryption was quite slow, on the order of a message or so per day. This led to Colossus, the first digital computer in existence. The National Museum of Computing in Bletchley Park has a working rebuild of a Mark 2 Colossus on display and we got to see it (and get the story behind it) on day 1 of Data Relay this year, so that was fun to see.
As an interesting side note, the British never told the Soviets that they had decrypted the Lorenz cipher, so when the Soviets took hold of these machines near the end of World War II, they assumed that nobody had cracked the code, so they continued to use these for a while, allowing the British access to certain sensitive communications for a time.