Eli5 how did the allies in ww2 decipher German enigma if there were millions of possibilities for letters and it was being changed every day?
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The enigma machine scrambled text, by outputting a different letter than what the input was, which was determined by settings on the machine, called keys. Inputting the scrambled text into an enigma machine with identical settings to the one that scrambled it would reverse the process and output the original text. It was a very clever system, and the inner workings of the machine itself are also very interesting, considering it was all mechanical and computers did not exist in the form they do today.
Polish scientists had actually managed to break the decryption of the machines long before the second world war but the machine itself was being updated, increasing its complexity, so a method that worked in the 30s for decrypting the enigma messages wouldn’t work on a machine from the 40s. It was also very difficult to share this information with the allies during war time since Poland was the first country occupied by the Nazi forces. They had shared their findings with the British near the start of the war, and would later aid them in decryption efforts again when they relocated in France, but the machines had been altered from what the Polish scientists knew and again, having teams of code breakers working together in secret, with some of them being in occupied territories and the ability to communicate and share information being very limited, it meant that despite their past success it still took a lot of effort to succeed. Of course the work of the Polish scientists was not in vain, since it was crucial in forming a baseline from where the efforts would begin, otherwise they’d be going in completely blind and with no idea where to start.
So, simplifying here, there were two main issues with cracking the encryption. One was that first they had to understand how the machines worked, and then even if they had that they had to know the encryption keys, the machine settings, in order to decrypt a message. However these two hurdles were also the encryption’s weakness and ultimately what led to its decryption. Even if teams of code breakers could decrypt individual messages or parts of them, this took tremendous work and trial and error and it wouldn’t help the next day when the encryption key changed. The encryption key was never included in the message, which meant the germans had to have that relayed to them in other ways. If it was relayed in non encrypted communications, those could be intercepted, and then a team with knowledge of how the machine operates could decrypt them. But since this very vulnerable, or impossible, what usually happened was that german units would receive tables of keys, usually covering large periods of time like months. This was especially important for naval and submarine crews, which spent months at sea so they would not be able to receive new keys in any easy and secure way. With those key tables they could keep all their long distance communication encrypted. But Nazi crews and units suffered defeats, and that meant that enigma machines or key tables made their way into allied hands. Ultimately it was careless handling on the Germans’ part that led to the encryption being broken.
I highly recommend watching [this](https://www.youtube.com/watch?v=ybkkiGtJmkM) video about how the machine operated.
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