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?
In: 14
Every message started (or ended, I forget the details) with “Heil Hitler”. Duplication is the enemy of encryption.
Enigma had two flaws and the Allies had one advantage.
The first flaw was design. Enigma could encode a letter as any other letter except itself. So a “H” could never be encoded as an “H” in the message.
The second flaw was human. The Germans would end most messages with “Heil Hitler”.
So if you know the message ends with the same two words every time. And you know the starting code can’t code for itself. You have a pretty good place to start cracking.
It still would take thousands of thousands of computations to decipher it. So this is the Allies advantage. Allen Turing invented a computer to do all those calculations. It could in a few hours solve what would take a team of mathematicians months to do on paper.
You should watch “The Imitation Game” which is a somewhat fictionalized story about how the Enigma codes were broken. It was much more involved than the movie makes it out, but it’s a decent telling of part of the story.
Spies were used to gain information regarding the machines and the messages sent. The Polish gave the allies important information on how the machines were constructed and the underlying code cypher principles. Code-breakers worked to decode individual repeating parts of messages, and then extrapolate the entire message from those fragments. That was what Turing’s machine did. Cipher books were seized by the allies at various times during the war, which made it possible to decode messages until those ciphers were updated.
But the basic reason that they were able to decode Enigma was humans. In theory, it should not have been breakable. But because humans are fallible, some procedures and working parts were compromised and that allowed others to break the codes.
They did it in a few ways. The first breakthrough was by looking for words they knew were likely to be there. Every morning the German military sent out a weather report. The codebreakers knew that the report was almost certain to contain the German word for weather, “wetter,” and that almost all messages contained the the phrase “heil Hitler.” This meant that, at a minimum, the weather report gave them the letters W, E, T, R, H, I, and L.
Once that was figured out there were able to look at past messages and find out two other major things:
The first is that a letter was never encoded as itself, the first letter in “wetter” would never be “W.” That eliminated one possibility for each letter, and going from 26 possibilities to 25 makes a huge difference.
The second is that numbers were always spelled out, it was “eins” rather than “1.” So if there were numbers involved, which there frequently were, you got those letters as well. So just from “wetter,” “heil Hitler,” and the numbers 0 to 12 you would have the letters W, E, T, R, H, I, L, N, U, S, Z, D, V, F, C, B, A, and O. That’s more than half the alphabet and every single vowel.
At that point it’s sort of like Wheel of Fortune because letters aren’t in random orders, they’re spelling out words and those words have context. For example “— — —– —-” could be anything, but once I have filled in “one two t_ree fo_r” it becomes obvious what the missing letters are.
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.
There are really good answers here, so I will just offer a very basic one.
With the exception of the German navy, often times security rules were ignored. People would start messages with the same thing every time. So allies could just look for those patterns which made it easy to decode. They were suppose to use something random at the beginning, but many ignored such rules and started the messages with the same thing each time. So once allies knew what works to try to look for, they could more easily decrypt the messages.
The navy however was much more strict about security which is why it was so much harder and the allies started building machines to try to decrypt them (Imitation game). All of this is likely more nuanced than I am saying, but for your EL5 version, it’s because the Germans did not always follow security protocol and used the same information at the start of every message.
Something else to add to the story. The Poles , with some help from the French, had managed to crack early versions of Enigma and then reverse engineer their own versions of the machine for further analysis. In July 1939 when it became apparent that things were going downhill the Poles invited French and British spooks to come to Warsaw and get briefed on the work to date, they also got given copies of the Engima machine as the Poles then understood it.
A lot of other work was required but the work by the Polish in the 1930’s was a significant part of what followed.
> Gordon Welchman, who became head of Hut 6 at Bletchley Park, has written: “Hut 6 Ultra would never have gotten off the ground if we had not learned from the Poles, in the nick of time, the details both of the German military version of the commercial Enigma machine, and of the operating procedures that were in use.” The Polish transfer of theory and technology at Pyry formed the crucial basis for the subsequent World War II British Enigma-decryption effort at Bletchley Park, where Welchman worked
https://en.wikipedia.org/wiki/Enigma_machine#Breaking_Enigma
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