I know that that was why the codebreakers could break Enigma, but why?
In: Mathematics
The Enigma machine was basically just a mechanism that changed circuitry in real time with each key press. It did this by moving rotors around, and those rotors had sets of inputs that mapped to an arbitrary different set of outputs. After the rotors, it went through a reflector, which mapped every output on the last rotor back through a output, putting the whole thing in reverse.
The same wiring was used for the return path, but because it had no key press, the circuit instead turned on a light. This is the crucial piece here — they reused the same wiring for compactness and production cost reasons. Nobody realized it was such a huge flaw.
If they made enigma machines with *separate* paths for the forward path and reverse path through the rotors, they could have potentially created some paths in the reflector that mapped letters back onto themselves.
And because letters couldn’t map onto themselves, the team deciphering the messages could look for messages that fit certain patterns, and immediately discard them if they had matching letters. For example, the Germans sent out daily weather reports using a specific format with 22 specific leading characters (something like WEUBYYNULLSEQSNULLNULL for “Weather 0600”). If you were to take an arbitrary message with random characters, there’s about a 60% chance that it will have one character in common with that pattern, which lets you immediately throw away so many messages to try and run through the deciphering mechanism.
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