eli5: Why could the Enigma Machine not map a letter onto itself?

Question

I know that that was why the codebreakers could break Enigma, but why?

In: Mathematics

Answer 1

The Enigma machine had a design rule where no letter could encrypt to itself because of the internal mechanics of the rotors and the plugboard. It actually messed with the pattern predictability, which ironically helped in codebreaking. It’s one of those quirks that was both a strength and a weakness.