Like all the best cryptography, the Enigma machine is simple to describe, but infuriating to break.
Straddling the border between mechanical and electrical, Enigma looked from the outside like an oversize typewriter. Enter the first letter of your message on the keyboard and a letter lights up showing what it has replaced within the encrypted message. At the other end, the process is the same: type in the “ciphertext” and the letters which light are the decoded missive.
Inside the box, the system is built around three physical rotors. Each takes in a letter and outputs it as a different one. That letter passes through all three rotors, bounces off a “reflector” at the end, and passes back through all three rotors in the other direction.
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