Enigma

keyboard → plugboard → rotors → reflector → rotors → plugboard → lamp

With every keystroke, the current runs through the plugboard, three wired rotors, strikes the reflector, runs back through the rotors and the plugboard — and lights a lamp: the enciphered letter.

Before each letter, the right rotor steps forward. On its notch, it carries the middle one along; and through a quirk of the mechanism — the double step — the middle rotor sometimes carries the left one as it moves itself. The electrical path changes with every keystroke: the same key never gives the same sequence twice.

The reflector always sends the current back along a different path: a letter is never enciphered as itself. That elegance was a flaw — at Bletchley Park it was used to rule out thousands of impossible settings.

Enciphering and deciphering are one and the same operation: same settings, type the ciphertext, and the plaintext reappears.

Verified historical configurations: the example from the 1930 Enigma instruction manual (rotors II·I·III, reflector A) and the first part of the Army Group North message of 7 July 1941 — Operation Barbarossa (rotors II·IV·V, ten plugs). These two published messages serve as this machine's test vectors: it deciphers them to the letter, and each preset reloads the original ciphertext.