The German military Enigma machine of World War II was an electro-mechanical cipher device. Although it looked like a typewriter, pressing a key triggered an intricate chain of electrical events inside. Let’s walk through what happens — electrically and mechanically — when an operator presses a single key.
When the operator presses a key (say G), it completes an electrical circuit powered by a 4.5V
battery inside the machine. The current begins its journey through the machine's various components.
The current first flows through the plugboard at the front of the machine. This is where letters can be swapped
in pairs. For example, if G is plugged into L, the current now proceeds as though the
user had pressed L instead.
The plugboard added an extra layer of permutation, significantly increasing cryptographic complexity.
The current passes from the plugboard into the fixed entry wheel, or ETW, which connects to the rightmost rotor. This simply maps the 26 letters in a fixed way — no scrambling happens here.
The signal now passes through the three rotors, from right to left:
Each rotor has 26 electrical contacts on each side, internally wired in a scrambled pattern. The rotors rotate with each keypress, causing the internal wiring alignment to change — that’s the “mechano” part of electro-mechanical.
The rotor sequence and starting positions were set daily using a key sheet. A ring setting (Ringstellung) could also offset the internal wiring relative to the rotor position.
Once the current leaves Rotor I, it hits the reflector. The reflector redirects the signal back through the rotors via a different path. It doesn’t encrypt — it simply ensures encryption is symmetrical: the same settings used to encrypt can decrypt.
The current now re-enters the rotors, passing through them in reverse order:
Because of the internal scrambling and rotor positions, the return path is entirely different from the first pass.
After exiting Rotor III, the signal goes back through the plugboard. If the output letter is also wired in the plugboard, another substitution happens.
Finally, the current lights up one of the 26 bulbs on the lampboard — say, Q. That’s the encrypted
letter. The operator notes it, then presses the next key.
With each keypress, the rightmost rotor steps forward one letter (like an odometer). The middle and left rotors step less frequently, determined by notch positions.
This mechanism introduces complexity: