Enigma Rotor

The Enigma machine was the German military's primary encryption device from the 1920s through World War II. Its security came not from a fixed substitution but from a constantly changing one: every keypress scrambled the alphabet differently. Understanding how it works explains both why it seemed unbreakable and why it wasn't.

Analogy

Imagine a row of combination dials on a safe, but wired so that turning the first dial by one click automatically nudges the second dial when it passes a marker — and nudges the third in turn. Every single character you type spins the dials into a fresh arrangement, so the same letter pressed twice in a row produces two different outputs. The whole machine is a self-rewiring scrambler; Bletchley Park didn't break it by guessing which combination the dials were set to — they broke it by noticing tiny structural quirks (like the fact that no letter was ever enciphered as itself) that leaked through regardless of the setting.

Rotors

Each rotor is a disc with 26 electrical contacts on each face and internal wiring that maps every input contact to a different output contact. That wiring is a substitution — but a mechanical one that rotates.

When a key is pressed:

1. The rightmost rotor steps forward one position.
2. If the rightmost rotor has passed its notch, the middle rotor steps.
3. If the middle rotor has passed its notch, the left rotor steps — and the middle rotor steps again (the double-stepping anomaly).

After the rotors step, the electrical signal from the pressed key travels through all three rotors, bounces off the reflector, and returns through the rotors in reverse.

The reflector

The reflector is a non-rotating disc that pairs every letter with another. Because it's self-paired, the electrical path through the machine is always its own reverse: the same machine, same settings, same starting positions encrypts and decrypts. Enigma is its own inverse.

A letter never encrypts to itself

Because the signal must pass through the reflector and back, it can never exit the same contact it entered. This is mathematically guaranteed by the reflector's construction. In practice it gave Bletchley Park a constraint to exploit: any guessed plaintext letter that matched the ciphertext letter at the same position could be immediately rejected.

The key space

A three-rotor Enigma with 5 available rotors, 26 starting positions each, and a plugboard connecting 10 letter pairs had roughly 10¹⁸ possible settings. That's why frequency analysis and brute force were both useless. Breaking Enigma required the Bombe — an electromechanical machine that exploited structural weaknesses, primarily the no-self-encryption property and the practice of repeating message settings at the start of every transmission.

Playground

Configure rotor order (I–V) and starting positions (A–Z). Type characters and watch each rotor step. The playground uses historical wiring tables and a UKW-B reflector. No plugboard — this version is educational, not combat-grade.

Visualizer

The rotor stack shows the live electrical path for each keypress: starting contact → rotor I → rotor II → rotor III → reflector → rotor III (reverse) → rotor II (reverse) → rotor I (reverse) → output lamp. Each segment lights up in sequence as you type.