I·II·III Enigma machine simulator

How to use it

The three rotors sit left to right: left, middle and right. For each one pick its number (I–V), its ring setting and the starting letter in its window. Below them are the reflector, B or C, and the plugboard: letter pairs separated by spaces, such as AV BS CG. A letter may appear in one pair only, and the tool flags a clash.

Type on the machine’s keyboard or on your own. A lamp lights up with the cipher letter, the rotors turn, and the current’s path is drawn underneath. For a longer text, paste it into the Text field: the result appears at once, in groups of five, the way it went out over the radio.

Decrypting is the same operation. Set the same rotors, rings, start letters and pairs, paste the ciphertext, and the plaintext comes out. To try it, open the example. It is a real German army signal from 1941 (rotors II-IV-V, rings B-U-L, start BLA) that begins AUFKL X ABTEILUNG: Aufklärungsabteilung, “reconnaissance unit”, with X for a space.

How it works

A key press first moves the right rotor on by one position, and only then closes the circuit. The current runs through the plugboard, through the three rotors right to left, into the reflector, back through the rotors and the plugboard again, and on to a lamp. Each rotor is a scrambled set of 26 wires; the reflector joins letters in pairs.

When the right rotor passes its notch, it pushes the middle one. When the middle rotor reaches its own notch, on the next key press it moves again and pushes the left rotor too. That is the double step: the middle rotor steps twice in a row. The notches differ by rotor: I turns over after Q, II after E, III after V, IV after J, V after Z.

The ring turns a rotor’s wiring against the letters printed on its rim. Rotor order, rings and plugboard pairs made up the daily key, which changed every day; the start letters were chosen afresh for each message.

Because of the reflector the machine is symmetric: if A goes to X, then with the same settings X goes to A. And a letter never encrypts to itself. That one property is what crib searching was built on.

Questions

Where did my spaces go?

Enigma has 26 Latin keys and nothing else. Operators typed X for a space. In “auto” mode the tool does the same with a plaintext, and simply drops the spaces in a ciphertext written in fives. Cyrillic is transliterated to Latin.

Is this the navy’s machine?

This is the Enigma I used by the army and the Luftwaffe. The navy’s M4 had a fourth, thin rotor and rotors VI–VIII, which are not included here.

How many keys are there?

The order of three rotors out of five, their start positions and ten plugboard pairs give about 1.59 × 10²⁰ settings. The rings of the middle and right rotors multiply that by another 676. Blind search is hopeless; the Enigma lesson tells how it was read anyway.

Is my text sent anywhere?

No. The machine runs in your browser.