Enigma Machine Simulator

How the Enigma machine works

Press a key and three things happen. First the rotors step. Then current runs through the plugboard, through the three rotors from right to left, into the reflector, back through the rotors from left to right, through the plugboard again, and lights a lamp. Because the rotors move before every letter, typing AAAAA gives five different letters.

Rotors and notches

Each rotor is a scrambled alphabet. The Enigma I came with five: I, II, III, IV and V, and the key sheet said which three to use and in what order. The right rotor steps on every key. Each rotor has a notch (Q, E, V, J and Z for I to V) that kicks its left neighbour on by one.

The double step

The middle rotor has a quirk. When it reaches its own notch, the pawl pushes it again on the next key press along with the left rotor, so it moves twice in a row. With rotors I, II and III starting at ADU you get ADV, AEW, then BFX. My simulator does the same.

Ring settings and the plugboard

The ring setting turns the wiring against the letters on the rotor's rim, so the same window letters give a different scramble. The plugboard swapped pairs of letters before and after the rotors. With ten cables, as the Army used, it added a vast number of extra settings.

The reflector, and the flaw

Reflector B sends the current back through the rotors. That makes Enigma its own inverse, so the same settings decipher what they encipher. It also means no letter can ever become itself, which the codebreakers used to line guessed words up against intercepts.

A short history

Enigma was a commercial machine from the 1920s that the German armed forces adopted and hardened. Marian Rejewski, Jerzy Różycki and Henryk Zygalski of the Polish Cipher Bureau broke it in the 1930s and handed their work to Britain and France in July 1939. At Bletchley Park, Alan Turing and Gordon Welchman designed the bombe, which found daily keys from 1940. The German high command's teleprinter traffic used a different machine, the Lorenz, which I've also built: try the Lorenz cipher game.

Questions

Is this simulator accurate?

It's the Enigma I with rotors I to V, their real wirings and notches, reflector B, rings, a plugboard and the double step, and I test it against known test vectors.

Why can't a letter encipher as itself?

The reflector always sends the current back by a different path.

Who broke Enigma?

The Polish Cipher Bureau first, then Bletchley Park with the bombe.

What's the double step?

The middle rotor moving on two key presses in a row when it reaches its notch.