Enigma Machine

What it's for

A working copy of the Enigma machines Germany used in the Second World War: the Enigma I of the Army and Air Force, the Navy’s M3 and the four-rotor M4. Enigma is its own inverse, so typing a ciphertext with the same settings gives back the message.

For example, when you’re reading about Bletchley Park and want to decrypt a real wartime message with its published settings, or send a friend a message they can decode with the same machine.

How to use it

Choose the model, reflector, three rotors (and the M4’s thin fourth rotor), the ring settings, the start position and any plugboard pairs. Settings are remembered. Ring settings and start position take letters (AAA) or numbers from 1 to 26 (01 01 01), left to right; on the M4, give four. Plugboard pairs are written like AV BS CG.

Type in Input and the output appears as you type, computed from the start position each time. The rotors’ positions after the last letter are shown, and the lamp for the last letter lights. Continue from here makes those positions the new start and clears the input. Letters only, in groups of five drops everything but letters and groups the result the way operators sent it.

Example

With the default settings (Enigma I, reflector B, rotors I, II and III, rings and start AAA, no plugs), type:

HELLOWORLD

The output is ILBDAAMTAZ and the rotors show A A K. Type ILBDAAMTAZ instead and you get HELLOWORLD.

Good to know

Includes the middle rotor’s double step, as the real machines had. Characters other than A to Z pass through unchanged and don’t turn the rotors; operators spelled out numbers and wrote X for spaces. Enigma never encrypts a letter to itself, which was one of the weaknesses that let it be broken.