Enigma M4: U-boat Cipher
On 1 February 1942 the U-boats switched to a new machine, and for the codebreakers the Atlantic went quiet.
The M4 squeezes in a fourth rotor: a thin Beta or Gamma wheel next to an even thinner reflector, in the space where the M3’s reflector used to sit on its own. The fourth rotor never steps. It doesn’t have to, because it multiplies the starting positions by 26, and that was enough. The Germans called the new network Triton. Bletchley Park called the traffic Shark, and couldn’t read it for the better part of a year.
M4 is a working machine, not an animation. Four windows instead of three, with the historical wiring, notches and stepping, double-step included. Encipher a message, wind back to the same ground setting, type the ciphertext in, and your original comes back out, because the machine is its own inverse.
To set it up, put Beta or Gamma in the thin slot, choose three of the eight rotors for the others, set the rings and the ground, and patch the plugboard. That’s a key that kept a lot of very clever people busy for a very long time.
Type straight into any app#
Switch to the Enigma keyboard in Messages, Mail, Notes, or anywhere else you type, and every letter is enciphered as you press it. The app you’re in only ever gets ciphertext. The rotors step as you go, and a reset key winds them back to the day’s ground setting.
Set it from the sheet#
A month of daily keys, one row per day: rotor order including the Greek rotor, ring and ground settings, and plugboard pairs. Tap the day and you’re set.
What’s here#
Four slots: Beta or Gamma, plus three rotors from I–VIII
Thin reflectors, and a thirteen-cable plugboard
The Enigma keyboard, for enciphering inside any other app
A month of daily keys, generated from a seed you can share
Share sheet support to encipher text you’ve already written
Home Screen and Lock Screen widgets carrying the day’s key
Shortcuts and Siri actions for enciphering and deciphering
Your own Triton#
To read each other’s signals, two people need the same key sheet, just like two boats needed the same printed one. Share yours by AirDrop, or hand it straight to a nearby device. It never touches a server, because there isn’t one.
Privacy#
Enigma M4 doesn’t collect anything. No analytics, no tracking, no advertising identifier, no account.
Nothing you type goes anywhere. Signals are enciphered on your device and stay there. Key sheets are made on your device too, and when you share one it goes straight to the device you choose. It never passes through a server of mine, because I don’t have one.
The keyboard has no network access and doesn’t ask for “full access”.
Support#
Mail support@backslashed.se and I’ll get back to you.
It helps to know which device and iOS version you’re on, what you expected, and what happened instead. If a signal wouldn’t decipher, include the rotor order (with the Greek rotor), ring setting and ground setting.