Enigma I: Army Cipher
This is where the story starts, so it’s where I started too.
The Enigma I was the German Army’s standard cipher machine. You picked three rotors from a box of five, set their rings, dropped them in and patched the plugboard. There were so many possible settings that the people using it were sure nobody could read their traffic. Marian Rejewski read it anyway, in 1932, years before Bletchley Park got involved.
I wanted a real machine, not an animation of one. Press a key and the rotors step, the current runs through the plugboard, the rotors and the reflector and back again, and a lamp lights. The wiring is the historical wiring, and the stepping is the historical stepping, double-step and all. Encipher a message, go back to the same ground setting, type the ciphertext in, and your original text comes back out. No trick. That’s just how the machine works.
Type straight into any app#
This is my favourite part. The app comes with an Enigma keyboard you can switch to in Messages, Mail, Notes, or anywhere else you type. Every letter is enciphered the moment you press it, so the app you’re typing in only ever receives ciphertext. A strip above the keys shows what you actually wrote, since the text field never sees it. The rotors step as you go, and a reset key winds them back to the day’s ground setting when you start a new message.
Set it like an operator#
The key sheet holds a month of daily keys, one row per day: rotor order, ring setting, ground setting and plugboard pairs. Tap today’s row and the machine is set, the way a signals clerk would have done it from a printed sheet.
What’s here#
The full machine: rotors I–V, 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
A network of your own#
To read each other’s messages, two people need the same key sheet, just like two operators 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. Whoever has your sheet can read you, and nobody else can.
A note on history#
I built this because the engineering is fascinating, and so is the maths that eventually beat it. The machine served a regime that used it to wage war. What interests me is how it worked, why it failed, and the people who broke it.
Privacy#
Enigma I doesn’t collect anything. No analytics, no tracking, no advertising identifier, no account.
Nothing you type goes anywhere. Messages 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, by AirDrop or the share sheet. 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 message wouldn’t decipher, I’ll probably ask for the rotor order, ring setting and ground setting first, so feel free to include them.