Enigma M4: U-boat Cipher

On 1 February 1942, the U-boat fleet changed machine and the Atlantic went quiet. The Enigma M4 added a fourth wheel — a thin Beta or Gamma rotor, paired with a thinner reflector, occupying the space the M3's reflector had filled on its own. The fourth wheel does not step. It does not need to: it multiplies the starting positions by twenty-six, and that was enough. The Germans called the new network Triton. Bletchley Park called the traffic Shark, and could not read it for the better part of a year. M4 is a working reconstruction of that machine. Four windows instead of three. Historical wiring, historical notches, historical stepping — including the double-step. Encipher a signal, wind back to the same ground setting, type the ciphertext, and the plaintext returns, because the machine is its own inverse. Set the thin wheel to Beta or Gamma, choose three rotors from eight for the other slots, set the rings and the ground, patch the plugboard, and you have a key that would have held up a continent's worth of codebreakers. TYPE STRAIGHT INTO ANY APP The Enigma keyboard is how a signal actually leaves this app. Switch to it in Messages, Mail, Notes or anywhere else you can type, and every letter is enciphered as you press it — the app you are in receives ciphertext, never the plaintext. A strip above the keys shows what you actually wrote, because the field itself never sees it. All four rotors step as you go, and a reset key winds them back to the day's ground setting when you start a fresh signal. SET IT FROM THE SHEET A month of daily keys, one row per day: wheel order including the Greek rotor, ring and ground settings, plugboard pairs. Tap the day and the machine is 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 have already written • Home Screen and Lock Screen widgets carrying the day's key • Shortcuts and Siri actions for enciphering and deciphering YOUR OWN TRITON Two people reading each other need the same key sheet, exactly as 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 is no server. Whoever holds it can read you; nobody else can. Nothing you type leaves your phone.