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…