Enigma M4
The four-rotor Enigma variant used by U-boats, which gave Bletchley Park its hardest stretch.
M4 was introduced in 1942 for the German naval network used by U-boats. A fourth rotor, thin and non-stepping, multiplied the number of possible settings by 26 compared with the usual three-rotor machine. Bletchley Park called the network Shark and lost access to it for most of 1942.