Four-rotor German Naval Enigma cypher machine, MK 4, 1942.

Introduced on Atlantic U- Boats in February 1942, the code produced by the MK 4 Enigma was not broken until December 1942, after the capture of cypher documents from U-559 in the Mediterranean. The cracking of German cypher codes by Allied intelligence was a major achievement in cryptanalysis and played a key role in the outcome of the North Atlantic U-boat engagements.