Editing Enigma rotor details (section)

==Fourth rotor==
{{See also|M4 (German Navy 4-rotor Enigma)}}
[[Image:Bletchley Park Naval Enigma IMG 3604.JPG|thumb|The German Navy 4-rotor [[Enigma machine]] (M4) which was introduced for U-boat traffic on 1 February 1942.]]
The introduction of the fourth rotor was anticipated because captured material dated January 1941 had made reference to the development of a fourth rotor wheel;<ref name="Mahon62">{{Harvnb|Mahon|1945|p=62}}</ref> indeed, the wiring of the new fourth rotor had already been worked out.

On 1 February 1942, the Enigma messages began to be encoded using a new Enigma version that had been brought into use. The previous [[3-rotor Enigma]] model had been modified with the old reflector replaced by a thin rotor and a new thin reflector. Breaking Shark on 3-rotor bombes would have taken 50 to 100 times as long as an average Air Force or Army message. It seemed, therefore, that effective, fast, 4-rotor bombes were the only way forward.  Encoding mistakes by cipher clerks allowed the British to determine the wiring of the new reflector and its rotor.<ref name="Mahon62"/>