Editing Cryptanalysis of the Enigma (section)

====M4 (German Navy 4-rotor Enigma)====
[[File: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]]
On 1 February 1942, the Enigma messages to and from Atlantic U-boats, which Bletchley Park called "Shark", became significantly different from the rest of the traffic, which they called "Dolphin".<ref>{{Harvnb|Alexander|c. 1945}} Ch. III Para. 20</ref>

This was because a new Enigma version had been brought into use. It was a development of the [[3-rotor Enigma]] with the reflector replaced by a thin rotor and a thin reflector. Eventually, there were two fourth-position rotors that were called Beta and Gamma and two thin reflectors, Bruno and Caesar, which could be used in any combination. These rotors were not advanced by the rotor to their right, in the way that rotors I through VIII were.

The introduction of the [[fourth rotor]] did not catch Bletchley Park by surprise, because captured material dated January 1941 had made reference to its development as an adaptation of the 3-rotor machine, with the fourth rotor wheel to be a reflector wheel.<ref>{{Harvnb|Mahon|1945|p=62}}</ref> Indeed, because of operator errors, the wiring of the new fourth rotor had already been worked out.

This major challenge could not be met by using existing methods and resources for a number of reasons.
# The work on the Shark cipher would have to be independent of the continuing work on messages in the Dolphin cipher.
# Solving Shark keys on 3-rotor bombes would have taken 50 to 100 times as long as an average Air Force or Army job.
# U-boat cribs at this time were extremely poor.<ref>{{Harvnb|Alexander|c. 1945}} Ch. III Para. 21</ref>

It seemed, therefore, that effective, fast, 4-rotor bombes were the only way forward. This was an immense problem and it gave a great deal of trouble. Work on a high speed machine had been started by [[C. E. Wynn-Williams|Wynn-Williams]] of the [[Telecommunications Research Establishment|TRE]] late in 1941 and some nine months later [[Harold Keen]] of BTM started work independently. Early in 1942, Bletchley Park were a long way from possessing a high speed machine of any sort.<ref>{{Harvnb|Mahon|1945|p=63}}</ref>

Eventually, after a long period of being unable to decipher U-boat messages, a source of cribs was found. This was the [[Kurzsignale|Kurzsignale (short signals)]], a code which the German navy used to minimise the duration of transmissions, thereby reducing the risk of being located by [[high-frequency direction finding]] techniques. The messages were only 22 characters long and were used to report sightings of possible Allied targets.<ref>{{citation |last=Sale |first=Tony |author-link=Anthony Sale |title=The Breaking of German Naval Enigma: U Boat Contact Signals |series=Codes and Ciphers in the Second World War: The history, science and engineering of cryptanalysis in World War II |url=https://www.codesandciphers.org.uk/virtualbp/navenigma/navenig10.htm |access-date=1 December 2008}}</ref> A copy of the code book had been captured from {{GS|U-110|1940|2}} on 9 May 1941. A similar coding system was used for weather reports from U-boats, the ''Wetterkurzschlüssel'', (Weather Short Code Book). A copy of this had been captured from {{GS|U-559||2}} on 29 or 30 October 1942.<ref>{{Harvnb|Budiansky|2000|pp=341–343}}</ref> These short signals had been used for deciphering 3-rotor Enigma messages and it was discovered that the new rotor had a neutral position at which it, and its matching reflector, behaved just like a 3-rotor Enigma reflector. This allowed messages enciphered at this neutral position to be deciphered by a 3-rotor machine, and hence deciphered by a standard bombe. Deciphered Short Signals provided good material for bombe menus for Shark.<ref>{{Harvnb|Mahon|1945|p=64}}</ref> Regular deciphering of U-boat traffic restarted in December 1942.<ref>{{Harvnb|Mahon|1945|p=77}}</ref>