Editing Cryptanalysis of the Enigma (section)

===''Abwehr'' Enigma===
[[File:Enigma-G.jpg|thumb|Enigma Model G, used by the ''[[Abwehr]]''. It had three ordinary rotors and a rotating reflector, multiple notches on the rotor rings, but no plugboard.]]

[[Dilly Knox]]'s last great cryptanalytical success, before his untimely death in February 1943, was the solving of the ''[[Abwehr]]'' Enigma in 1941. Intercepts of traffic which had an 8-letter indicator sequence before the usual 5-letter groups led to the suspicion that a 4-rotor machine was being used.<ref>{{Harvnb|Twinn|1993|p=127}}</ref> The assumption was correctly made that the indicator consisted of a 4-letter message key enciphered twice. The machine itself was similar to a [[Enigma machine#Military Enigma|Model G Enigma]], with three conventional rotors, though it did not have a plug board. The principal difference to the model G was that it was equipped with a reflector that was advanced by the stepping mechanism once it had been set by hand to its starting position (in all other variants, the reflector was fixed). Collecting a set of enciphered message keys for a particular day allowed ''cycles'' (or ''boxes'' as Knox called them) to be assembled in a similar way to the method used by the Poles in the 1930s.<ref name=FCarterAbwehr/>

Knox was able to derive, using his ''buttoning up'' procedure,<ref name=CarterButtoningUp/> some of the wiring of the rotor that had been loaded in the fast position on that day. Progressively he was able to derive the wiring of all three rotors. Once that had been done, he was able to work out the wiring of the reflector.<ref name=FCarterAbwehr>{{citation |last=Carter |first=Frank |title=The Abwehr Enigma Machine |url=http://www.bletchleypark.org.uk/resources/file.rhtm/261894/web+abwehr2.pdf |access-date=24 April 2009 |url-status=dead |archive-url=https://web.archive.org/web/20071002221359/http://www.bletchleypark.org.uk/resources/file.rhtm/261894/web+abwehr2.pdf |archive-date=2 October 2007}}</ref> Deriving the indicator setting for that day was achieved using Knox's time-consuming ''rodding'' procedure.<ref name=CarterRodding/> This involved a great deal of trial and error, imagination, and crossword puzzle-solving skills, but was helped by ''cillies''.

The ''Abwehr'' was the [[military intelligence|intelligence]] and [[espionage|counter-espionage]] service of the German High Command. The spies that it placed in enemy countries used a lower level cipher (which was broken by [[Oliver Strachey]]'s section at Bletchley Park) for their transmissions. However, the messages were often then re-transmitted word-for-word on the ''Abwehr's'' internal Enigma networks, which gave the best possible crib for deciphering that day's indicator setting. Interception and analysis of ''Abwehr'' transmissions led to the remarkable state of affairs that allowed MI5 to give a categorical assurance that all the German spies in Britain were controlled as [[double agents]] working for the Allies under the [[Double Cross System]].<ref name="Michael Smith 2007 p. 129"/>