Editing Double-Cross System (section)

== Methods of operation ==
The main form of communication that agents used with their handlers was [[secret writing]]. Letters were intercepted by the postal censorship authorities and some agents were caught. Later in the war, [[Radio|wireless set]]s were provided by the Germans. Eventually transmissions purporting to be from one [[double agent]] were facilitated by transferring the operation of the set to the main headquarters of MI5. On the British side, the fight against the Abwehr and SD was made much easier by the breaking of German [[Code (cryptography)|ciphers]]. Abwehr hand ciphers were cracked early in the war and SD hand ciphers and Abwehr [[Enigma cipher]]s followed on 8 November 1941 by [[Dilly Knox]], agents sent messages to the Abwehr in the simple code which was then sent on using an enigma machine, with the simple codes broken it helped break the daily enigma code.<ref name="Flem11">{{cite book |title=Ian Fleming's Commandos: The Story of 30 Assault Unit in WWII |last=Rankin |first=Nicholas |publisher=Oxford University Press |isbn=978-0199782826 |date=2011}}</ref> 

The [[Abwehr]] used a different version of Enigma machines, in November 1942 a machine was captured in Algiers during [[Operation Torch]], it was found to have no plug board however the three rotors had been changed to rotate 11, 15 and 19 times rather than once every 26 letters, plus a plate on the left acted as a fourth rotor. The capture of a machine greatly helped the quicker decoding of German messages.<ref name="Flem11"/> The [[signals intelligence]] allowed an accurate assessment of whether the double agents were really trusted by the Germans and what effect their information had.

A crucial aspect of the system was the need for genuine information to be sent along with the deception material. This need caused problems early in the war, with those who controlled the release of information being reluctant to provide even a small amount of relatively innocuous genuine material. Later in the war, as the system became better organised, genuine information was integrated into the deception system. It was used to disguise the development of "[[Gee (navigation)|Gee]]", the Allies' navigation aid for bombers.<ref name=jones>Jones RV; Most Secret War 1978</ref>{{rp|ch 25}} One of the agents sent genuine information about [[Operation Torch]] to the Germans. It was postmarked before the landing but due to delays deliberately introduced by the British authorities, the information did not reach the Germans until after the Allied troops were ashore. The information impressed the Germans as it appeared to date from before the attack, but it was militarily useless to them.