Double-Cross System - Methods of Operation

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 by this method. Later in the war, wireless sets 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 itself. On the British side, a critical aid in the fight against the Abwehr and SD was the breaking of the German ciphers. Abwehr hand ciphers were cracked early in the war, and SD hand ciphers and Abwehr Enigma ciphers followed thereafter. 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 on a regular basis early in the war, with those who controlled the release of information reluctant to provide even a small amount of relatively innocuous genuine material. Later in the war, as the system became a more coherent whole, genuine information was integrated into the deception system. For example, 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.