Blist dated 15/4/1940

Among the fragmented and torn wartime documents relating to Codebreaking processes that were discovered in the roof space of Hut 6 there was a B-List or ‘Blist’ dated 15th March 1940 and labelled 'Yellow'. Yellow was the cover-name for the German Enigma key used in the Norwegian campaign of April 1940.

Originally called a ‘Banister List’ or ‘B List after its designer Codebreaker Michael Banister, a Blist was a list compiled in the Hut 6 Registration Room 1 that dealt with Watch keys and was produced as a prelude to composing a Bombe menu.

This list contained sufficient technical details of each message to provide identity and continuity of originator for accurate Hut 6 processing, with details of all messages on any single key. The ‘Blist’ was designed to help in the detection of “cillies” and the identification of messages with cribs.