MASK is the codename for one of the most sensitive, long-term sources ever run by any British intelligence organization. It concealed the existence of a radio interception programme operated by the Government Code and Cipher School (GC&CS) which succeeded in monitoring, and reading, large quantities of encrypted wireless traffic exchanged between the headquarters of the Comintern in Moscow, and numerous Comintern representatives abroad, in countries as far apart as China, Austria and the United States. The content of these secret messages was of immense use to the very limited group of people who had access to it. Of greatest interest to MI5 and Stanley Baldwin's Cabinet was the material passing to and from the Communist Party of Great Britain (CPGB), which was monitored from a covert intercept station located on Denmark Hill, south London. Its principal target was the daily wireless traffic of a clandestine transmitter based in Wimbledon and operated by a member of the CPGB's underground cell, controlled by a Scot, Bob Stewart.

GC&CS was one of the most secret branches of Whitehall, under the control of the Secret Intelligence Service (SIS), and for years had supplied the Prime Minister and a handful of Cabinet ministers with summaries of decrypted foreign communications.