MASK is the codename for one of the most sensitive, long-term sources ever run by any British intelligence organisation. 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, 14,000 messages of encrypted wireless traffic exchanged between the imposing headquarters of the Communist International in Moscow, at 36 Mokovaia Street, directly opposite the Kremlin, and numerous Comintern representatives abroad, in countries as far apart as China, Austria and the United States. The source was so secret that its existence was not revealed to the Americans until 1946 when copies of the traffic were given to Cecil Phillips of the US Signals Security Agency when he was posted to GCHQ's postwar headquarters at Eastcote.