In November 1940 he was the first person to break a day's worth of the German Enigma code, deciphering 8 May 1940 by the method of Banburismus.
At Bletchley Park in World War II, Foss headed the Japanese Naval Section (Hut 7) from 1942 to 1943.
Gordon Welchman was told that Foss was highly esteemed by the Americans, and says that "before the war he was one of the most brilliant of the professional cryptographers of the Government Code and Cypher School".
Foss devised many Scottish country dances, including Fugal Fergus, John McAlpin, Polharrow Burn and The Wee Cooper o'Fife.
[3] Foss retired from GCHQ in 1953 to live at Glendarroch in St. John's Town of Dalry, Kirkcudbrightshire, Scotland.