Columbia subject headings:
Harvard subject headings:
Electronic surveillance--Great Britain--History.
Great Britain. Government Communications Headquarters--History.
Military intelligence--Great Britain--History--20th century
Princeton subject headings:
Espionage--Great Britain--History--20th century.
Great Britain. Government Communications Headquarters
Military intelligence--Great Britain.
World War, 1914-1918--Cryptography.
World War, 1939-1945--Cryptography.