Columbia subject headings:
Great Britain. Government Communications Headquarters--History.
World War, 1939-1945--Cryptography.
World War, 1939-1945--Electronic intelligence--Great Britain.
World War, 1939-1945--Secret service--Great Britain.
Harvard subject headings:
Bletchley (Buckinghamshire, England)--History.
Great Britain. Government Communications Headquarters--History.
Military intelligence--Great Britain--History--20th century
World War, 1939-1945--Cryptography.
Princeton subject headings:
Great Britain. Government Communications Headquarters--History.
World War, 1939-1945--Cryptography.
World War, 1939-1945--Electronic intelligence--Great Britain.
World War, 1939-1945--Secret service--Great Britain.