Columbia subject headings:
Harvard subject headings:
Cryptography--History--20th century.
Enigma cipher system--History.
Military intelligence--History--20th century.
Spain--History--Civil War, 1936-1939--Cryptography.
Spain--History--Civil War, 1936-1939--Electronic intelligence--Great Britain.
ULTRA (Intelligence system)
Princeton subject headings:
Cryptography--History--20th century.
Enigma cipher system--History.
Military intelligence--History--20th century.
Spain--History--Civil War, 1936-1939--Cryptography.
Spain--History--Civil War, 1936-1939--Electronic intelligence--Great Britain.
ULTRA (Intelligence system)