The secret Anglo-American debate over whether to make secure French diplomatic ciphers, 1948–1954

In 1948 the American and British COMINT agencies faced a dilemma: they were able to read the encrypted messages of the French Ministry of Foreign Affairs but suspected the Soviet Union could also read French traffic. Should the Americans and British warn France about its cipher insecurity and thereby lose a source of intelligence? Or should they remain silent and allow information to leak to the USSR? For six years the Anglo-Americans secretly debated the issue before eventually agreeing to tell the French. This article examines the debate to show how the US and UK balanced communications intelligence and security when dealing with NATO allies.