Room 40

In the history of cryptography, Room 40 (formally I.D. 25) was the room in the Admiralty which was the first location of the British cryptography effort during World War I (WWI). It was formed shortly after the start of the war in October 1914, and retained its informal name while it expanded during the war and moved into other offices. It closed in February 1919. It is estimated that Room 40 decrypted around 15,000 German communications. Room 40 was provided with copies of all interceptable communications traffic, specifically including cable traffic. One of those cables was from the German Foreign Office to its ambassador in Mexico. Nigel de Gray, and others, of Room 40 managed to decrypt that cable and discovered German Foreign Minister Zimmermann making an offer to Mexico of US territory as an enticement to join the War as a German ally. The cable was passed to the US by RN Captain Hall (the then director of Room 40), a scheme was devised to conceal how the plaintext had become available, and further to explain how the US had gotten possession of it and the cable/telegram was made public by the US. The whole affair has come to be called The Zimmermann Telegram. Shortly thereafter, the US entered the War on the Allied side. After WWI, some of its personnel were retained in Government service and merged with the British army's intelligence unit M.I.1b to from the Government Code and Cypher School, later housed at Bletchley Park during WWII; it is now called GCHQ and is in Cheltenham.

Further reading

\n* Room 40: British Naval Intelligence, 1914-1918, by Patrick Beesly. New York: Harcourt, Brace, Jovanovich, 1982 ISBN 0-15-178634-8 Category:Cryptography