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.

It was formed shortly after the start of the war in October 1914, as a result of codebooks and maps (containing coded squares) passed on to the Admiralty by the Russians who had seized them from the German cruiser Magdeburg when it had run aground. Two copies of the three which the warship had been carrying were recovered; one was retained by the Russians and the other passed to the British.

Room 40 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. It was provided with copies of all interceptable communications traffic, including wireless and telegraph traffic. For the whole of its existence it was under the command of Captain (later Admiral) William 'Blinker' Hall. It was managed until 1917 by James Alfred Ewing.

It played a key role in several naval engagements during the war, notably in detecting major German sorties into the North Sea that led to the battles of Dogger Bank and Jutland as the British fleet was sent out to intercept them. However probably its most important contribution was in decoding the Zimmermann Telegram, a cable from the German Foreign Office to its ambassador in Mexico.

In the cable, Nigel de Gray and other cryptanalysts discovered German Foreign Minister Arthur Zimmermann making an offer to Mexico of United States territory as an enticement to join the war as a German ally. The cable was passed to the U.S. by Captain Hall, and a scheme was devised to conceal how the plaintext had become available and how the U.S. had gotten possession of it. The cable was made public by the U.S., who shortly thereafter entered the war on the Allied side.

In 1919, Room 40 was run down and merged with the British army's intelligence unit MI1b to form the Government Code and Cypher School (GCCS), later housed at Bletchley Park during World War II and subsequently renamed Government Communications Headquarters (GCHQ) and relocated to Cheltenham.

Further reading

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