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In the history of cryptography, Room 40 (latterly NID25) was the room in the Admiralty most identified with the British cryptography effort during World War I.

It was formed shortly after the start of the war in October 1914. Admiral Oliver, the Director of Naval Intelligence, gave intercepts from the German radio station at Nauen near Berlin to Director of Naval Education Alfred Ewing, who constructed ciphers as a hobby. Ewing recruited civilians such as William Montgomery, a translator of theological works from German, and Nigel de Grey, a publisher.

A German naval codebook, the Signalbuch der Kaiserlichen Marine (SKM), and maps (containing coded squares) had been passed on to the Admiralty by the Russians, who had seized them from the German cruiser Magdeburg when it had run aground off the Estonian coast on 26 August 1914. Two copies of the four that the warship had been carrying were recovered; one was retained by the Russians and the other passed to the British. In October, 1914 the British also obtained the Imperial German Navy's Handelsschiffsverkehrsbuch (HVB), a codebook used by German naval warships, merchantmen, naval zeppelins and U-Boats. This had been captured from the German steamer Hobart by the Royal Australian Navy on 11 October. On 30 November a British trawler recovered a safe from the sunken German destroyer S-119, in which was found the Verkehrsbuch (VB), the code used by the Germans to communicate with naval attachés, embassies and warships overseas.1

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. It was managed until May 1917 by Alfred Ewing, when direct control passed to Captain (later Admiral) Reginald 'Blinker' Hall, assisted by William Milbourne James2.

It played an important 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 decrypting the Zimmermann Telegram, a cable from the German Foreign Office to its ambassador Heinrich von Eckardt in Mexico.

In the cable's plaintext, Nigel de Grey and William Montgomery discovered German Foreign Minister Arthur Zimmermann making an offer to Mexico of United States territory (Arizona, New Mexico, & Texas) 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 (involving a still unknown agent in Mexico and a burglary) to conceal how its plaintext had become available and also how the U.S. had gained possession of a copy. The cable was made public by the U.S., who shortly thereafter entered the war on the Allied side.

Other members of Room 40 were Frank Adcock, Frank Birch, William Nobby Clarke, Alastair Denniston and Dilly Knox.

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.

Notes

  1. ^ Massie. Castles of Steel. pp. 314–317. 
  2. ^ Johnson. British Sigint. pp. 32. 

References

  • Andrew, Christopher (1986). Her Majesty's Secret Service: The Making of the British Intelligence Community. New York: Viking. ISBN 0-670-80941-1. 
  • Beesly, Patrick (1982). Room 40: British Naval Intelligence, 1914–1918. New York: Harcourt, Brace Jovanovich. ISBN 0-15-178634-8. 
  • Johnson, John (1997). The Evolution of British Sigint, 1653–1939. London: H.M.S.O.. 
  • Tuchman, Barbara W. (1958). The Zimmerman Telegram. New York: Ballantine Books. ISBN 0-345-32425-0. 


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