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

Room 40 was formed in October, 1914, shortly after the start of the war. 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.

Purpose and initial activities

The basis of Room 40 operations evolved around a German naval codebook, the Signalbuch der Kaiserlichen Marine (SKM), and maps (containing coded squares), which had been passed on to the Admiralty by the Russians. The Russians had seized them from the German cruiser Magdeburg when it ran 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.

The function of the program was compromised by the Admiralty's insistence interpreting Room 40 information in its own way. Room 40 operators were permitted to decrypt, but not to interpret the information they acquired.

The section retained "Room 40" as its informal name even though it expanded during the war and moved into other offices. It has been estimated that Room 40 decrypted around 15,000 German communications, the section being provided with copies of all interceptable communications traffic, including wireless and telegraph traffic. Until May 1917 it was directed by Alfred Ewing, and then direct control passed to Captain (later Admiral) Reginald 'Blinker' Hall, assisted by William Milbourne James.

Zimmerman Telegram

Room 40 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 its most important contribution was probably in decrypting the Zimmermann Telegram, a cable from the German Foreign Office sent via Washington to its ambassador Heinrich von Eckardt in Mexico.

This interception had been made possible a few hours after Britain entered the war by the cable ship Teleconia, which stood off the German coast and cut the five telegraph cables connecting Germany with Spain, Tenerife and New York.

In the cable's plaintext, Nigel de Grey and William Montgomery learned of the German Foreign Minister Arthur Zimmermann's offer to Mexico of United States' territories of Arizona, New Mexico, and 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 United States, which declared war on Germany on April 6, 1917, entered the war on the Allied side.

Staffers

Other staff of Room 40 were Frank Adcock, Francis Birch, Walter Horace Bruford , William Nobby Clarke, Alastair Denniston and Dilly Knox.

Merger with Military Intelligence (MI)

In 1919, Room 40 was deactivated and its function merged with the British Army's intelligence unit MI1b to form the Government Code and Cypher School , later housed at Bletchley Park during World War II and subsequently renamed Government Communications Headquarters and relocated to Cheltenham.

Notes

1. http://otal.umd.edu/~mgk/blog/archives/000787.htm

References

External links

• The Papers of William Clarke, who worked in Room 40, are held at Churchill Archives Centre in Cambridge and are accessible to the public.
• The Papers of Alexander Denniston, second in command of Room 40, are also held at Churchill Archives Centre.
• The Papers of William Reginald Hall, joint founder of Room 40, are also held at Churchill Archives Centre.
• Original Documents from Room 40: LUSITANIA case; Naval Battle of Jutland/Skagerrak; The Zimmermann/Mexico Telegram; German Submarine Warfare and Room 40 Intelligence in general; PhotoCopies from the The National Archives, Kew, Richmond, UK.