Room 40

In the history of cryptanalysis, Room 40, also known as 40 O.B. (Old Building) (latterly NID25) was the section in the British Admiralty most identified with the British cryptanalysis effort during the First World War, in particular the interception and decoding of the Zimmermann Telegram which played a role in bringing the United States into the War.

Room 40 was a group 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.

The basis of Room 40 operations evolved around a German naval codebook, the Signalbuch der Kaiserlichen Marine (SKM), and around maps (containing coded squares), which Britain's Russian allies had passed on to the Admiralty. The Russians had seized this material from the German cruiser SMS Magdeburg when it ran aground off the Estonian coast on 26 August 1914. The Russians recovered two of the four copies that the warship had carried; they retained one and passed the other 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: the Royal Australian Navy seized a copy from the Australian-German steamer Hobart 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.

...
Wikipedia