Room 40

Room 40


Advances in communications technology such as the telephone and trans-Atlantic telegraph prompted the development of increasingly sophisticated cipher systems and codes. The telegraph facilitated communication between command and remote forces, but the lines were vulnerable to tapping, the interception of message traffic, on the wires. As codes became more mathematical and complicated, intelligence services enlisted professional cryptologists, or code breakers, and language translators.

At the outbreak of World War I in 1914, British intelligence began intercepting wire transmissions sent by the German military and government. The German cipher was unknown, so British intelligence quickly established a cryptography department to begin the task of breaking enemy code. The department, under the direction of intelligence officer Reginald "Blinker" Hall and code expert Alfred Ewing, was located in Room 40 of the Admiralty Building. The cryptology department housed in Room 40 was only a small branch of Britain's large intelligence system. However, after remarkable successes achieved by the team, Room 40 became a catch-all nickname for British military intelligence during the war.

While the cipher systems themselves were becoming more complex in the early twentieth century, the technology to decode them had not advanced at the same pace. Codes were still worked out by hand in long sequences to look for mathematical permutations and deviations from known ciphers that formed essential code patterns. The best means of breaking code was to capture an enemy codebook. Beginning in 1914, an extraordinary string of events led to the capture of not one, but three different German code books, allowing Room 40 to intercept, decode, and translate most German military and diplomatic transmissions.

Early in the war, a box recovered from a sunken German submarine yielded a copy of the German Foreign Office codebook. British intelligence was thus able to monitor diplomatic correspondence between the German government and its territories and embassies. Similarly fortuitous for Room 40, later that year a German cruiser was sunk by the Russian Navy. When the Russian fleet rescued surviving German sailors from the downed ship, one officer was found to have a copy of the German Naval codebook. The codebook was sent to British intelligence, and Room 40 was able to decipher wire traffic from German fleet commanders and ships. As most ships in the German fleet reported their positions daily, British intelligence learned individual ship identification codes and tracked the position of most German warships and submarines by the end of 1915.

While the two recovered codebooks let British military intelligence decipher nearly a quarter of German military transmissions, the capture of a third codebook in 1915 gave Room 40 the mathematical key to German cipher system. Wilhelm Wassmuss, the German consul in Persia, hastily fled his office to escape encroaching British forces, leaving behind his copy of the German diplomatic codebook. Room 40 cryptographers discovered that the first two codebooks recovered were standard permutations of the third code. Thus, several German codes were based on a single cipher system and, by applying systematic variations, British cryptographers were able to break the remaining codes.

In 1917, Room 40 had its greatest success. The United States, while holding Allied sympathies and aiding the transportation of ammunition across the Atlantic Ocean, held fast to a policy of non-intervention in the war. Increased German submarine warfare, the sinking of U.S. and Allied merchant and passenger ships, and the work of German saboteurs in the Black Tom explosion fostered a shift in American attitudes toward entering the war. On the morning of January 17, 1917, British military intelligence intercepted secret communication from German Foreign Minister Arthur Zimmerman to the German ambassador in Washington, D.C. The message took cryptographers nearly a month to decode in whole, but the importance of the telegram was realized almost immediately. The Zimmerman Telegram, as it became known, revealed German plans to begin unrestricted submarine warfare in the Atlantic. Knowing that this could bring America into the war, Germany planned to make alliances with Mexico and Japan to keep the U.S. occupied on its own ground instead of in Europe. The telegram not only spoke of driving England to surrender, but also promised Mexico the return of its former territories in Texas, New Mexico, and Arizona. British intelligence shared the contents of the memo with the American government, thwarting the German plan. Declaring war on Germany shortly after, America entered the war in Europe.

In 1918, Room 40 intercepted transmissions that revealed that a sizable group of German sailors had mutinied. News of a German surrender soon followed. The triumphs of Room 40 during the course of World War I convinced the British Admiralty that cryptography was a necessary tool of modern warfare. By the advent of World War II, however, the field of cryptography significantly changed with the introduction of cipher machines, teleprinters, and radio.


Black Tom Explosion
World War I
World War I: Loss of the German Codebook

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