Government Code and Cypher School

The Government Code and Cypher School (GC&CS) was a British signals intelligence agency set up in 1919. Before this, during the First World War, the British Army and Royal Navy each had their own groups for signals intelligence, called MI1b and NID25, with the latter originally known as Room 40. This new school became very important for breaking secret codes, especially at a special place called Bletchley Park. After the war, it changed its name and became what we now know as the Government Communications Headquarters (GCHQ).

Interwar period

In 1919, a group of leaders decided to create a special group to help protect important messages. This group, called the Government Code and Cypher School (GC&CS), was led by Hugh Sinclair and started with about 25–30 officers and some clerical staff. Alastair Denniston was chosen to run the day-to-day work. At first, it was located in a building in London and worked under the Admiralty, the part of the government in charge of the navy.

Before World War II, GC&CS stayed a small group. By 1922, its main job was to look at messages from other countries. In 1925, it moved to a new building near St James's Park. The group successfully read some secret messages from the Soviet Union, but in 1927, some of these messages were shared publicly because of a big problem with strikes and protests.

World War II

See also: Second World War activities of GC&CS referred to as 'Ultra'

During the Second World War, GC&CS worked mostly from Bletchley Park, in what is now Milton Keynes. Their job was to understand secret messages made using the German Enigma machine and Lorenz ciphers. By 1940, they were studying secret codes from 26 different countries, solving more than 150 different code systems. Important people who helped included Alastair Denniston, Oliver Strachey, Dilly Knox, John Tiltman, Edward Travis, Ernst Fetterlein, Josh Cooper, Donald Michie, Alan Turing, Gordon Welchman, Joan Clarke, Max Newman, William Tutte, I. J. (Jack) Good, Peter Calvocoressi, and Hugh Foss. In 1943, Britain and the United States made an agreement called BRUSA to share information about secret messages. They used special machines called computers, including the Colossus computer, which had ten linked computers to help break enemy codes.

There was also a group in the Far East called the Far East Combined Bureau. It started in Hong Kong in 1935 and moved to Singapore in 1939. When Japan invaded, some members went to the Wireless Experimental Centre in Delhi, India, while others moved to Colombo in Ceylon (now Sri Lanka), and later to Kilindini near Mombasa, Kenya.

GCHQ

GC&CS was renamed the Government Communications Headquarters (GCHQ) in June 1946.

The organisation was first based in Eastcote in northwest London, then in 1951 moved to the outskirts of Cheltenham. One of the major reasons for choosing Cheltenham was that the town had been the location of the headquarters of the United States Army Services of Supply for the European Theater during the War, which built up a telecommunications infrastructure in the region to carry out its logistics tasks.