TheGovernment Code and Cypher School (GC&CS) was a Britishsignals intelligence agency set up in 1919. During the First World War, theBritish Army andRoyal Navy had separate signals intelligence agencies,MI1b andNID25 (initially known as Room 40) respectively.[1][2] It was particularly known for its work on codebreaking atBletchley Park and after the war became theGovernment Communications Headquarters (GCHQ).
In 1919, the Cabinet's Secret Service Committee, chaired byLord Curzon, recommended that a peacetime codebreaking agency should be created, a task which was given to theDirector of Naval Intelligence,Hugh Sinclair.[3] Sinclair merged staff from NID25 and MI1b into the new organisation, which initially consisted of around 25–30 officers and a similar number of clerical staff.[4] It was titled the "Government Code and Cypher School" (GC&CS), a cover-name which was chosen by Victor Forbes of theForeign Office.[5]Alastair Denniston, who had been a member of NID25, was appointed as its operational head.[3] It was initially under the control of theAdmiralty and located in Watergate House, York Buildings,Adelphi, London.[3] Its public function was "to advise as to the security of codes and cyphers used by all Government departments and to assist in their provision", but also had a secret directive to "study the methods of cypher communications used by foreign powers".[6] GC&CS officially formed on 1 November 1919,[7] and produced its first decrypt prior to that date, on 19 October.[3]
Before the Second World War, GC&CS was a relatively small department. By 1922, the main focus of GC&CS was on diplomatic traffic, with "no service traffic ever worth circulating"[8] and so, at the initiative of Lord Curzon, it was transferred from the Admiralty to theForeign Office.[9] GC&CS came under the supervision ofHugh Sinclair, who by 1923 was both the Chief ofSIS and Director of GC&CS.[3] In 1925, both organisations were co-located on different floors ofBroadway Buildings, oppositeSt James's Park.[3] Messages decrypted by GC&CS were distributed in blue-jacketed files that became known as "BJs".[10] In the 1920s, GC&CS was successfully reading Soviet Union diplomatic cyphers. However, in May 1927, during a row over clandestine Soviet support for theGeneral Strike and the distribution of subversive propaganda, Prime MinisterStanley Baldwin made details from the decrypts public.[11]
During the Second World War, GC&CS was based largely atBletchley Park, in present-dayMilton Keynes, working on understanding the GermanEnigma machine andLorenz ciphers.[12] In 1940, GC&CS was working on the diplomatic codes and ciphers of 26 countries, tackling over 150 diplomatic cryptosystems.[13] Senior staff includedAlastair 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 andHugh Foss.[14] The 1943 British–US Communication Intelligence Agreement,BRUSA, connected the signal intercept networks of the GC&CS and the USNational Security Agency (NSA).[15][16] Equipment used to break enemy codes included theColossus computer.[17] Colossus consisted of ten networked computers.[18]
An outstation in the Far East, theFar East Combined Bureau, was set up in Hong Kong in 1935 and moved to Singapore in 1939. Subsequently, with the Japanese advance down the Malay Peninsula, the Army and RAF codebreakers went to theWireless Experimental Centre in Delhi, India. The Navy codebreakers in FECB went toColombo, Ceylon, then toKilindini, nearMombasa, Kenya.[19]
GC&CS was renamed the Government Communications Headquarters (GCHQ) in June 1946.[20]
The organisation was at first based inEastcote in northwest London, then in 1951[21] moved to the outskirts ofCheltenham. One of the major reasons for selecting Cheltenham was that the town had been the location of the headquarters of theUnited 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.[22]