Editing GCHQ (section)

===World War II===
During the Second World War, GC&CS was based largely at [[Bletchley Park]], in present-day [[Milton Keynes]], working on understanding the German [[Enigma machine]] and [[Lorenz cipher]]s.<ref>{{cite book |first=Paul |last=Gannon |title=Colossus: Bletchley Park's Greatest Secret |publisher=Atlantic Books |year=2006 |isbn=978-1-84354-331-2}}</ref> In 1940, GC&CS was working on the diplomatic codes and ciphers of 26 countries, tackling over 150 diplomatic cryptosystems.<ref>{{cite book |first=David |last=Alvarez |chapter=Most Helpful and Cooperative: GC&CS and the Development of American Diplomatic Cryptanalysis, 1941–1942 |title=Action This Day: Bletchley Park from the Breaking of the Enigma Code to the Birth of the Modern Computer |editor1-first=Michael |editor1-last=Smith |editor2-first=Ralph| editor2-last=Erskine |publisher=Bantam Press |year=2001 |isbn=978-0593049105}}</ref> Senior staff included [[Alastair Denniston]], [[Oliver Strachey]], [[Dilly Knox]], [[John Tiltman]], [[Edward Travis]], [[Ernst Fetterlein]], [[Josh Cooper (cryptographer)|Josh Cooper]], [[Donald Michie]], [[Alan Turing]], [[Gordon Welchman]], [[Joan Clarke]], [[Max Newman]], [[William Tutte]], [[I. J. Good|I. J. (Jack) Good]], [[Peter Calvocoressi]] and [[Hugh Foss]].<ref>{{Citation | editor-last = Erskine | editor-first = Ralph | editor2-last = Smith | editor2-first = Michael | editor2-link = Michael Smith (newspaper reporter) | title = The Bletchley Park Codebreakers | publisher = Biteback Publishing Ltd | year = 2011 | isbn = 978-1-84954-078-0}}</ref> The 1943 British–US Communication Intelligence Agreement, [[1943 BRUSA Agreement|BRUSA]], connected the signal intercept networks of the GC&CS and the US [[National Security Agency]] (NSA).<ref>{{Cite news |date=2016-02-08 |title=How the British and Americans started listening in |language=en-GB |work=BBC News |url=https://www.bbc.com/news/magazine-35491822 |access-date=2023-04-02 |archive-date=2 April 2023 |archive-url=https://web.archive.org/web/20230402082849/https://www.bbc.com/news/magazine-35491822 |url-status=live }}</ref><ref>{{Cite news |date=2021-03-05 |title=Diary reveals birth of secret UK-US spy pact that grew into Five Eyes |language=en-GB |work=BBC News |url=https://www.bbc.com/news/uk-56284453 |access-date=2023-04-02 |archive-date=2 April 2023 |archive-url=https://web.archive.org/web/20230402082850/https://www.bbc.com/news/uk-56284453 |url-status=live }}</ref> Equipment used to break enemy codes included the [[Colossus computer]].<ref>{{Citation |editor-last= Preneel |editor-first= Bart |title= Advances in Cryptology - EUROCRYPT 2000: International Conference on the Theory and Application of Cryptographic Techniques Bruges, Belgium May 14-18, 2000, Proceedings |publisher=  Springer |series= Lecture Notes in Computer Science |year= 2000 |volume= 1807 |isbn= 978-3540675174|contribution-url=https://www.iacr.org/archive/eurocrypt2000/1807/18070423-new.pdf |archive-url=https://web.archive.org/web/20081120221724/http://www.iacr.org/archive/eurocrypt2000/1807/18070423-new.pdf |archive-date=2008-11-20 |url-status=live |contribution=Colossus and the German Lorenz Cipher – Code Breaking in WW II |page=417 |doi=10.1007/3-540-45539-6_29 |doi-access= free }}</ref> Colossus consisted of ten networked computers.<ref>{{cite news|url=https://www.bbc.com/news/technology-67997406 |title=Unseen images of code breaking computer that helped win WW2|date= 17 January 2024|first= Shiona|last= McCallum|newspaper=BBC|access-date=21 January 2024}}</ref>

An outstation in the Far East, the [[Far 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 the [[Wireless Experimental Centre]] in Delhi, India. The Navy codebreakers in FECB went to [[Colombo]], Ceylon, then to [[Kilindini]], near [[Mombasa]], Kenya.<ref>{{cite web|url=http://www.coastweek.com/codes.htm|title=Mombasa was base for high-level UK espionage operation|publisher=Coastweek|access-date=25 February 2018|archive-date=15 May 2013|archive-url=https://web.archive.org/web/20130515123737/http://www.coastweek.com/codes.htm|url-status=usurped}}</ref>