CommanderAlexander "Alastair"Guthrie DennistonCBCMGCBERNVR (1 December 1881 – 1 January 1961) was a Scottish codebreaker, deputy head of theGovernment Code and Cypher School (GC&CS) andhockey player.^[1][2] Denniston was appointed operational head of GC&CS in 1919 and remained so until February 1942.^[3][4]

Denniston was born inGreenock,Renfrewshire, the son of a medical practitioner.^[3] He studied at theUniversity of Bonn and theUniversity of Paris.^[3] Denniston was a member of the Scottish Olympic hockey team in1908 and won a bronze medal. He played as a half-back, and his club team was listed as Edinburgh. In the IOC's official 1908 report, he is listed as Dennistoun rather than Denniston.^[2]

In 1914, Denniston helped formRoom 40 in the Admiralty, an organisation responsible for intercepting and decrypting enemy messages. In 1917, he married a fellow Room 40 worker, Dorothy Mary Gilliat.^[3]

AfterFirst World War, Denniston, recognising the strategic importance of codebreaking, kept the Room 40 activity functioning.^[5]: 8 Room 40 was merged with its counterpart in the Army,MI1b in 1919, renamed the Government Code and Cypher School (GC&CS) in 1920 and transferred from the Navy to the Foreign Office. Denniston was chosen to run the new organisation.^[6]

With the rise ofHitler, Denniston began making preparations. Following the practice of his superiors at Room 40, he contacted scientists from Oxford and Cambridge (includingAlan Turing andGordon Welchman) asking if they would be willing to serve if war broke out.Bletchley Park was chosen by MI6 chief AdmiralHugh Sinclair as the location for the codebreaking effort because it was at arail junction on the west coast main line 47 miles (76 km) north of London with good rail connections to Oxford and Cambridge. Sinclair acquired the Bletchley Park property and Denniston was assigned to prepare the site and design the huts to be built on the grounds. The GC&CS moved to the new location in August 1939, just before theInvasion of Poland and the start of theSecond World War. Its name changed toGovernment Communications Headquarters (GCHQ).^{[5]: 9–10}

On 26 July 1939, five weeks before the outbreak of war, Denniston was one of three Britons (along withDilly Knox and Humphrey Sandwith) who participated in the trilateral Polish-French-British conference held in theKabaty Woods south ofWarsaw, at which thePolishBiuro Szyfrów (Cipher Bureau) initiated the French and British into thedecryption ofGerman militaryEnigmaciphers.^[7]

Denniston remained in command until he was admitted to hospital in June 1940 for abladder stone. Although ill, he flew to the United States in 1941 to make contact with American cryptographers includingWilliam Friedman. Denniston returned to Bletchley Park for a while but moved to London later in 1941 to work on diplomatic traffic.^[5]: 10

Despite his knowledge of the success of Polish cryptologists against Enigma, Denniston shared the general pessimism about the prospects of breaking the more complex Naval Enigma encryption until as late as the summer of 1940, having told the Head of Naval Section at Bletchley: "You know, the Germans don't mean you to read their stuff, and I don't expect you ever will."^[8] The advent ofBanburismus shortly afterwards showed his pessimism to be misplaced.

In October 1941, the originator of the technique,Alan Turing, along with fellow senior cryptologistsGordon Welchman,Stuart Milner-Barry andHugh Alexander wrote to Churchill, over the head of Denniston, to alert Churchill to the fact that a shortage of staff at Bletchley Park was preventing them from deciphering many messages. An addition of personnel, small by military standards, could make a big difference to the effectiveness of the fighting effort. The slow response to previous requests had convinced them that the strategic value of their work was not understood in the right quarters. In the letter, there was praise for the 'energy and foresight' of CommanderEdward Travis.^[9]

Churchill reacted to the letter immediately, ordering "Action this day". Resources were transferred as fast as possible.^[10]

In February 1942, GC&CS was reorganised. Travis, Denniston's second in command and chief of the Naval section, succeeded Denniston at Bletchley Park, overseeing the work on military codes and ciphers.When Travis took over, he "presided over an administrative revolution which at last brought the management of Intelligence into line with its mode of production".^[9]

Denniston and his wife had two children: a son and daughter. Their son,Robin, was educated atWestminster School andChrist Church, Oxford. After Alastair's demotion and resulting decreased income, Robin's school fees were paid by benefactors. However, the Dennistons' daughter had to leave her school due to lack of funds.^[11]

Denniston retired in 1945, and later taught French and Latin inLeatherhead.^[3]

William Friedman, the American cryptographer who broke the JapanesePurple code, later wrote to Denniston's daughter: "Your father was a great man in whose debt all English-speaking people will remain for a very long time, if not forever. That so few should know exactly what he did ... is the sad part."^[5]: 11

Robin distinguished himself as a publisher. In 2007, he publishedThirty Secret Years, abiography of his father that consolidated his reputation in GCHQ history.^[11]

• 7 January 1918 appointed anOfficer of the Order of the British Empire (OBE)Commander Alexander Guthrie Denniston, R.N.V.R. Naval Intelligence Division, Admiralty.^[12]
• 2 January 1933 appointed aCommander of the Order of the British Empire (CBE)Commander Alexander Guthrie Denniston, O.B.E., R.N.V.R. Head of a Department, Foreign Office.^[13]
• 12 June 1941 appointed aCompanion of the Order of St Michael and St George (CMG)Commander Alexander Guthrie Denniston, C.B.E., R.N.V.R. (Retd.), Head of a Department of the Foreign Office.^[3][14]

In the 2014 filmThe Imitation Game, he is portrayed byCharles Dance.^[15]

• Robin Denniston,Churchill's Secret War: Diplomatic Decrypts, the Foreign Office and Turkey 1942–44 (1997)
• Robin Denniston,Thirty Secret Years: A.G. Denniston's work in signals intelligence 1914-1944 (2007)
• James Gannon,Stealing Secrets, Telling Lies: How Spies and Codebreakers Helped Shape the Twentieth Century, Washington, D.C., Brassey's, 2001,ISBN 1-57488-367-4.
• F. H. Hinsley andAlan Stripp, eds.,Codebreakers: the Inside Story of Bletchley Park, Oxford University Press, 1993,ISBN 0-19-820327-6.
• Władysław Kozaczuk,Enigma: How the German Machine Cipher Was Broken and How It Was Read by the Allies in World War II, edited and translated byChristopher Kasparek, Frederick, MD, University Publications of America, 1984,ISBN 0-89093-547-5, pp. 59–60.

• The Papers of Alexander Guthrie Denniston are held at theChurchill Archives Centre in Cambridge, and are accessible to the public.
• DatabaseOlympics.com profile
• Thirty Secret Years: A.G. Denniston's work in signals intelligence 1914-1944
• Alastair Denniston atOlympedia