Commander Alexander "Alastair" Guthrie Denniston CB CMG CBE RNVR (1 December 1881 – 1 January 1961) was a Scottish codebreaker, deputy head of the Government Code and Cypher School (GC&CS) and hockey player.
[2] In 1914, Denniston helped form Room 40 in the Admiralty, an organisation responsible for intercepting and decrypting enemy messages.
[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.
Following the practice of his superiors at Room 40, he contacted scientists from Oxford and Cambridge (including Alan Turing and Gordon Welchman) asking if they would be willing to serve if war broke out.
Sinclair acquired the Bletchley Park property and Denniston was assigned to prepare the site and design the huts to be built on the grounds.
[5]: 9–10 On 26 July 1939, five weeks before the outbreak of war, Denniston was one of three Britons (along with Dilly Knox and Humphrey Sandwith) who participated in the trilateral Polish-French-British conference held in the Kabaty Woods south of Warsaw, at which the Polish Biuro Szyfrów (Cipher Bureau) initiated the French and British into the decryption of German military Enigma ciphers.
[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.
In October 1941, the originator of the technique, Alan Turing, along with fellow senior cryptologists Gordon Welchman, Stuart Milner-Barry and Hugh 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.
[3] William Friedman, the American cryptographer who broke the Japanese Purple 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.