Alastair Graham Denniston was a British Cryptographer. He was a codebreaker at Room 40 during World War I, and was the first head of GC&CS, and was one of the leaders of GC&CS at Bletchley Park in World War II. Pre-19th century Leone Battista Alberti, polymath/universal genius, inventor of polyalphabetic substitution (see frequency analysis for the significance of this -- missed by most for a long time and dumbed down in the Vigenère cipher), and what may have been the first mechanical encryption aid. ... In the history of cryptography, Room 40 (formally I.D. 25) was the room in the Admiralty which was the first location of the British cryptography effort during World War I. It was formed shortly after the start of the war in October 1914, as a result of codebooks and... Combatants Entente Powers Central Powers Commanders {{{commander1}}} {{{commander2}}} Strength {{{strength1}}} {{{strength2}}} Casualties > 5 million military deaths > 3 million military deaths World War I, also known as the First World War and (before 1939) the Great War, the War of the Nations, War to End All Wars was a world conflict... The Government Communications Headquarters (GCHQ) (previously named the Government Code and Cipher School (GC&CS)) is the main British intelligence service providing signals intelligence (SIGINT). ... During World War II, British and American cryptographers at Bletchley Park broke a large number of Axis codes and ciphers, including the German Enigma machine. ... Combatants Allied Powers Axis Powers Commanders {{{commander1}}} {{{commander2}}} Strength {{{strength1}}} {{{strength2}}} Casualties 17 million military deaths 7 million military deaths World War II, also known as the Second World War (sometimes WW2 or WWII), was a mid-20th century conflict that engulfed much of the globe and is accepted as...