Harold Keen

Harold Hall "Doc" Keen (1894–1973) was a British engineer who produced the engineering design, and oversaw the construction of, the British bombe, a codebreaking machine used in World War II to read German messages sent using the Enigma machine. He was known as "Doc" Keen because of his habit of carrying tools and paperwork in a case resembling a doctor's bag. After the war he was awarded the O.B.E..

Keen was born in the borough of Shoreditch in east London in 1894. By age 18 he had moved to Kentish Town and began studying Electrical Engineering. In 1912 he joined the British Tabulating Machine Company (BTM), established to import and assemble American punched card technology. In 1916, Keen joined the Royal Flying Corps and was assigned to the ground staff of a bomber squadron in northern France. In 1919 he returned to BTM and married an Eva Burningham. In 1921, Keen moved with BTM to Letchworth in Hertfordshire. Two years later, he was appointed head of the Experimental Department, and his innovations there gained him the reputation as the leading British innovator of punched-card technology; Keen was granted more than sixty patents. In the 1930s he became Chief Engineer.

A machine called the cryptologic bomb (Polish: bomba kryptologiczna) had been produced by the Polish codebreakers in their successful breaking of Enigma before 1939. It was, however useful only as long as the Germans continued with particular weaknesses in operating procedure, which they gradually tightened up, so that the Polish success rate diminished dramatically. Alan Turing designed the British bombe on a more general principle, the assumption of the presence of text, called a crib, that cryptanalysts could predict was likely to be present at a defined point in the message. This technique is termed a known plaintext attack and had been used to a limited extent by the Poles, e.g., the Germans' use of "ANX" — German for "To," followed by "X" as a spacer.

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