Gordon Welchman
The man whose addition made the Bombe usable in practice — and who lost his security clearance forty years later for saying so in print.
Gordon Welchman (1906–1985) was a Cambridge mathematician who arrived at Bletchley Park in 1939 and built up and ran Hut 6, the section attacking army and air force Enigma traffic. Turing's original Bombe worked, but threw up so many false stops that it was close to useless operationally. Welchman added the diagonal board: a wiring matrix exploiting the fact that the Enigma plugboard is reciprocal — if A is plugged to B then B is plugged to A. False hits collapsed, and the Bombe became a production tool rather than an experiment.
He was also first to systematise traffic analysis: reading the enemy's order of battle out of call signs and transmission patterns, whether or not the messages themselves could be broken. When he published The Hut Six Story in 1982, both GCHQ and the NSA reacted. Welchman was 76, had worked in US defence electronics since the 1950s, and was stripped of his clearance and forbidden to discuss his own wartime work.