Hans-Thilo Schmidt
German cipher office clerk who sold Enigma documentation to French intelligence in 1931.
Hans-Thilo Schmidt worked at the German cipher office, the Chiffrierstelle in Berlin, and had access to Enigma documentation, including key tables. He was short of money and sold material through an intermediary to French intelligence. The documents reached France in 1931.
France passed the material on to Polish cryptologists, and in 1932 Marian Rejewski used it to reconstruct the internal wiring of the Enigma rotors. That was one of the reasons Poland could read German Enigma traffic before the war. Schmidt is a textbook example of how one person with access to the right paperwork can change the course of a war.