The plugboard does not hide all finger-prints

As an inveterate browser, even in airport and railway bookstalls, I frequently come across books with titles like Ten Spies Who Changed The World, but closer inspection reveals that the only common characteristic of the spies described is that their actions failed to change anything. It is, however, just possible that the disaffected employee of the German Cryptographic Agency who sold documents concerning Enigma to the French from the early 1930s onwards did indeed change the course of history. The documents gave the general structure of the Military Enigma, including the existence of the plugboard. They also gave the keys including the plugboard settings for certain periods (in effect, allowing a code-breaker to reduce the Military Enigma to a Commercial Enigma for those periods). What they did not give was the internal wiring of the Enigma and its rotors.

Unable to make much of this material, the French offered it to their British and Polish allies. The British, who did not yet see Germany as a major threat, seem to have given it a fairly cursory inspection before deciding that they too could make nothing of it. The Poles gave the material to one of their new ‘mathematical’ code-breakers named Rejewski who did what the French and British experts had considered impossible and recovered the complete wiring of the Enigma and its rotors: ‘a stunning achievement [and] one that elevates him to the pantheon of the greatest cryptanalysts of all time.’