Gordon Welchman was one of the original elite codebreakers of Bletchley Park. Without him, the German Enigma codes might not have been broken, and World War Two could have lasted two more years. His contribution was as great as Alan Turing's and he should be just as famous, yet very few have heard of him. But for a forgotten man, his secret legacy is everywhere - from whistleblower Edward Snowden's revelations about the NSA, to the hunt for Osama bin Laden and now the Islamic State.