Mathematician and cryptanalyst Gordon Welchman was one of the original elite codebreakers at Bletchley Park and played a crucial part in the Allies defeating Germany. After the war, he moved to America to work at the heart of the computer revolution and was employed by defence contractor the Mitre Corporation, where he was involved in top-secret research. In this programme, academic John Naughton and former CIA analyst Cynthia Storer examine how Welchman's pioneering work in the field of traffic analysis led directly to today's secret state surveillance, as revealed by whistleblower Edward Snowden