No history of the Second World War nowadays fails to mention the important role of signals intelligence (SIGINT) . By contrast, SIGINT is entirely absent from most studies of the Cold War. Newly declassified material in the West, as well as highly classified material exfiltrated from KGB archives by Vasili Mitrokhin, shows, however, that SIGINT continued to play a major role. The KGB supplied the Soviet leadership throughout the Cold War with far more high-grade diplomatic SIGINT (including decrypts from major NATO governments) than they could possibly read. In many cases agent penetration was able to resolve the problems caused by the increasing complexity of cipher systems. Among the revelations in recently declassified Western SIGINT is the identification of a Cambridge scientist as the youngest major spy of the twentieth century.