User .:Ajvol:. on en.wikipedia · CC BY-SA 3.0

Martin Hellman co-invented public-key cryptography with Whitfield Diffie and Ralph Merkle, a breakthrough that made secure communication over open networks — and eventually the modern internet — possible without parties ever having exchanged a secret key in advance. He and Diffie won the 2015 ACM Turing Award for the work, and Hellman has since applied risk analysis to the danger of nuclear deterrence failure.