Doug Tygar
Co-author of Why Johnny Can't Encrypt — the study that showed secure software nobody can use isn't secure.
J. D. Tygar was a professor at Carnegie Mellon and later UC Berkeley. In 1999 he and his student Alma Whitten published a user study of PGP 5.0: most participants failed to send a correctly encrypted email within 90 minutes, and several sent the secret in plaintext without noticing. The paper is generally counted as the starting gun for usable security as a research field.
He worked on protocols too. At CMU he helped build the NetBill payment system, and with Adrian Perrig and Dawn Song he developed TESLA, a scheme for authenticating broadcasts using keys disclosed after a delay. Later he turned to how machine learning behaves when the other side is actively trying to fool it.