Lauren Kohnfelder is best known for his MIT Msc thesis written in 1979 describing a practical means of applying public key cryptography to secure network communications.
The Kohnfelder thesis introduced the terms Certificate and Certificate Revocation List as well as introducing numerous other concepts now established as important parts of Public Key Infrastructure. The X.509 certificate specification that provides the basis for SSL, S/MIME and most modern PKI implementations is based on the Kohnfelder thesis.