In 1970 while at GCHQ Ellis wrote a paper describing what he called “non-secret encryption”. We now call this Public Key Cryptography.  His paper led to Clifford Cocks' discovery of what we now call the RSA algorithms and Malcolm Williamson's creation of what is now known as the Diffie-Hellman key exchange.  All of this work was classified, and although done before RSA and Diffie-Hellman, it was never executed at scale.  These works were not made public until 1997, after Ellis' death.

Wikipedia: http://en.wikipedia.org/wiki/James_H._Ellis

Bio at Virtual Exhibitions in Informatics: http://cs-exhibitions.uni-klu.ac.at/index.php?id=344