He worked on a number of cryptographic projects, but is credited with some of the original thinking that developed into the field of Public Key Cryptography (PKC).
[3] Ellis said that the idea first occurred to him after reading a paper from World War II by someone at Bell Labs describing the scheme named Project C43,[1] a way to protect voice communications by the receiver adding (and then later subtracting) random noise.
[4] Clifford Cocks and Malcom Williamson, two other GCHQ cryptographers, furthered Ellis' initial PKC related work.
As all of this work prior to 1997 was classified, it never became part of very significant mainstream initiatives that developed into modern PKC commercial endeavors, such as the work on Diffie–Hellman key exchange, RSA and other PKC linked initiatives which have become part of the modern world of Internet security.
[5] On 18 December 1997, Clifford Cocks delivered a public talk which contained a brief history of GCHQ's contribution to PKC.