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James H. Ellis (1924–November 1997) was an engineer and mathematician. In 1970, while working at GCHQ he conceived of the possibility of "non-secret encryption", more commonly termed public-key cryptography.

Ellis was born in Australia, although he was conceived in Britain and grew up in London. He studied physics at Imperial College London, and subsequently worked at the Post Office Research Station at Dollis Hill. In 1952, Ellis joined GCHQ in Eastcote, West London. In 1965, Ellis moved to Cheltenham to join the newly-formed Communications-Electronics Security Group (CESG), an arm of GCHQ.

Ellis said that the idea first occurred to him after reading a paper from World War II by someone at Bell Labs describing a way to protect voice communications by the receiver adding (and then later subtracting) random noise. However, he was unable to devise a way to implement the idea.

Shortly after joining GCHQ after he left Cambridge University, Clifford Cocks was told of Ellis' proof and that no one had been able to figure out a way to implement it. He went home, thought about it, and returned with the basic idea for what has become known as the RSA asymmetric key encryption algorithm. It was kept secret.

Not long thereafter, Cocks' friend, Malcolm Williamson, also working at GCHQ, after being told of Cocks' and Ellis' work, thought about the problem of key distribution and developed what has since become known as Diffie-Hellman key exchange. It was also kept secret.

When, a few years later, Diffie and Hellman published their 1976 paper, and shortly after that Rivest, Shamir, and Adleman announced their algorithm, Cocks, Ellis, and Williamson suggested that GCHQ announce that they had previously developed both. GCHQ decided against publication at the time and Ellis died the month before the public announcement was made.

• James Ellis' account of the invention of non-secret encryption