Key-agreement protocol

A key-agreement protocol is a protocol whereby two people can agree on a key in such a way that both influence the outcome. If properly done, this precludes a third-party from forcing a key choice on the communicating parties. Useful protocols also do not reveal to any eavesdropping party what the key agreed upon is.

The first publicly known key-agreement protocol that meets these criteria was Diffie-Hellman key exchange, in which the two people jointly exponentiate a generator with random numbers, in such a way that an eavesdropper has no way of guessing what the key is.

Diffie-Hellman was first developed by researchers at GCHQ, the UK equivalent to NSA. James Ellis[?] demonstrated that non-secret encryption was possible in the '60s and Malcolm Williamson developed what is now called Diffie-Hellman Key exchange in the early '70s. GCHQ did not allow publication, so Diffie and Hellman were the first to publish.

Reference

See the appendix to Crypto[?], by Steven Levy for more information on GCHQ's work, The Code Book[?] by Simon Singh[?], or the GCHQ Web page about 'non-secret encryption'. The latter contains an essay by James Ellis himself.