Key-agreement protocol
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In cryptography, 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. Protocols which are useful in practice also do not reveal to any eavesdropping party what key has been agreed upon.
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 1960s and Malcolm Williamson developed what is now called Diffie-Hellman Key exchange in the early 1970s. GCHQ did not allow publication, so Diffie and Hellman were the first to publish.
- See also : ISAKMP
Reference
See the appendix to Crypto: How the Code Rebels Beat the Government Saving Privacy in the Digital Age, 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.