Diffie–Hellman key exchange

Diffie-Hellman is a cryptographic (see Cryptography) protocol for key exchange, allowing Alice and Bob to agree on and construct a secret key over an insecure communication channel. The protocol is based on the Diffie-Hellman Problem related to discrete logarithms. It is considered to be secure if an appropriate mathematical group is used.

However it is not secure if a special type of attack called the "Man in the middle" attack is possible. This attack assumes the attacker is able to modify messages between Alice and Bob as well as read them.

Diffie-Hellman was proposed circa 1976 and was the first public proposal for a cryptographic method that did not rely on Alice and Bob already having a shared secret before they start. It had been discovered by Malcolm Williamson of GCHQ in the UK some years previously, but GCHQ chose not make it public. Cryptographic methods of this sort are often called

asymmetric algorithms. There are many others now proposed or in use, and some of them are apparently immune to "Man in the middle" attacks.