Diffie–Hellman Key Exchange

Published by Whitfield Diffie and Martin Hellman in 1976, the Diffie–Hellman (DH) key-exchange protocol allows two parties to agree on a shared secret over an untrusted network. Each party picks a private exponent, computes g^x mod p with public parameters (a generator g and large prime p), exchanges the resulting public value, and raises the received value to their own private exponent — both arrive at the same g^xy. Security rests on the computational Diffie–Hellman assumption, related to the discrete logarithm problem. Plain DH provides no authentication and is vulnerable to man-in-the-middle attacks, so production protocols (TLS, IPsec, SSH) authenticate the exchange with signatures or certificates and prefer ephemeral variants (DHE, ECDHE) to deliver perfect forward secrecy. Modern deployments use elliptic-curve forms (X25519, X448) for speed and small messages.