The Diffie-Hellman key exchange algorithm was introduced by Diffie and Hellman in the same 1976 paper that introduced the concept of public key cryptography to the world. It depends upon the discrete logarithm problem for its security. It can use keys of varying sizes, and provides forward secrecy. It is purely a key exchange algorithm, and cannot be used to encrypt data. It is vulnerable to man-in-the-middle attacks.
Although patented, the patent ran out in 1997. Diffie-Hellman can now be used freely.
Public Key Cryptography