Diffie-Hellman key exchange (alternatively key agreement or key negotiation) is a cryptographic protocol which allows two parties to agree on a secret key over an insecure communications channel. This key can then be used to encrypt subsequent communications using a symmetric key cipher.

The Diffie-Hellman algorithm specifies that the shared secret key

k = g^(ab) mod p

where a is one's own secret key and b is the other's public key.

You can find out more about the Diffie Hellman algorithm in the Wikipedia

-- MichaelJPan - 22 Mar 2005