Diffie and Hellman (
DH)
describe a method for two parties to agree upon a shared secret
number, called ZZ, in such a way that the secret will be unavailable
to eavesdroppers. This method requires that both the sender and
recipient of a message have key pairs (private and public). By combining one's
private key and the other party's public key, both parties can compute
the same shared secret number ZZ