Key Agreement Protocols

Key agreement, as the name implies, is a process in which principals cooperate in order to establish a session key. Amongst the class of public key protocols for key establishment without a server, key agreement has become much more popular than key transport in recent years. There is an intuitive feeling that key agreement is ‘fairer’ than key transport and can result in higher quality random keys than key transport. In addition, by basing key agreement on the Diffie—Hellman protocol, forward secrecy can often be achieved. We will consider these points further below. Notice that key agreement does not have to use public key cryptography but most examples do so. In this chapter we look only at key agreement based on public key cryptography; some examples of key agreement using symmetric cryptography are discussed in Chap. 3.