Certificates, key transport and key agreement

Abstract

We can now perform encryption and authentication using either public key techniques or symmetric key techniques. However, we have not addressed how parties actually obtain the public key of an entity, or obtain a shared symmetric key. When using hybrid ciphers in Section 16.3 we showed how a symmetric key could be transported to another party, and then that symmetric key used to encrypt a single message. However, we did not address what happens if we want to use the symmetric key many times, or use it for authentication etc. Nor did we address how the sender would know that the public key of the receiver he was using was genuine.