Chapters 3 and 4 discussed public and private keypairs and reviewed their importance to secure communications over insecure channels. Until now, where these keys come from and how they're exchanged has been mostly glossed over. Where the keys come from is the topic of this chapter. This chapter also includes some further discussion on authentication.
You're probably familiar with the term certificate, even if you're fuzzy on the details. You've undoubtedly visited web sites that have reported errors such as "this website's certificate is no longer valid" or "this website's host name does not match its certificate's host name" or "this certificate was not signed by a trusted CA." If you're like most Internet users, you generally ignore these warnings, although in some cases they can indicate something important.
Fundamentally, the certificate is a holder for a public key. Although it contains a lot more information about the subject of the public key — in the case of web sites, that would be the DNS name of the site which has the corresponding private key—the primary purpose of the certificate is to present the user agent with a public key that should then be used to encrypt a symmetric key that is subsequently used to protect the remainder of the connection's traffic.
At this point, you may have at least a hazy idea of how most of the concepts of the past three chapters can be put together to establish a secure communications ...