Implementing SSL/TLS Using Cryptography and PKI

CHAPTER 8

Advanced SSL Topics

The prior two chapters examined the TLS handshake process in detail, walking through each message that each side must send and receive. So far, you've looked at the most common use of SSL/TLS—a server-authenticated RSA key exchange. However, there are actually quite a few more options available to the user when performing a TLS handshake. Some potential scenarios are simpler, and some are more complex than those presented so far—it's possible to connect without authenticating the server, or to force the client to authenticate itself, or to employ different key exchange mechanisms. It's even possible to bypass the handshake completely, if secure parameters have already been negotiated. This chapter looks at the less common—or not strictly required—but still important aspects of the TLS handshake.

Passing Additional Information with Client Hello Extensions

Peek back to the definition of the handshake messages defined in Chapter 6. Although each one is prepended with a length, most of them—with the exception of the certificate message—have lengths that are fixed or that can easily be inferred from their structure. The client hello message, for instance, is a fixed two bytes of version information, 32 bytes of random data, and three variable-length structures each of which includes their length. The receiver can easily figure out where the record ends without being given an explicit length.

So, why restate the length? Well, the designers of the protocol ...