SSL architecture

To understand how SSL works, you need to consider how browsers and web servers actually send and receive HTTP messages. Browsers send HTTP requests by calling on the host systems' TCP/IP networking software, the software that does the work of sending and receiving data over the Internet. When a request is to be sent-for example when a user clicks on a hypertext link-the browser formulates the HTTP request in memory and uses the host's TCP/IP network service to send the request to the server. TCP/IP doesn't care that the message is HTTP; it is only responsible for getting the complete message to the destination. When a web server receives a message, data is read from its host's TCP/IP service and then interpreted as HTTP. We discuss the relationship between HTTP and TCP/IP in more detail in Appendix B.

As shown in Figure 9-4, The SSL protocol operates as a layer between the browser and the TCP/IP services provided by the host. A browser passes the HTTP message to the SSL layer to be encrypted before the message is passed to the host's TCP/IP service. The SSL layer, configured into the web server, decrypts the message from the TCP/IP service and then passes it to the web server. Once SSL is installed and the web server is configured correctly, the HTTP requests and responses are automatically encrypted. There is no scripting required to use the SSL services.

Figure 9-4. HTTP clients and servers, SSL, and the network layer that implements TCP/IP

Because SSL sits between HTTP and TCP/IP, secure web sites technically don't serve HTTP, at least not directly over TCP. URLs that locate resources on a secure server begin with https://, which means HTTP over SSL. The default port for an SSL service is 443, not port 80 as with HTTP; for example, when a browser connects to, it makes a TCP/IP connection to port 443 on Most browsers and web servers can support SSL, but keys and certificates need to be included in the configuration of the server (and possibly the browser, if client certification is required).