When asymmetric algorithms were first invented, it was suggested that people should publish their public keys in books like phone books. That idea didn't catch on. Instead, a new standard file type was invented, called a certificate.

A certificate contains its owner's public key and everyday elements of their identity such as their name, company, location, and the DNS host name of their computer. Your SSL software normally includes a function to create your certificate, but it's more usual to get an independent body, called a Certificate Authority, to create it. We'll go into this in the next chapter.

Anyone can have a certificate, and anyone needing to prove their identity online needs one. Notably, they are needed by organizations such as online banks and other Web sites that need clients or other organizations to communicate with them securely.

When you connect to such a Web site, the initial contact (called the handshake) between your SSL software and theirs includes their SSL software sending yours their certificate. This is how you get their public key, so that encrypted communication between you can begin, as described above.