This document discusses certificate authorities (CAs) and provides an example scenario for securing a web server using a CA. It defines a CA as an entity that issues digital certificates for use by other parties in public key infrastructure schemes. There are commercial CAs, as well as CAs run by institutions and governments. The document then describes the process a CA goes through to issue a certificate and how users can verify certificates. It provides a list of common CAs. Finally, it presents a scenario where a web server obtains a server certificate from a CA to secure its SSL port, and clients can obtain client certificates from the CA's website to access the secure site.