A couple weeks ago, we began an internal discussion about DNS security and X.509 certificates. It dawned on me that those of you who have never worked with certificates may not understand what they are or what they are for. Sure, you can go to the X.509 Wiki, where you get the rules for usage and certificate structure, but that’s a little like trying to figure out football by reading the rule book. If you are asking, “What the heck is it and what is it used for?”, you are not alone.

An X.509 certificate is used to make an authoritative statement about something. A real life equivalent would be “Hi, I’m David, and I live at 555 Main Street.” The certificate holder presents it to someone/something in order to prove they are who they say they are, in order to establish trust. X.509 and other certificates are useful because the certificate provides the necessary information to validate the presenter’s claim and the authenticate the certificate itself. Like a driver’s license with a hologram, but much better. The recipient examines the certificate’s contents to decide if the presenter is who they say they are, and them whether to trust them with some privilege.

Certificates are used primarily to establish trust on the web, and rely heavily on cryptography to provide the built-in validation. Certificates are always signed with a chain of authority. If the root of the chain is trusted, the user or application can extend that level of trust to some other domain/server/user. If the recipient doesn’t already trust the top signing authority, the certificate is ignored and no trust is established. In a way, an x.509 certificate is a basic embodiment of data centric security, as it contains both information and some rules of use.

Most certificates state within themselves what they are used for, and yes, they can be used for purposes other than validating web site identity/ownership, but in practice we don’t see diverse uses of X.509 certificates. You will hear that X.509 is an old format, that it’s not particularly flexible or adaptable. All of which is true and why we don’t see it used very often in different contexts. Considering that X.509 certificates are used primarily for network security, but were designed a decade before most people had even heard of the Internet, they have worked considerably better than we had any right to expect.