So far, we've assumed that you and the people you communicate with know and trust each other. In real life, this is unlikely to be the case. On the Internet, or a big organization's intranet, you are one of a great many people, all potentially communicating with each other, and unlikely ever to know each other personally.

In the previous chapter, we introduced certificates as a way of making an entity's public key available to anyone who wants it.

To prove that the entity is trustworthy, an entity's certificate is created and digitally signed by someone who knows the entity personally, or at least is able to check up on them personally, and who is in turn trusted by others. In this way the certificate also serves - like a traditional paper certificate - as a proof of the entity's trustworthiness. SSL software includes a function to display a certificate on the screen so a user can check the details of the owner and the signatory.

And the person who signed the certificate can in turn have a certificate signed by a third person, and that person can in turn have a certificate signed by a fourth, and so on. Each certificate includes a record of this chain of certificates.

This is called a "web-of-trust model", and in a small organization it may be sufficient, since it's fairly easy to ensure that anyone checking a certificate will find someone he or she knows and trusts in the chain of signatories on the certificate. But it's not scalable; that is, it won't work for a big organization consisting of thousands of people mostly unknown to each other - and certainly not for the Internet.

The solution to this is a PKI, which we describe later in this chapter.