Authentication is simple in principle. The client sends his name and password to Apache. Apache looks up its file of names and encrypted passwords to see whether the client is entitled to access. The webmaster can store a number of clients in a list — either as a simple text file or as a database — and thereby control access person by person.
It is also possible to group a number of people into named groups and
to give or deny access to these groups as a whole. So, throughout
this chapter, bill and ben
are in the group directors, and
daphne and sonia are in the
group cleaners. The webmaster can
require user so and so or
require group such and such, or even simply
require that visitors be registered users. If you
have to deal with large numbers of people, it is obviously easier to
group them in this way. To make the demonstration simpler, the
password is always theft. Naturally, you would not use so short and
obvious a password in real life, or one so open to a dictionary
attack.
Each username/password pair is valid for a particular realm, which is named when the passwords are created. The browser asks for a URL; the server sends back “Authentication Required” (code 401) and the realm. If the browser already has a username/password for that realm, it sends the request again with the username/password. If not, it prompts the user, usually including the realm’s name in the prompt, and sends that.
Of course, all this is worryingly insecure since the password is sent unencrypted over the Web (base64 encoding is easily reversed), and any malign observer simply has to watch the traffic to get the password — which is as good in his hands as in the legitimate client’s. Digest authentication improves on this by using a challenge/handshake protocol to avoid revealing the actual password. In the two earlier editions of this book, we had to report that no browsers actually supported this technique; now things are a bit better. Using SSL (see Chapter 11) also improves this.