form-based authentication with dynamically computed credentials

[Ch...@daifukuamerica.com]

I wish to use form-based authentication that accepts users with the 
following credentials:

username = any string (ex.: "test")
password = length of username (ex: "4")

This is a silly security system, I admit.  However, a solution to this 
problem will help me to solve my *real* problem.

Obviously, a database of any sort is inappropriate, since there are 
infinitely many valid credentials, and unnecessary, since the password can 
be computed from the username.

After reading through the archives, I have the impression that I must 
implement a custom realm.  Where can I go to learn how this is done?

Hints greatly appreciated.