form-based authentication with dynamically computed credentials

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,

sources for servlet-api.jar

I am trying to obtain sources for servlet-api.jar, but all my searching around on the web, including Sun's site, has failed to turn up what I'm looking for. Do you know where to look?