web service authentication - user tokens

Hi:

Im trying to replicate functionality similar to Google or Backpack 'API
Keys' for authentication. I am using ActionWebService for several web
services which I am exposing for external applications however I would
like to provide some form of authentication. Has anyone done or seen
this done? I have searched and cannot find information specifically in
this context.

thanks for any help
Ryan

It's just a hash of some unique string, like the domain, username, etc
plus maybe a timestamp thrown in.