Implementing Custom WebHttpBinding Authentication

Hello, I want to create a REST (Web Programming Model) style WCF service and
do the following:

1. Have my own code validate the "Authorization" HTTP header. Imagine Atom
Authentication (http://www.xml.com/pub/a/2003/12/17/dive.html) for example.
2. I want to have certain methods require an authenticated user while others
accept anonymous requests.

How can I achieve that?

Thank you,
Sammy