You might want to explore this solution for WCF REST, this is an interceptor for implementing basic authentication with a user user database.
http://weblogs.asp.net/cibrax/archive/2009/03/20/custom-basic-authentication-for-restful-services.aspx
This interceptor authenticates the user with a password and initializes the current principal, so you can use it from the service itself to perform authorization or implement IAuthorizationManager if you want this logic to be reused for something for several services.
There are other authentication methods on my blog, such as certificate authentication or OAuth, which are less common.
Thanks Pablo.
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