WS-Security

On April 2003 David Chappell published an article on MSDN Magazine regarding WS-SECURITY - New Technologies Help You Make Your Web Services More Secure

If you want to learn WSE basics:

Sending Security Tokens
Integrity
Confidentiality
Specifying Policies: WS-SecurityPolicy
Establishing Context: WS-SecureConversation
Using SAML and Other XML-based Tokens

check the article over here