Authentication to a "web service" ... Do you mean SOAP / HTTP (S) or a web page? The answers are different in two cases!
- For SOAP / HTTPS, you are talking using the WS-Security suite with SAML / XACML tokens. Permissions can be obtained in several ways, such as Kerberos or VOMS. This is clearly non-trivial, and you will need to find out what all the other parts are in the “ecosystem” of services, and ensure that you interact with it.
- - OpenID Shibboleth , . , OpenID , Shibboleth ( , - ).
-, -, , OpenID , . ! ( , , .)
[EDIT]: , , . , , -, , . , HTTPS, , HTTPS, -XSS ( SO!)