Approaches for Authentication through Web Services

Hi All,

I am calling a webservice through my portal service. This web service is on the (DI) Data Integration box which is not SAP.

I need to know approaches with which I can authenticate users when they use this webservice call. I mean how do I authenticate users who will be using this application which will call a web service on the DI box.

So when the user clicks on the submit button, this web service on the DI box is invoked, I want to authenticate users at this point, so that no unauthorized users are able to post data to DI box.

Also, I do not want any kind of login window popping up. It should be something like a SSO.

Please auggest approaches that I can use.

Thanks