I have a web service that needs to be able to verify the end-user's IP that called the server-script that is requesting the web service. Simple layout:

Person A goes to Webpage B. Webpage B calls Web Service C to get some info on Person A. Web Service C won't give Webpage B the requested information without confirmation that the request originated from Person A's IP and not someone who has stolen Person A's session.

I'm thinking that for a browser-based solution, the original site (Webpage B) can open an iframe that goes to the Web Service's authentication page. A key of some kind is passed to the browser which will some how indicate both the user's IP and Web Page B's IP, so that the Web Service can confirm that no one has nabbed anything.

I have two challenges, but I'll stick to the more immediate one first:

I'm not sure if my browser-based plan really makes sense. If someone steals the session cookie, how is the Web Service going to know? Would this cookie be held be Web Page B and thus be harder to steal? Is it a sound assumption that a cookie or key held by the server only and not the browser is safe?

Also, would the web service, based on the iframe initial connection, be expecting the server/user-ip combo? What I mean is, does the session key provided via the iframe get stored by the web service and the Web Site B shows it has a match? Or is the session key more generic, meaning the web service is passed the key by Website B and the Web Service verifies that this is a valid session key based on what a valid session key should look like?