calling WCF Service from client side app but not directly by user

I have a silverlight application that sends some tracking information back to server using WCF service. I do not want users to call this service directly.

For example I need to track when a user started watching a video or when they stopped it. I can call service on start and stop events to log the information back on server. But I dont want user to call the service directly because then they can pass in any value without even watching the video.

I am thinking about creating some sort of unique id (may be a GUID) when user logs in and save the ecrypted version of this id in the cookie. When my app will calls the service I ll get this encrypted  id, decrypt it and after verification continue the service call. If user directly calls the service, it wont work because they can not get the encrypted id.

Is there any problem with this approach?
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