title: Authenticate user from mobile app
body: <p>I have mobile online game and my server need user_id for the player. But facebook only gives me access_token to request data from user account and doesn't provide signed user_id. It's important to have SIGNED user_id so no other user can pretend to be another just by replacing user_id in request to my server. Siging on client is no secure because anyone who cares can find secret key in client binary. </p>
  
  <p>So now I will have to create intermediate step in authentication process - request to my server with access_token, which in turn request facebook for user info (which is sloooow and depends on my server geoposition (constant), not clients), create signature with MY_SERVER_PRIVATE_KEY + user_id, and send it back to client. This not a big problem, but create latency during authentication.</p>
  
  <p>Do I miss something? Is there no way to receive user_id from facebook signed with my application secret key?</p>