title: How do popular apps authenticate user requests from their mobile app to their server?
body: <p>Say I have an Android application that connects to a .Net API for receiving/setting data. The confusion that I have is regarding how to sign-up/login the user first time and authenticate it every time they make a request to the API.</p>
  
  <ul>
  <li>If I just use username/password based authentication they won't be safe
  enough?</li>
  <li>And I can't save that username/password in the device
  for of course security reasons?</li>
  <li>Should I issue a GUID for every user at the sign-up, save it in their device and retrieve every time during an API request?</li>
  </ul>
  
  <p>What other patterns are available and which are most efficient and secure, I just need a process flow for it.
  Can someone tell me what method famous android applications like Facebook, FourSquare, or Twitter use to authenticate every request coming from their mobile application to their server?</p>
  
  <p>Sorry in advance if that's not some public information.</p>