title: iOS unique user identifier
body: <p>I'm writting an application for iphone, which communicates with my server using REST. The main problem is, I need to identify user somehow. Not so long ago, we were allowed to use UDID, but now its not allowed anymore. So what should I use instead? I need some kind of identifier on iphone, so user will delete application, install it again, and he will get same id.</p>