title: CloudKit user authentication with single sign-on
body: <p>How does one implement Sign sign-on with CloudKit in iOS8?</p>
  
  <p>I understand that you can get a <a href="https://medium.com/@skreutzb/ios-onboarding-without-signup-screens-cb7a76d01d6e#.u0w6szgdp" rel="nofollow">string that represents your logged-in user</a> that is unique to your app but how can I verify that string in my backend?</p>
  
  <p>I understand that the Apple ID / iCloud email address is hidden for privacy reasons.</p>
  
  <p>My backend is ASP.NET Web Api 2 in Azure but I could implement it in another technology if easier.</p>
  
  <p>I am looking for the equivalent of Google's single sign-on on Android.</p>
  
  <p>I realise that I could use the <a href="https://developer.apple.com/library/ios/documentation/Accounts/Reference/AccountsFrameworkRef/index.html#//apple_ref/doc/uid/TP40011024" rel="nofollow">Accounts framework</a> but from what I understand this allows the user to log-in with services like Twitter or Google. I want the user to log-in with their Apple ID as they are guaranteed to have one.</p>