As you said CloudKit provides a very efficient approach for this. With CloudKit the app get an unique persistant identifier related to the iCloud account. The

As you said CloudKit provides a very efficient approach for this. With CloudKit the app get an unique persistant identifier related to the iCloud account. The cons are it only works through Apple devices for now. But if you have an iOS only app, it's so easy to identify the user through all his devices without asking him anything. And if it loose his phone or use the app on another device, his CloudKit ID is persistant (and related to your app(s) only, not global).