Mobile devices don’t have user accounts by default. You can use GameCenter/Facebook/etc as an account proxy, which is what Infinity Blade 3 does. If they have none of those social accounts available, you can use the Identifier for Vendor, which is a unique id created for this user and your application. Note that this is device specific so you can’t use it as a way to create your own cloud storage because the ID will be different on each device the user uses. If you want cross device support, then it is best to use a social network id of some form. The code to get the IDFV looks like:
NSUUID* Id = [[UIDevice currentDevice] identifierForVendor];
if (Id != nil)
{
do something with the ID
}