title: iOS individual constant identifier
body: <p>I want to create my app with encryption for some properties, that I will save in CoreData
  So I want use AES256. What the best solution for <code>key</code> ? So I can use some identifier from device, but I want use iCloud, so what individual constant I can use for each user? I don't want to hardcode <code>key</code>. But I don't know what identifier (like UUID in iPhone), that will be always constant and similar for one user in iCloud.</p>