Always get a unique device id in iOS 7

Question

Our iOS application is for specific users. So, we used device unique identifier for user identification. This approach works fine till iOS 6, because we are

Answer 1

To add something to nerowolfe's answer, there is a great Keychain Wrapper named MCSMKeychainItem, that, on top of several other things, allows you to generate and retrieve Unique Device ID with single line of code:

[MCSMApplicationUUIDKeychainItem applicationUUID];

so the basic usage will be something like

+ (NSString *)deviceId {
  // load unique device ID or generate new one
  return [MCSMApplicationUUIDKeychainItem applicationUUID];
}

It works on the basis of Keychain, where it stores once-generated unique identifier (as nerowolf suggested). It's open-source and you can download it here on github.

Note: I am not the author of the extension, nor do I in any way know him/her.