This research was originally performed by researchers from iSec Partners (now NCC Group), and has been migrated to research.nccgroup.com for posterity.
iOS Secure State Preservation
18 Sep 2013 – Tom Daniels
iOS 6 introduced the concept of application state preservation. The purpose of state preservation is to hide unexpected application termination from users. Regardless of why the application was terminated (e.g., the user explicitly kills the app or the system terminates it to free up memory for the foreground application), state preservation allows users to return directly to the last point of use within the app to provide as smooth a user experience as possible. This is achieved by serializing the state of your app’s view controllers and views when it is backgrounded and storing them to disk.
For a detailed explanation of the preservation and restoration process, refer to Apple’s State Preservation and Restoration Programming Guide.
Developers need to be careful when persisting view states, as there is the potential to expose sensitive application information since the serialized data is in plaintext. Therefore, any data that should remain private needs to be appropriately protected from offline recovery. As such, iSEC created a sample application that implements a more secure process for state preservation and restoration by encrypting objects before encoding and writing them to disk. The goal was to provide developers with a reference for securely implementing state preservation so they can protect their application data while still providing users with an uninterrupted experience.
The NSCoder subclasses for key-based encoding, NSKeyedArchiver and NSKeyedUnarchiver, define delegate protocols that allow applications to customize the encoding and decoding processes, including manipulation of the objects to be (de)serialized. By using a custom class that implements these delegate protocols we can serialize objects, encrypt them with a key from the Keychain, and then pass them back off to UIKit for preservation. When the app is relaunched the data is decrypted and deserialized before being reinstantiated by the view controller. Through implementing the delegate protocol, developers can take fine grained control over which objects are encrypted or simply encrypt all state information.
Check out the project page on GitHub and try out secure state preservation in your apps!