Comment Re:What's he on, today? (Score 5, Informative) 364
Apple devices have an additional "trick" beyond just PBKDF2 - There's a random AES key burned into the CPU, and it's wired such that it can be set/erased, but not directly read - it can only be fed as the key into an AES engine.
I am not sure if Apple's PBKDF2 has this AES engine as part of the loop, or if it just feeds the key that comes out of PBKDF2 through the AES engine, but the end result is, on any given device, the AES key that results from a given passphrase is unique to that device and cannot be reproduced off-device.
So if someone just clones the device's flash contents, they have to resort to brute-forcing AES directly, as opposed to trying to brute-force passcodes.
So you can only brute-force passcodes on-device (something like 80ms per try on this model, newer models have a 5 seconds per try limitation), and Apple's software doesn't even allow you to do that. The FBI wants to at LEAST get on-device brute-force capability.
Which might still take years if the user had a reasonably strong passphrase.