The iOS Security document describes several security mechanisms implemented in iOS 9. I am specifically interested in the actual security of a 4 digits PIN on iOS. I am looking for tangible information specific to iOS.
The document above states on page 10:
(...) so brute-force attempts must be performed on the device under attack. A large iteration count is used to make each attempt slower. The iteration count is calibrated so that one attempt takes approximately 80 milliseconds. This means it would take more than 5½ years to try all combinations of a six-character alphanumeric passcode with lowercase letters and numbers.
To further discourage brute-force passcode attacks, there are escalating time delays after the entry of an invalid passcode at the Lock screen.
With this in mind and putting aside possible vulnerabilities, how secure is a 4 digits PIN on iOS 9? Specifically with the assumption that:
- the attack is possible only via the crypto-chip (described in the doc)
- ... which enforces the restrictions and time delays
is a PIN a mechanism secure enough to sustain two years of attacks (with the knowledge we have today)? (*)
The social engineering part and human nature suggests that some combinations must be avoided (
2222, ... ,
4321, some visual ones as a cross or a box made out of the digits, etc.). We are now past the 10 tries and the device is at that point throttled or wiped.
(*) two years is an example, data recovered after this time may not be that sensitive anymore. I am aware of the fact that one can find tomorrow a vulnerability or a flaw in the architecture which breaks everything in iOS - this is why I am interested in the technical capacities of today.