TouchID and !simple passcodes

2013.12.02
Credit: Brandon Perry
Risk: Medium
Local: Yes
Remote: No
CVE: N/A
CWE: N/A

So, playing around with my new handy-dandy iPhone 5s, enabled a strong passcode > 20 characters long. I notice however, if I use TouchID to login while on the passcode screen (slide over to it after unlocking, then log in with TouchID), ~10 characters are entered into the passcode text box before I am logged in. Has anyone ever researched this behaviour? I have heard arguments that it is simply a cosmetic feature with random text or something, but that really makes no sense to me. My main concern is if this short little string that I have no control over can also unlock my phone, it will be bruteforced before my actual passcode (couldn't care less about the iphone personally, a toy). That would mean TouchID is actually making my phone less secure under the guise of being more secure (gasp!). Not saying this is some Apple backdoor, could just be a design flaw. Also, if it is a hash of some kind limited to a-f0-9, that greatly decreases the space needed to bruteforce a string this length. :/ But that is wild ass speculation. I am not being too conspiratorial, as I just find it very curious behaviour and don't find something like that being just cosmetic realistic. Totally willing to eat crow though. Any thoughts?

References:

http://seclists.org/fulldisclosure/2013/Dec/0


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