full-disclosure - TouchID and !simple passcodes

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Message-ID: <529ABE11.8060701@gmail.com>
Date: Sat, 30 Nov 2013 22:41:53 -0600
From: Brandon Perry <bperry.volatile@...il.com>
To: full-disclosure@...ts.grok.org.uk
Subject: TouchID and !simple passcodes

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?

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