Re: Graphic cards as Wi-Fi crypto crackers

[David Farber <dave () farber net>]

Begin forwarded message:

From: Christian Huitema <huitema () windows microsoft com>
Date: October 12, 2008 5:46:22 PM EDT
To: "dave () farber net" <dave () farber net>, ip <ip () v2 listbox com>
Subject: RE: [IP] Graphic cards as Wi-Fi crypto crackers


> I hadn't seen any mention of this issue here, so I thought I'd pass
> it along.  The story is that graphics cards' GPUs have been used to
> massively speed brute force cracking of Wi-Fi (WPA) crypto keys.
>
> http://www.dslreports.com/shownews/Using-GPUs-To-Speed-Up-WPA-Hacks-
> 98321
>
> I definitely agree that the devil is in the details, and details are
> sorely lacking.  But keeping on eye on this attack vector clearly
> makes sense.

The paper describes a brute force attack against WPA keys. Normally, a  
brute force attack should not succeed against the 128 bit keys, so the  
attack has to concentrate on the human element. The 128 keys are  
derived from a "pass phrase". The attack works by trying to guess the  
passphrase. The eventual success of the attack depends a lot on how  
the pass phrase is generated, and what methods are used to copy the  
pass phrase between computers.

The simplest systems ask the users to enter a passphrase. People- 
generated passwords, or even pass phrases, are notoriously weak. You  
will be happy if you get 40 bits of entropy. Such people generated  
passwords are no match for a serious computer, even less a network of  
computers using the parallel processing powers of their GPU.

The "good" pass phrases are the ones generated by computers, but the  
strength of the computer generated phrases is limited by the "human  
vector". You can only pile up so much complexity before humans find  
the phrase impossible to remember. Even if you accept that people will  
write it down on paper, there are still limits of the complexity you  
can pile up before the human-copying errors become too frequent to  
properly manage.

WPA and WPA2 are not the only system that combine a secret password or  
passphrase with some kind of public hash used for verification. All of  
these systems are vulnerable to elaborate "dictionary" attacks, in  
which attackers use combination of words from dictionaries, digits,  
and the occasional special characters. If the password or passphrase  
was generated by a human, it is most certainly already broken. If the  
passphrase can be remembered by a human, it is probably broken. If the  
passphrase can be copied by a human, it is suspect.

-- Christian Huitema






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