WEP-Open Key Auth - detection of Decryption Failures???

Dan Williams dcbw at redhat.com
Mon Feb 20 21:00:55 EST 2006


On Mon, 2006-02-20 at 13:21 -0500, Tony Espy wrote:
> Is there anyway to detect decryption failures after association with a 
> WEP Open Key access point other than keeping track of the discarded 
> undecryptable RX packets via /proc/net/wireless or 
> /proc/net/hostap/wlanN/stats?

Not without driver support, but it's unclear what further information
you want here...

> Also, are there any other causes of decryption failure besides an 
> invalid key?

Not really, except RF interference from cordless phones, microwaves,
etc.

> We frequently get customers calling in stating that they can't get an IP 
> address for their Pepper Pad and quite often it can be traced back to an 
> invalid WEP key entered.
> 
> It seems like Apple has some kind of magic in their laptop WiFi 
> driver(s) and/or UI that figures out that a WEP key is incorrect and 
> displays a popup message almost immediately.

There is _no_ way to determine whether a key is incorrect with Open
System.  If there is, I'd love to hear it.  I've talked to the original
lead of the Airport software, and asked him this exact question.  He
said the the Airport drivers will always try to do Shared Key when the
AP is WEP encrypted.

Beyond that, there's no way to know that your WEP key is wrong in Open
System besides not being able to decrypt frames from the AP.  In SK at
least, you must prove you know the correct key before you are
authenticated/associated, but there's no such thing in Open System.  And
with Open System, if you don't have the right WEP key, your packets get
dropped at the AP and you get no response.  So it pretty much can't be
done.

> The only way I can think of detecting a bad key from user space would be 
> to modify the driver to reset the 'rx_discards_wep_undecryptable' 
> counter to 0 on association, and then monitor the counter, displaying a 
> popup dialog if the counter starts increasing.

Except that you're never guaranteed to get traffic from the AP at all,
even if your WEP key is correct.  If the AP is dropping badly-encrypted
packets, it will never get your traffic for DHCP or whatever.  And you
can't rely on traffic coming from the AP, because there might not be
any.

The long and short of it is that counting badly-encrypted packets from a
certain AP to get an idea of whether or not the WEP key is incorrect is
an /optimization/, not a solution.  It /might/ give you an idea of
whether or not the key is bad, but it might not.  You never know.

Dan





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