wpa keyhandshake question / bug

Sebastian Weitzel togg
Tue Apr 19 11:10:11 PDT 2005


I am investigating a wpa bug together with hostapd (tested mit madwifi 
but should also apply to others) and serveral types of clients 
(windows, prism, madwifi). On reauthentication (WPA, EAP-TLS) clients 
lose connection from time to time, especially when there is traffic 
between client and ap. I narrowed down the problem into one direction, 
and I now want to get some feedback here to hopefully solve it soon.

I will refer to the 802.11i-2004 standard to describe my problem.
On page 89 step 4 of the 4-way handshake is described. When the 
Authenticator received message 4 it checks some things and then goes to 
PTKINITDONE and sets new key to the driver. After this a group key 
handshake should occur.

This does not always work for me. In my scenario the message 4 under 
some circumstance does not reach the authenticator. One possibility is 
it is lost on air. Another harmfull possibilty is that the message 4 is 
already encrypted with the new sta key and is dropped by the MAC layer 
as crypto error (linux queuing issue maybe, that means the message 4 is 
stuck in linux queue while the .set_key function allready activated the 
new key in the driver). In first case a retransmit on mac layer 
will/should happen?

That is the point where I am very confused. Does wpa_supplicant 
recognize that the message 4 did not arrived at the ap and will 
retransmit the packet? And whats with the driver interface? If the 
.set_key function is done (directly after sending message 4) all 
packets from this time on would be encrypted with the new key and 
dropped from the ap because the 4-way is not done.

Hmm I want to solve this issue, but am also thinking about a workaround.
It maybe would be an idea to check failing packets against the old key, 
but that would mean that the driver has to be modified what is not 
always possible (windows, broadcom, ndis). Another idea is to simply 
disable the ptk derivation on reauth, as it is stated on page 76 that 
this should only occur once per session (but nonce has to be 

Thanks for reading and much regards,
Sebastian Weitzel

p.s.: I just noticed the comment 2 on on page 75, please also have a 
look on this!

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