Re: WEP attacks based on IV Collisions

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To: pen-test@securityfocus.com
Date: Sat, 1 May 2004 11:14:39 +1000

> First, correct me if I am wrong, but it seems like a non-trivial task
> to actually determine the WEP key if you have zero knowledge about the
> target network, i.e. IP addressing, AND can't readily inject 802.11b
> frames into the target network just because you have a usable
> keystream? Has anyone found differently?

Well in this case you essentially have:

        Random WEP bitstream XOR Random data

Good luck. Statistical methods might work if you have LOTS of data for each
possible IV (of which there are 16 million). I don't know of anyone that has
bothered to look into this seriously.

> follows directly, since all the pairwise XORs are known." But that's just
> my confusion - if you have the
> keystream (IV + Secret key run through RC4) and you have the original
> plaintext, then why can't you determine the secret key as well?

The (40 or 104 bit) WEP key is merged with the IV (stored in plaintext int he
packet) to give a 64 or 128bit number. This is used to seed a pseudo-random
number generator built around RC4. All you get when you know the plaintext is
a section of the pseudo-random number sequence. Going from that sequence back
to the secret key is non-trivial. That said, the CRC at the end of the wep
packet can be used to verify decryption (check out the wep_tools.tgz package)
and/or to do an offline brute-force crack.

> Last, what types of traffic or methods are used to determine a
> plaintext? I've seen one method mentioned: inject an ARP packet to the
> AP encrypted with the known keystream. But this seems to be based on
> having information such as IP addressing on the target network, which
> isn't known in this case.

I've used ping packets of known length to a known IP (WEP doesn't pad packets
so its easy to determine your traffic). Its fairly trivial to pick out your
traffic if you know a valid IP address on the network.

For something off-the-wall that I have always wanted to try - There are
various fields in IP/ethernet traffic that are always constant or can be
calculated easily (Protocol IDs, length fields, etc). It should therefore be
trivial to find the WEP PRN sequence for these parts of the packets - even if
their content is unknown. It might be possible to use that information to
launch an offline brute-force attack that is faster than using the CRC attack
(less processing).

>
> [1] "Security of the WEP algorithm"
> http://www.isaac.cs.berkeley.edu/isaac/wep-faq.html
>
>
>
>
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