Subject: [Capwap] Fw: Comments on "Security Analysis of LWAPP" by Cheng, Nistazakis and Comley

From: "James Kempf" <[email-masked]>
Date: Fri, 16 Apr 2004 14:02:35 -0700

I took a look through this paper and I had a couple of comments.

I agree that CRL checks are necessary, but I believe that there are already
protocols for doing CRL checks, so I'm not sure having LWAPP explicitly
support them is really required. It does require that the AP have an
Internet connection, however. The other alternative would be to reprovision
the certificate frequently, so that the lifetime is relatively short. An
attempt by a compromised AP to reprovision would fail, resulting in having
the certificate time out. In addition, failure of a reprovision would notify
the network administrator that the AP had  been compromised, allowing
them to send out a truck to remove it. On the downside, there is still a
window of vulnerability, but that would need to be weighed against the
risk of compromise.

The paper presents three additional key exchange protocols but then mentions
that Identity Based Cryptosystems might be used. While I agree that IDCs
have some advantages, they still don't remove the need for a PKI-like server
to do parameter distribution, and they still require encrypted key
distribution. They do remove the need for CRL checks, at least on the
identity-based cryptosystem keys. In addition, the best ID-crypto algorithms
like Boneh-Franklin use pairing-based methods and are computationally
expensive.

I think a better solution would be to have the two sides engage in
authenticated Diffie-Hellman (see RFC 2631). Both sides have a certificate
and a public key, so they should be able to authenticate the exchange. DH
does not
require revealing any cryptomaterial over the wire. I think DH keys can be
used in AES. There is still the possibility of DoS attack, but that could be
handled by making the protocol a three message protocol where the two sides
have to exchange a nonce first, like IKE uses. Would there any problem with
using authenticated DH?

            jak

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