eap Mailing List: Re: FW: [eap] Re: EAP key binding discussion

[Bernard Aboba (aboba]

> Yes, if the AAA-Key is same for all authenticators.

I think one issue is exactly what the definition of an "authenticator" is.

>From the AAA perspective, an authenticator is defined by attributes such
as NAS-IP-Address, NAS-IPv6-Address, and NAS-Identifier.  One issue is
that a NAS may have more than one IP address (e.g. APs can be members of
multiple VLANs and will need an IP address on each VLAN).  So it seems
like NAS-Identifier is perhaps the best identifier to use for the purpose
of defining the NAS identity from the point of view of the EAP peer,
server and authenticator.

Assuming that the NAS advertises its NAS-Identifier to the peer, securely
confirms it in an exchange with the peer (such as in the Secure
Association Protocol) and sends this to the server, it is possible for the
EAP peer, server and authenticator to confirm that the NAS has accurately
represented its identity.  The peer and server can confirm that the NAS
has told them the same NAS-Identifier via Channel Bindings.

If the peer and server confirm the NAS-Identifier value told to them is
the same, then the peer can assume that the server has verified that the
NAS is not impersonating another NAS.  The key framework discusses how
this can be done -- it basically requires verification by the first hop
proxy.

Note that I believe it is outside the scope of the document to describe
what hardware configurations constitute a valid NAS.  That is, a large NAS
could involve multiple processors, could be a WLAN switch or a stand alone
AP, could come in a plastic box, a metal box or no box at all (wirewrap
board in the open air).  A single NAS could even be a cluster.

Obviously it is a good idea for the NAS to not leak keying material beyond
its boundaries.  But that leakage can occur via poor circuit design,
inadequate shielding, as well as security vulnerabilities within the AP
itself.  So I think it's out of scope to describe all the precautions that
should be taken.