Overview: This version of the document still has some issues remaining.
Section 1
The so-called "lying NAS" problem is a well-documented problem with
the current Extensible Authentication Protocol (EAP) architecture
[RFC3748] when used in pass-through authenticator mode. Here, a
Network Access Server (NAS), or pass-through authenticator, may
represent one set of information (e.g. identity, capabilities,
configuration, etc) to the backend Authentication, Authorization, and
Accounting (AAA) infrastructure, while representing contrary
information to EAP clients.
[BA] As noted in the review of -00, the issue isn't just whether the
NAS is sending different information to the EAP peer and
AAA server. It also is possible that the NAS will send the same
information to the peer and AAA server, but that both could be
wrong.
Section 3
There are two different types of networks to consider: enterprise
networks and service provider networks. In enterprise networks, we
assume a single administrative domain, making it feasible for an EAP
server to have information about all the authenticators in the
network. In service provider networks, global knowledge is
infeasible due to indirection via roaming. When a client is outside
its home administrative domain, the goal is to ensure that the level
of service received by the client is consistent with the contractual
agreement between the two service providers.
[BA] While the AAA server might have information about all the
authenticators
in the enterprise case, if it is more than one hop removed from the NAS,
then
it might not be able to check the validity of the AAA attributes. For
example,
a first hop AAA server can check if the NAS-IP-Address/NAS-IPv6-Address
attributes match the IP source address corresponding to the shared secret.
A AAA server multiple hops away cannot verify this.
o Service Provider Network: An EAP-enabled mobile phone provider
operating along a geo-political boundary could boost their cell
towers' transmission power and advertise the network identity of
the neighboring country's indigenous provider. This would cause
unknowing handsets to associate with an unintended operator, and
consequently be subject to high roaming fees without realizing
they had roamed off their home provider's network.
[BA] This seems like a good example. My understanding is that
power boosting actually does occur. It might be worthwhile to consider
adding an Appendix to talk about how channel bindings might address
this or other potential examples.
o It allows for fuzzy comparisons of network properties, rather than
requiring absolute comparisons. This allows for a broader
definition of consistency, rather than bitwise equality.
[BA] As discussed during the EMU WG meeting, a term other than "fuzzy"
would probably be better. Also, there probably needs to be more discussion
on why enabling bit-by-bit comparisons is undesirable or not important.
Section 4
o Given it doesn't affect the key derivation, exact use of the
results can be subject to policy, to facilitate debugging,
incremental deployment, and backward compatibility.
[BA] I think the major issue with the key derivation approach is that in
practice, "canonicalization" and formatting issues are highly likely in
a channel bindings implementation, even if formats are well specified.
The implication of this is that requiring enforcement may not be
practical; rather
logging, or evidence gathering may be all that can be achieved. The key
derivation approach can't support such a "logging only" mode; enforcement
is required.
The scope of EAP channel bindings differs somewhat depending on the
type of deployment in which they are being used. In enterprise
networks, they can be used to authenticate very specific properties
of the authenticator (e.g. MAC address, supported link types and
data rates, etc), while in service provider networks they can
generally only authenticate broader information about a roaming
partner's network (e.g. network name, roaming information, link
security requirements, etc). The reason for the difference has to do
with the amount of information you expect your home EAP server to
know about the authenticator and/or network to which you are
connected. In roaming cases, they are likely to only know
information contained in their roaming agreements.
[BA] It would probably also be worth talking about the inability to directly
verify the correctness of some parameters in the multi-hop case (in either
enterprise or service provider scenarios).
Section 5
Channel bindings are always provided between two communication
endpoints, here the EAP client and server, who communicate through an
authenticator in pass-trough mode. During network advertisement,
selection, and authentication, the authenticator presents
unauthenticated information, labeled i1 for convenience, about the
network to the peer. As there is no established trust relationship
between the peer and authenticator, there is no way for the peer to
validate this information. Our goal is to transport i1 from the peer
to the server, and allow the server to validate it against
information i2 it has stored in its local database, labeled DB.
[BA] As we discussed in the EMU WG meeting, is Channel Bindings only about
the AAA server verifying the info sent from the NAS to the EAP peer, or
is it
also about comparing the AAA attributes against that information? For
example,
would it be ok if the peer got the correct information, but the AAA
attributes
are incorrect?
This information, i1, could include the identity of the authenticator
and the network it represents, in addition to advertised network
information such as offered services and roaming information. To
prevent attacks by rogue authenticators, the EAP server must be able
to verify that i1 either matches its knowledge of the network
(enterprise model) or is consistent with the contractual agreement
between itself and the roaming partner network to which the client is
connected (service provider model).
[BA] The term "authenticator identity" is used in multiple places, without
being defined. RFC 5247 defines this as the NAS-Identifier/NAS-IP-Address/
NAS-IPv6-Address attributes, but I'm not sure this is what you mean here.
One issue with highlighting this particular problem is that even in IEEE
802.11r,
the NAS-Identifier is not advertised in the Beacon.
Note that in addition to just returning a validation result
indicating whether i1 and i2 are consistent, the EAP server can
optionally return i2 in its entirety. This would allow the EAP
server to provide additional, authenticated information about network
or authenticator to which the client has connected that the client
may wish to use in deciding whether the authenticator is authorized
to provide the type of service the client desires. This goes beyond
the general definition of channel binding, but allows for additional
flexibility.
[BA] How does the EAP server/AAA server obtain the information i2? For
example, is it indexed via some key AAA attributes (e.g. NAS-Identifier)?
One way to transport the single round-trip exchange is as a series of
Diameter AVPs formatted and encapsulated in EAP methods per
[I-D.clancy-emu-aaapay]. For each lower layer, this document defines
the parameters of interest, and the appropriate Diameter AVPs for
transporting them. Additionally, guidance on how to perform
consistency checks on those values will be provided.
[BA] One potential complicating factor will be RADIUS extended attributes.
These will be encoded as Diameter vendor-specific AVPs, potentially with
grouping. It might make sense to explicitly state that attributes
useful for
Channel Bindings should probably be allocated in the standard RADIUS space,
to avoid this potential "gotcha". It also might be useful to state how the
comparison is to be done (e.g. ignore Diameter AVP 'M' bit).
Section 7.1
The client SHOULD transmit to the server the following fields,
encapsulated within the appropriate Diameter AVP:
SSID
BSSID
RSN IE (if present)
The server MAY send the Cost-Information AVP from the Diameter
Credit-Control Application [RFC4006] to the peer indicating how much
peers will be billed for service.
[BA] There is currently no AAA attribute for sending the SSID by itself,
only the combination BSSID/SSID (e.g. Called-Station-Id). The RSN IE
currently isn't carried either. You might want to review
draft-aboba-radext-wlan-08.txt for a list of the new attributes that
are being proposed for usage with various IEEE 802 media (including
IEEE 802.11, 802.16, 802.1X-REV, etc.).
Having the server send the Cost-Information AVP is an interesting idea.
However, is this information 802.11-specific or might it be useful in many
media?
Section 9
The EAP peer will need an API between the EAP lower layer and the EAP
method that exposes the necessary information from the NAS to be
validated to the EAP peer, which can then feed that information into
the EAP methods for transport. For example, an IEEE 802.11 system
would need to make available the various information elements that
require validation to the EAP peer which would properly format them
and pass them to the EAP method. Additionally, the EAP peer will
require updated EAP methods that support transporting channel binding
information. While most method documents are written modularly to
allow incorporating arbirary protected information, implementations
of those methods would need to be revised to support these
extensions.
[BA] The issue is not just about modification of the EAP lower layer --
in many cases, drivers will need to be changed as well to provide the
required information, and in the interim before this happens, there may
be cased where an EAP method that is Channel-Binding capable will not
be able to obtain channel binding information.
Overall, I'd suggest that a section be added laying out the cost/benefit
analysis. Although multiple vendors have attempted to implement Channel
Bindings, to my knowledge none of those implementations have shipped,
because
the cost are easy to identify (and are high) and the benefits are more
difficult
to get a handle on (and are less amenable to quantification).
Additionally, an interface is necessary for populating the EAP server
database with the appropriate parameters. In the enterprise case,
when a NAS is provisioned, information about what it should be
advertising to peers needs to be entered into the database. In the
service provider case, there should be a mechanism for entering
contractual information about roaming partners.
[BA] Do we really expect operators to enter in all potential AAA
parameters into the database? This seems like a substantial
operational burden. Instead, I'd suggest that for some parameters,
auto-registration might be helpful -- allowing the database to be
populated based on the AAA attributes first obtained from the NAS when
it is provisioned. While this trusts that the NAS isn't sent to the
operator in a compromised state, but only becomes compromised later,
it would ease the operational burden.
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