Hi Steve,
Thanks for taking time to perform a detailed review the document. Comments inline below:
On Apr 13, 2012, at 11:26 AM, Stephen Hanna wrote:
> I have reviewed this document as part of the security directorate's
> ongoing effort to review all IETF documents being processed by the
> IESG. These comments were written primarily for the benefit of the
> security area directors. Document editors and WG chairs should treat
> these comments just like any other last call comments. I apologize
> for submitting these comments a day after the IETF LC closed.
> Other commitments delayed my work. I hope these are still useful.
>
> Thanks,
>
> Steve
>
> ------
>
> secdir review of draft-ietf-emu-chbind-14
>
> This document defines channel bindings for EAP methods, providing
> a way to address the lying NAS and lying provider problems.
>
> Overall, I have no objections to this document. However,
> I do have some comments. I have divided these comments
> into substantive and non-substantive ones.
>
> Substantive Comments
> --------------------
>
> In the Introduction, the second paragraph says that the
> problem "results when the same credentials are used to
> access multiple services that differ in some interesting
> property". Do you mean client or server credentials?
> I think you mean EAP server credentials. Please be more
> explicit to make this clearer, since many people will
> assume that you mean client (EAP peer) credentials. If
> I'm correct and you do mean EAP server credentials,
> I suggest that you say so in the first sentence of
> this paragraph and also in the last sentence.
>
[Joe] The case here is both client and server credentials. If different credentials are required for each type of service then the authenticator will not be able to lie about the type of service it is representing to the client because the client credentials are bound to the service.
> In the next paragraph, you give an example where a low
> security network is used to read email and a high security
> network is use to access valuable confidential information.
> A better example would be to use the low security network
> for web surfing. Reading email can require a lot of security.
>
[Joe] OK
> In the first paragraph of the Problem Statement, the second
> sentence says "However, when operating in pass-through mode,
> the EAP server can be far removed from the authenticator."
> While this is true, I think the more relevant statement here
> is that the EAP server can be far removed from the EAP peer.
> This paragraph is all about problems that can arise when
> parties between the EAP peer and the EAP server (including
> the authenticator) are malicious or compromised. So the
> important thing to point out at the start of the paragraph
> is the large number of parties that may come between the
> EAP server and the EAP peer.
>
[Joe] I think I see your point, but I'm not sure. Traditionally, we have often thought of the path between EAP peer and EAP authenticator as being vulnerable to having multiple parties able to insert themselves into the conversation. However it may be the case that the authenticator the peer is talking to isn't the one it thinks it is and the "real" authenticator may be somewhere in the path as well. The conversation between the EAP peer and EAP server will not be compromised, however the result of the conversation may not have its intended effect.
> In the second bullet of section 3 (on page 7), the EAP GSS-API
> mechanism is a good example. However, I wonder if this attack
> might take a slightly different form. Could the IM server
> pretend to be the mail server? I think so. That's just one
> specific example of a relatively untrusted service pretending
> to be a relatively trusted service. I suggest that you add
> an example like that since the current language is quite
> abstract.
>
[Joe] I agree having a more concrete example would be helpful.
> Overall, I found the document too abstract for my taste. When
> I read an RFC that's defining a protocol to be implemented,
> I prefer to read a description of the problem to be solved
> and then a clear definition of the protocol. This document
> reads like a combination of an academic paper and a protocol
> definition. For example, section 4.1 describes two categories
> of approach to EAP channel bindings: exchanging the actual
> network information or encoding the network information into
> a blob that is used to derive EAP session keys. The rest of
> section 4.1 goes on to discuss the pros and cons of these
> approaches. Then section 4.2 defines a bunch of requirements
> for transporting channel bindings in a lower layer's secure
> association protocol. While that's interesting, the actual
> protocol defined in this document sends the actual network
> information in EAP. Why bother spending several pages talking
> about things that this protocol doesn't actually do?
[Joe] THis is historical and reflects the discussion we had in the working as the document was being developed.
> I see
> several downsides to including that text in this document.
> First, you're making it harder for the reader to understand
> what they actually need to do to implement this protocol.
> You've probably decreased by half the number of people who
> will actually read all of this document. Second, some people
> will use that digression to support arguments like ("My
> incompatible implementation is compliant with RFC XXXX
> because I encoded the network information into a blob
> and used that to generate EAP session keys"). IETF is an
> engineering organization. We should make our specs as clear
> and simple as possible and no simpler. This document includes
> lots of interesting theory that's not needed for its purpose.
> If you're interested in seeing this document widely implemented,
> I suggest that you remove as much of that as possible and focus
> the document on explaining the problem and defining a protocol
> that solves it. Or you can leave it as is. I'm sure some people
> will implement it. Just not as many. Friendly suggestion. Take
> it or leave it.
>
[Joe] I can see your point, but I think some members of the working group would object if we removed some of this material. If its not clear perhaps we can so a better job of indicating what is normative and what is informative.
> In the first paragraph of section 5.1, you say that "the
> EAP server needs to be able to access information from the
> AAA server that is utilized during the EAP session and a
> local database". Which information? A parenthetical example
> (an e.g.) would help with understanding what you mean.
>
[Joe] OK
> In section 7.1, you explain that this document isn't useful
> without an accompanying document defining what information
> should be included in i1 for each lower layer. Are those
> documents being prepared for all or at least some of the
> lower layers? I couldn't find any in a quick search. I know
> we can't wait for everything to be ready but it would be
> nice to see at least one or two of those documents so that
> we can have confidence that this protocol can support all
> the things that those documents will need.
>
[Joe] That not how I interpret the text in section 7.1. This document does specify a general attribute that can be used to specify the type of lower layer used to carry EAP. Section 7.1 states that additional documents will be needed to specify attributes specific to a particular lower layer. ABFAB is working on a document that contains this information for their lower layer, draft-ietf-abfab-gss-eap.
> In section 7.2, you define a new RADIUS attribute that
> identifies the EAP lower layer in use. But you say in
> the first sentence of that section that this attribute
> is defined to "carry the EAP lower layer". That sounds
> like all the lower layer traffic will be encapsulated
> in this attribute and carried in it. I think you should
> change the word "carry" in that text to "identify".
> Unless I misunderstood you.
>
[Joe] OK
> As I read section 8, I wonder whether include the User-Name
> attribute in i2 could open the system up to attacks where
> an authenticator or intermediate proxy could remove the
> anonymity of a user who's using a pseudonym for the
> username by changing the value of the User-Name attribute
> to the username of the user suspected of being responsible
> for this authentication. If the channel bindings checks
> fail, the authenticator will know they were wrong but if
> the channel bindings checks succeed, the authenticator
> will have confirmation of the user's identity.
>
[Joe] Perhaps, on the one hand I would think the system would not behave this way, the server would be expecting a pseudonym or token and fail if it got something else. On the other hand, I don' think the text for the user-name check or the test itself are that useful. We could either warn against the possible identity disclosure or remove the user-name check.
> I didn't understand that sending i2 from the server to
> the peer is optional until I got to section 9.1. In section
> 5.3, you said that "For success, the server returns the
> attributes that were considered by the server in making
> the determination that channel bindings are successfully
> validated". Which of these is right?
>
[Joe] The server is required to return the channel-binding attributes it verified because the client may require certain attributes to be checked. This list of verified attributes is not equivalent to i2. i2 is what attributes are sent in the AAA protocol and I don't think it should be referenced here. Instead it should probably say:
"sending the result from server to peer over integrity-protected channel"
> At the top of page 23, you say that "In many EAP deployments
> today attacks where one NAS can impersonate another are
> out of scope." Do you mean that these attacks are generally
> not considered but they are a potential problem? Or that
> they are not a problem? I think they are always a possible
> problem. Even when all the NASes are owned and managed by
> the same organization that runs the AAA server and no proxies
> are used, there's still the potential for a NAS to become
> compromised. So I think you should say these attacks are
> "not considered although a real concern" not that they are
> "out of scope".
[Joe] As EAP is deployed in most cases today the authenticator is not identified, it is merely authorized as being part of the network. The peer does not expect differentiated service based on the NAS it is connected to. Since the service is the same it does not matter to the peer if one authenticator impersonates another. When we start discussing use case such as ABFAB where different services are being provided did the identity of the authenticator really become an issue.
Perhaps the following text is better
"In many EAP deployments today attacks where one authenticator can impersonate another not a real concern since different authenticators provide the same service. In these situations an authenticator does not gain significant advantage in impersonating another authenticator. "
> And then the following sentence says that
> "Channel bindings brings these attacks into scope". Again,
> I think those attacks are always a potential issue. Channel
> bindings provide a good way to address them, whereas there
> were few ways to do so before. Maybe it would be better to
> say that.
>
[Joe] I see your point here, how about:
"The use of EAP in situations where different authenticators provide different services makes the ability to authorize different authenticators more important. The system as a whole needs to be analyzed to
evaluate cases where one authenticator may impersonate another and to evaluate the impact of this impersonation. Channel bindings provides a way to address these issues. "
> In the next paragraph, you say that when cryptographic binding
> is used in a tunnel method, the MSK is disclosed to the NAS.
> I don't think that's right. The MSK that's disclosed to the
> NAS is the one generated by the outer method. The MSK that's
> used in cryptographic binding is the one that's generated by
> the inner method. I don't think there's a problem there.
>
[Joe] OK, this test is referring to where there is separate between the terminating server of the inner and outer methods. Maybe it would be clearer if the text said the following:
"Even when cryptographic binding does attempt to confirm that the inner and outer server are the same, the Master Session Key (MSK) from the inner method is typically used to protect the binding. However, if the outer method tunnel terminates on the authenticator the inner MSK is disclosed to the authenticator, which can attack cryptographic binding."
> Later in that paragraph, you say that an attack where the NAS
> terminates an EAP tunnel method is not in scope for existing
> models for cryptographic binding. I think that's wrong also.
> EAP tunnel methods protect against just this sort of attack.
> The first step in these methods is for the EAP peer and the
> EAP server to build a TLS tunnel. This requires the EAP peer
> to authenticate the EAP server and decide whether it's trusted.
> If the NAS has credentials that will cause the EAP peer to
> trust it as an EAP server, a MITM attack is possible. Of course,
> that's true for any secure communications and it's a very bad
> situation. That's why the EAP peer must be very careful about
> who it trusts and how it authenticates them and the EAP server
> (and any CAs or other TTPs) must also be similarly careful.
> I don't see that any of this has much to do with channel bindings.
> Am I missing something here? If so, please explain it. And maybe
> you can clarify the text so that other folks get it also.
>
[Joe] The issue is when you have different services the client may not have enough information to determine what an authenticator is authorized for based on its identity alone. In this case it would like help from the server that terminates the inner method. However current crypto binding is based on the MSK so the authenticator can generate whatever channel binding quantities it wants because the authenticator has all the necessary key material. In order for channel bindings to determine if the authenticator is authorized or not, the method needs protect the channel bindings with a key generated from the inner method EMSK that the authenticator does not possess. Here is some text that may help:
"If the authenticator controls the cryptographic binding then it also controls the channel binding parameters and results. If the channel binding process is used to differentiate one authenticator from another then the authenticator can claim to support services that it was not authorized to. This attack was not in scope for existing threat models for cryptographic binding because differentiated authenticators was not a consideration. Thus, existing cryptographic binding does not typically provide mutual authentication of the inner method server required for channel binding."
> In section 9.2, this paragraph appears:
>
> Dishonest peers can only manipulate the first message i1 of the
> channel binding protocol. In this scenario, a peer sends i1' to the
> server. If i1' is invalid, the channel binding validation will fail.
> On the other hand if i1' passes the validation, either the original
> i1 was wrong and i1' corrected the problem or both i1 and i1'
> constitute valid information. A peer could potentially gain an
> advantage in auditing or charging if both are valid and information
> from i1 is used for auditing or charging. Such peers can be detected
> by including the information in i2 and checking i1 against i2.
>
> In the penultimate sentence, I think you mean "information from
> i1' is used for auditing or charging." There's no problem if
> information from i1 is used for auditing. If that happens, the
> bad peer's information is being ignored.
>
[Joe] I think you are correct.
> That paragraph does make me wonder about another attack that
> could be enabled by channel bindings. What if a malicious peer
> gets perfectly good i1 from a NAS but then sends bad i1' to
> the EAP server with a goal of damaging the reputation of the
> NAS? The EAP peer could be working for a competitor of the
> organization that runs the NAS or just be malicious. Is that
> a valid concern? If so, maybe you should cite it and suggest
> a countermeasure.
>
[Joe] I think this is a valid concern for some environments. I think we should add a section for that.
> In section 9.4, you refer to "the optional result message".
> I didn't know before this that the result message was optional.
> Could you make that clear earlier?
>
[Joe] The result message is not intended to be optional. This text needs to be fixed.
> In the IANA Considerations, you talk about creating a new
> "EAP Channel Binding Parameters" top level registry. That's
> fine. But then you talk about a "Channel Binding Codes"
> registry. Didn't you mean a "sub registry"? If you want
> the Channel Binding Codes registry to be in the EAP Channel
> Binding Paramters registry, I think the first one is really
> a sub registry.
[Joe] Yes
> Also, you say that "Early allocation is
> allowed" for that registry. What does that mean? I don't
> see any description of "early allocation" in RFC 5226.
> I expect that IANA will want to know what they need to
> do to provide "early allocation". How early? Etc.
>
[Joe] OK, I need to check if there is any convention here.
> In the definition of the "Channel Binding Namespaces"
> registry, did you want to include a reference column
> as you did in the "Channel Binding Codes" registry?
> If so, maybe you should say so.
>
[Joe] OK
> At the end of section 11.1, a sentence says that
> "Values skipped in the above table are available
> for assignment." I don't think any values were skipped
> so I don't think you need that sentence.
>
[Joe] Yup, left over from previous version.
> Appendix A is pretty similar to section 1. Might you
> be able to merge them somehow?
>
[Joe] I think the authors attempted to do this and then decided it was better to keep them separate.
> In section A.3, you describe downgrading attacks and
> how channel bindings can prevent them. However, I think
> this will only work if the EAP peer rejects all methods
> that don't include channel bindings. Otherwise, the NAS
> can just offer an unauthorized EAP method that permits
> it to obtain the user's credentials and off to the bank.
>
[Joe] I believe you are correct.
> Non-Substantive Comments
> ------------------------
>
> In the top right header of the first page, the organization
> for T. Clancy should probably be Virginia Tech not Electrical
> and Computer Engineering.
>
> In the fourth paragraph of the Introduction, I think you should
> add the word "the" before "lying provider" problem. In the last
> sentence of that paragraph, I'd suggest adding "also" before
> "gain access". That should make things clearer.
>
> In the first bullet in section 3 (at the bottom of page 6),
> "virtual Lads" should probably be "virtual LANs". Unless you
> are talking about some other phenomenon! ;-)
>
> In the second paragraph of section 4.3, the phrase "More often
> it is useful" can be confusing. The word "it" seems to refer
> to the subject of the previous sentence ("verification of the
> MAC or IP address of the NAS"). Actually, the word "it" refers
> to the idea of making policy decisions about services but that
> idea doesn't appear until later in the sentence. I suggest that
> you reword the sentence to say "More often the best approach is
> to make policy decisions [...]".
>
> In the first sentence of section 8, the phrase "a AAA Accept-
> Request messages" can't be right. Is it singular or plural?
> I think it's plural so the word "a" should be removed.
>
> In section 12 (Acknowledgements), I think Sam Hartman's last
> name should be capitalized.
>
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