Re: Gen-ART and OPS-Dir review of draft-ietf-httpauth-hoba-07

Stephen Farrell <> Wed, 24 December 2014 11:15 UTC

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Subject: Re: Gen-ART and OPS-Dir review of draft-ietf-httpauth-hoba-07
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Hi David,

Sorry for the slow response, end of year rush and all that...

And thanks for the review, responses inline and a proto-08
version (handling the various IETF LC comments) is at [1]
with a diff vs. 07 at [2]. I'll plan to publish that as an
official -08 in a few days if there are no comments.



On 15/12/14 23:25, Black, David wrote:
> This is a combined Gen-ART and OPS-Dir review.  Boilerplate for both follows ...
> I am the assigned Gen-ART reviewer for this draft. For background on
> Gen-ART, please see the FAQ at:
> <>.
> Please resolve these comments along with any other Last Call comments
> you may receive.
> I have reviewed this document as part of the Operational directorate's ongoing
> effort to review all IETF documents being processed by the IESG.  These comments
> were written primarily for the benefit of the operational area directors.
> Document editors and WG chairs should treat these comments just like any other
> last call comments.
> Document: draft-ietf-httpauth-hoba-07
> Reviewer: David Black
> Review Date: Dec XX, 2014
> IETF LC End Date: Dec 24, 2014
> Summary: This draft is on the right track, but has open issues
>  		described in the review.
> This draft specifies a signature-based authentication mechanism for HTTP
> that is based on asymmetric (public-private) key pairs without using
> certificates.  A different key pair is used by each user for each for
> each host ("web-origin", see [RFC6454]), and each signature is based
> on a server-generated challenge.
> The draft is generally well-written and clear on the authentication
> mechanics, but has weaknesses in specifying the environment that
> surrounds use of this mechanism.  Some of these issues are basic things
> that anyone working on web technology or applied crypto "just knows",
> but even the obvious needs to be stated when it's integral to security.
> -- Major issues: --
> [1] Challenge size and predictability
> In the ABNF:
>    challenge = 1*base64urlchars
> I did not see any other discussion of minimum length of challenge
> or amount of entropy in the challenge.  For the latter, do the challenges
> need to be unpredictable?  I would think so.
> I think a requirement on minimum challenge size and a recommendation on
> minimum amount of entropy would be good ideas.

Fair point. And you're right that this is assumed and ought be
explicit, so good catch.


   o  challenge: MUST be a base64url encoded challenge value that the
      server chose to send to the client


   o  challenge: MUST be a base64url encoded challenge value that the
      server chose to send to the client. The challenge MUST be chosen
      so that it is infeasible to guess, and SHOULD be indistinguishable
      from (the base64url encoding of) an at least 128-bit long random

> [2] Challenge verification and reuse
> This sentence on p.6 strikes me as subtly dangerous:
>    The challenge however is sent over
>    the network so as to make it simpler for a server to be stateless.
>    (One form of stateless challenge might be a ciphertext that the
>    server decrypts and checks, but that is an implementation detail.)
> The draft elsewhere asserts or assumes that challenge reuse can be
> prevented by the server or be time-bounded by max-age, and relies on
> those reuse limits for security properties, e.g., as in this sentence
> near the end of Section 6.1:
>    Note that replay of registration (and other HOBA) messages is quite
>    possible.  That however can be counteracted if challenge freshness is
>    ensured.
> The sentence quoted from p.6 appears to allow (or even encourage)
> stateless servers to implement rather weak checks on not only challenge
> reuse, but also challenge validity (e.g., what if the challenge validity
> test were "challenge mod 1024 == 7" ?).
> Allowing such weak checks could enable an attacker to choose or influence
> the choice of challenge values, which could be useful (e.g., for
> differential cryptanalysis attack on the hash function), as the
> attacker (client) already controls the nonce.
> As part of this, the server needs to check whether a challenge presented
> on one connection or session has been reused on another.  I think the
> following sentence in section 3 is intended to prevent this, but it's
> weakened by the "stateless" language above:
>       The challenge MUST be
>       unique for every HTTP 401 response in order to prevent replay
>       attacks from passive observers.
> Explicit server checks for challenge reuse ought to be REQUIRED, and
> the above use of "stateless" needs to be qualified.

Hmm. I think that HOBA ought not REQUIRE full replay protection
in all cases, that is, it SHOULD be possible to deploy a server
that is vulnerable to some replay. The reason is that we're trying
to displace passwords here and I believe (without much hard evidence
I'll grant you) that we'd lose some deployment chances if we were
to insist on every single challenge being fresh.

So I'd argue for no-change here, but would welcome further discussionm
evidence or suggestions for text that's not going to be so onerous
as to weaken our chances of getting this deployed.

> [3] Web origin checks
> I did not see text mandating that TLS (server certificate), HTTP (accessed
> URL) and HOBA (signature input) use the same web origin, and stating that
> the server has to check/enforce this.  Did I miss something?
> I realize that this is an obvious requirement, but it does need to be
> stated.

Fair enough, I agree folks have been known to not implement even the
most obvious things:-) I added the text below to section 3:

   The server MUST check that the same web origin is used in all of the
   server's TLS server certificate, the URL being accessed and the HOBA
   signature.  If any of those checks fail, the server treats the
   signature as being cryptographically incorrect.

> [4] TLS requirement level
> TLS is only a "SHOULD" requirement for HOBA, not a "MUST" requirement -
> first paragraph in Section 8.1 (Privacy Considerations).
> If TLS is not a "MUST" requirement, at least a discussion of man-in-the-
> middle (MITM) attacks seems to be in order.  There's no need for an extensive
> discussion of all of the security properties of TLS, but MITM attacks are
> definitely worth a mention, and that could be complemented by a pointer to
> suitable HTTP/TLS security considerations text elsewhere.

I think that's covered where we note the potential for replay, which
is what a MitM would/could do in this case. That is noted in both
section 3 and in 6.1 and in section 8. I think we have it covered to
be honest.

> [5] Section 5.3 - unrecognized key
> Paragraphs 4 and 5 in this section don't seem to belong here, and appear to
> be written in a somewhat loose fashion, particularly around user verification.
> A server that does what is suggested here could have significant security
> weaknesses.

Which specific weaknesses do you mean?

> I strongly recommend replacing those two paragraphs with a short sentence that
> says:
> 	- if a kid is unrecognized or the server otherwise determines
> 		that the CPK is not to be used for this HTTP authentication,
> 	- then the Registration (see 6.1) and/or Associating Additional Keys to
> 		an Exiting Account (see 6.2) processes MAY be used instead of
> 		treating this situation as an authentication failure.
> and moving all discussion of user verification into Sections 6.1 and 6.2.

Sorry, I don't think that is needed at all. The current text simply
reflects the kind of thing that we think deployments will do and will
need to do and I don't see what specific weaknesses you are claiming
might ensue. I'd be happy to note any such of course.

Also, from my reading your suggested text has the same impact and
only differs in using a 2119 MAY instead of could, might etc. But if
you meant it to differ more substantively you'll need to explain that
to me some more.

> -- Minor issues: --
> [A] ABNF encoding of alg
> In section 2, I see:
>    alg = 1*2DIGIT
>    o  alg: specifies the signature algorithm being used encoded as an
>       ASCII character as defined in Section 9.3.
> Section 9.3 specifies a single digit.  How does that become 1*2DIGIT ?

Huh? The current algs only use one digit, but the abnf allows for
more. Nothing odd there that I can see.

> I suggest: "as an ASCII character" -> "as one or two ASCII digits based
> on the IANA registry established by Section 9.3"

I think it's ok as is.

> This may seem like a nit, but any imprecision in signature algorithm
> input can lead to interoperability problems.

Yes it can. But we're not being imprecise that I can see.

> [B] Section 5.3 - CPK vs. kid
>    In the authentication phase, the server extracts the CPK from the
>    signing phase and decides if it recognizes the CPK.  If the server
>    recognizes the CPK, the server may finish the client authentication
>    process.
> That seems wrong.  IIUC, the HOBA client sends:
>    HOBA-RES = kid "." challenge "." nonce "." sig
> If so, I don't understand how the server extracts a CPK from that.

Via the kid yes.

> I suspect that what was intended is that the server extracts a key identifier
> (kid) and then determines a) whether it knows a CPK for that kid and b) whether
> it allows use of that kid/CPK for this authentication.  At least a) seems to
> be indicated by this text in Section 3:
>    The server MUST check the cryptographic correctness of the signature
>    based on a public key it knows for the kid in the signatures,

I'm not seeing an actionable issue that needs a change.

> [C] RSA signature algorithm
> Section 7 says:
>    RSA is defined in
>    Section 8.2 of [RFC3447], and SHA-1 and SHA-256 are defined in [SHS].
> Section 8.2 of RFC 3447 specifies RSASSA-PKCS1-v1_5.  At a minimum, that
> algorithm name should be stated here (which is a nit).  

Disagree (also as a nit:-)

> The minor issue is
> whether that is the signature algorithm that's wanted, on which I'll defer
> to the crypto experts on that (e.g., I seem to recall a negative saag
> comment on PKCS 1.5 mechanisms in the past couple of months).

Yes, it's fine for security and is correct for interop/deployment
reasons. PKCSv1.5 encryption is what's less desirable if not done
well and attempting to mandate PSS here would be a bad plan.

> [D] Javascript local storage.
> The first paragraph of Section 8.2 on this topic concludes with:
>    It's not clear that we
>    introduce unique threats from which clear text passwords don't
>    already suffer.
> That's at odds with the use of "all" in this sentence in the abstract:
>    HOBA is an
>    alternative to HTTP authentication schemes that require passwords and
>    therefore avoids all problems related to passwords, such as leakage
>    of server-side password databases.
> One of those two paragraphs needs some editing.

I don't think so actually, the first quoted sentence above is in the
context of a web site with XSS vulnerabilities. Omitting that context
would make for bad text, but the immediately preceding sentence is
fairly explicit on that.

(In passing, I did edit the top of that para a bit, since using the
WebCrypto API is now possible so a tweak was needed.)

> -- Nits/editorial comments: --
> This is buried near the end of section 6.1
>    Note that replay of registration (and other HOBA) messages is quite
>    possible.  That however can be counteracted if challenge freshness is
>    ensured.
> That's not a good place for these two sentences.  HOBA registration
> messages don't contain a challenge, and the general discussion of
> replay attacks belongs under Security Considerations, not Registration.

They do contain a challenge in the header, though not in the body.
I think the sentence is ok where it is.

> In Section 9.4 and 9.5, I see the following proposed as part of IANA
> registry contents:
> 	an unformatted string, at the user's/UA's whim
> I get the point, but it's still not appropriate for an IANA registry
> - remove ", at the user's/UA's whim".

I whimsically disagree:-) IANA shouldn't be against whimsy.

> Appendix A appears  to assume that human-memorable passwords are stored in
> the clear in server databases.  It should also briefly discuss the use
> of one-way functions, especially computationally intensive ones, to
> generate stored password verifiers.  HOBA is still superior by comparison,
> as the one-way functions only increase the difficulty of dictionary
> attack, whereas dictionary attack on HOBA keys is pointless when
> sufficiently large keys are used.

I'd prefer to not get into more analysis of passwords, and even hashed
passwords are essentially guessable.

> idnits 2.13.01 found the references to W3C's web site ( in
> Section 4, e.g.,
> 389	   One element is required for HOBA-js: localStorage (see http://
> 390 from HTML5 can be used for persistent key
> 391	   storage.
> I think idnits can be ignored, even though the "right way" to fix this is to
> move each URL to a reference and cite the reference in Section 4.
> OTOH, idnits did not complain about the normative reference to RFC 20 ;-).


> --- Selected RFC 5706 Appendix A Q&A for OPS-Dir review ---
> Most of these questions are n/a because this draft describes an extension
> to HTTP authentication, and RFC 5706's concerns would apply primarily to
> HTTP and HTTP authentication.
> A.1.1    Has deployment been discussed?
> Yes, section 6 discusses how a user would deploy this for her access to web
> servers.
> A.1.4.  Have the Requirements on other protocols and functional
>        components been discussed?
> Yes, the discussion of how this functions within HTTP is sufficient.
> Major issues [3] and [4] fall into this category, but in both cases,
> I think they're probably text oversights that are easy to fix.
> A.2.7 Is security management discussed?  
> Not really, but this is an Experimental RFC.  I would expect that the
> experimental use of HOBA will yield insight into server key database
> structure/management, key lifecycle management (e.g., revocation),
> account management techniques/processes, etc.
> A.3.  Documentation
> I do not expect significant operational impact.
> Section 10 notes the existence of two implementations of HOBA-JS and
> one implementation of HOBA-http.  That's a good start for an experiment.

And now there's a 3rd!

Thanks again for the fine review,

> Thanks,
> --David
> ----------------------------------------------------
> David L. Black, Distinguished Engineer
> EMC Corporation, 176 South St., Hopkinton, MA  01748
> +1 (508) 293-7953             FAX: +1 (508) 293-7786
>        Mobile: +1 (978) 394-7754
> ----------------------------------------------------