Re: [Gen-art] Gen-ART Last Call review of draft-ietf-ace-dtls-authorize-12

Benjamin Kaduk <kaduk@mit.edu> Wed, 29 July 2020 10:16 UTC

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Date: Wed, 29 Jul 2020 03:16:39 -0700
From: Benjamin Kaduk <kaduk@mit.edu>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>
Cc: draft-ietf-ace-dtls-authorize.all@ietf.org, General Area Review Team <gen-art@ietf.org>
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Subject: Re: [Gen-art] Gen-ART Last Call review of draft-ietf-ace-dtls-authorize-12
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Hi Paul,

Also inline.

On Tue, Jul 28, 2020 at 11:36:48AM -0400, Paul Kyzivat wrote:
> Hi Ben,
> 
> Please find my comments inline.
> 
> On 7/27/20 3:10 PM, Benjamin Kaduk wrote:
> > Hi Paul,
> > 
> > Just a couple notes in addition to what Jim already mentioned.
> > 
> > On Sun, Jul 19, 2020 at 04:23:46PM -0400, Paul Kyzivat wrote:
> >> I am the assigned Gen-ART reviewer for this draft. The General Area
> >> Review Team (Gen-ART) reviews all IETF documents being processed
> >> by the IESG for the IETF Chair.  Please treat these comments just
> >> like any other last call comments.
> >>
> >> For more information, please see the FAQ at
> >>
> >> <https://trac.ietf.org/trac/gen/wiki/GenArtfaq>.
> >>
> >> Document: draft-ietf-ace-dtls-authorize-12
> >> Reviewer: Paul Kyzivat
> >> Review Date: 2020-07-19
> >> IETF LC End Date: 2020-07-20
> >> IESG Telechat date: ?
> >>
> >> Summary:
> >>
> >> This draft is on the right track but has open issues, described in the
> >> review.
> >>
> >> General:
> >>
> >> TBD
> >>
> >> Issues:
> >>
> >> Major: 2
> >> Minor: 1
> >> Nits:  1
> >>
> >> 1) MAJOR: Management of token storage in RS
> >>
> >> There seems to be an expectation that when the client uploads an access
> >> token that the RS will retain it until the client attempts to establish
> >> a DTLS association. This seems to require some sort of management of
> >> token lifetime in the RS. The only discussion I can find of this issue
> >> is the following in section 7:
> >>
> >>      ... A similar issue exists with the
> >>      unprotected authorization information endpoint where the resource
> >>      server needs to keep valid access tokens until their expiry.
> >>      Adversaries can fill up the constrained resource server's internal
> >>      storage for a very long time with interjected or otherwise retrieved
> >>      valid access tokens.
> >>
> >> This seems to imply a normative requirement to keep tokens until their
> >> expiry. But I find no supporting normative requirements about this. And,
> >> this section only presents it as a DoS attack, rather than a potential
> >> problem with valid usage.
> >>
> >> ISTM that there is an implied requirement that the RS have the capacity
> >> to store one access token for every PoP key of every authorized client.
> >> If so, that ought to be stated. If not, then some other way of bounding
> >> storage ought to be discussed.
> >>
> >> 2) MAJOR: Missing normative language
> >>
> >> I found several places where the text seems to suggest required behavior
> >> but fails to do so using normative language:
> >>
> >> * In section 3.3:
> >>
> >>      ... Instead of
> >>      providing the keying material in the access token, the authorization
> >>      server includes the key identifier in the "kid" parameter, see
> >>      Figure 7.  This key identifier enables the resource server to
> >>      calculate the symmetric key used for the communication with the
> >>      client using the key derivation key and a KDF to be defined by the
> >>      application, for example HKDF-SHA-256.  The key identifier picked by
> >>      the authorization server needs to be unique for each access token
> >>      where a unique symmetric key is required.
> >>      ...
> >>      Use of a unique (per resource server) "kid" and the use of a key
> >>      derivation IKM that is unique per authorization server/resource
> >>      server pair as specified above will ensure that the derived key is
> >>      not shared across multiple clients.
> >>
> >> The uniqueness seems to be a requirement. Perhaps "needs to be unique"
> >> should be "MUST be unique". (And something similar for the IKM.)
> >>
> >> * Also in section 3.3:
> >>
> >>      All CBOR data types are encoded in CBOR using preferred serialization
> >>      and deterministic encoding as specified in Section 4 of
> >>      [I-D.ietf-cbor-7049bis].  This implies in particular that the "type"
> >>      and "L" components use the minimum length encoding.  The content of
> >>      the "access_token" field is treated as opaque data for the purpose of
> >>      key derivation.
> >>
> >> IIUC the type of serialization and encoding is a requirement. Will need
> >> some rewording to make it so.
> > 
> > Both this and the previous item are scoped by the following text:
> > 
> >     The method for how the resource server determines the symmetric key
> >     from an access token containing only a key identifier is application-
> >     specific; the remainder of this section provides one example.
> > 
> > So it's not entirely clear that normative language is appropriate in the
> > discussion of the example behavior.
> 
> OK. That makes sense.
> 
> >> * In section 3.3.1:
> >>
> >>      ... To
> >>      be consistent with the recommendations in [RFC7252] a client is
> >>      expected to offer at least the ciphersuite TLS_PSK_WITH_AES_128_CCM_8
> >>      [RFC6655] to the resource server.
> >>
> >> I think "is expected" should be "MUST".
> >>
> >> * Also in section 3.3.1:
> >>
> >>      ... This
> >>      specification assumes that the access token is a PoP token as
> >>      described in [I-D.ietf-ace-oauth-authz] unless specifically stated
> >>      otherwise.
> >>
> >> I think "assumes ... unless" should be "MUST ... unless".
> > 
> > My understanding is that this is just talking about the text in the
> > document itself.  But as far as I remember we always require PoP tokens, so
> > this could just be removed.
> 
> It gets simpler if you always require PoP tokens. Does it state that 
> normatively somewhere?
> 
> The "unless" construct opens a can of worms about how things are to work 
> in that case.

Hmm, I didn't find a clear and unambiguous statement in a quick check.
Similar language about "in this document the access token is assumed to be
a PoP token unless specified otherwise" in the core framework document,
draft-ietf-ace-oauth-authz.  But that document uses "access token" some
130-odd times, and while I didn't see any mention of
non-proof-of-possession in there, I may have missed one.  (There is
discussion of not using symmetric proof-of-possession keys in a
group-audience context, which is supposed to mean use asymmetric proof of
possesion keys in that case.)

Assuming that I remember correctly (and the WG should correct me if I'm
wrong), it might be easiest to change both this document and the core
framework to flatly assert that PoP tokens are always required.

> >> * Also in section 3.3.1:
> >>
> >>      ... New access tokens issued by the
> >>      authorization server are supposed to replace previously issued access
> >>      tokens for the respective client.
> >>
> >> Is this normative? Should "are supposed to" be "MUST"?
> > 
> > I don't think this is a "MUST"; it refers to some behavior in the core
> > framework that is merely suggested and not mandatory.
> 
> I commented on this in my reply to Jim.

Okay, I'll follow that thread.

Thanks again,

Ben

> 	Thanks,
> 	Paul
> 
> > Thanks for the review; it brought up some good points!
> > 
> > -Ben
> > 
> >> 3) MINOR: Insufficient specification
> >>
> >> (I'm waffling whether this is minor or major.)
> >>
> >> There are a couple of places where what seem to be requirements are
> >> stated too vaguely to be implemented consistently:
> >>
> >> * In the previously mentioned paragraph in 3.3.1:
> >>
> >>      ... This
> >>      specification assumes that the access token is a PoP token as
> >>      described in [I-D.ietf-ace-oauth-authz] unless specifically stated
> >>      otherwise.
> >>
> >> The "unless specifically stated otherwise" is too vague to be normative.
> >> How would the alternative be indicated? Is this an escape hatch for
> >> future extensions? If so, it needs more work to make that clear and to
> >> open a path for that future work.
> >>
> >> * Also in section 3.3.1:
> >>
> >>      ... The resource server therefore must
> >>      have a common understanding with the authorization server how access
> >>      tokens are ordered.
> >>
> >> The last statement ("must have a common understanding") is mysterious.
> >> IIUC section 4 is covering the same topic in a less mysterious way.
> >>
> >> 4) NIT: Subsection organization
> >>
> >> Both 3.2 and 3.3 share a common structure:
> >>
> >> * The section begins with discussion of the interaction between the
> >> client and the AS.
> >>
> >> * it is followed by a subsection discussing the interaction between the
> >> client and the RS.
> >>
> >> It is odd to have a section with a single subsection. And the structure
> >> isn't easily discerned from the TOC.
> >>
> >> I suggest it would be clearer if each of these sections had *two*
> >> subsections, one covering the AS interactions and the other the RS
> >> interactions. IOW, put the material covering the AS interactions into a
> >> new subsection.
>