[Ace] AD review of draft-ietf-ace-cwt-proof-of-possession-06

Benjamin Kaduk <kaduk@mit.edu> Tue, 30 July 2019 15:56 UTC

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Date: Tue, 30 Jul 2019 10:56:06 -0500
From: Benjamin Kaduk <kaduk@mit.edu>
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Subject: [Ace] AD review of draft-ietf-ace-cwt-proof-of-possession-06
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We should be consistent across examples about whether the use of CBOR
diagnostic notation also requires a disclaimer about "with linebreaks
for readability".

Section 2

      Party that proves possession of a private key (for asymmetric key
      cryptography) or secret key (for symmetric key cryptography) to a

nit: it might be worth either capitalizing Recipient to point to the
following item more clearly, or specifying "recipient of a CWT" for
parallelism with Recipient.
(If we do the former we should capitalize Presenter in the following
definition.  But since we don't use the capitalized terms throughout the
text, it wouldn't be my own preference.)

Section 3.2

This example key expired in 2013.  While the example will "always" be
expired for an archival document, it might be worth making it more
timely with respect to the publication date.  (This holds for basically
all time values in the document's examples.)

Value 2 for 'kty' should have diagnostic notation /EC2/, not /EC/.

[I did not check that the example x and y coordinates are in fact points
on P256.]

   The "COSE_Key" member MAY also be used for a COSE_Key representing a
   symmetric key, provided that the CWT is encrypted so that the key is
   not revealed to unintended parties.  The means of encrypting a CWT is
   explained in [RFC8392].  If the CWT is not encrypted, the symmetric
   key MUST be encrypted as described in Section 3.3.

It's hard for me to escape the conclusion that this paragraph needs to
be a dedicated section with a bit more discussion about how exactly this
usage is performed and encoded.

Section 3.3

Is the /HMAC256/ the conventional diagnostic notation for alg value 5
(noting that RFC 8152 calls it both "HMAC256/256" and "HMAC w/ SHA-256")?

   The following example CWT Claims Set of a CWT (using CBOR diagnostic
   notation, with linebreaks for readability) illustrates the use of an
   encrypted symmetric key as the "Encrypted_COSE_Key" member value:

   /iss/ 1 : "coaps://server.example.com",
   /sub/ 2 : "24400320",
   /aud/ 3: "s6BhdRkqt3",
   /exp/ 4 : 1311281970,
   /iat/ 5 : 1311280970,
   /cnf/ 8 : {
   /COSE_Encrypt0/ 2 : [

Should this be "/Encrypted_COSE_Key/" and not "/COSE_Enrypt0/"?

[I did not validate the COSE_Encrypt0 output]

Section 3.4

      /iss/ 1 : "coaps://server.example.com",
      /aud/ 3 : "coaps://client.example.org",

Is it in any way confusing to use client.example.org as opposed to, say,
resource.example.org as the audience?

      /exp/ 4 : 1361398824,
      /cnf/ 8 : {
        /kid/ 2 : h'dfd1aa976d8d4575a0fe34b96de2bfad'

/kid/ in the 'cnf' map has key 3, not 2.

   Note that the use of a Key ID to identify a proof-of-possesion key
   needs to be carefully circumscribed, as described below and in
   Section 6.  Where the Key ID is not a cryptographic value derived
   from the key or where all of the parties involved are not validating
   the cryptographic derivation, it is possible to get into situations
   where the same Key ID is being used for multiple keys.  The

I don't think this quite covers the needed properties, since we also
need some assurance that all parties are interpreting the kid value in
the same way. It may be possible to construct such a scenario via an
attacker replaying a stolen token or even just inadvertent confusion by
some party (quite plausible when we consider scenarios that involve the
same key being described by different 'kid' values in messages with
different recipients).  In particular, if the situation arises where an
attacker is able to choose the 'kid' value that will be used, they could
deliberately cause a collision with a legitimate value used by some
other party.

   In the world of constrained Internet of Things (IoT) devices, there
   is frequently a restriction on the size of Key IDs, either because of
   table constraints or a desire to keep message sizes small.  These
   restrictions are going to protocol dependent.  For example, DTLS can

nit: "going to be" or maybe even just "are protocol dependent".

   use a Key ID of any size.  However, if the key is being used with
   COSE encrypted message, then the length of the key needs to be
   minimized and may have a limit as small as one byte.

nit: is this "length of the key" or "length of the key ID"?

   Note that the value of a Key ID is not always the same for different
   parties.  When sending a COSE encrypted message with a shared key,
   the Key ID may be different on both sides of the conversation, with
   the appropriate one being included in the message based on the
   recipient of the message.

While this recipient-dependence is probably unavoidable (as the
requisite namespacing would use more space on the wire than is
reasonable for many applications), it does merit some security
considerations text, noting that such messages are context-dependant and
could be misinterpreted if presented in a different context.
Audience restrictions, as mentioned in Section 4, provide substantial
protection in this regard, but are not necessarily a complete solution.

   o  A recipient can decide not to use a CWT based on a created Key ID
      if it does not fit the recipient's requirements.

I'm not sure I understand the semantics being described here.  Are we
saying that the issuer might give a presenter a CWT, and by the time the
presenter presents the CWT to the recipient, the recipient says "this is
no good" and denies the transaction in question, forcing the presenter
to go back to the issuer and try again?  (How do we know that the issuer
would make any different choices the second time around?)

   o  If an issuer is going to use the Key ID confirmation method and is
      not going to guarantee that serial number uniqueness is going to
      be preserved, the recipient needs to have that information
      configured into it so that appropriate actions can be taken.

nit: this is the only place in the document that talks about "serial
Do we say what the "appropriate actions" are somewhere else in the

Section 3.5

   Note that another means of proving possession of the key when it is a
   symmetric key is to encrypt the key to the recipient.  The means of
   obtaining a key for the recipient is likewise protocol specific.

While true, there are some subtleties here, namely that the recipient
needs some way of knowing that it is the presenter (as opposed to some
other party) that has actually performed that encryption.  In Kerberos
we perform this key confirmation by using the encrypted (session) key
for subsequent protocol exchanges, and if the presenter fails to process
those exchanges then it is clear the presenter does not actually possess
the key.  This document does not seem to describe the need for such
subsequent confirmation (or, alternately, the consequences of failing to
do so.)

Section 4

   A recipient might not understand the "cnf" claim.  Applications that
   require the proof-of-possession keys communicated with it to be
   understood and processed MUST ensure that the parts of this
   specification that they use are implemented.

nit(?): I'm not sure I'm parsing this sentence correctly: is
"communicated with it" referring to "CWTs that include the 'cnf' claim"?

                To avoid replay attacks when the proof-of-possession
   tokens are sent to presenters, a security protocol, which uses
   mechansims such as nonces or timestamps, has to be utilized.  [...]

I suspect the RFC Editor will want to reword this sentence to avoid the
excessive parentheticals and awkward passive-voice construction.  I'll
be bold and suggest a more dramatic rewording to preempt that: "Proof of
possession only provides the intended security gains when the proof is
known to be current and not subject to replay attacks; security
protocols using mechanisms such as nonces and timestamps can be used to
avoid this risk of replay when sending proof-of-possession tokens to
[presenters or recipients]".  Note that I am not sure if the original
was intended to refer to the transfer of PoP tokens to presenters (as
written) or recipients (where risk of replay is more traditionally

   from changing any elements conveyed within the CWT payload.  Special
   care has to be applied when carrying symmetric keys inside the CWT
   since those not only require integrity protection but also
   confidentiality protection.

Do we want to reiterate the common mechanisms for providing
confidentiality protection here, or just leave the existing text earlier
in the document to cover it?

Section 5

This sort of correlation can occur even for subsequent connections
between the same two parties if observed by a passive observer (e.g., in
the case of a mobile client that changes location).  I forget if we have
strong enough guarantees on the use of transport-level encryption that
would prevent such CWTs from being observed in this fashion.

Section 6

   ensure correct processing.  The recipient needs to be able to use
   credentials to verify the authenticity, integrity, and potentially
   the confidentiality of the CWT and its content.  This requires the

Just from a rhetorical point of view, can you help me understand how
credentials would be used to verify the confidentiality of a CWT?  It
seems like this depends on either (or both of) how the CWT is
transmitted or how it is prepared, and I am not sure how the recipient's
credentials come into play.

   recipient to know information about the issuer.  Likewise, there
   needs to be agreement between the issuer and the recipient about the
   claims being used (which is also true of CWTs in general).

We briefly discuss the presenter (in the guise of the subject) in the
next paragraph when we talk about key IDs, but are there any other cases
where there needs to be coordination with the presenter?  Would
distributing a symmetric PoP key be considered to fall under this

   When an issuer creates a CWT containing a Key ID claim, it needs to
   make sure that it does not issue another CWT containing the same Key
   ID with a different content, or for a different subject, within the
   lifetime of the CWTs, unless intentionally desired.  Failure to do so
   may allow one party to impersonate another party, with the potential
   to gain additional privileges.  Likewise, if PoP keys are used for

nit: "same Key ID with a different content" doesn't seem quite right, as
the "content" of a Key ID is surely the key-ID value itself.  Perhaps
"referring to" would be more appropriate.
I think we should probably give the reader some indication of what
semantics might be attributed to the "intentionally desired" case.  It
doesn't necessarily need to be a specific example, but we could say what
the relevant properties/attributes that might lead to such a situation
would be.

   multiple different kinds of CWTs in an application and the PoP keys
   are identified by Key IDs, care must be taken to keep the keys for
   the different kinds of CWTs segregated so that an attacker cannot
   cause the wrong PoP key to be used by using a valid Key ID for the
   wrong kind of CWT.

Audience restrictions can come into play here, too, right?  In that a
restricted audience of all the CWTs in turn limits the scope of
administration in which this care/segregation needs to be enforced.

Section 7

   Criteria that should be applied by the Designated Experts include
   determining whether the proposed registration duplicates existing
   functionality, determining whether it is likely to be of general
   applicability or whether it is useful only for a single application,
   and evaluating the security properties of the item being registered
   and whether the registration makes sense.

I know we've been using (variations of) this text for a while, but it
seems to me that it could be more clear than it currently is -- is
duplication of functionality grounds for denial of registration?  What
about general vs. specific applicability?  The latter seems more clearly
applicable for determining which range from which to allocate, since
that has impact on the encoding size.  Can the experts insist on updates
to the security considerations text of a specification prior to granting
approval, or are they limited to denying registration of values with
poor security properties or insufficient documentation thereof?

Section 7.2.1

   Change Controller:
      For Standards Track RFCs, list the "IESG".  For others, give the
      name of the responsible party.  Other details (e.g., postal
      address, email address, home page URI) may also be included.

In light of the GDPR and similar regulations (and, as is done in RFC 8602), we
may not want to keep all of these, especially postal address, given the lack of
a clear need.

   Specification Document(s):
      Reference to the document or documents that specify the parameter,
      preferably including URIs that can be used to retrieve copies of

Is the reference required to be publicly accessible?  If not, is it
required that a copy be made available to the experts and IANA?

Section 7.2.2

   o  Confirmation Method Name: "Encrypted_COSE_Key"
   o  Confirmation Method Description: Encrypted COSE_Key
   o  JWT Confirmation Method Name: "jwe"
   o  Confirmation Key: 2
   o  Confirmation Value Type(s): COSE_Encrypt or COSE_Encrypt0
      structure (with an optional corresponding COSE_Encrypt or
      COSE_Encrypt0 tag)

Do we want to say something about how, in the case when the tag is
omitted, the application protocol specification needs to indicate how to
determine which one is in use?

Section 8.2

I think [JWT] needs to be normative, as we have a SHOULD-level
requirement for the audience restriction procedures it specifies.

I just want to check that it's intentional/desired to mix descriptive
reference tags (e.g., [JWS]) and RFC-number tags (e.g., [RFC7800]) for
the referenced RFCs.

Acknowledgements, Authors

The datatracker is currently accepting XML v3 format drafts, and the RFC
Editor's target cutover date for the end of August is quite soon, so
feel free to consider using an XML v3 submission with UTF-8
representations of names not well represented by the ASCII subset.