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

Jim Schaad <> Tue, 30 July 2019 17:10 UTC

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From: Jim Schaad <>
To: 'Benjamin Kaduk' <>, <>
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Date: Tue, 30 Jul 2019 10:10:12 -0700
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Subject: Re: [Ace] AD review of draft-ietf-ace-cwt-proof-of-possession-06
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Comments inline.

-----Original Message-----
From: Benjamin Kaduk <> 
Sent: Tuesday, July 30, 2019 8:56 AM
Subject: AD review of draft-ietf-ace-cwt-proof-of-possession-06

We should be consistent across examples about whether the use of CBOR
diagnostic notation also requires a disclaimer about "with linebreaks for

[JLS] I don't believe that this disclaimer needs to be present.  Unlike the
JSON document where what is being presented is in fact JSON, what is being
presented here is simply a more user friendly readable version of the binary
data.  (A different question would be if the hex should be presented but
that is not what the ACE group is doing in general.)

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

   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")?

[JLS] HMAC 256/256 is the algorithm name while HMAC w/ SHA-256 is
descriptive.  I believe that this is completely consistent in RFC 8152.

   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://",
   /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://",
      /aud/ 3 : "coaps://",

Is it in any way confusing to use as opposed to, say, 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.

[JLS] I don't agree with that.   It should be assumed by all implementations
that there will be collisions between kid values.  Except in very specific
cases such as all of the kids being assigned by a group keying server, there
is no way for the assumption of a single kid value being assigned once on a
recipient basis will ever be a true statement.  

   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.

[JLS] Again I don't understand this, but I think that your concern goes away
if you do not assume that kid values are going to be unique for an entity.

   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 document?

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.

[JLS] No we don't.  Most of the time we are sending CWTs in the clear and
not over a transport-level encrypted channel.

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

   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 mantle?

   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?

[JLS] It is not clear to me that this is an Designated Expert registry.  It
is only referred to once in the JWT Confirmation Method Name and that could
refer to the JWT registry and not this one.

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?

[JLS] This is easy as the number of elements in the array is different
between the two.

Section 8.2

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

[JLS] Should this be indirect through the CWT spec rather than via the JWT

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.

[JLS] I have no problem with this.  I find descriptive reference tags to be
easier to identify than RFC number tags for which I only know  couple of

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.