[Ace] Francesca Palombini's Discuss on draft-ietf-ace-oauth-authz-38: (with DISCUSS and COMMENT)

[Francesca Palombini via Datatracker <[e-mail]>]

Francesca Palombini has entered the following ballot position for
draft-ietf-ace-oauth-authz-38: Discuss

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DISCUSS:
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Thank you for this document. I think this is an important document which should
move forward, but I would like to discuss some points before it does so. These
might result in simple clarifications, or might require more discussion, but I
do hope they help improve the document.

General comments:

I found confusing to understand how optional or mandatory is the use of CBOR
for profiles of this specification compared to the transport used. I understand
the need for flexibility, but maybe it should be clarified the implication of
using CoAP (is CBOR mandatory then?) vs HTTP (is CBOR always permitted? How is
the encoding in that case? Is the same media type application/ace+cbor used in
that case?). Note also that while requests include the content type to use,
both in case HTTP or CoAP+CBOR are used, the response don't seem to include
this information.

I would like it to be clarified what requirements (or even just
recommendations) are there to use CoAP vs HTTP for different legs of the
exchange - not necessarily remove the flexibility but to clarify for
implementers what can be done and what would be the reasoning to do that: for
example if both endpoints support HTTP with the AS, most likely you can have
HTTP between C and RS, so does it really make sense to run this instead of
OAuth 2.0? Right now all is permitted, but does it all make sense? I feel like
this type of considerations are missing. As a note - I am not sure what
allowing a different encoding than CBOR for any leg of the exchange adds to the
specification - it makes things more confusing, and if needed it could be
specified in another document.

While going through and thinking about encodings (assuming we keep the doc as
is with regards to allowing more than just CBOR), I wondered if it would be
better to define a new media type to use when the ACE framework is used with
HTTP, to differentiate from OAuth 2.0, since some of the endpoints used are the
same (/token and /introspect at the AS). I am interested to hear more from my
co-AD as well if this would be an OK use of a new media type - I am thinking of
the case where AS is supporting both OAuth 2.0 and the Ace framework - or if it
is unnecessary, since the encodings are the same, and the parameters are
registered in OAuth 2.0 registry.

More detailed comments below.
Francesca


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COMMENT:
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Detailed comments:

1. -----

   sufficiently compact.  CBOR is a binary encoding designed for small
   code and message size, which may be used for encoding of self
   contained tokens, and also for encoding payloads transferred in
   protocol messages.

FP: "may be used" make it seem as if CBOR is always optional (even when CoAP is
used). See points 11. and 13. for examples of text that seem to imply that CBOR
is mandatory in some cases.

2. -----

      Refresh tokens are issued to the client by the authorization
      server and are used to obtain a new access token when the current
      access token becomes invalid or expires, or to obtain additional

FP: token validity - it is not clear what it means for a token to become
invalid (vs expiring). As this is the first time it is mentioned, it should be
explained here or referenced.

3. -----

         An asymmetric key pair is generated on the token's recipient
         and the public key is sent to the AS (if it does not already

          inside the token and sent back to the requesting party.  The
         consumer of the token can identify the public key from the

 FP: "token's recipient" and "consumer of the token" - why not talk about
 client and resource server here? It would fit better with the terminology used
 in the rest of the document.

 4. -----

    This framework RECOMMENDS the use of CoAP as replacement for HTTP for
   use in constrained environments.  For communication security this

FP: This was already stated in the introduction in the following sentence:

   of processing capabilities, available memory, etc.  For web
   applications on constrained nodes, this specification RECOMMENDS the
   use of the Constrained Application Protocol (CoAP) [RFC7252] as
   replacement for HTTP.

Not sure repeating is useful.

5. -----

   OAuth 2.0 (such as [RFC7521] and [RFC8628]).  What grant types works
   best depends on the usage scenario and [RFC7744] describes many
   different IoT use cases but there are two preferred grant types,
   namely the Authorization Code Grant (described in Section 4.1 of

FP: nit: s/works/work . I think "preferred" is probably not the right term
here, and should be rephrased or clarified - preferred respect to?

6. -----

      In addition to the response parameters defined by OAuth 2.0 and
      the PoP access token extension, this framework defines parameters
      that can be used to inform the client about capabilities of the
      RS, e.g. the profiles the RS supports.  More information about

FP: I believe this is a leftover from a previous version of the document, but
should be "profile" and not "profiles" as the AS is only allowed to communicate
one profile to the client and RS - see for example the following sentences:

      The Client and the RS mutually authenticate using the security
      protocol specified in the profile (see step B) and the keys

      The AS informs the client of the selected profile using the
      "ace_profile" parameter in the token response.

7. -----

         (1) the client sends the access token containing, or
         referencing, the authorization information to the RS, that may
         be used for subsequent resource requests by the client, and

FP: s/may/will

8. -----

      While the structure and encoding of the access token varies
      throughout deployments, a standardized format has been defined
      with the JSON Web Token (JWT) [RFC7519] where claims are encoded
      as a JSON object.  In [RFC8392], an equivalent format using CBOR
      encoding (CWT) has been defined.

FP: Would it make sense to RECOMMEND the use of JWT when HTTP is used? Or is
CWT always RECOMMENDED?

9. -----

   parameters.  When profiles of this framework use CoAP instead, it is
   REQUIRED to use of the following alternative instead of Uri-query
   parameters: The sender (client or RS) encodes the parameters of its
   request as a CBOR map and submits that map as the payload of the POST
   request.

FP: I think it should be better defined (or at least hinted in this paragraph)
the mapping between the CBOR encoded parameters and the Uri-query parameters:
are they all encoded as CBOR text strings?

10. -----

   Applications that use this media type: The type is used by
   authorization servers, clients and resource servers that support the
   ACE framework as specified in [this document].

FP: I suggest adding "that support the ACE framework with CBOR encoding, as
specified ..."

11. -----

   The OAuth 2.0 AS uses a JSON structure in the payload of its
   responses both to client and RS.  If CoAP is used, it is REQUIRED to
   use CBOR [RFC8949] instead of JSON.  Depending on the profile, the
   CBOR payload MAY be enclosed in a non-CBOR cryptographic wrapper.

FP: This sentence seems to add a requirement (when CoAP is used, then CBOR must
be used) only for communication from AS to C or RS. I could not find in the
rest of the specification the same type of requirement for the other legs of
communication (C to AS, RS to AS, C to RS). Please let me know if I missed it.

12. -----

   the AS authorization is not in scope for this document.  C may, e.g.,
   ask it's owner if this AS is authorized for this RS.  C may also use
   a mechanism that addresses both problems at once.

FP: nit - s/it's/its . Also "C may also use ... at once" such as? I think it
would be good to give an example here.

13. -----

   valid access token.  The AS Request Creation Hints message is a CBOR
   map, with an OPTIONAL element "AS" specifying an absolute URI (see

FP: another case where CBOR seem mandatory.. Is this the case, even if HTTP or
other protocol is used?

14. -----

      MUST discard the access token.  If this was an interaction with
      the authz-info endpoint the RS MUST also respond with an error
      message using a response code equivalent to the CoAP code 4.01
      (Unauthorized).

FP: This seems to imply that another endpoint could be used to receive an
access token. Is that the case, and if so where is this defined?

15. -----

Section 5.8:

  If HTTPS is used, JSON or CBOR payloads may be supported.  If JSON
   payloads are used, the semantics of Section 4 of the OAuth 2.0
   specification MUST be followed (with additions as described below).

FP: I suggest to point to the specific subsection in OAuth - namely 4.1.3 and
4.1.4

16. -----

   If CBOR is used then these parameters MUST be provided as a CBOR map.

FP: s/as/in . I suggest to reference Figure 12.

17. -----

   the HTTP request entity-body, as defined in section 3.2 of [RFC6749].

FP: Section 3.2 of OAuth 2.0 specifies:

   The endpoint URI MAY include an "application/x-www-form-urlencoded"
   formatted (per Appendix B) query component ([RFC3986] Section 3.4),
   which MUST be retained when adding additional query parameters.  The
   endpoint URI MUST NOT include a fragment component.

which explicitly specifies that the parameters are transported as query
components. I either suggest to change this reference to Appendix B of RFC6749.

18. -----

         "x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8'

FP: noted here since this is the first time it appears, but same comment for
the rest of the document. I think it would make more sense to show examples
where CBOR byte strings are used instead of Base64.

19. -----

   Figure 8 summarizes the parameters that can currently be part of the
   Access Information.  Future extensions may define additional
   parameters.

FP: This is useful! Why not having the same type of figure listing all
acceptable parameters for the request (section 5.8.1)?

20. -----

   Header: Created (Code=2.01)
   Content-Format: "application/ace+cbor"
   Payload:
   {
     "access_token" : b64'SlAV32hkKG ...
      (remainder of CWT omitted for brevity;
      CWT contains COSE_Key in the "cnf" claim)',
     "ace_profile" : "coap_dtls",
     "expires_in" : "3600",
     "cnf" : {
       "COSE_Key" : {
         "kty" : "Symmetric",
         "kid" : b64'39Gqlw',
         "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
       }
     }
   }

       Figure 9: Example AS response with an access token bound to a
                              symmetric key.

FP: Section 3.1 states:

      Symmetric PoP key:
         The AS generates a random symmetric PoP key.  The key is either
         stored to be returned on introspection calls or encrypted and
         included in the token.  The PoP key is also encrypted for the
         token recipient and sent to the recipient together with the
         token.

But in the example the key is not encrypted.

21. -----

   o  When using CBOR the raw payload before being processed by the
      communication security protocol MUST be encoded as a CBOR map.

FP: I don't understand "before being processed by the communication security
protocol"

22. -----

   o  The Content-Format (for CoAP-based interactions) or media type
      (for HTTP-based interactions) "application/ace+cbor" MUST be used
      for the error response.

FP: Does this mean that CBOR is always used for errors? However in the same
section:

   o  The parameters "error", "error_description" and "error_uri" MUST
      be abbreviated using the codes specified in Figure 12, when a CBOR
      encoding is used.

"when a CBOR encoding is used" so not always?

23. -----

Sections 5.8.2, 5.8.3, 5.9.1, 5.9.2, 5.9.3, 5.10.1

FP: I found useful that Section 5.8.1 spelled out the encoding when HTTP is
used (See sentence below). I think it would be as useful to have the same type
of phrasing in these and following sections (wherever it is missing now). I
think in general it is very clear in the document what format the message has
when CBOR is used, and it seems that CBOR can be used with HTTP according to
this specification (but not sure it is actually recommended, what are the pros
and cons). In some sections (5.8.2, 5.8.3, etc) it is left to implicit
references to OAuth 2.0 for the implementers to figure out what the encoding is
(and what media type is used), although modifications are added to it.

   When HTTP is used as a transport then the client makes a request to
   the token endpoint by sending the parameters using the "application/
   x-www-form-urlencoded" format with a character encoding of UTF-8 in
   the HTTP request entity-body, as defined in section 3.2 of [RFC6749].

24. -----

      including the error code "unsupported_pop_key" defined in
      Figure 10.

      including the error code "incompatible_ace_profiles" defined in
      Figure 10.

FP: nit - these error codes are not "defined" in figure 10.

25. -----

   In this framework the "pop" value for the "token_type" parameter is
   the default.  The AS may, however, provide a different value.

FP: I would change the sentence to:

   In this framework the "pop" value for the "token_type" parameter is
   the default.  The AS may, however, provide a different value from those
   registered in [IANA registry].

26. -----

   Clients that want the AS to provide them with the "ace_profile"
   parameter in the access token response can indicate that by sending a
   ace_profile parameter with a null value (for CBOR-based interactions)
   or an empty string (for JSON based interactions) in the access token
   request.

      Hints Section 5.3.  The parameter is encoded as a byte string for
   CBOR-based interactions, and as a string (Base64 encoded binary) for
   JSON-based interactions.

FP: OAuth 2.0 section 4.1.3 says that JSON is not used, the parameters are
encoded using "application/x-www-form-urlencoded"
 format in the request entity-body.

27. -----

   The figures of this section uses CBOR diagnostic notation without the

FP: Nit - s/use

28. -----

   A client using this method MUST make a POST request to the authz-info
   endpoint at the RS with the access token in the payload.  The RS

FP: The formatting of the request is unclear - could you clarify that it
depends on the access token used, and the media type (or content format) needs
to reflect that? I.e. if CWT is used, the media type MUST be application/cwt.

29. -----

   o  If token or claim verification fails, the RS MUST discard the
      token and, if this was an interaction with authz-info, return an
      error message with a response code equivalent to the CoAP code

FP: Same comment as before, "if this was an interaction with authz-info" - the
alternative needs to be clarified.

30. -----

   Errors may happen during this initial processing stage:

   o  If token or claim verification fails, the RS MUST discard the
      token and, if this was an interaction with authz-info, return an
      error message with a response code equivalent to the CoAP code
      4.01 (Unauthorized).

      ...

   Next, the RS MUST verify claims, if present, contained in the access
   token.  Errors are returned when claim checks fail, in the order of
   priority of this list:

FP: It seems weird to read first about errors due to claim verification, and
then "Next" discuss claim verification - unless this is two different claim
verifications, in which case I think this needs to be clarified. Also in each
claim:

      the RS MUST discard the token.  If this was an interaction with
      authz-info, the RS MUST also respond with a response code
      equivalent to the CoAP code 4.01 (Unauthorized).

Seems like a repeat of the sentence above.

31. -----

   on the authz-info endpoint and on it's children (if any).

FP: nit - s/it's/its

32. -----

   The POST method SHOULD NOT be allowed on children of the authz-info
   endpoint.

FP: What children? They do not seem to be defined, so I do not understand this
sentence.

33. -----

         +  When creating the token, the AS MUST add a 'cti' claim ( or
            'jti' for JWTs) to the access token.  The value of this

FP: Since the use of CWT and JWT are only recommended, it might be good to
rephrase this as to allow for other access token's encodings too.

34. -----

         +  When creating the token, the AS MUST add a 'cti' claim ( or
            'jti' for JWTs) to the access token.  The value of this
            claim MUST be created as the binary representation of the
            concatenation of the identifier of the RS with a sequence
            number counting the tokens containing an 'exi' claim, issued
            by this AS for the RS.

         +  The RS MUST store the highest sequence number of an expired
            token containing the "exi" claim that it has seen, and treat
            tokens with lower sequence numbers as expired.

FP: A question rather than a comment - I am not sure I understand this. An
"exi" value might be higher for a different token (with a lower sequence
number), so how does the counter of the tokens help? Wouldn't that make the RS
discard a valid token just because it has a lower sequence number?

35. -----

   RS.  Therefore, C must not communicate with an AS if it cannot
   determine that this AS has the authority to issue access tokens for
   this RS.  Otherwise, a compromised RS may use this to perform a

FP: How is C supposed to determine that? Doesn't this sentence make the whole
AS Creation Hints response useless - either the client already knows, or it
doesn't and it must not communicate with it regardless of RS says?

36. -----

   compromised.  In order to prevent this, an RS may use the nonce-based
   mechanism defined in Section 5.3 to ensure freshness of an Access

FP: please mention "cnonce" to make it easier to find.

37. -----

   There may be use cases were different profiles of this framework are
   combined.  For example, an MQTT-TLS profile is used between the
   client and the RS in combination with a CoAP-DTLS profile for
   interactions between the client and the AS.  The security of a
   profile MUST NOT depend on the assumption that the profile is used
   for all the different types of interactions in this framework.

FP: This seems strange - maybe I just don't understand how this is supposed to
work, but each profile defines exactly at least C - RS communication and
security, if several are combined, it seems that at least the C-RS would
conflict.