Internet Engineering Task Force (IETF)                          L. Seitz
Request for Comments: 9201                                     Combitech
Category: Standards Track                                     March 2022
ISSN: 2070-1721

  Additional OAuth Parameters for Authentication and Authorization for
                     Constrained Environments (ACE)

Abstract

   This specification defines new parameters and encodings for the OAuth
   2.0 token and introspection endpoints when used with the framework
   for Authentication and Authorization for Constrained Environments
   (ACE).  These are used to express the proof-of-possession (PoP) key
   the client wishes to use, the proof-of-possession PoP key that the authorization server
   has selected, and the proof-of-possession PoP key the resource server uses to
   authenticate to the client.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9201.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Parameters for the Token Endpoint
     3.1.  Client-to-AS Request
     3.2.  AS-to-Client Response
   4.  Parameters for the Introspection Endpoint
   5.  Confirmation Method Parameters
   6.  CBOR Mappings
   7.  Requirements When Using Asymmetric Keys
   8.  Security Considerations
   9.  Privacy Considerations
   10. IANA Considerations
     10.1.  OAuth Parameter Registration
     10.2.  OAuth Parameters CBOR Mappings Registration
     10.3.  OAuth Token Introspection Response CBOR Mappings
            Registration
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgments
   Author's Address

1.  Introduction

   The Authentication and Authorization for Constrained Environments
   (ACE) specification [RFC9200] requires some new parameters for
   interactions with the OAuth 2.0 [RFC6749] token and introspection
   endpoints, as well as some new claims to be used in access tokens.
   These parameters and claims can also be used in other contexts and
   have therefore been put into a dedicated document to facilitate their
   use in a manner independent of [RFC9200].

   Note that although all examples are shown in Concise Binary Object
   Representation (CBOR) [RFC8949], JSON [RFC8259] MAY be used as an
   alternative for HTTP-based communications, as specified in [RFC9200].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are assumed to be familiar with the terminology from
   [RFC9200], especially the terminology for entities in the
   architecture such as client (C), resource server (RS), and
   authorization server (AS).

   Terminology from [RFC8152] is used in the examples, especially
   COSE_Key, which is defined in Section 7 of [RFC8152].

   Note that the term "endpoint" is used here following its OAuth 2.0
   [RFC6749] definition, which is to denote resources such as token and
   introspection at the AS and authz-info at the RS.  The Constrained
   Application Protocol (CoAP) [RFC7252] definition, which is "[a]n
   entity participating in the CoAP protocol", is not used in this
   specification.

3.  Parameters for the Token Endpoint

   This section defines additional parameters for the interactions with
   the token endpoint in the ACE framework [RFC9200].

3.1.  Client-to-AS Request

   This section defines the "req_cnf" req_cnf parameter allowing clients to
   request a specific proof-of-possession PoP key in an access token from a token endpoint
   in the ACE framework [RFC9200]:

   req_cnf
      OPTIONAL.  This field contains information about the key the
      client would like to bind to the access token for proof of
      possession.  It is RECOMMENDED that an AS rejects a request
      containing a symmetric key value in the "req_cnf" req_cnf field
      (kty=Symmetric), since the AS is expected to be able to generate
      better symmetric keys than a constrained client.  (Note: this does
      not apply to key identifiers referencing a symmetric key.)  The AS
      MUST verify that the client really is in possession of the
      corresponding key.  Profiles of [RFC9200] using this specification
      MUST define the proof-of-possession PoP method used by the AS if they allow clients to
      use this request parameter.  Values of this parameter follow the
      syntax and semantics of the "cnf" cnf claim either from Section 3.1 of
      [RFC8747] for CBOR-based interactions or from Section 3.1 of
      [RFC7800] for JSON-based interactions.

   Figure 1 shows a request for an access token using the "req_cnf" req_cnf
   parameter to request a specific public key as a proof-of-possession PoP key.  The content
   is displayed in CBOR diagnostic notation without abbreviations and
   with line breaks for better readability.

   Header: POST (Code=0.02)
   Uri-Host: "as.example.com"
   Uri-Path: "token"
   Content-Format: "application/ace+cbor"
   Payload:
   {
      "req_cnf" : {
         "COSE_Key" : {
            "kty" : "EC2",
            "kid" : h'11',
            "crv" : "P-256",
            "x" : h'BAC5B11CAD8F99F9C72B05CF4B9E26D24
                    4DC189F745228255A219A86D6A09EFF',
            "y" : h'20138BF82DC1B6D562BE0FA54AB7804A3
                    A64B6D72CCFED6B6FB6ED28BBFC117E'
         }
      }
    }

         Figure 1: Example Request for an Access Token Bound to an
                               Asymmetric Key

3.2.  AS-to-Client Response

   This section defines the following additional parameters for an AS
   response to a request to the token endpoint:

   cnf
      REQUIRED if the token type is "pop" and a symmetric key is used.
      MAY be present for asymmetric proof-of-possession PoP keys.  This field contains the proof-of-possession
      PoP key that the AS selected for the token.  Values of this
      parameter follow the syntax and semantics of the "cnf" cnf claim either
      from Section 3.1 of [RFC8747] for CBOR-based interactions or from
      Section 3.1 of [RFC7800] for JSON-based interactions.  See
      Section 5 for additional discussion of the usage of this
      parameter.

   rs_cnf
      OPTIONAL if the token type is "pop" and asymmetric keys are used.
      MUST NOT be present otherwise.  This field contains information
      about the public key used by the RS to authenticate.  If this
      parameter is absent, either the RS does not use a public key or
      the AS knows that the RS can authenticate itself to the client
      without additional information.  Values of this parameter follow
      the syntax and semantics of the "cnf" cnf claim either from Section 3.1
      of [RFC8747] for CBOR-based interactions or from Section 3.1 of
      [RFC7800] for JSON-based interactions.  See Section 5 for
      additional discussion of the usage of this parameter.

   Figure 2 shows an AS response containing a token and a "cnf" cnf parameter
   with a symmetric proof-of-possession PoP key.

   Header: Created (Code=2.01)
   Content-Format: "application/ace+cbor"
   Payload:
   {
     "access_token" : h'4A5015DF686428 ...
      (remainder of CWT omitted for brevity;
      CWT contains COSE_Key in the "cnf" claim)',
     "cnf" : {
       "COSE_Key" : {
         "kty" : "Symmetric",
         "kid" : h'DFD1AA97',
         "k" : h'849B5786457C1491BE3A76DCEA6C427108'
       }
     }
   }

       Figure 2: Example AS Response with an Access Token Bound to a
                               Symmetric Key

   Figure 3 shows an AS response containing a token bound to a
   previously requested asymmetric proof-of-possession PoP key (not shown) and an "rs_cnf" rs_cnf
   parameter containing the public key of the RS.

   Header: Created (Code=2.01)
   Content-Format: "application/ace+cbor"
   Payload:
   {
     "access_token" : h'D08343A1010AA1054D2A45DF6FBC5A5A ...
      (remainder of CWT omitted for brevity)',
     "rs_cnf" : {
       "COSE_Key" : {
         "kty" : "EC2",
         "kid" : h'12',
         "crv" : "P-256",
         "x" : h'BCEE7EAAC162F91E6F330F5771211E220
                 B8B546C96589B0AC4AD0FD24C77E1F1',
         "y" : h'C647B38C55EFBBC4E62E651720F002D5D
                 75B2E0C02CD1326E662BCA222B90416'
       }
     }
   }

        Figure 3: Example AS Response Including the RS's Public Key

4.  Parameters for the Introspection Endpoint

   This section defines the use of CBOR instead of JSON for the "cnf" cnf
   introspection response parameter specified in Section 9.4 of
   [RFC8705].

   If CBOR is used instead of JSON in an interaction with the
   introspection endpoint, the AS MUST use the parameter mapping
   specified in Table 1 and the value must follow the syntax of "cnf" cnf
   claim values from Section 3.1 of [RFC8747].

   Figure 4 shows an AS response to an introspection request including
   the "cnf" cnf parameter to indicate the proof-of-possession PoP key bound to the token.

   Header: Created (Code=2.01)
   Content-Format: "application/ace+cbor"
   Payload:
   {
     "active" : true,
     "scope" : "read",
     "aud" : "tempSensor4711",
     "cnf" : {
       "COSE_Key" : {
         "kty" : "EC2",
         "kid" : h'11',
         "crv" : "P-256",
         "x" : h'BAC5B11CAD8F99F9C72B05CF4B9E26D24
                 4DC189F745228255A219A86D6A09EFF',
         "y" : h'20138BF82DC1B6D562BE0FA54AB7804A3
                 A64B6D72CCFED6B6FB6ED28BBFC117E'
       }
     }
   }

                  Figure 4: Example Introspection Response

5.  Confirmation Method Parameters

   The confirmation method parameters are used in [RFC9200] as follows:

   *  "req_cnf"  req_cnf in the access token request C -> AS, OPTIONAL to indicate
      the client's raw public key or the key identifier of a previously
      established key between the C and RS that the client wishes to use
      for proof of possession of the access token.

   *  "cnf"  cnf in the token response AS -> C, OPTIONAL if using an asymmetric
      key or a key that the client requested via a key identifier in the
      request.  REQUIRED if the client didn't specify a "req_cnf" req_cnf and
      symmetric keys are used.  Used to indicate the symmetric key
      generated by the AS for proof of possession of the access token.

   *  "cnf"  cnf in the introspection response AS -> RS, REQUIRED if the access
      token that was subject to introspection is a proof-of-
      possession PoP token, absent
      otherwise.  Indicates the proof-of-
      possession PoP key bound to the access token.

   *  "rs_cnf"  rs_cnf in the token response AS -> C, OPTIONAL to indicate the
      public key of the RS if it uses one to authenticate itself to the
      client and the binding between the key and RS identity is not
      established through other means.

   Note that the COSE_Key structure in a confirmation claim or parameter
   may contain an "alg" alg or "key_ops" key_ops parameter.  If such parameters are
   present, a client MUST NOT use a key that is incompatible with the
   profile or proof-of-possession PoP algorithm according to those parameters.  An RS MUST
   reject a proof of possession using such a key with a response code
   equivalent to the CoAP code 4.00 (Bad Request).

   If an access token is issued for an audience that includes several
   RSs, the "rs_cnf" rs_cnf parameter MUST NOT be used, since the client cannot
   determine for which RS the key applies.  This document recommends to
   specify a different endpoint that the client can use to acquire RS
   authentication keys in such cases.  The specification of such an
   endpoint is out of scope for this document.

6.  CBOR Mappings

   If CBOR is used, the new parameters and claims defined in this
   document MUST be mapped to CBOR types, as specified in Table 1, using
   the given integer abbreviation for the map key.

   +=========+==========+============+========================+
   | Name    | CBOR Key | Value Type | Usage                  |
   +=========+==========+============+========================+
   | req_cnf | 4        | map        | token request          |
   +---------+----------+------------+------------------------+
   | cnf     | 8        | map        | token response         |
   +---------+----------+------------+------------------------+
   | cnf     | 8        | map        | introspection response |
   +---------+----------+------------+------------------------+
   | rs_cnf  | 41       | map        | token response         |
   +---------+----------+------------+------------------------+

       Table 1: CBOR Mappings for New Parameters and Claims

7.  Requirements When Using Asymmetric Keys

   An RS using asymmetric keys to authenticate to the client MUST NOT
   hold several different asymmetric key pairs applicable to the same
   authentication algorithm.  For example, when using DTLS, the RS MUST
   NOT hold several asymmetric key pairs applicable to the same cipher
   suite.  The reason for this restriction is that the RS has no way of
   determining which key to use before the client's identity is
   established.  Therefore, authentication attempts by the RS could
   randomly fail based on which key the RS selects, unless the algorithm
   negotiation produces a unique choice of key pair for the RS.

8.  Security Considerations

   This document is an extension to [RFC9200].  All security
   considerations from that document apply here as well.

9.  Privacy Considerations

   This document is an extension to [RFC9200].  All privacy
   considerations from that document apply here as well.

10.  IANA Considerations

10.1.  OAuth Parameter Registration

   This section registers the following parameters in the "OAuth
   Parameters" registry [IANA.OAuthParameters]:

   Name:  req_cnf
   Parameter Usage Location:  token request
   Change Controller:  IETF
   Reference:  Section 5 of RFC 9201

   Name:  rs_cnf
   Parameter Usage Location:  token response
   Change Controller:  IETF
   Reference:  Section 5 of RFC 9201

   Name:  cnf
   Parameter Usage Location:  token response
   Change Controller:  IETF
   Reference:  Section 5 of RFC 9201

10.2.  OAuth Parameters CBOR Mappings Registration

   This section registers the following parameter mappings in the "OAuth
   Parameters CBOR Mappings" registry established in Section 8.10 of
   [RFC9200].

   Name:  req_cnf
   CBOR Key:  4
   Value Type:  map
   Reference:  Section 3.1 of RFC 9201
   Original specification:  RFC 9201

   Name:  cnf
   CBOR Key:  8
   Value Type:  map
   Reference:  Section 3.2 of RFC 9201
   Original specification:  RFC 9201

   Name:  rs_cnf
   CBOR Key:  41
   Value Type:  map
   Reference:  Section 3.2 of RFC 9201
   Original specification:  RFC 9201

10.3.  OAuth Token Introspection Response CBOR Mappings Registration

   This section registers the following parameter mapping in the "OAuth
   Token Introspection Response CBOR Mappings" registry established in
   Section 8.12 of [RFC9200].

   Name:  cnf
   CBOR Key:  8
   Value Type:  map
   Reference:  Section 4 of RFC 9201
   Original specification:  [RFC8705]

11.  References

11.1.  Normative References

   [IANA.OAuthParameters]
              IANA, "OAuth Parameters",
              <https://www.iana.org/assignments/oauth-parameters>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC7800]  Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
              Possession Key Semantics for JSON Web Tokens (JWTs)",
              RFC 7800, DOI 10.17487/RFC7800, April 2016,
              <https://www.rfc-editor.org/info/rfc7800>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", RFC 8705,
              DOI 10.17487/RFC8705, February 2020,
              <https://www.rfc-editor.org/info/rfc8705>.

   [RFC8747]  Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
              2020, <https://www.rfc-editor.org/info/rfc8747>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [RFC9200]  Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) Using the OAuth 2.0
              Framework (ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200,
              March 2022, <https://www.rfc-editor.org/info/rfc9200>.

11.2.  Informative References

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

Acknowledgments

   This document is a product of the ACE Working Group of the IETF.
   Special thanks to Brian Campbell for his thorough review of this
   document.

   Ludwig Seitz worked on this document as part of the CelticNext
   projects CyberWI and CRITISEC with funding from Vinnova.

Author's Address

   Ludwig Seitz
   Combitech
   Djäknegatan 31
   SE-211 35 Malmö
   Sweden
   Email: ludwig.seitz@combitech.com