Internet Engineering Task Force (IETF)                          L. Seitz
Request for Comments: 9201                                     Combitech
Category: Standards Track                                    August 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 PoP key that the authorization server
  has selected, and the 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
  Provisions Relating to IETF Documents
  (https://trustee.ietf.org/license-info) in effect on the date of
  publication of this document.  Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Revised BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  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 parameter allowing clients to
  request a specific 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 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 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 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
  parameter to request a specific public key as a PoP key.  The content
  is displayed in CBOR diagnostic notation 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 / 4 : {
       / COSE_Key / 1 : {
       / kty /  1 : 2 /EC2/,
       / kid /  2 : h'11',
       / crv / -1 : 1 /P-256/,
       / x /   -2 : h'BAC5B11CAD8F99F9C72B05CF4B9E26D24
                    4DC189F745228255A219A86D6A09EFF',
       / y /   -3 : 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 PoP keys.  This field contains the
     PoP key that the AS selected for the token.  Values of this
     parameter follow the syntax and semantics of the 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 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 parameter
  with a symmetric PoP key.

  Header: Created (Code=2.01)
  Content-Format: application/ace+cbor
  Payload:
  {
    / access_token / 1 : h'4A5015DF686428/...
     (remainder of CWT omitted for brevity;
     CWT contains COSE_Key in the "cnf" claim)/',
    / cnf / 8 : {
     / COSE_Key / 1 : {
        / kty / 1 : 4 / Symmetric /,
        / kid / 2 : h'DFD1AA97',
        / k /  -1 : 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 PoP key (not shown) and an rs_cnf
  parameter containing the public key of the RS.

  Header: Created (Code=2.01)
  Content-Format: application/ace+cbor
  Payload:
  {
    / access_token / 1 : h'D08343A1010AA1054D2A45DF6FBC5A5A/...
     (remainder of CWT omitted for brevity)/',
    / rs_cnf / 41 : {
      / COSE_Key / 1 : {
       / kty /  1 : 2 /EC2/,
       / kid /  2 : h'12',
       / crv / -1 : 1 /P-256/,
        / x /  -2 : h'BCEE7EAAC162F91E6F330F5771211E220
                    B8B546C96589B0AC4AD0FD24C77E1F1',
        / y /  -3 : 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
  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
  claim values from Section 3.1 of [RFC8747].

  Figure 4 shows an AS response to an introspection request including
  the cnf parameter to indicate the PoP key bound to the token.

  Header: Created (Code=2.01)
  Content-Format: application/ace+cbor
  Payload:
  {
    / active / 10 : true,
    / scope / 9 : "read",
    / aud / 3 : "tempSensor4711",
    / cnf / 8 : {
      / COSE_Key / 1 : {
        / kty /  1 : 2 /EC2/,
        / kid /  2 : h'11',
        / crv / -1 : 1 /P-256/,
        / x /   -2 : h'BAC5B11CAD8F99F9C72B05CF4B9E26D24
                     4DC189F745228255A219A86D6A09EFF',
        / y /   -3 : h'20138BF82DC1B6D562BE0FA54AB7804A3
                     A64B6D72CCFED6B6FB6ED28BBFC117E'
      }
    }
  }

                 Figure 4: Example Introspection Response

5.  Confirmation Method Parameters

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

  *  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 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 and
     symmetric keys are used.  Used to indicate the symmetric key
     generated by the AS for proof of possession of the access token.

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

  *  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 or key_ops parameter.  If such parameters are
  present, a client MUST NOT use a key that is incompatible with the
  profile or 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 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,
             August 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: [email protected]

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