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.¶

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Table of Contents

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].¶

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'
      }
   }
 }

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'
    }
  }
}

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'
    }
  }
}

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 proof-of-possession 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'
    }
  }
}

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 proof-of-possession token, absent otherwise. Indicates the proof-of-possession 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 proof-of-possession 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.¶

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

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