RFC 9201 | OAuth Parameters for ACE | August 2022 |
Seitz | Standards Track | [Page] |
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.¶
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 (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.¶
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].¶
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.¶
This section defines additional parameters for the interactions with the token endpoint in the ACE framework [RFC9200].¶
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
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.¶
This section defines the following additional parameters for an AS response to a request to the token endpoint:¶
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.¶
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.¶
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.¶
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.¶
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.¶
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 |
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.¶
This document is an extension to [RFC9200]. All security considerations from that document apply here as well.¶
This document is an extension to [RFC9200]. All privacy considerations from that document apply here as well.¶
This section registers the following parameters in the "OAuth Parameters" registry [IANA.OAuthParameters]:¶
req_cnf
¶
This section registers the following parameter mappings in the "OAuth Parameters CBOR Mappings" registry established in Section 8.10 of [RFC9200].¶
req_cnf
¶
This section registers the following parameter mapping in the "OAuth Token Introspection Response CBOR Mappings" registry established in Section 8.12 of [RFC9200].¶
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.¶