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Security ACE CoAP OAuth 2.0 Access Control Authorization Internet of Things 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.

Introduction The Authentication and Authorization for Constrained Environments (ACE) specification requires some new parameters for interactions with the OAuth 2.0 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 . Note that although all examples are shown in Concise Binary Object Representation (CBOR) , JSON MAY be used as an alternative for HTTP-based communications, as specified in .

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 when, and only when, they appear in all capitals, as shown here. Readers are assumed to be familiar with the terminology from , especially the terminology for entities in the architecture such as client (C), resource server (RS), and authorization server (AS). Terminology from is used in the examples, especially COSE_Key, which is defined in . Note that the term "endpoint" is used here following its OAuth 2.0 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) definition, which is "[a]n entity participating in the CoAP protocol", is not used in this specification.

Parameters for the Token Endpoint This section defines additional parameters for the interactions with the token endpoint in the ACE framework .

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 :

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 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 for CBOR-based interactions or from for JSON-based interactions.

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.

Example Request for an Access Token Bound to an Asymmetric Key

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 for CBOR-based interactions or from for JSON-based interactions. See 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 for CBOR-based interactions or from for JSON-based interactions. See for additional discussion of the usage of this parameter.

shows an AS response containing a token and a cnf parameter with a symmetric PoP key.

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

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.

Example AS Response Including the RS's Public Key

Parameters for the Introspection Endpoint This section defines the use of CBOR instead of JSON for the cnf introspection response parameter specified in . If CBOR is used instead of JSON in an interaction with the introspection endpoint, the AS MUST use the parameter mapping specified in and the value must follow the syntax of cnf claim values from . shows an AS response to an introspection request including the cnf parameter to indicate the PoP key bound to the token.

Example Introspection Response

Confirmation Method Parameters The confirmation method parameters are used in 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.

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

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

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.

Security Considerations This document is an extension to . All security considerations from that document apply here as well.

Privacy Considerations This document is an extension to . All privacy considerations from that document apply here as well.

IANA Considerations

OAuth Parameter Registration This section registers the following parameters in the "OAuth Parameters" registry :

Name:

req_cnf

Parameter Usage Location:

token request

Change Controller:

IETF

Reference:

of RFC 9201

Name:

rs_cnf

Parameter Usage Location:

token response

Change Controller:

IETF

Reference:

of RFC 9201

Name:

cnf

Parameter Usage Location:

token response

Change Controller:

IETF

Reference:

of RFC 9201

OAuth Parameters CBOR Mappings Registration This section registers the following parameter mappings in the "OAuth Parameters CBOR Mappings" registry established in .

Name:

req_cnf

CBOR Key:

4

Value Type:

map

Reference:

of RFC 9201

Original Specification:

RFC 9201

Name:

cnf

CBOR Key:

8

Value Type:

map

Reference:

of RFC 9201

Original Specification:

RFC 9201

Name:

rs_cnf

CBOR Key:

41

Value Type:

map

Reference:

of RFC 9201

Original Specification:

RFC 9201

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 .

Name:

cnf

CBOR Key:

8

Value Type:

map

Reference:

of RFC 9201

Original Specification:

References Normative References IANA Informative References

Acknowledgments This document is a product of the ACE Working Group of the IETF. Special thanks to for his thorough review of this document. worked on this document as part of the CelticNext projects CyberWI and CRITISEC with funding from Vinnova.