Additional OAuth Parameters for Authorization in Constrained Environments (ACE)

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 .

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 defined in section 7 of . 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 "An 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 .

This section defines the "req_cnf" parameter allowing clients to request a specific proof-of-possession key in an access token from a token endpoint in the ACE framework : 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 proof-of-possession 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 for CBOR-based interactions or from section 3.1 of for JSON-based interactions. shows a request for an access token using the "req_cnf" parameter to request a specific public key as proof-of-possession key. The content is displayed in CBOR diagnostic notation, without abbreviations and with line-breaks for better readability.

This section defines the following additional parameters for an AS response to a request to the token endpoint: REQUIRED if the token type is "pop" and a symmetric key is used. MAY be present for asymmetric proof-of-possession keys. This field contains the proof-of-possession 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 for CBOR-based interactions or from section 3.1 of for JSON-based interactions. See for additional discussion of the usage of this parameter. 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 for CBOR-based interactions or from section 3.1 of 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 proof-of-possession key.

shows an AS response containing a token bound to a previously requested asymmetric proof-of-possession key (not shown) and a "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 . 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 section 3.1 of . shows an AS response to an introspection request including the "cnf" parameter to indicate the proof-of-possession key bound to the token.

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 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 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 RS, 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 , using the given integer abbreviation for the map key.

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 . All security considerations from that document apply here as well.

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

This section registers the following parameters in the "OAuth Parameters" registry : Name: req_cnf Parameter Usage Location: token request Change Controller: IETF Reference: of [this document] Name: rs_cnf Parameter Usage Location: token response Change Controller: IETF Reference: of [this document] Name: cnf Parameter Usage Location: token response Change Controller: IETF Reference: of [this document]

This section registers the following parameter mappings in the "OAuth Parameters CBOR Mappings" registry established in section 8.9. of . Name: req_cnf CBOR key: TBD (suggested: 4) Value Type: map Reference: of [this document] Original specification: [this document] Name: cnf CBOR key: TBD (suggested: 8) Value Type: map Reference: of [this document] Original specification: [this document] Name: rs_cnf CBOR key: TBD (suggested: 41) Value Type: map Reference: of [this document] Original specification: [this document]

This section registers the following parameter mapping in the "OAuth Token Introspection Response CBOR Mappings" registry established in section 8.11. of . Name: cnf CBOR key: TBD (suggested: 8) Value Type: map Reference: of [this document] Original specification:

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.