]> RISE AB

Isafjordsgatan 22 Kista 16440 Sweden marco.tiloca@ri.se

Ericsson AB

Torshamnsgatan 23 Kista 16440 Stockholm Sweden goran.selander@ericsson.com

Security ACE Working Group Internet-Draft This document updates the Authentication and Authorization for Constrained Environments Framework (ACE, RFC 9200) as follows. First, it defines a new, alternative workflow that the Authorization Server can use for uploading an access token to a Resource Server on behalf of the Client. Second, it defines new parameters and encodings for the OAuth 2.0 token endpoint at the Authorization Server. Discussion Venues Discussion of this document takes place on the Authentication and Authorization for Constrained Environments Working Group mailing list (ace@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction The Authentication and Authorization for Constrained Environments (ACE) framework defines an architecture to enforce access control for constrained devices. A Client (C) requests an assertion of granted permissions from an Authorization Server (AS) in the form of an access token, then uploads the access token to the target Resource Server (RS), and finally accesses protected resources at the RS according to the permissions specified in the access token. The framework has as main building blocks the OAuth 2.0 framework , the Constrained Application Protocol (CoAP) for message transfer, CBOR for compact encoding, and COSE for self-contained protection of access tokens. In addition, separate profile documents define in detail how the participants in the ACE architecture communicate, especially as to the security protocols that they use. This document updates as follows.

• It defines a new, alternative protocol workflow for the ACE framework (see ), according to which the AS uploads the access token to the RS on behalf of C, and then informs C about the outcome. The new workflow is especially convenient in deployments where the communication leg between C and the RS is constrained, but the communication leg between the AS and the RS is not. The new workflow has no ambition to replace the original workflow. The AS can use one workflow or the other depending, for example, on the specific RS for which an access token has been issued and the nature of the communication leg with that RS.
• It defines additional parameters and encodings for the OAuth 2.0 token endpoint at the AS (see ). These include:
  • "token_uploaded", used by the AS to inform C about the outcome of the token uploading to the RS, as per the new ACE workflow.
  • "rs_cnf2", used by the AS to provide C with the public keys of the RSs in the group-audience for which the access token is issued (see ).
  • "aud2", used by the AS to provide C with the identifiers of the RSs in the group-audience for which the access token is issued.
  • "anchor_cnf", used by the AS to provide C with the public keys of trust anchors, which C can use to validate the public key of an RS (e.g., as provided in the parameter "rs_cnf" defined in or in the parameter "rs_cnf2" defined in this document).

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 expected to be familiar with the terms and concepts described in the ACE framework for Authentication and Authorization , as well as with terms and concepts related to CBOR Web Tokens (CWTs) and CWT Confirmation Methods . The terminology for entities in the considered architecture is defined in OAuth 2.0 . In particular, this includes Client (C), Resource Server (RS), and Authorization Server (AS). Readers are also expected to be familiar with the terms and concepts related to the CoAP protocol , CBOR , and COSE . Note that, unless otherwise indicated, the term "endpoint" is used here following its OAuth definition, aimed at denoting resources such as /token and /introspect at the AS, and /authz-info at the RS. This document does not use the CoAP definition of "endpoint", which is "An entity participating in the CoAP protocol." Examples throughout this document are expressed in CBOR diagnostic notation without the tag and value abbreviations.

New ACE Workflow As defined in , the ACE framework considers what is shown in as its basic protocol workflow. That is, the Client first sends an access token request to the token endpoint at the AS (step A), specifying permissions that it seeks to obtain for accessing protected resources at the RS, possibly together with information on its own credentials. Then, if the request has been successfully verified, authenticated, and authorized, the AS replies to the Client (step B), providing an access token and possibly additional parameters as access information including the actually granted permissions. Finally, the Client uploads the access token to the RS and, consistently with the permissions granted according to the access token, accesses a resource at the RS (step C), which replies with the result of the resource access (step F). Details about what protocol the Client and RS use to establish a secure association, mutually authenticate and secure their communications are defined in the specifically used profile of ACE, e.g., . Further interactions are possible between the AS and RS, i.e., the exchange of an introspection request and response where the AS validates a previously issued access token for RS (steps D and E).

ACE Basic Protocol Workflow | | | | | Authorization | | |<--(B)-- Access Token ---------| Server | | | + Access Information | | | | + Refresh Token (optional) +---------------+ | | ^ | | | Introspection Request (D)| | | Client | Response | |(E) | | (optional exchange) | | | | | v | | +--------------+ | |---(C)-- Token + Request ----->| | | | | Resource | | |<--(F)-- Protected Resource ---| Server | | | | | +--------+ +--------------+ ]]>

This section defines a new, alternative protocol workflow shown in , which MAY be supported by the AS. Unlike in the original protocol workflow, the AS uploads the access token to the RS on behalf of the Client, and then informs the Client about the outcome. If the token uploading has been successfully completed, the AS does not provide the access token to the Client altogether. Instead, the Client simply establishes a secure association with the RS (if that has not happened already), and then accesses protected resources at the RS according to the permissions granted per the access token and specified by the AS as access information.

ACE Alternative Protocol Workflow | | | | | Authorization | | |<--(B)-- Token Response -------| Server | | | + Access Information | | | | + Access Token (optional) +----------------------------+ | | + Refresh Token (optional) ^ | | ^ | | | | | | Token-Upload | | Introspection Request (D)| | (A1)| | Request | Client | Response | |(E) | |(A2) Response | | (optional exchange) | | | | | | | v v | | | +----------------------------+ | |---(C1)-- Token (Optional) --->| | | | | | | |---(C2)-- Protected Request -->| Resource | | | | Server | | |<--(F)--- Protected Resource --| | | | | | +--------+ +----------------------------+ ]]>

More specifically, the new workflow consists of the following steps.

• Step A - This step is as in the original workflow, with the Client sending an Access Token Request to the token endpoint at the AS.
• Step A1 - This new step consists of the AS uploading the access token to the RS, typically at the authz-info endpoint, just like the Client does in the original workflow.
• Step A2 - This new step consists of the RS replying to the AS, following the uploading of the access token at step A1.
• Step B - In the Access Token Response, the AS tells the Client that it has attempted to upload the access token to the RS, specifying the outcome of the token uploading based on the reply received from the RS at step A2. As defined in , this information is conveyed by the "token_uploaded" parameter. If the token uploading has succeeded, the AS does not provide the Client with the access token. Otherwise, the AS provides the Client with the access token.
• Step C1 - This step occurs only if the token uploading from the AS has failed, and the AS has provided the Client with the access token at step B. In such a case, the Client uploads the access token to the RS just like at step C of the original workflow.
• Step C2 - The Client attempts to access a protected resource at the RS, according to the permissions granted per the access token and specified by the AS as access information at step B.
• Steps D, E and F are as in the original workflow.

The new workflow has no ambition to replace the original workflow defined in . The AS can use one workflow or the other depending, for example, on the specific RS for which the access token has been issued and the nature of the communication leg with that RS.

New ACE Parameters The rest of this section defines a number of additional parameters and encodings for the OAuth 2.0 token endpoint at the AS.

token_uploaded This section defines the additional parameter "token_uploaded" for an Access Token Response, sent by the AS in reply to a request to the token endpoint from C. The parameter "token_uploaded" is REQUIRED in a successful Access Token Response with response code 2.01 (Created), if the AS has issued an access token and has attempted to upload it to the RS on behalf of C as per the ACE alternative protocol workflow defined in , irrespective of the result of the token upload. Otherwise, the parameter "token_uploaded" MUST NOT be present. If present, the parameter "token_uploaded" MUST encode the CBOR simple value "true" (0xf5) if the token upload at the RS was successful, or the CBOR simple value "false" (0xf4) otherwise. If the parameter "token_upload" encodes the CBOR simple value "true", the access token MUST NOT be included in the Access Token response. Otherwise, the access token MUST be included.

Examples shows an example of Access Token Response from the AS to C, following the issue of an access token bound to a symmetric PoP key. The Access Token Response specifies the parameter "token_uploaded" with value "true", which indicates that the AS has successfully uploaded the access token to the RS on behalf of C. Consistently, the Access Token Response does not include the access token, while it still includes the parameter "cnf" specifying the symmetric PoP key bound to the access token.

Example of Access Token Response, including the parameter "token_uploaded" but not the access token, which is bound to a symmetric key and was uploaded to the RS by the AS

shows another example of Access Token Response from the AS to C, also following the issue of an access token bound to a symmetric PoP key. In this example, the Access Token Response includes the parameter "token_uploaded" with value "false", which indicates that the AS has failed to upload the access token to the RS on behalf of C. The Access Token Response also includes the access token and the parameter "cnf" specifying the symmetric PoP key bound to the access token. Note that, even though the AS has failed to upload the access token to the RS, the response code 2.01 (Created) is used when replying to C, since the Access Token Request as such has been successfully processed at the AS, with the following issue of the access token.

Example of Access Token Response, including the parameter "token_uploaded" together with the access token, which is bound to a symmetric key and which the AS failed to upload to the RS

rs_cnf2 and aud2 This section defines the additional parameters "rs_cnf2" and "aud2" for an Access Token Response, sent by the AS in reply to a request to the token endpoint from C.

• The parameter "rs_cnf2" is OPTIONAL if the token type is "pop", asymmetric keys are used, and the access token is issued for an audience that includes multiple RSs (i.e., a group-audience, see ). Otherwise, the parameter "rs_cnf2" MUST NOT be present. This parameter specifies information about the public keys used by the RSs of a group-audience for authenticating themselves to C, and is used in case the binding between the public keys and the corresponding RS identities are not established through other means. If this parameter is absent, either the RSs in the group-audience do not use a public key, or the AS knows that the RSs can authenticate themselves to C without additional information. If present, this parameter MUST encode a non-empty CBOR array of N elements, where N is the number of RSs in the group-audience for which the access token is issued. Each element of the CBOR array specifies the public key of one RS in the group-audience, and MUST follow the syntax and semantics of the "cnf" claim either from for CBOR-based interactions, or from for JSON-based interactions. It is not required that all the elements of the CBOR array rely on the same confirmation method. Each of the public keys may contain parameters specifying information such as the public key algorithm and use (e.g., by means of the parameters "alg" or "key_ops" in a COSE_Key structure). If such information is specified, a Client MUST NOT use a public key that is incompatible with the profile or PoP algorithm according to that information. 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).
• The parameter "aud2" is OPTIONAL and specifies the identifiers of the RSs in the group-audience for which the access token is issued. If present, this parameter MUST encode a non-empty CBOR array of N elements, where N is the number of RSs in the group-audience for which the access token is issued. Each element of the CBOR array in the "aud2" parameter MUST be a CBOR text string, with value the identifier of one RS in the group-audience. The element of the CBOR array referring to an RS in the group-audience SHOULD have the same value that would be used to identify that RS through the parameter "aud" of an Access Token Request to the AS (see ) and an Access Token Response from the AS (see ), when requesting and issuing an access token for that individual RS. The parameter "aud2" is REQUIRED if the parameter "rs_cnf2" is present. The i-th element of the CBOR array in the "aud2" parameter MUST be the identifier of the RS whose public key is specified as the i-th element of the CBOR array in the "rs_cnf2" parameter.

Example shows an example of Access Token Response from the AS to C, following the issue of an access token for a group-audience composed of two RSs "rs1" and "rs2", and bound to C's public key as asymmetric PoP key. The Access Token Response includes the access token, as well as the parameters "rs_cnf2" and "aud2". These specify the public key of the two RSs as intended recipients of the access token and the identifiers of those two RSs, respectively.

Example of Access Token Response with an access token bound to an asymmetric key, using the parameters "rs_cnf2" and "aud2"

anchor_cnf This section defines the additional parameter "anchor_cnf" for an Access Token Response, sent by the AS in reply to a request to the token endpoint from C. The parameter "anchor_cnf" is OPTIONAL if the token type is "pop" and asymmetric keys are used. Otherwise, the parameter "anchor_cnf" MUST NOT be present. This parameter specifies information about the public keys of trust anchors, which C can use to validate the public key of the RS/RSs included in the audience for which the access token is issued. This parameter can be used when the access token is issued for an audience including one RS or multiple RSs. If this parameter is absent, either the RS/RSs in the audience do not use a public key, or the AS knows that C can validate the public key of such RS/RSs without additional information (e.g., C has already obtained the required public keys of the involved trust anchors from the AS or through other means). If present, this parameter MUST encode a non-empty CBOR array that MUST be treated as a set, i.e., the order of its elements has no meaning. Each element of the CBOR array specifies the public key of one trust anchor, which can be used to validate the public key of at least one RS included in the audience for which the access token is issued. Each element of the CBOR array MUST follow the syntax and semantics of the "cnf" claim either from for CBOR-based interactions, or from for JSON-based interactions. It is not required that all the elements of the CBOR array rely on the same confirmation method. Each of the public keys specified in the parameter "anchor_cnf" may contain parameters specifying information such as the public key algorithm and use (e.g., by means of the parameters "alg" or "key_ops" in a COSE_Key structure). If such information is specified, a Client MUST NOT use a public key that is incompatible with the profile, or with the public keys to validate and the way to validate those. The presence of this parameter does not require that the Access Token Response also includes the parameter "req_cnf" defined in or the parameter "req_cnf2" defined in of this document. That is, C may be able to obtain the public keys of the RS/RSs for which the access token is issued through other means. When the Access Token Response includes both the parameter "anchor_cnf" and the parameter "aud2" defined in , then C MUST make sure that a public key PK_RS is associated with an RS identified by an element of "aud2", before using any of the public keys specified in "anchor_cnf" to validate PK_RS. When the Access Token Response includes the parameter "anchor_cnf" but not the parameter "aud2", then C can use the issued access token with any RS in the targeted audience for which "anchor_cnf" can be used to validate PK_RS. This allows C to use the access token with an RS that is deployed later on as part of the same audience, which is particularly useful in the case of a group-audience.

Example shows an example of Access Token Response from the AS to C, following the issue of an access token for a group-audience, and bound to C's public key as asymmetric PoP key. The identifier of the group-audience was specified by the "aud" parameter of the Access Token Request to the AS and is specified by the "aud" claim of the issued access token, and is not repeated in the Access Token Response from the AS. The Access Token Response includes the parameter "anchor_cnf". This specifies the public key of a trust anchor that C can use to validate the public keys of any RS with which the access token is going to be used. The public key of the trust anchor is here conveyed within an X.509 certificate used as authentication credential for that trust anchor, by means of the CWT confirmation method "x5chain" defined in .

Example of Access Token Response with an access token bound to an asymmetric key, using the parameter "anchor_cnf"

Security Considerations TBD

IANA Considerations This document has the following actions for IANA.

OAuth Parameters Registry IANA is asked to add the following entries to the "OAuth Parameters" registry.

• Name: "token_uploaded"
• Parameter Usage Location: token response
• Change Controller: IESG
• Reference: [RFC-XXXX]

 

• Name: "rs_cnf2"
• Parameter Usage Location: token response
• Change Controller: IESG
• Reference: [RFC-XXXX]

 

• Name: "aud2"
• Parameter Usage Location: token response
• Change Controller: IESG
• Reference: [RFC-XXXX]

 

• Name: "anchor_cnf"
• Parameter Usage Location: token response
• Change Controller: IESG
• Reference: [RFC-XXXX]

OAuth Parameters CBOR Mappings Registry IANA is asked to add the following entries to the "OAuth Parameters CBOR Mappings" following the procedure specified in .

• Name: "token_uploaded"
• CBOR Key: TBD
• Value Type: simple value "true" / simple type "false"
• Reference: [RFC-XXXX]

 

• Name: "rs_cnf2"
• CBOR Key: TBD
• Value Type: array
• Reference: [RFC-XXXX]

 

• Name: "aud2"
• CBOR Key: TBD
• Value Type: array
• Reference: [RFC-XXXX]

 

• Name: "anchor_cnf"
• CBOR Key: TBD
• Value Type: array
• Reference: [RFC-XXXX]

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The OAuth 2.0 authorization framework enables a third-party application to obtain limited access to an HTTP service, either on behalf of a resource owner by orchestrating an approval interaction between the resource owner and the HTTP service, or by allowing the third-party application to obtain access on its own behalf. This specification replaces and obsoletes the OAuth 1.0 protocol described in RFC 5849. [STANDARDS-TRACK] The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. This specification describes how to declare in a JSON Web Token (JWT) that the presenter of the JWT possesses a particular proof-of- possession key and how the recipient can cryptographically confirm proof of possession of the key by the presenter. Being able to prove possession of a key is also sometimes described as the presenter being a holder-of-key. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. This specification describes how to declare in a CBOR Web Token (CWT) (which is defined by RFC 8392) that the presenter of the CWT possesses a particular proof-of-possession key. Being able to prove possession of a key is also sometimes described as being the holder-of-key. This specification provides equivalent functionality to "Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)" (RFC 7800) but using Concise Binary Object Representation (CBOR) and CWTs rather than JavaScript Object Notation (JSON) and JSON Web Tokens (JWTs). The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol (RFC 9052). This document, along with RFC 9052, obsoletes RFC 8152. This specification defines a framework for authentication and authorization in Internet of Things (IoT) environments called ACE-OAuth. The framework is based on a set of building blocks including OAuth 2.0 and the Constrained Application Protocol (CoAP), thus transforming a well-known and widely used authorization solution into a form suitable for IoT devices. Existing specifications are used where possible, but extensions are added and profiles are defined to better serve the IoT use cases. 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. Ericsson Ericsson RISE RISE This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. It utilizes Ephemeral Diffie-Hellman Over COSE (EDHOC) for achieving mutual authentication between an OAuth 2.0 Client and Resource Server, and it binds an authentication credential of the Client to an OAuth 2.0 access token. EDHOC also establishes an Object Security for Constrained RESTful Environments (OSCORE) Security Context, which is used to secure communications when accessing protected resources according to the authorization information indicated in the access token. This profile can be used to delegate management of authorization information from a resource-constrained server to a trusted host with less severe limitations regarding processing power and memory. Informative References This specification defines a profile of the Authentication and Authorization for Constrained Environments (ACE) framework that allows constrained servers to delegate client authentication and authorization. The protocol relies on DTLS version 1.2 or later for communication security between entities in a constrained network using either raw public keys or pre-shared keys. A resource-constrained server can use this protocol to delegate management of authorization information to a trusted host with less-severe limitations regarding processing power and memory. This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. It utilizes Object Security for Constrained RESTful Environments (OSCORE) to provide communication security and proof-of-possession for a key owned by the client and bound to an OAuth 2.0 access token. RISE AB Ericsson AB CMU SEI CMU SEI This document specifies a method of the Authentication and Authorization for Constrained Environments (ACE) framework, which allows an Authorization Server to notify Clients and Resource Servers (i.e., registered devices) about revoked access tokens. As specified in this document, the method allows Clients and Resource Servers to access a Token Revocation List on the Authorization Server by using the Constrained Application Protocol (CoAP), with the possible additional use of resource observation. Resulting (unsolicited) notifications of revoked access tokens complement alternative approaches such as token introspection, while not requiring additional endpoints on Clients and Resource Servers. RISE AB RISE AB Combitech Ericsson AB This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. The profile uses Group Object Security for Constrained RESTful Environments (Group OSCORE) to provide communication security between a Client and one or multiple Resource Servers that are members of an OSCORE group. The profile securely binds an OAuth 2.0 Access Token to the public key of the Client associated with the private key used by that Client in the OSCORE group. The profile uses Group OSCORE to achieve server authentication, as well as proof-of-possession for the Client's public key. Also, it provides proof of the Client's membership to the OSCORE group by binding the Access Token to information from the Group OSCORE Security Context, thus allowing the Resource Server(s) to verify the Client's membership upon receiving a message protected with Group OSCORE from the Client. Effectively, the profile enables fine-grained access control paired with secure group communication, in accordance with the Zero Trust principles. Ericsson AB Ericsson AB Ericsson AB This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios and a main use case is to establish an OSCORE security context. By reusing COSE for cryptography, CBOR for encoding, and CoAP for transport, the additional code size can be kept very low.

Benefits for ACE Transport Profiles For any transport profile of ACE, the following holds.

• The new ACE workflow defined in is effectively possible to use. This is beneficial for deployments where the communication leg between C and the RS is constrained, but the communication leg between the AS and RS is not.
• When the new ACE workflow is used, the parameter "token_uploaded" defined in is used to inform C that the AS has attempted to upload the access token to the RS, specifying whether the uploading has succeeded or failed.

DTLS Profile When the RPK mode of the DTLS profile is used (see ), the parameters "rs_cnf2" and "aud2" defined in enable the effective issue of an access token intended to an audience that includes multiple RSs.

EDHOC and OSCORE Profile When the EDHOC and OSCORE profile is used , the parameters "rs_cnf2" and "aud2" defined in enable the effective issue of an access token intended to an audience that includes multiple RSs.

Open Points

New Workflow The following discusses open points related to the use of the new ACE workflow defined in .

Allow the Dynamic Update of Access Rights In some profiles of ACE, C can request a new access token to update its access rights, while preserving the same secure association with the RS. The new access token supersedes the current one stored at the RS, as they are both part of the same "token series". A token series comprises all the access tokens issued by the same AS for the same pair (Client, Resource Server). Specific profiles can provide a more specialized definition, e.g., by further taking into account the public authentication credentials of C and the RS. When using the original ACE workflow, C uploads the new access token to the RS by protecting the message exchange through the secure association with the RS. This allows the RS to determine that the upload of such access token is for updating the access rights of C. When using the new ACE workflow, the AS uploads the new access token to the RS also when an update of access rights for C is to be performed. This message exchange would be protected through the secure association between the AS and RS. However, this secure association does not help the RS retrieve the stored access token to supersede, as that is rather bound to the secure association with C. In order for the new ACE workflow to also allow the dynamic update of access rights, it is required that the new access token updating the access rights of C includes an explicit indication for the RS. Such an indication can point the RS to the token series in question (hence to the current access token to supersede), irrespective of the secure association used to protect the token uploading. In some profiles of ACE, such an indication is in fact already present in issued access tokens:

• In the PSK mode of the DTLS profile , the token series is indicated by the parameter "kid" within the claim "cnf" of the new access token. This has the same value of the parameter "kid" in the COSE_Key within the claim "cnf" from the first access token of the token series.
• In the OSCORE profile , the token series is indicated by the parameter "kid" within the claim "cnf" of the new access token. This has the same value of the parameter "id" in the OSCORE_Input_Material object within the claim "cnf" from the first access token of the token series.
• In the EDHOC and OSCORE profile , the token series is indicated by the parameter "kid" within the claim "cnf" of the new access token. This has the same value of the parameter "id" in the EDHOC_Information object within the claim "cnf" from the first access token of the token series. Note: version -01 of the EDHOC and OSCORE profile says that an update of access rights is not possible when using the new workflow. However, it is actually possible as discussed above.

In the three cases above, the update of access rights is possible because there is a value used as de facto "token series ID". This value does not change throughout the lifetime of a token series, and it is used to associate the new access token with the previous one in the same series to be superseded. Such a token series ID is required to have a unique value from a namespace/pool that the AS exclusively control. This is in fact what happens in the profiles of ACE above, where the AS is the entity creating the mentioned objects or COSE Key included in the first access token of a token series. However, this may generally not hold and it is not what happens in other known cases, i.e., the DTLS profile in RPK mode and the Group OSCORE profile . At the moment, the dynamic update of access rights is not possible for those, neither in the original nor in the new ACE workflow. In order to make the update of access rights possible also for such cases, as well as both in the original and in the new ACE workflow, those cases can rely on a new parameter and claim "token_series_id" (see ), which specifies a unique identifier of the token series which an access token belongs to. As to existing profiles of ACE, the above has no intention to change the current behavior when the update of access rights occurs, irrespective of the used ACE workflow and especially when using the original workflow. If future profiles rely on a construction where the AS creates the object or key included in the claim "cnf" of the first access token in a token series, and a unique ID generated by the AS is included in such object or key, then that ID must be used as de facto "token series ID", rather than the new parameter "token_series_id".

Allow the Re-uploading of the Access Token After the AS has successfully uploaded the access token to the RS when using the new ACE workflow, C does not obtain the access token altogether. It follows that C cannot re-upload the Access Token to the RS by itself, e.g., in order to perform a key update like defined for the OSCORE profile . Even in such a case, the token re-uploading can be allowed by relying on a new parameter "token_hash", which the AS provides to C and specifies the hash of the access token (see ). Then, C can practically "re-upload" the access token to the RS, by sending a request to the authz-info endpoint that includes the parameter "token_hash" instead of the parameter "access_token". Such a request may include further parameters, depending on what is defined for the used transport profile. If the RS still stores the access token in question, then the RS can identify it by means of the received token hash, and take the same actions that would have been taken in case the full access token was re-uploaded to the authz-info endpoint.

Ensure Applicability to Any ACE Profile Some profiles of ACE require that C and the RS generate information to be exchanged when uploading the access token. For example:

• In the OSCORE profile , C and RS are required to exchange the nonces N1 and N2, when uploading to the RS the first access token of a token series, as well as when re-uploading any access token (e.g., in order to perform a key update).
• In the EDHOC and OSCORE profile , C and the RS are required to exchange the nonces N1 and N2, when re-uploading any access token in order to perform a key update through the function EDHOC_KeyUpdate (see ).

Evidently, using the new ACE workflow prevents C and the RS from directly performing the required exchanges above, since the uploading of the access token does not rely on a direct interaction between C and RS like in the original ACE workflow. For some profiles of ACE, this may prevent the use of the new ACE workflow altogether. This issue can be solved by having the AS acting as intermediary also for the exchange of C- and RS-generated information, by relying on two new parameters "to_rs" and "from_rs" (see ). In particular, C can use "to_rs" for providing the AS with C-generated information, to be relayed to the RS when uploading the access token. Also, the RS can use "from_rs" for providing the AS with RS-generated information when replying to the token uploading, and to be relayed to C. With reference to the two cases mentioned above, "to_rs" can specify the nonce N1 generated by C, while "from_rs" can specify the nonce N2 generated by RS.

New Parameters

token_uploaded The parameter "token_uploaded" defined in builds on the assumption that C can operate in the presence of the alternative ACE workflow defined in . In particular, it assumes that C accepts as valid an Access Token Response that includes the parameter "token_uploaded" encoding the CBOR simple value "true" but not the access token, and that, in such a case, C continues by sending a protected request to the RS. In turn, this assumes that the AS knows that C can operate in the presence of the alternative ACE workflow. This can be part of the information that C provided to the AS at its registration. An alternative design choice would instead require C to opt-in when sending the Access Token Request to the AS. That is, C can include the parameter "token_uploaded" in the Access Token Request, encoding the CBOR simple value "true", hence explicitly signaling its understanding of the alternative workflow. Only if the AS supports the alternative workflow and the Access Token Request includes the parameter "token_uploaded" encoding the CBOR simple value "true", can the AS attempt to upload the access token to the RS on behalf of C as per the alternative workflow.

Further New Parameters to Consider The following discusses possible, further new parameters that can be defined for addressing the open points raised earlier in .

• "token_series_id" - This parameter specifies the unique identifier of a token series, thus ensuring that C can dynamically update its access rights, irrespective of the used ACE workflow (see ). When issuing the first access token of a token series, the AS specifies this parameter in the Access Token Response to C, with value TS_ID. Also, the AS includes a claim "token_series_id" with the same value in the access token. When C requests a new access token in the same tokes series for dynamically updating its access rights, C specifies the value TS_ID as "kid" within the parameter "req_cnf" of the Access Token Request (see ). The AS specifies the same value as "kid" within the claim "cnf" of the new access token.
• "token_hash" - This parameter specifies the hash of an access token that the AS has successfully issued and uploaded to the RS as per the new ACE workflow, and thus that the AS does not provide to C (see ). The AS specifies this parameter in a successful Access Token Response, in case the parameter "token_uploaded" is also specified as encoding the CBOR simple value "true" (see ). The parameter value is the hash computed over the value that the parameter "access_token" would have had in that same response message, if it was included therein specifying the access token. C specifies this parameter in the request sent to the authz-info endpoint at the RS for "re-uploading" the same access token, e.g., in order to perform a key update (see ). This parameter also allows C to seamlessly use the method defined in for learning of revoked access tokens, even when the new ACE workflow is used and C does not obtain the access token, which makes it impossible for C to compute the token hash by itself.
• "to_rs" - When using the new ACE workflow, this parameter specifies C-generated information that, according to the used profile of ACE, C has to provide to the RS together with the access token if using the original ACE workflow. This allows the AS to relay such information to the RS upon uploading the access token on behalf of C (see ). First, C specifies this parameter in the Access Token Request sent to the AS. Then, the AS specifies this parameter in the request to the RS sent for uploading the access token on behalf of C, by simply relaying the value received from C. The used profile of ACE has to define the detailed content and semantics of the information specified in the parameter value.
• "from_rs" - When using the new ACE workflow, this parameter specifies RS-generated information that, according to the used profile of ACE, the RS has to provide to C after the uploading of an access token if using the original ACE workflow. This allows the AS to relay such information to C after having uploaded the access token on behalf of C (see ). First, the RS specifies this parameter in the response sent to the AS, after the upload of an access token through a request from the AS. Then, the AS specifies this parameter in the Access Token Response to C, by simply relaying the value received from the RS. The used profile of ACE has to define the detailed content and semantics of the information specified in the parameter value.

Acknowledgments The author sincerely thanks and for their comments and feedback. The work on this document has been partly supported by the H2020 project SIFIS-Home (Grant agreement 952652).