RFC 9203 | OSCORE Profile of ACE | August 2022
Palombini, et al. | Standards Track | [Page]

Stream:
  Internet Engineering Task Force (IETF)
RFC:
  9203
Category:
  Standards Track
Published:
  August 2022
ISSN:
  2070-1721
Authors:
  F. Palombini
  Ericsson AB
  L. Seitz
  Combitech
  G. Selander
  Ericsson AB
  M. Gunnarsson
  RISE

# RFC 9203

# The Object Security for Constrained RESTful Environments (OSCORE) Profile of the Authentication and Authorization for Constrained Environments (ACE) Framework

## Abstract

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.¶

## 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/rfc9203.¶

## Copyright Notice

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.¶

## 1. Introduction

This document specifies the `coap_oscore` profile of the ACE framework [RFC9200]. In this profile, a client (C) and a resource server (RS) use the Constrained Application Protocol (CoAP) [RFC7252] to communicate. The client uses an access token, bound to a symmetric key (the proof-of-possession (PoP) key) to authorize its access to the resource server. Note that this profile uses a symmetric-crypto-based scheme, where the symmetric secret is used as input material for keying material derivation. In order to provide communication security and PoP, the client and resource server use Object Security for Constrained RESTful Environments (OSCORE) as defined in [RFC8613]. Note that the PoP is not achieved through a dedicated protocol element but rather occurs after the first message exchange using OSCORE.¶

OSCORE specifies how to use CBOR Object Signing and Encryption (COSE) [RFC9052] [RFC9053] to secure CoAP messages. Note that OSCORE can be used to secure CoAP messages, as well as HTTP and combinations of HTTP and CoAP; a profile of ACE similar to the one described in this document, with the difference of using HTTP instead of CoAP as the communication protocol, could be specified analogously to this one.¶

### 1.1. 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.¶

Certain security-related terms such as "authentication", "authorization", "confidentiality", "(data) integrity", "Message Authentication Code (MAC)", "Hash-based Message Authentication Code (HMAC)", and "verify" are taken from [RFC4949].¶

RESTful terminology follows HTTP [RFC9110].¶

Readers are expected to be familiar with the terms and concepts defined in OSCORE [RFC8613], such as "security context" and "Recipient ID".¶

Terminology for entities in the architecture is defined in OAuth 2.0 [RFC6749], such as client (C), resource server (RS), and authorization server (AS). It is assumed in this document that a given resource on a specific RS is associated to a unique AS.¶

Concise Binary Object Representation (CBOR) [RFC8949] and Concise Data Definition Language (CDDL) [RFC8610] are used in this document. CDDL predefined type names, especially "bstr" for CBOR byte strings and "tstr" for CBOR text strings, are used extensively in this document.¶

Note that the term "endpoint" is used as in [RFC9200], following its OAuth definition, which is to denote resources such as token and introspect at the AS and authz-info at the RS. The CoAP definition, which is "[a]n entity participating in the CoAP protocol" [RFC7252], is not used in this document.¶

Throughout this document, examples for CBOR data items are expressed in CBOR extended diagnostic notation as defined in Section 8 of [RFC8949] and Appendix G of [RFC8610] ("diagnostic notation"), unless noted otherwise. We often use diagnostic notation comments to provide a textual representation of the numeric parameter names and values.¶

In this document, the term "base64-encoded" refers to URL-Safe base64 encoding (see Section 5 of [RFC4648]) without padding.¶

## 2. Protocol Overview

This section gives an overview of how to use the ACE Framework [RFC9200] to secure the communication between a client and a resource server using OSCORE [RFC8613]. The parameters needed by the client to negotiate the use of this profile with the AS, as well as the OSCORE setup process, are described in detail in the following sections.¶

The RS maintains a collection of OSCORE security contexts with associated authorization information for all the clients that it is communicating with. The authorization information is maintained as policy that is used as input to processing requests from those clients.¶

This profile requires a client to retrieve an access token from the AS for the resource it wants to access on an RS, by sending an access token request to the token endpoint, as specified in Section 5.8 of [RFC9200]. The access token request and response MUST be confidentiality protected and ensure authenticity. The use of OSCORE between the client and AS is RECOMMENDED in this profile, to reduce the number of libraries the client has to support, but other protocols fulfilling the security requirements defined in Section 5 of [RFC9200] MAY alternatively be used, such as TLS [RFC8446] or DTLS [RFC9147].¶

Once the client has retrieved the access token, it generates a nonce N1, as defined in this document (see Section 4.1.1). The client also generates its own OSCORE Recipient ID, ID1 (see Section 3.1 of [RFC8613]), for use with the keying material associated to the RS. The client posts the token, N1, and its Recipient ID to the RS using the authz-info endpoint and mechanisms specified in Section 5.8 of [RFC9200] and Content-Format = application/ace+cbor. When using this profile, the communication with the authz-info endpoint is not protected, except for the update of access rights.¶

If the access token is valid, the RS replies to this request with a 2.01 (Created) response with Content-Format = application/ace+cbor, which contains a nonce N2 and its newly generated OSCORE Recipient ID, ID2, for use with the keying material associated to the client. Moreover, the server concatenates the input salt received in the token, N1, and N2 to obtain the Master Salt of the OSCORE security context (see Section 3 of [RFC8613]). The RS then derives the complete security context associated with the received token from the Master Salt; the OSCORE Recipient ID generated by the client (set as its OSCORE Sender ID); its own OSCORE Recipient ID; plus the parameters received in the access token from the AS, following Section 3.2 of [RFC8613].¶

In a similar way, after receiving the nonce N2, the client concatenates the input salt, N1, and N2 to obtain the Master Salt of the OSCORE security context. The client then derives the complete security context from the Master Salt; the OSCORE Recipient ID generated by the RS (set as its OSCORE Sender ID); its own OSCORE Recipient ID; plus the parameters received from the AS.¶

Finally, the client starts the communication with the RS by sending a request protected with OSCORE to the RS. If the request is successfully verified, the server stores the complete security context state that is ready for use in protecting messages and uses it in the response, and in further communications with the client, until token deletion due to, for example, expiration. This security context is discarded when a token (whether the same or a different one) is used to successfully derive a new security context for that client.¶

The use of nonces N1 and N2 during the exchange prevents the reuse of an Authenticated Encryption with Associated Data (AEAD) nonce/key pair for two different messages. Reuse might otherwise occur when the client and RS derive a new security context from an existing (non-expired) access token, as might occur when either party has just rebooted, and that might lead to loss of both confidentiality and integrity. Instead, by using the exchanged nonces N1 and N2 as part of the Master Salt, the request to the authz-info endpoint posting the same token results in a different security context, by OSCORE construction, since even though the Master Secret, Sender ID, and Recipient ID are the same, the Master Salt is different (see Section 3.2.1 of [RFC8613]). If the exchanged nonces were reused, a node reusing a non-expired old token would be susceptible to on-path attackers provoking the creation of an OSCORE message using an old AEAD key and nonce.¶

After the whole message exchange has taken place, the client can contact the AS to request an update of its access rights, sending a similar request to the token endpoint that also includes an identifier so that the AS can find the correct OSCORE Input Material it has previously shared with the client. This specific identifier, encoded as a byte string, is assigned by the AS to be unique in the sets of its OSCORE Input Materials, and it is not used as input material to derive the full OSCORE security context.¶

An overview of the profile flow for the OSCORE profile is given in Figure 1. The names of messages coincide with those of [RFC9200] when applicable.¶

       C                            RS                   AS
       |                            |                     |
       | ----- POST /token  ----------------------------> |
       |                            |                     |
       | <---------------------------- Access Token ----- |
       |                           + Access Information   |
       | ---- POST /authz-info ---> |                     |
       |   (access_token, N1, ID1)  |                     |
       |                            |                     |
       | <- 2.01 Created (N2, ID2)- |                     |
       |                            |                     |
     /Sec Context             /Sec Context                |
       derivation/              derivation/               |
       |                            |                     |
       | ---- OSCORE Request -----> |                     |
       |                            |                     |
       |                    /proof-of-possession          |
       |                    Sec Context storage/          |
       |                            |                     |
       | <--- OSCORE Response ----- |                     |
       |                            |                     |
    /proof-of-possession            |                     |
    Sec Context storage/            |                     |
       |                            |                     |
       | ---- OSCORE Request -----> |                     |
       |                            |                     |
       | <--- OSCORE Response ----- |                     |
       |                            |                     |
       |           ...              |                     |

Figure 1: Protocol Overview

## 3. Client-AS Communication

The following subsections describe the details of the POST request and response to the token endpoint between the client and AS. Section 3.2 of [RFC8613] defines how to derive a security context based on a shared Master Secret and a set of other parameters, established between the client and server, which the client receives from the AS in this exchange. The PoP key included in the response from the AS MUST be used as a Master Secret in OSCORE.¶

### 3.1. C-to-AS: POST to Token Endpoint

The client-to-AS request is specified in Section 5.8.1 of [RFC9200].¶

The client must send this POST request to the token endpoint over a secure channel that guarantees authentication, message integrity, and confidentiality (see Section 5).¶

An example of such a request is shown in Figure 2.¶

        Header: POST (Code=0.02)
        Uri-Host: "as.example.com"
        Uri-Path: "token"
        Content-Format: application/ace+cbor
        Payload:
        {
          / audience / 5 : "tempSensor4711",
             / scope / 9 : "read"
        }

Figure 2: Example C-to-AS POST /token Request for an Access Token Bound to a Symmetric Key

If the client wants to update its access rights without changing an existing OSCORE security context, it MUST include a `req_cnf` object in its POST request to the token endpoint, with the `kid` field carrying a CBOR byte string containing the OSCORE Input Material identifier (assigned as discussed in Section 3.2). This identifier, together with other information such as audience (see Section 5.8.1 of [RFC9200]), can be used by the AS to determine the shared secret bound to the proof-of-possession token; therefore, it MUST identify a symmetric key that was previously generated by the AS as a shared secret for the communication between the client and the RS. The AS MUST verify that the received value identifies a proof-of-possession key that has previously been issued to the requesting client. If that is not the case, the client-to-AS request MUST be declined with the error code `invalid_request` as defined in Section 5.8.3 of [RFC9200].¶

An example of such a request is shown in Figure 3.¶

        Header: POST (Code=0.02)
        Uri-Host: "as.example.com"
        Uri-Path: "token"
        Content-Format: application/ace+cbor
        Payload:
        {
          / audience / 5 : "tempSensor4711",
             / scope / 9 : "write",
           / req_cnf / 4 : {
                 / kid / 3 : h'01'
          }
        }

Figure 3: Example C-to-AS POST /token Request for Updating Rights to an Access Token Bound to a Symmetric Key

### 3.2. AS-to-C: Access Token

After verifying the POST request to the token endpoint and that the client is authorized to obtain an access token corresponding to its access token request, the AS responds as defined in Section 5.8.2 of [RFC9200]. If the client request was invalid, or not authorized, the AS returns an error response as described in Section 5.8.3 of [RFC9200].¶

The AS can signal that the use of OSCORE is REQUIRED for a specific access token by including the `ace_profile` parameter with the value `coap_oscore` in the access token response. This means that the client MUST use OSCORE towards all resource servers for which this access token is valid, and follow Section 4.3 to derive the security context to run OSCORE. Usually, it is assumed that constrained devices will be preconfigured with the necessary profile, so that this kind of profile signaling can be omitted.¶

Moreover, the AS MUST send the following data:¶

- a Master Secret¶
- an identifier of the OSCORE Input Material¶

Additionally, the AS MAY send the following data, in the same response.¶

- a context identifier¶
- an AEAD algorithm¶
- an HMAC-based key derivation function (HKDF) algorithm [RFC5869]. It is specified by the HMAC algorithm value; see Section 3.1 of [RFC9053].¶
- a salt¶
- the OSCORE version number¶

This data is transported in the OSCORE_Input_Material. The OSCORE_Input_Material is a CBOR map object, defined in Section 3.2.1. This object is transported in the `cnf` parameter of the access token response, as defined in Section 3.2 of [RFC9201], as the value of a field named `osc`, which is registered in Sections 9.5 and 9.6.¶

The AS MAY assign an identifier to the context (context identifier). This identifier is used as ID Context in the OSCORE context as described in Section 3.1 of [RFC8613]. If assigned, these parameters MUST be communicated as the `contextId` field in the OSCORE_Input_Material. The application needs to consider that this identifier is sent in the clear and may reveal information about the endpoints, as mentioned in Section 12.8 of [RFC8613].¶

The Master Secret and the identifier of the OSCORE_Input_Material MUST be communicated as the `ms` and `id` field in the `osc` field in the `cnf` parameter of the access token response. If included, the following are sent: the AEAD algorithm in the `alg` parameter in the OSCORE_Input_Material; the HKDF algorithm in the `hkdf` parameter of the OSCORE_Input_Material; a salt in the `salt` parameter of the OSCORE_Input_Material; and the OSCORE version in the `version` parameter of the OSCORE_Input_Material.¶

The same parameters MUST be included in the claims associated with the access token. The OSCORE Master Secret MUST be encrypted by the authorization server so that only the resource server can decrypt it (see Section 6.1 of [RFC9200]). The use of a CBOR Web Token (CWT) protected with COSE_Encrypt/COSE_Encrypt0 as specified in [RFC8392] is RECOMMENDED in this profile. If the token is a CWT, the same OSCORE_Input_Material structure defined above MUST be placed in the `osc` field of the `cnf` claim of this token.¶

The AS MUST send a different OSCORE_Input_Material (and therefore different access tokens) to different authorized clients, in order for the RS to differentiate between clients.¶

Figure 4 shows an example of an AS response. The access token has been truncated for readability.¶

        Header: Created (Code=2.01)
        Content-Type: application/ace+cbor
        Payload:
        {
          / access_token / 1 : h'8343a1010aa2044c53/...
           (remainder of access token (CWT) omitted for brevity)/',
          / ace_profile / 38 : / coap_oscore / 2,
            / expires_in / 2 : 3600,
                   / cnf / 8 : {
                     / osc / 4 : {
                        / id / 0 : h'01',
                        / ms / 2 : h'f9af838368e353e78888e1426bd94e6f'
            }
          }
        }

Figure 4: Example AS-to-C Access Token Response with an OSCORE Profile

Figure 5 shows an example CWT Claims Set, including the relevant OSCORE parameters in the `cnf` claim.¶

    {
        / aud / 3 : "tempSensorInLivingRoom",
        / iat / 6 : 1360189224,
        / exp / 4 : 1360289224,
      / scope / 9 :  "temperature_g firmware_p",
        / cnf / 8 : {
          / osc / 4 : {
             / id / 0 : h'01',
             / ms / 2 : h'f9af838368e353e78888e1426bd94e6f'
        }
      }
    }

Figure 5: Example CWT Claims Set with OSCORE Parameters

The same CWT Claims Set as in Figure 5, using the value abbreviations defined in [RFC9200] and [RFC8747] and encoded in CBOR, is shown in Figure 6. The bytes in hexadecimal are reported in the first column, while their corresponding CBOR meaning is reported after the `#` sign on the second column, for readability.¶

    A5                              # map(5)
       03                           # unsigned(3)
       76                           # text(22)
          74656D7053656E736F72496E4C6976696E67526F6F6D
                                    # "tempSensorInLivingRoom"
       06                           # unsigned(6)
       1A 5112D728                  # unsigned(1360189224)
       04                           # unsigned(4)
       1A 51145DC8                  # unsigned(1360289224)
       09                           # unsigned(9)
       78 18                        # text(24)
          74656D70657261747572655F67206669726D776172655F70
                                    # "temperature_g firmware_p"
       08                           # unsigned(8)
          A1                        # map(1)
             04                     # unsigned(4)
                A2                  # map(2)
                   00               # unsigned(0)
                   41               # bytes(1)
                      01
                   02               # unsigned(2)
                   50               # bytes(16)
                      F9AF838368E353E78888E1426BD94E6F

Figure 6: Example CWT Claims Set with OSCORE Parameters Using CBOR Encoding

If the client has requested an update to its access rights using the same OSCORE security context, which is valid and authorized, the AS MUST omit the `cnf` parameter in the response and MUST carry the OSCORE Input Material identifier in the `kid` field in the `cnf` claim of the token. This identifier needs to be included in the token in order for the RS to identify the correct OSCORE Input Material.¶

Figure 7 shows an example of such an AS response. The access token has been truncated for readability.¶

        Header: Created (Code=2.01)
        Content-Type: application/ace+cbor
        Payload:
        {
          / access_token / 1 : h'8343a1010aa2044c53/ ...
           (remainder of access token (CWT) omitted for brevity)/',
          / ace_profile / 38 : / coap_oscore / 2,
            / expires_in / 2 : 3600
        }

Figure 7: Example AS-to-C Access Token Response with an OSCORE Profile for the Update of Access Rights

Figure 8 shows an example CWT Claims Set that contains the necessary OSCORE parameters in the `cnf` claim for the update of access rights.¶

      {
          / aud / 3 : "tempSensorInLivingRoom",
          / iat / 6 : 1360189224,
          / exp / 4 : 1360289224,
        / scope / 9 :  "temperature_h",
          / cnf / 8 : {
            / kid / 3 : h'01'
        }
      }

Figure 8: Example CWT Claims Set with OSCORE Parameters for the Update of Access Rights

#### 3.2.1. The OSCORE_Input_Material

An OSCORE_Input_Material is an object that represents the input material to derive an OSCORE security context, i.e., the local set of information elements necessary to carry out the cryptographic operations in OSCORE (Section 3.1 of [RFC8613]). In particular, the OSCORE_Input_Material is defined to be serialized and transported between nodes, as specified by this document, but it can also be used by other specifications if needed. The OSCORE_Input_Material can be encoded as either a JSON object or a CBOR map. The set of common parameters that can appear in an OSCORE_Input_Material can be found in the IANA "OSCORE Security Context Parameters" registry (Section 9.4), defined for extensibility, and the initial set of parameters defined in this document is specified below. All parameters are optional. Table 1 provides a summary of the OSCORE_Input_Material parameters defined in this section.¶

Table 1: OSCORE_Input_Material Parameters
name | CBOR label | CBOR type | registry | description
--- | --- | --- | --- | ---
id | 0 | byte string |  | OSCORE Input Material identifier
version | 1 | unsigned integer |  | OSCORE version
ms | 2 | byte string |  | OSCORE Master Secret value
hkdf | 3 | text string / integer | [COSE.Algorithms] values (HMAC-based) | OSCORE HKDF value
alg | 4 | text string / integer | [COSE.Algorithms] values (AEAD) | OSCORE AEAD Algorithm value
salt | 5 | byte string |  | an input to OSCORE Master Salt value
contextId | 6 | byte string |  | OSCORE ID Context value

id:
  This parameter identifies the OSCORE_Input_Material and is encoded as a byte string. In JSON, the `id` value is a base64-encoded byte string. In CBOR, the `id` type is a byte string, and it has label 0.¶
version:
  This parameter identifies the OSCORE version number, which is an unsigned integer. For more information about this field, see Section 5.4 of [RFC8613]. In JSON, the `version` value is an integer. In CBOR, the `version` type is an integer, and it has label 1.¶
ms:
  This parameter identifies the OSCORE Master Secret value, which is a byte string. For more information about this field, see Section 3.1 of [RFC8613]. In JSON, the `ms` value is a base64-encoded byte string. In CBOR, the `ms` type is byte string, and it has label 2.¶
hkdf:
  This parameter identifies the OSCORE HKDF Algorithm. For more information about this field, see Section 3.1 of [RFC8613]. The values used MUST be registered in the IANA "COSE Algorithms" registry (see [COSE.Algorithms]) and MUST be HMAC-based HKDF algorithms (see Section 3.1 of [RFC9053]). The value can be either the integer or the text-string value of the HMAC-based HKDF algorithm in the "COSE Algorithms" registry. In JSON, the `hkdf` value is a case-sensitive ASCII string or an integer. In CBOR, the `hkdf` type is a text string or integer, and it has label 3.¶
alg:
  This parameter identifies the OSCORE AEAD Algorithm. For more information about this field, see Section 3.1 of [RFC8613]. The values used MUST be registered in the IANA "COSE Algorithms" registry (see [COSE.Algorithms]) and MUST be AEAD algorithms. The value can be either the integer or the text-string value of the HMAC-based HKDF algorithm in the "COSE Algorithms" registry. In JSON, the `alg` value is a case-sensitive ASCII string or an integer. In CBOR, the `alg` type is a text string or integer, and it has label 4.¶
salt:
  This parameter identifies an input to the OSCORE Master Salt value, which is a byte string. For more information about this field, see Section 3.1 of [RFC8613]. In JSON, the `salt` value is a base64-encoded byte string. In CBOR, the `salt` type is a byte string, and it has label 5.¶
contextId:
  This parameter identifies the security context as a byte string. This identifier is used as OSCORE ID Context. For more information about this field, see Section 3.1 of [RFC8613]. In JSON, the `contextID` value is a base64-encoded byte string. In CBOR, the `contextID` type is a byte string, and it has label 6.¶

An example of JSON OSCORE_Input_Material is given in Figure 9.¶

                    "osc" : {
                      "alg" : "AES-CCM-16-64-128",
                       "id" : "AQ",
                       "ms" : "-a-Dg2jjU-eIiOFCa9lObw"
                    }

Figure 9: Example JSON OSCORE_Input_Material

The CDDL grammar describing the CBOR OSCORE_Input_Material is shown in Figure 10.¶

    OSCORE_Input_Material = {
        ? 0 => bstr,              ; id
        ? 1 => int,               ; version
        ? 2 => bstr,              ; ms
        ? 3 => tstr / int,        ; hkdf
        ? 4 => tstr / int,        ; alg
        ? 5 => bstr,              ; salt
        ? 6 => bstr,              ; contextId
        * (int / tstr) => any
    }

Figure 10: CDDL Grammar of the OSCORE_Input_Material

## 4. Client-RS Communication

The following subsections describe the details of the POST request and response to the authz-info endpoint between the client and RS. The client generates a nonce N1 and an identifier ID1 that is unique in the sets of its own Recipient IDs and posts them together with the token that includes the materials (e.g., OSCORE parameters) received from the AS to the RS. The RS then generates a nonce N2 and an identifier ID2 that is unique in the sets of its own Recipient IDs and uses Section 3.2 of [RFC8613] to derive a security context based on a shared Master Secret, the two exchanged nonces, and the two identifiers, established between the client and server. The exchanged nonces and identifiers are encoded as a CBOR byte string if CBOR is used and as a base64 string if JSON is used. This security context is used to protect all future communication between the client and RS using OSCORE, as long as the access token is valid.¶

Note that the RS and client authenticate each other by generating the shared OSCORE security context using the PoP key as the Master Secret. An attacker posting a valid token to the RS will not be able to generate a valid OSCORE security context and thus will not be able to prove possession of the PoP key. Additionally, the mutual authentication is only achieved after the client has successfully verified a response from the RS protected with the generated OSCORE security context.¶

### 4.1. C-to-RS: POST to authz-info Endpoint

The client MUST generate a nonce value N1 that is very unlikely to have been previously used with the same input keying material. The use of a 64-bit long random number as the nonce's value is RECOMMENDED in this profile. The client MUST store the nonce N1 as long as the response from the RS is not received and the access token related to it is still valid (to the best of the client's knowledge).¶

The client generates its own Recipient ID, ID1, for the OSCORE security context that it is establishing with the RS. By generating its own Recipient ID, the client makes sure that it does not collide with any of its Recipient IDs, nor with any other identifier ID1 if the client is executing this exchange with a different RS at the same time.¶

The client MUST use CoAP and the authorization information resource as described in Section 5.8.1 of [RFC9200] to transport the token, N1, and ID1 to the RS.¶

Note that the use of the payload and the Content-Format is different from what is described in Section 5.8.1 of [RFC9200], which only transports the token without any CBOR wrapping. In this profile, the client MUST wrap the token, N1, and ID1 in a CBOR map. The client MUST use the Content-Format application/ace+cbor defined in Section 8.16 of [RFC9200]. The client MUST include the access token using the `access_token` parameter; N1 using the `nonce1` parameter defined in Section 4.1.1; and ID1 using the `ace_client_recipientid` parameter defined in Section 4.1.2.¶

The communication with the authz-info endpoint does not have to be protected, except for the update of access rights case described below.¶

Note that a client may be required to repost the access token in order to complete a request, since an RS may delete a stored access token (and associated security context) at any time, for example, due to all storage space being consumed. This situation is detected by the client when it receives an AS Request Creation Hints response. Reposting the same access token will result in deriving a new OSCORE security context to be used with the RS, as different exchanged nonces will be used.¶

The client may also choose to repost the access token in order to update its OSCORE security context. In that case, the client and the RS will exchange newly generated nonces, renegotiate identifiers, and derive new keying material. The client and RS might decide to keep the same identifiers or renew them during the renegotiation.¶

Figure 11 shows an example of the request sent from the client to the RS. The access token has been truncated for readability.¶

          Header: POST (Code=0.02)
          Uri-Host: "rs.example.com"
          Uri-Path: "authz-info"
          Content-Format: application/ace+cbor
          Payload:
            {
                         / access_token / 1 : h'8343a1010aa2044c53/...
           (remainder of access token (CWT) omitted for brevity)/',
                              / nonce1 / 40 : h'018a278f7faab55a',
              / ace_client_recipientid / 43 : h'1645'
            }

Figure 11: Example C-to-RS POST /authz-info Request Using CWT

If the client has already posted a valid token, has already established a security association with the RS, and wants to update its access rights, the client can do so by posting the new token (retrieved from the AS and containing the update of access rights) to the /authz-info endpoint. The client MUST protect the request using the OSCORE security context established during the first token exchange. The client MUST only send the `access_token` field in the CBOR map in the payload; no nonce or identifier is sent. After proper verification (see Section 4.2), the RS will replace the old token with the new one, maintaining the same security context.¶

#### 4.1.1. The Nonce 1 Parameter

The `nonce1` parameter MUST be sent from the client to the RS, together with the access token, if the ACE profile used is `coap_oscore`, and the message is not an update of access rights, protected with an existing OSCORE security context. The parameter is encoded as a byte string for CBOR-based interactions and as a string (base64-encoded binary) for JSON-based interactions. This parameter is registered in Section 9.2.¶

#### 4.1.2. The ace_client_recipientid Parameter

The `ace_client_recipientid` parameter MUST be sent from the client to the RS, together with the access token, if the ACE profile used is `coap_oscore`, and the message is not an update of access rights, protected with an existing OSCORE security context. The parameter is encoded as a byte string for CBOR-based interactions and as a string (base64-encoded binary) for JSON-based interactions. This parameter is registered in Section 9.2.¶

### 4.2. RS-to-C: 2.01 (Created)

The RS MUST follow the procedures defined in Section 5.8.1 of [RFC9200]: the RS must verify the validity of the token. If the token is valid, the RS must respond to the POST request with 2.01 (Created). If the token is valid but is associated to claims that the RS cannot process (e.g., an unknown scope), or if any of the expected parameters are missing (e.g., any of the mandatory parameters from the AS or the identifier ID1), or if any parameters received in the `osc` field are unrecognized, the RS must respond with an error response code equivalent to the CoAP code 4.00 (Bad Request). In the latter two cases, the RS may provide additional information in the error response, in order to clarify what went wrong. The RS may make an introspection request (see Section 5.9.1 of [RFC9200]) to validate the token before responding to the POST request to the authz-info endpoint.¶

Additionally, the RS MUST generate a nonce N2 that is very unlikely to have been previously used with the same input keying material and its own Recipient ID, ID2. The RS makes sure that ID2 does not collide with any of its Recipient IDs. The RS MUST ensure that ID2 is different from the value received in the `ace_client_recipientid` parameter. The RS sends N2 and ID2 within the 2.01 (Created) response. The payload of the 2.01 (Created) response MUST be a CBOR map containing the `nonce2` parameter defined in Section 4.2.1, set to N2, and the `ace_server_recipientid` parameter defined in Section 4.2.2, set to ID2. The use of a 64-bit long random number as the nonce's value is RECOMMENDED in this profile. The RS MUST use the Content-Format application/ace+cbor defined in Section 8.16 of [RFC9200].¶

Figure 12 shows an example of the response sent from the RS to the client.¶

          Header: Created (Code=2.01)
          Content-Format: application/ace+cbor
          Payload:
            {
                              / nonce2 / 42 : h'25a8991cd700ac01',
              / ace_server_recipientid / 44 : h'0000'
            }

Figure 12: Example RS-to-C 2.01 (Created) Response

As specified in Section 5.8.3 of [RFC9200], the RS must notify the client with an error response with code 4.01 (Unauthorized) for any long running request before terminating the session, when the access token expires.¶

If the RS receives the token in an OSCORE-protected message, it means that the client is requesting an update of access rights. The RS MUST ignore any nonce and identifiers in the request, if any were sent. The RS MUST check that the `kid` of the `cnf` claim of the new access token matches the identifier of the OSCORE Input Material of the context used to protect the message. If that is the case, the RS MUST overwrite the old token and associate the new token to the security context identified by the `kid` value in the `cnf` claim. The RS MUST respond with a 2.01 (Created) response protected with the same security context, with no payload. If any verification fails, the RS MUST respond with a 4.01 (Unauthorized) error response.¶

As specified in Section 5.8.1 of [RFC9200], when receiving an updated access token with updated authorization information from the client (see Section 3.1), it is recommended that the RS overwrites the previous token; that is, only the latest authorization information in the token received by the RS is valid. This simplifies the process needed by the RS to keep track of authorization information for a given client.¶

#### 4.2.1. The Nonce 2 Parameter

The `nonce2` parameter MUST be sent from the RS to the client if the ACE profile used is `coap_oscore` and the message is not a response to an update of access rights, protected with an existing OSCORE security context. The parameter is encoded as a byte string for CBOR-based interactions and as a string (base64-encoded binary) for JSON-based interactions. This parameter is registered in Section 9.2¶

#### 4.2.2. The ace_server_recipientid Parameter

The `ace_server_recipientid` parameter MUST be sent from the RS to the client if the ACE profile used is `coap_oscore` and the message is not a response to an update of access rights, protected with an existing OSCORE security context. The parameter is encoded as a byte string for CBOR-based interactions and as a string (base64-encoded binary) for JSON-based interactions. This parameter is registered in Section 9.2¶

### 4.3. OSCORE Setup

Once the 2.01 (Created) response is received from the RS, following the POST request to authz-info endpoint, the client MUST extract the bstr nonce N2 from the `nonce2` parameter in the CBOR map in the payload of the response. Then, the client MUST set the Master Salt of the security context created to communicate with the RS to the concatenation of salt, N1, and N2 in this order: Master Salt = salt | N1 | N2, where | denotes byte string concatenation, salt is the CBOR byte string received from the AS in Section 3.2, and N1 and N2 are the two nonces encoded as CBOR byte strings. An example of Master Salt construction using CBOR encoding is given in Figure 13.¶

    N1, N2, and input salt expressed in CBOR diagnostic notation:
          nonce1 = h'018a278f7faab55a'
          nonce2 = h'25a8991cd700ac01'
          input salt = h'f9af838368e353e78888e1426bd94e6f'

    N1, N2, and input salt as CBOR encoded byte strings:
          nonce1 = 0x48018a278f7faab55a
          nonce2 = 0x4825a8991cd700ac01
          input salt = 0x50f9af838368e353e78888e1426bd94e6f

    Master Salt = 0x50 f9af838368e353e78888e1426bd94e6f
                    48 018a278f7faab55a 48 25a8991cd700ac01

Figure 13: Example of Master Salt Construction Using CBOR Encoding

If JSON is used instead of CBOR, the Master Salt of the security context is the base64 encoding of the concatenation of the same parameters, each of them prefixed by their size, encoded in 1 byte. When using JSON, the nonces and input salt have a maximum size of 255 bytes. An example of Master Salt construction using base64 encoding is given in Figure 14.¶

    N1, N2, and input salt values:
          nonce1 = 0x018a278f7faab55a (8 bytes)
          nonce2 = 0x25a8991cd700ac01 (8 bytes)
          input salt = 0xf9af838368e353e78888e1426bd94e6f (16 bytes)

    Input to base64 encoding: 0x10 f9af838368e353e78888e1426bd94e6f
                                08 018a278f7faab55a 08 25a8991cd700ac01

    Master Salt = b64'EPmvg4No41PniIjhQmvZTm8IAYonj3+qtVoIJaiZHNcArAE='

Figure 14: Example of Master Salt Construction Using Base64 Encoding

The client MUST set the Sender ID to the `ace_server_recipientid` received in Section 4.2 and set the Recipient ID to the `ace_client_recipientid` sent in Section 4.1. The client MUST set the Master Secret from the parameter received from the AS in Section 3.2. The client MUST set the AEAD algorithm, ID Context, HKDF, and OSCORE version from the parameters received from the AS in Section 3.2, if present. In case an optional parameter is omitted, the default value SHALL be used as described in Sections 3.2 and 5.4 of [RFC8613]. After that, the client MUST derive the complete security context following Section 3.2.1 of [RFC8613]. From this point on, the client MUST use this security context to communicate with the RS when accessing the resources as specified by the authorization information.¶

If any of the expected parameters are missing (e.g., any of the mandatory parameters from the AS or the RS), or if `ace_client_recipientid` equals `ace_server_recipientid` (and as a consequence, the Sender and Recipient Keys derived would be equal; see Section 3.3 of [RFC8613]), then the client MUST stop the exchange and MUST NOT derive the security context. The client MAY restart the exchange, to get the correct security material.¶

The client then uses this security context to send requests to the RS using OSCORE.¶

After sending the 2.01 (Created) response, the RS MUST set the Master Salt of the security context created to communicate with the client to the concatenation of salt, N1, and N2 in the same way described above. An example of Master Salt construction using CBOR encoding is given in Figure 13 and using base64 encoding is given in Figure 14. The RS MUST set the Sender ID from the `ace_client_recipientid` received in Section 4.1 and set the Recipient ID from the `ace_server_recipientid` sent in Section 4.2. The RS MUST set the Master Secret from the parameter received from the AS and forwarded by the client in the access token in Section 4.1 after validation of the token as specified in Section 4.2. The RS MUST set the AEAD algorithm, ID Context, HKDF, and OSCORE version from the parameters received from the AS and forwarded by the client in the access token in Section 4.1 after validation of the token as specified in Section 4.2, if present. In case an optional parameter is omitted, the default value SHALL be used as described in Sections 3.2 and 5.4 of [RFC8613]. After that, the RS MUST derive the complete security context following Section 3.2.1 of [RFC8613] and MUST associate this security context with the authorization information from the access token.¶

The RS then uses this security context to verify requests and send responses to the client using OSCORE. If OSCORE verification fails, error responses are used, as specified in Section 8 of [RFC8613]. Additionally, if OSCORE verification succeeds, the verification of access rights is performed as described in Section 4.4. The RS MUST NOT use the security context after the related token has expired and MUST respond with an unprotected 4.01 (Unauthorized) error message to requests received that correspond to a security context with an expired token.¶

Note that the ID Context can be assigned by the AS, communicated and set in both the RS and client after the exchange specified in this profile is executed. Subsequently, the client and RS can update their ID Context by running a mechanism such as the one defined in Appendix B.2 of [RFC8613] if they both support it and are configured to do so. In that case, the ID Context in the OSCORE security context will not match the `contextId` parameter of the corresponding OSCORE_Input_Material. Running Appendix B.2 results in the keying material being updated in the security contexts of the client and RS; this same result can also be achieved by the client reposting the access token to the unprotected /authz-info endpoint at the RS, as described in Section 4.1, but without updating the ID Context.¶

### 4.4. Access Rights Verification

The RS MUST follow the procedures defined in Section 5.8.2 of [RFC9200]: if an RS receives an OSCORE-protected request from a client, then the RS processes it according to [RFC8613]. If OSCORE verification succeeds, and the target resource requires authorization, the RS retrieves the authorization information using the access token associated to the security context. The RS then must verify that the authorization information covers the resource and the action requested.¶

## 5. Secure Communication with AS

As specified in the ACE framework (Section 5.9 of [RFC9200]), the requesting entity (RS and/or client) and the AS communicates via the introspection or token endpoint. The use of CoAP and OSCORE [RFC8613] for this communication is RECOMMENDED in this profile; other protocols fulfilling the security requirements defined in Section 5 of [RFC9200] (such as HTTP and DTLS or TLS) MAY be used instead.¶

If OSCORE is used, the requesting entity and the AS are expected to have preestablished security contexts in place. How these security contexts are established is out of scope for this profile. Furthermore, the requesting entity and the AS communicate through the introspection endpoint as specified in Section 5.9 of [RFC9200] and through the token endpoint as specified in Section 5.8 of [RFC9200].¶

## 6. Discarding the Security Context

There are a number of scenarios where a client or RS needs to discard the OSCORE security context and acquire a new one.¶

The client MUST discard the current security context associated with an RS when any of the following occurs:¶

- the sequence number space ends.¶
- the access token associated with the context becomes invalid due to, for example, expiration.¶
- the client receives a number of 4.01 Unauthorized responses to OSCORE requests using the same security context. The exact number needs to be specified by the application.¶
- the client receives a new nonce in the 2.01 (Created) response (see Section 4.2) to a POST request to the authz-info endpoint, when reposting a (non-expired) token associated to the existing context.¶

The RS MUST discard the current security context associated with a client when any of the following occurs:¶

- the sequence number space ends.¶
- the access token associated with the context expires.¶
- the client has successfully replaced the current security context with a newer one by posting an access token to the unprotected /authz-info endpoint at the RS, e.g., by reposting the same token, as specified in Section 4.1.¶

Whenever one more access token is successfully posted to the RS, and a new security context is derived between the client and RS, messages in transit that were protected with the previous security context might not pass verification, as the old context is discarded. That means that messages sent shortly before the client posts one more access tokens to the RS might not successfully reach the destination. Analogously, implementations may want to cancel CoAP observations at the RS registered before the security context is replaced, or conversely, they will need to implement a mechanism to ensure that those observations are to be protected with the newly derived security context.¶

## 7. Security Considerations

This document specifies a profile for the ACE framework [RFC9200]. Thus, the general security considerations from the framework also apply to this profile.¶

Furthermore, the general security considerations of OSCORE [RFC8613] also apply to this specific use of the OSCORE protocol.¶

As previously stated, the proof of possession in this profile is performed by both parties verifying that they have established the same security context, as specified in Section 4.3, which means that both the OSCORE request and the OSCORE response passes verification. RS authentication requires both that the client trusts the AS and that the OSCORE response from the RS passes verification.¶

OSCORE is designed to secure point-to-point communication, providing a secure binding between the request and the response(s). Thus, the basic OSCORE protocol is not intended for use in point-to-multipoint communication (e.g., multicast, publish-subscribe). Implementers of this profile should make sure that their use case corresponds to the expected use of OSCORE, to prevent weakening the security assurances provided by OSCORE.¶

Since the use of nonces N1 and N2 during the exchange guarantees uniqueness of AEAD keys and nonces, it is REQUIRED that the exchanged nonces are not reused with the same input keying material even in case of reboots. The exchange of 64-bit random nonces is RECOMMENDED in this document. Considering the birthday paradox, the average collision for each nonce will happen after 232 messages, which is considerably more token provisionings than would be expected for intended applications. If applications use something else, such as a counter, they need to guarantee that reboot and loss of state on either node does not provoke reuse. If that is not guaranteed, nodes are susceptible to reuse of AEAD (nonce, key) pairs, especially since an on-path attacker can cause the use of a previously exchanged client nonce N1 for security context establishment by replaying the corresponding client-to-server message.¶

In this profile, it is RECOMMENDED that the RS maintains a single access token for each client. The use of multiple access tokens for a single client increases the strain on the resource server as it must consider every access token and calculate the actual permissions of the client. Also, tokens indicating different or disjoint permissions from each other may lead the server to enforce wrong permissions. If one of the access tokens expires earlier than others, the resulting permissions may offer insufficient protection. Developers SHOULD avoid using multiple access tokens for the same client.¶

If a single OSCORE Input Material is used with multiple RSs, the RSs can impersonate the client to one of the other RSs and impersonate another RS to the client. If a Master Secret is used with several clients, the clients can impersonate RS to one of the other clients. Similarly, if symmetric keys are used to integrity protect the token between AS and RS and the token can be used with multiple RSs, the RSs can impersonate AS to one of the other RSs. If the token key is used for any other communication between the RSs and AS, the RSs can impersonate each other to the AS.¶

## 8. Privacy Considerations

This document specifies a profile for the ACE framework [RFC9200]. Thus, the general privacy considerations from the framework also apply to this profile.¶

As this document uses OSCORE, the privacy considerations from [RFC8613] apply here as well.¶

An unprotected response to an unauthorized request may disclose information about the resource server and/or its existing relationship with the client. It is advisable to include as little information as possible in an unencrypted response. When an OSCORE security context already exists between the client and the resource server, more detailed information may be included.¶

The token is sent in the clear to the authz-info endpoint, so if a client uses the same single token from multiple locations with multiple resource servers, it can risk being tracked by the token's value even when the access token is encrypted.¶

The nonces exchanged in the request and response to the authz-info endpoint are also sent in the clear, so using random nonces is best for privacy (as opposed to, e.g., a counter, which might leak some information about the client).¶

The identifiers used in OSCORE, negotiated between the client and RS, are privacy sensitive (see Section 12.8 of [RFC8613]) and could reveal information about the client, or they may be used for correlating requests from one client.¶

Note that some information might still leak after OSCORE is established, due to observable message sizes, the source, and the destination addresses.¶

## 9. IANA Considerations

### 9.1. ACE Profile Registry

The following registration has been made in the "ACE Profiles" registry following the procedure specified in Section 8.8 of [RFC9200]:¶

Name:
  coap_oscore¶
Description:
  Profile for using OSCORE to secure communication between constrained nodes using the Authentication and Authorization for Constrained Environments framework.¶
CBOR Value:
  2¶
Reference:
  RFC 9203¶

### 9.2. OAuth Parameters Registry

The following registrations have been made in the "OAuth Parameters" registry [IANA.OAuthParameters] following the procedure specified in Section 11.2 of [RFC6749]:¶

Parameter name:
  nonce1¶
Parameter usage location:
  client-rs request¶
Change Controller:
  IETF¶
Specification Document(s):
  RFC 9203¶

Parameter name:
  nonce2¶
Parameter usage location:
  rs-client response¶
Change Controller:
  IETF¶
Specification Document(s):
  RFC 9203¶

Parameter name:
  ace_client_recipientid¶
Parameter usage location:
  client-rs request¶
Change Controller:
  IETF¶
Specification Document(s):
  RFC 9203¶

Parameter name:
  ace_server_recipientid¶
Parameter usage location:
  rs-client response¶
Change Controller:
  IETF¶
Specification Document(s):
  RFC 9203¶

### 9.3. OAuth Parameters CBOR Mappings Registry

The following registrations have been made in the "OAuth Parameters CBOR Mappings" registry following the procedure specified in Section 8.10 of [RFC9200]:¶

Name:
  nonce1¶
CBOR Key:
  40¶
Value Type:
  bstr¶
Reference:
  RFC 9203¶

Name:
  nonce2¶
CBOR Key:
  42¶
Value Type:
  bstr¶
Reference:
  RFC 9203¶

Name:
  ace_client_recipientid¶
CBOR Key:
  43¶
Value Type:
  bstr¶
Reference:
  RFC 9203¶

Name:
  ace_server_recipientid¶
CBOR Key:
  44¶
Value Type:
  bstr¶
Reference:
  RFC 9203¶

### 9.4. OSCORE Security Context Parameters Registry

IANA has created a new registry entitled "OSCORE Security Context Parameters". The registration procedure depends on the range of CBOR label values, following [RFC8126]. Guidelines for the experts are provided in Section 9.7.¶

The columns of the registry are:¶

Name:
  The JSON name requested (e.g., "ms"). Because a core goal of this document is for the resulting representations to be compact, it is RECOMMENDED that the name be short. This name is case sensitive. Names may not match other registered names in a case-insensitive manner unless the designated experts determine that there is a compelling reason to allow an exception. The name is not used in the CBOR encoding.¶
CBOR Label:
  The value to be used to identify this name. Map key labels MUST be unique. The label can be a positive integer, a negative integer, or a string. Integer values between -256 and 255 and strings of length 1 are designated as Standards Track document required. Integer values from -65536 to -257 and from 256 to 65535 and strings of length 2 are designated as Specification Required. Integer values greater than 65535 and strings of length greater than 2 are designated as Expert Review. Integer values less than -65536 are marked as Private Use.¶
CBOR Type:
  This field contains the CBOR type for the field.¶
Registry:
  This field denotes the registry that values may come from, if one exists.¶
Description:
  This field contains a brief description for the field.¶
Reference:
  This contains a pointer to the public specification for the field, if one exists.¶

This registry has been initially populated by the values in Table 1. The Reference column for all of these entries is this document.¶

### 9.5. CWT Confirmation Methods Registry

The following registration has been made in the "CWT Confirmation Methods" registry [IANA.CWTConfirmationMethods] following the procedure specified in Section 7.2.1 of [RFC8747]:¶

Confirmation Method Name:
  osc¶
Confirmation Method Description:
  OSCORE_Input_Material carrying the parameters for using OSCORE per-message security with implicit key confirmation¶
JWT Confirmation Method Name:
  osc¶
Confirmation Key:
  4¶
Confirmation Value Type(s):
  map¶
Change Controller:
  IETF¶
Specification Document(s):
  Section 3.2.1 of RFC 9203¶

### 9.6. JWT Confirmation Methods Registry

The following registration has been made in the "JWT Confirmation Methods" registry [IANA.JWTConfirmationMethods] following the procedure specified in Section 6.2.1 of [RFC7800]:¶

Confirmation Method Value:
  osc¶
Confirmation Method Description:
  OSCORE_Input_Material carrying the parameters for using OSCORE per-message security with implicit key confirmation¶
Change Controller:
  IETF¶
Specification Document(s):
  Section 3.2.1 of RFC 9203¶

### 9.7. Expert Review Instructions

The IANA registry established in this document is defined to use the Expert Review registration policy. This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason, so they should be given substantial latitude.¶

Expert reviewers should take into consideration the following points:¶

- Point squatting should be discouraged. Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered and that the point is likely to be used in deployments. The zones tagged as Private Use are intended for testing purposes and closed environments. Code points in other ranges should not be assigned for testing.¶
- Specifications are required for the Standards Track range of point assignment. Specifications should exist for specification required ranges, but early assignment before a specification is available is considered to be permissible. Specifications are needed for the First Come First Served range if they are expected to be used outside of closed environments in an interoperable way. When specifications are not provided, the description provided needs to have sufficient information to identify what the point is being used for.¶
- Experts should take into account the expected usage of fields when approving point assignment. The fact that there is a range for Standards Track documents does not mean that a Standards Track document cannot have points assigned outside of that range. The length of the encoded value should be weighed against how many code points of that length are left, the size of device it will be used on, and the number of code points left that encode to that size.¶

## 10. References

### 10.1. Normative References

[COSE.Algorithms]
  IANA, "COSE Algorithms", <https://www.iana.org/assignments/cose>.
[IANA.CWTConfirmationMethods]
  IANA, "CWT Confirmation Methods", <https://www.iana.org/assignments/cwt>.
[IANA.JWTConfirmationMethods]
  IANA, "JWT Confirmation Methods", <https://www.iana.org/assignments/jwt>.
[IANA.OAuthParameters]
  IANA, "OAuth Parameters", <https://www.iana.org/assignments/oauth-parameters>.
[RFC2119]
  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC4648]
  Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, <https://www.rfc-editor.org/info/rfc4648>.
[RFC5869]
  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010, <https://www.rfc-editor.org/info/rfc5869>.
[RFC7252]
  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, <https://www.rfc-editor.org/info/rfc7252>.
[RFC8174]
  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8392]
  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, May 2018, <https://www.rfc-editor.org/info/rfc8392>.
[RFC8610]
  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[RFC8613]
  Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, <https://www.rfc-editor.org/info/rfc8613>.
[RFC8949]
  Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, December 2020, <https://www.rfc-editor.org/info/rfc8949>.
[RFC9052]
  Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, August 2022, <https://www.rfc-editor.org/info/rfc9052>.
[RFC9053]
  Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, August 2022, <https://www.rfc-editor.org/info/rfc9053>.
[RFC9200]
  Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "Authentication and Authorization for Constrained Environments Using the OAuth 2.0 Framework (ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022, <https://www.rfc-editor.org/info/rfc9200>.
[RFC9201]
  Seitz, L., "Additional OAuth Parameters for Authentication and Authorization for Constrained Environments (ACE)", RFC 9201, DOI 10.17487/RFC9201, August 2022, <https://www.rfc-editor.org/info/rfc9201>.

### 10.2. Informative References

[RFC4949]
  Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, <https://www.rfc-editor.org/info/rfc4949>.
[RFC6749]
  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012, <https://www.rfc-editor.org/info/rfc6749>.
[RFC7800]
  Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)", RFC 7800, DOI 10.17487/RFC7800, April 2016, <https://www.rfc-editor.org/info/rfc7800>.
[RFC8126]
  Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>.
[RFC8446]
  Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.
[RFC8747]
  Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. Tschofenig, "Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March 2020, <https://www.rfc-editor.org/info/rfc8747>.
[RFC9110]
  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/info/rfc9110>.
[RFC9147]
  Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022, <https://www.rfc-editor.org/info/rfc9147>.

## Appendix A. Profile Requirements

This section lists the specifications of this profile based on the requirements of the framework, as requested in Appendix C of [RFC9200].¶

- Optionally, define new methods for the client to discover the necessary permissions and AS for accessing a resource, different from the one proposed in [RFC9200]: Not specified¶
- Optionally, specify new grant types: Not specified¶
- Optionally, define the use of client certificates as client credential type: Not specified¶
- Specify the communication protocol the client and RS must use: CoAP¶
- Specify the security protocol the client and RS must use to protect their communication: OSCORE¶
- Specify how the client and the RS mutually authenticate: Implicitly by possession of a common OSCORE security context. Note that the mutual authentication is not completed before the client has verified an OSCORE response using this security context.¶
- Specify the proof-of-possession protocol(s) and how to select one, if several are available. Also specify which key types (e.g., symmetric/asymmetric) are supported by a specific proof-of- possession protocol: OSCORE algorithms; preestablished symmetric keys¶
- Specify a unique ace_profile identifier: coap_oscore¶
- If introspection is supported, specify the communication and security protocol for introspection: HTTP/CoAP (+ TLS/DTLS/OSCORE)¶
- Specify the communication and security protocol for interactions between client and AS: HTTP/CoAP (+ TLS/DTLS/OSCORE)¶
- Specify if/how the authz-info endpoint is protected, including how error responses are protected: Not protected¶
- Optionally, define methods of token transport other than the authz-info endpoint: Not defined¶

## Acknowledgments

The authors wish to thank Jim Schaad and Marco Tiloca for the substantial input to this document, as well as Carsten Bormann, Elwyn Davies, Linda Dunbar, Roman Danyliw, Martin Duke, Lars Eggert, Murray Kucherawy, and Zaheduzzaman Sarker for their reviews and feedback. Special thanks to the responsible area director Benjamin Kaduk for his extensive review and contributed text. Ludwig Seitz worked on this document as part of the CelticNext projects CyberWI and CRITISEC with funding from Vinnova. The work on this document has been partly supported also by the H2020 project SIFIS-Home (Grant agreement 952652).¶

## Authors' Addresses