Publish-Subscribe Profile for Authentication and Authorization for Constrained Environments (ACE)

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.¶

Readers are expected to be familiar with:¶

• The terms and concepts described in the ACE framework for Authentication and Authorization [RFC9200]. The terminology for entities in the considered architecture is defined in OAuth 2.0 [RFC6749]. In particular, this includes Client, Resource Server (RS), and Authorization Server (AS).¶

• The Authorization Information Format (AIF) [RFC9237] used to express authorization information.¶

• The terms and concept related to the message formats and processing specified in [I-D.ietf-ace-key-groupcomm], for provisioning and renewing keying material in group communication scenarios. These include the abbreviations REQx and OPTx denoting the numbered mandatory-to-address and optional-to-address requirements, respectively.¶

• The terms and concepts described in CDDL [RFC8610], CBOR [RFC8949], and COSE [RFC9052][RFC9053][RFC9338].¶

• The terms and concepts described in CoAP [RFC7252]. Note that 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".¶

• The terms and concepts of Pub-Sub group communication with CoAP, as described in [I-D.ietf-core-coap-pubsub].¶

A party interested in participating in group communication as well as already participating as a group member is interchangeably denoted as "Client", "Pub-Sub client", or "node".¶

• Group: a set of nodes that share common keying material and security parameters to protect their communications with one another. That is, the term refers to a "security group".¶

  This is not to be confused with an "application group", which has relevance at the application level and whose members are in this case the Clients acting as Publishers and/or Subscribers for a topic.¶

Examples throughout this document are expressed in CBOR diagnostic notation as defined in Section 8 of [RFC8949] and Appendix G of [RFC8610]. Diagnostic notation comments are often used to provide a textual representation of the numeric parameter names and values.¶

3.1. Topic Discovery at the Broker (Optional)

The discovery of a topic at the Broker can be performed by discovering the corresponding topic resource hosted at the Broker. For example, the Client can send a lookup request to /.well-known/core at the Broker, specifying as lookup criterion the resource type "core.ps.conf" (see Section 2.3.3 of [I-D.ietf-core-coap-pubsub]).¶

Although the links to the topic resources are also specified in the representation of the collection resource at the Broker (see Section 2.4 of [I-D.ietf-core-coap-pubsub]), the Client is not supposed to access such a resource, as intended for administrative operations that are out of the scope of this document.¶

3.2. AS Discovery at the Broker (Optional)

Complementary to what is defined in Section 5.1 of [RFC9200] for AS discovery, the Broker MAY send the address of the AS to the Client in the 'AS' parameter of the AS Request Creation Hints, as a response to an Unauthorized Resource Request (see Section 5.2 of [RFC9200]). An example using the CBOR diagnostic notation and CoAP is given below.¶

    4.01 Unauthorized
    Content-Format: application/ace+cbor
    Payload:
    {
     / AS / 1 : "coaps://as.example.com/token"
    }

3.3. KDC Discovery at the Broker (Optional)

Once a Client has obtained an access token from the AS and accordingly established a secure association with the Broker, the Client has the permission to access the topic resources at the Broker that pertain to the topics on which the Client is authorized to operate.¶

In particular the Client is authorized to retrieve the representation of a topic resource, from which the Client can retrieve information related to the topic in question, as specified in Section 2.5 of [I-D.ietf-core-coap-pubsub].¶

This profile extends the set of CoAP Pub-Sub Parameters that is possible to specify within the representation of a topic resource, as originally defined in Section 3 of [I-D.ietf-core-coap-pubsub]. In particular, this profile defines the following two parameters that the Broker can specify in a response from a topic resource (see Section 2.5 of [I-D.ietf-core-coap-pubsub]). Note that, when these parameters are transported in their respective fields of the message payload, the Content-Format application/core-pubsub+cbor defined in [I-D.ietf-core-coap-pubsub] MUST be used.¶

• 'kdc_uri', with value the URI of the group membership resource at the KDC, where Clients can send a request to join the security group associated with the topic in question. The URI is encoded as a CBOR text string. Clients will have to obtain an access token from the AS to upload to the KDC, before starting the process to join the security group at the KDC.¶

• 'sec_gp', specifying the name of the security group associated with the topic in question, as a stable and invariant identifier. The name of the security group is encoded as a CBOR text string.¶

Furthermore, the Resource Type (rt=) Link Target Attribute value "core.ps.gm" is registered in Section 9.3 (REQ10), and can be used to describe group-membership resources at KDC, e.g., by using a link-format document [RFC6690]. As an alternative to the discovery approach defined above and provided by the Broker, applications can use this common resource type to discover links to group-membership resources at the KDC for joining security groups associated with Pub-Sub topics.¶

3.4. Authorisation Request/Response for the KDC and the Broker

A Client sends two Authorisation Requests to the AS, targeting two different audiences, i.e., the Broker and the KDC.¶

As to the former, the AS handles Authorisation Requests related to a topic for which the Client is allowed to perform topic data operations at the Broker, as corresponding to an application group.¶

As to the latter, the AS handles Authorization Requests for security groups that the Client is allowed to join, in order to obtain the group keying material for protecting end-to-end and verifying the content of exchanged messages on the associated Pub-Sub topics.¶

This section builds on Section 3 of [I-D.ietf-ace-key-groupcomm] and defines only additions or modifications to that specification.¶

Both Authorisation Requests include the following fields (see Section 3.1 of [I-D.ietf-ace-key-groupcomm]):¶

• 'scope': Optional. If present, it specifies the following information, depending on the specifically targeted audience.¶

  If the audience is the Broker, the scope specifies the name of the topics that the Client wishes to access, together with the corresponding requested permissions. If the audience is the KDC, the scope specifies the name of the security groups that the Client wishes to join, together with the corresponding requested permissions.¶

  This parameter is encoded as a CBOR byte string, whose value is the binary encoding of a CBOR array. The format of the encoded scope MUST follow the data model AIF-PUBSUB-GROUPCOMM defined in Section 3.4.1.¶

• 'audience': Required identifier corresponding to either the Broker or the KDC.¶

Other additional parameters can be included if necessary, as defined in [RFC9200].¶

When using this profile, it is expected that a one-to-one mapping is enforced between the application group and the security group (see Section 2). If this is not the case, the correct access policies for both sets of scopes have to be available to the AS.¶

      AIF-Generic<Toid, Tperm> = [* [Toid, Tperm]]

  AIF-PUBSUB-GROUPCOMM = AIF-Generic<pubsub-group, pubsub-perm>
   pubsub-group = tstr ; name of Pub-Sub topic or of
                       ; the associated security group

   pubsub-perm = uint .bits pubsub-perm-details

   pubsub-perm-details = &(
    Admin: 0,
    AppGroup: 1
    Publish: 2,
    Read: 3,
    Delete: 4
   )

   scope_entry = [pubsub-group, pubsub-perm]

3.5. Authorisation response

The AS responds with an Authorization Response as defined in Section 5.8.2 of [RFC9200] and Section 3.2 of [I-D.ietf-ace-key-groupcomm], with the following additions:¶

• In the Authorization Response, the AS MUST include the 'expires_in' parameter. Other means for the AS to specify the lifetime of access tokens are out of the scope of this document.¶

• In the Authorization Response, the AS MUST include the 'scope' parameter, when the value included in the access token differs from the one specified by the Client in the Authorization Request. In such a case, the second element of each scope entry specifies the set of operations that the Client is authorized for that scope entry, encoded as specified in Section 3.4.¶

Furthermore, the AS MAY use the extended format of scope defined in Section 7 of [I-D.ietf-ace-key-groupcomm] for the 'scope' claim of the access token. In such a case, the AS MUST use the CBOR tag with tag number TAG_NUMBER, associated with the CoAP Content-Format CF_ID for the media type application/aif+cbor registered in Section 9.5 of this document (REQ28).¶

Note to RFC Editor: In the previous paragraph, please replace "TAG_NUMBER" with the CBOR tag number computed as TN(ct) in Section 4.3 of [RFC9277], where ct is the ID assigned to the CoAP Content-Format registered in Section 9.5 of this document. Then, please replace "CF_ID" with the ID assigned to that CoAP Content-Format. Finally, please delete this paragraph.¶

This indicates that the binary encoded scope follows the scope semantics defined for this application profile in Section 3.4.1 of this document.¶

3.6. Token Transfer to KDC

The Client transfers its access token to the KDC using one of the methods defined in the Section 3.3 of [I-D.ietf-ace-key-groupcomm]. This typically includes sending a POST request to the authz-info endpoint. However, if the DTLS transport profile of ACE [RFC9202] is used and the Client uses a symmetric proof-of-possession key in the DTLS handshake, the Client MAY provide the access token to the KDC in the "psk_identity" field of the DTLS ClientKeyExchange message when using DTLS 1.2 [RFC6347], or in the "identity" field of a PskIdentity within the PreSharedKeyExtension of the ClientHello message when using DTLS 1.3 [RFC9147]. In addition to that, the following applies.¶

In the Token Transfer Response to the Publishers, i.e., the Clients whose scope of the access token includes the "Publish" permission for at least one scope entry, the KDC MUST include the parameter 'kdcchallenge' in the CBOR map. 'kdcchallenge' is a challenge N_S generated by the KDC, and is RECOMMENDED to be an 8-byte long random nonce. Later when joining the group, the Publisher can use the 'kdcchallenge' as part of proving possession of its private key (see [I-D.ietf-ace-key-groupcomm]). If a Publisher provides the access token to the KDC through an authz-info endpoint, the Client MUST support the parameter 'kdcchallenge'.¶

If 'sign_info' is included in the Token Transfer Request, the KDC SHOULD include the 'sign_info' parameter in the Token Transfer Response. Note that the joining node may have obtained such information by alternative means, e.g., the 'sign_info' may have been pre-configured (OPT3).¶

The following applies for each element 'sign_info_entry'.¶

• 'sign_alg' MUST take its value from the "Value" column of one of the recommended algorithms in the "COSE Algorithms" registry [IANA.cose_algorithms] (REQ3).¶

• 'sign_parameters' is a CBOR array. Its format and value are the same of the COSE capabilities array for the algorithm indicated in 'sign_alg' under the "Capabilities" column of the "COSE Algorithms" registry [IANA.cose_algorithms] (REQ4).¶

• 'sign_key_parameters' is a CBOR array. Its format and value are the same of the COSE capabilities array for the COSE key type of the keys used with the algorithm indicated in 'sign_alg', as specified for that key type in the "Capabilities" column of the "COSE Key Types" registry [IANA.cose_key-type] (REQ5).¶

• 'cred_fmt' takes value from the "Label" column of the "COSE Header Parameters" registry [IANA.cose_header-parameters] (REQ6). Acceptable values denote a format of authentication credential that MUST explicitly provide the public key as well as the comprehensive set of information related to the public key algorithm, including, e.g., the used elliptic curve (when applicable). Acceptable formats of authentication credentials include CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs) [RFC8392], X.509 certificates [RFC7925], and C509 certificates [I-D.ietf-cose-cbor-encoded-cert]. Future formats would be acceptable to use as long as they comply with the criteria defined above.¶

4.1. Joining a Security Group

This section describes the interactions between a Client and the KDC to join a security group. Source authentication of a message sent within the group is ensured by means of a digital signature embedded in the message. Subscribers must be able to retrieve Publishers' authentication credentials from a trusted repository, to verify source authentication of received messages. Hence, on joining a security group, a Publisher is expected to provide its own authentication credential to the KDC.¶

On a successful join, the Clients receive from the KDC the symmetric COSE Key used as shared group key to protect the payload of a published topic data.¶

The message exchange between the joining node and the KDC follows what is defined in Section 4.3.1.1 of [I-D.ietf-ace-key-groupcomm], and only additions or modifications to that specification are defined in this document.¶

4.2. Other Group Operations through the KDC

6.1. Using COSE to Protect the Published Topic Data

The Publisher uses the symmetric COSE Key received from the KDC to protect the payload of the Publish operation (see Section 4.3 of [I-D.ietf-core-coap-pubsub]). Specifically, the Publisher creates a COSE_Encrypt0 object [RFC9052][RFC9053] by means of the COSE Key currently used as group key. The encryption algorithm and Base IV to use are specified by the 'alg' and 'Base IV' parameters of the COSE Key, together with its key identifier in the 'kid' parameter.¶

Also, the Publisher uses its private key corresponding to the public key sent to the KDC, in order to countersign the COSE_Encrypt0 object as specified in [RFC9338]. The countersignature is specified in the 'Countersignature version 2' parameter, within the 'unprotected' field of the COSE_Encrypt0 object.¶

Finally, the Publisher sends the COSE_Encrypt0 object conveying the countersignature to the Broker, as payload of the PUT request sent to the topic-data of the topic targeted by the Publish operation.¶

Upon receiving a response from the topic-data resource at the Broker, the Subscriber uses the 'kid' parameter from the 'Countersignature version 2' parameter within the 'unprotected' field of the COSE_Encrypt0 object, in order to retrieve the Publisher's public key from the Broker or from its local storage. Then, the Subscriber uses that public key to verify the countersignature.¶

In case of successful verification, the Subscriber uses the 'kid' parameter from the 'unprotected' field of the COSE_Encrypt0 object, in order to retrieve the COSE Key used as current group key from its local storage. Then, the Subscriber uses that group key to verify and decrypt the COSE_Encrypt0 object. In case of successful verification, the Subscriber delivers the received topic data to the application.¶

The COSE_Encrypt0 object is constructed as follows.¶

The 'protected' field MUST include:¶

• The 'alg' parameter, with value the identifier of the AEAD algorithm specified in the 'alg' parameter of the COSE Key used as current group key.¶

The 'unprotected' field MUST include:¶

• The 'kid' parameter, with the same value specified in the 'kid' parameter of the COSE Key used as current group key. This value represents the current Group ID (Gid) of the security group associated with the application group (topic).¶

• The 'Partial IV' parameter, with value set to the current Sender Sequence Number of the Publisher. All leading bytes of value zero SHALL be removed when encoding the Partial IV, except in the case of Partial IV value 0, which is encoded to the byte string 0x00.¶

  The Publisher MUST initialize the Sender Sequence Number to 0 upon joining the security group, and MUST reset it to 0 upon receiving a new group key as result of a group rekeying (see Section 5). The Publisher MUST increment its Sender Sequence Number value by 1, after having completed an encryption operation by means of the current group key.¶

  When the Publisher exhausts its Sender Sequence Numbers, the Publisher MUST NOT protect further topic data by using the current group key while still retaining its current Sender ID, and MUST send a Key Renewal Request message to the KDC (see Section 4.2.2). This will result in the KDC rekeying the group and distributing a new group key, or in the KDC providing the Publisher with a new Sender ID. The Publisher MUST reset its Sender Sequence Number to 0 upon receiving a new Sender ID from the KDC.¶

• The 'Countersignature version 2' parameter, specifying the countersignature of the COSE_Encrypt0 object. In particular:¶

  • The 'protected' field includes the 'alg' parameter, with value the identifier of the Signature Algorithm used in the security group.¶

  • The 'unprotected' field includes the 'kid' parameter, with value the Publisher's Sender ID that the Publisher obtained from the KDC when joining the security group, as value of the 'group_SenderId' parameter of the Join Response (see Section 4.1.2).¶

  • The 'signature' field, with value the countersignature.¶

  The countersignature is computed as defined in [RFC9338], by using the private key of the Publisher as signing key, and by means of the Signature Algorithm used in the group. The fields of the Countersign_structure are populated as follows:¶

  • 'context' takes "CounterSignature".¶

  • 'body_protected' takes the serialized parameters from the 'protected' field of the COSE_Encrypt0 object, i.e., the 'alg' parameter.¶

  • 'sign_protected' takes the serialized parameters from the 'protected' field of the 'Countersignature version 2' parameter, i.e., the 'alg' parameter.¶

  • 'external_aad is not supplied.¶

  • 'payload' is the ciphertext of the COSE_Encrypt0 object (see below).¶

The 'ciphertext' field specifies the ciphertext computed over the topic data to publish. The ciphertext is computed as defined in [RFC9052][RFC9053], by using the current group key as encryption key, the AEAD Nonce computed as defined in Section 6.2, the topic data to publish as plaintext, and the Enc_structure populated as follows:¶

• 'context' takes "Encrypt0".¶

• 'protected' takes the serialization of the protected parameter 'alg' from the 'protected' field of the COSE_Encrypt0 object.¶

• 'external_aad is not supplied.¶

6.2. AEAD Nonce

This section defines how to generate the AEAD nonce used for encrypting and decrypting the COSE_Encrypt0 object protecting the published topic data. This construction is analogous to that used to generate the AEAD nonce in the OSCORE security protocol (see Section 5.2 of [RFC8613]).¶

The AEAD nonce for producing or consuming the COSE_Encrypt0 object is constructed as defined below and also shown in Figure 9.¶

1. Left-pad the Partial IV (PIV) with zeroes to exactly 5 bytes.¶

2. Left-pad the Sender ID of the Publisher that generated the Partial IV (ID_PIV) with zeroes to exactly the nonce length of the AEAD algorithm minus 6 bytes.¶

3. Concatenate the size of the ID_PIV (a single byte S) with the padded ID_PIV and the padded PIV.¶

4. XOR the result from the previous step with the Base IV.¶

     <- nonce length minus 6 B -> <-- 5 bytes -->
+---+-------------------+--------+---------+-----+
| S |      padding      | ID_PIV | padding | PIV |-----+
+---+-------------------+--------+---------+-----+     |
                                                       |
 <---------------- nonce length ---------------->      |
+------------------------------------------------+     |
|                    Base IV                     |-->(XOR)
+------------------------------------------------+     |
                                                       |
 <---------------- nonce length ---------------->      |
+------------------------------------------------+     |
|                     Nonce                      |<----+
+------------------------------------------------+

The construction above only supports AEAD algorithms that use nonces with length equal or greater than 7 bytes. At the same time, it makes it easy to verify that the nonces will be unique when used with the same group key, even though this is shared and used by all the Publishers in the security group. In fact:¶

• Since Publisher's Sender IDs are unique within the security group and they are not reassigned until a group rekeying occurs (see Section 4.1.2 and Section 5), two Publisher Clients cannot share the same tuple (S, padded ID_PIV) by construction.¶

• Since a Publisher increments by 1 its Sender Sequence Number after each use that it makes of the current group key, the Publisher never reuses the same tuple (S, padded ID_PIV, padded PIV) together with the same group key.¶

• Therefore neither the same Publisher reuses the same AEAD nonce with the same group key, nor any two Publishers use the same AEAD nonce with the same group key.¶

6.3. Replay Checks

In order to protect from replay of published topic data, every Subscriber maintains a Replay Window for each different Publisher in the same group. It is RECOMMENDED that the Replay Window has a default size of 32.¶

Upon receiving a topic data published by a given Publisher P, the Subscriber retrieves the Sender ID of P conveyed as 'kid' in the 'Countersignature version 2' parameter of the COSE_Encrypt0 object (see Section 6.1), and determines the Replay Window W_P associated with P.¶

The Subcriber MUST verify that, according to W_P, the Sender Sequence Number SN_P specified by the 'Partial IV' parameter of the COSE_Encrypt0 object has not been received before from P.¶

If the verification above fails, the Subscriber MUST stop processing the COSE_Encrypt0 object conveying the topic data. If the value of SN_P is strictly smaller than the currently smallest value in W_P, then the Subscriber MUST stop processing the COSE_Encrypt0 object.¶

If the verification above succeeds, the Subscriber proceeds with processing the COSE_Encrypt0 object, by verifying the countersignature from P using P's public key as well as by decrypting the COSE_Encrypt0 object using the group key. If both operations succeed, the Subscriber updates W_P as follows:¶

• If SN_P is strictly greater than the currently largest value in W_P, then W_P is updated in order to set SN_P as its largest value.¶

• SN_P is marked to denote that it has been received.¶

The operation of validating the 'Partial IV' and updating the Replay Window MUST be atomic.¶

Upon installing a new group key (e.g., due to a group rekeying performed by the KDC, see Section 5) or upon receiving published topic data from a given Publisher for the first time, the Subscriber initializes the Replay Window corresponding to that Publisher, i.e., the smallest value of the Replay Window is set to 0.¶

9.1. ACE Groupcomm Key Types

IANA is asked to register the following entry in the "ACE Groupcomm Key Types" registry defined in Section 11.8 of [I-D.ietf-ace-key-groupcomm].¶

• Name: Group_PubSub_Keying_Material¶

• Key Type Value: GROUPCOMM_KEY_TBD¶

• Profile: coap_group_pubsub_app or mqtt_pubsub_app (Section 9.2 of [RFC-XXXX]).¶

• Description: Encoded as described in Section 4.1.2 of [RFC-XXXX].¶

• References: [RFC9052], [RFC9053], [RFC-XXXX]¶

9.2. ACE Groupcomm Profiles

IANA is asked to register the following entries in the "ACE Groupcomm Profiles" registry defined in Section 11.9 of [I-D.ietf-ace-key-groupcomm].¶

• Name: coap_group_pubsub_app¶

• Description: Application profile to provision keying material for participating in group communication based on the Pub-Sub architecture [I-D.ietf-core-coap-pubsub] for CoAP [RFC7252] and protected with COSE [RFC9052][RFC9053][RFC9338].¶

• CBOR Value: TBD¶

• Reference: [RFC-XXXX]¶

• Name: mqtt_pubsub_app¶

• Description: Application profile to provision keying material for participating in group communication based on the Pub-Sub MQTT protocol MQTT protocol [MQTT-OASIS-Standard-v5] and protected with COSE [RFC9052][RFC9053][RFC9338].¶

• CBOR Value: TBD¶

• Reference: [RFC-XXXX]¶

9.3. CoRE Resource Type

IANA is asked to register the following entry in the "Resource Type (rt=) Link Target Attribute Values" registry within the "Constrained Restful Environments (CoRE) Parameters" registry group.¶

• Value: "core.ps.gm"¶

• Description: Group-membership resource for Pub-Sub communication.¶

• Reference: [RFC-XXXX]¶

Clients can use this resource type to discover a group membership resource at the KDC.¶

9.4. AIF Media-Type Sub-Parameters

For the media-types application/aif+cbor and application/aif+json defined in Section 5.1 of [RFC9237], IANA is requested to register the following entries for the two media-type parameters Toid and Tperm, in the respective sub-registry defined in Section 5.2 of [RFC9237] within the "MIME Media Type Sub-Parameter" registry group.¶

• Parameter: Toid¶

• Name: pubsub-topic¶

• Description/Specification: Name of one application group (topic) or of a corresponding security group.¶

• Reference: [RFC-XXXX]¶

• Parameter: Tperm¶

• Name: pubsub-perm¶

• Description/Specification: Permissions pertaining to application groups (topics) or to corresponding security groups.¶

• Reference: [RFC-XXXX]¶

9.5. CoAP Content-Formats

IANA is asked to register the following entries to the "CoAP Content-Formats" registry within the "Constrained RESTful Environments (CoRE) Parameters" registry group.¶

• Content Type: application/aif+cbor;Toid="pubsub-topic",Tperm="pubsub-perm"¶

• Content Coding: -¶

• ID: 294 (suggested)¶

• Reference: [RFC-XXXX]¶

• Content Type: application/aif+json;Toid="pubsub-topic",Tperm="pubsub-perm"¶

• Content Coding: -¶

• ID: 295 (suggested)¶

• Reference: [RFC-XXXX]¶

9.6. TLS Exporter Labels

IANA is asked to register the following entry to the "TLS Exporter Labels" registry defined in Section 6 of [RFC5705] and updated in Section 12 of [RFC8447].¶

• Value: EXPORTER-ACE-Sign-Challenge-pubsub-app¶

• DTLS-OK: Y¶

• Recommended: N¶

• Reference: Section 4.1.1.2 of [RFC-XXXX]¶

A.1. Mandatory-to-Address Requirements

• REQ1: Specify the format and encoding of 'scope'. This includes defining the set of possible roles and their identifiers, as well as the corresponding encoding to use in the scope entries according to the used scope format: see Section 3.4.1.¶

• REQ2: If the AIF format of 'scope' is used, register its specific instance of "Toid" and "Tperm" as Media Type parameters and a corresponding Content-Format, as per the guidelines in [RFC9237]: see Section 9.4 and Section 9.5.¶

• REQ3: If used, specify the acceptable values for 'sign_alg': values from the "Value" column of the "COSE Algorithms" registry [IANA.cose_algorithms].¶

• REQ4: If used, specify the acceptable values for 'sign_parameters': format and values from the COSE algorithm capabilities as specified in the "COSE Algorithms" registry [IANA.cose_algorithms].¶

• REQ5: If used, specify the acceptable values for 'sign_key_parameters': format and values from the COSE key type capabilities as specified in the "COSE Key Types" registry [IANA.cose_key-type].¶

• REQ6: Specify the acceptable formats for authentication credentials and, if used, the acceptable values for 'cred_fmt': acceptable formats explicitly provide the public key as well as the comprehensive set of information related to the public key algorithm (see Section 3.6 and Section 4.1.2). Consistent acceptable values for 'cred_fmt' are taken from the "Label" column of the "COSE Header Parameters" registry [IANA.cose_header-parameters].¶

• REQ7: If the value of the GROUPNAME URI path and the group name in the access token scope (gname) are not required to coincide, specify the mechanism to map the GROUPNAME value in the URI to the group name: not applicable; a perfect matching is required.¶

• REQ8: Define whether the KDC has an authentication credential and if this has to be provided through the 'kdc_cred' parameter: optional, see Section 4.1.2.¶

• REQ9: Specify if any part of the KDC interface as defined in [I-D.ietf-ace-key-groupcomm] is not supported by the KDC: some are left optional, see Section 4.¶

• REQ10: Register a Resource Type for the group-membership resource, which is used to discover the correct URL for sending a Join Request to the KDC: the Resource Type (rt=) Link Target Attribute value "core.ps.gm" is registered in Section Section 9.3.¶

• REQ11: Define what specific actions (e.g., CoAP methods) are allowed on each resource provided by the KDC interface, depending on whether the Client is a current group member; the roles that a Client is authorized to take as per the obtained access token; and the roles that the Client has as current group member: see Section 4 of this document.¶

• REQ12: Categorize possible newly defined operations for Clients into primary operations expected to be minimally supported and secondary operations, and provide accompanying considerations: none added.¶

• REQ13: Specify the encoding of group identifier: CBOR byte string, with value used also to identify the current group key used in the security group (see Section 4.1.2).¶

• REQ14: Specify the approaches used to compute and verify the PoP evidence to include in 'client_cred_verify', and which of those approaches is used in which case: see Section 4.1.1.2.¶

• REQ15: Specify how the nonce N_S is generated, if the token is not provided to the KDC through the Token Transfer Request to the authz-info endpoint (e.g., if it is used directly to validate TLS instead): see Section 4.1.1.2.¶

• REQ16: Define the initial value of the 'num' parameter: the initial value MUST be set to 0 (see Section 4.1.2).¶

• REQ17: Specify the format of the 'key' parameter and register a corresponding entry in the "ACE Groupcomm Key Types" IANA registry: see Section 4.1.2 and Section 9.1.¶

• REQ18: Specify the acceptable values of the 'gkty' parameter: Group_PubSub_Keying_Material, see Section 4.1.2.¶

• REQ19: Specify and register the application profile identifier: coap_group_pubsub_app (see Section 4.1.2 and Section 9.2) and mqtt_pubsub_app (see Section 7 and Section 9.2).¶

• REQ20: If used, specify the format and content of 'group_policies' and its entries. Specify the policies default values: not applicable.¶

• REQ21: Specify the approaches used to compute and verify the PoP evidence to include in 'kdc_cred_verify', and which of those approaches is used in which case. If external signature verifiers are supported, specify how those provide a nonce to the KDC to be used for computing the PoP evidence: see Section 4.1.2.¶

• REQ22: Specify the communication protocol that the members of the group must use: CoAP [RFC7252], used for Pub-Sub communications as defined in [I-D.ietf-core-coap-pubsub].¶

• REQ23: Specify the security protocol the group members must use to protect their communication. This must provide encryption, integrity, and replay protection: Publishers in a group use a symmetric group key to protect published topic data as a COSE_Encrypt0 object, per the AEAD algorithm specified by the KDC. A Publisher also produces a COSE countersignature of the COSE_Encrypt0 object by using its private key, per the signature algorithm specified by the KDC.¶

• REQ24: Specify how the communication is secured between Client and KDC. Optionally, specify a transport profile of ACE [RFC9200] to use between Client and KDC: ACE transport profile such as for DTLS [RFC9202] or OSCORE [RFC9203].¶

• REQ25: Specify the format of the identifiers of group members: the Sender ID defined in Section 4.1.2.¶

• REQ26: Specify policies at the KDC to handle ids that are not included in 'get_creds': see Section 4.2.1.¶

• REQ27: Specify the format of newly-generated individual keying material for group members, or of the information to derive it, and corresponding CBOR label: see Section 4.2.2.¶

• REQ28: Specify which CBOR tag is used for identifying the semantics of binary scopes, or register a new CBOR tag if a suitable one does not exist already: see Section 3.5 and Section 9.5.¶

• REQ29: Categorize newly defined parameters according to the same criteria of Section 8 of [I-D.ietf-ace-key-groupcomm]: a Publisher Client MUST support 'group_SenderId' in 'key'; see Section 4.1.2.¶

• REQ30: Define whether Clients must, should, or may support the conditional parameters defined in Section 8 of [I-D.ietf-ace-key-groupcomm], and under which circumstances: a Publisher Client MUST support the client_cred', 'cnonce', and 'client_cred_verify' parameters (see Section 4.1.1). A Publisher Client that provides the access token to the KDC through the authz-info endpoint MUST support the parameter 'kdcchallenge' (see Section 3.6).¶

A.2. Optional-to-Address Requirements

• OPT1: Optionally, if the textual format of 'scope' is used, specify CBOR values to use for abbreviating the role identifiers in the group: not applicable.¶

• OPT2: Optionally, specify the additional parameters used in the exchange of Token Transfer Request and Response: none are defined.¶

• OPT3: Optionally, specify the negotiation of parameter values for signature algorithm and signature keys, if 'sign_info' is not used: see Section 3.6.¶

• OPT4: Optionally, specify possible or required payload formats for specific error cases: see Section 4.1.3.¶

• OPT5: Optionally, specify additional identifiers of error types, as values of the 'error-id' field within the Custom Problem Detail entry 'ace-groupcomm-error': no.¶

• OPT6: Optionally, specify the encoding of 'creds_repo' if the default is not used: no.¶

• OPT7: Optionally, specify the functionalities implemented at the 'control_uri' resource hosted at the Client, including message exchange encoding and other details: no.¶

• OPT8: Optionally, specify the behavior of the handler in case of failure to retrieve an authentication credential for the specific node: The KDC MUST reply with a 4.00 (Bad Request) error response to the Join Request (see Section 4.1.3).¶

• OPT9: Optionally, define a default group rekeying scheme, to refer to in case the 'rekeying_scheme' parameter is not included in the Join Response: the "Point-to-Point" rekeying scheme registered in Section 11.12 of [I-D.ietf-ace-key-groupcomm].¶

• OPT10: Optionally, specify the functionalities implemented at the 'control_group_uri' resource hosted at the Client, including message exchange encoding and other details: no.¶

• OPT11: Optionally, specify policies that instruct Clients to retain messages and for how long, if they are unsuccessfully decrypted: no.¶

• OPT12: Optionally, specify for the KDC to perform group rekeying (together or instead of renewing individual keying material) when receiving a Key Renewal Request: the KDC SHOULD NOT perform a group rekeying, unless already scheduled to occur shortly (see Section 4.2.2).¶

• OPT13: Optionally, specify how the identifier of a group member's authentication credential is included in requests sent to other group members: no.¶

• OPT14: Optionally, specify additional information to include in rekeying messages for the "Point-to-Point" group rekeying scheme (see Section 6.1 of [I-D.ietf-ace-key-groupcomm]): no.¶

B.1. Version -09 to -10

• More details on the scope format.¶

• More details in the encoding of the 'key' parameter in the Join Response.¶

• More details on exchanges between group members and KDC.¶

• More details on the rekeying process and rekeying messages.¶

• Defined replay checks at the Subscriber.¶

• Improved examples.¶

• Improved security considerations.¶

• Revised IANA considerations.¶

• Aligned the list of profile requirements with draft-ietf-ace-key-groupcomm.¶

• Clarifications and editorial improvements.¶

B.2. Version -08 to -09

• Improved terminology section.¶

• Generalized scope format for future, admin-related extensions.¶

• Improved definition of permissions in the format of scope.¶

• Clarified alternative computing of N_S Challenge when DTLS is used.¶

• Use of the parameter 'exi' in the Join Response.¶

• Use of RFC 9290 instead of the custom format of error responses.¶

• Fixed construction of the COSE_Encrypt0 object.¶

• Fixed use of the resource type "core.ps.gm".¶

• Updated formulation of profile requirements.¶

• Clarification and editorial improvements.¶

B.3. Version -07 to -08

• Revised presentation of the scope format.¶

• Revised presentation of the Join Request-Response exchange.¶

• The 'cnonce' parameter must be present in the Join Request.¶

• The 'kid' of the group key is used as Group Identifier.¶

• Relaxed inclusion of the 'peer_roles' parameter.¶

• More detailed description of the encryption and signing operations.¶

• Defined construction of the AEAD nonce.¶

• Clarifications and editorial improvements.¶

B.4. Version -06 to -07

• Revised abstract and introduction.¶

• Clarified use of "application groups".¶

• Revised use of protocols and transport profiles with Broker and KDC.¶

• Revised overview of the profile and its security associations.¶

• Revised presentation of authorization flow.¶

• Subscribers cannot be anonymous anymore.¶

• Revised scope definition.¶

• Revised Join Response.¶

• Revised COSE countersignature, COSE encrypt objects.¶

• Further clarifications, fixes and editorial improvements.¶