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 [RFC9200], and the Authorization Information Format (AIF) [RFC9237] to express authorization information. In particular, analogously to [RFC9200], terminology for entities in the architecture such as Client (C), Resource Server (RS), and Authorization Server (AS) is defined in OAuth 2.0 [RFC6749].¶
• 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]. Unless otherwise indicated, the term "endpoint" is used here following its OAuth definition, aimed at denoting resources such as /token and /introspect at the AS, and /authz-info at the RS. This document does not use the CoAP definition of "endpoint", which is "An entity participating in the CoAP protocol".¶
• 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 without the tag and value abbreviations.¶

3.1. Topic Discovery at the Broker (Optional)

The discovery of a topic at the Broker can be performed by discovering the corresponding topic resources 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 2.3.2 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 CBOR diagnostic notation and CoAP is given below:¶

    4.01 Unauthorized
    Content-Format: application/ace+cbor
    Payload:
    {
     "AS": "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 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 8.3 (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 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 permissions.¶

  This parameter is encoded as a CBOR byte string, whose value is the binary encoding of a CBOR array. The format 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 to each request, containing claims, as defined in Section 5.8.2 of [RFC9200] and Section 3.2 of [I-D.ietf-ace-key-groupcomm] with the following additions:¶

• 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.¶
• 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 interactions 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 8.5 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 8.5 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. 'kdcchallange' 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

5.1. Using COSE Objects To Protect The Resource Representation

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 for countersigning 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 4.2.4). The Publisher MUST increment its Sender Sequence Number value by 1, after having completed an encryption operation by means of the current group key.¶

• 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 5.2, 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.¶

5.2. 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 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 4.2.4), 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 nor any two Publishers use the same AEAD nonce with the same group key.¶

5.3. Replay Checks

This section defines how a Subscriber Client checks whether the topic data conveyed in a received message from the Broker is a replay.¶

TBD¶

8.1. ACE Groupcomm Key Types Registry

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, defined in Section 8.2 of this document.¶
• Description: Encoded as described in the Section 4.1.2 of this document.¶
• References: [RFC9052], [RFC9053], [RFC-XXXX]¶

8.2. ACE Groupcomm Profiles Registry

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: Profile for delegating client authentication and authorization for publishers and subscribers in a CoAP pub/sub setting scenario in a constrained environment.¶
• CBOR Value: TBD¶
• Reference: [RFC-XXXX]¶

• Name: mqtt_pubsub_app¶
• Description: Profile for delegating client authentication and authorization for publishers and subscribers in a MQTT pub/sub setting scenario in a constrained environment.¶
• CBOR Value: TBD¶
• Reference: [RFC-XXXX]¶

8.3. 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.¶

8.4. 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: Pub/sub topic name, corresponding to the security group¶
• Reference: [RFC-XXXX]¶

• Parameter: Tperm¶
• Name: pubsub-perm¶
• Description/Specification: Permissions corresponding to the topic data interactions in the pub/sub group¶
• Reference: [RFC-XXXX]¶

8.5. 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]¶

8.6. 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-coap-group-pubsub-app¶
• DTLS-OK: Y¶
• Recommended: N¶
• Reference: [RFC-XXXX]¶

B.1. Version -08 to -09

This section is to be removed before publishing as an RFC.¶

• 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.2. 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.3. 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.¶