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

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Table of Contents

1. Introduction

In a publish-subscribe (pub/sub) scenario, devices with limited reachability communicate via a Broker, which enables store-and-forward messaging between these devices. This effectively enables a form of group communication, where all the Publishers and Subscribers participating in the same pub/sub topic are considered members of the same group associated with that topic.¶

With a focus on the pub/sub architecture defined in [I-D.ietf-core-coap-pubsub] for the Constrained Application Protocol (CoAP) [RFC7252], this document defines an application profile of the Authentication and Authorization for Constrained Environments (ACE) framework [RFC9200], which enables pub/sub communication where a group of Publishers and Subscribers securely communicate through a Broker using CoAP.¶

Building on the message formats and processing defined in [I-D.ietf-ace-key-groupcomm], this document specifies the provisioning and enforcement of authorization information for Clients to act as Publishers and/or Subscribers at the Broker, as well as the provisioning of keying material and security parameters that Clients use for protecting end-to-end their communications via the Broker.¶

In order to protect the pub/sub operations at the Broker as well as the provisioning of keying material and security parameters, this profile relies on protocol-specific transport profiles of ACE (e.g., [RFC9202], [RFC9203], or [I-D.ietf-ace-edhoc-oscore-profile]) to achieve communication security, server authentication, and proof-of-possession for a key owned by the Client and bound to an OAuth 2.0 Access Token.¶

Furthermore, the content of published messages that are circulated by the Broker is protected end-to-end between the corresponding Publisher and the intended Subscribers. To this end, this profile relies on COSE [RFC9052][RFC9053] and on keying material provided to the Publishers and Subscribers participating in the same pub/sub topic. In particular, source authentication of published content is achieved by means of the corresponding Publisher signing such content with its own private key.¶

While this profile focuses on the pub/sub architecture for CoAP, this document also describes how it can be applicable to MQTT [MQTT-OASIS-Standard-v5]. Similar adaptations can also extend to further transport protocols and pub/sub architectures.¶

2. Application Profile Overview

This document describes how to use [RFC9200] and [I-D.ietf-ace-key-groupcomm] to perform authentication, authorization, and key distribution operations as overviewed in Section 2 of [I-D.ietf-ace-key-groupcomm], where the considered group is the security group including the pub/sub Clients that exchange end-to-end protected content through the Broker.¶

Pub/sub Clients communicate within their application groups, each of which is mapped to a pub/sub topic. Depending on the application, a pub/sub topic may consist of one or more sub-topics, which may have their own sub-topics and so on, thus forming a hierarchy. A security group SHOULD be associated with a single application group. However, the same application group MAY be associated with multiple security groups. Further details and considerations on the mapping between the two types of groups are out of the scope of this document.¶

This profile considers the architecture shown in Figure 1. A Client can act as a Publisher, or a Subscriber, or both, e.g., by publishing to some topics and subscribing to others. However, for the simplicity of presentation, this profile describes Publisher and Subscriber Clients separately.¶

Both Publishers and Subscribers Clients act as ACE Clients. The Broker acts as an ACE RS, and corresponds to the Dispatcher in [I-D.ietf-ace-key-groupcomm]. The Key Distribution Center (KDC) also acts as an ACE RS, and builds on what is defined for the KDC in [I-D.ietf-ace-key-groupcomm]. From a high-level point of view, the Clients interact with the KDC in order to join security groups, upon which they obtain the group keying material to use for protecting and verifying the content published in the corresponding pub/sub topics and protected end-to-end.¶

             +---------------+   +--------------+
             | Authorization |   |     Key      |
             |    Server     |   | Distribution |
             |      (AS)     |   |    Center    |
             |               |   |    (KDC)     |
             +---------------+   +--------------+
                      ^                  ^
                      |                  |
     +---------(A)----+                  |
     |                                   |
     |   +--------------------(B)--------+
     v   v
+------------+             +------------+
|            | <-- (O) --> |            |
|  Pub/Sub   |             |   Broker   |
|  Client    | <-- (C)---> |            |
|            |             |            |
+------------+             +------------+

Both Publisher and Subscriber Clients MUST use the same pub/sub communication protocol for their interaction with the Broker. When using the profile defined in this document, such a protocol MUST be CoAP, which is used as described in [I-D.ietf-core-coap-pubsub]. What is specified in this document can apply to other pub/sub protocols such as MQTT [MQTT-OASIS-Standard-v5], or to further transport protocols.¶

All Publisher and Subscriber Clients MUST use CoAP when communicating with the KDC.¶

Furthermore, both Publisher and Subscriber Clients MUST use the same transport profile of ACE (e.g., [RFC9202] for DTLS; or [RFC9203] or [I-D.ietf-ace-edhoc-oscore-profile] for OSCORE) in their interaction with the Broker. In order to reduce the number of libraries that Clients have to support, it is RECOMMENDED that the same transport profile of ACE is used also for the interaction between the Clients and the KDC.¶

All communications between the involved entities MUST be secured.¶

The Client and the Broker MUST have a secure association, which they establish with the help of the AS and using a transport profile of ACE. This is shown by the interactions A and C in Figure 1. During this process, the Client obtains an Access Token from the AS and uploads it to the Broker, thus providing an evidence of the pub-sub topics that it is authorized to participate in, and with which permissions.¶

The Client and the KDC MUST have a secure association, which they also establish with the help of the AS and using a transport profile of ACE. This is shown by the interactions A and B in Figure 1. During this process, the Client obtains an Access Token from the AS and uploads it to the KDC, thus providing an evidence of the security groups that it can join, as corresponding to the pub/sub topics of interest at the Broker. Based on the permissions specified in the Access Token, the Client can request the KDC to join a security group, after which the Client obtains from the KDC the keying material to use for communicating with the other group members. This builds on the process for joining security groups with ACE defined in [I-D.ietf-ace-key-groupcomm] and further specified in this document.¶

In addition, this profile allows an anonymous Client to perform some of the discovery operations defined in Section 2.3 of [I-D.ietf-core-coap-pubsub] through the Broker, as shows by the interaction O in Figure 1. That is, an anonymous Client can discover:¶

• the Broker itself, by relying on the resource type "core.ps" (see Section 2.3.1 of [I-D.ietf-core-coap-pubsub]); and¶
• topics of interest at the Broker (i.e., the corresponding topic resources hosted at the Broker), by relying on the resource type "core.ps.conf" (see Section 2.3.2 of [I-D.ietf-core-coap-pubsub]).¶

However, an anonymous Client is not allowed to access topic resources at the Broker and obtain from those any additional information or metadata about the corresponding topic (e.g., the topic status, the URI of the topic-data resource where to publish or subscribe for that topic, or the URI to the KDC).¶

As highlighted in Figure 2, each Client maintains two different security associations pertaining to the pub/sub group communication. On the one hand, the Client has a pairwise security association with the Broker, which, as the ACE RS, verifies that the Client is authorized to publish and/or subscribe on a certain set of topics (Security Association 1). As discussed above, this security association is set up with the help of the AS and using a transport profile of ACE, when the Client obtains the Access Token to upload to the Broker.¶

On the other hand, separately for each topic, all the Publisher and Subscribers for that topic have a common, group security association, through which the published content sent through the Broker is protected end-to-end (Security Association 2). As discussed above, this security association is set up and maintained as the different Clients request the KDC to join the security group, upon which they obtain from the KDC the corresponding group keying material to use for protecting end-to-end and verifying the content of their pub/sub group communication.¶

+------------+             +------------+              +------------+
|            |             |            |              |            |
| Publisher  |             |   Broker   |              | Subscriber |
|            |             |            |              |            |
|            |             |            |              |            |
+------------+             +------------+              +------------+
      :   :                     :   :                      :   :
      :   '----- Security ------'   '------ Security ------'   :
      :        Association 1              Association 1        :
      :                                                        :
      '---------------------- Security ------------------------'
                            Association 2

In summary, this profile specifies the following functionalities.¶

1. A Client obtains the authorization to participate in a pub-sub topic at the Broker with certain permissions. This pertains operations defined in [I-D.ietf-core-coap-pubsub] for taking part in pub/sub group communication with CoAP.¶
2. A Client obtains the authorization to join a security group with certain permissions. This allows the Client to obtain from the KDC the group keying material for communicating with other group members, i.e., to protect end-to-end and verify the content published at the Broker on pub/sub topics associated with the security group.¶
3. A Client obtains from the KDC the authentication credentials of other group members, and provides the KDC with its own (updated) authentication credential.¶
4. A Client leaves the group or is removed from the group by the KDC.¶
5. The KDC renews and redistributes the group keying material (rekeying), e.g., due to a membership change in the group.¶

Appendix A lists the specifications on this application profile of ACE, based on the requirements defined in Appendix A of [I-D.ietf-ace-key-groupcomm].¶

3. Getting Authorisation to Join a Pub/sub security group (A)

Figure 3 provides a high level overview of the message flow for a Client getting authorisation to join a group. Square brackets denote optional steps. The message flow is expanded in the subsequent sections.¶

Client                                          Broker   AS   KDC
   |                                                 |    |     |
   |[<-------- Discovery of Topic Resource -------->]|    |     |
   |                                                 |    |     |
   |[--------------- Resource Request ------------->]|    |     |
   |[<--------------- AS Information ---------------]|    |     |
   |                                                 |    |     |
   |                                                 |    |     |
   |----- Authorisation Request (Audience: Broker) ------>|     |
   |<---- Authorisation Response (Audience: Broker) ------|     |
   |                                                 |    |     |
   |                                                 |    |     |
   |------ Upload of authorisation information ----->|    |     |
   |<----- Establishment of secure association ----->|    |     |
   |                                                 |    |     |
   |                                                 |    |     |
   |[<-- Discovery of KDC and name of sec. group -->]|    |     |
   |                                                 |    |     |
   |                                                 |    |     |
   |------- Authorisation Request (Audience: KDC) ------->|     |
   |<------ Authorisation Response (Audience: KDC) -------|     |
   |                                                 |    |     |
   |                                                 |    |     |
   |--------- Upload of authorisation information ------------->|
   |<-------- Establishment of secure association ------------->|
   |                                                 |    |     |
   |                                                 |    |     |
   |----- Request to join the security group for the topic ---->|
   |<-------- Keying material for the security group -----------|
   |                                                 |    |     |
   |                                                 |    |     |
   |---------------- Resource Request -------------->|    |     |
   |                                                 |    |     |

Since [RFC9200] recommends the use of CoAP and CBOR, this document describes the exchanges assuming CoAP and CBOR are used.¶

However, using HTTP instead of CoAP is possible, by leveraging the corresponding parameters and methods. Analogously, JSON [RFC8259] can be used instead of CBOR, using the conversion method specified in Sections 6.1 and 6.2 of [RFC8949]. In case JSON is used, the Content-Format of the message has to be changed accordingly. Exact definitions of these exchanges are out of scope for this document.¶

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

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

  AIF-PUBSUB-GROUPCOMM = AIF-Generic<pubsub-topic, pubsub-perm>
   pubsub-topic = tstr ; Pub/sub topic name
                       ; (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-topic, pubsub-perm]

4. Client Group Communication Interface at the KDC

The Pub/Sub clients uses the following KDC resources to enable group communication:¶

Note that the use of these resources follows what is defined in [I-D.ietf-ace-key-groupcomm], and only additions or modifications to that specification are defined in this document.¶

   Client                               KDC
      |----- Join Request (CoAP) ------>|
      |                                 |
      |<-----Join Response (CoAP) ------|

5. PubSub Protected Communication (C)

+------------+             +------------+              +------------+
|            |             |            |              |            |
| Publisher  | ----(D)---> |   Broker   |              | Subscriber |
|            |             |            | <----(E)---- |            |
|            |             |            | -----(F)---> |            |
+------------+             +------------+              +------------+

(D) corresponds to the publication of a topic on the Broker, using a CoAP PUT. The publication (the resource representation) is protected with COSE ([RFC9052][RFC9053]) by the Publisher. The (E) message is the subscription of the Subscriber, and uses a CoAP GET with the Observe option set to 0 (zero) [I-D.ietf-core-coap-pubsub]. The (F) message is the response from the Broker, where the publication is protected with COSE by the Publisher. (ToDo: Add Delete to the flow?)¶

  Publisher                Broker               Subscriber
      | --- PUT /topic ----> |                       |
      |  protected with COSE |                       |
      |                      | <--- GET /topic ----- |
      |                      |      Observe:0        |
      |                      | ---- response ------> |
      |                      |  protected with COSE  |

6. Applicability to MQTT PubSub Profile

The steps MQTT clients go through would be similar to the CoAP clients, and the payload of the MQTT PUBLISH messages will be protected using COSE. The MQTT clients needs to use CoAP to communicate to the KDC, to join security groups, and be part of the pair-wise rekeying initiated by the KDC.¶

Authorisation Server (AS) Discovery is defined in Section 2.2.6.1 of [I-D.ietf-ace-mqtt-tls-profile] for MQTT v5 clients (and not supported for MQTT v3 clients). $SYS/ has been widely adopted as a prefix to topics that contain Server-specific information or control APIs, and may be used for topic and KDC discovery.¶

When the Client sends an authorisation request to the AS using the AIF-PUBSUB-GROUPCOMM data model, in the authorisation response, the 'profile' claim is set to "mqtt_pubsub_app" as defined in Section 8.2.¶

Both Publisher and Subscriber Clients MUST authorise to the Broker with their respective tokens (described in [I-D.ietf-ace-mqtt-tls-profile]) i.e., anonymous Subscribers are not supported in the profile. A Publisher Client sends PUBLISH messages for a given topic and protects the payload with the corresponding key for the associated security group. The Broker validates the PUBLISH message by verifying its topic in the stored token. A Subscriber Client may send SUBSCRIBE messages with one or multiple topic filters. A topic filter may correspond to multiple topics. The Broker validates the SUBSCRIBE message by checking the stored token for the Client. The Broker forwards all PUBLISH messages to all authorised Subscribers, including the retained messages.¶

7. Security Considerations

All the security considerations in [I-D.ietf-ace-key-groupcomm] apply.¶

In the profile described above, the Publisher and Subscriber use asymmetric crypto, which would make the message exchange quite heavy for small constrained devices. Moreover, all Subscribers must be able to access the public keys of all the Publishers to a specific topic to verify the publications.¶

Even though Access Tokens have expiration times, an Access Token may need to be revoked before its expiration time (see [I-D.draft-ietf-ace-revoked-token-notification] for a list of possible circumstances). Clients can be excluded from future publications through re-keying for a certain topic. This could be set up to happen on a regular basis, for certain applications. How this could be done is out of scope for this work. The method described in [I-D.draft-ietf-ace-revoked-token-notification] MAY be used to allow an Authorization Server to notify the KDC about revoked Access Tokens.¶

The Broker is only trusted with verifying that the Publisher is authorized to publish, but is not trusted with the publications itself, which it cannot read nor modify.¶

With respect to the reusage of nonces for Proof-of-Possession input, the same considerations apply as in the [I-D.ietf-ace-key-groupcomm-oscore].¶

TODO: expand on security and privacy considerations¶

8. IANA Considerations

Note to RFC Editor: Please replace "[RFC-XXXX]" with the RFC number of this document and delete this paragraph.¶

This document has the following actions for IANA.¶

Appendix A. Requirements on Application Profiles

This section lists the specifications on this profile based on the requirements defined in Appendix A of [I-D.ietf-ace-key-groupcomm].¶

• REQ1: Specify the format and encoding of 'scope'. : 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 8.4.¶
• 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' : 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].¶
• 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. Takes value 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 of this document.¶
• 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 left optional, see Section 4 of this document.¶
• REQ10: Register a Resource Type for the root url-path, which is used to discover the correct url to access at the KDC : the Resource Type (rt=) Link Target Attribute value "core.ps.gm" is registered in Section Section 8.3. ToDo: This possibly will not stay as the final method, KDC discovery done differently through topic discovery?¶
• 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, see Section 4.2.1.¶
• 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 in this document.¶
• 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 in this document.¶
• REQ16: Define the initial value of the 'num' parameter: The initial value MUST be set to 0.¶
• REQ17: Specify the format of the 'key' parameter: See Section 4.1.2.¶
• REQ18: Specify the acceptable values of the 'gkty' parameter: Group_PubSub_Keying_Material, see Section 8.1.¶
• REQ19: Specify and register the application profile identifier: coap_group_pubsub_app, see Section 8.2.¶
• REQ20: If used, specify the format and content of 'group_policies' and its entries. Specify the policies default values: ToDo.¶
• 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: see Section 4.1.2.¶
• REQ22: Specify the communication protocol the members of the group must use.: CoAP [RFC7252], and for pub/sub communication [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.: Symmetric COSE Key is used to create a COSE_Encrypt0 object with an AEAD algorithm specified by the KDC.¶
• REQ24: Specify how the communication is secured between Client and KDC. Optionally, specify transport profile of ACE [RFC9200] to use between Client and KDC.: ACE transport profile such as 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.: Not applicable.¶
• 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 8.5 of this document.¶
• 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 client_cred', 'cnonce', 'client_cred_verify' parameters; see Section 4.1.1. A Publisher Client that provides the token to the KDC, through the authz-info endpoint, MUST support the parameter 'kdcchallenge'; see Section 3.6.¶
• OPT1: Optionally, if the textual format of 'scope' is used, specify CBOR values to use for abbreviating the role identifiers in the group: No.¶
• OPT2: Optionally, specify the additional parameters used in the exchange of Token Transfer Request and Response : No.¶
• 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' field in an error response from the KDC: 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: ToDo.¶
• 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: ToDo.¶
• OPT15: Optionally, specify if Clients must or should support any of the parameters defined as optional in [I-D.ietf-ace-key-groupcomm]: No.¶

Appendix B. Document Updates

RFC EDITOR: PLEASE REMOVE THIS SECTION.¶

Acknowledgments

The authors wish to thank Ari Keränen, John Preuß Mattsson, Jim Schaad, Ludwig Seitz, and Göran Selander for the useful discussion and reviews that helped shape this document.¶

The work on this document has been partly supported by the H2020 project SIFIS-Home (Grant agreement 952652).¶

Authors' Addresses