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

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 14 September 2023.¶

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

1. Introduction

In the publish-subscribe (pub/sub) scenario, devices with limited reachability communicate via a broker, which enables store-and-forward messaging between these devices. This document specifies how to request, distribute and renew the keying material and configuration parameters to protect message exchanges for pub/sub communication, using [I-D.ietf-ace-key-groupcomm], which expands from the ACE framework ([RFC9200]). Message exchanges among the participants as well as message formats and processing follow the specifications for provisioning and renewing keying material in group communication scenarios in [I-D.ietf-ace-key-groupcomm].¶

The pub/sub communication using the Constrained Application Protocol (CoAP) [RFC7252] is specified in [I-D.ietf-core-coap-pubsub].This document gives detailed specifications for CoAP pub/sub, and describes how it can be applicable to MQTT [MQTT-OASIS-Standard-v5]; similar adaptations can extend to other transport protocols as well.¶

2. Application Profile Overview

This document describes how to use [I-D.ietf-ace-key-groupcomm] and [RFC9200] to perform authentication, authorization and key distribution actions as overviewed in Section 2 of [I-D.ietf-ace-key-groupcomm], when the considered group is pub/sub clients belonging to the same security group.¶

Pub/sub clients communicate within their application groups mapped to a collection of pub/sub topics. The pub/sub topics may consist of one or more sub-topics, which may have their own sub-topics, forming a hierarchy. The applications decide how to map this hierarchy into different application groups, and 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.¶

The architecture of the scenario is shown in Figure 1. A Client can act both as a publisher and a subscriber, publishing to some topics, and subscribing to others. However, for the simplicity of presentation, this profile describes Publisher and Subscriber Clients separately. The Broker acts as the ACE RS, and also corresponds to the Dispatcher in [I-D.ietf-ace-key-groupcomm]. The Clients communicate with The Key Distribution Center (KDC) to join security groups, and obtain the group keying material.¶

Both Publisher and Subscriber Clients use the same pub/sub communication protocol and the same transport profile of ACE in their interaction with the broker. The pub/sub communication protocol considered in this document is CoAP, as described in [I-D.ietf-core-coap-pubsub], but the specification can apply to other pub/sub protocols such as MQTT [MQTT-OASIS-Standard-v5], or other transport protocols. All clients MUST use CoAP when communicating to the KDC.¶

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

All communications between the involved entities MUST be secured. This profile expects the establishment of a secure connection between a Client and Broker, using an ACE transport profile such as DTLS [RFC9202] or OSCORE [RFC9203] (A and C). Once the client establishes a secure association with KDC with the help of AS, it can request to join the security groups of its pub/sub topics (A and B), and can communicate securely with the other group members, using the keying material provided by the KDC.¶

(C) corresponds to the exchange between the Client and the Broker, where the Client sends its access token to the Broker and establishes a secure connection with the Broker. Depending on the Information received in (A), the connection set-up may involve, for example, a DTLS handshake, or other protocols. Depending on the application, the set up phase may be skipped: for example, if OSCORE is used directly.¶

In addition, this document describes an Optional Discovery though Broker (O), where an anonymous Clients MAY discover the topic categories, topics resources, the AS and the KDC from the Broker.¶

It must be noted that Clients maintain two different security associations. On the one hand, the Publisher and the Subscriber clients have a security association with the Broker, which, as the ACE RS, verifies that the Clients are authorized (Security Association 1). On the other hand, the Publisher has a security association with the Subscriber, to protect the publication content (Security Association 2) while sending it through the broker. The Security Association 1 is set up using AS and a transport profile of [RFC9200], the Security Association 2 is set up using AS, KDC and [I-D.ietf-ace-key-groupcomm].¶

Given that the publication content is protected, the Broker MAY accept unauthorised Subscribers. In this case, the Subscriber client MAY skip setting up Security Association 1 with the Broker and connect to it as an anonymous client to subscribe to topics of interest at the Broker.¶

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

In summary, this profile describes how:¶

1. A Client gets the authorization to join a security group, and providing it with the group keying material to communicate with other group members.¶
2. A Client retrieves group keying material to publish protected publications to the Broker or read protected publications.¶
3. A Client retrieves authentication credentials of other group members, and provides and updates own authentication credentials.¶
4. A Client is removed from the group.¶
5. The KDC renews and redistributes the group keying (rekeying) material due to membership change in the group.¶

Appendix 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 Figure 3 provides a high level overview of the message flow for a node getting authorisation to join a group. This message flow is expanded in the subsequent sections.¶

   Client                                       Broker   AS
      | [--Resource Request (CoAP/MQTT or other)-->] |    |
      | [<----AS Information (CoAP/MQTT or other)--] |    |
      |[<--Topic Collection/Resource/KDC Discovery->]|
      |                                              |
      | --Broker Authorisation Req (CoAP/HTTP or other)-->|
      | <---Authorisation Response (CoAP/HTTP or other) --|
      |                                                   |
      | --KDC Authorisation Req (CoAP/HTTP or other)----->|
      | <---Authorisation Response (CoAP/HTTP or other) --|

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, using 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 or Media Type of the message has to be changed accordingly. Exact definition of these exchanges are considered out of scope for this document.¶

    4.01 Unauthorized
    Content-Format: application/ace-groupcomm+cbor
    {"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-roles

   pubsub-roles = &(
    Admin: 0,
    Pub: 1,
    Sub: 2,
    Delete: 3
   )

   scope_entry = [pubsub-topic, pubsub-perm]

4. Client Group Communication Interface at the KDC

The 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] applies, and only additions or modifications to that specification are defined in this document.¶

   Client                                  KDC
      |----- Joining Request (CoAP) ------>|
      |                                    |
      |<-----Joining 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 subscription MAY be unprotected. 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.¶

Differently for MQTT, the Client sends an authorisation request to the AS using AIF-MQTT data model for representing the requested scopes is described in Section 3 of the [I-D.ietf-ace-mqtt-tls-profile]. In the authorisation response, the 'profile' claim is set to "mqtt_pubsub_app" as defined in Section 8.1.2.¶

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, when 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-03] 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-03] 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. In this setting, caching of publications on the Broker is still allowed.¶

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

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 3.3.1.¶
• 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_COSE_Key, see Section 8.2.¶
• REQ19: Specify and register the application profile identifier: coap_group_pubsub_app, see Section 8.1.1.¶
• 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 3.4 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 3.5.¶
• 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 3.5.¶
• 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.¶

Acknowledgments

The author wishes to thank Ari Keränen, John Preuß Mattsson, Ludwig Seitz, Göran Selander, and Jim Schaad 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