ACE Working Group                                           F. Palombini
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               C. Sengul
Expires: 1 January 2022                                Brunel University
                                                            30 June 2021


  Pub-Sub Profile for Authentication and Authorization for Constrained
                           Environments (ACE)
                    draft-ietf-ace-pubsub-profile-03

Abstract

   This specification defines an application profile for authentication
   and authorization for publishers and subscribers in a constrained
   pub-sub scenario, using the ACE framework.  This profile relies on
   transport layer or application layer security to authorize the pub-
   sub clients to the broker.  Moreover, it describes application layer
   security for publisher-subscriber communication going through the
   broker.

Note to Readers

   Source for this draft and an issue tracker can be found at
   https://github.com/ace-wg/pubsub-profile (https://github.com/ace-wg/
   pubsub-profile).

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 1 January 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Application Profile Overview  . . . . . . . . . . . . . . . .   3
   3.  PubSub Authorisation  . . . . . . . . . . . . . . . . . . . .   5
     3.1.  AS Discovery (Optional) . . . . . . . . . . . . . . . . .   6
     3.2.  Authorising to the Broker . . . . . . . . . . . . . . . .   6
   4.  Key Distribution for PubSub Content Protection  . . . . . . .   7
     4.1.  Token POST  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Join Request  . . . . . . . . . . . . . . . . . . . . . .   8
   5.  PubSub Protected Communication  . . . . . . . . . . . . . . .  11
     5.1.  Using COSE Objects To Protect The Resource
           Representation  . . . . . . . . . . . . . . . . . . . . .  12
   6.  Profile-specific Considerations . . . . . . . . . . . . . . .  13
     6.1.  CoAP PubSub Application Profile . . . . . . . . . . . . .  13
     6.2.  MQTT PubSub Application Profile . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  ACE Groupcomm Profile Registry  . . . . . . . . . . . . .  16
       8.1.1.  CoAP Profile Registration . . . . . . . . . . . . . .  16
       8.1.2.  MQTT Profile Registration . . . . . . . . . . . . . .  17
     8.2.  ACE Groupcomm Key Registry  . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Requirements on Application Profiles . . . . . . . .  19
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   In the publish-subscribe (pub-sub) scenario, devices with limited
   reachability communicate via a broker, which enables store-and-
   forward messaging between the devices.  This document defines a way
   to authorize pub-sub clients using the ACE framework
   [I-D.ietf-ace-oauth-authz], and to provide the keys for protecting
   the communication between them.  The pub-sub communication using the
   Constrained Application Protocol (CoAP) is specified in



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   [I-D.ietf-core-coap-pubsub], while the one using MQTT is specified in
   [MQTT-OASIS-Standard-v5].  This document gives detailed
   specifications for MQTT and CoAP pub-sub, but can easily be adapted
   for other transport protocols as well.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Readers are expected to be familiar with the terms and concepts
   described in [I-D.ietf-ace-oauth-authz],
   [I-D.ietf-ace-key-groupcomm].  In particular, analogously to
   [I-D.ietf-ace-oauth-authz], terminology for entities in the
   architecture such as Client (C), Resource Server (RS), and
   Authorization Server (AS) is defined in OAuth 2.0 [RFC6749] and
   [I-D.ietf-ace-actors], and terminology for entities such as the Key
   Distribution Center (KDC) and Dispatcher in
   [I-D.ietf-ace-key-groupcomm].

   Readers are expected to be familiar with terms and concepts of pub-
   sub group communication, as described in [I-D.ietf-core-coap-pubsub],
   or MQTT [MQTT-OASIS-Standard-v5].

2.  Application Profile Overview

   The objective of this document is to specify how to authorize nodes,
   provide keys, and protect a pub-sub communication, using
   [I-D.ietf-ace-key-groupcomm], which expands from the ACE framework
   ([I-D.ietf-ace-oauth-authz]), and transport profiles
   ([I-D.ietf-ace-dtls-authorize], [I-D.ietf-ace-oscore-profile],
   [I-D.ietf-ace-mqtt-tls-profile]).  The pub-sub communication protocol
   can be based on CoAP, as described in [I-D.ietf-core-coap-pubsub],
   MQTT [MQTT-OASIS-Standard-v5] , or other transport.  Note that both
   publishers and subscribers use the same profiles.

   The architecture of the scenario is shown in Figure 1.













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

       Figure 1: Architecture for Pub-Sub with Authorization Servers

   Publisher or Subscriber Clients is referred to as Client in short.
   This profile specifies:

   1.  The establishment of a secure connection between a Client and
       Broker, using an ACE transport profile such as DTLS
       [I-D.ietf-ace-dtls-authorize], OSCORE
       [I-D.ietf-ace-oscore-profile], or MQTT-TLS
       [I-D.ietf-ace-mqtt-tls-profile] (A and C).

   2.  The Clients retrieval of keying material for the Publisher Client
       to publish protected publications to the Broker, and for the
       Subscriber Client to read protected publications (B).

   These exchanges aim at setting up two different security
   associations.  On the one hand, the Publisher and the Subscriber
   clients have a security association with the Broker (i.e.  RS), so
   that RS can authorize the Clients (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 (i.e. here, the broker
   corresponds to the Dispatcher in [I-D.ietf-ace-key-groupcomm]).  The
   Security Association 1 is set up using AS and a transport profile of
   [I-D.ietf-ace-oauth-authz], the Security Association 2 is set up
   using AS, KDC and [I-D.ietf-ace-key-groupcomm].  Note that, given
   that the publication content is protected, the Broker MAY accept
   unauthorised Subscribers.  In this case, the Subscriber client can
   skip setting up Security Association 1 with the Broker.




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

         Figure 2: Security Associations between Publisher, Broker,
                             Subscriber pairs.

3.  PubSub Authorisation

   Since [I-D.ietf-ace-oauth-authz] 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.

   Figure 3 shows the message flow for authorisation purposes.

      Client                                       Broker   AS      KDC
         | [--Resource Request (CoAP/MQTT/other)-->] |       |       |
         |                                           |       |       |
         | [<----AS Information (CoAP/MQTT/other)--] |       |       |
         |                                                   |       |
         | ----- Authorisation Request (CoAP/HTTP/other)---->|       |
         |                                                   |       |
         | <------Authorisation Response (CoAP/HTTP/other) --|       |
         |                                                           |
         |----------------------Token Post (CoAP)------------------->|
         |                                                           |
         |------------------- Joining Request (CoAP) --------------->|
         |                                                           |
         |------------------ Joining Response (CoAP) --------------->|

                        Figure 3: Authorisation Flow






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3.1.  AS Discovery (Optional)

   Complementary to what is defined in [I-D.ietf-ace-oauth-authz]
   (Section 5.1) for AS discovery, the Broker MAY send the address of
   the AS to the Client in the 'AS' parameter in the AS Information as a
   response to an Unauthorized Resource Request (Section 5.2).  An
   example using CBOR diagnostic notation and CoAP is given below:

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

                      Figure 4: AS Information example

   Authorisation Server (AS) Discovery is also 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).

3.2.  Authorising to the Broker

   After retrieving the AS address, the Client sends an Authorisation
   Request to the AS for the KDC and the Broker.  Note that the AS
   authorises:

   1.  What endpoints are allowed to Publish or Subscribe to the Broker.

   2.  What endpoints are allowed to access to which topic(s).

   The request includes the following fields from the Authorization
   Request (Section 3.1 of [I-D.ietf-ace-key-groupcomm]):

   *  'scope', containing the topic identifier, that the Client wishes
      to access

   *  'audience', an array with identifiers of the KDC and the Broker.

   Other additional parameters can be included if necessary, as defined
   in [I-D.ietf-ace-oauth-authz].

   The 'scope' parameter is encoded as follows, where 'gname' is treated
   as topic identifier or filter.










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              gname = tstr

              role = tstr

              scope_entry = [ gname , ? ( role / [ 2*role ] ) ]

              scope = << [ + scope_entry ] >>

      Figure 5: CDLL definition of scope, using as example group name
                     encoded as tstr and role as tstr.

   Other scope representations are also possible and are described in
   (Section 3.1 of [I-D.ietf-ace-key-groupcomm]).  Note that in the AIF-
   MQTT data model is described in Section 3 of the
   [I-D.ietf-ace-mqtt-tls-profile], the role values have been further
   constrained to "pub" and "sub".

   The AS responds with an Authorization Response as defined in
   Section 5.8.2 of [I-D.ietf-ace-oauth-authz] and Section 3.2 of
   [I-D.ietf-ace-key-groupcomm].  If a token is returned, then the
   audience of this token are the KDC and the Broker, and the client
   uses the same token for both.  In case CoAP PubSub is used as
   communication protocol, 'profile' is set to "coap_pubsub_app" as
   defined in Section 8.1.1.  In case MQTT PubSub is used as
   communication protocol, 'profile' is set to "mqtt_pubsub_app" as
   defined in Section 8.1.2.

4.  Key Distribution for PubSub Content Protection

4.1.  Token POST

   After receiving a token from the AS, the Client posts the token to
   the KDC (Section 3.3 [I-D.ietf-ace-key-groupcomm]).  In addition to
   the token post, a Subscriber Client MAY ask for the public keys in
   the group, used for source authentication, as well as any other group
   parameters.  In this case, the message MUST have Content-Format set
   to "application/ace+cbor" defined in Section 8.16 of
   [I-D.ietf-ace-oauth-authz].  The message payload MUST be formatted as
   a CBOR map, which MUST include the access token and the 'sign_info'
   parameter.  The details for the 'sign_info' parameter can be found in
   Section 3.3 of [I-D.ietf-ace-key-groupcomm].  Alternatively, the
   joining node may retrieve this information by other means as
   described in [I-D.ietf-ace-key-groupcomm].








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   The KDC verifies that the Client is authorized to access the topic
   with the requested role.  After successful verification, the Client
   is authorized to receive the group keying material from the KDC and
   join the group.  The KDC replies to the Client with a 2.01 (Created)
   response, using Content-Format "application/ace+cbor".  The payload
   of the 2.01 response is a CBOR map.

   A Publisher Client MUST send its own public key to the KDC when
   joining the group.  Since the access token from a Publisher Client
   will have "pub" role, the KDC MUST include 'kdcchallenge' in the CBOR
   map, specifying a dedicated challenge N_S generated by the KDC.  The
   Client uses this challenge to prove possession of its own private key
   (see [I-D.ietf-ace-key-groupcomm] for details).

4.2.  Join Request

   In the next step, a joining node MUST have a secure communication
   association established with the KDC, before starting to join a group
   under that KDC.  Possible ways to provide a secure communication
   association are described in the DTLS transport profile
   [I-D.ietf-ace-dtls-authorize] and OSCORE transport profile
   [I-D.ietf-ace-oscore-profile] of ACE.

   After establishing a secure communication, the Client sends a Joining
   Request to the KDC as described in Section 4.3 of
   [I-D.ietf-ace-key-groupcomm].  More specifically, the Client sends a
   POST request to the /ace-group/GROUPNAME endpoint on KDC, with
   Content-Format = "application/ace+cbor" that MUST contain in the
   payload (formatted as a CBOR map, Section 4.1.2.1 of
   [I-D.ietf-ace-key-groupcomm]):

   *  'scope' parameter as defined earlier

   *  'get_pub_keys' parameter set to the empty array if the Client
      needs to retrieve the public keys of the other pubsub members,

   *  'client_cred' parameter containing the Client's public key
      formatted as a COSE_Key (as defined in Section 8.2), if the Client
      is a Publisher,

   *  'cnonce', encoded as a CBOR byte string, and including a dedicated
      nonce N_C generated by the Client, if 'client_cred' is present,

   *  'client_cred_verify', set to a singature computed over the
      'rsnonce' concatenated with cnonce, if 'client_cred' is present,

   *  OPTIONALLY, if needed, the 'pub_keys_repos' parameter




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   TODO: Check 'cnonce'

   Note that for a Subscriber-only Client, the Joining Request MUST NOT
   contain the 'client_cred parameter', the role element in the 'scope'
   parameter MUST be set to "sub".  The Subscriber MUST have access to
   the public keys of all the Publishers; this MAY be achieved in the
   Joining Request by using the parameter 'get_pub_keys' set to receive
   the public key of all Publishers using "pub" as the 'role_filter' (as
   described in Section 4.1.2.1 of [I-D.ietf-ace-key-groupcomm]).

   If the 'client_cred' parameter is present, KDC stores the public key
   of the Client.  Note that the alg parameter in the 'client_cred'
   COSE_Key MUST be a signing algorithm, as defined in section 8 of
   [RFC8152], and that it is the same algorithm used to compute the
   signature sent in 'client_cred_verify'.

   The KDC response to Joining Response has the Content-Format =
   "application/ace+cbor".  The payload (formatted as a CBOR map) MUST
   contain the following fields from the Joining Response (Section 4.2
   of [I-D.ietf-ace-key-groupcomm]):

   *  'kty' identifies a key type "COSE_Key".

   *  'key', which contains a "COSE_Key" object (defined in [RFC8152],
      containing:

      -  'kty' with value 4 (symmetric)

      -  'alg' with value defined by the AS2 (Content Encryption
         Algorithm)

      -  'Base IV' with value defined by the AS2

      -  'k' with value the symmetric key value

      -  OPTIONALLY, 'kid' with an identifier for the key value

   *  OPTIONALLY, 'exp' with the expiration time of the key

   *  'pub_keys', containing the public keys of all authorized signing
      members formatted as COSE_Keys, if the 'get_pub_keys' parameter
      was present and set to the empty array in the Key Distribution
      Request.  For Subscriber Clients, the Joining Response MUST
      contain the 'pub_keys' parameter.







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   An example of the Joining Request and corresponding Response for a
   CoAP Publisher using CoAP and CBOR is specified in Figure 6 and
   Figure 7, where SIG is a signature computed using the private key
   associated to the public key and the algorithm in 'client_cred'.

   {
     "scope" : ["Broker1/Temp", "pub"],
     "client_cred" :
       { / COSE_Key /
         / type / 1 : 2, / EC2 /
         / kid / 2 : h'11',
         / alg / 3 : -7, / ECDSA with SHA-256 /
         / crv / -1 : 1 , / P-256 /
         / x / -2 : h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de1
         08de439c08551d',
         / y /-3 : h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e
         9eecd0084d19c',
     "cnonce" : h'd36b581d1eef9c7c,
     "client_cred_verify" : SIG
       }
   }

             Figure 6: Joining Request payload for a Publisher

         {
           "kty" : "COSE_Key",
           "key" : {1: 4, 2: h'1234', 3: 12, 5: h'1f389d14d17dc7',
                   -1: h'02e2cc3a9b92855220f255fff1c615bc'}
         }

             Figure 7: Joining Response payload for a Publisher

   An example of the payload of a Joining Request and corresponding
   Response for a Subscriber using CoAP and CBOR is specified in
   Figure 8 and Figure 9.

                  {
                    "scope" : ["Broker1/Temp", "sub"],
                    "get_pub_keys" : [true, ["pub"], []]
                  }

             Figure 8: Joining Request payload for a Subscriber









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   {
     "scope" : ["Broker1/Temp", "sub"],
     "kty" : "COSE_Key"
     "key" : {1: 4, 2: h'1234', 3: 12, 5: h'1f389d14d17dc7',
             -1: h'02e2cc3a9b92855220f255fff1c615bc'},
     "pub_keys" : [
      {
         1 : 2, / type EC2 /
         2 : h'11', / kid /
         3 : -7, / alg ECDSA with SHA-256 /
         -1 : 1 , / crv P-256 /
         -2 : h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de43
         9c08551d', / x /
         -3 : h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd
         0084d19c' / y /
       }
     ]
   }

            Figure 9: Joining Response payload for a Subscriber

5.  PubSub Protected Communication

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

      Figure 10: Secure communication between Publisher and Subscriber

   (D) corresponds to the publication of a topic on the Broker.  The
   publication (the resource representation) is protected with COSE
   ([RFC8152]).  The (E) message is the subscription of the Subscriber.
   The subscription MAY be unprotected.  The (F) message is the response
   from the Broker, where the publication is protected with COSE.

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

   Figure 11: (E), (F), (G): Example of protected communication for CoAP




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   The flow graph is presented below for CoAP.  The message flow is
   similar for MQTT, where PUT corresponds to a PUBLISH message, and GET
   corresponds to a SUBSCRIBE message.  Whenever a Client publishes a
   new message, the Broker sends this message to all valid subscribers.

5.1.  Using COSE Objects To Protect The Resource Representation

   The Publisher uses the symmetric COSE Key received from the KDC
   (Section 4) to protect the payload of the PUBLISH operation
   (Section 4.3 of [I-D.ietf-core-coap-pubsub] and
   [MQTT-OASIS-Standard-v5]).  Specifically, the COSE Key is used to
   create a COSE_Encrypt0 with algorithm specified by KDC.  The
   Publisher uses the private key corresponding to the public key sent
   to the KDC in exchange B (Section 4) to countersign the COSE Object
   as specified in Section 4.5 of [RFC8152].  The payload is replaced by
   the COSE object before the publication is sent to the Broker.

   The Subscriber uses the 'kid' in the 'countersignature' field in the
   COSE object to retrieve the right public key to verify the
   countersignature.  It then uses the symmetric key received from KDC
   to verify and decrypt the publication received in the payload from
   the Broker (in the case of CoAP the publication is received by the
   CoAP Notification and for MQTT, it is received as a PUBLISH message
   from the Broker to the subscribing client).

   The COSE object is constructed in the following way:

   *  The protected Headers (as described in Section 3 of [RFC8152]) MAY
      contain the kid parameter, with value the kid of the symmetric
      COSE Key received in Section 4 and MUST contain the content
      encryption algorithm.

   *  The unprotected Headers MUST contain the Partial IV, with value a
      sequence number that is incremented for every message sent, and
      the counter signature that includes:

      -  the algorithm (same value as in the asymmetric COSE Key
         received in (B)) in the protected header;

      -  the kid (same value as the kid of the asymmetric COSE Key
         received in (B)) in the unprotected header;

      -  the signature computed as specified in Section 4.5 of
         [RFC8152].

   *  The ciphertext, computed over the plaintext that MUST contain the
      message payload.




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   The 'external_aad' is an empty string.

   An example is given in Figure 12:

        16(
          [
            / protected / h'a2010c04421234' / {
                \ alg \ 1:12, \ AES-CCM-64-64-128 \
                \ kid \ 4: h'1234'
              } / ,
            / unprotected / {
              / iv / 5:h'89f52f65a1c580',
              / countersign / 7:[
                / protected / h'a10126' / {
                  \ alg \ 1:-7
                } / ,
                / unprotected / {
                  / kid / 4:h'11'
                },
                / signature / SIG / 64 bytes signature /
              ]
            },
            / ciphertext / h'8df0a3b62fccff37aa313c8020e971f8aC8d'
          ]
        )

         Figure 12: Example of COSE Object sent in the payload of a
                             PUBLISH operation

   The encryption and decryption operations are described in sections
   5.3 and 5.4 of [RFC8152].

6.  Profile-specific Considerations

   This section summarises the CoAP and MQTT specific pub-sub
   communications, and considerations respectively.

6.1.  CoAP PubSub Application Profile

   A CoAP Pub-Sub Client and Broker use an ACE transport profile such as
   DTLS [I-D.ietf-ace-dtls-authorize], OSCORE
   [I-D.ietf-ace-oscore-profile].

   As shown in Figure 1, (A) is an Access Token Request and Response
   exchange between Publisher and Authorization Server to retrieve the
   Access Token and RS (Broker) Information.  As specified, the Client
   has the role of a CoAP client, the Broker has the role of the CoAP
   server.



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   (B) corresponds to the retrieval of the keying material to protect
   the publication end-to-end (see Section 5.1), and uses
   [I-D.ietf-ace-key-groupcomm].  The details are defined in Section 4.

   (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), this can be for example DTLS handshake, or other
   protocols.  Depending on the application, there may not be the need
   for this set up phase: for example, if OSCORE is used directly.  Note
   that, in line with what defined in the ACE transport profile used,
   the access token includes the scope (i.e. pubsub topics on the
   Broker) the Publisher is allowed to publish to.  For implementation
   simplicity, it is RECOMMENDED that the ACE transport profile used and
   this specification use the same format of "scope".

   After the previous phases have taken place, the pub-sub communication
   can commence.  The operations of publishing and subscribing are
   defined in [I-D.ietf-core-coap-pubsub].

6.2.  MQTT PubSub Application Profile

   The steps MQTT clients go through are similar to the CoAP clients as
   described in Section 6.1.  The payload that is carried in MQTT
   messages will be protected using COSE.

   In MQTT, topics are organised as a tree, and in the
   [I-D.ietf-ace-mqtt-tls-profile] 'scope' captures permissions for not
   a single topic but a topic filter.  Therefore, topic names (i.e.,
   group names) may include wildcards spanning several levels of the
   topic tree.  Hence, it is important to distinguish application groups
   and security groups defined in [I-D.ietf-core-groupcomm-bis].  An
   application group has relevance at the application level - for
   example, in MQTT an application group could denote all topics stored
   under ""home/lights/".  On the other hand, a security group is a
   group of endpoints that each store group security material to
   exchange secure communication within the group.  The group
   communication in [I-D.ietf-ace-key-groupcomm] refers to security
   groups.

   To be able join the right security group associated with requested
   topics (application groups), the client needs to discover the
   (application group, security group) association.  In MQTT, $SYS/ has
   been widely adopted as a prefix to topics that contain broker-
   specific information, and hence, can be used by the broker for this
   purpose.  In typical implementations, Clients that subscribe to one
   or more SYS-Topics receive the current value on the SYS topics as
   soon as they subscribe, and then after periodically.



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   For an MQTT client we envision the following steps to take place:

   1.  Client learns the (application group, security group)
       associations from the $SYS topic (this topic is RECOMMENDED to be
       a protected topic).  These associations MAY be published under
       another topic.

   2.  Client computes the corresponding security groups for its
       application groups, and sends token requests for the security
       groups to AS.

   3.  Client sends join requests to KDC to gets the keys for these
       security groups.

   4.  Client authorises to the Broker with the token (described in
       [I-D.ietf-ace-mqtt-tls-profile]).

   5.  A Publisher Client sends PUBLISH messages for a given topic and
       protects the payload with the corresponding key for the
       associated security group.  RS validates the PUBLISH message by
       checking the topic's security group association and the stored
       token.

   6.  A Subscriber Client may send SUBSCRIBE messages with one or
       multiple topic filters.  A topic filter may correspond to
       multiple topics but MUST belong to a single security group.  If
       requested topics are in multiple security groups, then these
       topics SHOULD be separated into the corresponding topic filters
       in the SUBSCRIBE message.

7.  Security Considerations

   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 be able to verify the publications.  Such a database could
   be set up and managed by the same entity having control of the topic,
   i.e. KDC.












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   An application where it is not critical that only authorized
   Publishers can publish on a topic may decide not to make use of the
   asymmetric crypto and only use symmetric encryption/MAC to
   confidentiality and integrity protection of the publication.
   However, this is not recommended since, as a result, any authorized
   Subscribers with access to the Broker may forge unauthorized
   publications without being detected.  In this symmetric case the
   Subscribers would only need one symmetric key per topic, and would
   not need to know any information about the Publishers, that can be
   anonymous to it and the Broker.

   Subscribers 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 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.

   TODO: expand on security and privacy considerations

8.  IANA Considerations

8.1.  ACE Groupcomm Profile Registry

   The following registrations are done for the "ACE Groupcomm Profile"
   Registry following the procedure specified in
   [I-D.ietf-ace-key-groupcomm].

   Note to RFC Editor: Please replace all occurrences of "[[This
   document]]" with the RFC number of this specification and delete this
   paragraph.

8.1.1.  CoAP Profile Registration

   Name: coap_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 Key: TBD

   Reference: [[This document]]





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8.1.2.  MQTT Profile Registration

   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 Key: TBD

   Reference: [[This document]]

8.2.  ACE Groupcomm Key Registry

   The following registrations are done for the ACE Groupcomm Key
   Registry following the procedure specified in
   [I-D.ietf-ace-key-groupcomm].

   Note to RFC Editor: Please replace all occurrences of "[[This
   document]]" with the RFC number of this specification and delete this
   paragraph.

   Name: COSE_Key

   Key Type Value: TBD

   Profile: coap_pubsub_app

   Description: COSE_Key object

   References: [RFC8152], [[This document]]

9.  References

9.1.  Normative References

   [I-D.ietf-ace-key-groupcomm]
              Palombini, F. and M. Tiloca, "Key Provisioning for Group
              Communication using ACE", Work in Progress, Internet-
              Draft, draft-ietf-ace-key-groupcomm-11, 22 February 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-key-
              groupcomm-11.txt>.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,



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              draft-ietf-ace-oauth-authz-40, 26 April 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
              authz-40.txt>.

   [I-D.ietf-core-coap-pubsub]
              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", Work in Progress, Internet-Draft, draft-ietf-
              core-coap-pubsub-09, 30 September 2019,
              <https://www.ietf.org/archive/id/draft-ietf-core-coap-
              pubsub-09.txt>.

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", Work in
              Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
              03, 22 February 2021, <https://www.ietf.org/archive/id/
              draft-ietf-core-groupcomm-bis-03.txt>.

   [MQTT-OASIS-Standard-v5]
              Banks, A., Briggs, E., Borgendale, K., and R. Gupta,
              "OASIS Standard MQTT Version 5.0", 2017,
              <http://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-
              v5.0-os.html>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

9.2.  Informative References

   [I-D.ietf-ace-actors]
              Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An
              architecture for authorization in constrained



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              environments", Work in Progress, Internet-Draft, draft-
              ietf-ace-actors-07, 22 October 2018,
              <https://www.ietf.org/archive/id/draft-ietf-ace-actors-
              07.txt>.

   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE)", Work in Progress,
              Internet-Draft, draft-ietf-ace-dtls-authorize-16, 8 March
              2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
              dtls-authorize-16.txt>.

   [I-D.ietf-ace-mqtt-tls-profile]
              Sengul, C. and A. Kirby, "Message Queuing Telemetry
              Transport (MQTT)-TLS profile of Authentication and
              Authorization for Constrained Environments (ACE)
              Framework", Work in Progress, Internet-Draft, draft-ietf-
              ace-mqtt-tls-profile-11, 14 April 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-mqtt-tls-
              profile-11.txt>.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE Profile of the Authentication and Authorization
              for Constrained Environments Framework", Work in Progress,
              Internet-Draft, draft-ietf-ace-oscore-profile-18, 14 April
              2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
              oscore-profile-18.txt>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

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 encoding and value of the identifier of group or
      topic of 'scope': see Section 4).

   *  REQ2: Specify the encoding and value of roles of 'scope': see
      Section 4).





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   *  REQ3: Optionally, specify the acceptable values for 'sign_alg':
      TODO

   *  REQ4: Optionally, specify the acceptable values for
      'sign_parameters': TODO

   *  REQ5: Optionally, specify the acceptable values for
      'sign_key_parameters': TODO

   *  REQ6: Optionally, specify the acceptable values for 'pub_key_enc':
      TODO

   *  REQ7: Specify the exact format of the 'key' value: COSE_Key, see
      Section 4.

   *  REQ8: Specify the acceptable values of 'kty' : "COSE_Key", see
      Section 4.

   *  REQ9: Specity the format of the identifiers of group members: TODO

   *  REQ10: Optionally, specify the format and content of
      'group_policies' entries: not defined

   *  REQ11: Specify the communication protocol the members of the group
      must use: CoAP pub/sub.

   *  REQ12: Specify the security protocol the group members must use to
      protect their communication.  This must provide encryption,
      integrity and replay protection: Object Security of Content using
      COSE, see Section 5.1.

   *  REQ13: Specify and register the application profile identifier :
      "coap_pubsub_app", see Section 8.1.

   *  REQ14: Optionally, specify the encoding of public keys, of
      'client_cred', and of 'pub_keys' if COSE_Keys are not used: NA.

   *  REQ15: Specify policies at the KDC to handle id that are not
      included in get_pub_keys: TODO

   *  REQ16: Specify the format and content of 'group_policies': TODO

   *  REQ17: Specify the format of newly-generated individual keying
      material for group members, or of the information to derive it,
      and corresponding CBOR label : not defined






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   *  REQ18: Specify how the communication is secured between Client and
      KDC.  Optionally, specify tranport profile of ACE
      [I-D.ietf-ace-oauth-authz] to use between Client and KDC: pre-set,
      as KDC is AS.

   *  OPT1: Optionally, specify the encoding of public keys, of
      'client_cred', and of 'pub_keys' if COSE_Keys are not used: NA

   *  OPT2: Optionally, specify the negotiation of parameter values for
      signature algorithm and signature keys, if 'sign_info' and
      'pub_key_enc' are not used: NA

   *  OPT3: Optionally, specify the format and content of
      'mgt_key_material': not defined

   *  OPT4: Optionally, specify policies that instruct clients to retain
      unsuccessfully decrypted messages and for how long, so that they
      can be decrypted after getting updated keying material: not
      defined

Acknowledgments

   The author wishes to thank Ari Keraenen, John Mattsson, Ludwig Seitz,
   Goeran Selander, Jim Schaad and Marco Tiloca for the useful
   discussion and reviews that helped shape this document.

Authors' Addresses

   Francesca Palombini
   Ericsson

   Email: francesca.palombini@ericsson.com


   Cigdem Sengul
   Brunel University

   Email: csengul@acm.org













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