Pub-Sub Profile for Authentication and Authorization for Constrained Environments (ACE)
draft-ietf-ace-pubsub-profile-01

ACE Working Group                                           F. Palombini
Internet-Draft                                                  Ericsson
Intended status: Standards Track                           July 03, 2020
Expires: January 4, 2021


  Pub-Sub Profile for Authentication and Authorization for Constrained
                           Environments (ACE)
                    draft-ietf-ace-pubsub-profile-01

Abstract

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

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 January 4, 2021.

Copyright Notice

   Copyright (c) 2020 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
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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   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 Application Profiles . . . . . . . . . . . . . . . . .   5
     3.1.  Retrieval of COSE Key for protection of content . . . . .   6
     3.2.  coap_pubsub_app Application Profile . . . . . . . . . . .   9
     3.3.  mqtt_pubsub_app Application Profile . . . . . . . . . . .   9
   4.  Publisher . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  CoAP Publisher  . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  MQTT Publisher  . . . . . . . . . . . . . . . . . . . . .  12
   5.  Subscriber  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  CoAP Subscriber . . . . . . . . . . . . . . . . . . . . .  13
     5.2.  MQTT Subscriber . . . . . . . . . . . . . . . . . . . . .  13
   6.  Pub-Sub Protected Communication . . . . . . . . . . . . . . .  13
     6.1.  Using COSE Objects To Protect The Resource Representation  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  ACE Groupcomm Profile Registry  . . . . . . . . . . . . .  17
       8.1.1.  CoAP Profile Registration . . . . . . . . . . . . . .  17
       8.1.2.  CoAP Profile Registration . . . . . . . . . . . . . .  17
     8.2.  ACE Groupcomm Key Registry  . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Appendix A.  Requirements on Application Profiles . . . . . . . .  19
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The publisher-subscriber setting allows for devices with limited
   reachability to communicate via a broker that enables store-and-
   forward messaging between the devices.  The pub-sub scenario using
   the Constrained Application Protocol (CoAP) is specified in
   [I-D.ietf-core-coap-pubsub], while the one using MQTT is specified in
   REF MQTT.  This document defines a way to authorize nodes in a CoAP
   pub-sub type of setting, using the ACE framework
   [I-D.ietf-ace-oauth-authz], and to provide the keys for protecting
   the communication between these nodes.  This document gives detailed
   specifications for MQTT and CoAP pub-sub, but can easily be adapted
   for other transport protocol as well.




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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 (REF MQTT pubsub).

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 itself expands the Ace framework
   ([I-D.ietf-ace-oauth-authz]), and transport profiles
   ([I-D.ietf-ace-dtls-authorize], [I-D.ietf-ace-oscore-profile], REF
   MQTT profile).  The pub-sub communication protocol can be based on
   CoAP, as described in [I-D.ietf-core-coap-pubsub], MQTT (see REF MQTT
   comm), or other transport.

   The architecture of the scenario is shown in Figure 1.



















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                +----------------+   +----------------+
                |                |   |                |
                | Authorization  |   | Authorization  |
                |    Server 1    |   |    Server 2    |
                |                |   |                |
                +----------------+   +----------------+
                         ^                  ^  ^
                         |                  |  |
        +---------(A)----+                  |  +-----(D)------+
        |   +--------------------(B)--------+                 |
        v   v                                                 v
   +------------+             +------------+              +------------+
   |            |             |            |              |            |
   | Publisher  | ----(E)---> |   Broker   |              | Subscriber |
   |            |             |            | <----(F)---- |            |
   |            |             |            | -----(G)---> |            |
   +------------+             +------------+              +------------+

       Figure 1: Architecture CoAP pubsub with Authorization Servers

   The RS is the broker, which contains the topic.  This node
   corresponds to the Dispatcher, in [I-D.ietf-ace-key-groupcomm].  The
   AS1 hosts the policies about the Broker: what endpoints are allowed
   to Publish on the Broker.  The Clients access this node to get write
   access to the Broker.  The AS2 hosts the policies about the topic:
   what endpoints are allowed to access what topic.  This node
   represents both the AS and Key Distribution Center roles from
   [I-D.ietf-ace-key-groupcomm].

   There are four phases, the first three can be done in parallel.

   1.  The Publisher requests publishing access to the Broker at the
       AS1, and communicates with the Broker to set up security.

   2.  The Publisher requests access to a specific topic at the AS2

   3.  The Subscriber requests access to a specific topic at the AS2.

   4.  The Publisher and the Subscriber securely post to and get
       publications from the Broker.

   This exchange aims at setting up 2 different security associations:
   on the one hand, the Publisher has a security association with the
   Broker, to protect the communication and securely authorize the
   Publisher to publish on a topic (Security Association 1).  On the
   other hand, the Publisher has a security association with the
   Subscriber, to protect the publication content itself (Security
   Association 2).  The Security Association 1 is set up using AS1 and a



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   transport profile of [I-D.ietf-ace-oauth-authz], the Security
   Association 2 is set up using AS2 and [I-D.ietf-ace-key-groupcomm].

   Note that, analogously to the Publisher, the Subscriber can also set
   up an additional security association with the Broker, using an AS,
   in the same way the Publisher does with AS1.  In this case, only
   authorized Subscribers would be able to get notifications from the
   Broker.  The overhead would be that each Subscriber should access the
   AS and get all the information to start a secure exchange with the
   Broker.

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

   Note that AS1 and AS2 might either be co-resident or be 2 separate
   physical entities, in which case access control policies must be
   exchanged between AS1 and AS2, so that they agree on rights for
   joining nodes about specific topics.  How the policies are exchanged
   is out of scope for this specification.

3.  PubSub Application Profiles

   Each profile defined in this document uses
   [I-D.ietf-ace-key-groupcomm], which expands the ACE framework.  This
   section defines which exact parameters from
   [I-D.ietf-ace-key-groupcomm] have to be used, and the values for each
   parameter.  Since [I-D.ietf-ace-oauth-authz] recommends the use of
   CoAP anc 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 4.1 and 4.2 of [RFC7049].  In case JSON is
   used, the Content Format or Media Type of the message has to be
   changed accordingly.

   The Publisher and the Subscriber map to the Client in
   [I-D.ietf-ace-key-groupcomm], the AS2 maps to the AS and to the KDC,
   the Broker maps to the Dispatcher.




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   Note that both publishers and subscribers use the same profile.

3.1.  Retrieval of COSE Key for protection of content

   This phase is common to both Publisher and Subscriber.  To maintain
   the generality, the Publisher or Subscriber is referred as Client in
   this section.

           Client                            Broker             AS2
              | [----- Resource Request ---->] |                 |
              |                                |                 |
              | [<-- AS1, AS2 Information ---] |                 |
              |                                                  |
              | [------ Pub Key Format Negociation Request --->] |
              |                                                  |
              | [<---- Pub Key Format Negociation Response ----] |
              |                                                  |
              | -- Authorization + Key Distribution Request ---> |
              |                                                  |
              | <-- Authorization + Key Distribution Response -- |
              |                                                  |

                  Figure 2: B: Access request - response

   Complementary to what is defined in [I-D.ietf-ace-oauth-authz]
   (Section 5.1.1), to determine the AS2 in charge of a topic hosted at
   the Broker, the Broker MAY send the address of both the AS in charge
   of the topic back to the Client in the 'AS' parameter in the AS
   Information, as a response to an Unauthorized Resource Request
   (Section 5.1.2).  The uri of AS2 is concatenated to the uri of AS1,
   and separated by a comma.  An example using CBOR diagnostic notation
   and CoAP is given below:

                    4.01 Unauthorized
                    Content-Format: application/ace+cbor
                    {"AS": "coaps://as1.example.com/token,
                    coaps://as2.example.com/pubsubkey"}

                  Figure 3: AS1, AS2 Information example

   After retrieving the AS2 address, the Client MAY send a request to
   the AS, in order to retrieve necessary information concerning the
   public keys in the group, as well as concerning the algorithm and
   related parameters for computing signatures in the group.  This
   request is a subset of the Token POST request defined in Section 3.3
   of [I-D.ietf-ace-key-groupcomm], specifically a CoAP POST request to
   a specific resource at the AS, including only the parameters
   'sign_info' and 'pub_key_enc' in the CBOR map in the payload.  The



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   default url-path for this resource is /ace-group/gid/cs-info, where
   "gid" is the topic identifier, but implementations are not required
   to use this name, and can use their own instead.  The AS MUST respond
   with the response defined in Section 3.3 of
   [I-D.ietf-ace-key-groupcomm], specifically including the parameters
   'sign_info', 'pub_key_enc', and 'rsnonce' (8 bytes pseudo-random
   nonce generated by the AS).

   After that, the Client sends an Authorization + Joining Request,
   which is an Authorization Request merged with a Joining Request, as
   described in [I-D.ietf-ace-key-groupcomm], Sections 3.1 and 4.2.  The
   reason for merging these two messages is that the AS2 is both the AS
   and the KDC, in this setting, so the Authorization Response and the
   Post Token message are not necessary.

   More specifically, the Client sends a POST request to the /ace-group/
   gid endpoint on AS2, with Content-Format = "application/ace+cbor"
   that MUST contain in the payload (formatted as a CBOR map):

   o  the following fields from the Joining Request (Section 4.2 of
      [I-D.ietf-ace-key-groupcomm]):

      *  'scope' parameter set to a CBOR array containing:

         +  the broker's topic as first element, and

         +  the text string "publisher" if the client request to be a
            publisher, "subscriber" if the client request to be a
            subscriber, or a CBOR array containing both, if the client
            request to be both.

      *  '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, if the Client needs to directly send
         that to the AS2,

      *  'cnonce', set to a 8 bytes long pseudo-random nonce, 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

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



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      *  OPTIONALLY, if needed, additional parameters such as
         'client_id'

   TODO: 'cnonce' might change name.  TODO: register media type ace+json
   for HTTP?

   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'.

   Examples of the payload of a Authorization + Joining Request are
   specified in Figure 5 and Figure 8.

   The AS2 verifies that the Client is authorized to access the topic
   and, if the 'client_cred' parameter is present, stores the public key
   of the Client.

   The AS2 response is an Authorization + Joining Response, with
   Content-Format = "application/ace+cbor".  The payload (formatted as a
   CBOR map) MUST contain:

   o  the following fields from the Joining Response (Section 4.2 of
      [I-D.ietf-ace-key-groupcomm]):

      *  'kty' identifies a key type "COSE_Key", as defined in
         Section 8.2.

      *  '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
         Authorization + Key Distribution Request



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   o  the following fields from the Authorization Response (Section 3.2
      of [I-D.ietf-ace-key-groupcomm]):

      *  'profile' set to the corresponding value, see Section 3.2 or
         Section 3.3

      *  OPTIONALLY 'scope', set to a CBOR array containing:

         +  the broker's topic as first element, and

         +  the string "publisher" if the client is an authorized
            publisher, "subscriber" if the client is an authorized
            subscriber, or a CBOR array containing both, if the client
            is authorized to be both.

   Examples for the response payload are detailed in Figure 6 and
   Figure 9.

3.2.  coap_pubsub_app Application Profile

   In case CoAP PubSub is used as communication protocol:

   o  'profile' set to "coap_pubsub_app", as specified in Section 8.1.1.

3.3.  mqtt_pubsub_app Application Profile

   In case mQTT PubSub is used as communication protocol:

   o  'profile' set to "mqtt_pubsub_app", as specified in Section 8.1.2.

4.  Publisher

   In this section, it is specified how the Publisher requests, obtains
   and communicates to the Broker the access token, as well as the
   retrieval of the keying material to protect the publication.
















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                        +----------------+   +----------------+
                        |                |   |                |
                        | Authorization  |   | Authorization  |
                        |    Server 1    |   |    Server 2    |
                        |                |   |                |
                        +----------------+   +----------------+
                                 ^                  ^
                                 |                  |
                +---------(A)----+                  |
                |   +--------------------(B)--------+
                v   v
           +------------+             +------------+
           |            | ----(C)---> |            |
           | Publisher  |             |   Broker   |
           |            |             |            |
           |            |             |            |
           +------------+             +------------+

                     Figure 4: Phase 1: Publisher side

   This is a combination of two independent phases:

   o  one is the establishment of a secure connection between Publisher
      and Broker, using an ACE transport profile such as DTLS
      [I-D.ietf-ace-dtls-authorize], OSCORE
      [I-D.ietf-ace-oscore-profile] or REF MQTT Profile.  (A)(C)

   o  the other is the Publisher's retrieval of keying material to
      protect the publication.  (B)

   In detail:

   (A) corresponds to the Access Token Request and Response between
   Publisher and Authorization Server to retrieve the Access Token and
   RS (Broker) Information.  As specified, the Publisher has the role of
   a CoAP client, the Broker has the role of the CoAP server.

   (C) corresponds to the exchange between Publisher and Broker, where
   the Publisher 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
   semplicity, it is RECOMMENDED that the ACE transport profile used and
   this specification use the same format of "scope".



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   (A) and (C) details are specified in the profile used.

   (B) corresponds to the retrieval of the keying material to protect
   the publication end-to-end with the subscribers (see Section 6.1),
   and uses [I-D.ietf-ace-key-groupcomm].  The details are defined in
   Section 3.1.

4.1.  CoAP Publisher

   An example of the payload of an Authorization + Joining Request and
   corresponding Response for a CoAP Publisher using CoAP and CBOR is
   specified in Figure 5 and Figure 6, where SIG is a signature computed
   using the private key associated to the public key and the algorithm
   in "client_cred".

   {
     "scope" : ["Broker1/Temp", "publisher"],
     "client_id" : "publisher1",
     "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 5: Authorization + Joining Request payload for a Publisher

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

    Figure 6: Authorization + Joining Response payload for a Publisher








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4.2.  MQTT Publisher

   TODO

5.  Subscriber

   In this section, it is specified how the Subscriber retrieves the
   keying material to protect the publication.

                                     +----------------+
                                     |                |
                                     | Authorization  |
                                     |    Server 2    |
                                     |                |
                                     +----------------+
                                               ^
                                               |
                                               +-----(D)------+
                                                              |
                                                              v
                                                          +------------+
                                                          |            |
                                                          | Subscriber |
                                                          |            |
                                                          |            |
                                                          +------------+

                    Figure 7: Phase 2: Subscriber side

   Step (D) between Subscriber and AS2 corresponds to the retrieval of
   the keying material to verify the publication end-to-end with the
   publishers (see Section 6.1).  The details are defined in Section 3.1

   This step is the same as (B) between Publisher and AS2 (Section 3.1),
   with the following differences:

   o  The Authorization + Joining Request MUST NOT contain the
      'client_cred parameter', the role element in the 'scope' parameter
      MUST be set to "subscriber".  The Subscriber MUST have access to
      the public keys of all the Publishers; this MAY be achieved in the
      Authorization + Joining Request by using the parameter
      'get_pub_keys' set to empty array.

   o  The Authorization + Key Distribution Response MUST contain the
      'pub_keys' parameter.






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5.1.  CoAP Subscriber

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

                {
                  "scope" : ["Broker1/Temp", "subscriber"],
                  "get_pub_keys" : [ ]
                }

    Figure 8: Authorization + Joining Request payload for a Subscriber

   {
     "profile" : "coap_pubsub_app",
     "scope" : ["Broker1/Temp", "subscriber"],
     "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: Authorization + Joining Response payload for a Subscriber

5.2.  MQTT Subscriber

   TODO

6.  Pub-Sub Protected Communication

   This section specifies the communication Publisher-Broker and
   Subscriber-Broker, after the previous phases have taken place.  The
   operations of publishing and subscribing are defined in
   [I-D.ietf-core-coap-pubsub].






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

      Figure 10: Phase 3: Secure communication between Publisher and
                                Subscriber

   The (E) message corresponds to the publication of a topic on the
   Broker.  The publication (the resource representation) is protected
   with COSE ([RFC8152]).  The (F) message is the subscription of the
   Subscriber, which is unprotected, unless a profile of ACE
   [I-D.ietf-ace-oauth-authz] is used between Subscriber and Broker.
   The (G) message is the response from the Broker, where the
   publication is protected with COSE.

   The flow graph is presented below.

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

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

6.1.  Using COSE Objects To Protect The Resource Representation

   The Publisher uses the symmetric COSE Key received from AS2 in
   exchange B (Section 3.1) to protect the payload of the PUBLISH
   operation (Section 4.3 of [I-D.ietf-core-coap-pubsub] and REF MQTT).
   Specifically, the COSE Key is used to create a COSE_Encrypt0 with
   algorithm specified by AS2.  The Publisher uses the private key
   corresponding to the public key sent to the AS2 in exchange B
   (Section 3.1) to countersign the COSE Object as specified in
   Section 4.5 of [RFC8152].  The CoAP 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 AS2
   to verify and decrypt the publication received in the payload of the
   CoAP Notification from the Broker.




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   The COSE object is constructed in the following way:

   o  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 3.1 and MUST contain the content
      encryption algorithm.

   o  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].

   o  The ciphertext, computed over the plaintext that MUST contain the
      CoAP payload.

   The external_aad is an empty string.

   An example is given in Figure 12

























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        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].

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. AS2.

   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 protect the publication, but 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.



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   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]]

8.1.2.  CoAP 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]]




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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", draft-ietf-ace-key-groupcomm-07
              (work in progress), June 2020.

   [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)", draft-ietf-ace-oauth-authz-35
              (work in progress), June 2020.

   [I-D.ietf-core-coap-pubsub]
              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", draft-ietf-core-coap-pubsub-09 (work in
              progress), September 2019.

   [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>.





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   [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>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

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

9.2.  Informative References

   [I-D.ietf-ace-actors]
              Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An
              architecture for authorization in constrained
              environments", draft-ietf-ace-actors-07 (work in
              progress), October 2018.

   [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)", draft-ietf-ace-dtls-
              authorize-11 (work in progress), June 2020.

   [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", draft-ietf-ace-
              oscore-profile-11 (work in progress), June 2020.

   [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]

   o  REQ1: Specify the encoding and value of the identifier of group or
      topic of 'scope': see Section 3.1).

   o  REQ2: Specify the encoding and value of roles of 'scope': see
      Section 3.1).



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

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

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

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

   o  REQ7: Specify the exact format of the 'key' value: COSE_Key, see
      Section 3.1.

   o  REQ8: Specify the acceptable values of 'kty' : "COSE_Key", see
      Section 3.1.

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

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

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

   o  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 6.1.

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

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

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

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

   o  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

   o  REQ18: Specify how the communication is secured between Client and
      KDC.  Optionally, specify tranport profile of ACE



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      [I-D.ietf-ace-oauth-authz] to use between Client and KDC: pre-set,
      as KDC is AS.

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

   o  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

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

   o  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, Cigdem Sengul, Jim Schaad and Marco Tiloca for the
   useful discussion and reviews that helped shape this document.

Author's Address

   Francesca Palombini
   Ericsson

   Email: francesca.palombini@ericsson.com





















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