ACE Working Group                                           F. Palombini
Internet-Draft                                               Ericsson AB
Intended status: Standards Track                               M. Tiloca
Expires: 28 April 2022                                           RISE AB
                                                         25 October 2021


           Key Provisioning for Group Communication using ACE
                    draft-ietf-ace-key-groupcomm-14

Abstract

   This document defines how to use the Authentication and Authorization
   for Constrained Environments (ACE) framework to distribute keying
   material and configuration parameters for secure group communication.
   Candidate group members acting as Clients and authorized to join a
   group can do so by interacting with a Key Distribution Center (KDC)
   acting as Resource Server, from which they obtain the keying material
   to communicate with other group members.  While defining general
   message formats as well as the interface and operations available at
   the KDC, this document supports different approaches and protocols
   for secure group communication.  Therefore, details are delegated to
   separate application profiles of this document, as specialized
   instances that target a particular group communication approach and
   define how communications in the group are protected.  Compliance
   requirements for such application profiles are also specified.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/ace-wg/ace-key-groupcomm.

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."



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   This Internet-Draft will expire on 28 April 2022.

Copyright Notice

   Copyright (c) 2021 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
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Authorization to Join a Group . . . . . . . . . . . . . . . .  10
     3.1.  Authorization Request . . . . . . . . . . . . . . . . . .  11
     3.2.  Authorization Response  . . . . . . . . . . . . . . . . .  13
     3.3.  Token Transferring  . . . . . . . . . . . . . . . . . . .  15
       3.3.1.  'sign_info' Parameter . . . . . . . . . . . . . . . .  17
       3.3.2.  'kdcchallenge' Parameter  . . . . . . . . . . . . . .  19
   4.  KDC Functionalities . . . . . . . . . . . . . . . . . . . . .  19
     4.1.  Interface at the KDC  . . . . . . . . . . . . . . . . . .  20
       4.1.1.  Operations Supported by Clients . . . . . . . . . . .  23
       4.1.2.  Error Handling  . . . . . . . . . . . . . . . . . . .  24
     4.2.  /ace-group  . . . . . . . . . . . . . . . . . . . . . . .  26
       4.2.1.  FETCH Handler . . . . . . . . . . . . . . . . . . . .  26
         4.2.1.1.  Retrieve Group Names  . . . . . . . . . . . . . .  27
     4.3.  /ace-group/GROUPNAME  . . . . . . . . . . . . . . . . . .  28
       4.3.1.  POST Handler  . . . . . . . . . . . . . . . . . . . .  28
         4.3.1.1.  Join the Group  . . . . . . . . . . . . . . . . .  41
       4.3.2.  GET Handler . . . . . . . . . . . . . . . . . . . . .  43
         4.3.2.1.  Retrieve Group Keying Material  . . . . . . . . .  44
     4.4.  /ace-group/GROUPNAME/pub-key  . . . . . . . . . . . . . .  45
       4.4.1.  FETCH Handler . . . . . . . . . . . . . . . . . . . .  45
         4.4.1.1.  Retrieve a Subset of Public Keys in the Group . .  47
       4.4.2.  GET Handler . . . . . . . . . . . . . . . . . . . . .  48
         4.4.2.1.  Retrieve All Public Keys in the Group . . . . . .  48
     4.5.  ace-group/GROUPNAME/kdc-pub-key . . . . . . . . . . . . .  49
       4.5.1.  GET Handler . . . . . . . . . . . . . . . . . . . . .  49
         4.5.1.1.  Retrieve the KDC's Public Key . . . . . . . . . .  50
     4.6.  /ace-group/GROUPNAME/policies . . . . . . . . . . . . . .  51



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       4.6.1.  GET Handler . . . . . . . . . . . . . . . . . . . . .  51
         4.6.1.1.  Retrieve the Group Policies . . . . . . . . . . .  52
     4.7.  /ace-group/GROUPNAME/num  . . . . . . . . . . . . . . . .  53
       4.7.1.  GET Handler . . . . . . . . . . . . . . . . . . . . .  53
         4.7.1.1.  Retrieve the Keying Material Version  . . . . . .  54
     4.8.  /ace-group/GROUPNAME/nodes/NODENAME . . . . . . . . . . .  54
       4.8.1.  GET Handler . . . . . . . . . . . . . . . . . . . . .  55
         4.8.1.1.  Retrieve Group and Individual Keying Material . .  56
       4.8.2.  PUT Handler . . . . . . . . . . . . . . . . . . . . .  57
         4.8.2.1.  Request to Change Individual Keying Material  . .  59
       4.8.3.  DELETE Handler  . . . . . . . . . . . . . . . . . . .  60
         4.8.3.1.  Leave the Group . . . . . . . . . . . . . . . . .  60
     4.9.  /ace-group/GROUPNAME/nodes/NODENAME/pub-key . . . . . . .  61
       4.9.1.  POST Handler  . . . . . . . . . . . . . . . . . . . .  61
         4.9.1.1.  Uploading a New Public Key  . . . . . . . . . . .  62
   5.  Removal of a Group Member . . . . . . . . . . . . . . . . . .  63
   6.  Group Rekeying Process  . . . . . . . . . . . . . . . . . . .  65
     6.1.  Point-to-Point Group Rekeying . . . . . . . . . . . . . .  66
     6.2.  One-to-Many Group Rekeying  . . . . . . . . . . . . . . .  67
       6.2.1.  Protection of Rekeying Messages . . . . . . . . . . .  69
   7.  Extended Scope Format . . . . . . . . . . . . . . . . . . . .  72
   8.  ACE Groupcomm Parameters  . . . . . . . . . . . . . . . . . .  74
   9.  ACE Groupcomm Error Identifiers . . . . . . . . . . . . . . .  77
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  79
     10.1.  Secure Communication in the Group  . . . . . . . . . . .  79
     10.2.  Update of Group Keying Material  . . . . . . . . . . . .  80
       10.2.1.  Misalignment of Group Keying Material  . . . . . . .  82
     10.3.  Block-Wise Considerations  . . . . . . . . . . . . . . .  83
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  83
     11.1.  Media Type Registrations . . . . . . . . . . . . . . . .  83
     11.2.  CoAP Content-Formats . . . . . . . . . . . . . . . . . .  84
     11.3.  OAuth Parameters . . . . . . . . . . . . . . . . . . . .  84
     11.4.  OAuth Parameters CBOR Mappings . . . . . . . . . . . . .  85
     11.5.  Interface Description (if=) Link Target Attribute
             Values  . . . . . . . . . . . . . . . . . . . . . . . .  85
     11.6.  CBOR Tags  . . . . . . . . . . . . . . . . . . . . . . .  86
     11.7.  ACE Groupcomm Parameters . . . . . . . . . . . . . . . .  86
     11.8.  ACE Groupcomm Key Types  . . . . . . . . . . . . . . . .  87
     11.9.  ACE Groupcomm Profiles . . . . . . . . . . . . . . . . .  87
     11.10. ACE Groupcomm Policies . . . . . . . . . . . . . . . . .  88
     11.11. Sequence Number Synchronization Methods  . . . . . . . .  89
     11.12. ACE Scope Semantics  . . . . . . . . . . . . . . . . . .  89
     11.13. ACE Groupcomm Errors . . . . . . . . . . . . . . . . . .  90
     11.14. ACE Groupcomm Rekeying Schemes . . . . . . . . . . . . .  90
     11.15. Expert Review Instructions . . . . . . . . . . . . . . .  91
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  92
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  92
     12.2.  Informative References . . . . . . . . . . . . . . . . .  94



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   Appendix A.  Requirements on Application Profiles . . . . . . . .  96
     A.1.  Mandatory-to-Address Requirements . . . . . . . . . . . .  96
     A.2.  Optional-to-Address Requirements  . . . . . . . . . . . .  99
   Appendix B.  Extensibility for Future COSE Algorithms . . . . . . 100
     B.1.  Format of 'sign_info_entry' . . . . . . . . . . . . . . . 100
   Appendix C.  Document Updates . . . . . . . . . . . . . . . . . . 101
     C.1.  Version -13 to -14  . . . . . . . . . . . . . . . . . . . 101
     C.2.  Version -05 to -13  . . . . . . . . . . . . . . . . . . . 102
     C.3.  Version -04 to -05  . . . . . . . . . . . . . . . . . . . 102
     C.4.  Version -03 to -04  . . . . . . . . . . . . . . . . . . . 103
     C.5.  Version -02 to -03  . . . . . . . . . . . . . . . . . . . 103
     C.6.  Version -01 to -02  . . . . . . . . . . . . . . . . . . . 104
     C.7.  Version -00 to -01  . . . . . . . . . . . . . . . . . . . 105
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 105
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 106

1.  Introduction

   This document builds on the Authentication and Authorization for
   Constrained Environments (ACE) framework and defines how to request,
   distribute and renew keying material and configuration parameters to
   protect message exchanges in a group communication environment.

   Candidate group members acting as Clients and authorized to join a
   group can interact with the Key Distribution Center (KDC) acting as
   Resource Server and responsible for that group, in order to obtain
   the necessary keying material and parameters to communicate with
   other group members.

   In particular, this document defines the operations and interface
   available at the KDC, as well as general message formats for the
   interactions between Clients and KDC.  At the same time,
   communications in the group can rely on different approaches, e.g.,
   based on multicast [I-D.ietf-core-groupcomm-bis] or on publish-
   subscribe messaging [I-D.ietf-core-coap-pubsub], and can be protected
   in different ways.

   Therefore, this document delegates details on the communication and
   security approaches used in a group to separate application profiles.
   These are specialized instances of this document, targeting a
   particular group communication approach and defining how
   communications in the group are protected, as well as the specific
   keying material and configuration parameters provided to group
   members.  In order to ensure consistency and aid the development of
   such application profiles, this document defines a number of related
   compliance requirements (see Appendix A).





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   If the application requires backward and forward security, new keying
   material is generated and distributed to the group upon membership
   changes (rekeying).  A group rekeying scheme performs the actual
   distribution of the new keying material, by rekeying the current
   group members when a new Client joins the group, and the remaining
   group members when a Client leaves the group.  This can rely on
   different approaches, including efficient group rekeying schemes such
   as [RFC2093], [RFC2094] and [RFC2627].

   Consistently with what is recommeded in the ACE framework, this
   document uses CBOR [RFC8949] for data encoding.  However, using JSON
   [RFC8259] instead of CBOR is possible, by relying on the conversion
   method specified in Sections 6.1 and 6.2 of [RFC8949].

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are expected to be familiar with:

   *  The terms and concepts described in the ACE framework
      [I-D.ietf-ace-oauth-authz] and in the Authorization Information
      Format (AIF) [I-D.ietf-ace-aif] to express authorization
      information.  The terminology for entities in the considered
      architecture is defined in OAuth 2.0 [RFC6749].  In particular,
      this includes Client (C), Resource Server (RS), and Authorization
      Server (AS).

   *  The terms and concepts described in CoAP [RFC7252].  Unless
      otherwise indicated, the term "endpoint" is used here following
      its OAuth definition, aimed at denoting resources such as /token
      and /introspect at the AS, and /authz-info at the RS.  This
      document does not use the CoAP definition of "endpoint", which is
      "An entity participating in the CoAP protocol".

   *  The terms and concepts described in CBOR [RFC8949] and COSE [I-D.i
      etf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs][I-D.iet
      f-cose-countersign].

   A principal interested to participate in group communication as well
   as already participating as a group member is interchangeably denoted
   as "Client" or "node".





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   Furthermore, this document uses "names" or "identifiers" for groups
   and nodes.  Their different meanings are summarized below.

   *  Group: a set of nodes that share common keying material and
      security parameters used to protect their communications with one
      another.  That is, the term refers to a "security group".

      This is not to be confused with an "application group", which has
      relevance at the application level and whose members share a
      common pool of resources or content.  Examples of application
      groups are the set of all nodes deployed in a same physical room,
      or the set of nodes registered to a pub-sub topic.

      The same security group might be associated to multiple
      application groups.  Also, the same application group can be
      associated to multiple security groups.  Further details and
      considerations on the mapping between the two types of group are
      out of the scope of this document.

   *  Key Distribution Center (KDC): the entity responsible for managing
      one or multiple groups, with particular reference to the group
      membership and the keying material to use for protecting group
      communications.

   *  Group name: the invariant once established identifier of a group.
      It is used in the interactions between Client, AS and RS to
      identify a group.  A group name is always unique among the group
      names of the existing groups under the same KDC.

   *  GROUPNAME: the invariant once established text string used in
      URIs.  GROUPNAME uniquely maps to the group name of a group,
      although they do not necessarily coincide.

   *  Group identifier: the identifier of the group keying material used
      in a group.  Unlike group name and GROUPNAME, this identifier
      changes over time, when the group keying material is updated.

   *  Node name: the invariant once established identifier of a node.
      It is used in the interactions between Client and RS and to
      identify a member of a group.  Within the same group, a node name
      is always unique among the node names of all the current members
      of that group.

   *  NODENAME: the invariant once established text string used in URIs
      to identify a member a group.  Its value coincides with the node
      name of the associated group member.

   This document additionally uses the following terminology:



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   *  Transport profile, to indicate a profile of ACE as per
      Section 5.8.4.3 of [I-D.ietf-ace-oauth-authz].  A transport
      profile specifies the communication protocol and communication
      security protocol between an ACE Client and Resource Server, as
      well as proof-of-possession methods, if it supports proof-of-
      possession access tokens, etc.  Transport profiles of ACE include,
      for instance, [I-D.ietf-ace-oscore-profile],
      [I-D.ietf-ace-dtls-authorize] and [I-D.ietf-ace-mqtt-tls-profile].

   *  Application profile, that defines how applications enforce and use
      supporting security services they require.  These services may
      include, for instance, provisioning, revocation and distribution
      of keying material.  An application profile may define specific
      procedures and message formats.

2.  Overview

   The full procedure can be separated in two phases: the first one
   follows the ACE Framework, between Client, AS and KDC; the second one
   is the key distribution between Client and KDC.  After the two phases
   are completed, the Client is able to participate in the group
   communication, via a Dispatcher entity.

    +------------+               +-----------+
    |     AS     |               |    KDC    |
    |            |        .----->|           |
    +------------+       /       +-----------+
          ^             /
          |            /
          v           /                                 +-----------+
    +------------+   /           +------------+         |+-----------+
    |   Client   |<-'            | Dispatcher |         ||+-----------+
    |            |<------------->|            |<------->|||   Group   |
    +------------+               +------------+         |||  members  |
                                                        +++-----------+

                  Figure 1: Key Distribution Participants

   The following participants (see Figure 1) take part in the
   authorization and key distribution.

   *  Client (C): node that wants to join a group and take part in group
      communication with other group memebrs.  Within the group, the
      Client can have different roles.

   *  Authorization Server (AS): as per the AS defined in the ACE
      Framework, it enforces access policies, and knows if a node is
      allowed to join a given group with write and/or read rights.



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   *  Key Distribution Center (KDC): maintains the keying material to
      protect group communications, and provides it to Clients
      authorized to join a given group.  During the first part of the
      exchange (Section 3), it takes the role of the RS in the ACE
      Framework.  During the second part (Section 4), which is not based
      on the ACE Framework, it distributes the keying material.  In
      addition, it provides the latest keying material to group members
      when requested or, if required by the application, when membership
      changes.

   *  Dispatcher: entity through which the Clients communicate with the
      group, when sending a message intended to multiple group members.
      That is, the Dispatcher distributes such a one-to-many message to
      the group members as intended recipients.  A single-recipient
      message intended to only one group member may be delivered by
      alternative means, with no assistance from the Dispatcher.

      Examples of a Dispatcher are: the Broker in a pub-sub setting; a
      relayer for group communication that delivers group messages as
      multiple unicast messages to all group members; an implicit entity
      as in a multicast communication setting, where messages are
      transmitted to a multicast IP address and delivered on the
      transport channel.

   This document specifies a mechanism for:

   *  Authorizing a Client to join the group (Section 3), and providing
      it with the group keying material to communicate with the other
      group members (Section 4).

   *  Allowing a group member to retrieve group keying material
      (Section 4.8.1.1 and Section 4.8.2.1).

   *  Allowing a group member to retrieve public keys of other group
      members (Section 4.4.1.1) and to provide an updated public key
      (Section 4.9.1.1).

   *  Allowing a group member to leave the group (Section 5).

   *  Evicting a group member from the group (Section 5).

   *  Renewing and re-distributing the group keying material (rekeying)
      upon a membership change in the group (Section 4.8.3.1 and
      Section 5).

   Figure 2 provides a high level overview of the message flow for a
   node joining a group.  The message flow can be expanded as follows.




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   1.  The joining node requests an access token from the AS, in order
       to access one or more group-membership resources at the KDC and
       hence join the associated groups.

       This exchange between Client and AS MUST be secured, as specified
       by the transport profile of ACE used between Client and KDC.
       Based on the response from the AS, the joining node will
       establish or continue using a secure communication association
       with the KDC.

   2.  The joining node transfers authentication and authorization
       information to the KDC, by transferring the obtained access
       token.  This is typically achieved by including the access token
       in a request sent to the /authz-info endpoint at the KDC.

       Once this exchange is completed, the joining node MUST have a
       secure communication association established with the KDC, before
       joining a group under that KDC.

       This exchange and the following secure communications between the
       Client and the KDC MUST occur in accordance with the transport
       profile of ACE used between Client and KDC, such as the DTLS
       transport profile [I-D.ietf-ace-dtls-authorize] and OSCORE
       transport profile [I-D.ietf-ace-oscore-profile] of ACE.

   3.  The joining node starts the joining process to become a member of
       the group, by sending a request to the related group-membership
       resource at the KDC.  Based on the application requirements and
       policies, the KDC may perform a group rekeying, by generating new
       group keying material and distributing it to the current group
       members through the rekeying scheme used in the group.

       At the end of the joining process, the joining node has received
       from the KDC the parameters and group keying material to securely
       communicate with the other group members.  Also, the KDC has
       stored the association between the authorization information from
       the access token and the secure session with the joining node.

   4.  The joining node and the KDC maintain the secure association, to
       support possible future communications.  These especially include
       key management operations, such as retrieval of updated keying
       material or participation to a group rekeying process.

   5.  The joining node can communicate securely with the other group
       members, using the keying material provided in step 3.






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           C                            AS  KDC                   Group
           |                             |   |                   Members
         / |                             |   |                      |
        |  |--- Authorization Request -->|   |                      |
        |  |                             |   |                      |
        |  |<-- Authorization Response --|   |                      |
   (*) <   |                             |   |                      |
        |  |                             |   |                      |
        |  |---  Token Transfer Request ---->|                      |
        |  |                                 |                      |
        |  |<--- Token Transfer Response-----|                      |
         \ |                             |   |                      |
           |                             |   |                      |
           |------ Joining Request --------->|                      |
           |                             |   |                      |
           |                             |   | -- Group rekeying -->|
           |                             |   |      (optional)      |
           |<----- Joining Response ---------|                      |
           |                             |   |                      |
           |                             |   |                      |
           |                             |   |       Dispatcher     |
           |                                             |          |
           |<======= Secure group communication =========|=========>|
           |                                             |          |

   (*) Defined in the ACE framework

               Figure 2: Message Flow Upon New Node's Joining

3.  Authorization to Join a Group

   This section describes in detail the format of messages exchanged by
   the participants when a node requests access to a given group.  This
   exchange is based on ACE [I-D.ietf-ace-oauth-authz].

   As defined in [I-D.ietf-ace-oauth-authz], the Client requests the AS
   for the authorization to join the group through the KDC (see
   Section 3.1).  If the request is approved and authorization is
   granted, the AS provides the Client with a proof-of-possession access
   token and parameters to securely communicate with the KDC (see
   Section 3.2).

   Communications between the Client and the AS MUST be secured,
   according to what is defined by the used transport profile of ACE.
   The Content-Format used in the message also depends on the used
   transport profile of ACE.  For example, it can be application/
   ace+cbor for the first two messages and application/cwt for the third
   message, which are defined in the ACE framework.



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   The transport profile of ACE also defines a number of details such as
   the communication and security protocols used with the KDC (see
   Appendix C of [I-D.ietf-ace-oauth-authz]).

   Figure 3 gives an overview of the exchange described above.

       Client                                             AS    KDC
          |                                                |     |
          |---- Authorization Request: POST /token ------->|     |
          |                                                |     |
          |<--- Authorization Response: 2.01 (Created) ----|     |
          |                                                |     |
          |---- Token Transfer Request: POST /authz-info ------->|
          |                                                |     |
          |<--- Token Transfer Response: 2.01 (Created) -------->|
          |                                                |     |

                Figure 3: Message Flow of Join Authorization

3.1.  Authorization Request

   The Authorization Request sent from the Client to the AS is defined
   in Section 5.8.1 of [I-D.ietf-ace-oauth-authz] and MAY contain the
   following parameters, which, if included, MUST have format and value
   as specified below.

   *  'scope', specifying the name of the groups that the Client
      requests to access, and optionally the roles that the Client
      requests to have in those groups.

      This parameter is encoded as a CBOR byte string, which wraps a
      CBOR array of one or more scope entries.  All the scope entries
      are specified according to a same format, i.e. either the AIF
      format or the textual format defined below.

      -  If the AIF format is used, each scope entry is encoded as
         specified in [I-D.ietf-ace-aif].  The object identifier "Toid"
         corresponds to the group name and MUST be encoded as a CBOR
         text string.  The permission set "Tperm" indicates the roles
         that the Client wishes to take in the group.

         The AIF format is the default format for application profiles
         of this specification, and is preferable for those that aim to
         a compact encoding of scope.  This is desirable especially for
         application profiles defining several roles, with the Client
         possibly requesting for multiple roles combined.





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         Figure 4 shows an example in CDDL notation [RFC8610] where
         scope uses the AIF format.

      -  If the textual format is used, each scope entry is a CBOR array
         formatted as follows.

         o  As first element, the group name, encoded as a CBOR text
            string.

         o  Optionally, as second element, the role or CBOR array of
            roles that the Client wishes to take in the group.  This
            element is optional since roles may have been pre-assigned
            to the Client, as associated to its verifiable identity
            credentials.  Alternatively, the application may have
            defined a single, well-known role for the target resource(s)
            and audience(s).

         Figure 5 shows an example in CDDL notation where scope uses the
         textual format, with group name and role identifiers encoded as
         CBOR text strings.

      It is REQUIRED of application profiles of this specificaton to
      specify the exact format and encoding of scope (REQ1).  This
      includes defining the set of possible roles and their identifiers,
      as well as the corresponding encoding to use in the scope entries
      according to the used scope format.

      If the application profile uses the AIF format, it is also
      REQUIRED to register its specific instance of "Toid" and "Tperm",
      as well as the corresponding Media Type and Content-Format, as per
      the guidelines in [I-D.ietf-ace-aif] (REQ2).

      If the application profile uses the textual format, it MAY
      additionally specify CBOR values to use for abbreviating the role
      identifiers (OPT1).

   *  'audience', with an identifier of the KDC.

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











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   gname = tstr

   permissions = uint . bits roles

   roles = &(
      Requester: 1,
      Responder: 2,
      Monitor: 3,
      Verifier: 4
   )

   scope_entry = AIF_Generic<gname, permissions>

   scope = << [ + scope_entry ] >>

              Figure 4: Example of scope using the AIF format

   gname = tstr

   role = tstr

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

   scope = << [ + scope_entry ] >>

       Figure 5: Example of scope using the textual format, with the
          group name and role identifiers encoded as text strings

3.2.  Authorization Response

   The AS processes the Authorization Request as defined in
   Section 5.8.2 of [I-D.ietf-ace-oauth-authz], especially verifying
   that the Client is authorized to access the specified groups with the
   requested roles, or possibly a subset of those.

   In case of successful verification, the Authorization Response sent
   from the AS to the Client is also defined in Section 5.8.2 of
   [I-D.ietf-ace-oauth-authz].  Note that the parameter 'expires_in' MAY
   be omitted if the application defines how the expiration time is
   communicated to the Client via other means, or if it establishes a
   default value.

   Additionally, when included, the following parameter MUST have the
   corresponding values:







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   *  'scope' has the same format and encoding of 'scope' in the
      Authorization Request, defined in Section 3.1.  If this parameter
      is not present, the granted scope is equal to the one requested in
      Section 3.1.

   The proof-of-possession access token (in 'access_token' above) MUST
   contain the following parameters:

   *  a confirmation claim (see for example 'cnf' defined in Section 3.1
      of [RFC8747] for CWT);

   *  an expiration time claim (see for example 'exp' defined in
      Section 3.1.4 of [RFC8392] for CWT);

   *  a scope claim (see for example 'scope' registered in Section 8.14
      of [I-D.ietf-ace-oauth-authz] for CWT).

      This claim specifies the same access control information as in the
      'scope' parameter of the Authorization Response, if the parameter
      is present in the message, or as in the 'scope' parameter of the
      Authorization Request otherwise.

      By default, this claim has the same encoding as the 'scope'
      parameter in the Authorization Request, defined in Section 3.1.

      Optionally, an alternative extended format of scope defined in
      Section 7 can be used.  This format explicitly signals the
      semantics used to express the actual access control information,
      and according to which this has to be parsed.  This enables a
      Resource Server to correctly process a received access token, also
      in case:

      -  The Resource Server implements a KDC that supports multiple
         application profiles of this specification, using different
         scope semantics; and/or

      -  The Resource Server implements further services beyond a KDC
         for group communication, using different scope semantics.

      If the Authorization Server is aware that this applies to the
      Resource Server for which the access token is issued, the
      Authorization Server SHOULD use the extended format of scope
      defined in Section 7.

   The access token MAY additionally contain other claims that the
   transport profile of ACE requires, or other optional parameters.





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   When receiving an Authorization Request from a Client that was
   previously authorized, and for which the AS still owns a valid non-
   expired access token, the AS MAY reply with that token.  Note that it
   is up to application profiles of ACE to make sure that re-posting the
   same token does not cause re-use of keying material between nodes
   (for example, that is done with the use of random nonces in
   [I-D.ietf-ace-oscore-profile]).

3.3.  Token Transferring

   The Client sends a Token Transfer Request to the KDC, i.e., a CoAP
   POST request including the access token and targeting the authz-info
   endpoint (see Section 5.10.1 of [I-D.ietf-ace-oauth-authz]).

   Note that this request deviates from the one defined in
   [I-D.ietf-ace-oauth-authz], since it allows to ask the KDC for
   additional information concerning the public keys used in the group
   to ensure source authentication, as well as for possible additional
   group parameters.

   The joining node MAY ask for this information from the KDC through
   the same Token Transfer Request.  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], and the message payload MUST be formatted
   as a CBOR map, which MUST include the access token.  The CBOR map MAY
   additionally include the following parameter, which, if included,
   MUST have format and value as specified below.

   *  'sign_info' defined in Section 3.3.1, specifying the CBOR simple
      value 'null' (0xf6) to request information about the signature
      algorithm, signature algorithm parameters, signature key
      parameters and about the exact encoding of public keys used in the
      groups that the Client has been authorized to join.

   Alternatively, such information may be pre-configured on the joining
   node, or may be retrieved by alternative means.  For example, the
   joining node may have performed an early group discovery process and
   obtained the link to the associated group-membership resource at the
   KDC, together with attributes descriptive of the group configuration
   (see, e.g., [I-D.tiloca-core-oscore-discovery]).

   After successful verification, the Client is authorized to receive
   the group keying material from the KDC and join the group.  Hence,
   the KDC replies to the Client with a Token Transfer Response, i.e., a
   CoAP 2.01 (Created) response.






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   The Token Transfer Response MUST have Content-Format "application/
   ace+cbor", and its payload is a CBOR map.  Note that this deviates
   from what is defined in the ACE framework, where the response from
   the authz-info endpoint is defined as conveying no payload (see
   Section 5.10.1 of [I-D.ietf-ace-oauth-authz]).

   If the access token contains a role that requires the Client to send
   its own public key to the KDC when joining the group, the CBOR map
   MUST include the parameter 'kdcchallenge' defined in Section 3.3.2,
   specifying a dedicated challenge N_S generated by the KDC.

   Later, when joining the group (see Section 4.3.1.1), the Client uses
   the 'kdcchallenge' value and additional information to build a proof-
   of-possession (PoP) input.  This is in turn used to compute a PoP
   evidence, which the Client also provides to the Group Manager in
   order to prove possession of its own private key (see the
   'client_cred_verify' parameter in Section 4.3.1).

   The KDC MUST store the 'kdcchallenge' value associated to the Client
   at least until it receives a Joining Request from it (see
   Section 4.3.1.1), to be able to verify the PoP evidence provided
   during the join process, and thus that the Client possesses its own
   private key.

   The same 'kdcchallenge' value MAY be reused several times by the
   Client, to generate a new PoP evidence, e.g., in case the Client
   provides the Group Manager with a new public key while being a group
   member (see Section 4.9.1.1), or joins a different group where it
   intends to use a different public key.  Therefore, it is RECOMMENDED
   that the KDC keeps storing the 'kdcchallenge' value after the first
   join is processed as well.  If the KDC has already discarded the
   'kdcchallenge' value, that will trigger an error response with a
   newly generated 'kdcchallenge' value that the Client can use to
   restart the join process, as specified in Section 4.3.1.1.

   If 'sign_info' is included in the Token Transfer Request, the KDC
   SHOULD include the 'sign_info' parameter in the Token Transfer
   Response, as per the format defined in Section 3.3.1.  Note that the
   field 'id' of each 'sign_info_entry' specifies the name, or array of
   group names, for which that 'sign_info_entry' applies to.  As an
   exception, the KDC MAY NOT include the 'sign_info' parameter in the
   Token Transfer Response even if 'sign_info' is included in the Token
   Transfer Request, in case none of the groups that the Client is
   authorized to join uses signatures to achieve source authentication.







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   Note that the CBOR map specified as payload of the 2.01 (Created)
   response may include further parameters, e.g., according to the used
   transport profile of ACE.  Application profiles of this specification
   MAY define additional parameters to use within this exchange (OPT2).

   Application profiles of this specification MAY define alternative
   specific negotiations of parameter values for the signature algorithm
   and signature keys, if 'sign_info' is not used (OPT3).

   If allowed by the used transport profile of ACE, the Client may
   provide the Access Token to the KDC by other means than the Token
   Transfer Request.  An example is the DTLS transport profile of ACE,
   where the Client can provide the access token to the KDC during the
   secure session establishment (see Section 3.3.2 of
   [I-D.ietf-ace-dtls-authorize]).

3.3.1.  'sign_info' Parameter

   The 'sign_info' parameter is an OPTIONAL parameter of the request and
   response messages exchanged between the Client and the authz-info
   endpoint at the RS (see Section 5.10.1. of
   [I-D.ietf-ace-oauth-authz]).

   This parameter allows the Client and the RS to exchange information
   about a signature algorithm and about public keys to accordingly use
   for signature verification.  Its exact semantics and content are
   application specific.

   In this specification and in application profiles building on it,
   this parameter is used to exchange information about the signature
   algorithm and about public keys to be used with it, in the groups
   indicated by the transferred acces token as per its 'scope' claim
   (see Section 3.2).

   When used in the Token Transfer Request sent to the KDC (see
   Section 3.3), the 'sign_info' parameter specifies the CBOR simple
   value 'null' (0xf6).  This is done to ask for information about the
   signature algorithm and about the public keys used in the groups that
   the Client has been authorized to join - or to have a more restricted
   interaction as per its granted roles (e.g., the Client is an external
   signature verifier).










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   When used in the following Token Transfer Response from the KDC (see
   Section 3.3), the 'sign_info' parameter is a CBOR array of one or
   more elements.  The number of elements is at most the number of
   groups that the Client has been authorized to join - or to have a
   more restricted interaction (see above).  Each element contains
   information about signing parameters and about public keys for one or
   more groups, and is formatted as follows.

   *  The first element 'id' is a group name or an array of group names,
      associated to groups for which the next four elements apply.  In
      the following, each specified group name is referred to as
      'gname'.

   *  The second element 'sign_alg' is an integer or a text string if
      the POST request included the 'sign_info' parameter with value the
      CBOR simple value 'null' (0xf6), and indicates the signature
      algorithm used in the groups identified by the 'gname' values.  It
      is REQUIRED of the application profiles to define specific values
      that this parameter can take (REQ3), selected from the set of
      signing algorithms of the COSE Algorithms registry
      [COSE.Algorithms].

   *  The third element 'sign_parameters' is a CBOR array indicating the
      parameters of the signature algorithm used in the groups
      identified by the 'gname' values.  Its content depends on the
      value of 'sign_alg'.  It is REQUIRED of the application profiles
      to define the possible values and structure for the elements of
      this parameter (REQ4).

   *  The fourth element 'sign_key_parameters' is a CBOR array
      indicating the parameters of the key used with the signature
      algorithm, in the groups identified by the 'gname' values.  Its
      content depends on the value of 'sign_alg'.  It is REQUIRED of the
      application profiles to define the possible values and structure
      for the elements of this parameter (REQ5).

   *  The fifth element 'pub_key_enc' parameter is either a CBOR integer
      indicating the encoding of public keys used in the groups
      identified by the 'gname' values, or has value the CBOR simple
      value 'null' (0xf6) indicating that the KDC does not act as
      repository of public keys for group members.  Its acceptable
      integer values are taken from the 'Label' column of the "COSE
      Header Parameters" registry [COSE.Header.Parameters].  It is
      REQUIRED of the application profiles to define specific values to
      use for this parameter, consistently with the acceptable formats
      of public keys (REQ6).





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   The CDDL notation [RFC8610] of the 'sign_info' parameter is given
   below.

   sign_info = sign_info_req / sign_info_resp

   sign_info_req  = nil                   ; in the Token Transfer
                                          ; Request to the KDC

   sign_info_resp = [ + sign_info_entry ] ; in the Token Transfer
                                          ; Response from the KDC

   sign_info_entry =
   [
     id : gname / [ + gname ],
     sign_alg : int / tstr,
     sign_parameters : [ any ],
     sign_key_parameters : [ any ],
     pub_key_enc = int / nil
   ]

   gname = tstr

   This format is consistent with every signature algorithm currently
   defined in [I-D.ietf-cose-rfc8152bis-algs], i.e., with algorithms
   that have only the COSE key type as their COSE capability.
   Appendix B describes how the format of each 'sign_info_entry' can be
   generalized for possible future registered algorithms having a
   different set of COSE capabilities.

3.3.2.  'kdcchallenge' Parameter

   The 'kdcchallenge' parameter is an OPTIONAL parameter of response
   message returned from the authz-info endpoint at the RS, as defined
   in Section 5.10.1 of [I-D.ietf-ace-oauth-authz].  This parameter
   contains a challenge generated by the RS and provided to the Client.

   In this specification and in application profiles building on it, the
   Client may use this challenge to prove possession of its own private
   key in the Joining Request (see the 'client_cred_verify' parameter in
   Section 4.3.1).

4.  KDC Functionalities

   This section describes the functionalities provided by the KDC, as
   related to the provisioning of the keying material as well as to the
   group membership management.





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   In particular, this section defines the interface available at the
   KDC; specifies the handlers of each resource provided by the KDC
   interface; and describes how Clients interact with those resources to
   join a group and to perform additional operations as group members.

   As most important operation after trasferring the access token to the
   KDC, the Client can perform a "Joining" exchange with the KDC, by
   specifying the group it requests to join (see Section 4.3.1.1).
   Then, the KDC verifies the access token and that the Client is
   authorized to join the specified group.  If so, the KDC provides the
   Client with the keying material to securely communicate with the
   other members of the group.

   Later on as a group member, the Client can also rely on the interface
   at the KDC to perform additional operations, consistently with the
   roles it has in the group.

4.1.  Interface at the KDC

   The KDC provides its interface by hosting the following resources.
   Note that the root url-path "ace-group" used hereafter is a default
   name; implementations are not required to use this name, and can
   define their own instead.  The Interface Description (if=) Link
   Target Attribute value "ace.group" is registered in Section 11.5 and
   can be used to describe this interface.

   If request messages sent to the KDC as well as success response
   messages from the KDC include a payload and specify a Content-Format,
   those messages MUST have Content-Format set to application/ace-
   groupcomm+cbor, defined in Section 11.2.  CBOR labels for the message
   parameters are defined in Section 8.

   *  /ace-group : this resource is invariant once established, and
      indicates that this specification is used.  If other applications
      run on a KDC implementing this specification and use this same
      resource, those applications will collide, and a mechanism will be
      needed to differentiate the endpoints.

      A Client can access this resource in order to retrieve a set of
      group names, each corresponding to one of the specified group
      identifiers.  This operation is described in Section 4.2.1.1.

   *  /ace-group/GROUPNAME : one such sub-resource to /ace-group is
      hosted for each group with name GROUPNAME that the KDC manages,
      and contains the symmetric group keying material for that group.






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      A Client can access this resource in order to join the group with
      name GROUPNAME, or later as a group member to retrieve the current
      group keying material.  These operations are described in
      Section 4.3.1.1 and Section 4.3.2.1, respectively.

      If the value of the GROUPNAME URI path and the group name in the
      access token scope ('gname' in Section 3.2) are not required to
      coincide, the KDC MUST implement a mechanism to map the GROUPNAME
      value in the URI to the group name, in order to refer to the
      correct group (REQ7).

   *  /ace-group/GROUPNAME/pub-key : this resource is invariant once
      established, and contains the public keys of all the members of
      the group with name GROUPNAME.

      This resource is created only in case the KDC acts as repository
      of public keys for group members.

      A Client can access this resource in order to retrieve the public
      keys of other group members, in addition to when joining the
      group.  That is, the Client can retrieve the public keys of all
      the current group members, or a subset of them by specifying
      filter criteria.  These operations are described in
      Section 4.4.2.1 and Section 4.4.1.1, respectively.

      Clients may be authorized to access this resource even without
      being group members, e.g., if authorized to be external signature
      verifiers for the group.

   *  ace-group/GROUPNAME/kdc-pub-key : this resource is invariant once
      established, and contains the public key of the KDC for the group
      with name GROUPNAME.

      This resource is created only in case the KDC has an associated
      public key and this is required for the correct group operation.
      It is REQUIRED of application profiles to define whether the KDC
      has such an associated public key (REQ8).

      A Client can interact with this resource in order to retrieve the
      current public key of the KDC, in addition to when joining the
      group.

      Clients may be authorized to access this resource even without
      being group members, e.g., if authorized to be external signature
      verifiers for the group.






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   *  /ace-group/GROUPNAME/policies : this resource is invariant once
      established, and contains the group policies of the group with
      name GROUPNAME.

      A Client can access this resource as a group member in order to
      retrieve the group policies.  This operation is described in
      Section 4.6.1.1.

   *  /ace-group/GROUPNAME/num : this resource is invariant once
      established, and contains the current version number for the
      symmetric group keying material of the group with name GROUPNAME.

      A Client can access this resource as a group member in order to
      retrieve the version number of the keying material currently used
      in the group.  This operation is described in Section 4.7.1.1.

   *  /ace-group/GROUPNAME/nodes/NODENAME : one such sub-resource of
      /ace-group/GROUPNAME is hosted for each group member of the group
      with name GROUPNAME.  Each of such resources is identified by the
      node name NODENAME of the associated group member, and contains
      the group keying material and the individual keying material for
      that group member.

      A Client as a group member can access this resource in order to
      retrieve the current group keying material together with its the
      individual keying material; request new individual keying material
      to use in the group; and leave the group.  These operations are
      described in Section 4.8.1.1, Section 4.8.2.1, and
      Section 4.8.3.1, respectively.

   *  /ace-group/GROUPNAME/nodes/NODENAME/pub-key : this resource is
      invariant once established, and contains the individual public
      keying material for the node with name NODENAME, as group member
      of the group with name GROUPNAME.

      A Client can access this resource in order to upload at the KDC a
      new public key to use in the group.  This operation is described
      in Section 4.9.1.1.

      This resource is not created if the group member does not have
      individual public keying material to use in the group, or if the
      KDC does not store the public keys of group members.









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   The KDC is expected to fully provide the interface defined above.  It
   is otherwise REQUIRED of the application profiles of this
   specification to indicate which resources are not hosted, i.e., which
   parts of the interface defined in this section are not supported by
   the KDC (REQ9).  Application profiles of this specification MAY
   extend the KDC interface, by defining additional resources and their
   handlers.

   It is REQUIRED of the application profiles of this specification to
   register a Resource Type for the root url-path (REQ10).  This
   Resource Type can be used to discover the correct url to access at
   the KDC.  This Resource Type can also be used at the GROUPNAME sub-
   resource, to indicate different application profiles for different
   groups.

   It is REQUIRED of the application profiles of this specification to
   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 (see
   Section 3.1); and the roles that the Client has as current group
   member (REQ11).

4.1.1.  Operations Supported by Clients

   It is expected that a Client minimally supports the following set of
   primary operations and corresponding interactions with the KDC.

   *  FETCH request to ace-group/ , in order to retrieve group names
      associated to group identifiers.

   *  POST and GET requests to ace-group/GROUPNAME/ , in order to join a
      group (POST) and later retrieve the current group key material as
      a group member (GET).

   *  GET and FETCH requests to ace-group/GROUPNAME/pub-key , in order
      to retrieve the public keys of all the other group members (GET)
      or only some of them by filtering (FETCH).  While retrieving
      public keys remains possible by using GET requests, retrieval by
      filtering allows to greatly limit the size of exchanged messages.

   *  GET request to ace-group/GROUPNAME/num , in order to retrieve the
      current version of the group key material as a group member.

   *  DELETE request to ace-group/GROUPNAME/nodes/NODENAME , in order to
      leave the group.





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   In addition, some Clients may rather not support the following set of
   secondary operations and corresponding interactions with the KDC.
   This can be specified, for instance, in compliance documents defining
   minimalistic Clients and their capabilities in specific deployments.
   In turn, these might also have to consider the used application
   profile of this specification.

   *  GET request to ace-group/GROUPNAME/kdc-pub-key , in order to
      retrieve the current public key of the KDC, in addition to when
      joining the group.  This is relevant only if the KDC has an
      associated public key and this is required for the correct group
      operation.

   *  GET request to ace-group/GROUPNAME/policies , in order to retrieve
      the current group policies as a group member, in addition to when
      joining the group.

   *  GET request to ace-group/GROUPNAME/nodes/NODENAME, in order to
      retrieve the current group keying material and individual keying
      material.  The former can also be retrieved through a GET request
      to ace-group/GROUPNAME/ (see above).  The latter would not be
      possible to re-obtain as a group member.

   *  PUT request to ace-group/GROUPNAME/nodes/NODENAME , in order to
      ask for new individual keying material.  The Client would have to
      alternatively re-join the group through a POST request to ace-
      group/GROUPNAME/ (see above).  Furthermore, depending on its roles
      in the group or on the application profile of this specification,
      the Client might simply not be associated to any individual keying
      material.

   *  POST request to ace-group/GROUPNAME/nodes/NODENAME/pub-key , in
      order to provide the KDC with a new public key.  The Client would
      have to alternatively re-join the group through a POST request to
      ace-group/GROUPNAME/ (see above).  Furthermore, depending on its
      roles in the group, the Client might simply not have an associated
      public key to provide.

   It is REQUIRED of application profiles of this specification to
   categorize possible newly defined operations for Clients into primary
   operations and secondary operations, and to provide accompanying
   considerations (REQ12).

4.1.2.  Error Handling

   Upon receiving a request from a Client, the KDC MUST check that it is
   storing a valid access token from that Client.  If this is not the
   case, the KDC MUST reply with a 4.01 (Unauthorized) error response.



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   Unless the request targets the /ace-group resource, the KDC MUST
   check that it is storing a valid access token from that Client such
   that:

   *  The scope specified in the access token includes a scope entry
      related to the group name GROUPNAME associated to targeted
      resource; and

   *  The set of roles specified in that scope entry allows the Client
      to perform the requested operation on the targeted resource
      (REQ11).

   In case the KDC stores a valid access token but the verifications
   above fail, the KDC MUST reply with a 4.03 (Forbidden) error
   response.  This response MAY be an AS Request Creation Hints, as
   defined in Section 5.3 of [I-D.ietf-ace-oauth-authz], in which case
   the Content-Format MUST be set to application/ace+cbor.

   If the request is not formatted correctly (e.g., required fields are
   not present or are not encoded as expected), the handler MUST reply
   with a 4.00 (Bad Request) error response.

   If the request includes unknown or non-expected fields, the handler
   MUST silently ignore them and continue processing the request.
   Application profiles of this specification MAY define optional or
   mandatory payload formats for specific error cases (OPT4).

   Some error responses from the KDC can have Content-Format set to
   application/ace-groupcomm+cbor.  In such a case, the paylod of the
   response MUST be a CBOR map, which includes the following fields.

   *  'error', with value a CBOR integer specifying the error occurred
      at the KDC.  The value is taken from the "Value" column of the
      "ACE Groupcomm Errors" registry defined in Section 11.13 of this
      specification.  This field MUST be present.

   *  'error_description', with value a CBOR text string specifying a
      human-readable diagnostic description of the error occurred at the
      KDC, written in English.  The diagnostic text is intended for
      software engineers as well as for device and network operators, in
      order to aid debugging and provide context for possible
      intervention.  The diagnostic message SHOULD be logged by the KDC.
      This field MAY be present, and it is unlikely relevant in an
      unattended setup where human intervention is not expected.







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   The 'error' and 'error_description' fields are defined as OPTIONAL to
   support for Clients (see Section 8).  A Client supporting the 'error'
   parameter and able to understand the specified error may use that
   information to determine what actions to take next.

   Section 9 of this specification defines an initial set of error
   identifiers, as possible values for the 'error' field.  Application
   profiles of this specification inherit this initial set or error
   identifiers and MAY define additional value (OPT5).

4.2.  /ace-group

   This resource implements the FETCH handler.

4.2.1.  FETCH Handler

   The FETCH handler receives group identifiers and returns the
   corresponding group names and GROUPNAME URIs.

   The handler expects a request with payload formatted as a CBOR map,
   which MUST contain the following fields:

   *  'gid', whose value is encoded as a CBOR array, containing one or
      more group identifiers.  The exact encoding of group identifier
      MUST be specified by the application profile (REQ13).  The Client
      indicates that it wishes to receive the group names and GROUPNAMEs
      of all groups having these identifiers.

   The handler identifies the groups that are secured by the keying
   material identified by those group identifiers.

   If all verifications succeed, the handler replies with a 2.05
   (Content) response, whose payload is formatted as a CBOR map that
   MUST contain the following fields:

   *  'gid', whose value is encoded as a CBOR array, containing zero or
      more group identifiers.  The handler indicates that those are the
      identifiers it is sending group names and GROUPNAMEs for.  This
      CBOR array is a subset of the 'gid' array in the FETCH request.

   *  'gname', whose value is encoded as a CBOR array, containing zero
      or more group names.  The elements of this array are encoded as
      text strings.  Each element of index i of this CBOR array
      corresponds to the element of group identifier i in the 'gid'
      array.






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   *  'guri', whose value is encoded as a CBOR array, containing zero or
      more URIs, each indicating a GROUPNAME resource.  The elements of
      this array are encoded as text strings.  Each element of index i
      of this CBOR array corresponds to the element of group identifier
      i in the 'gid' array.

   If the KDC does not find any group associated to the specified group
   identifiers, the handler returns a response with payload formatted as
   a CBOR byte string of zero length.

   Note that the KDC only verifies that the node is authorized by the AS
   to access this resource.  Nodes that are not members of the group but
   are authorized to do signature verification on the group messages may
   be allowed to access this resource, if the application needs it.

4.2.1.1.  Retrieve Group Names

   In case the joining node only knows the group identifier of the group
   it wishes to join or about which it wishes to get update information
   from the KDC, the node can contact the KDC to request the
   corresponding group name and joining resource URI.  The node can
   request several group identifiers at once.  It does so by sending a
   CoAP FETCH request to the /ace-group endpoint at the KDC formatted as
   defined in Section 4.2.1.

   Figure 6 gives an overview of the exchanges described above, and
   Figure 7 shows an example.

      Client                                                     KDC
         |                                                        |
         |-------- Group Name and URI Retrieval Request: -------->|
         |                   FETCH /ace-group                     |
         |                                                        |
         |<-Group Name and URI Retrieval Response: 2.05 (Content)-|
         |                                                        |

      Figure 6: Message Flow of Group Name and URI Retrieval Request-
                                  Response













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   Request:

   Header: FETCH (Code=0.05)
   Uri-Host: "kdc.example.com"
   Uri-Path: "ace-group"
   Content-Format: "application/ace-groupcomm+cbor"
   Payload (in CBOR diagnostic notation):
     { "gid": [01, 02] }

   Response:

   Header: Content (Code=2.05)
   Content-Format: "application/ace-groupcomm+cbor"
   Payload (in CBOR diagnostic notation):
     { "gid": [01, 02], "gname": ["group1", "group2"],
       "guri": ["ace-group/g1", "ace-group/g2"] }

     Figure 7: Example of Group Name and URI Retrieval Request-Response

4.3.  /ace-group/GROUPNAME

   This resource implements the POST and GET and handlers.

4.3.1.  POST Handler

   The POST handler processes the Joining Request sent by a Client to
   join a group, and returns a Joining Response as successful result of
   the joining process (see Section 4.3.1.1).  At a high level, the POST
   handler adds the Client to the list of current group members, adds
   the public key of the Client to the list of the group members' public
   keys, and returns the symmetric group keying material for the group
   identified by GROUPNAME.

   The handler expects a request with payload formatted as a CBOR map,
   which MAY contain the following fields, which, if included, MUST have
   format and value as specified below.

   *  'scope', with value the specific group that the Client is
      attempting to join, i.e., the group name, and the roles it wishes
      to have in the group.  This value is a CBOR byte string wrapping
      one scope entry, as defined in Section 3.1.










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   *  'get_pub_keys', if the Client wishes to receive the public keys of
      the current group members from the KDC.  This parameter may be
      included in the Joining Request if the KDC stores the public keys
      of the group members, while it is not useful to include it if the
      Client obtains those public keys through alternative means, e.g.,
      from the AS.  Note that including this parameter might result in a
      following Joining Response of large size, which can be
      inconvenient for resource-constrained devices.

      If the Client wishes to retrieve the public keys of all the
      current group members, the 'get_pub_keys' parameter MUST encode
      the CBOR simple value 'null' (0xf6).  Otherwise, the
      'get_pub_keys' parameter MUST encode a non-empty CBOR array,
      containing the following three elements formatted as defined
      below.

      -  The first element, namely 'inclusion_flag', encodes the CBOR
         simple value True.  That is, the Client indicates that it
         wishes to receive the public keys of all group members having
         their node identifier specified in the third element of the
         'get_pub_keys' array, namely 'id_filter' (see below).

      -  The second element, namely 'role_filter', is a non-empty CBOR
         array.  Each element of the array contains one role or a
         combination of roles for the group identified by GROUPNAME.
         That is, when the Joining Request includes a non-Null
         'get_pub_keys' parameter, the Client filters public keys based
         on node identifiers.

         In particular, the Client indicates that it wishes to retrieve
         the public keys of all the group members having any of the
         single roles, or at least all of the roles indicated in any
         combination of roles.  For example, the array ["role1",
         "role2+role3"] indicates that the Client wishes to receive the
         public keys of all group members that have at least "role1" or
         at least both "role2" and "role3".

      -  The third element, namely 'id_filter', is an empty CBOR array.
         That is, when the Joining Request includes a non-Null
         'get_pub_keys' parameter, the Client does not filter public
         keys based on node identifiers.










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         In fact, when first joining the group, the Client is not
         expected or capable to express a filter based on node
         identifiers of other group members.  Instead, when already a
         group member and sending a Joining Request to re-join, the
         Client is not expected to include the 'get_pub_keys' parameter
         in the Joining Request altogether, since it can rather retrieve
         public keys associated to specific group identifiers as defined
         in Section 4.4.1.1.

      The CDDL definition [RFC8610] of 'get_pub_keys' is given in
      Figure 8, using as example encoding: node identifier encoded as a
      CBOR byte string; role identifier encoded as a CBOR text string,
      and combination of roles encoded as a CBOR array of roles.

      Note that, for this handler, 'inclusion_flag' is always set to
      true, the array of roles 'role_filter' is always non-empty, while
      the array of node identifiers 'id_filter' is always empty.
      However, this is not necessarily the case for other handlers using
      the 'get_pub_keys' parameter.

   inclusion_flag = bool

   role = tstr
   comb_role = [ 2*role ]
   role_filter = [ *(role / comb_role) ]

   id = bstr
   id_filter = [ *id ]

   get_pub_keys = null / [ inclusion_flag, role_filter, id_filter]

      Figure 8: CDLL definition of get_pub_keys, using as example node
                identifier encoded as bstr and role as tstr

   *  'client_cred', encoded as a CBOR byte string, with value the
      original binary representation of the Client's public key.  This
      parameter is used if the KDC is managing (collecting from/
      distributing to the Client) the public keys of the group members,
      and if the Client's role in the group will require for it to send
      messages to one or more group members.  It is REQUIRED of the
      application profiles to define the specific formats that are
      acceptable to use for encoding public keys in the group (REQ6).

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





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   *  'client_cred_verify', encoded as a CBOR byte string.  This
      parameter MUST be present if the 'client_cred' parameter is
      present and no public key associated to the Client's token can be
      retrieved for that group.

      This parameter contains a proof-of-possession (PoP) evidence
      computed by the Client over the following PoP input: the scope
      (encoded as CBOR byte string), concatenated with N_S (encoded as
      CBOR byte string) concatenated with N_C (encoded as CBOR byte
      string), where:

      -  scope is the CBOR byte string either specified in the 'scope'
         parameter above, if present, or as a default scope that the
         handler is expected to understand, if omitted.

      -  N_S is the challenge received from the KDC in the
         'kdcchallenge' parameter of the 2.01 (Created) response to the
         Token Transfer Request (see Section 3.3), encoded as a CBOR
         byte string.

      -  N_C is the nonce generated by the Client and specified in the
         'cnonce' parameter above, encoded as a CBOR byte string.

      An example of PoP input to compute 'client_cred_verify' using CBOR
      encoding is given in Figure 9.

      A possible type of PoP evidence is a signature, that the Client
      computes by using its own private key, whose corresponding public
      key is specified in the 'client_cred' parameter.  Application
      profiles of this specification MUST specify the exact approaches
      used to compute the PoP evidence to include in
      'client_cred_verify', and MUST specify which of those approaches
      is used in which case (REQ14).

      If the token was not provided to the KDC through a Token Transfer
      Request (e.g., it is used directly to validate TLS instead), it is
      REQUIRED of the specific application profile to define how the
      challenge N_S is generated (REQ15).

   *  'pub_keys_repos', which can be present if the format of the
      Client's public key in the 'client_cred' parameter is a
      certificate.  In such a case, this parameter has as value the URI
      of the certificate.  This parameter is encoded as a CBOR text
      string.  Alternative specific encodings of this parameter MAY be
      defined in applications of this specification (OPT6).






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   *  'control_uri', with value a full URI, encoded as a CBOR text
      string.  A default url-path is /ace-group/GROUPNAME/node, although
      implementations can use different ones instead.  The URI MUST NOT
      have url-path ace-group/GROUPNAME.

      If 'control_uri' is specified in the Joining Request, the Client
      acts as a CoAP server and hosts a resource at this specific URI.
      The KDC MAY use this URI to send CoAP requests to the Client
      (acting as CoAP server in this exchange), for example for one-to-
      one provisioning of new group keying material when performing a
      group rekeying (see Section 4.8.1.1), or to inform the Client of
      its removal from the group Section 5.

      In particular, this resource is intended for communications
      concerning exclusively the group whose group name GROUPNAME is
      specified in the 'scope' parameter.  If the KDC does not implement
      mechanisms using this resource for that group, it can ignore this
      parameter.  Other additional functionalities of this resource MAY
      be defined in application profiles of this specifications (OPT7).

   scope, N_S, and N_C expressed in CBOR diagnostic notation:
     scope = h'826667726F7570316673656E646572'
     N_S   = h'018a278f7faab55a'
     N_C   = h'25a8991cd700ac01'


   scope, N_S, and N_C as CBOR encoded byte strings:
     scope = 0x4f826667726F7570316673656E646572
     N_S   = 0x48018a278f7faab55a
     N_C   = 0x4825a8991cd700ac01

   PoP input:
     0x4f 826667726F7570316673656E646572
       48 018a278f7faab55a 48 25a8991cd700ac01

       Figure 9: Example of PoP input to compute 'client_cred_verify'
                            using CBOR encoding

   If the request does not include a 'scope' field, the KDC is expected
   to understand with what roles the Client is requesting to join the
   group.  For example, as per the access token, the Client might have
   been granted access to the group with only one role.  If the KDC
   cannot determine which exact scope should be considered for the
   Client, it MUST reply with a 4.00 (Bad Request) error response.

   The handler considers the scope specified in the access token
   associated to the Client, and checks the scope entry related to the
   group with name GROUPNAME associated to the endpoint.  In particular,



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   the handler checks whether the set of roles specified in that scope
   entry includes all the roles that the Client wishes to have in the
   group as per the Joining Request.  If this is not the case, the KDC
   MUST reply with a 4.03 (Forbidden) error response.

   If the KDC manages the group members' public keys, the handler checks
   if one is included in the 'client_cred' field.  If so, the KDC
   retrieves the public key and performs the following actions.

   *  If the access token was provided through a Token Transfer Request
      (see Section 3.3) but the KDC cannot retrieve the 'kdcchallenge'
      associated to this Client (see Section 3.3), the KDC MUST reply
      with a 4.00 Bad Request error response, which MUST also have
      Content-Format application/ace-groupcomm+cbor.  The payload of the
      error response is a CBOR map including a newly generated
      'kdcchallenge' value.  This is specified in the 'kdcchallenge'
      parameter.

   *  The KDC checks the public key to be valid for the group identified
      by GROUPNAME.  That is, it checks that the public key is encoded
      according to the format used in the group, is intended for the
      public key algorithm used in the group, and is aligned with the
      possible associated parameters used in the group.

      If this verification fails, the handler MUST reply with a 4.00
      (Bad Request) error response.  The response MUST have Content-
      Format set to application/ace-groupcomm+cbor and is formatted as
      defined in Section 4.  The value of the 'error' field MUST be set
      to 2 ("Public key incompatible with the group configuration").

   *  The KDC verifies the PoP evidence contained in the
      'client_cred_verify' field.  Application profiles of this
      specification MUST specify the exact approaches used to verify the
      PoP evidence, and MUST specify which of those approaches is used
      in which case (REQ14).

      If the PoP evidence does not pass verification, the handler MUST
      reply with a 4.00 (Bad Request) error response.  The response MUST
      have Content-Format set to application/ace-groupcomm+cbor and is
      formatted as defined in Section 4.  The value of the 'error' field
      MUST be set to 3 ("Invalid Proof-of-Possession evidence").

   If no public key is included in the 'client_cred' field, the handler
   checks if a public key is already associated to the received access
   token and to the group identified by GROUPNAME (see also
   Section 4.3.1.1).  Note that the same joining node may use different
   public keys in different groups, and all those public keys would be
   associate to the same access token.



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   If an eligible public key for the Client is neither present in the
   'client_cred' field nor retrieved from the stored ones at the KDC, it
   is RECOMMENDED that the handler stops the processing and replies with
   a 4.00 (Bad Request) error response.  Applications profiles MAY
   define alternatives (OPT8).

   If, regardless the reason, the KDC replies with a 4.00 (Bad Request)
   error response, this response MAY have Content-Format set to
   application/ace-groupcomm+cbor and have a CBOR map as payload.  For
   instance, the CBOR map can include a 'sign_info' parameter formatted
   as 'sign_info_res' defined in Section 3.3.1, with the 'pub_key_enc'
   element set to the CBOR simple value 'null' (0xf6) if the Client sent
   its own public key and the KDC is not set to store public keys of the
   group members.

   If all the verifications above succeed, the KDC proceeds as follows.

   First, only in case the Client is not already a group member, the
   handler performs the following actions:

   *  The handler adds the Client to the list of current members of the
      group.

   *  The handler assigns a name NODENAME to the Client, and creates a
      sub-resource to /ace-group/GROUPNAME at the KDC, i.e., "/ace-
      group/GROUPNAME/nodes/NODENAME".

   *  The handler associates the node identifier NODENAME to the access
      token and the secure session for the Client.

   Then, the handler performs the following actions.

   *  If the KDC manages the group members' public keys:

      -  The handler associates the retrieved Client's public key to the
         tuple composed of the node name NODENAME, the group name
         GROUPNAME and the received access token.

      -  The handler adds the retrieved Client's public key to the
         stored list of public keys stored for the group identified by
         GROUPNAME.  If such list already includes a public key for the
         Client, but a different public key is specified in the
         'client_cred' field, then the handler MUST replace the old
         public key in the list with the one specified in the
         'client_cred' field.






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   *  If the application requires backward security or if the used
      application profile prescribes so, the KDC MUST generate new group
      keying material and securely distribute it to the current group
      members (see Section 6).

   *  The handler returns a successful Joining Response as defined
      below, containing the symmetric group keying material; the group
      policies; and the public keys of the current members of the group,
      if the KDC manages those and the Client requested them.

   The Joining Response MUST have response code 2.01 (Created) if the
   Client has been added to the list of group members in this joining
   exchange (see above), or 2.04 (Changed) otherwise, i.e., if the
   Client is re-joining the group without having left it.

   The Joining Response message MUST include the Location-Path CoAP
   option, specifying the URI path to the sub-resource associated to the
   Client, i.e. "/ace-group/GROUPNAME/nodes/NODENAME".

   The Joining Response message MUST have Content-Format application/
   ace-groupcomm+cbor.  The payload of the response is formatted as a
   CBOR map, which MUST contain the following fields and values.

   *  'gkty', identifying the key type of the 'key' parameter.  The set
      of values can be found in the "Key Type" column of the "ACE
      Groupcomm Key Types" registry.  Implementations MUST verify that
      the key type matches the application profile being used, if
      present, as registered in the "ACE Groupcomm Key Types" registry.

   *  'key', containing the keying material for the group communication,
      or information required to derive it.

   *  'num', containing the version number of the keying material for
      the group communication, formatted as an integer.  This is a
      strictly monotonic increasing field.  The application profile MUST
      define the initial version number (REQ16).

   The exact format of the 'key' value MUST be defined in applications
   of this specification (REQ17), as well as values of 'gkty' accepted
   by the application (REQ18).  Additionally, documents specifying the
   key format MUST register it in the "ACE Groupcomm Key Types" registry
   defined in Section 11.8, including its name, type and application
   profile to be used with.








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     +----------+----------------+---------+-------------------------+
     | Name     | Key Type Value | Profile | Description             |
     +----------+----------------+---------+-------------------------+
     | Reserved | 0              |         | This value is reserved  |
     +----------+----------------+---------+-------------------------+

                         Figure 10: Key Type Values

   The response SHOULD contain the following parameter:

   *  'exp', with value the expiration time of the keying material for
      the group communication, encoded as a CBOR unsigned integer.  This
      field contains a numeric value representing the number of seconds
      from 1970-01-01T00:00:00Z UTC until the specified UTC date/time,
      ignoring leap seconds, analogous to what specified for NumericDate
      in Section 2 of [RFC7519].  Group members MUST stop using the
      keying material to protect outgoing messages and retrieve new
      keying material at the time indicated in this field.

   Optionally, the response MAY contain the following parameters, which,
   if included, MUST have format and value as specified below.

   *  'ace-groupcomm-profile', with value a CBOR integer that MUST be
      used to uniquely identify the application profile for group
      communication.  Applications of this specification MUST register
      an application profile identifier and the related value for this
      parameter in the "ACE Groupcomm Profiles" registry (REQ19).

   *  'pub_keys', MUST be present if 'get_pub_keys' was present in the
      request, otherwise it MUST NOT be present.  This parameter is a
      CBOR array specifying the public keys of the group members, i.e.,
      of all of them or of the ones selected according to the
      'get_pub_keys' parameter in the request.  In particular, each
      element of the array is a CBOR byte string, with value the
      original binary representation of a group member's public key.  It
      is REQUIRED of the application profiles to define the specific
      formats of public keys that are acceptable to use in the group
      (REQ6).

   *  'peer_roles', MUST be present if 'pub_keys' is also present,
      otherwise it MUST NOT be present.  This parameter is a CBOR array
      of n elements, with n the number of public keys included in the
      'pub_keys' parameter (at most the number of members in the group).
      The i-th element of the array specifies the role (or CBOR array of
      roles) that the group member associated to the i-th public key in
      'pub_keys' has in the group.  In particular, each array element is
      encoded as the role element of a scope entry, as defined in
      Section 3.1.



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   *  'peer_identifiers', MUST be present if 'pub_keys' is also present,
      otherwise it MUST NOT be present.  This parameter is a CBOR array
      of n elements, with n the number of public keys included in the
      'pub_keys' parameter (at most the number of members in the group).
      The i-th element of the array specifies the node identifier that
      the group member associated to the i-th public key in 'pub_keys'
      has in the group.  In particular, the i-th array element is
      encoded as a CBOR byte string, with value the node identifier of
      the group member.

   *  'group_policies', with value a CBOR map, whose entries specify how
      the group handles specific management aspects.  These include, for
      instance, approaches to achieve synchronization of sequence
      numbers among group members.  The elements of this field are
      registered in the "ACE Groupcomm Policies" registry.  This
      specification defines the three elements "Sequence Number
      Synchronization Methods", "Key Update Check Interval" and
      "Expiration Delta", which are summarized in Figure 11.
      Application profiles that build on this document MUST specify the
      exact content format and default value of included map entries
      (REQ20).






























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 +--------------+-------+----------+----------------------+------------+
 |     Name     | CBOR  |   CBOR   |     Description      | Reference  |
 |              | label |   type   |                      |            |
 +--------------+-------+----------+----------------------+------------+
 | Sequence     | TBD   | tstr/int | Method for recipient | [[this     |
 | Number       |       |          | group members to     | document]] |
 | Synchroniza- |       |          | synchronize with     |            |
 | tion Method  |       |          | sequence numbers of  |            |
 |              |       |          | of sender group      |            |
 |              |       |          | members. Its value   |            |
 |              |       |          | is taken from the    |            |
 |              |       |          | 'Value' column of    |            |
 |              |       |          | the Sequence Number  |            |
 |              |       |          | Synchronization      |            |
 |              |       |          | Method registry      |            |
 +--------------+-------+----------+----------------------+------------+
 | Key Update   | TBD   | int      | Polling interval in  | [[this     |
 | Check        |       |          | seconds, for  group  | document]] |
 | Interval     |       |          | members to check at  |            |
 |              |       |          | the KDC if the       |            |
 |              |       |          | latest group keying  |            |
 |              |       |          | material is the one  |            |
 |              |       |          | that they own        |            |
 +--------------+-------+----------+----------------------+------------+
 | Expiration   | TBD   | uint     | Number of seconds    | [[this     |
 | Delta        |       |          | from 'exp' until the | document]] |
 |              |       |          | specified UTC        |            |
 |              |       |          | date/time after      |            |
 |              |       |          | which group members  |            |
 |              |       |          | MUST stop using the  |            |
 |              |       |          | group keying         |            |
 |              |       |          | material they own to |            |
 |              |       |          | verify incoming      |            |
 |              |       |          | messages             |            |
 +--------------+-------+----------|----------------------|------------+

                   Figure 11: ACE Groupcomm Policies

   *  'kdc_cred', encoded as a CBOR byte string, with value the original
      binary representation of the KDC's public key.  This parameter is
      used if the KDC has an associated public key and this is required
      for the correct group operation.  It is REQUIRED of application
      profiles to define whether the KDC has a public key and if this
      has to be provided through the 'kdc_cred' parameter (REQ8).







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      In such a case, the KDC's public key MUST have the same format
      used for the public keys of the group members.  It is REQUIRED of
      the application profiles to define the specific formats that are
      acceptable to use for encoding public keys in the group (REQ6).

   *  'kdc_nonce', encoded as a CBOR byte string, and including a
      dedicated nonce N_KDC generated by the KDC.  This parameter MUST
      be present if the 'kdc_cred' parameter is present.

   *  'kdc_cred_verify' parameter, encoded as a CBOR byte string.  This
      parameter MUST be present if the 'kdc_cred' parameter is present.

      This parameter contains a proof-of-possession (PoP) evidence
      computed by the KDC over the nonce N_KDC, which is specified in
      the 'kdc_nonce' parameter and taken as PoP input.

      A possible type of PoP evidence is a signature, that the KDC
      computes by using its own private key, whose corresponding public
      key is specified in the 'kdc_cred' parameter.  Application
      profiles of this specification MUST specify the exact approaches
      used by the KDC to compute the PoP evidence to include in
      'kdc_cred_verify', and MUST specify which of those approaches is
      used in which case (REQ21).

   *  'rekeying_scheme', identifying the rekeying scheme that the KDC
      uses to provide new group keying meterial to the group members.
      This parameter is encoded as a CBOR integer, whose value is taken
      from the "Value" column of the "ACE Groupcomm Rekeying Schemes"
      registry defined in Section 11.14 of this specification.

  +-------+----------------+-------------------------------+-----------+
  | Value |      Name      |          Description          | Reference |
  +-------+----------------+-------------------------------+-----------+
  |   0   | Point-to-Point | The KDC individually targets  | [this     |
  |       |                | each node to rekey, using the | document] |
  |       |                | pairwise secure communication |           |
  |       |                | association with that node    |           |
  +-------+----------------+-------------------------------+-----------+

                Figure 12: ACE Groupcomm Rekeying Schemes

   Application profiles of this specification MAY define a default group
   rekeying scheme, to refer to in case the 'rekeying_scheme' parameter
   is not included in the Joining Response (OPT9).

   *  'mgt_key_material', encoded as a CBOR byte string and containing
      the specific administrative keying material that the joining node
      requires in order to participate in the group rekeying process



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      performed by the KDC.  This parameter MUST NOT be present if the
      'rekeying_scheme' parameter is not present and the application
      profile does not specify a default group rekeying scheme to use in
      the group.  Some simple rekeying scheme may not require specific
      administrative keying material to be provided, e.g., the basic
      "Point-to-Point" group rekeying scheme (see Section 6.1).

      In more advanced group rekeying schemes, the administrative keying
      material can be composed of multiple keys organized, for instance,
      into a logical tree hierarchy, whose root key is the only
      administrative key shared by all the group members.  In such a
      case, each group member is exclusively associated to one leaf key
      in the hierarchy, and owns only the administrative keys from the
      associated leaf key all the way up along the path to the root key.
      That is, different group members can be provided with a different
      subset of the overall administrative keying material.

      It is expected from separate documents to define how the advanced
      group rekeying scheme possibly indicated in the 'rekeying_scheme'
      parameter is used by an application profile of this specification.
      This includes defining the format of the administrative keying
      material to specify in 'mgt_key_material', consistently with the
      group rekeying scheme and the application profile in question.

   *  'control_group_uri', with value a full URI, encoded as a CBOR text
      string.  The URI MUST specify addressing information intended to
      reach all the members in the group.  For example, this can be a
      multicast IP address, optionally together with a port number
      (which defaults to 5683 if omitted).  The URI MUST include
      GROUPNAME in the url-path.  A default url-path is /ace-group/
      GROUPNAME, although implementations can use different ones
      instead.  The URI MUST NOT have url-path ace-group/GROUPNAME/node.

      If 'control_group_uri' is included in the Joining Response, the
      Clients supporting this parameter act as CoAP servers, host a
      resource at this specific URI, and listen to the specified
      addressing information.

      The KDC MAY use this URI to send one-to-many CoAP requests to the
      Client group members (acting as CoAP servers in this exchange),
      for example for one-to-many provisioning of new group keying
      material when performing a group rekeying (see Section 4.8.1.1),
      or to inform the Clients of their removal from the group
      Section 5.

      In particular, this resource is intended for communications
      concerning exclusively the group whose group name GROUPNAME is
      specified in the 'scope' parameter.  If the KDC does not implement



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      mechanisms using this resource for that group, it can ignore this
      parameter.  Other additional functionalities of this resource MAY
      be defined in application profiles of this specifications (OPT10).

   If the Joining Response includes the 'kdc_cred_verify' parameter, the
   Client verifies the conveyed PoP evidence and considers the group
   joining unsuccessful in case of failed verification.  Application
   profiles of this specification MUST specify the exact approaches used
   by the Client to verify the PoP evidence in 'kdc_cred_verify', and
   MUST specify which of those approaches is used in which case (REQ21).

   Specific application profiles that build on this document MUST
   specify the communication protocol that members of the group use to
   communicate with each other (REQ22) and how exactly the keying
   material is used to protect the group communication (REQ23).

4.3.1.1.  Join the Group

   Figure 13 gives an overview of the Joining exchange between Client
   and KDC, when the Client first joins a group, while Figure 14 shows
   an example.

      Client                                                     KDC
         |                                                        |
         |----- Joining Request: POST /ace-group/GROUPNAME ------>|
         |                                                        |
         |<--------- Joining Response: 2.01 (Created) ----------- |
         | Location-Path = "/ace-group/GROUPNAME/nodes/NODENAME"  |

              Figure 13: Message Flow of the Joining Exchange





















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 Request:

 Header: POST (Code=0.02)
 Uri-Host: "kdc.example.com"
 Uri-Path: "ace-group"
 Uri-Path: "g1"
 Content-Format: "application/ace-groupcomm+cbor"
 Payload (in CBOR diagnostic notation,
          with PUB_KEY and POP_EVIDENCE being CBOR byte strings):
   { "scope": << [ "group1", ["sender", "receiver"] ] >> ,
     "get_pub_keys": [true, ["sender"], []], "client_cred": PUB_KEY,
     "cnonce": h'6df49c495409a9b5', "client_cred_verify": POP_EVIDENCE }

 Response:

 Header: Created (Code=2.01)
 Content-Format: "application/ace-groupcomm+cbor"
 Location-Path: "kdc.example.com"
 Location-Path: "g1"
 Location-Path: "nodes"
 Location-Path: "c101"
 Payload (in CBOR diagnostic notation,
          with KEY being a CBOR byte strings):
   { "gkty": 13, "key": KEY, "num": 12, "exp": 1609459200,
     "pub_keys": [ PUB_KEY1, PUB_KEY2 ],
     "peer_roles": ["sender", ["sender", "receiver"]],
     "peer_identifiers": [ ID1, ID2 ] }

         Figure 14: Example of First Exchange for Group Joining

   If not previously established, the Client and the KDC MUST first
   establish a pairwise secure communication channel (REQ24).  This can
   be achieved, for instance, by using a transport profile of ACE.  The
   Joining exchange MUST occur over that secure channel.  The Client and
   the KDC MAY use that same secure channel to protect further pairwise
   communications that must be secured.

   The secure communication protocol is REQUIRED to establish the secure
   channel between Client and KDC by using the proof-of-possession key
   bound to the access token.  As a result, the proof-of-possession to
   bind the access token to the Client is performed by using the proof-
   of-possession key bound to the access token for establishing secure
   communication between the Client and the KDC.

   To join the group, the Client sends a CoAP POST request to the /ace-
   group/GROUPNAME endpoint at the KDC, where GROUPNAME is the group
   name of the group to join, formatted as specified in Section 4.3.1.
   This group name is the same as in the scope entry corresponding to



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   that group, specified in the 'scope' parameter of the Authorization
   Request/Response, or it can be retrieved from it.  Note that, in case
   of successful joining, the Client will receive the URI to retrieve
   individual keying material and to leave the group in the Location-
   Path option of the response.

   If the node is joining a group for the first time, and the KDC
   maintains the public keys of the group members, the Client is
   REQUIRED to send its own public key and proof-of-possession (PoP)
   evidence in the Joining Request (see the 'client_cred' and
   'client_cred_verify' parameters in Section 4.3.1).  The request is
   accepted only if both public key is provided and the PoP evidence is
   successfully verified.

   If a node re-joins a group as authorized by the same access token and
   using the same public key, it can omit the public key and the PoP
   evidence, or just the PoP evidence, from the Joining Request.  Then,
   the KDC will be able to retrieve the node's public key associated to
   the access token for that group.  If the public key has been
   discarded, the KDC replies with 4.00 (Bad Request) error response, as
   specified in Section 4.3.1.  If a node re-joins a group but wants to
   update its own public key, it needs to include both its public key
   and the PoP evidence in the Joining Request like when it joined the
   group for the first time.

4.3.2.  GET Handler

   The GET handler returns the symmetric group keying material for the
   group identified by GROUPNAME.

   The handler expects a GET request.

   In addition to what is defined in Section 4.1.2, the handler verifies
   that the Client is a current member of the group.  If the
   verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
   response.  The response MUST have Content-Format set to application/
   ace-groupcomm+cbor and is formatted as defined in Section 4.  The
   value of the 'error' field MUST be set to 0 ("Operation permitted
   only to group members").

   If all verifications succeed, the handler replies with a 2.05
   (Content) response containing the symmetric group keying material.
   The payload of the response is formatted as a CBOR map which MUST
   contain the parameters 'gkty', 'key' and 'num' specified in
   Section 4.3.1.






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   Each of the following parameters specified in Section 4.3.1 MUST also
   be included in the payload of the response, if they are included in
   the payload of the Joining Responses sent for the group:
   'rekeying_scheme', 'mgt_key_material'.

   The payload MAY also include the parameters 'ace-groupcomm-profile'
   and 'exp' parameters specified in Section 4.3.1.

4.3.2.1.  Retrieve Group Keying Material

   A node in the group can contact the KDC to retrieve the current group
   keying material, by sending a CoAP GET request to the /ace-group/
   GROUPNAME endpoint at the KDC, where GROUPNAME is the group name.

   Figure 15 gives an overview of the Joining exchange between Client
   and KDC, when the Client first joins a group, while Figure 16 shows
   an example.

 Client                                                              KDC
    |                                                                 |
    |----- Key Distribution Request: POST /ace-group/GROUPNAME ------>|
    |                                                                 |
    |<----------- Key Distribution Response: 2.05 (Content) --------- |
    |                                                                 |

      Figure 15: Message Flow of Key Distribution Request-Response

   Request:

   Header: GET (Code=0.01)
   Uri-Host: "kdc.example.com"
   Uri-Path: "ace-group"
   Uri-Path: "g1"
   Payload: -

   Response:

   Header: Content (Code=2.05)
   Content-Format: "application/ace-groupcomm+cbor"
   Payload (in CBOR diagnostic notation,
            with KEY being a CBOR byte strings):
     { "gkty": 13, "key": KEY, "num": 12 }

          Figure 16: Example of Key Distribution Request-Response







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4.4.  /ace-group/GROUPNAME/pub-key

   This resource implements the GET and FETCH handlers.

4.4.1.  FETCH Handler

   The FETCH handler receives identifiers of group members for the group
   identified by GROUPNAME and returns the public keys of such group
   members.

   The handler expects a request with payload formatted as a CBOR map,
   that MUST contain the following field.

   *  'get_pub_keys', whose value is encoded as in Section 4.3.1 with
      the following modifications.

      -  The arrays 'role_filter' and 'id_filter' MUST NOT both be
         empty, i.e., in CBOR diagnostic notation: [ bool, [ ], [ ] ].
         If the 'get_pub_keys' parameter has such a format, the request
         MUST be considered malformed, and the KDC MUST reply with a
         4.00 (Bad Request) error response.

         Note that a group member can retrieve the public keys of all
         the current group members by sending a GET request to the same
         KDC resource instead (see Section 4.4.2.1).

      -  The element 'inclusion_flag' encodes the CBOR simple value True
         if the third element 'id_filter' specifies an empty CBOR array,
         or if the Client wishes to receive the public keys of the nodes
         having their node identifier specified in 'id_filter' (i.e,
         selection by inclusive filtering).  Instead, this element
         encodes the CBOR simple value False if the Client wishes to
         receive the public keys of the nodes not having the node
         identifiers specified in the third element 'id_filter' (i.e.,
         selection by exclusive filtering).

      -  The array 'role_filter' can be empty, if the Client does not
         wish to filter the requested public keys based on the roles of
         the group members.

      -  The array 'id_filter' contains zero or more node identifiers of
         group members, for the group identified by GROUPNAME.  The
         Client indicates that it wishes to receive the public keys of
         the nodes having or not having these node identifiers, in case
         the 'inclusion_flag' element encodes the CBOR simple value True
         or False, respectively.  The array 'id_filter' may be empty, if
         the Client does not wish to filter the requested public keys
         based on the node identifiers of the group members.



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   Note that, in case the 'role_filter' array and the 'id_filter' array
   are both non-empty:

   *  If the 'inclusion_flag' encodes the CBOR simple value True, the
      handler returns the public keys of group members whose roles match
      with 'role_filter' and/or having their node identifier specified
      in 'id_filter'.

   *  If the 'inclusion_flag' encodes the CBOR simple value False, the
      handler returns the public keys of group members whose roles match
      with 'role_filter' and, at the same time, not having their node
      identifier specified in 'id_filter'.

   The specific format of public keys as well as identifiers, roles and
   combination of roles of group members MUST be specified by It is
   REQUIRED of application profiles of this specification (REQ1, REQ6,
   REQ25).

   The handler identifies the public keys of the current group members
   for which either:

   *  the role identifier matches with one of those indicated in the
      request; note that the request can contain a "combination of
      roles", where the handler select all group members who have all
      roles included in the combination.

   *  the node identifier matches with one of those indicated in the
      request.

   If all verifications succeed, the handler returns a 2.05 (Content)
   message response with payload formatted as a CBOR map, containing
   only the following parameters from Section 4.3.1.

   *  'num', which encodes the version number of the current group
      keying material.

   *  'pub_keys', which encodes the list of public keys of the selected
      group members.

   *  'peer_roles', which encodes the role (or CBOR array of roles) that
      each of the selected group members has in the group.

   *  'peer_identifiers', which encodes the node identifier that each of
      the selected group members has in the group.

   The specific format of public keys as well as of node identifiers of
   group members is specified by the application profile (REQ6, REQ25).




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   If the KDC does not store any public key associated to the specified
   node identifiers, the handler returns a response with payload
   formatted as a CBOR byte string of zero length.

   The handler MAY enforce one of the following policies, in order to
   handle possible node identifiers that are included in the 'id_filter'
   element of the 'get_pub_keys' parameter of the request but are not
   associated to any current group member.  Such a policy MUST be
   specified by the application profile (REQ26).

   *  The KDC silently ignores those node identifiers.

   *  The KDC retains public keys of group members for a given amount of
      time after their leaving, before discarding them.  As long as such
      public keys are retained, the KDC provides them to a requesting
      Client.

      If the KDC adopts this policy, the application profile MUST also
      specify the amount of time during which the KDC retains the public
      key of a former group member after its leaving, possibly on a per-
      member basis.

   Note that this resource handler only verifies that the node is
   authorized by the AS to access this resource.  Nodes that are not
   members of the group but are authorized to do signature verifications
   on the group messages may be allowed to access this resource, if the
   application needs it.

4.4.1.1.  Retrieve a Subset of Public Keys in the Group

   In case the KDC maintains the public keys of group members, a node in
   the group can contact the KDC to request the public keys, roles and
   node identifiers of a specified subset of group members, by sending a
   CoAP FETCH request to the /ace-group/GROUPNAME/pub-key endpoint at
   the KDC, where GROUPNAME is the group name, and formatted as defined
   in Section 4.4.1.

   Figure 17 gives an overview of the exchange mentioned above, while
   Figure 18 shows an example of such an exchange.

      Client                                                      KDC
         |                                                         |
         |-Public Key Request: FETCH /ace-group/GROUPNAME/pub-key->|
         |                                                         |
         |<--------- Public Key Response: 2.05 (Created) ----------|
         |                                                         |





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       Figure 17: Message Flow of Public Key Exchange to Request the
                   Public Keys of Specific Group Members

   Request:

   Header: FETCH (Code=0.05)
   Uri-Host: "kdc.example.com"
   Uri-Path: "ace-group"
   Uri-Path: "g1"
   Uri-Path: "pub-key"
   Content-Format: "application/ace-groupcomm+cbor"
   Payload:
     { "get_pub_keys": [true, [], [ ID3 ]] }

   Response:

   Header: Content (Code=2.05)
   Content-Format: "application/ace-groupcomm+cbor"
   Payload (in CBOR diagnostic notation):
     { "pub_keys": [ PUB_KEY3 ],
       "peer_roles": [ "receiver" ],
       "peer_identifiers": [ ID3 ] }

      Figure 18: Example of Public Key Exchange to Request the Public
                       Keys of Specific Group Members

4.4.2.  GET Handler

   The handler expects a GET request.

   If all verifications succeed, the KDC replies with a 2.05 (Content)
   response as in the FETCH handler in Section 4.4.1, but specifying in
   the payload the public keys of all the group members, together with
   their roles and node identifiers.

4.4.2.1.  Retrieve All Public Keys in the Group

   In case the KDC maintains the public keys of group members, a group
   or an external signature verifier can contact the KDC to request the
   public keys, roles and node identifiers of all the current group
   members, by sending a CoAP GET request to the /ace-group/GROUPNAME/
   pub-key endpoint at the KDC, where GROUPNAME is the group name.

   Figure 19 gives an overview of the message exchange, while Figure 20
   shows an example of such an exchange.






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      Client                                                     KDC
         |                                                        |
         |--Public Key Request: GET /ace-group/GROUPNAME/pub-key->|
         |                                                        |
         |<--------- Public Key Response: 2.05 (Content) ---------|
         |                                                        |

       Figure 19: Message Flow of Public Key Exchange to Request the
                    Public Keys of all the Group Members

   Request:

   Header: GET (Code=0.01)
   Uri-Host: "kdc.example.com"
   Uri-Path: "ace-group"
   Uri-Path: "g1"
   Uri-Path: "pub-key"
   Payload: -

   Response:

   Header: Content (Code=2.05)
   Content-Format: "application/ace-groupcomm+cbor"
   Payload (in CBOR diagnostic notation):
     { "num": 5,
       "pub_keys": [ PUB_KEY1, PUB_KEY2, PUB_KEY3 ],
       "peer_roles": ["sender", ["sender", "receiver"], "receiver"],
       "peer_identifiers": [ ID1, ID2, ID3 ] }

      Figure 20: Example of Public Key Exchange to Request the Public
                       Keys of all the Group Members

4.5.  ace-group/GROUPNAME/kdc-pub-key

   This resource implements a GET handler.

4.5.1.  GET Handler

   The handler expects a GET request.

   If all verifications succeed, the handler returns a 2.05 (Content)
   message containing the KDC's public key together with a proof-of-
   possession (PoP) evidence.  The response MUST have Content-Format set
   to application/ace-groupcomm+cbor.  The payload of the response is a
   CBOR map, which includes the following fields.






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   *  The 'kdc_cred' parameter, specifying the KDC's public key.  This
      parameter is encoded like the 'kdc_cred' parameter in the Joining
      Response (see Section 4.3.1).

   *  The 'kdc_nonce' parameter, specifying a nonce generated by the
      KDC.  This parameter is encoded like the 'kdc_nonce' parameter in
      the Joining Response (see Section 4.3.1).

   *  The 'kdc_cred_verify' parameter, specifying a PoP evidence
      computed by the KDC.  This parameter is encoded like the
      'kdc_cred_verify' parameter in the Joining Response (see
      Section 4.3.1).

      The PoP evidence is computed over the nonce specified in the
      'kdc_nonce' parameter and taken as PoP input, by means of the same
      method used when preparing the Joining Response (see
      Section 4.3.1).  Application profiles of this specification MUST
      specify the exact approaches used by the KDC to compute the PoP
      evidence to include in 'kdc_cred_verify', and MUST specify which
      of those approaches is used in which case (REQ21).

4.5.1.1.  Retrieve the KDC's Public Key

   In case the KDC has an associated public key as required for the
   correct group operation, a group member or an external signature
   verifier can contact the KDC to request the KDC's public key, by
   sending a CoAP GET request to the /ace-group/GROUPNAME/kdc-pub-key
   endpoint at the KDC, where GROUPNAME is the group name.

   Upon receiving the 2.05 (Content) response, the Client retrieves the
   KDC's public key from the 'kdc_cred' parameter, and MUST verify the
   proof-of-possession (PoP) evidence specified in the 'kdc_cred_verify'
   parameter.  In case of successful verification of the PoP evidence,
   the Client MUST store the obtained KDC's public key and replace the
   currently stored one.

   The PoP evidence is verified by means of the same method used when
   processing the Joining Response (see Section 4.3.1).  Application
   profiles of this specification MUST specify the exact approaches used
   by the Client to verify the PoP evidence in 'kdc_cred_verify', and
   MUST specify which of those approaches is used in which case (REQ21).

   Figure 21 gives an overview of the exchange described above, while
   Figure 22 shows an example.







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    Group
    Member                                                         KDC
      |                                                             |
      |                    KDC Public Key Request                   |
      |------------------------------------------------------------>|
      |              GET ace-group/GROUPNAME/gm-pub-key             |
      |                                                             |
      |<--------- KDC Public Key Response: 2.05 (Content) ----------|
      |                                                             |

         Figure 21: Message Flow of KDC Public Key Request-Response

   Request:

   Header: GET (Code=0.01)
   Uri-Host: "kdc.example.com"
   Uri-Path: "ace-group"
   Uri-Path: "g1"
   Uri-Path: "kdc-pub-key"
   Payload: -

   Response:

   Header: Content (Code=2.05)
   Content-Format: "application/ace-groupcomm+cbor"
   Payload (in CBOR diagnostic notation, with PUB_KEY_KDC
            and POP_EVIDENCE being CBOR byte strings):
     {
       "kdc_nonce": h'25a8991cd700ac01',
       "kdc_cred": PUB_KEY_KDC,
       "kdc_cred_verify": POP_EVIDENCE
     }

           Figure 22: Example of KDC Public Key Request-Response

4.6.  /ace-group/GROUPNAME/policies

   This resource implements the GET handler.

4.6.1.  GET Handler

   The handler expects a GET request.









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   In addition to what is defined in Section 4.1.2, the handler verifies
   that the Client is a current member of the group.  If the
   verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
   response.  The response MUST have Content-Format set to application/
   ace-groupcomm+cbor and is formatted as defined in Section 4.  The
   value of the 'error' field MUST be set to 0 ("Operation permitted
   only to group members").

   If all verifications succeed, the handler replies with a 2.05
   (Content) response containing the list of policies for the group
   identified by GROUPNAME.  The payload of the response is formatted as
   a CBOR map including only the parameter 'group_policies' defined in
   Section 4.3.1 and specifying the current policies in the group.  If
   the KDC does not store any policy, the payload is formatted as a
   zero-length CBOR byte string.

   The specific format and meaning of group policies MUST be specified
   in the application profile (REQ20).

4.6.1.1.  Retrieve the Group Policies

   A node in the group can contact the KDC to retrieve the current group
   policies, by sending a CoAP GET request to the /ace-group/GROUPNAME/
   policies endpoint at the KDC, where GROUPNAME is the group name, and
   formatted as defined in Section 4.6.1

   Figure 23 gives an overview of the exchange described above, while
   Figure 24 shows an example.

       Client                                                   KDC
          |                                                      |
          |-Policies Request: GET ace-group/GROUPNAME/policies ->|
          |                                                      |
          |<--------- Policies Response: 2.05 (Content) ---------|
          |                                                      |

            Figure 23: Message Flow of Policies Request-Response














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   Request:

   Header: GET (Code=0.01)
   Uri-Host: "kdc.example.com"
   Uri-Path: "ace-group"
   Uri-Path: "g1"
   Uri-Path: "policies"
   Payload: -

   Response:

   Header: Content (Code=2.05)
   Content-Format: "application/ace-groupcomm+cbor"
   Payload(in CBOR diagnostic notation):
     { "group_policies": {"exp-delta": 120} }

              Figure 24: Example of Policies Request-Response

4.7.  /ace-group/GROUPNAME/num

   This resource implements the GET handler.

4.7.1.  GET Handler

   The handler expects a GET request.

   In addition to what is defined in Section 4.1.2, the handler verifies
   that the Client is a current member of the group.  If the
   verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
   response.  The response MUST have Content-Format set to application/
   ace-groupcomm+cbor and is formatted as defined in Section 4.  The
   value of the 'error' field MUST be set to 0 ("Operation permitted
   only to group members").

   If all verifications succeed, the handler returns a 2.05 (Content)
   message containing an integer that represents the version number of
   the symmetric group keying material.  This number is incremented on
   the KDC every time the KDC updates the symmetric group keying
   material, before the new keying material is distributed.  This number
   is stored in persistent storage.

   The payload of the response is formatted as a CBOR integer.









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4.7.1.1.  Retrieve the Keying Material Version

   A node in the group can contact the KDC to request information about
   the version number of the symmetric group keying material, by sending
   a CoAP GET request to the /ace-group/GROUPNAME/num endpoint at the
   KDC, where GROUPNAME is the group name, formatted as defined in
   Section 4.7.1.  In particular, the version is incremented by the KDC
   every time the group keying material is renewed, before it's
   distributed to the group members.

   Figure 25 gives an overview of the exchange described above, while
   Figure 26 shows an example.

       Client                                                    KDC
          |                                                       |
          |---- Version Request: GET ace-group/GROUPNAME/num ---->|
          |                                                       |
          |<--------- Version Response: 2.05 (Content) -----------|
          |                                                       |

            Figure 25: Message Flow of Version Request-Response

   Request:

   Header: GET (Code=0.01)
   Uri-Host: "kdc.example.com"
   Uri-Path: "ace-group"
   Uri-Path: "g1"
   Uri-Path: "num"
   Payload: -

   Response:

   Header: Content (Code=2.05)
   Content-Format: "application/ace-groupcomm+cbor"
   Payload(in CBOR diagnostic notation):
     13

               Figure 26: Example of Version Request-Response

4.8.  /ace-group/GROUPNAME/nodes/NODENAME

   This resource implements the GET, PUT and DELETE handlers.

   In addition to what is defined in Section 4.1.2, each of the handlers
   performs the following two verifications.





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   *  The handler verifies that the Client is a current member of the
      group.  If the verification fails, the KDC MUST reply with a 4.03
      (Forbidden) error response.  The response MUST have Content-Format
      set to application/ace-groupcomm+cbor and is formatted as defined
      in Section 4.  The value of the 'error' field MUST be set to 0
      ("Operation permitted only to group members").

   *  The handler verifies that the node name of the Client is equal to
      NODENAME used in the url-path.  If the verification fails, the
      handler replies with a 4.03 (Forbidden) error response.

4.8.1.  GET Handler

   The handler expects a GET request.

   If all verifications succeed, the handler replies with a 2.05
   (Content) response containing both the group keying material and the
   individual keying material for the Client, or information enabling
   the Client to derive it.  The payload of the response is formatted as
   a CBOR map.  The format for the group keying material is the same as
   defined in the response of Section 4.3.2.  The specific format of
   individual keying material for group members, or of the information
   to derive it, and corresponding CBOR label, MUST be specified in the
   application profile (REQ27) and registered in Section 11.7.

   Optionally, the KDC can make the sub-resource at ace-
   group/GROUPNAME/nodes/NODENAME also Observable [RFC7641] for the
   associated node.  In case the KDC removes that node from the group
   without having been explicitly asked for it, this allows the KDC to
   send an unsolicited 4.04 (Not Found) response to the node as a
   notification of eviction from the group (see Section 5).

   Note that the node could have been observing also the resource at
   ace-group/GROUPNAME, in order to be informed of changes in the keying
   material.  In such a case, this method would result in largely
   overlapping notifications received for the resource at ace-group/
   GROUPNAME and the sub-resource at ace-group/GROUPNAME/nodes/NODENAME.

   In order to mitigate this, a node that supports the No-Response
   option [RFC7967] can use it when starting the observation of the sub-
   resource at ace-group/GROUPNAME/nodes/NODENAME.  In particular, the
   GET observation request can also include the No-Response option, with
   value set to 2 (Not interested in 2.xx responses).








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4.8.1.1.  Retrieve Group and Individual Keying Material

   When any of the following happens, a node MUST stop using the owned
   group keying material to protect outgoing messages, and SHOULD stop
   using it to decrypt and verify incoming messages.

   *  Upon expiration of the keying material, according to what
      indicated by the KDC with the 'exp' parameter in a Joining
      Response, or to a pre-configured value.

   *  Upon receiving a notification of revoked/renewed keying material
      from the KDC, possibly as part of an update of the keying material
      (rekeying) triggered by the KDC.

   *  Upon receiving messages from other group members without being
      able to retrieve the keying material to correctly decrypt them.
      This may be due to rekeying messages previously sent by the KDC,
      that the Client was not able to receive or decrypt.

   In either case, if it wants to continue participating in the group
   communication, the node has to request the latest keying material
   from the KDC.  To this end, the Client sends a CoAP GET request to
   the /ace-group/GROUPNAME/nodes/NODENAME endpoint at the KDC,
   formatted as specified in Section 4.8.1.

   Note that policies can be set up, so that the Client sends a Key Re-
   Distribution request to the KDC only after a given number of received
   messages could not be decrypted (because of failed decryption
   processing or inability to retrieve the necessary keying material).

   It is application dependent and pertaining to the particular message
   exchange (e.g., [I-D.ietf-core-oscore-groupcomm]) to set up these
   policies for instructing Clients to retain incoming messages and for
   how long (OPT11).  This allows Clients to possibly decrypt such
   messages after getting updated keying material, rather than just
   consider them non valid messages to discard right away.

   The same Key Distribution Request could also be sent by the Client
   without being triggered by a failed decryption of a message, if the
   Client wants to be sure that it has the latest group keying material.
   If that is the case, the Client will receive from the KDC the same
   group keying material it already has in memory.

   Figure 27 gives an overview of the exchange described above, while
   Figure 28 shows an example.






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    Client                                                          KDC
       |                                                             |
       |------------------ Key Distribution Request: --------------->|
       |           GET ace-group/GROUPNAME/nodes/NODENAME            |
       |                                                             |
       |<-------- Key Distribution Response: 2.05 (Content) ---------|
       |                                                             |

        Figure 27: Message Flow of Key Distribution Request-Response

   Request:

   Header: GET (Code=0.01)
   Uri-Host: "kdc.example.com"
   Uri-Path: "ace-group"
   Uri-Path: "g1"
   Uri-Path: "nodes"
   Uri-Path: "c101"
   Payload: -

   Response:

   Header: Content (Code=2.05)
   Content-Format: "application/ace-groupcomm+cbor"
   Payload (in CBOR diagnostic notation,
            with KEY and IND_KEY being CBOR byte strings,
            and "ind-key" the profile-specified label
            for individual keying material):
     { "gkty": 13, "key": KEY, "num": 12, "ind-key": IND_KEY }

          Figure 28: Example of Key Distribution Request-Response

4.8.2.  PUT Handler

   The PUT handler processes requests from a Client that asks for new
   individual keying material, as required to process messages exchanged
   in the group.

   The handler expects a PUT request with empty payload.

   In addition to what is defined in Section 4.1.2 and at the beginning
   of Section 4.8, the handler verifies that this operation is
   consistent with the set of roles that the Client has in the group
   (REQ11).  If the verification fails, the KDC MUST reply with a 4.00
   (Bad Request) error response.  The response MUST have Content-Format
   set to application/ace-groupcomm+cbor and is formatted as defined in
   Section 4.  The value of the 'error' field MUST be set to 1 ("Request
   inconsistent with the current roles").



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   If the KDC is currently not able to serve this request, i.e., to
   generate new individual keying material for the requesting Client,
   the KDC MUST reply with a 5.03 (Service Unavailable) error response.
   The response MUST have Content-Format set to application/ace-
   groupcomm+cbor and is formatted as defined in Section 4.  The value
   of the 'error' field MUST be set to 4 ("No available node
   identifiers").

   If all verifications succeed, the handler reply with a 2.05 (Content)
   response containing newly generated, individual keying material for
   the Client.  The payload of the response is formatted as a CBOR map.
   The specific format of newly-generated individual keying material for
   group members, or of the information to derive it, and corresponding
   CBOR label, MUST be specified in the application profile (REQ27) and
   registered in Section 11.7.

   The typical successful outcome consists in replying with newly
   generated, individual keying material for the Client, as defined
   above.  However, application profiles of this specification MAY also
   extend this handler in order to achieve different akin outcomes
   (OPT12), for instance:

   *  Not providing the Client with newly generated, individual keying
      material, but rather rekeying the whole group, i.e., providing all
      the current group members with newly generated group keying
      material.

   *  Both providing the Client with newly generated, individual keying
      material, as well as rekeying the whole group, i.e., providing all
      the current group members with newly generated group keying
      material.

   In either case, the handler may specify the new group keying material
   as part of the 2.05 (Content) response.

   Note that this handler is not intended to accommodate requests from a
   group member to trigger a group rekeying, whose scheduling and
   execution is an exclusive prerogative of the KDC.













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4.8.2.1.  Request to Change Individual Keying Material

   A Client may ask the KDC for new, individual keying material.  For
   instance, this can be due to the expiration of such individual keying
   material, or to the exhaustion of AEAD nonces, if an AEAD encryption
   algorithm is used for protecting communications in the group.  An
   example of individual keying material can simply be an individual
   encryption key associated to the Client.  Hence, the Client may ask
   for a new individual encryption key, or for new input material to
   derive it.

   To this end, the Client performs a Key Renewal Request/Response
   exchange with the KDC, i.e., it sends a CoAP PUT request to the /ace-
   group/GROUPNAME/nodes/NODENAME endpoint at the KDC, where GROUPNAME
   is the group name and NODENAME is its node name, and formatted as
   defined in Section 4.8.1.

   Figure 29 gives an overview of the exchange described above, while
   Figure 30 shows an example.

       Client                                                    KDC
          |                                                       |
          |------------------ Key Renewal Request: -------------->|
          |           PUT ace-group/GROUPNAME/nodes/NODENAME      |
          |                                                       |
          |<-------- Key Renewal Response: 2.05 (Content) --------|
          |                                                       |

          Figure 29: Message Flow of Key Renewal Request-Response

   Request:

   Header: PUT (Code=0.03)
   Uri-Host: "kdc.example.com"
   Uri-Path: "ace-group"
   Uri-Path: "g1"
   Uri-Path: "nodes"
   Uri-Path: "c101"
   Payload: -

   Response:

   Header: Content (Code=2.05)
   Content-Format: "application/ace-groupcomm+cbor"
   Payload (in CBOR diagnostic notation, with IND_KEY being
            a CBOR byte string, and "ind-key" the profile-specified
            label for individual keying material):
     { "ind-key": IND_KEY }



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             Figure 30: Example of Key Renewal Request-Response

   Note the difference between the Key Renewal Request in this section
   and the Key Distribution Request in Section 4.8.1.1.  The former asks
   the KDC for new individual keying material, while the latters asks
   the KDC for the current group keying material together with the
   current individual keying material.

   As discussed in Section 4.8.2, application profiles of this
   specification may define alternative outcomes for the Key Renewal
   Request-Response exchange (OPT12), where the provisioning of new
   individual keying material is replaced by or combined with the
   execution of a whole group rekeying.

4.8.3.  DELETE Handler

   The DELETE handler removes the node identified by NODENAME from the
   group identified by GROUPNAME.

   The handler expects a DELETE request with empty payload.

   In addition to what is defined in Section 4.1.2, the handler verifies
   that the Client is a current member of the group.  If the
   verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
   response.  The response MUST have Content-Format set to application/
   ace-groupcomm+cbor and is formatted as defined in Section 4.  The
   value of the 'error' field MUST be set to 0 ("Operation permitted
   only to group members").

   If all verification succeeds, the handler performs the actions
   defined in Section 5 and replies with a 2.02 (Deleted) response with
   empty payload.

4.8.3.1.  Leave the Group

   A Client can actively request to leave the group.  In this case, the
   Client sends a CoAP DELETE request to the endpoint /ace-
   group/GROUPNAME/nodes/NODENAME at the KDC, where GROUPNAME is the
   group name and NODENAME is its node name, formatted as defined in
   Section 4.8.3

   Note that, after having left the group, the Client may wish to join
   it again.  Then, as long as the Client is still authorized to join
   the group, i.e., the associated access token is still valid, the
   Client can request to re-join the group directly to the KDC (see
   Section 4.3.1.1), without having to retrieve a new access token from
   the AS.




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4.9.  /ace-group/GROUPNAME/nodes/NODENAME/pub-key

   This resource implements the POST handler.

4.9.1.  POST Handler

   The POST handler is used to replace the stored public key of this
   Client (identified by NODENAME) with the one specified in the request
   at the KDC, for the group identified by GROUPNAME.

   The handler expects a POST request with payload as specified in
   Section 4.3.1, with the difference that it includes only the
   parameters 'client_cred', 'cnonce' and 'client_cred_verify'.  In
   particular, the PoP evidence included in 'client_cred_verify' is
   computed in the same way considered in Section 4.3.1 and defined by
   the specific application profile (REQ14), with a newly generated N_C
   nonce and the previously received N_S.  It is REQUIRED of the
   application profiles to define the specific formats of public keys
   that are acceptable to use in the group (REQ6).

   In addition to what is defined in Section 4.1.2 and at the beginning
   of Section 4.8, the handler verifies that this operation is
   consistent with the set of roles that the node has in the group.  If
   the verification fails, the KDC MUST reply with a 4.00 (Bad Request)
   error response.  The response MUST have Content-Format set to
   application/ace-groupcomm+cbor and is formatted as defined in
   Section 4.  The value of the 'error' field MUST be set to 1 ("Request
   inconsistent with the current roles").

   If the KDC cannot retrieve the 'kdcchallenge' associated to this
   Client (see Section 3.3), the KDC MUST reply with a 4.00 (Bad
   Request) error response, which MUST also have Content-Format
   application/ace-groupcomm+cbor.  The payload of the error response is
   a CBOR map including a newly generated 'kdcchallenge' value.  This is
   specified in the 'kdcchallenge' parameter.  In such a case the KDC
   MUST store the newly generated value as the 'kdcchallenge' value
   associated to this Client, possibly replacing the currently stored
   value.













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   Otherwise, the handler checks that the public key specified in the
   'client_cred' field is valid for the group identified by GROUPNAME.
   That is, the handler checks that the public key is encoded according
   to the format used in the group, is intended for the public key
   algorithm used in the group, and is aligned with the possible
   associated parameters used in the group.  If that cannot be
   successfully verified, the handler MUST reply with a 4.00 (Bad
   Request) error response.  The response MUST have Content-Format set
   to application/ace-groupcomm+cbor and is formatted as defined in
   Section 4.  The value of the 'error' field MUST be set to 2 ("Public
   key incompatible with the group configuration").

   Otherwise, the handler verifies the PoP evidence contained in the
   'client_cred_verify' field of the request, by using the public key
   specified in the 'client_cred' field, as well as the same way
   considered in Section 4.3.1 and defined by the specific application
   profile (REQ14).  If the PoP evidence does not pass verification, the
   handler MUST reply with a 4.00 (Bad Request) error response.  The
   response MUST have Content-Format set to application/ace-
   groupcomm+cbor and is formatted as defined in Section 4.  The value
   of the 'error' field MUST be set to 3 ("Invalid Proof-of-Possession
   evidence").

   If all verifications succeed, the handler performs the following
   actions.

   *  The handler associates the public key from the 'client_cred' field
      of the request to the node identifier NODENAME and to the access
      token associated to the node identified by NODENAME.

   *  In the stored list of group members' public keys for the group
      identified by GROUPNAME, the handler replaces the public key of
      the node identified by NODENAME with the public key specified in
      the 'client_cred' field of the request.

   Then, the handler replies with a 2.04 (Changed) response, which does
   not include a payload.

4.9.1.1.  Uploading a New Public Key

   In case the KDC maintains the public keys of group members, a node in
   the group can contact the KDC to upload a new public key to use in
   the group, and replace the currently stored one.

   To this end, the Client performs a Public Key Update Request/Response
   exchange with the KDC, i.e., it sends a CoAP POST request to the
   /ace-group/GROUPNAME/nodes/NODENAME/pub-key endpoint at the KDC,
   where GROUPNAME is the group name and NODENAME is its node name.



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   The request is formatted as specified in Section 4.9.1.

   Figure Figure 31 gives an overview of the exchange described above,
   while Figure 32 shows an example.

   Client                                                           KDC
   |                                                                 |
   |-------------- Public Key Update Request: ---------------------->|
   |      POST ace-group/GROUPNAME/nodes/NODENAME/pub-key            |
   |                                                                 |
   |<------- Public Key Update Response: 2.04 (Changed) -------------|
   |                                                                 |

       Figure 31: Message Flow of Public Key Update Request-Response

   Request:

   Header: POST (Code=0.02)
   Uri-Host: "kdc.example.com"
   Uri-Path: "ace-group"
   Uri-Path: "g1"
   Uri-Path: "nodes"
   Uri-Path: "c101"
   Uri-Path: "pub-key"
   Content-Format: "application/ace-groupcomm+cbor"
   Payload (in CBOR diagnostic notation, with PUB_KEY
            and POP_EVIDENCE being CBOR byte strings):
     { "client_cred": PUB_KEY, "cnonce": h'9ff7684414affcc8',
       "client_cred_verify": POP_EVIDENCE }

   Response:

   Header: Changed (Code=2.04)
   Payload: -

          Figure 32: Example of Public Key Update Request-Response

   Additionally, after updating its own public key, a group member MAY
   send a number of requests including an identifier of the updated
   public key, to notify other group members that they have to retrieve
   it.  How this is done depends on the group communication protocol
   used, and therefore is application profile specific (OPT13).

5.  Removal of a Group Member

   A Client identified by NODENAME may be removed from a group
   identified by GROUPNAME where it is a member, due to the following
   reasons.



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   1.  The Client explicitly asks to leave the group, as defined in
       Section 4.8.3.1.

   2.  The node has been found compromised or is suspected so.

   3.  The Client's authorization to be a group member with the current
       roles is not valid anymore, i.e., the access token has expired or
       has been revoked.  If the AS provides token introspection (see
       Section 5.9 of [I-D.ietf-ace-oauth-authz]), the KDC can
       optionally use it and check whether the Client is still
       authorized.

   In either case, the KDC performs the following actions.

   *  The KDC removes the Client from the list of current members or the
      group.

   *  In case of forced eviction, i.e., for cases 2 and 3 above, the KDC
      deletes the public key of the removed Client, if it acts as
      repository of public keys for group members.

   *  If the removed Client is registered as an observer of the group-
      membership resource at ace-group/GROUPNAME, the KDC removes the
      Client from the list of observers of that resource.

   *  If the sub-resource nodes/NODENAME was created for the removed
      Client, the KDC deletes that sub-resource.

      In case of forced eviction, i.e., for cases 2 and 3 above, the KDC
      MAY explicitly inform the removed Client, by means of the
      following methods.

      -  If the evicted Client implements the 'control_uri' resource
         specified in Section 4.3.1, the KDC sends a DELETE request,
         targeting the URI specified in the 'control_uri' parameter of
         the Joining Request (see Section 4.3.1).

      -  If the evicted Client is observing its associated sub-resource
         at ace-group/GROUPNAME/nodes/NODENAME (see Section 4.8.1), the
         KDC sends an unsolicited 4.04 (Not Found) error response, which
         does not include the Observe option and indicates that the
         observed resource has been deleted (see Section 3.2 of
         [RFC7641]).

         The response MUST have Content-Format set to application/ace-
         groupcomm+cbor and is formatted as defined in Section 4.  The
         value of the 'error' field MUST be set to 5 ("Group membership
         terminated").



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   *  If the application requires forward security or the used
      application profile requires so, the KDC MUST generate new group
      keying material and securely distribute it to all the current
      group members except the leaving node (see Section 6).

6.  Group Rekeying Process

   A group rekeying is started and driven by the KDC.  The KDC is not
   intended to accommodate explicit requests from group members to
   trigger a group rekeying.  That is, the scheduling and execution of a
   group rekeying is an exclusive prerogative of the KDC.  Reasons that
   can trigger a group rekeying are a change in the group membership,
   the current group keying material approaching its expiration time, or
   a regularly scheduled update of the group keying material.

   The KDC MUST increment the version number NUM of the current keying
   material, before distributing the newly generated keying material
   with version number NUM+1 to the group.  Once completed the group
   rekeying, the KDC MUST delete the old keying material and SHOULD
   store the newly distributed keying material in persistent storage.

   Distributing the new group keying material requires the KDC to send
   multiple rekeying messages to the group members.  Depending on the
   rekeying scheme used in the group and the reason that has triggered
   the rekeying process, each rekeying message can be intended to one or
   multiple group members, hereafter referred to as target group
   members.  The KDC MUST support at least the "Point-to-Point" group
   rekeying scheme in Section 6.1 and MAY support additional ones.

   Each rekeying message MUST have Content-Format set to application/
   ace-groupcomm+cbor and its payload formatted as a CBOR map, which
   MUST include at least the information specified in the Key
   Distribution Response message (see Section 4.3.2), i.e., the
   parameters 'gkty', 'key' and 'num' defined in Section 4.3.1.  The
   CBOR map MAY include the parameter 'exp', as well as the parameter
   'mgt_key_material' specifying new administrative keying material for
   the target group members, if relevant for the used rekeying scheme.














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   A rekeying message may include additional information, depending on
   the rekeying scheme used in the group, the reason that has triggered
   the rekeying process and the specific target group members.  In
   particular, if the group rekeying is performed due to one or multiple
   Clients that have joined the group and the KDC acts as repository of
   public keys of the group members, then a rekeying message MAY also
   include the public keys that those Clients use in the group, together
   with the roles and node identifier that the corresponding Client has
   in the group.  It is RECOMMENDED to specify this information by means
   of the parameters 'pub_keys', 'peer_roles' and 'peer_identifiers',
   like done in the Joining Response message (see Section 4.3.1).

   The complete format of a rekeying message, including the encoding and
   content of the 'mgt_key_material' parameter, has to be defined in
   separate specifications aimed at profiling the used rekeying scheme
   in the context of the used application profile of this specification.
   As a particular case, an application profile of this specification
   MAY define additional information to include in rekeying messages for
   the "Point-to-Point" group rekeying scheme in Section 6.1 (OPT14).

   Consistently with the used group rekeying scheme, the actual delivery
   of rekeying messages can occur through different approaches, as
   discussed in the following.

6.1.  Point-to-Point Group Rekeying

   This approach consists in the KDC sending one individual rekeying
   message to each target group member.  In particular, the rekeying
   message is protected by means of the security association between the
   KDC and the target group member in question, as per the used
   application profile of this specification and the used transport
   profile of ACE.

   This is the approach taken by the basic "Point-to-Point" group
   rekeying scheme, that the KDC can explicitly signal in the Joining
   Response (see Section 4.3.1), through the 'rekeying_scheme' parameter
   specifying the value 0.

   When taking this approach in the group identified by GROUPNAME, the
   KDC can practically deliver the rekeying messages to the target group
   members in different, co-existing ways.

   *  The KDC SHOULD make the ace-group/GROUPNAME resource Observable
      [RFC7641].  Thus, upon performing a group rekeying, the KDC can
      distribute the new group keying material through individual
      notification responses sent to the target group members that are
      also observing that resource.




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      In case the KDC deletes the group, this also allows the KDC to
      send an unsolicited 4.04 (Not Found) response to each observer
      group member, as a notification of group termination.  The
      response MUST have Content-Format set to application/ace-
      groupcomm+cbor and is formatted as defined in Section 4.  The
      value of the 'error' field MUST be set to 6 ("Group deleted").

   *  If a target group member specified a URI in the 'control_uri'
      parameter of the Joining Request upon joining the group (see
      Section 4.3.1), the KDC can provide that group member with the new
      group keying material by sending a unicast POST request to that
      URI.

      A Client that does not plan to observe the ace-group/GROUPNAME
      resource at the KDC SHOULD provide a URI in the 'control_uri'
      parameter of the Joining Request upon joining the group.

   If the KDC has to send a rekeying message to a target group member,
   but this did not include the 'control_uri' parameter in the Joining
   Request and is not a registered observer for the ace-group/GROUPNAME
   resource, then that target group member would not be able to
   participate to the group rekeying.  Later on, after having repeatedly
   failed to successfully exchange secure messages in the group, that
   group member can retrieve the current group keying material from the
   KDC, by sending a GET request to ace-group/GROUPNAME or ace-
   group/GROUPNAME/nodes/NODENAME (see Section 4.3.2 and Section 4.8.1,
   respectively).

6.2.  One-to-Many Group Rekeying

   This section provides high-level recommendations on how the KDC can
   rekey a group by means of a more efficient and scalable group
   rekeying scheme, e.g., [RFC2093][RFC2094][RFC2627].  That is, each
   rekeying message might be, and likely is, intended to multiple target
   group members, and thus can be delivered to the whole group, although
   possible to decrypt only for the actual target group members.

   This yields an overall lower number of rekeying messages, thus
   potentially reducing the overall time required to rekey the group.
   On the other hand, it requires the KDC to provide and use additional
   administrative keying material to protect the rekeying messages, and
   to additionally sign them to ensure source authentication (see
   Section 6.2.1).  Typically, this pays off in large-scale groups,
   where the introduced performance overhead is less than what
   experienced by rekeying the group in a point-to-point fashion (see
   Section 6.1).





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   The exact set of rekeying messages to send, their content and format,
   the administrative keying material to use to protect them, as well as
   the set of target group members depend on the specific group rekeying
   scheme, and are typically affected by the reason that has triggered
   the group rekeying.  Details about the data content and format of
   rekeying messages have to be defined by separate documents profiling
   the use of the group rekeying scheme, in the context of the used
   application profile of this specification.

   When one of these group rekeying schemes is used, the KDC provides a
   number of related information to a Client joining the group in the
   Joining Response message (see Section 4.3.1).  In particular,
   'rekeying_scheme' identifies the rekeying scheme used in the group
   (if no default can be assumed); 'control_group_uri', if present,
   specifies a URI with a multicast address where the KDC will send the
   rekeying messages for that group; 'mgt_key_material' specifies a
   subset of the administrative keying material intended for that
   particular joining Client to have, as used to protect the rekeying
   messages sent to the group when intended also to that joining Client.

   Rekeying messages can be protected at the application layer, by using
   COSE and the administrative keying material as prescribed by the
   specific group rekeying scheme (see Section 6.2.1).  After that, the
   delivery of protected rekeying messages to the intended target group
   members can occur in different ways, such as the following ones.

   *  Over multicast - In this case, the KDC simply sends a rekeying
      message as a CoAP request addressed to the multicast URI specified
      in the 'control_group_uri' parameter of the Joining Response (see
      Section 4.3.1).

      If a particular rekeying message is intended to a single target
      group member, the KDC may alternatively protect the message using
      the security association with that group member, and deliver the
      message like when using the "Point-to-Point" group rekeying scheme
      (see Section 6.1).

   *  Through a pub-sub communication model - In this case, the KDC acts
      as publisher and publishes each rekeying message to a specific
      "rekeying topic", which is associated to the group and is hosted
      at a broker server.  Following their group joining, the group
      members subscribe to the rekeying topic at the broker, thus
      receiving the group rekeying messages as they are published by the
      KDC.

      In order to make such message delivery more efficient, the
      rekeying topic associated to a group can be further organized into
      subtopics.  For instance, the KDC can use a particular subtopic to



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      address a particular set of target group members during the
      rekeying process, as possibly aligned to a similar organization of
      the administrative keying material (e.g., a key hierarchy).

      The setup of rekeying topics at the broker as well as the
      discovery of the topics at the broker for group members are
      application specific.  A possible way is for the KDC to provide
      such information in the Joining Response message (see
      Section 4.3.1), by means of a new parameter analogous to
      'control_group_uri' and specifying the URI(s) of the rekeying
      topic(s) that a group member has to subscribe to at the broker.

   Regardless the specifically used delivery method, the group rekeying
   scheme can perform a possible roll-over of the administrative keying
   material through the same sent rekeying messages.  Actually, such a
   roll-over occurs every time a group rekeying is performed upon the
   leaving of group members, which have to be excluded from future
   communications in the group.

   From a high level point of view, each group member owns only a subset
   of the overall administrative keying material, obtained upon joining
   the group.  Then, when a group rekeying occurs:

   *  Each rekeying message is protected by using a (most convenient)
      key from the administrative keying material such that: i) the used
      key is not owned by any node leaving the group, i.e. the key is
      safe to use and does not have to be renewed; and ii) the used key
      is owned by all the target group members, that indeed have to be
      provided with new group keying material to protect communications
      in the group.

   *  Each rekeying message includes not only the new group keying
      material intended to all the rekeyed group members, but also any
      new administrative keys that: i) are pertaining to and supposed to
      be owned by the target group members; and ii) had to be updated
      since leaving group members own the previous version.

   Further details depend on the specific rekeying scheme used in the
   group.

6.2.1.  Protection of Rekeying Messages

   When using a group rekeying scheme relying on one-to-many rekeying
   messages, the actual data content of each rekeying message is
   prepared according to what the rekeying scheme prescribes.






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   Then, the KDC can protect the rekeying message as defined below.  The
   used encryption algorithm which SHOULD be the same one used to
   protect communications in the group.  The method defined below
   assumes that the following holds for the management keying material
   specified in the 'mgt_key_material' parameter of the Joining Response
   (see Section 4.3.1).

   *  The included symmetric encryption keys are accompanied by a
      corresponding and unique key identifier assigned by the KDC.

   *  A Base IV is also included, with the same size of the AEAD nonce
      considered by the encryption algorithm to use.

   First, the KDC computes a COSE_Encrypt0 object as follows.

   *  The encryption key to use is selected from the administrative
      keying material, as defined by the rekeying scheme used in the
      group.

   *  The plaintext is the actual data content of the rekeying message.

   *  The Additional Authenticated Data (AAD) is empty, unless otherwise
      specified by separate documents profiling the use of the group
      rekeying scheme.

   *  Since the KDC is the only sender of rekeying messages, the AEAD
      nonce can be computed as follows, where NONCE_SIZE is the size in
      bytes of the AEAD nonce.  Separate documents profiling the use of
      the group rekeying scheme may define alternative ways to compute
      the AEAD nonce.

      The KDC considers the following values.

      -  COUNT, as a 1-byte unsigned integer associated to the used
         encryption key.  Its value is set to 0 when starting to perform
         a new group rekeying instance, and is incremented after each
         use of the encryption key.

      -  NEW_NUM, as the version number of the new group keying material
         to distribute in this rekeying instance, left-padded with
         zeroes to exactly NONCE_SIZE - 1.

      Then, the KDC computes a Partial IV as the byte string
      concatenation of COUNT and NEW_NUM, in this order.  Finally, the
      AEAD nonce is computed as the XOR between the Base IV and the
      Partial IV.





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   *  The protected header of the COSE_Encrypt0 object MUST include the
      following parameters.

      -  'alg', specifying the used encryption algorithm.

      -  'kid', specifying the identifier of the encryption key from the
         administrative keying material used to protect this rekeying
         message.

   *  The unprotected header of the COSE_Encrypt0 object MUST include
      the 'Partial IV' parameter, with value the Partial IV computed
      above.

   In order to ensure source authentication, each rekeying message
   protected with the administrative keying material MUST be signed by
   the KDC.  To this end, the KDC computes a countersignature of the
   COSE_Encrypt0 object, as described in Sections 3.2 and 3.3 of
   [I-D.ietf-cose-countersign].  In particular, the following applies
   when computing the countersignature.

   *  The Countersign_structure contains the context text string
      "CounterSignature0".

   *  The private key of the KDC is used as signing key.

   *  The payload is the ciphertext of the COSE_Encrypt0 object.

   *  The Additional Authenticated Data (AAD) is empty, unless otherwise
      specified by separate documents profiling the use of a group
      rekeying scheme.

   *  The protected header of the signing object MUST include the
      parameter 'alg', specifying the used signature algorithm.

   If source authentication of messages exchanged in the group is also
   ensured by means of signatures, then rekeying messages MUST be signed
   using the same signature algorithm and related parameters.  Also, the
   KDC's public key used for signature verification MUST me provided in
   the Joining Response through the 'kdc_cred' parameter, together with
   the corresponding proof-of-possession (PoP) evidence in the
   'kdc_cred_verify' parameter.

   If source authentication of messages exchanged in the group is not
   ensured by means of signatures, then the KDC MUST provide its public
   key together with a corresponding PoP evidence as part of the
   management keying material specified in the 'mgt_key_material'
   parameter of the Joining Response (see Section 4.3.1).  It is
   RECOMMENDED to specify this information by using the same format and



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   encoding used for the parameters 'kdc_cred', 'kdc_nonce' and
   'kdc_cred_verify' in the Joining Response.  It is up to separate
   documents profiling the use of the group rekeying scheme to specify
   such details.

   After that, the KDC specifies the computed countersignature in the
   'COSE_Countersignature0' header parameter of the COSE_Encrypt0
   object.

   Finally, the KDC specifies the COSE_Encrypt0 object as payload of a
   CoAP request, which is sent to the target group members as per the
   used message delivery method.

7.  Extended Scope Format

   This section defines an extended format of binary encoded scope,
   which additionally specifies the semantics used to express the same
   access control information from the corresponding original scope.

   As also discussed in Section 3.2, this enables a Resource Server to
   unambiguously process a received access token, also in case the
   Resource Server runs multiple applications or application profiles
   that involve different scope semantics.

   The extended format is intended only for the 'scope' claim of access
   tokens, for the cases where the claim takes as value a CBOR byte
   string.  That is, the extended format does not apply to the 'scope'
   parameter included in ACE messages, i.e., the Authorization Request
   and Authorization Response exchanged between the Client and the
   Authorization Server (see Sections 5.8.1 and 5.8.2 of
   [I-D.ietf-ace-oauth-authz]), the AS Request Creation Hints message
   from the Resource Server (see Section 5.3 of
   [I-D.ietf-ace-oauth-authz]), and the Introspection Response from the
   Authorization Server (see Section 5.9.2 of
   [I-D.ietf-ace-oauth-authz]).

   The value of the 'scope' claim following the extended format is
   composed as follows.  Given the original scope using a semantics SEM
   and encoded as a CBOR byte string, the corresponding extended scope
   is encoded as a tagged CBOR byte string, wrapping a CBOR sequence
   [RFC8742] of two elements.  In particular:

   *  The first element of the sequence is a CBOR integer, and
      identifies the semantics SEM used for this scope.  The value of
      this element has to be taken from the "Value" column of the "ACE
      Scope Semantics" registry defined in Section 11.12 of this
      specification.




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      When defining a new semantics for a binary scope, it is up to the
      applications and application profiles to define and register the
      corresponding integer identifier (REQ28).

   *  The second element of the sequence is the original scope using the
      semantics SEM, encoded as a CBOR byte string.

   Finally, the CBOR byte string wrapping the CBOR sequence is tagged,
   and identified by the CBOR tag TBD_TAG "ACE Extended Scope Format",
   defined in Section 11.6 of this specification.

   The resulting tagged CBOR byte string is used as value of the 'scope'
   claim of the access token.

   The usage of the extended scope format is not limited to application
   profiles of this specification or to applications based on group
   communication.  Rather, it is generally applicable to any application
   and application profile where access control information in the
   access token is expressed as a binary encoded scope.

   Figure 33 and Figure 34 build on the examples in Section 3.2, and
   show the corresponding extended scopes.

   gname = tstr

   permissions = uint . bits roles

   roles = &(
      Requester: 1,
      Responder: 2,
      Monitor: 3,
      Verifier: 4
   )

   scope_entry = AIF_Generic<gname, permissions>

   scope = << [ + scope_entry ] >>

   semantics = int

   ; This defines an array, the elements
   ; of which are to be used in a CBOR Sequence:
   sequence = [semantics, scope]

   extended_scope = #6.TBD_TAG(<< sequence >>)

       Figure 33: Example CDLL definition of scope, using the default
                      Authorization Information Format



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   gname = tstr

   role = tstr

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

   scope = << [ + scope_entry ] >>

   semantics = int

   ; This defines an array, the elements
   ; of which are to be used in a CBOR Sequence:
   sequence = [semantics, scope]

   extended_scope = #6.TBD_TAG(<< sequence >>)

      Figure 34: CDLL definition of scope, using as example group name
                      encoded as tstr and role as tstr

8.  ACE Groupcomm Parameters

   This specification defines a number of parameters used during the
   second part of the message exchange, after the exchange of Token
   Transfer Request and Response.  The table below summarizes them, and
   specifies the CBOR key to use instead of the full descriptive name.

   Note that the media type application/ace-groupcomm+cbor MUST be used
   when these parameters are transported in the respective message
   fields.

   +-----------------------+------+-----------------+-----------------+
   | Name                  | CBOR | CBOR Type       | Reference       |
   |                       | Key  |                 |                 |
   +-----------------------+------+-----------------+-----------------+
   | error                 | TBD  | int             | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | error_description     | TBD  | tstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | gid                   | TBD  | array           | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | gname                 | TBD  | array of tstr   | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | guri                  | TBD  | array of tstr   | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | scope                 | TBD  | bstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | get_pub_keys          | TBD  | array / nil     | [this document] |
   +-----------------------+------+-----------------+-----------------+



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   | client_cred           | TBD  | bstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | cnonce                | TBD  | bstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | client_cred_verify    | TBD  | bstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | pub_keys_repos        | TBD  | tstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | control_uri           | TBD  | tstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | gkty                  | TBD  | int / tstr      | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | key                   | TBD  | See the "ACE    | [this document] |
   |                       |      | Groupcomm Key   |                 |
   |                       |      | Types" registry |                 |
   +-----------------------+------+-----------------+-----------------+
   | num                   | TBD  | int             | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | ace-groupcomm-profile | TBD  | int             | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | exp                   | TBD  | int             | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | pub_keys              | TBD  | array           | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | peer_roles            | TBD  | array           | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | peer_identifiers      | TBD  | array           | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | group_policies        | TBD  | map             | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | kdc_cred              | TBD  | bstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | kdc_nonce             | TBD  | bstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | kdc_cred_verify       | TBD  | bstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | rekeying_scheme       | TBD  | int             | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | mgt_key_material      | TBD  | bstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | control_group_uri     | TBD  | tstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | sign_info             | TBD  | array           | [this document] |
   +-----------------------+------+-----------------+-----------------+
   | kdcchallenge          | TBD  | bstr            | [this document] |
   +-----------------------+------+-----------------+-----------------+

                    Figure 35: ACE Groupcomm Parameters



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   The KDC is expected to support and understand all the parameters
   above.  Instead, a Client can support and understand only a subset of
   such parameters, depending on the roles it expects to take in the
   joined groups or on other conditions defined in application profiles
   of this specification.

   In the following, the parameters are categorized according to the
   support expected by Clients.  That is, a Client that supports a
   parameter is able to: i) use and specify it in a request message to
   the KDC; and ii) understand and process it if specified in a response
   message from the KDC.  It is REQUIRED of application profiles of this
   specification to sort their newly defined parameters according to the
   same categorization (REQ29).

   Note that the actual use of a parameter and its inclusion in a
   message depends on the specific exchange, the specific Client and
   group involved, as well as what is defined in the used application
   profile of this specification.

   A Client MUST support the following parameters.

   *  'scope', 'gkty', 'key', 'num', 'exp', 'gid', 'gname', 'guri',
      'pub_keys', 'peer_identifiers', 'ace_groupcomm_profile',
      'control_uri', 'rekeying_scheme'.

   A Client SHOULD support the following parameter.

   *  'get_pub_keys'.  That is, not supporting this parameter would
      yield the inconvenient and undesirable behavior where: i) the
      Client does not ask for the other group members' public keys upon
      joining the group (see Section 4.3.1.1); and ii) later on as a
      group member, the Client only retrieves the public keys of all
      group members (see Section 4.4.2.1).

   A Client MAY support the following optional parameters.  Application
   profiles of this specification MAY define that Clients must or should
   support these parameters instead (OPT15).

   *  'error', 'error_description'.

   The following conditional parameters are relevant only if specific
   conditions hold.  It is REQUIRED of application profiles of this
   specification to define whether Clients must, should or may support
   these parameters, and under which circumstances (REQ30).

   *  'client_cred', 'cnonce', 'client_cred_verify'.  These parameters
      are relevant for a Client that has a public key to use in a joined
      group.



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   *  'kdcchallenge'.  This parameter is relevant for a Client that has
      a public key to use in a joined group and that provides the access
      token to the KDC through a Token Transfer Request (see
      Section 3.3).

   *  'pub_keys'repo'.  This parameter is relevant for a Client that has
      a public key to use in a joined group and that makes it available
      from a key repository different than the KDC.

   *  'group_policies'.  This parameter is relevant for a Client that is
      interested in the specific policies used in a group, but it does
      not know them or cannot become aware of them before joining that
      group.

   *  'peer_roles'.  This parameter is relevant for a Client that has to
      know about the roles of other group members, especially when
      retrieving and handling their corresponding public keys.

   *  'kdc_nonce', 'kdc_cred', 'kdc_cred_verify'.  These parameters are
      relevant for a Client that joins a group for which, as per the
      used application profile of this specification, the KDC has an
      associated public key and this is required for the correct group
      operation.

   *  'mgt_key_material'.  This parameter is relevant for a Client that
      supports an advanced rekeying scheme possibly used in the group,
      such as based on one-to-many rekeying messages sent over IP
      multicast.

   *  'control_group_uri'.  This parameter is relevant for a Client that
      supports the hosting of local resources each associated to a group
      (hence acting as CoAP server) and the reception of one-to-many
      requests sent to those resources by the KDC (e.g., over IP
      multicast), targeting multiple members of the corresponding group.
      Examples of related management operations that the KDC can perform
      by this means are the eviction of group members and the execution
      of a group rekeying process through an advanced rekeying scheme,
      such as based on one-to-many rekeying messages.

9.  ACE Groupcomm Error Identifiers

   This specification defines a number of values that the KDC can
   include as error identifiers, in the 'error' field of an error
   response with Content-Format application/ace-groupcomm+cbor.







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     +-------+------------------------------------------------------+
     | Value |                   Description                        |
     +-------+------------------------------------------------------+
     |   0   | Operation permitted only to group members            |
     +-------+------------------------------------------------------+
     |   1   | Request inconsistent with the current roles          |
     +-------+------------------------------------------------------+
     |   2   | Public key incompatible with the group configuration |
     +-------+------------------------------------------------------+
     |   3   | Invalid proof-of-possession evidence                 |
     +-------+------------------------------------------------------+
     |   4   | No available node identifiers                        |
     +-------+------------------------------------------------------+
     |   5   | Group membership terminated                          |
     +-------+------------------------------------------------------+
     |   6   | Group deleted                                        |
     +-------+------------------------------------------------------+

                 Figure 36: ACE Groupcomm Error Identifiers

   A Client supporting the 'error' parameter (see Section 4.1.2 and
   Section 8) and able to understand the specified error may use that
   information to determine what actions to take next.  If it is
   included in the error response and supported by the Client, the
   'error_description' parameter may provide additional context.

   In particular, the following guidelines apply, and application
   profiles of this specification can define more detailed actions for
   the Client to take when learning that a specific error has occurred.

   *  In case of error 0, the Client should stop sending the request in
      question to the KDC.  Rather, the Client should first join the
      targeted group.  If it has not happened already, this first
      requires the Client to obtain an appropriate access token
      authorizing access to the group and provide it to the KDC.

   *  In case of error 1, the Client as a group member should re-join
      the group with all the roles needed to perform the operation in
      question.  This might require the Client to first obtain a new
      access token and provide it to the KDC, if the current access
      token does not authorize to take those roles in the group.  For
      operations admitted to a Client which is not a group member (e.g.,
      an external signature verifier), the Client should first obtain a
      new access token authorizing to also have the missing roles.







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   *  In case of error 2, the Client has to obtain or self-generate a
      different asymmetric key pair, as aligned to the publick key
      algorithms, parameters and encoding used in the targeted group.
      After that, the Client should provide its new consistent public
      key to the KDC.

   *  In case of error 3, the Client should ensure to be computing its
      proof-of-possession evidence by correctly using the parameters and
      procedures defined in the used application profile of this
      specification.  In an unattended setup, it might be not possible
      for a Client to autonomously diagnose the error and take an
      effective next action to address it.

   *  In case of error 4, the Client should wait for a certain (pre-
      configured) amount of time, before trying re-sending its request
      to the KDC.

   *  In case of error 5, the Client may try joining the group again.
      This might require the Client to first obtain a new access token
      and provide it to the KDC, e.g., if the current access token has
      expired.

   *  In case of error 6, the Client should clean up its state regarding
      the group, just like if it has left the group with no intention to
      re-join it.

10.  Security Considerations

   Security considerations are inherited from the ACE framework
   [I-D.ietf-ace-oauth-authz], and from the specific transport profile
   of ACE used between the Clients and the KDC, e.g.,
   [I-D.ietf-ace-dtls-authorize] and [I-D.ietf-ace-oscore-profile].

   Furthermore, the following security considerations apply.

10.1.  Secure Communication in the Group

   When a group member receives a message from a certain sender for the
   first time since joining the group, it needs to have a mechanism in
   place to avoid replayed messages, e.g., Appendix B.2 of [RFC8613] or
   Appendix E of [I-D.ietf-core-oscore-groupcomm].  Such a mechanism
   aids the recipient group member also in case it has rebooted and lost
   the security state used to protect previous group communications with
   that sender.

   By its nature, the KDC is invested with a large amount of trust,
   since it acts as generator and provider of the symmetric keying
   material used to protect communications in each of its groups.  While



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   details depend on the specific communication and security protocols
   used in the group, the KDC is in the position to decrypt messages
   exchanged in the group as if it was also a group member, as long as
   those are protected through commonly shared group keying material.

   A compromised KDC would thus put the attacker in the same position,
   which also means that:

   *  The attacker can generate and control new group keying material,
      hence possibly rekeying the group and evicting certain group
      members as part of a broader attack.

   *  The attacker can actively participate to communications in a group
      even without been authorized to join it, and can allow further
      unauthorized entities to do so.

   *  The attacker can build erroneous associations between node
      identifiers and group members' public keys.

   On the other hand, as long as the security protocol used in the group
   ensures source authentication of messages (e.g., by means of
   signatures), the KDC is not able to impersonate group members since
   it does now own their private keys.

   Further security considerations are specific of the communication and
   security protocols used in the group, and thus have to be provided by
   those protocols and complemented by the application profiles of this
   specification using them.

10.2.  Update of Group Keying Material

   Due to different reasons, the KDC can generate new group keying
   material and provide it to the group members (rekeying) through the
   rekeying scheme used in the group, as discussed in Section 6.

   In particular, the KDC must renew the group keying material latest
   upon its expiration.  Before then, the KDC may also renew the group
   keying material on a regular or periodical fashion.

   The KDC should renew the group keying material upon a group
   membership change.  Since the minimum number of group members is one,
   the KDC should provide also a Client joining an empty group with new
   keying material never used before in that group.  Similarly, the KDC
   should provide new group keying material also to a Client that
   remains the only member in the group after the leaving of other group
   members.





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   Note that the considerations in Section 10.1 about dealing with
   replayed messages still hold, even in case the KDC rekeys the group
   upon every single joining of a new group member.  However, if the KDC
   has renewed the group keying material upon a group member's joining,
   and the time interval between the end of the rekeying process and
   that member's joining is sufficiently small, then that group member
   is also on the safe side, since it would not accept replayed messages
   protected with the old group keying material previous to its joining.

   The KDC may enforce a rekeying policy that takes into account the
   overall time required to rekey the group, as well as the expected
   rate of changes in the group membership.  That is, the KDC may not
   rekey the group at each and every group membership change, for
   instance if members' joining and leaving occur frequently and
   performing a group rekeying takes too long.  Instead, the KDC might
   rekey the group after a minimum number of group members have joined
   or left within a given time interval, or after a maximum amount of
   time since the last group rekeying was completed, or yet during
   predictable network inactivity periods.

   However, this would result in the KDC not constantly preserving
   backward and forward security in the group.  That is:

   *  Newly joining group members would be able to access the keying
      material used before their joining, and thus they could access
      past group communications if they have recorded old exchanged
      messages.  This might still be acceptable for some applications
      and in situations where the new group members are freshly deployed
      through strictly controlled procedures.

   *  The leaving group members would remain able to access upcoming
      group communications, as protected with the current keying
      material that has not been updated.  This is typically
      undesirable, especially if the leaving group member is compromised
      or suspected to be, and it might have an impact or compromise the
      security properties of the protocols used in the group to protect
      messages exchanged among the group member.

   The KDC should renew the group keying material in case it has
   rebooted, even in case it stores the whole group keying material in
   persistent storage.  This assumes that the secure associations with
   the current group members as well as any administrative keying
   material required to rekey the group are also stored in persistent
   storage.

   However, if the KDC relies on Observe notifications to distribute the
   new group keying material, the KDC would have lost all the current
   ongoing Observations with the group members after rebooting, and the



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   group members would continue using the old group keying material.
   Therefore, the KDC will rather rely on each group member asking for
   the new group keying material (see Section 4.3.2.1 and
   Section 4.8.1.1), or rather perform a group rekeying by actively
   sending rekeying messages to group members as discussed in Section 6.

   The KDC needs to have a mechanism in place to detect DoS attacks from
   nodes repeatedly performing actions that might trigger a group
   rekeying.  Such actions can include leaving and/or re-joining the
   group at high rates, or often asking the KDC for new indidivual
   keying material.  Ultimately, the KDC can resort to removing these
   nodes from the group and (temperorarily) preventing them from joining
   the group again.

   The KDC also needs to have a congestion control mechanism in place,
   in order to avoid network congestion upon distributing new group
   keying material.  For example, CoAP and Observe give guidance on such
   mechanisms, see Section 4.7 of [RFC7252] and Section 4.5.1 of
   [RFC7641].

   A node that has left the group should not expect any of its outgoing
   messages to be successfully processed, if received by other nodes
   after its leaving, due to a possible group rekeying occurred before
   the message reception.

10.2.1.  Misalignment of Group Keying Material

   A group member can receive a message shortly after the group has been
   rekeyed, and new keying material has been distributed by the KDC (see
   Section 6).  In the following two cases, this may result in
   misaligned keying material between the group members.

   In the first case, the sender protects a message using the old group
   keying material.  However, the recipient receives the message after
   having received the new group keying material, hence not being able
   to correctly process it.  A possible way to ameliorate this issue is
   to preserve the old, recent group keying material for a maximum
   amount of time defined by the application, during which it is used
   solely for processing incoming messages.  By doing so, the recipient
   can still temporarily process received messages also by using the
   old, retained group keying material.  Note that a former
   (compromised) group member can take advantage of this by sending
   messages protected with the old, retained group keying material.
   Therefore, a conservative application policy should not admit the
   storage of old group keying material.  Eventually, the sender will
   have obtained the new group keying material too, and can possibly re-
   send the message protected with such keying material.




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   In the second case, the sender protects a message using the new group
   keying material, but the recipient receives that message before
   having received the new group keying material.  Therefore, the
   recipient would not be able to correctly process the message and
   hence discards it.  If the recipient receives the new group keying
   material shortly after that and the application at the sender
   endpoint performs retransmissions, the former will still be able to
   receive and correctly process the message.  In any case, the
   recipient should actively ask the KDC for the latest group keying
   material according to an application-defined policy, for instance
   after a given number of unsuccessfully decrypted incoming messages.

10.3.  Block-Wise Considerations

   If the Block-Wise CoAP options [RFC7959] are used, and the keying
   material is updated in the middle of a Block-Wise transfer, the
   sender of the blocks just changes the group keying material to the
   updated one and continues the transfer.  As long as both sides get
   the new group keying material, updating group the keying material in
   the middle of a transfer will not cause any issue.  Otherwise, the
   sender will have to transmit the message again, when receiving an
   error message from the recipient.

   Compared to a scenario where the transfer does not use Block-Wise,
   depending on how fast the group keying material is changed, the group
   members might consume a larger amount of the network bandwidth by
   repeatedly resending the same blocks, which might be problematic.

11.  IANA Considerations

   This document has the following actions for IANA.

11.1.  Media Type Registrations

   This specification registers the 'application/ace-groupcomm+cbor'
   media type for messages of the protocols defined in this document
   following the ACE exchange and carrying parameters encoded in CBOR.
   This registration follows the procedures specified in [RFC6838].

   Type name: application

   Subtype name: ace-groupcomm+cbor

   Required parameters: N/A

   Optional parameters: N/A





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   Encoding considerations: Must be encoded as CBOR map containing the
   protocol parameters defined in [this document].

   Security considerations: See Section 10 of this document.

   Interoperability considerations: n/a

   Published specification: [this document]

   Applications that use this media type: The type is used by
   Authorization Servers, Clients and Resource Servers that support the
   ACE groupcomm framework as specified in [this document].

   Fragment identifier considerations: N/A

   Additional information: N/A

   Person & email address to contact for further information:
   iesg@ietf.org (mailto:iesg@ietf.org)

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Francesca Palombini francesca.palombini@ericsson.com
   (mailto:francesca.palombini@ericsson.com)

   Change controller: IESG

11.2.  CoAP Content-Formats

   IANA is asked to register the following entry to the "CoAP Content-
   Formats" registry within the "CoRE Parameters" registry group.

   Media Type: application/ace-groupcomm+cbor

   Encoding: -

   ID: TBD

   Reference: [this document]

11.3.  OAuth Parameters

   IANA is asked to register the following entries in the "OAuth
   Parameters" registry following the procedure specified in
   Section 11.2 of [RFC6749].




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   *  Parameter name: sign_info

   *  Parameter usage location: client-rs request, rs-client response

   *  Change Controller: IESG

   *  Specification Document(s): [[This specification]]


   *  Parameter name: kdcchallenge

   *  Parameter usage location: rs-client response

   *  Change Controller: IESG

   *  Specification Document(s): [[This specification]]

11.4.  OAuth Parameters CBOR Mappings

   IANA is asked to register the following entries in the "OAuth
   Parameters CBOR Mappings" registry following the procedure specified
   in Section 8.10 of [I-D.ietf-ace-oauth-authz].

   *  Name: sign_info

   *  CBOR Key: TBD (range -256 to 255)

   *  Value Type: Simple value null / array

   *  Reference: [[This specification]]


   *  Name: kdcchallenge

   *  CBOR Key: TBD (range -256 to 255)

   *  Value Type: Byte string

   *  Reference: [[This specification]]

11.5.  Interface Description (if=) Link Target Attribute Values

   IANA is asked to register the following entry in the "Interface
   Description (if=) Link Target Attribute Values" registry within the
   "CoRE Parameters" registry group.

   *  Attribute Value: ace.group




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   *  Description: The 'ace group' interface is used to provision keying
      material and related information and policies to members of a
      group using the Ace framework.

   *  Reference: [This Document]

11.6.  CBOR Tags

   IANA is asked to register the following entry in the "CBOR Tags"
   registry.

   *  Tag : TBD_TAG

   *  Data Item: byte string

   *  Semantics: Extended ACE scope format, including the identifier of
      the used scope semantics.

   *  Reference: [This Document]

11.7.  ACE Groupcomm Parameters

   This specification establishes the "ACE Groupcomm Parameters" IANA
   registry.  The registry has been created to use the "Expert Review"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 11.15.

   The columns of this registry are:

   *  Name: This is a descriptive name that enables easier reference to
      the item.  The name MUST be unique.  It is not used in the
      encoding.

   *  CBOR Key: This is the value used as CBOR key of the item.  These
      values MUST be unique.  The value can be a positive integer, a
      negative integer, or a string.

   *  CBOR Type: This contains the CBOR type of the item, or a pointer
      to the registry that defines its type, when that depends on
      another item.

   *  Reference: This contains a pointer to the public specification for
      the item.

   This registry has been initially populated by the values in
   Section 8.  The Reference column for all of these entries refers to
   sections of this document.




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11.8.  ACE Groupcomm Key Types

   This specification establishes the "ACE Groupcomm Key Types" IANA
   registry.  The registry has been created to use the "Expert Review"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 11.15.

   The columns of this registry are:

   *  Name: This is a descriptive name that enables easier reference to
      the item.  The name MUST be unique.  It is not used in the
      encoding.

   *  Key Type Value: This is the value used to identify the keying
      material.  These values MUST be unique.  The value can be a
      positive integer, a negative integer, or a text string.

   *  Profile: This field may contain one or more descriptive strings of
      application profiles to be used with this item.  The values should
      be taken from the Name column of the "ACE Groupcomm Profiles"
      registry.

   *  Description: This field contains a brief description of the keying
      material.

   *  References: This contains a pointer to the public specification
      for the format of the keying material, if one exists.

   This registry has been initially populated by the values in
   Figure 10.  The specification column for all of these entries will be
   this document.

11.9.  ACE Groupcomm Profiles

   This specification establishes the "ACE Groupcomm Profiles" IANA
   registry.  The registry has been created to use the "Expert Review"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 11.15.  It should be noted that, in addition to
   the expert review, some portions of the registry require a
   specification, potentially a Standards Track RFC, to be supplied as
   well.

   The columns of this registry are:

   *  Name: The name of the application profile, to be used as value of
      the profile attribute.





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   *  Description: Text giving an overview of the application profile
      and the context it is developed for.

   *  CBOR Value: CBOR abbreviation for the name of this application
      profile.  Different ranges of values use different registration
      policies [RFC8126].  Integer values from -256 to 255 are
      designated as Standards Action.  Integer values from -65536 to
      -257 and from 256 to 65535 are designated as Specification
      Required.  Integer values greater than 65535 are designated as
      Expert Review.  Integer values less than -65536 are marked as
      Private Use.

   *  Reference: This contains a pointer to the public specification of
      the abbreviation for this application profile, if one exists.

11.10.  ACE Groupcomm Policies

   This specification establishes the "ACE Groupcomm Policies" IANA
   registry.  The registry has been created to use the "Expert Review"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 11.15.  It should be noted that, in addition to
   the expert review, some portions of the registry require a
   specification, potentially a Standards Track RFC, to be supplied as
   well.

   The columns of this registry are:

   *  Name: The name of the group communication policy.

   *  CBOR label: The value to be used to identify this group
      communication policy.  Key map labels MUST be unique.  The label
      can be a positive integer, a negative integer or a string.
      Integer values between 0 and 255 and strings of length 1 are
      designated as Standards Track Document required.  Integer values
      from 256 to 65535 and strings of length 2 are designated as
      Specification Required.  Integer values greater than 65535 and
      strings of length greater than 2 are designated as expert review.
      Integer values less than -65536 are marked as private use.

   *  CBOR type: the CBOR type used to encode the value of this group
      communication policy.

   *  Description: This field contains a brief description for this
      group communication policy.

   *  Reference: This field contains a pointer to the public
      specification providing the format of the group communication
      policy, if one exists.



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   This registry will be initially populated by the values in Figure 11.

11.11.  Sequence Number Synchronization Methods

   This specification establishes the "Sequence Number Synchronization
   Methods" IANA registry.  The registry has been created to use the
   "Expert Review" registration procedure [RFC8126].  Expert review
   guidelines are provided in Section 11.15.  It should be noted that,
   in addition to the expert review, some portions of the registry
   require a specification, potentially a Standards Track RFC, to be
   supplied as well.

   The columns of this registry are:

   *  Name: The name of the sequence number synchronization method.

   *  Value: The value to be used to identify this sequence number
      synchronization method.

   *  Description: This field contains a brief description for this
      sequence number synchronization method.

   *  Reference: This field contains a pointer to the public
      specification describing the sequence number synchronization
      method.

11.12.  ACE Scope Semantics

   This specification establishes the "ACE Scope Semantics" IANA
   registry.  The registry has been created to use the "Expert Review"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 11.15.  It should be noted that, in addition to
   the expert review, some portions of the registry require a
   specification, potentially a Standards Track RFC, to be supplied as
   well.

   The columns of this registry are:

   *  Value: The value to be used to identify this scope semantics.  The
      value MUST be unique.  The value can be a positive integer or a
      negative integer.  Integer values between 0 and 255 are designated
      as Standards Track Document required.  Integer values from 256 to
      65535 are designated as Specification Required.  Integer values
      greater than 65535 are designated as expert review.  Integer
      values less than -65536 are marked as private use.

   *  Description: This field contains a brief description of the scope
      semantics.



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   *  Reference: This field contains a pointer to the public
      specification defining the scope semantics, if one exists.

11.13.  ACE Groupcomm Errors

   This specification establishes the "ACE Groupcomm Errors" IANA
   registry.  The registry has been created to use the "Expert Review"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 11.15.  It should be noted that, in addition to
   the expert review, some portions of the registry require a
   specification, potentially a Standards Track RFC, to be supplied as
   well.

   The columns of this registry are:

   *  Value: The value to be used to identify the error.  The value MUST
      be unique.  The value can be a positive integer or a negative
      integer.  Integer values between 0 and 255 are designated as
      Standards Track Document required.  Integer values from 256 to
      65535 are designated as Specification Required.  Integer values
      greater than 65535 are designated as expert review.  Integer
      values less than -65536 are marked as private use.

   *  Description: This field contains a brief description of the error.

   *  Reference: This field contains a pointer to the public
      specification defining the error, if one exists.

   This registry has been initially populated by the values in
   Section 9.  The Reference column for all of these entries refers to
   this document.

11.14.  ACE Groupcomm Rekeying Schemes

   This specification establishes the "ACE Groupcomm Rekeying Schemes"
   IANA registry.  The registry has been created to use the "Expert
   Review" registration procedure [RFC8126].  Expert review guidelines
   are provided in Section 11.15.  It should be noted that, in addition
   to the expert review, some portions of the registry require a
   specification, potentially a Standards Track RFC, to be supplied as
   well.

   The columns of this registry are:

   *  Value: The value to be used to identify the group rekeying scheme.
      The value MUST be unique.  The value can be a positive integer or
      a negative integer.  Integer values between 0 and 255 are
      designated as Standards Track Document required.  Integer values



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      from 256 to 65535 are designated as Specification Required.
      Integer values greater than 65535 are designated as expert review.
      Integer values less than -65536 are marked as private use.

   *  Name: The name of the group rekeying scheme.

   *  Description: This field contains a brief description of the group
      rekeying scheme.

   *  Reference: This field contains a pointer to the public
      specification defining the group rekeying scheme, if one exists.

   This registry has been initially populated by the value in Figure 12.

11.15.  Expert Review Instructions

   The IANA Registries established in this document are defined as
   expert review.  This section gives some general guidelines for what
   the experts should be looking for, but they are being designated as
   experts for a reason so they should be given substantial latitude.

   Expert reviewers should take into consideration the following points:

   *  Point squatting should be discouraged.  Reviewers are encouraged
      to get sufficient information for registration requests to ensure
      that the usage is not going to duplicate one that is already
      registered and that the point is likely to be used in deployments.
      The zones tagged as private use are intended for testing purposes
      and closed environments, code points in other ranges should not be
      assigned for testing.

   *  Specifications are required for the standards track range of point
      assignment.  Specifications should exist for specification
      required ranges, but early assignment before a specification is
      available is considered to be permissible.  Specifications are
      needed for the first-come, first-serve range if they are expected
      to be used outside of closed environments in an interoperable way.
      When specifications are not provided, the description provided
      needs to have sufficient information to identify what the point is
      being used for.











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   *  Experts should take into account the expected usage of fields when
      approving point assignment.  The fact that there is a range for
      standards track documents does not mean that a standards track
      document cannot have points assigned outside of that range.  The
      length of the encoded value should be weighed against how many
      code points of that length are left, the size of device it will be
      used on, and the number of code points left that encode to that
      size.

12.  References

12.1.  Normative References

   [COSE.Algorithms]
              IANA, "COSE Algorithms",
              <https://www.iana.org/assignments/cose/
              cose.xhtml#algorithms>.

   [COSE.Header.Parameters]
              IANA, "COSE Header Parameters",
              <https://www.iana.org/assignments/cose/cose.xhtml#header-
              parameters>.

   [I-D.ietf-ace-aif]
              Bormann, C., "An Authorization Information Format (AIF)
              for ACE", Work in Progress, Internet-Draft, draft-ietf-
              ace-aif-03, 24 June 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-aif-
              03.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,
              draft-ietf-ace-oauth-authz-45, 29 August 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
              authz-45.txt>.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
              and J. Park, "Group OSCORE - Secure Group Communication
              for CoAP", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-groupcomm-13, 25 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-core-oscore-
              groupcomm-13.txt>.





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   [I-D.ietf-cose-countersign]
              Schaad, J. and R. Housley, "CBOR Object Signing and
              Encryption (COSE): Countersignatures", Work in Progress,
              Internet-Draft, draft-ietf-cose-countersign-05, 23 June
              2021, <https://www.ietf.org/archive/id/draft-ietf-cose-
              countersign-05.txt>.

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-algs-12.txt>.

   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-struct-15.txt>.

   [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>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7967]  Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
              Bose, "Constrained Application Protocol (CoAP) Option for
              No Server Response", RFC 7967, DOI 10.17487/RFC7967,
              August 2016, <https://www.rfc-editor.org/info/rfc7967>.







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   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8742]  Bormann, C., "Concise Binary Object Representation (CBOR)
              Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
              <https://www.rfc-editor.org/info/rfc8742>.

   [RFC8747]  Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
              2020, <https://www.rfc-editor.org/info/rfc8747>.

   [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>.

12.2.  Informative References

   [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-18, 4 June
              2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
              dtls-authorize-18.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-13, 23 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-mqtt-tls-
              profile-13.txt>.



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   [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-19, 6 May
              2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
              oscore-profile-19.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-
              05, 25 October 2021, <https://www.ietf.org/archive/id/
              draft-ietf-core-groupcomm-bis-05.txt>.

   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsuess, C., and P. V. D. Stok, "Discovery of
              OSCORE Groups with the CoRE Resource Directory", Work in
              Progress, Internet-Draft, draft-tiloca-core-oscore-
              discovery-10, 25 October 2021,
              <https://www.ietf.org/archive/id/draft-tiloca-core-oscore-
              discovery-10.txt>.

   [RFC2093]  Harney, H. and C. Muckenhirn, "Group Key Management
              Protocol (GKMP) Specification", RFC 2093,
              DOI 10.17487/RFC2093, July 1997,
              <https://www.rfc-editor.org/info/rfc2093>.

   [RFC2094]  Harney, H. and C. Muckenhirn, "Group Key Management
              Protocol (GKMP) Architecture", RFC 2094,
              DOI 10.17487/RFC2094, July 1997,
              <https://www.rfc-editor.org/info/rfc2094>.

   [RFC2627]  Wallner, D., Harder, E., and R. Agee, "Key Management for
              Multicast: Issues and Architectures", RFC 2627,
              DOI 10.17487/RFC2627, June 1999,
              <https://www.rfc-editor.org/info/rfc2627>.






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   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [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>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

Appendix A.  Requirements on Application Profiles

   This section lists the requirements on application profiles of this
   specification, for the convenience of application profile designers.

A.1.  Mandatory-to-Address Requirements

   *  REQ1: Specify the format and encoding of 'scope'.  This includes
      defining the set of possible roles and their identifiers, as well
      as the corresponding encoding to use in the scope entries
      according to the used scope format (see Section 3.1).

   *  REQ2: If the AIF format of 'scope' is used, register its specific
      instance of "Toid" and "Tperm", as well as the corresponding Media
      Type and Content-Format, as per the guidelines in
      [I-D.ietf-ace-aif].

   *  REQ3: If used, specify the acceptable values for 'sign_alg' (see
      Section 3.3).




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   *  REQ4: If used, specify the acceptable values for 'sign_parameters'
      (see Section 3.3).

   *  REQ5: If used, specify the acceptable values for
      'sign_key_parameters' (see Section 3.3).

   *  REQ6: Specify the acceptable formats for encoding public keys and,
      if used, the acceptable values for 'pub_key_enc' (see
      Section 3.3).

   *  REQ7: If the value of the GROUPNAME URI path and the group name in
      the access token scope (gname in Section 3.2) are not required to
      coincide, specify the mechanism to map the GROUPNAME value in the
      URI to the group name (see Section 4.1).

   *  REQ8: Define whether the KDC has a public key and if this has to
      be provided through the 'kdc_cred' parameter, see Section 4.3.1.

   *  REQ9: Specify if any part of the KDC interface as defined in this
      document is not supported by the KDC (see Section 4.1).

   *  REQ10: Register a Resource Type for the root url-path, which is
      used to discover the correct url to access at the KDC (see
      Section 4.1).

   *  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 (see
      Section 3.1); and the roles that the Client has as current group
      member.

   *  REQ12: Categorize possible newly defined operations for Clients
      into primary operations expected to be minimally supported and
      secondary operations, and provide accompanying considerations (see
      Section 4.1.1).

   *  REQ13: Specify the encoding of group identifier (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 4.3.1).

   *  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).



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   *  REQ16 Define the initial value of the 'num' parameter (see
      Section 4.3.1).

   *  REQ17: Specify the format of the 'key' parameter (see
      Section 4.3.1).

   *  REQ18: Specify the acceptable values of the 'gkty' parameter (see
      Section 4.3.1).

   *  REQ19: Specify and register the application profile identifier
      (see Section 4.3.1).

   *  REQ20: If used, specify the format and content of 'group_policies'
      and its entries.  Specify the policies default values (see
      Section 4.3.1).

   *  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 4.3.1).

   *  REQ22: Specify the communication protocol the members of the group
      must use (e.g., multicast CoAP).

   *  REQ23: Specify the security protocol the group members must use to
      protect their communication (e.g., group OSCORE).  This must
      provide encryption, integrity and replay protection.

   *  REQ24: Specify how the communication is secured between Client and
      KDC.  Optionally, specify transport profile of ACE
      [I-D.ietf-ace-oauth-authz] to use between Client and KDC (see
      Section 4.3.1.1.

   *  REQ25: Specify the format of the identifiers of group members (see
      Section 4.3.1).

   *  REQ26: Specify policies at the KDC to handle ids that are not
      included in 'get_pub_keys' (see Section 4.4.1).

   *  REQ27: Specify the format of newly-generated individual keying
      material for group members, or of the information to derive it,
      and corresponding CBOR label (see Section 4.8.1).

   *  REQ28: Specify and register the identifier of newly defined
      semantics for binary scopes (see Section 7).

   *  REQ29: Categorize newly defined parameters according to the same
      criteria of Section 8.




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   *  REQ30: Define whether Clients must, should or may support the
      conditional parameters defined in Section 8, and under which
      circumstances.

A.2.  Optional-to-Address Requirements

   *  OPT1: Optionally, if the textual format of 'scope' is used,
      specify CBOR values to use for abbreviating the role identifiers
      in the group (see Section 3.1).

   *  OPT2: Optionally, specify the additional parameters used in the
      exchange of Token Transfer Request and Response (see Section 3.3).

   *  OPT3: Optionally, specify the negotiation of parameter values for
      signature algorithm and signature keys, if 'sign_info' is not used
      (see Section 3.3).

   *  OPT4: Optionally, specify possible or required payload formats for
      specific error cases.

   *  OPT5: Optionally, specify additional identifiers of error types,
      as values of the 'error' field in an error response from the KDC.

   *  OPT6: Optionally, specify the encoding of 'pub_keys_repos' if the
      default is not used (see Section 4.3.1).

   *  OPT7: Optionally, specify the functionalities implemented at the
      'control_uri' resource hosted at the Client, including message
      exchange encoding and other details (see Section 4.3.1).

   *  OPT8: Optionally, specify the behavior of the handler in case of
      failure to retrieve a public key for the specific node (see
      Section 4.3.1).

   *  OPT9: Optionally, define a default group rekeying scheme, to refer
      to in case the 'rekeying_scheme' parameter is not included in the
      Joining Response (see Section 4.3.1).

   *  OPT10: Optionally, specify the functionalities implemented at the
      'control_group_uri' resource hosted at the Client, including
      message exchange encoding and other details (see Section 4.3.1).

   *  OPT11: Optionally, specify policies that instruct Clients to
      retain messages and for how long, if they are unsuccessfully
      decrypted (see Section 4.8.1.1).  This makes it possible to
      decrypt such messages after getting updated keying material.





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   *  OPT12: Optionally, specify for the KDC to perform group rekeying
      (together or instead of renewing individual keying material) when
      receiving a Key Renewal Request (see Section 4.8.2.1).

   *  OPT13: Optionally, specify how the identifier of a group members's
      public key is included in requests sent to other group members
      (see Section 4.9.1.1).

   *  OPT14: Optionally, specify additional information to include in
      rekeying messages for the "Point-to-Point" group rekeying scheme
      (see Section 6).

   *  OPT15: Optionally, specify if Clients must or should support any
      of the parameters defined as optional in this specification (see
      Section 8).

Appendix B.  Extensibility for Future COSE Algorithms

   As defined in Section 8.1 of [I-D.ietf-cose-rfc8152bis-algs], future
   algorithms can be registered in the "COSE Algorithms" registry
   [COSE.Algorithms] as specifying none or multiple COSE capabilities.

   To enable the seamless use of such future registered algorithms, this
   section defines a general, agile format for each 'sign_info_entry' of
   the 'sign_info' parameter in the Token Transfer Response, see
   Section 3.3.1.

   If any of the currently registered COSE algorithms is considered,
   using this general format yields the same structure of
   'sign_info_entry' defined in this document, thus ensuring retro-
   compatibility.

B.1.  Format of 'sign_info_entry'

   The format of each 'sign_info_entry' (see Section 3.3.1) is
   generalized as follows.  Given N the number of elements of the
   'sign_parameters' array, i.e., the number of COSE capabilities of the
   signature algorithm, then:

   *  'sign_key_parameters' is replaced by N elements 'sign_capab_i',
      each of which is a CBOR array.

   *  The i-th array following 'sign_parameters', i.e., 'sign_capab_i'
      (i = 0, ..., N-1), is the array of COSE capabilities for the
      algorithm capability specified in 'sign_parameters'[i].






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   sign_info_entry =
   [
     id : gname / [ + gname ],
     sign_alg : int / tstr,
     sign_parameters : [ alg_capab_1 : any,
                         alg_capab_2 : any,
                         ...,
                         alg_capab_N : any],
     sign_capab_1 : [ any ],
     sign_capab_2 : [ any ],
     ...,
     sign_capab_N : [ any ],
     pub_key_enc = int / nil
   ]

   gname = tstr

              Figure 37: 'sign_info_entry' with general format

Appendix C.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

C.1.  Version -13 to -14

   *  Clarified scope and goal of the document in abstract and
      introduction.

   *  Overall clarifications on semantics of operations and parameters.

   *  Major restructuring in the presentation of the KDC interface.

   *  Revised error handling, also removing redundant text.

   *  Imported parameters and KDC resource about the KDC's public key
      from draft-ietf-ace-key-groupcomm-oscore.

   *  New parameters 'group_rekeying_scheme' and 'control_group_uri'.

   *  Provided example of administrative keying material transported in
      'mgt_key_material'.

   *  Reasoned categorization of parameters, as expected support by ACE
      Clients.

   *  Reasoned categorization of KDC functionalities, as minimally/
      optional to support for ACE Clients.




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   *  Guidelines on enhanced error responses using 'error' and
      'error_description'.

   *  New section on group rekeying, discussing at a high-level a basic
      one-to-one approach and possible one-to-many approaches.

   *  Revised and expanded security considerations, also about the KDC.

   *  Updated list of requirements for application profiles.

   *  Several further clarifications and editorial improvements.

C.2.  Version -05 to -13

   *  Incremental revision of the KDC interface.

   *  Removed redundancy in parameters about signature algorithm and
      signature keys.

   *  Node identifiers always indicated with 'peer_identifiers'.

   *  Format of public keys changed from raw COSE Keys to be
      certificates, CWTs or CWT Claims Set (CCS).  Adapted parameter
      'pub_key_enc'.

   *  Parameters and functionalities imported from draft-ietf-key-
      groupcomm-oscore where early defined.

   *  Possible provisioning of the KDC's Diffie-Hellman public key in
      response to the Token transferring to /authz-info.

   *  Generalized proof-of-possession evidence, to be not necessarily a
      signature.

   *  Public keys of group members may be retrieved filtering by role
      and/or node identifier.

   *  Enhanced error handling with error code and error description.

   *  Extended "typed" format for the 'scope' claim, optional to use.

   *  Editorial improvements.

C.3.  Version -04 to -05

   *  Updated uppercase/lowercase URI segments for KDC resources.

   *  Supporting single Access Token for multiple groups/topics.



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   *  Added 'control_uri' parameter in the Joining Request.

   *  Added 'peer_roles' parameter to support legal requesters/
      responders.

   *  Clarification on stopping using owned keying material.

   *  Clarification on different reasons for processing failures,
      related policies, and requirement OPT11.

   *  Added a KDC sub-resource for group members to upload a new public
      key.

   *  Possible group rekeying following an individual Key Renewal
      Request.

   *  Clarified meaning of requirement REQ3; added requirement OPT12.

   *  Editorial improvements.

C.4.  Version -03 to -04

   *  Revised RESTful interface, as to methods and parameters.

   *  Extended processing of joining request, as to check/retrieval of
      public keys.

   *  Revised and extended profile requirements.

   *  Clarified specific usage of parameters related to signature
      algorithms/keys.

   *  Included general content previously in draft-ietf-ace-key-
      groupcomm-oscore

   *  Registration of media type and content format application/ace-
      group+cbor

   *  Editorial improvements.

C.5.  Version -02 to -03

   *  Exchange of information on the signature algorithm and related
      parameters, during the Token POST (Section 3.3).

   *  Restructured KDC interface, with new possible operations
      (Section 4).




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   *  Client PoP signature for the Joining Request upon joining
      (Section 4.1.2.1).

   *  Revised text on group member removal (Section 5).

   *  Added more profile requirements (Appendix A).

C.6.  Version -01 to -02

   *  Editorial fixes.

   *  Distinction between transport profile and application profile
      (Section 1.1).

   *  New parameters 'sign_info' and 'pub_key_enc' to negotiate
      parameter values for signature algorithm and signature keys
      (Section 3.3).

   *  New parameter 'type' to distinguish different Key Distribution
      Request messages (Section 4.1).

   *  New parameter 'client_cred_verify' in the Key Distribution Request
      to convey a Client signature (Section 4.1).

   *  Encoding of 'pub_keys_repos' (Section 4.1).

   *  Encoding of 'mgt_key_material' (Section 4.1).

   *  Improved description on retrieval of new or updated keying
      material (Section 6).

   *  Encoding of 'get_pub_keys' in Public Key Request (Section 7.1).

   *  Extended security considerations (Sections 10.1 and 10.2).

   *  New "ACE Public Key Encoding" IANA registry (Section 11.2).

   *  New "ACE Groupcomm Parameters" IANA registry (Section 11.3),
      populated with the entries in Section 8.

   *  New "Ace Groupcomm Request Type" IANA registry (Section 11.4),
      populated with the values in Section 9.

   *  New "ACE Groupcomm Policy" IANA registry (Section 11.7) populated
      with two entries "Sequence Number Synchronization Method" and "Key
      Update Check Interval" (Section 4.2).





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   *  Improved list of requirements for application profiles
      (Appendix A).

C.7.  Version -00 to -01

   *  Changed name of 'req_aud' to 'audience' in the Authorization
      Request (Section 3.1).

   *  Defined error handling on the KDC (Sections 4.2 and 6.2).

   *  Updated format of the Key Distribution Response as a whole
      (Section 4.2).

   *  Generalized format of 'pub_keys' in the Key Distribution Response
      (Section 4.2).

   *  Defined format for the message to request leaving the group
      (Section 5.2).

   *  Renewal of individual keying material and methods for group
      rekeying initiated by the KDC (Section 6).

   *  CBOR type for node identifiers in 'get_pub_keys' (Section 7.1).

   *  Added section on parameter identifiers and their CBOR keys
      (Section 8).

   *  Added request types for requests to a Join Response (Section 9).

   *  Extended security considerations (Section 10).

   *  New IANA registries "ACE Groupcomm Key registry", "ACE Groupcomm
      Profile registry", "ACE Groupcomm Policy registry" and "Sequence
      Number Synchronization Method registry" (Section 11).

   *  Added appendix about requirements for application profiles of ACE
      on group communication (Appendix A).

Acknowledgments

   The following individuals were helpful in shaping this document:
   Christian Amsuess, Carsten Bormann, Rikard Hoeglund, Ben Kaduk,
   Watson Ladd, John Mattsson, Daniel Migault, Jim Schaad, Ludwig Seitz,
   Goeran Selander, Cigdem Sengul and Peter van der Stok.







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Internet-Draft  Key Provisioning for Group Communication    October 2021


   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC; by the H2020 project SIFIS-Home
   (Grant agreement 952652); and by the EIT-Digital High Impact
   Initiative ACTIVE.

Authors' Addresses

   Francesca Palombini
   Ericsson AB
   Torshamnsgatan 23
   SE-16440 Stockholm Kista
   Sweden

   Email: francesca.palombini@ericsson.com


   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   SE-16440 Stockholm Kista
   Sweden

   Email: marco.tiloca@ri.se




























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