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Versions: (draft-palombini-ace-key-groupcomm) 00 01 02 03

ACE Working Group                                           F. Palombini
Internet-Draft                                               Ericsson AB
Intended status: Standards Track                               M. Tiloca
Expires: May 7, 2020                                             RISE AB
                                                       November 04, 2019


           Key Provisioning for Group Communication using ACE
                    draft-ietf-ace-key-groupcomm-03

Abstract

   This document defines message formats and procedures for requesting
   and distributing group keying material using the ACE framework, to
   protect communications between group members.

Status of This Memo

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

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

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

   This Internet-Draft will expire on May 7, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Authorization to Join a Group . . . . . . . . . . . . . . . .   6
     3.1.  Authorization Request . . . . . . . . . . . . . . . . . .   6
     3.2.  Authorization Response  . . . . . . . . . . . . . . . . .   7
     3.3.  Token Post  . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Keying Material Provisioning and Group Membership Management   11
     4.1.  Interface at KDC  . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Joining Exchange  . . . . . . . . . . . . . . . . . . . .  21
     4.3.  Retrieval of Updated Keying Material  . . . . . . . . . .  22
     4.4.  Retrieval of New Keying Material  . . . . . . . . . . . .  24
     4.5.  Retrieval of Public Keys for Group Members  . . . . . . .  24
     4.6.  Retrieval of Group Policies . . . . . . . . . . . . . . .  25
     4.7.  Retrieval of Keying Material Version  . . . . . . . . . .  25
     4.8.  Group Leaving Request . . . . . . . . . . . . . . . . . .  26
   5.  Removal of a Node from the Group  . . . . . . . . . . . . . .  26
   6.  ACE Groupcomm Parameters  . . . . . . . . . . . . . . . . . .  27
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
     7.1.  Update of Keying Material . . . . . . . . . . . . . . . .  29
     7.2.  Block-Wise Considerations . . . . . . . . . . . . . . . .  29
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
     8.1.  ACE Authorization Server Request Creation Hints Registry   30
     8.2.  ACE Groupcomm Parameters Registry . . . . . . . . . . . .  30
     8.3.  ACE Groupcomm Key Registry  . . . . . . . . . . . . . . .  31
     8.4.  ACE Groupcomm Profile Registry  . . . . . . . . . . . . .  32
     8.5.  ACE Groupcomm Policy Registry . . . . . . . . . . . . . .  32
     8.6.  Sequence Number Synchronization Method Registry . . . . .  33
     8.7.  Expert Review Instructions  . . . . . . . . . . . . . . .  34
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  34
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  35
   Appendix A.  Requirements on Application Profiles . . . . . . . .  37
   Appendix B.  Document Updates . . . . . . . . . . . . . . . . . .  39
     B.1.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  39
     B.2.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  39
     B.3.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  40
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction

   This document expands the ACE framework [I-D.ietf-ace-oauth-authz] to
   define the message exchanges used to request, distribute and renew
   the keying material in a group communication scenario, e.g. based on
   multicast [RFC7390][I-D.dijk-core-groupcomm-bis] or on publishing-



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   subscribing [I-D.ietf-core-coap-pubsub].  The ACE framework is based
   on CBOR [RFC7049], so CBOR is the format used in this specification.
   However, using JSON [RFC8259] instead of CBOR is possible, using the
   conversion method specified in Sections 4.1 and 4.2 of [RFC7049].

   Profiles that use group communication can build on this document, by
   defining a number of details such as the exact group communication
   protocol and security protocols used.  The specific list of details a
   profile needs to define is in Appendix A.

   If the application requires backward and forward security, updated
   keying material is generated and distributed to the group members
   (rekeying), when membership changes.  A key management scheme
   performs the actual distribution of the updated keying material to
   the group.  In particular, the key management scheme rekeys the
   current group members when a new node joins the group, and the
   remaining group members when a node leaves the group.  Rekeying
   mechanisms can be based on [RFC2093], [RFC2094] and [RFC2627].

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 [I-D.ietf-ace-oauth-authz] and [RFC8152], such as
   Authorization Server (AS) and Resource Server (RS).

   This document additionally uses the following terminology:

   o  Transport profile, to indicate a profile of ACE as per
      Section 5.6.4.3 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.  Tranport 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].

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




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2.  Overview

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

                  Figure 1: Key Distribution Participants

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

   o  Client (C): node that wants to join the group communication.  It
      can request write and/or read rights.

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

   o  Key Distribution Center (KDC): maintains the keying material to
      protect group communications, and provides it to Clients
      authorized to join the 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.  If required by the application, the KDC renews
      and re-distributes the keying material in the group when
      membership changes.

   o  Dispatcher: entity through which the Clients communicate with the
      group and which distributes messages to the group members.
      Examples of dispatchers are: the Broker node in a pub-sub setting;
      a relayer node 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:



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

   o  A node to leave the group of for the KDC to remove a current
      member of the group (Section 5).

   o  Retrieving keying material as a current group member (Section 4.3
      and Section 4.4).

   o  Renewing and re-distributing the group keying material (rekeying)
      upon a membership change in the group (Section 4.8 and Section 5).

   Figure 2 provides a high level overview of the message flow for a
   node joining a group communication setting.

            C                             AS  KDC                 Group
            |                             |    |                  Member
          / |                             |    |                     |
          | |    Authorization Request    |    |                     |
 Defined  | |---------------------------->|    |                     |
 in the   | |                             |    |                     |
   ACE    | |    Authorization Response   |    |                     |
framework | |<----------------------------|    |                     |
          | |                                  |                     |
          \ |----------- Token Post ---------->|                     |
            |                                  |                     |
            |-------- Joining Request -------->|                     |
            |                                  |                     |
            |<------- Joining Response --------|-- Group Rekeying -->|
            |                                  |                     |
            |                                       Dispatcher       |
            |                                           |            |
            |<====== Secure group communication ========|===========>|
            |                                           |            |

              Figure 2: Message Flow Upon New Node's Joining

   The exchange of Authorization Request and Authorization Response
   between Client and AS MUST be secured, as specified by the transport
   profile of ACE used between Client and KDC.

   The exchange of Joining Request and Joining Response between Client
   and KDC MUST be secured, as a result of the transport profile of ACE
   used between Client and KDC.






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   All further communications between the Client and the KDC MUST be
   secured, for instance with the same security mechanism used for the
   Key Distribution exchange.

   All communications between a Client and the other group members MUST
   be secured using the keying material provided in Section 4.

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 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 from
   the AS an 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, as
   defined by the transport profile of ACE used.  The Content-Format
   used in the messages is the one specified by the used transport
   profile of ACE (e.g. application/ace+cbor for the first two messages
   and application/cwt for the third message, depending on the format of
   the access token).  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) ---|   |
            |                                               |   |
            |----- POST Token: POST /authz-info --------------->|
            |                                                   |

               Figure 3: Message Flow of Join Authorization

3.1.  Authorization Request

   The Authorization Request sent from the Client to the AS is as
   defined in Section 5.6.1 of [I-D.ietf-ace-oauth-authz] and MAY
   contain the following parameters, which, if included, MUST have the
   corresponding values:



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   o  'scope', containing the identifier of the specific group (or topic
      in the case of pub-sub) that the Client wishes to access, and
      optionally the role(s) that the Client wishes to take.  This value
      is a CBOR array encoded as a byte string, which contains:

      *  As first element, the identifier of the specific group or
         topic.

      *  Optionally, as second element, the role (or CBOR array of
         roles) the Client wishes to take in the group.

      The encoding of the group or topic identifier (REQ1) and of the
      role identifiers (REQ2) is application specific, and part of the
      requirements for the application profile.

   o  'audience', with an identifier of a KDC.

   o  'req_cnf', as defined in Section 3.1 of
      [I-D.ietf-ace-oauth-params], optionally containing the public key
      or a reference to the public key of the Client, if it wishes to
      communicate that to the AS.

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

   As in [I-D.ietf-ace-oauth-authz], these parameters are included in
   the payload, which is formatted as a CBOR map.  The Content-Format
   "application/ace+cbor" defined in Section 8.14 of
   [I-D.ietf-ace-oauth-authz] is used.

3.2.  Authorization Response

   The Authorization Response sent from the AS to the Client is as
   defined in Section 5.6.2 of [I-D.ietf-ace-oauth-authz] and MUST
   contain the following parameters:

   o  'access_token', containing the proof-of-possession access token.

   o  'cnf' if symmetric keys are used, not present if asymmetric keys
      are used.  This parameter is defined in Section 3.2 of
      [I-D.ietf-ace-oauth-params] and contains the symmetric proof-of-
      possession key that the Client is supposed to use with the KDC.

   o  'rs_cnf' if asymmetric keys are used, not present if symmetric
      keys are used.  This parameter is as defined in Section 3.2 of
      [I-D.ietf-ace-oauth-params] and contains information about the
      public key of the KDC.




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   o  'exp', contains the lifetime in seconds of the access token.  This
      parameter 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, the Authorization Response MAY contain the following
   parameters, which, if included, MUST have the corresponding values:

   o  'scope', which mirrors the 'scope' parameter in the Authorization
      Request (see Section 3.1).  Its value is a CBOR array encoded as a
      byte string, containing:

      *  As first element, the identifier of the specific group or topic
         the Client is authorized to access.

      *  Optionally, as second element, the role (or CBOR array of
         roles) the Client is authorized to take in the group.

      The encoding of the group or topic identifier and of the role
      identifiers is the same as in Section 3.1.

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

   The access token MUST contain all the parameters defined above
   (including the same 'scope' as in this message, if present, or the
   'scope' of the Authorization Request otherwise), and additionally
   other optional parameters that the transport profile of ACE requires.

   As in [I-D.ietf-ace-oauth-authz], these parameters are included in
   the payload, which is formatted as a CBOR map.  The Content-Format
   "application/ace+cbor" is used.

   When receiving an Authorization Request from a Client that was
   previously authorized, and which still owns a valid non expired
   access token, the AS replies with an Authorization Response with a
   new access token.

3.3.  Token Post

   The Client sends a CoAP POST request including the access token to
   the KDC, as specified in Section 5.8.1 of [I-D.ietf-ace-oauth-authz].
   If the specific transport profile of ACE defines it, the Client MAY
   use a different endpoint than /authz-info at the KDC to post the
   access token to.

   Optionally, the Client might want to request necessary information
   concerning the public keys in the group, as well as concerning the



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   algorithm and related parameters for computing signatures in the
   group.  In such a case, the joining node MAY ask for that information
   to the KDC in this same request.  To this end, it sends the CoAP POST
   request to the /authz-info endpoint using the Content-Format
   "application/ace+cbor".  The payload of the message MUST be formatted
   as a CBOR map, including the access token and the following
   parameters:

   o  'sign_info' defined in Section 3.3.1, encoding the CBOR simple
      value Null, to require information and parameters on the signature
      algorithm and on the public keys used in the group.

   o  'pub_key_enc' defined in Section 3.3.2, encoding the CBOR simple
      value Null, to require information on the exact encoding of public
      keys used in the group.

   The CDDL notation of the 'sign_info' and 'pub_key_enc' parameters
   formatted as in the request is given below.

      sign_info_req = nil

      pub_key_enc_req = nil

   Alternatively, the joining node may retrieve this information by
   other means.

   After successful verification, the Client is authorized to receive
   the group keying material from the KDC and join the group.  In
   particular, the KDC replies to the Client with a 2.01 (Created)
   response, using Content-Format "application/ace+cbor" defined in
   Section 8.14 of [I-D.ietf-ace-oauth-authz].

   The payload of the 2.01 response is a CBOR map, which MUST include
   the parameter 'rsnonce' defined in Section Section 3.3.3, specifying
   a dedicated nonce N_S generated by the KDC.  The Client may use this
   nonce for proving possession of its own private key (see the
   'client_cred_verify' parameter in Section 4).

   Optionally, if they were included in the request, the AS MAY include
   the 'sign_info' parameter as well as the 'pub_key_enc' parameter
   defined in Section 3.3.1 and Section 3.3.2 of this specification,
   respectively.

   The 'sign_info' parameter MUST be present if the POST request
   included the 'sign_info' parameter with value Null.  If present, the
   'sign_info' parameter of the 2.01 (Created) response is a CBOR array
   formatted as follows.




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   o  The first element 'sign_alg' is an integer or a text string,
      indicating the signature algorithm used in the group.  It is
      REQUIRED of the application profiles to define specific values for
      this parameter (REQ3).

   o  The second element 'sign_parameters' indicates the parameters of
      the signature algorithm.  Its structure depends on the value of
      'sign_alg'.  It is REQUIRED of the application profiles to define
      specific values for this parameter (REQ4).  If no parameters of
      the signature algorithm are specified, 'sign_parameters' MUST be
      encoded as the CBOR simple value Null.

   o  The third element 'sign_key_parameters' indicates the parameters
      of the key used with the signature algorithm.  Its structure
      depends on the value of 'sign_alg'.  It is REQUIRED of the
      application profiles to define specific values for this parameter
      (REQ5).  If no parameters of the key used with the signature
      algorithm are specified, 'sign_key_parameters' MUST be encoded as
      the CBOR simple value Null.

   The 'pub_key_enc' parameter MUST be present if the POST request
   included the 'pub_key_enc' parameter with value Null.  If present,
   the 'pub_key_enc' parameter of the 2.01 (Created) response is a CBOR
   integer, indicating the encoding of public keys used in the group.
   Its acceptable values are taken from the "CWT Confirmation Method"
   Registry defined in [I-D.ietf-ace-cwt-proof-of-possession].  It is
   REQUIRED of the application profiles to define specific values to use
   for this parameter (REQ6).

   The CDDL notation of the 'sign_info' and 'pub_key_enc' parameters
   formatted as in the response is given below.

      sign_info_res = [
        sign_alg : int / tstr,
        sign_parameters : any / nil,
        sign_key_parameters : any / nil
      ]

      pub_key_enc_res = int

   Note that the CBOR map specified as payload of the 2.01 (Created)
   response may include further parameters, e.g. according to the
   signalled transport profile of ACE.








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3.3.1.  'sign_info' Parameter

   The 'sign_info' parameter is an OPTIONAL parameter of the AS Request
   Creation Hints message defined in Section 5.1.2. of
   [I-D.ietf-ace-oauth-authz].  This parameter contains information and
   parameters about the signature algorithm and the public keys to be
   used between the Client and the RS.  Its exact content is application
   specific.

   In this specification and in application profiles building on it,
   this parameter is used to ask and retrieve from the KDC information
   about the signature algorithm and related parameters used in the
   group.

3.3.2.  'pub_key_enc' Parameter

   The 'pub_key_enc' parameter is an OPTIONAL parameter of the AS
   Request Creation Hints message defined in Section 5.1.2. of
   [I-D.ietf-ace-oauth-authz].  This parameter contains information
   about the exact encoding of public keys to be used between the Client
   and the RS.  Its exact content is application specific.

   In this specification and in application profiles building on it,
   this parameter is used to ask and retrieve from the KDC information
   about the encoding of public keys used in the group.

3.3.3.  'rsnonce' Parameter

   The 'rsnonce' parameter is an OPTIONAL parameter of the AS Request
   Creation Hints message defined in Section 5.1.2. of
   [I-D.ietf-ace-oauth-authz].  This parameter contains a nonce
   generated by the RS and provided to the Client.  Its exact content is
   application specific.

   In this specification and in application profiles building on it,
   this parameter is used to provide a nonce that the Client may use to
   prove possession of its own private key in the Joining Request ((see
   the 'client_cred_verify' parameter in Section 4).

4.  Keying Material Provisioning and Group Membership Management

   This section defines the interface available at the KDC.  Moreover,
   this section specifies how the clients can use this interface to join
   a group, leave a group, retrieve new keying material or policies.

   During the first exchange with the KDC ("Joining"), the Client sends
   a request to the KDC, specifying the group it wishes to join (see
   Section 4.2).  Then, the KDC verifies the access token and that the



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   Client is authorized to join that group.  If so, it provides the
   Client with the keying material to securely communicate with the
   other members of the group.  Whenever used, the Content-Format in
   messages containing a payload is set to application/cbor.

   TODO: Do we need to define a new Content-Format cbor+ace-groupcomm?

   When the Client is already a group member, the Client can use the
   interface at the KDC to perform the following actions:

   o  The Client can (re-)get the current keying material, for cases
      such as expiration, loss or suspected mismatch, due to e.g. reboot
      or missed group rekeying.  This is described in Section 4.3.

   o  The Client can retrieve a new individual key, or new input
      material to derive it.  This is described in Section 4.4.

   o  The Client can (re-)get the public keys of other group members,
      e.g. if it is aware of new nodes joining the group after itself.
      This is described in Section 4.5.

   o  The Client can (re-)get the policies currently enforced in the
      group.  This is described in Section 4.6.

   o  The Client can (re-)get the version number of the keying material
      currently used in the group.  This is described in Section 4.7.

   o  The Client can request to leave the group.  This is further
      discussed in Section 4.8.

   Upon receiving a request from a Client, the KDC MUST check that it is
   storing a valid access token from that Client for the group
   identifier assiciated to the endpoint.  If that is not the case, i.e.
   the KDC does not store a valid access token or this is not valid for
   that Client for the group identifier at hand, the KDC MUST respond to
   the Client with a 4.01 (Unauthorized) error message.

4.1.  Interface at KDC

   The KDC is configured with the following resources:

   o  /ace-group : this resource is fixed and indicates that this
      specification is used.  Other applications that run on a KDC
      implementing this specification MUST NOT use this same resource.

   o  /ace-group/gid : one sub-resource to /ace-group is implemented for
      each group the KDC manages.  These resources are identified by the
      group identifiers of the groups the KDC manages (in this example,



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      the group identifier has value "gid").  These resources support
      GET and POST method.

   o  /ace-group/gid/pub-key : this sub-resource is fixed and supports
      GET and POST methods.

   o  /ace-group/gid/policies: this sub-resource is fixed and supports
      the GET method.

   o  /ace-group/gid/ctx-num: this sub-resource is fixed and supports
      the GET method.

   o  /ace-group/gid/node: this sub-resource is fixed and supports GET
      and POST methods.

   The details for the handlers of each resource are given in the
   following sections.  These endpoints are used to perform the
   operations introduced in Section 4.  Note that the url-path given
   here are default names: implementations are not required to use these
   names, and can define their own instead.

4.1.1.  ace-group

   No handlers are implemented for this resource.

4.1.2.  ace-group/gid

   This resource implements GET and POST handlers.

4.1.2.1.  POST Handler

   The POST handler 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 "gid".

   The handler expects a request with payload formatted as a CBOR map
   which MAY contain the following fields, which, if included, MUST have
   the corresponding values:

   o  'scope', with value the specific resource that the Client is
      authorized to access (i.e. group or topic identifier) and role(s),
      encoded as in Section 3.1.

   o  'get_pub_keys', if the Client wishes to receive the public keys of
      the other nodes in the group from the KDC.  The value is an empty
      CBOR array.  This parameter may be present if the KDC stores the
      public keys of the nodes in the group and distributes them to the
      Client; it is useless to have here if the set of public keys of



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      the members of the group is known in another way, e.g. it was
      provided by the AS.

   o  'client_cred', with value the public key or certificate of the
      Client, encoded as a CBOR byte string.  If the KDC is managing
      (collecting from/distributing to the Client) the public keys of
      the group members, this field contains the public key of the
      Client.  The default encoding for public keys is COSE Keys.
      Alternative specific encodings of this parameter MAY be defined in
      applications of this specification (OPT1).

   o  'cnonce', as defined in Section 5.1.2 of
      [I-D.ietf-ace-oauth-authz], and including a dedicated nonce N_C
      generated by the Client.  This parameter MUST be present if the
      'client_cred' parameter is present.

   o  'client_cred_verify', encoded as a CBOR byte string.  This
      parameter MUST be present if the 'client_cred' parameter is
      present.  This parameter contains a signature computed by the
      Client over N_S concatenated with N_C, where N_S is the nonce
      received from the KDC in the 'rsnonce' parameter of the 2.01
      (Created) response to the token POST request (see Section 3.3),
      while N_C is the nonce generated by the Client and specified in
      the 'cnonce' parameter above.  The Client computes the signature
      by using its own private key, whose corresponding public key is
      either directly specified in the 'client_cred' parameter or
      included in the certificate specified in the 'client_cred'
      parameter.

   o  'pub_keys_repos', can be present if a certificate is present in
      the 'client_cred' field, with value a list of public key
      repositories storing the certificate of the Client.  This
      parameter is encoded as a CBOR array of CBOR text strings, each of
      which specifies the URI of a key repository.

   The handler verifies that the group identifier of the /ace-group/gid
   path is a subset of the 'scope' stored in the access token associated
   to this client.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.

   If the request is not formatted correctly (e.g. unknown fields
   present), the handler MUST respond with 4.00 (Bad Request) error
   message.

   If verification succeeds, the handler adds the public key indicated
   in "client_cred" to the list of public keys stored for the group
   identified by "gid".  The handler returns a 2.01 (Created) message
   containing the symmetric group keying material, the group policies



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   and all the public keys of the current members of the group, if the
   KDC manages those and the Client requested them.  The payload of the
   response is formatted as a CBOR map which MAY contain the following
   fields, which, if included, MUST have the corresponding values:

   o  'kty', 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" Registry.  Implementations MUST verify that the key
      type matches the application profile being used, if present, as
      registered in the "ACE Groupcomm Key" registry.

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

   o  'num', containing the version number of the keying material for
      the group communication, formatted as an integer.  The initial
      version MUST be set to 0 at the KDC.

   The exact format of the 'key' value MUST be defined in applications
   of this specification (REQ7), as well as accepted values of 'kty' by
   the application (REQ8).  Additionally, documents specifying the key
   format MUST register it in the "ACE Groupcomm Key" registry defined
   in Section 8.3, including its name, type and application profile to
   be used with.

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

                         Figure 4: Key Type Values

   Optionally, the response MAY contain the following parameters, which,
   if included, MUST have the corresponding values:

   o  'profile', with value a CBOR integer that MUST be used to uniquely
      identify the application profile for group communication.  The
      value MUST be registered in the "ACE Groupcomm Profile" Registry.

   o  'exp', with value the expiration time of the keying material for
      the group communication, encoded as a CBOR unsigned integer or
      floating-point number.  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 in Section 2 of [RFC7519].





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   o  'pub_keys', may only be present if 'get_pub_keys' was present in
      the request.  This parameter is a CBOR byte string, which encodes
      the public keys of all the group members paired with the
      respective member identifiers.  The default encoding for public
      keys is COSE Keys, so the default encoding for 'pub_keys' is a
      CBOR byte string wrapping a COSE_KeySet (see [RFC8152]), which
      contains the public keys of all the members of the group.  In
      particular, each COSE Key in the COSE_KeySet includes the
      identifier of the corresponding group member as value of its 'kid'
      key parameter.  Alternative specific encodings of this parameter
      MAY be defined in applications of this specification (OPT2).  The
      specific format of the identifiers of group members MUST be
      specified in the application profile (REQ8).

   o  '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 Policy" Registry.  This
      specification defines the two elements "Sequence Number
      Synchronization Method" and "Key Update Check Interval", which are
      summarized in Figure 5.  Application profiles that build on this
      document MUST specify the exact content format of included map
      entries (REQ9).



























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+-----------------+-------+----------|--------------------|------------+
|      Name       | CBOR  |   CBOR   |    Description     | Reference  |
|                 | label |   type   |                    |            |
|-----------------+-------+----------|--------------------|------------|
| Sequence Number | TBD1  | tstr/int | Method for a re-   | [[this     |
| Synchronization |       |          | cipient node to    | document]] |
| Method          |       |          | synchronize with   |            |
|                 |       |          | sequence numbers   |            |
|                 |       |          | of a sender node.  |            |
|                 |       |          | Its value is taken |            |
|                 |       |          | from the 'Value'   |            |
|                 |       |          | column of the      |            |
|                 |       |          | Sequence Number    |            |
|                 |       |          | Synchronization    |            |
|                 |       |          | Method registry    |            |
|                 |       |          |                    |            |
| Key Update      | TBD2  |   int    | Polling interval   | [[this     |
| Check Interval  |       |          | in seconds, to     | document]] |
|                 |       |          | check for new      |            |
|                 |       |          | keying material at |            |
|                 |       |          | the KDC            |            |
+-----------------+-------+----------|--------------------|------------+

                     Figure 5: ACE Groupcomm Policies

   o  'mgt_key_material', encoded as a CBOR byte string and containing
      the administrative keying material to participate in the group
      rekeying performed by the KDC.  The exact format and content
      depend on the specific rekeying scheme used in the group, which
      MAY be specified in the application profile (OPT3).

   Specific application profiles that build on this document MUST
   specify how exactly the keying material is used to protect the group
   communication (REQ10).

   CBOR labels for these fields are defined in Section 6.

4.1.2.2.  GET Handler

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

   The handler expects a GET request.

   The handler verifies that the group identifier of the /ace-group/gid
   path is a subset of the 'scope' stored in the access token associated
   to this client.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.



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   If verification succeeds, the handler returns a 2.05 (Content)
   message containing the symmetric group keying material, the group
   policies and all the public keys of the current members of the group.
   The payload of the response is formatted as a CBOR map which MUST
   contain the parameters 'kty','key' and 'num' specified in
   Section 4.1.2.1.

   The payload MAY also include the parameters 'profile', 'exp' and
   'mgt_key_material' parameters specified in Section 4.1.2.1.

4.1.3.  ace-group/gid/pub-key

   This resource implements GET and POST handlers.

4.1.3.1.  POST Handler

   The POST handler receives identifiers of group members for the group
   identified by "gid" and returns the public keys of such group
   members.

   The handler expects a request with payload formatted as a CBOR map.
   The payload of this request is a CBOR Map that MUST contain the
   following fields:

   o  'get_pub_keys', whose value is a non-empty CBOR array.  Each
      element of the array is the identifier of a group member for the
      group identified by "gid".  The specific format of public keys as
      well as identifiers of group members MUST be specified by the
      application profile (REQ11, REQ8).

   The handler verifies that the group identifier of the /ace-group/gid
   path is a subset of the 'scope' stored in the access token associated
   to this client.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.

   The handler verifies that the 'get_pub_keys' parameter is not an
   empty CBOR Array.  If verification fails, the KDC MUST treat the
   request as malformed and respond with a 4.00 (Bad Request) error
   message.

   If verification succeeds, the handler identifies the public keys of
   the current group members for which the identifier matches with one
   of those indicated in the request.  Then, the handler returns a 2.05
   (Content) message response with payload formatted as a CBOR map
   containing only the 'pub_keys' parameter from Section 4.1.2.1, which
   encodes the list of public keys of those group members including the
   respective member identifiers.  If the KDC does not store any public
   key associated with the specified member identifiers, the handler



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   returns a response with payload formatted as a CBOR byte string of
   zero length.  The specific format of public keys as well as of
   identifiers of group members is specified by the application profile
   (REQ11, REQ8).

   The handler MAY enforce one of the following policies, in order to
   handle possible identifiers that are included in 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
   (REQ12)

   o  The KDC silently ignores those identifiers.

   o  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.

4.1.3.2.  GET Handler

   The handler expects a GET request.

   The handler verifies that the group identifier of the /ace-group/gid
   path is a subset of the 'scope' stored in the access token associated
   to this client.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.

   If verification succeeds, the handler returns a 2.05 (Content)
   message containing the public keys of all the current group members,
   for the group identified by "gid".  The payload of the response is
   formatted as a CBOR map containing only the 'pub_keys' parameter from
   Section 4.1.2.1, which encodes the list of public keys of all the
   group members including the respective member identifiers.  If the
   KDC does not store any public key for the group, the handler returns
   a response with payload formatted as a CBOR byte string of zero
   length.  The specific format of public keys as well as of identifiers
   of group members is specified by the application profile (REQ11,
   REQ8).

4.1.4.  ace-group/gid/policies

   This resource implements a GET handler.

4.1.4.1.  GET Handler

   The handler expects a GET request.





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   The handler verifies that the group identifier of the /ace-group/gid
   path is a subset of the 'scope' stored in the access token associated
   to this client.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.

   If verification succeeds, the handler returns a 2.05 (Content)
   message containing the list of policies for the group identified by
   "gid".  The payload of the response is formatted as a CBOR map
   including only the parameter 'group_policies' defined in
   Section 4.1.2.1 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 (REQ13).

4.1.5.  ace-group/gid/ctx-num

   This resource implements a GET handler.

4.1.5.1.  GET Handler

   The handler expects a GET request.

   The handler verifies that the group identifier of the /ace-group/gid
   path is a subset of the 'scope' stored in the access token associated
   to this client.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.

   If verification succeeds, 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.  The payload of the response is formatted as a CBOR
   integer.

4.1.6.  ace-group/gid/node

   This resource implements GET and POST handlers.

4.1.6.1.  POST Handler

   The POST handler removes the node from the group, for the group
   identified by "gid".

   The handler expects a request with payload formatted as a CBOR map.
   The payload of this request is a CBOR Map that MAY contain only the
   'scope' field as specified in Section 4.1.2.1.



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   The handler verifies that the group identifier of the /ace-group/gid
   path is a subset of the 'scope' stored in the access token associated
   to this client.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.

   If the request contained a 'scope' field, the handler MUST extract
   the roles for that client.  If the value is such that the KDC cannot
   extract all the necessary information to understand and process it
   correctly (e.g. unrecognized roles), the KDC MUST respond with a 4.00
   (Bad Request) error message.

   If verification succeeds, the handler removes the client from the
   group identified by "gid", for specific roles if roles were specified
   in the 'scope' field, or for all roles.  That includes removing the
   public key of the client if the KDC keep tracks of that.  Then, the
   handler returns a 2.05 (Content) message with empty payload.

4.1.6.2.  GET Handler

   The handler expects a GET request.

   The handler verifies that the group identifier of the /ace-group/gid
   path is a subset of the 'scope' stored in the access token associated
   to this client.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.

   If verification succeeds, the handler returns a 2.05 (Content)
   message containing newly-generated 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 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 (REQ14) and registered in
   Section 8.2.

4.2.  Joining Exchange

   Figure 6 gives an overview of the Joining exchange between Client and
   KDC, when the Client first joins a group.

      Client                                                     KDC
         |                                                        |
         |-------- Joining Request: POST /ace-group/gid --------->|
         |                                                        |
         |<--------- Joining Response: 2.01 (Created) ----------- |
         |                                                        |

        Figure 6: Message Flow of First Exchange for Group Joining



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   If not previously established, the Client and the KDC MUST first
   establish a pairwise secure communication channel (REQ15).  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 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/gid endpoint at the KDC, where gid is the group identifier of
   the group to join, formatted as specified in Section 4.1.2.1.  This
   group identifier is the same as the 'scope' value of the
   Authorization Request/Response, or it can be retrieved from it.

   If the application requires backward security, the KDC MUST generate
   new group keying material and securely distribute it to all the
   current group members, upon a new node's joining the group.  To this
   end, the KDC uses the message format of the Joining Response (see
   Section 4.1.2.1).  Application profiles may define alternative ways
   of retrieving the keying material, such as sending separate requests
   to different resources at the KDC (Section 4.1.2.2, Section 4.1.3.2,
   Section 4.1.4.1).  After distributing the new group keying material,
   the KDC MUST increment the version number of the keying material.

4.3.  Retrieval of Updated Keying Material

   A node stops using the group keying material upon its expiration,
   according to what indicated by the KDC with the 'exp' parameter in a
   Joining response, or to a pre-configured value.  Then, if it wants to
   continue participating in the group communication, the node has to
   request new updated keying material from the KDC.

   The Client may need to request the latest group keying material also
   upon receiving messages from other group members without being able
   to retrieve the material to correctly decrypt them.  This may be due
   to a previous update of the group keying material (rekeying)
   triggered by the KDC, that the Client was not able to receive or
   decrypt.  To this end, the Client sends a CoAP GET request to the
   /ace-group/gid endpoint at the KDC, formatted as specified in
   Section 4.1.2.2.





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   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
   unsuccessfully decrypted incoming messages.  It is application
   dependent and pertaining to the particular message exchange (e.g.
   [I-D.ietf-core-oscore-groupcomm]) to set up policies that instruct
   clients to retain unsuccessfully decrypted messages and for how long,
   so that they can be decrypted after getting updated keying material,
   rather than just considered non valid messages to discard right away
   (OPT4).

   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 7 gives an overview of the exchange described above.

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

        Figure 7: Message Flow of Key Distribution Request-Response

   Alternatively, the re-distribution of keying material can be
   initiated by the KDC, which e.g.:

   o  Can make the ace-group/gid resource Observable, and send
      notifications to Clients when the keying material is updated.

   o  Can send the Key Distribution Response as one or multiple
      multicast requests to the members of the group, using secure
      rekeying schemes such as [RFC2093][RFC2094][RFC2627].

   o  Can send unicast requests to each Client over a secure channel,
      with the same payload as the Key Distribution Response.

   o  Can act as a publisher in a pub-sub scenario, and update the
      keying material by publishing on a specific topic on a broker,
      which all the members of the group are subscribed to.

   Note that these methods of KDC-initiated key distribution have
   different security properties and require different security
   associations.




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4.4.  Retrieval of New Keying Material

   Beside possible expiration and depending on what part of the keying
   material is no longer eligible to be used, the client may need to
   communicate to the KDC its need for that part to be renewed.  For
   example, if the Client uses an individual key to protect outgoing
   traffic and has to renew it, the node may request a new one, or new
   input material to derive it, without renewing the whole group keying
   material.  To this end, the client performs a Key Renewal Request/
   Response exchange with the KDC, that is a CoAP GET request to the
   /ace-group/gid/node endpoint at the KDC, where gid is the group
   identifier, and formatted as defined in Section 4.1.6.2.

   Figure 8 gives an overview of the exchange described above.

        Client                                                  KDC
           |                                                     |
           |---- Key Renewal Request: GET ace-group/gid/node --->|
           |                                                     |
           |<----- Key Renewal Response: 2.05 (Content) ---------|
           |                                                     |

          Figure 8: Message Flow of Key Renewal Request-Response

   Note the difference between the Key Distribution Request and the Key
   Renewal Request: while the first one only triggers distribution (the
   renewal might have happened independently, e.g. because of
   expiration), the second one triggers the KDC to produce new
   individual keying material for the requesting node.

4.5.  Retrieval of Public Keys for Group Members

   In case the KDC maintains the public keys of group members, a node in
   the group can contact the KDC to request public keys of either all
   group members or a specified subset, by sending a CoAP GET or POST
   request to the /ace-group/gid/pub-key endpoint at the KDC, where gid
   is the group identifier, and formatted as defined in Section 4.1.3.2
   and Section 4.1.3.1.

   Figure 9 and Figure 10 give an overview of the exchanges described
   above.










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

   Figure 9: Message Flow of Public Key Exchange to Request All Members
                                Public Keys

      Client                                                     KDC
         |                                                        |
         |--- Public Key Request: POST /ace-group/gid/pub-key --->|
         |                                                        |
         |<--------- Public Key Response: 2.01 (Created) ---------|
         |                                                        |

    Figure 10: Message Flow of Public Key Exchange to Request Specific
                            Members Public Keys

4.6.  Retrieval of 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/gid/
   policies endpoint at the KDC, where gid is the group identifier, and
   formatted as defined in Section 4.1.4.1

   Figure 11 gives an overview of the exchange described above.

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

           Figure 11: Message Flow of Policies Request-Response

4.7.  Retrieval of 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/gid/ctx-num endpoint at the KDC,
   where gid is the group identifier, formatted as defined in
   Section 4.1.5.1.  In particular, the version is incremented by the
   KDC every time the group keying material is renewed.

   Figure 12 gives an overview of the exchange described above.



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

            Figure 12: Message Flow of Version Request-Response

4.8.  Group Leaving Request

   A node can actively request to leave the group.  In this case, the
   Client sends a CoAP POST request to the endpoint /ace-group/gid/node
   at the KDC, where gid is the group identifier, formatted as defined
   in Section 4.1.6.1

   Alternatively, a node may be removed by the KDC, without having
   explicitly asked for it.  This is further discussed in Section 5.

5.  Removal of a Node from the Group

   This section describes the different scenarios according to which a
   node ends up being removed from the group.

   If the application requires forward security, the KDC MUST generate
   new group keying material and securely distribute it to all the
   current group members but the leaving node, using the message format
   of the Key Distribution Response (see Section 4.3).  Application
   profiles may define alternative message formats.  Once distributed
   the new group keying material, the KDC MUST increment the version
   number of the keying material.

   Note that, after having left the group, a node may wish to join it
   again.  Then, as long as the node is still authorized to join the
   group, i.e. it still has a valid access token, it can re-request to
   join the group directly to the KDC without needing to retrieve a new
   access token from the AS.  This means that the KDC might decide to
   keep track of nodes with valid access tokens, before deleting all
   information about the leaving node.

   A node may be evicted from the group in the following cases.

   1.  The node explicitly asks to leave the group, as defined in
       Section 4.8.

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





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   3.  The node's authorization to be a group member is expired.  If the
       AS provides Token introspection (see Section 5.7 of
       [I-D.ietf-ace-oauth-authz]), the KDC can optionally use and check
       whether:

       *  the node is not authorized anymore;

       *  the access token is still valid, upon its expiration.

       Either case, once aware that a node is not authorized anymore,
       the KDC has to remove the unauthorized node from the list of
       group members, if the KDC keeps track of that.

6.  ACE Groupcomm Parameters

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

   +--------------------+--------+-----------------------+-------------+
   | Name               | CBOR   | CBOR Type             | Reference   |
   |                    | Key    |                       |             |
   +--------------------+--------+-----------------------+-------------+
   | scope              | TBD    | array                 | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | get_pub_keys       | TBD    | array                 | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | client_cred        | TBD    | byte string           | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | cnonce             | TBD    | byte string           | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | client_cred_verify | TBD    | byte string           | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | pub_keys_repos     | TBD    | array                 | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | kty                | TBD    | int / byte string     | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | key                | TBD    | see "ACE Groupcomm    | Section     |
   |                    |        | Key" Registry         | 4.1.2.1     |
   |                    |        |                       |             |



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   | num                | TBD    | int                   | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | profile            | TBD    | int                   | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | exp                | TBD    | int / float           | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | pub_keys           | TBD    | byte string           | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | group_policies     | TBD    | map                   | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | mgt_key_material   | TBD    | byte string           | Section     |
   |                    |        |                       | 4.1.2.1     |
   |                    |        |                       |             |
   | get_pub_keys       | TBD    | array                 | Section     |
   |                    |        |                       | 4.1.3.1     |
   +--------------------+--------+-----------------------+-------------+

7.  Security Considerations

   When a Client receives a message from a sender for the first time, it
   needs to have a mechanism in place to avoid replay, e.g.
   Appendix B.2 of [RFC8613].

   The KDC must renew the group keying material upon its expiration.

   The KDC should renew the keying material upon group membership
   change, and should provide it to the current group members through
   the rekeying scheme used in the group.

   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 every membership change,
   for instance if members' joining and leaving occur frequently and
   performing a group rekeying takes too long.  Instead, the KDC may
   rekey the group after a minum number of group members have joined or
   left within a given time interval, or during predictable network
   inactivity periods.

   However, this would result in the KDC not constantly preserving
   backward and forward security.  In fact, newly joining group members
   could be able to access the keying material used before their



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   joining, and thus could access past group communications.  Also,
   until the KDC performs a group rekeying, the newly leaving nodes
   would still be able to access upcoming group communications that are
   protected with the keying material that has not yet been updated.

7.1.  Update of 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.  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 keying
   material.  However, the recipient receives the message after having
   received the new keying material, hence not being able to correctly
   process it.  A possible way to ameliorate this issue is to preserve
   the old, recent, keying material for a maximum amount of time defined
   by the application.  By doing so, the recipient can still try to
   process the received message using the old retained keying material
   as second attempt.  Note that a former (compromised) group member can
   take advantage of this by sending messages protected with the old
   retained keying material.  Therefore, a conservative application
   policy should not admit the storage of old keying material.

   In the second case, the sender protects a message using the new
   keying material, but the recipient receives that request before
   having received the new keying material.  Therefore, the recipient
   would not be able to correctly process the request and hence discards
   it.  If the recipient receives the new keying material shortly after
   that and the sender endpoint uses CoAP 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 an updated
   keying material according to an application-defined policy, for
   instance after a given number of unsuccessfully decrypted incoming
   messages.

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

7.2.  Block-Wise Considerations

   If the block-wise 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 keying material to the updated one and
   continues the transfer.  As long as both sides get the new keying
   material, updating the keying material in the middle of a transfer



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   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 keying material is changed, the nodes might
   consume a larger amount of the network bandwidth resending the blocks
   again and again, which might be problematic.

8.  IANA Considerations

   This document has the following actions for IANA.

8.1.  ACE Authorization Server Request Creation Hints Registry

   IANA is asked to register the following entries in the "ACE
   Authorization Server Request Creation Hints" Registry defined in
   Section 8.1 of [I-D.ietf-ace-oauth-authz].

   o  Name: sign_info

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

   o  Value Type: any

   o  Reference: [[This specification]]

   o  Name: pub_key_enc

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

   o  Value Type: integer

   o  Reference: [[This specification]]

   o  Name: rsnonce

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

   o  Value Type: byte string

   o  Reference: [[This specification]]

8.2.  ACE Groupcomm Parameters Registry

   This specification establishes the "ACE Groupcomm Parameters" IANA
   Registry.  The Registry has been created to use the "Expert Review




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   Required" registration procedure [RFC8126].  Expert review guidelines
   are provided in Section 8.7.

   The columns of this Registry are:

   o  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.

   o  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.

   o  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.

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

   This Registry has been initially populated by the values in
   Section 6.  The Reference column for all of these entries refers to
   sections of this document.

8.3.  ACE Groupcomm Key Registry

   This specification establishes the "ACE Groupcomm Key" IANA Registry.
   The Registry has been created to use the "Expert Review Required"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 8.7.

   The columns of this Registry are:

   o  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.

   o  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 string.

   o  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 Profile"
      Registry.

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



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   o  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 4.
   The specification column for all of these entries will be this
   document.

8.4.  ACE Groupcomm Profile Registry

   This specification establishes the "ACE Groupcomm Profile" IANA
   Registry.  The Registry has been created to use the "Expert Review
   Required" registration procedure [RFC8126].  Expert review guidelines
   are provided in Section 8.7.  It should be noted that, in addition to
   the expert review, some portions of the Registry require a
   specification, potentially a Standards Track RFC, be supplied as
   well.

   The columns of this Registry are:

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

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

   o  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.

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

8.5.  ACE Groupcomm Policy Registry

   This specification establishes the "ACE Groupcomm Policy" IANA
   Registry.  The Registry has been created to use the "Expert Review
   Required" registration procedure [RFC8126].  Expert review guidelines
   are provided in Section 8.7.  It should be noted that, in addition to
   the expert review, some portions of the Registry require a
   specification, potentially a Standards Track RFC, be supplied as
   well.

   The columns of this Registry are:



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   o  Name: The name of the group communication policy.

   o  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 of 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.

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

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

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

   This registry will be initially populated by the values in Figure 5.

8.6.  Sequence Number Synchronization Method Registry

   This specification establishes the "Sequence Number Synchronization
   Method" IANA Registry.  The Registry has been created to use the
   "Expert Review Required" registration procedure [RFC8126].  Expert
   review guidelines are provided in Section 8.7.  It should be noted
   that, in addition to the expert review, some portions of the Registry
   require a specification, potentially a Standards Track RFC, be
   supplied as well.

   The columns of this Registry are:

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

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

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

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




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8.7.  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:

   o  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.

   o  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.

   o  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.

9.  References

9.1.  Normative References

   [I-D.ietf-ace-cwt-proof-of-possession]
              Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of-
              possession-11 (work in progress), October 2019.






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   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-25
              (work in progress), October 2019.

   [I-D.ietf-ace-oauth-params]
              Seitz, L., "Additional OAuth Parameters for Authorization
              in Constrained Environments (ACE)", draft-ietf-ace-oauth-
              params-05 (work in progress), March 2019.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Group OSCORE - Secure Group Communication for CoAP",
              draft-ietf-core-oscore-groupcomm-05 (work in progress),
              July 2019.

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

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

   [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>.

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

   [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>.

9.2.  Informative References

   [I-D.dijk-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              dijk-core-groupcomm-bis-01 (work in progress), July 2019.





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   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-dtls-
              authorize-08 (work in progress), April 2019.

   [I-D.ietf-ace-mqtt-tls-profile]
              Sengul, C., Kirby, A., and P. Fremantle, "MQTT-TLS profile
              of ACE", draft-ietf-ace-mqtt-tls-profile-01 (work in
              progress), October 2019.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE profile of the Authentication and Authorization
              for Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-08 (work in progress), July 2019.

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

   [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>.

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <https://www.rfc-editor.org/info/rfc7390>.

   [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>.




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   [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>.

   [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

   TODO: fix req numbers in the text.

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

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

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

   o  REQ3: Optionally, specify the acceptable values for 'sign_alg'
      (see Section 3.3).

   o  REQ4: Optionally, specify the acceptable values for
      'sign_parameters' (see Section 3.3).

   o  REQ5: Optionally, specify the acceptable values for
      'sign_key_parameters' (see Section 3.3).

   o  REQ6: Optionally, specify the acceptable values for 'pub_key_enc'
      (see Section 3.3).

   o  REQ7: Specify the exact format of the 'key' value (see
      Section 4.1.2.1).

   o  REQ8: Specify the acceptable values of 'kty' (see
      Section 4.1.2.1).

   o  REQ9: Specity the format of the identifiers of group members (see
      Section 4.1.2.1).



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   o  REQ10: Optionally, specify the format and content of
      'group_policies' entries (see Section 4.1.2.1).

   o  REQ11: Specify the communication protocol the members of the group
      must use (e.g., multicast CoAP).

   o  REQ12: 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.

   o  REQ13: Specify and register the application profile identifier
      (see Section 4.1.2.1).

   o  REQ14: Optionally, specify the encoding of public keys, of
      'client_cred', and of 'pub_keys' if COSE_Keys are not used (see
      Section 4.1.2.1).

   o  REQ15: Specify policies at the KDC to handle id that are not
      included in get_pub_keys (see Section 4.1.3.1).

   o  REQ16: Specify the format and content of 'group_policies' (see
      Section 4.1.2.1).

   o  REQ17: Specify the format of newly-generated individual keying
      material for group members, or of the information to derive it,
      and corresponding CBOR label (see Section 4.1.6.2).

   o  REQ18: Specify how the communication is secured between Client and
      KDC.  Optionally, specify tranport profile of ACE
      [I-D.ietf-ace-oauth-authz] to use between Client and KDC (see
      Section 4.2.

   o  OPT1: Optionally, specify the encoding of public keys, of
      'client_cred', and of 'pub_keys' if COSE_Keys are not used (see
      Section 4.1.2.1).

   o  OPT2: Optionally, specify the negotiation of parameter values for
      signature algorithm and signature keys, if 'sign_info' and
      'pub_key_enc' are not used (see Section 3.3).

   o  OPT3: Optionally, specify the format and content of
      'mgt_key_material' (see Section 4.1.2.1).

   o  OPT4: Optionally, specify policies that instruct clients to retain
      unsuccessfully decrypted messages and for how long, so that they
      can be decrypted after getting updated keying material (OPT4).





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Appendix B.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

B.1.  Version -02 to -03

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

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

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

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

   o  Added more profile requirements (Appendix A).

B.2.  Version -01 to -02

   o  Editorial fixes.

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

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

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

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

   o  Encoding of 'pub_keys_repos' (Section 4.1).

   o  Encoding of 'mgt_key_material' (Section 4.1).

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

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

   o  Extended security considerations (Sections 10.1 and 10.2).

   o  New "ACE Public Key Encoding" IANA Registry (Section 11.2).



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

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

   o  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).

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

B.3.  Version -00 to -01

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

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

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

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

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

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

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

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

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

   o  Extended security considerations (Section 10).

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

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




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Acknowledgments

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

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC; and by the EIT-Digital High Impact
   Initiative ACTIVE.

Authors' Addresses

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

   Email: francesca.palombini@ericsson.com


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

   Email: marco.tiloca@ri.se






















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