ACE Working Group                                           F. Palombini
Internet-Draft                                               Ericsson AB
Intended status: Standards Track                               M. Tiloca
Expires: 13 January 2022                                         RISE AB
                                                            12 July 2021


           Key Provisioning for Group Communication using ACE
                    draft-ietf-ace-key-groupcomm-13

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.

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."

   This Internet-Draft will expire on 13 January 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.



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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Authorization to Join a Group . . . . . . . . . . . . . . . .   8
     3.1.  Authorization Request . . . . . . . . . . . . . . . . . .   8
     3.2.  Authorization Response  . . . . . . . . . . . . . . . . .  10
     3.3.  Token Post  . . . . . . . . . . . . . . . . . . . . . . .  12
   4.  Keying Material Provisioning and Group Membership
           Management  . . . . . . . . . . . . . . . . . . . . . . .  15
     4.1.  Interface at the KDC  . . . . . . . . . . . . . . . . . .  17
     4.2.  Retrieval of Group Names and URIs . . . . . . . . . . . .  39
     4.3.  Joining Exchange  . . . . . . . . . . . . . . . . . . . .  40
     4.4.  Retrieval of Updated Keying Material  . . . . . . . . . .  42
     4.5.  Requesting a Change of Keying Material  . . . . . . . . .  44
     4.6.  Retrieval of Public Keys and Roles for Group Members  . .  45
     4.7.  Update of Public Key  . . . . . . . . . . . . . . . . . .  47
     4.8.  Retrieval of Group Policies . . . . . . . . . . . . . . .  48
     4.9.  Retrieval of Keying Material Version  . . . . . . . . . .  49
     4.10. Group Leaving Request . . . . . . . . . . . . . . . . . .  50
   5.  Removal of a Node from the Group  . . . . . . . . . . . . . .  50
   6.  Extended Scope Format . . . . . . . . . . . . . . . . . . . .  52
   7.  ACE Groupcomm Parameters  . . . . . . . . . . . . . . . . . .  54
   8.  ACE Groupcomm Error Identifiers . . . . . . . . . . . . . . .  55
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  56
     9.1.  Update of Keying Material . . . . . . . . . . . . . . . .  57
     9.2.  Block-Wise Considerations . . . . . . . . . . . . . . . .  58
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  58
     10.1.  Media Type Registrations . . . . . . . . . . . . . . . .  58
     10.2.  CoAP Content-Formats Registry  . . . . . . . . . . . . .  59
     10.3.  OAuth Parameters Registry  . . . . . . . . . . . . . . .  60
     10.4.  OAuth Parameters CBOR Mappings Registry  . . . . . . . .  60
     10.5.  ACE Groupcomm Parameters Registry  . . . . . . . . . . .  61
     10.6.  ACE Groupcomm Key Registry . . . . . . . . . . . . . . .  61
     10.7.  ACE Groupcomm Profile Registry . . . . . . . . . . . . .  62
     10.8.  ACE Groupcomm Policy Registry  . . . . . . . . . . . . .  63
     10.9.  Sequence Number Synchronization Method Registry  . . . .  63
     10.10. Interface Description (if=) Link Target Attribute Values
             Registry  . . . . . . . . . . . . . . . . . . . . . . .  64
     10.11. CBOR Tags Registry . . . . . . . . . . . . . . . . . . .  64
     10.12. ACE Scope Semantics  . . . . . . . . . . . . . . . . . .  65



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     10.13. ACE Groupcomm Errors . . . . . . . . . . . . . . . . . .  65
     10.14. Expert Review Instructions . . . . . . . . . . . . . . .  66
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  66
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  66
     11.2.  Informative References . . . . . . . . . . . . . . . . .  69
   Appendix A.  Requirements on Application Profiles . . . . . . . .  70
   Appendix B.  Extensibility for Future COSE Algorithms . . . . . .  73
     B.1.  Format of 'sign_info_entry' . . . . . . . . . . . . . . .  74
   Appendix C.  Document Updates . . . . . . . . . . . . . . . . . .  74
     C.1.  Version -04 to -05  . . . . . . . . . . . . . . . . . . .  74
     C.2.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  75
     C.3.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  75
     C.4.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  76
     C.5.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  76
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  77
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  77

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 [I-D.ietf-core-groupcomm-bis] or on publishing-subscribing
   [I-D.ietf-core-coap-pubsub].  The ACE framework is based on CBOR
   [RFC8949], 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 6.1 and 6.2 of [RFC8949].

   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 shown in Appendix A.

   If the application requires backward and forward security, new keying
   material is generated and distributed to the group upon membership
   changes.  A key management scheme performs the actual distribution of
   the new 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].










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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][I-D.ietf-cose-rfc8152bis-stru
   ct][I-D.ietf-cose-rfc8152bis-algs], such as Authorization Server (AS)
   and Resource Server (RS).

   This document uses names or identifiers for groups and nodes.  Their
   different meanings are summarized here:

   *  "Group name" is the invariant once established identifier of the
      group.  It is used in the communication between AS, RS and Client
      to identify the group.

   *  "GROUPNAME" is the invariant once established text string used in
      URIs.  GROUPNAME maps to the group name of a group, although it is
      not necessarily the same.

   *  "Group identifier" is the identifier of the group keying material.
      Opposite to group name and GROUPNAME, this identifier changes over
      time, when the keying material is updated.

   *  "Node name" is the invariant once established identifier of the
      node.  It is used in the communication between AS, RS and Client
      to identify a member of the group.

   *  "NODENAME" is the invariant once established text string used in
      URIs.  NODENAME is used to identify a node in a group.

   This document additionally uses the following terminology:

   *  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.  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].






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   *  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 the group communication.  It
      can request write and/or read rights.

   *  Authorization Server (AS): same as AS 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.

   *  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.



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   *  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:

   *  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).

   *  Allowing a group member to retrieve group keying material
      (Section 4.4 and Section 4.5).

   *  Allowing a group member to retrieve public keys of other group
      members (Section 4.6) and to provide an updated public key
      (Section 4.7).

   *  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 4.10 and
      Section 5).

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

   1.  The joining node requests an Access Token from the AS, in order
       to access a specific group-membership resource on the KDC and
       hence join the associated group.  This exchange between Client
       and AS MUST be secured, as specified by the transport profile of
       ACE used between Client and KDC.  The joining node will start or
       continue using a secure communication association with the KDC,
       according to the response from the AS.

   2.  The joining node transfers authentication and authorization
       information to the KDC, by posting the obtained Access Token to
       the /authz-info endpoint at the KDC.  This exchange, and all
       further communications between the Client and the KDC, MUST occur
       over the secure channel established as a result of the transport
       profile of ACE used between Client and KDC.  After that, a



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

   3.  The joining node starts the joining process to become a member of
       the group, by accessing the related group-membership resource at
       the KDC.  At the end of the joining process, the joining node has
       received from the KDC the parameters and keying material to
       securely communicate with the other members of the group, and 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.

               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






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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 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 message depends on the used transport profile of ACE.
   For example, this can be application/ace+cbor for the first two
   messages and application/cwt for the third message, which are defined
   in the ACE framework.  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 defined
   in Section 5.8.1 of [I-D.ietf-ace-oauth-authz] and MAY contain the
   following parameters, which, if included, MUST have the corresponding
   values:

   *  'scope', containing the identifier of the specific groups, or
      topics in the case of pub-sub, that the Client wishes to access,
      and optionally the roles that the Client wishes to take.

      This value is a CBOR byte string, wrapping a CBOR array of one or
      more entries.



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      By default, each entry is encoded as specified by
      [I-D.ietf-ace-aif].  The object identifier Toid corresponds to the
      group name and MUST be encoded as a tstr.  The permission set
      Tperm indicates the roles that the client wishes to take in the
      group.  It is up to the application profiles to define Tperm
      (REQ2) and register Toid and Tperm to fit the use case.  An
      example of scope using the AIF format is given in Figure 4.

      Otherwise, each scope entry can be defined as a CBOR array, which
      contains:

      -  As first element, the identifier of the specific group or
         topic, encoded as a tstr.

      -  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).

      In each entry, the encoding of the role identifiers is application
      specific, and part of the requirements for the application profile
      (REQ2).  In particular, the application profile may specify CBOR
      values to use for abbreviating role identifiers (OPT7).

      An example of CDDL definition [RFC8610] of scope using the format
      above, with group name and role identifiers encoded as text
      strings is given in Figure 5.

   *  'audience', with an identifier of a 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 CDLL definition of scope, using the default
                      Authorization Information Format

   gname = tstr

   role = tstr

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

   scope = << [ + scope_entry ] >>

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

3.2.  Authorization Response

   The Authorization Response sent from the AS to the Client is 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:

   *  '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:




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   *  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 6 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 6.

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

   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]).





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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].

   This request differs from the one defined in
   [I-D.ietf-ace-oauth-authz], because it allows to transport additional
   encoding information about the public keys in the group, used for
   source authentication, as well as any other group parameters.

   The joining node MAY ask for this information from the KDC in the
   same message it uses to POST the token to the RS.  In such a case,
   the message MUST have Content-Format set to application/ace+cbor
   defined in Section 8.16 of [I-D.ietf-ace-oauth-authz].  The message
   payload MUST be formatted as a CBOR map, which MUST include the
   access token.  The CBOR map MAY additionally include the following
   parameter, which, if included, MUST have the corresponding values:

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

   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.

   The KDC replies to the Client with a 2.01 (Created) response, using
   Content-Format "application/ace+cbor".

   The payload of the 2.01 response is a CBOR map.  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.  The Client uses this
   challenge to prove possession of its own private key (see the
   'client_cred_verify' parameter in Section 4).  Note that the payload
   format of the response deviates from the one defined in the ACE
   framework (see Section 5.10.1 of [I-D.ietf-ace-oauth-authz]), which
   has no payload.

   The KDC MUST store the 'kdcchallenge' value associated to the Client
   at least until it receives a join request from it (see Section 4.3),
   to be able to verify that the Client possesses its own private key.
   The same challenge MAY be reused several times by the Client, to
   generate a new proof of possession, e.g., in case of update of the



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   public key, or to join a different group with a different signing
   key, so it is RECOMMENDED that the KDC keeps storing the
   'kdcchallenge' after the first join is processed as well.  If the KDC
   has already discarded the 'kdcchallenge', that will trigger an error
   response with a newly generated 'kdcchallenge' that the Client can
   use to restart the join process, as specified in Section 4.3.

   If 'sign_info' is included in the request, the KDC MAY include the
   'sign_info' parameter defined in Section 3.3.1, with the same
   encoding.  Note that the field 'id' takes the value of the group name
   for which the 'sign_info_entry' applies to.

   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.  Application profiles MAY define
   the 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 (OPT1).

3.3.1.  'sign_info' Parameter

   The 'sign_info' parameter is an OPTIONAL parameter of the Token Post
   response message defined in Section 5.10.1. 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.

   When used in the request, the 'sign_info' encodes the CBOR simple
   value Null, to require information and parameters on the signature
   algorithm and on the public keys used.

   The CDDL notation [RFC8610] of the 'sign_info' parameter formatted as
   in the request is given below.

      sign_info_req = nil








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   The 'sign_info' parameter of the 2.01 (Created) response 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.  Each
   element contains information about signing parameters and keys for
   one or more group or topic, 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
      Null, 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 Null 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).

   The CDDL notation [RFC8610] of the 'sign_info' parameter formatted as
   in the response is given below.






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      sign_info_res = [ + sign_info_entry ]

      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
   considered 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 the Token
   Post response message defined in Section 5.10.1 of
   [I-D.ietf-ace-oauth-authz].  This parameter contains a challenge
   generated by the KDC and provided to the Client.  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).

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 the group policies or the group
   keying material.

   During the first exchange with the KDC ("Joining") after posting the
   Token, the Client sends a request to the KDC, specifying the group it
   wishes to join (see Section 4.3).  Then, the KDC verifies the access
   token and that the 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.

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





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   *  The Client can 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 4.4.

   *  The Client can retrieve new keying material for itself.  This is
      described in Section 4.5.

   *  The Client can get the public keys of other group members.  This
      is described in Section 4.6.

   *  The Client can upload a new, updated public key at the KDC.  This
      is described in Section 4.7.

   *  The Client can get the group policies.  This is described in
      Section 4.8.

   *  The Client can get the version number of the keying material
      currently used in the group.  This is described in Section 4.9.

   *  The Client can request to leave the group.  This is further
      discussed in Section 4.10.

   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 name
   associated 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 name, the KDC MUST respond to the Client with a
   4.01 (Unauthorized) error message.

   If they include a payload and specify a Content-Format, requests sent
   to the KDC and success responses from the KDC MUST have Content-
   Format set to application/ace-groupcomm+cbor, defined in
   Section 10.2.

   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 10.13 of this
      specification.  This field MUST be present.

   *  'error_description', with value a CBOR text string specifying a
      human-readable description of the error occurred at the KDC.  This
      field MAY be present.





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   CBOR labels for the 'error' and 'error_description' fields are
   defined in Section 7.

   Section 8 of this specification defines an initial set of error
   identifiers, as possible values for the 'error' field.  Application
   profiles of this specification MAY define additional value (OPT11).

4.1.  Interface at the KDC

   The KDC is configured with the following resources.  Note that the
   root url-path "ace-group" given here are default names:
   implementations are not required to use these names, and can define
   their own instead.  Each application profile of this specification
   MUST register a Resource Type for the root url-path (REQ7), and that
   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.  The Interface Description (if=) Link Target Attribute value
   ace.group is registered (Section 10.10) and can be used to describe
   this interface.

   *  /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, these applications will collide, and a mechanism will be
      needed to differentiate the endpoints.  This resource supports the
      FETCH method.

   *  /ace-group/GROUPNAME: one sub-resource to /ace-group is
      implemented for each group the KDC manages.

      If the value of the GROUPNAME URI path and the group name in the
      access token scope ('gname' in Section 3.2) do not match, the KDC
      MUST implement a mechanism to map the GROUPNAME value in the URI
      to the group name, in order to retrieve the right group (REQ1).
      Each resource contains the symmetric group keying material for
      that group.  These resources support the GET and POST methods.

   *  /ace-group/GROUPNAME/pub-key: this resource is invariant once
      established and contains the public keys of all group members.
      This resource supports the GET and FETCH methods.

   *  /ace-group/GROUPNAME/policies: this resource is invariant once
      established and contains the group policies.  This resource
      supports the GET method.






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   *  /ace-group/GROUPNAME/num: this resource is invariant once
      established and contains the version number for the symmetric
      group keying material.  This sub-resource supports the GET method.

   *  /ace-group/GROUPNAME/nodes/NODENAME: one sub-resource to /ace-
      group/GROUPNAME is implemented for each node in the group the KDC
      manages.  These resources are identified by the node name (in this
      example, the node name has value NODENAME).  Each resource
      contains the group and individual keying material for that node.
      These resources support the GET, PUT and DELETE methods.

   *  /ace-group/GROUPNAME/nodes/NODENAME/pub-key: one sub-resource to
      /ace-group/GROUPNAME/nodes/NODENAME is implemented for each node
      in the group the KDC manages.  These resources are identified by
      the node name (in this example, the node name has value NODENAME).
      Each resource contains the individual public keying material for
      that node.  These resources support the POST method.

   It is REQUIRED of the application profiles of this specification to
   define what operations (e.g., CoAP methods) are allowed on each
   resource, for each role defined in Section 3.1 according to REQ2
   (REQ8).

   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.

4.1.1.  ace-group

   This resource implements a FETCH handler.

4.1.1.1.  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 (REQ9).  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.




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   Then, the handler returns a 2.05 (Content) message response with
   payload 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.

   *  '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.

4.1.2.  ace-group/GROUPNAME

   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 GROUPNAME.  Note that the group
   joining exchange is done by the client via this operation, as
   described in Section 4.3.

   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:






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   *  'scope', with value the specific resource at the KDC that the
      Client is authorized to access, i.e., group or topic name, and
      role(s).  This value is a CBOR byte string wrapping one scope
      entry, as defined in Section 3.1.

   *  'get_pub_keys', if the Client wishes to receive the public keys of
      the other nodes in the group from the KDC.  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 the members of the group is
      known in another way, e.g., it was provided by the AS.  Note that
      including this parameter may result in a large message size for
      the following response, which can be inconvenient for resource-
      constrained devices.

      The parameter's value is either the CBOR simple value Null, or a
      non-empty CBOR array containing the following three elements.

      -  The first element, namely 'inclusion_flag', encodes the CBOR
         simple value True.

      -  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.
         The Client indicates that it wishes to receive the public keys
         of all 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.

      If the Client wishes to receive all public keys of all group
      members, it encodes the 'get_pub_key' parameter as the CBOR simple
      value Null.

      The CDDL definition [RFC8610] of 'get_pub_keys' is given in
      Figure 6, 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 the array of roles 'role_filter' is non-empty for this
      handler, but this is not necessarily the case for other handlers
      using this parameter: if this array is empty, it means that the
      client is not filtering public keys based on roles.




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      Also note that the array of node identifiers 'id_filter' is empty
      for this handler, because the joining node is not expected or
      capable to express a filter based on node identifiers at this
      point in time.  Consistently, the 'inclusion_flag' element is set
      to the CBOR simple value True.  However, the 'id_filter' array is
      not necessarily empty for the value of 'get_pub_keys' received by
      the handler of FETCH to ace-group/GROUPNAME/pub-key (see
      Section 4.1.3.1).

      Finally, the 'get_pub_keys' parameter MUST NOT have the arrays
      'role_filter' and 'id_filter' as both empty, i.e., in CBOR
      diagnostic notation: [ bool, [ ], [ ] ].  Thus, if this parameter
      is received as formatted in that way, it has to be considered
      malformed.

   id = bstr

   role = tstr

   comb_role = [ 2*role ]

   inclusion = bool

   get_pub_keys = null / [ [ inclusion, *(role / comb_role) ], [ *id ] ]

      Figure 6: 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, which wraps 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.

   *  '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.






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      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 POST 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 7.

      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 (REQ20).

      If the token was not posted (e.g., if it is used directly to
      validate TLS instead), it is REQUIRED of the specific profile to
      define how the challenge N_S is generated (REQ21).

   *  '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 (OPT3).

   *  'control_uri', with value a full URI, encoded as a CBOR text
      string.  If 'control_uri' is supported by the Client, 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
      individual provisioning of new keying material when performing a
      group rekeying (see Section 4.4), or to inform the Client of its
      removal from the group Section 5.  If the KDC does not implement



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      mechanisms using this resource, it can just ignore this parameter.
      Other additional functionalities of this resource MAY be defined
      in application profiles of this specifications (OPT9).  In
      particular, this resource is intended for communications
      concerning exclusively the group or topic specified in the 'scope'
      parameter.

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

   The handler extracts the granted scope from the access token, and
   checks the requested one against the token one.  If the requested one
   is not a subset of the token one, the KDC MUST respond with a 4.01
   (Unauthorized) error message.

   If the request does not include a 'scope' field, the KDC is expected
   to understand which group and role(s) the Client is requesting (e.g.,
   there is only one the Client has been granted).  If the KDC can not
   recognize which scope the Client is requesting, it MUST respond with
   a 4.00 (Bad Request) error message.

   The KDC verifies that the group name of the /ace-group/GROUPNAME path
   is a subset of the 'scope' stored in the access token associated to
   this client.  The KDC also verifies that the roles the client is
   granted in the group allow it to perform this operation on this
   resource (REQ8).  If either verification fails, the KDC MUST respond
   with a 4.01 (Unauthorized) error message.  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.






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   If the request is not formatted correctly (i.e., required fields non
   received or received with incorrect format), the handler MUST respond
   with a 4.00 (Bad Request) error message.  The 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 Null if the Client sent its own
   public key and the KDC is not set to store public keys of the group
   members.

   If the request contained unknown or non-expected fields present, the
   handler MUST silently drop them and continue processing.  Application
   profiles MAY define optional or mandatory payload formats for
   specific error cases (OPT5).

   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 posted but the KDC cannot retrieve the
      'kdcchallenge' associated to this Client (see Section 3.3), the
      KDC MUST respond 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, whose CBOR label is defined in
      Section 7.

   *  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 respond with a 4.00
      (Bad Request) error message.  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 (REQ20).






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      If the PoP evidence does not pass verification, the handler MUST
      respond with a 4.01 (Unauthorized) error message.  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 one public key is already associated to the received access
   token (see Section 4.3 for an example) and to the group identified by
   GROUPNAME.

   If an eligible public key for the Client is neither present in the
   'client_cred' field nor already stored, it is RECOMMENDED that the
   handler stops the processing and responds with a 4.00 (Bad Request)
   error message.  Applications profiles MAY define alternatives (OPT6).

   If all the verifications above succeed, 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 identifier NODENAME to the Client, and
      creates a sub-resource to /ace-group/GROUPNAME/ at the KDC (e.g.,
      "/ace-group/GROUPNAME/nodes/NODENAME").

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

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

      -  The handler associates the retrieved Client's public key to the
         node identifier NODENAME and to the 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.

   *  The handler returns a 2.01 (Created) response, 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.





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   The response message also contains the URI path to the sub-resource
   created for that node in a Location-Path CoAP option.  The response
   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" Registry.  Implementations MUST verify that the key
      type matches the application profile being used, if present, as
      registered in the "ACE Groupcomm Key" 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 (REQ23).

   The exact format of the 'key' value MUST be defined in applications
   of this specification (REQ10), as well as values of 'gkty' accepted
   by the application (REQ11).  Additionally, documents specifying the
   key format MUST register it in the "ACE Groupcomm Key" registry
   defined in Section 10.6, including its name, type and application
   profile to be used with.

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

                         Figure 8: 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 the corresponding values:



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   *  '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 Profile" Registry (REQ15).

   *  '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, which wraps 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.

   *  '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 wrapping 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 Policy" Registry.  This
      specification defines the three elements "Sequence Number
      Synchronization Method", "Key Update Check Interval" and
      "Expiration Delta", which are summarized in Figure 9.  Application
      profiles that build on this document MUST specify the exact
      content format and default value of included map entries (REQ17).




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  +--------------+-------+----------|---------------------|------------+
  |      Name    | CBOR  |   CBOR   |    Description      | Reference  |
  |              | label |   type   |                     |            |
  |--------------+-------+----------|---------------------|------------|
  | Sequence     | TBD1  | tstr/int | Method for a re-    | [[this     |
  | Number       |       |          | cipient node to     | document]] |
  | Synchroniza- |       |          | synchronize with    |            |
  | tion Method  |       |          | 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        |       |          | in seconds, to      | document]] |
  | Interval     |       |          | check for new       |            |
  |              |       |          | keying material at  |            |
  |              |       |          | the KDC             |            |
  |              |       |          |                     |            |
  | Expiration   | TBD3  |   uint   | Number of seconds   | [[this     |
  | Delta        |       |          | from 'exp' until    | document]] |
  |              |       |          | the specified UTC   |            |
  |              |       |          | date/time after     |            |
  |              |       |          | which group members |            |
  |              |       |          | MUST stop using the |            |
  |              |       |          | keying material to  |            |
  |              |       |          | verify incoming     |            |
  |              |       |          | messages.           |            |
  +--------------+-------+----------|---------------------|------------+

                     Figure 9: ACE Groupcomm Policies

   *  '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 application profile MUST
      define if this field is used, and if used then MUST specify the
      exact format and content which depend on the specific rekeying
      scheme used in the group.  If the usage of 'mgt_key_material' is
      indicated and its format defined for a specific key management
      scheme, that format must explicitly indicate the key management
      scheme itself.  If a new rekeying scheme is defined to be used for
      an existing 'mgt_key_material' in an existing profile, then that
      profile will have to be updated accordingly, especially with
      respect to the usage of 'mgt_key_material' related format and
      content (REQ22).



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   Specific application profiles that build on this document MUST
   specify the communication protocol that members of the group use to
   communicate with each other (REQ13) and how exactly the keying
   material is used to protect the group communication (REQ14).

   CBOR labels for these fields are defined in Section 7.

4.1.2.2.  GET Handler

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

   The handler expects a GET request.

   The KDC verifies that the group name of the /ace-group/GROUPNAME path
   is a subset of the 'scope' stored in the access token associated to
   this client.  The KDC also verifies that the roles the client is
   granted in the group allow it to perform this operation on this
   resource (REQ8).  If either verification fails, the KDC MUST respond
   with a 4.01 (Unauthorized) error message.  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.

   Additionally, the handler verifies that the node is a current member
   of the group.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.  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 verification succeeds, the handler returns a 2.05 (Content)
   message 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 4.1.2.1.

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

4.1.3.  ace-group/GROUPNAME/pub-key

   If the KDC does not maintain public keys for the group, the handler
   for any request on this resource returns a 4.05 (Method Not Allowed)
   error message.  If it does, the rest of this section applies.

   This resource implements GET and FETCH handlers.




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4.1.3.1.  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 fields:

   *  'get_pub_keys', whose value is encoded as in Section 4.1.2.1 with
      the following modification:

      -  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'.
         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'.

      -  The array 'role_filter' may 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' parameter encodes the CBOR simple value
         True or False, respectively.  The array may be empty, if the
         Client does not wish to filter the requested public keys based
         on the node identifiers of the group members.

   Note that, in case both the 'role_filter' array and the 'id_filter'
   array are not 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'.





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   Finally, as mentioned in Section 4.1.2.1, both arrays 'role_filter'
   and 'id_filter' MUST NOT be both empty.

   The specific format of public keys as well as identifiers, roles and
   combination of roles of group members MUST be specified by the
   application profile (OPT1, REQ2, REQ12).

   The KDC verifies that the group name of the /ace-group/GROUPNAME path
   is a subset of the 'scope' stored in the access token associated to
   this client.  The KDC also verifies that the roles the client is
   granted in the group allow it to perform this operation on this
   resource (REQ8).  If either verification fails, the KDC MUST respond
   with a 4.01 (Unauthorized) error message.

   If verification succeeds, 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.

   Then, the handler returns a 2.05 (Content) message response with
   payload formatted as a CBOR map, containing only the following
   parameters from Section 4.1.2.1.

   *  '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, REQ12).

   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.




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   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 (REQ16).

   *  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.

   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.

4.1.3.2.  GET Handler

   The handler expects a GET request.

   The KDC performs the same verifications as the FETCH handler in
   Section 4.1.3.1, and if successful returns the same response as in
   Section 4.1.3.1 but without filtering based on roles or node
   identifiers: all the group members' public keys are returned.

   Note that this resource handler, as the FETCH handler for the same
   resource, 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.

4.1.4.  ace-group/GROUPNAME/policies

   This resource implements a GET handler.

4.1.4.1.  GET Handler

   The handler expects a GET request.

   The KDC verifies that the group name of the /ace-group/GROUPNAME path
   is a subset of the 'scope' stored in the access token associated to
   this client.  The KDC also verifies that the roles the client is
   granted in the group allow it to perform this operation on this
   resource (REQ8).  If either verification fails, the KDC MUST respond
   with a 4.01 (Unauthorized) error message.



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   Additionally, the handler verifies that the node is a current member
   of the group.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.  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 verification succeeds, the handler returns a 2.05 (Content)
   message 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 4.1.2.1 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 (REQ17).

4.1.5.  ace-group/GROUPNAME/num

   This resource implements a GET handler.

4.1.5.1.  GET Handler

   The handler expects a GET request.

   The KDC verifies that the group name of the /ace-group/GROUPNAME path
   is a subset of the 'scope' stored in the access token associated to
   this client.  The KDC also verifies that the roles the client is
   granted in the group allow it to perform this operation on this
   resource (REQ8).  If either verification fails, the KDC MUST respond
   with a 4.01 (Unauthorized) error message.

   Additionally, the handler verifies that the node is a current member
   of the group.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.  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 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, 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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4.1.6.  ace-group/GROUPNAME/nodes/NODENAME

   This resource implements GET, PUT and DELETE handlers.

4.1.6.1.  PUT Handler

   The PUT handler is used to get the KDC to produce and return
   individual keying material to protect outgoing messages for the node
   (identified by NODENAME) for the group identified by GROUPNAME.
   Application profiles MAY also use this handler to rekey the whole
   group.  It is up to the application profiles to specify if this
   handler supports renewal of individual keying material, renewal of
   the group keying material or both (OPT8).

   The handler expects a request with empty payload.  In case the
   request has a non-empty payload, the KDC MUST respond with a 4.00
   (Bad Request) error message.

   The KDC verifies that the group name of the /ace-group/GROUPNAME path
   is a subset of the 'scope' stored in the access token associated to
   the client identified by NODENAME.  The KDC also verifies that the
   roles the client is granted in the group allow it to perform this
   operation on this resource (REQ8).  If either verification fails, the
   KDC MUST respond with a 4.01 (Unauthorized) error message.

   The handler also verifies that the node sending the request and the
   node name used in the Uri-Path match.  If that is not the case, the
   handler responds with a 4.01 (Unauthorized) error response.

   Additionally, the handler verifies that the node is a current member
   of the group.  If the verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.  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").

   Also, 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 respond with a 4.00 (Bad Request) error message.
   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 is currently not able to serve this request, i.e., to
   generate new individual keying material for the requesting client,
   the KDC MUST respond with a 5.03 (Service Unavailable) error message.
   The response MUST have Content-Format set to application/ace-



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   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 returns a 2.05 (Content)
   message containing newly-generated keying material for the Client,
   and/or, if the application profiles requires it (OPT8), starts the
   complete group rekeying.  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 (REQ18) and registered in Section 10.5.

4.1.6.2.  GET Handler

   The handler expects a GET request.

   The KDC verifies that the group name of the /ace-group/GROUPNAME path
   is a subset of the 'scope' stored in the access token associated to
   the client identified by NODENAME.  The KDC also verifies that the
   roles the client is granted in the group allow it to perform this
   operation on this resource (REQ8).  If either verification fails, the
   KDC MUST respond with a 4.01 (Unauthorized) error message.

   The handler also verifies that the node sending the request and the
   node name used in the Uri-Path match.  If that is not the case, the
   handler responds with a 4.01 (Unauthorized) error response.

   Additionally, the handler verifies that the node is a current member
   of the group.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.  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 verification succeeds, the handler returns a 2.05 (Content)
   message 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 4.1.2.2.  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 (REQ18) and registered in Section 10.5.

   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



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   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).

4.1.6.3.  DELETE Handler

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

   The handler expects a request with method DELETE (and empty payload).

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

   The handler also verifies that the node sending the request and the
   node name used in the Uri-Path match.  If that is not the case, the
   handler responds with a 4.01 (Unauthorized) error response.

   Additionally, the handler verifies that the node is a current member
   of the group.  If verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.  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 verification succeeds, the handler removes the client from the
   group identified by GROUPNAME.  That includes removing the public key
   of the client if the KDC keep tracks of that, and possibly removing
   the evicted node from the list of observers of the resource at ace-
   group/GROUPNAME (if observable).  Then, the handler deletes the sub-
   resource nodes/NODENAME and returns a 2.02 (Deleted) message with
   empty payload.





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

   This resource implements a POST handler, if the KDC stores the public
   key of group members.  If the KDC does not store the public keys of
   group members, the handler does not implement any method, and every
   request returns a 4.05 Method Not Allowed error.

4.1.7.1.  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 4.1.2.1, 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 4.1.2.1 and defined by
   the specific application profile (REQ20), 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).

   The handler verifies that the group name GROUPNAME is a subset of the
   'scope' stored in the access token associated to the client
   identified by NODENAME.  The KDC also verifies that the roles the
   client is granted in the group allow it to perform this operation on
   this resource (REQ8).  If either verification fails, the KDC MUST
   respond with a 4.01 (Unauthorized) error message.

   The handler also verifies that the node sending the request and the
   node name used in the Uri-Path match.  If that is not the case, the
   handler responds with a 4.01 (Unauthorized) error response.

   Additionally, the handler verifies that the node is a current member
   of the group.  If the verification fails, the KDC MUST respond with a
   4.01 (Unauthorized) error message.  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").

   Also, 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 respond with a 4.00 (Bad Request) error message.
   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 request is not formatted correctly (i.e., required fields non
   received or received with incorrect format), the handler MUST respond
   with a 4.00 (Bad Request) error message.  If the request contains
   unknown or non-expected fields present, the handler MUST silently
   ignore them and continue processing.  Application profiles MAY define
   optional or mandatory payload formats for specific error cases
   (OPT5).

   If the KDC cannot retrieve the 'kdcchallenge' associated to this
   Client (see Section 3.3), the KDC MUST respond 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, whose CBOR label is
   defined in Section 7.  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.

   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 respond with a 4.00 (Bad
   Request) error message.  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 4.1.2.1 and defined by the specific application
   profile (REQ20).  If the PoP evidence does not pass verification, the
   handler MUST respond with a 4.01 (Unauthorized) error message.  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 verification succeeds, 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.





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   *  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.

4.2.  Retrieval of Group Names and URIs

   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 4.1.1.1.

   Figure 10 gives an overview of the exchanges described above, and
   Figure 11 shows an example.

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

      Figure 10: Message Flow of Group Name and URI Retrieval Request-
                                  Response

            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"] }



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    Figure 11: Example of Group Name and URI Retrieval Request-Response

4.3.  Joining Exchange

   Figure 12 gives an overview of the Joining exchange between Client
   and KDC, when the Client first joins a group, while Figure 13 shows
   an example.

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

        Figure 12: Message Flow of First Exchange for Group Joining

 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 13: Example 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 (REQ19).  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 4.1.2.1.
   This group name is the same as in the scope entry corresponding to
   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
   group 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 ("client_cred" and "client_cred_verify" in Section 4.1.2.1).
   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
   with the same access token and the same public key, it can omit to
   send the public key and the PoP evidence, or just omit the PoP
   evidence, and the KDC will be able to retrieve its public key
   associated to its token for that group (if the key has been
   discarded, the KDC will reply with 4.00 Bad Request, as specified in
   Section 4.1.2.1).  If a node re-joins a group but wants to update its
   own public key, it needs to send both its public key and the PoP
   evidence.













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   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 response defined in
   Section 4.1.2.2.  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).  The KDC MUST increment the version number of the
   current keying material, before distributing the newly generated
   keying material to the group.  After that, the KDC SHOULD store the
   distributed keying material in persistent storage.

4.4.  Retrieval of Updated 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 4.1.6.2.

   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 (OPT4).  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.



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   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 14 gives an overview of the exchange described above, while
   Figure 15 shows an example.

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

        Figure 14: 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 15: Example of Key Distribution Request-Response

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

   *  Can make the ace-group/GROUPNAME resource Observable [RFC7641],
      and send notifications to observer Clients when the keying
      material is updated.




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      In case the KDC deletes the group identified by GROUPNAME, 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").

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

   *  Can send unicast POST requests to each Client over a secure
      channel, with the same payload as the Key Distribution Response.
      When sending such requests, the KDC can target the URI path
      provided by the intended recipient upon joining the group, as
      specified in the 'control_uri' parameter of the Joining Request
      (see Section 4.1.2.1).

   *  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.

4.5.  Requesting a Change of Keying Material

   Beside possible expiration, the client may need to communicate to the
   KDC its need for the keying material to be renewed, e.g., due to
   exhaustion of AEAD nonces, if AEAD is used for protecting group
   communication.  Depending on the application profile (OPT8), this can
   result in renewal of individual keying material, group keying
   material, or both.

   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, 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 4.1.6.2.





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   Figure 16 gives an overview of the exchange described above, while
   Figure 17 shows an example.

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

          Figure 16: 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 }

             Figure 17: Example 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.

4.6.  Retrieval of Public Keys and Roles 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 and roles of
   either all group members or a specified subset, by sending a CoAP GET
   or 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 4.1.3.2 and Section 4.1.3.1.



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   Figure 18 and Figure 20 give an overview of the exchanges described
   above, while Figure 19 and Figure 21 show an example for each
   exchange.

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

       Figure 18: Message Flow of Public Key Exchange to Request All
                            Members Public Keys

     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 19: Example of Public Key Exchange to Request All Members
                                Public Keys

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

         Figure 20: Message Flow of Public Key Exchange to Request
                        Specific Members Public Keys






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             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 21: Example of Public Key Exchange to Request Specific
                            Members Public Keys

4.7.  Update of 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.

   The request is formatted as specified in Section 4.1.7.1.

   Figure Figure 22 gives an overview of the exchange described above,
   while Figure 23 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 22: Message Flow of Public Key Update Request-Response



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        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 23: Example of Public Key Update Request-Response

   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 group member updating its own public
   key.  To this end, the KDC uses the message format of the response
   defined in Section 4.1.2.2.  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).  The KDC MUST increment the
   version number of the current keying material, before distributing
   the newly generated keying material to the group.  After that, the
   KDC SHOULD store the distributed keying material in persistent
   storage.

   Additionally, after updating its own public key, a group member MAY
   send a number of the later requests including an identifier of the
   updated public key, to signal nodes that they need to retrieve it.
   How that is done depends on the group communication protocol used,
   and therefore is application profile specific (OPT10).

4.8.  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/GROUPNAME/
   policies endpoint at the KDC, where GROUPNAME is the group name, and
   formatted as defined in Section 4.1.4.1




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   Figure 24 gives an overview of the exchange described above, while
   Figure 25 shows an example.

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

            Figure 24: Message Flow of Policies Request-Response

             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 25: Example of Policies Request-Response

4.9.  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/GROUPNAME/num endpoint at the
   KDC, where GROUPNAME is the group name, 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, before it's
   distributed to the group members.

   Figure 26 gives an overview of the exchange described above, while
   Figure 27 shows an example.









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

            Figure 26: 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: text/plain
                   Payload(in CBOR diagnostic notation):
                     13

               Figure 27: Example of Version Request-Response

4.10.  Group Leaving Request

   A node 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 4.1.6.3

   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 4.4).  Application
   profiles may define alternative message formats.  The KDC MUST



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   increment the version number of the current keying material, before
   distributing the newly generated keying material to the group.  After
   that, the KDC SHOULD store the distributed keying material in
   persistent storage.

   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 request to re-
   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 4.10.

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

   3.  The node's authorization to be a group member is not valid
       anymore, either because the access token has expired, or it 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 node is still authorized
       for that group in that role.

       In 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.

   Furthermore, in case of forced eviction, the KDC removes the public
   key of the evicted node if the KDC keep tracks of that, and possibly
   removes the evicted node from the list of observers of the resource
   at ace-group/GROUPNAME (if observable).

   Then, the KDC deletes the sub-resource ace-group/GROUPNAME/nodes/
   NODENAME associated to the evicted node.  After that, the KDC MAY
   explicitly inform the evicted node, by means of the following
   methods.

   *  If the evicted node implements the 'control_uri' resource
      specified in Section 4.1.2.1, the KDC sends a DELETE request,
      targeting the URI specified in the 'control_uri' parameter of the
      Joining Request (see Section 4.1.2.1).






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   *  If the evicted node is observing its associated sub-resource at
      ace-group/GROUPNAME/nodes/NODENAME (see Section 4.1.6.2), the KDC
      sends an unsolicited 4.04 (Not Found) 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").

      Consistently, the KDC also removes the node's entry from the list
      of observers of the sub-resource.

6.  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 10.12 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 (REQ24).

   *  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 10.11 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 28 and Figure 29 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 28: 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 29: CDLL definition of scope, using as example group name
                      encoded as tstr and role as tstr

7.  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.

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

   +=======================+======+================+==================+
   | Name                  | CBOR | CBOR Type      | Reference        |
   |                       | Key  |                |                  |
   +=======================+======+================+==================+
   | error                 | TBD  | int            | Section 4        |
   +-----------------------+------+----------------+------------------+
   | error_description     | TBD  | text string    | Section 4        |
   +-----------------------+------+----------------+------------------+
   | scope                 | TBD  | byte string    | Section 4.1.2.1  |
   +-----------------------+------+----------------+------------------+
   | get_pub_keys          | TBD  | array / simple | Section 4.1.2.1, |
   |                       |      | value null     | Section 4.1.3.1  |
   +-----------------------+------+----------------+------------------+
   | 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  |
   +-----------------------+------+----------------+------------------+



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   | pub_keys_repos        | TBD  | text string    | Section 4.1.2.1  |
   +-----------------------+------+----------------+------------------+
   | control_uri           | TBD  | text string    | Section 4.1.2.1  |
   +-----------------------+------+----------------+------------------+
   | gkty                  | TBD  | integer / text | Section 4.1.2.1  |
   |                       |      | string         |                  |
   +-----------------------+------+----------------+------------------+
   | key                   | TBD  | see "ACE       | Section 4.1.2.1  |
   |                       |      | Groupcomm Key" |                  |
   |                       |      | Registry       |                  |
   +-----------------------+------+----------------+------------------+
   | num                   | TBD  | int            | Section 4.1.2.1  |
   +-----------------------+------+----------------+------------------+
   | ace-groupcomm-profile | TBD  | int            | Section 4.1.2.1  |
   +-----------------------+------+----------------+------------------+
   | exp                   | TBD  | int            | Section 4.1.2.1  |
   +-----------------------+------+----------------+------------------+
   | pub_keys              | TBD  | array          | Section 4.1.2.1  |
   +-----------------------+------+----------------+------------------+
   | peer_roles            | TBD  | array          | Section 4.1.2.1  |
   +-----------------------+------+----------------+------------------+
   | peer_identifiers      | TBD  | array          | 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  |
   +-----------------------+------+----------------+------------------+
   | sign_info             | TBD  | array          | Section 4.1.2.1  |
   +-----------------------+------+----------------+------------------+
   | gid                   | TBD  | array          | Section 4.1.1.1  |
   +-----------------------+------+----------------+------------------+
   | gname                 | TBD  | array of text  | Section 4.1.1.1  |
   |                       |      | strings        |                  |
   +-----------------------+------+----------------+------------------+
   | guri                  | TBD  | array of text  | Section 4.1.1.1  |
   |                       |      | strings        |                  |
   +-----------------------+------+----------------+------------------+
   | kdcchallenge          | TBD  | byte string    | Section 4.1.7.1  |
   +-----------------------+------+----------------+------------------+

                                 Table 1

8.  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                                        |
     +-------+------------------------------------------------------+

                                 Table 2

9.  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].  In case the Client rebooted and lost the
   security state used to protect previous communication with that
   sender, such a mechanism is useful for the recipient to be on the
   safe side.

   Besides, if the KDC has renewed the group keying material, and the
   time interval between the end of the rekeying process and the joining
   of the Client is sufficiently small, that Client is also on the safe
   side, since replayed older messages protected with the previous
   keying material will not be accepted.

   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 should renew the group keying material after rebooting, even
   in the case where all keying material is stored in persistent
   storage.  However, if the KDC relies on Observe responses to notify
   the group of renewed keying material, after rebooting the KDC will
   have lost all the current ongoing Observations with the group
   members, and the previous keying material will be used to protect
   messages in the group anyway.  The KDC will rely on each node



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   requesting updates of the group keying material to establish the new
   keying material in the nodes, or, if implemented, it can push the
   update to the nodes in the group using the 'control_uri' resource.

   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.  The KDC may rekey the
   group after a minimum number of group members have joined or left
   within a given time interval, or after maximum amount of time since
   the last rekeying was completed, or yet during predictable network
   inactivity periods.

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

   The KDC needs to have a mechanism in place to detect DoS attacks from
   nodes constantly initiating rekey events (for example by updating
   their public key), such as removing these nodes from the group.

   The KDC also needs to have a congestion control mechanism in place to
   avoid network congestion when the KDC renews the group keying
   material; CoAP and Observe give guidance on such mechanisms, see
   Section 4.7 of [RFC7252] and Section 4.5.1 of [RFC7641].

9.1.  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.
   Note that a former (compromised) group member can take advantage of



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   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 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 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.

9.2.  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
   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.

10.  IANA Considerations

   This document has the following actions for IANA.

10.1.  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].




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   Type name: application

   Subtype name: ace-groupcomm+cbor

   Required parameters: N/A

   Optional parameters: N/A

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

   Security considerations: See Section 9 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

10.2.  CoAP Content-Formats Registry

   This specification registers the following entry to the "CoAP
   Content-Formats" registry, within the "CoRE Parameters" registry:

   Media Type: application/ace-groupcomm+cbor

   Encoding: -

   ID: TBD




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   Reference: [this document]

10.3.  OAuth Parameters Registry

   The following registrations are done for the OAuth Parameters
   Registry following the procedure specified in Section 11.2 of
   [RFC6749]:

   *  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]]

10.4.  OAuth Parameters CBOR Mappings Registry

   The following registrations are done for 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]]




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10.5.  ACE Groupcomm Parameters Registry

   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 10.14.

   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 7.  The Reference column for all of these entries refers to
   sections of this document.

10.6.  ACE Groupcomm Key Registry

   This specification establishes the "ACE Groupcomm Key" IANA Registry.
   The Registry has been created to use the "Expert Review" registration
   procedure [RFC8126].  Expert review guidelines are provided in
   Section 10.14.

   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.







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   *  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.

   *  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 8.
   The specification column for all of these entries will be this
   document.

10.7.  ACE Groupcomm Profile Registry

   This specification establishes the "ACE Groupcomm Profile" IANA
   Registry.  The Registry has been created to use the "Expert Review"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 10.14.  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.

   *  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.







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10.8.  ACE Groupcomm Policy Registry

   This specification establishes the "ACE Groupcomm Policy" IANA
   Registry.  The Registry has been created to use the "Expert Review"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 10.14.  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.

   This registry will be initially populated by the values in Figure 9.

10.9.  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" registration procedure [RFC8126].  Expert review
   guidelines are provided in Section 10.14.  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:




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   *  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.

10.10.  Interface Description (if=) Link Target Attribute Values
        Registry

   This specification registers the following entry to the "Interface
   Description (if=) Link Target Attribute Values Registry" registry,
   within the "CoRE Parameters" registry:

   *  Attribute Value: ace.group

   *  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]

10.11.  CBOR Tags Registry

   This specification registers the following entry to 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]











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10.12.  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 10.14.  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.

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

10.13.  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 10.14.  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.




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   *  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 8.  The Reference column for all of these entries refers to
   this document.

10.14.  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.

   *  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.

11.  References

11.1.  Normative References





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   [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-43, 10 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
              authz-43.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-12, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-core-oscore-
              groupcomm-12.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>.




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   [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>.

   [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>.




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   [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>.

11.2.  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-12, 11 May 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-mqtt-tls-
              profile-12.txt>.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE Profile of the Authentication and Authorization
              for Constrained Environments Framework", Work in Progress,
              Internet-Draft, draft-ietf-ace-oscore-profile-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-
              04, 12 July 2021, <https://www.ietf.org/archive/id/
              draft-ietf-core-groupcomm-bis-04.txt>.




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   [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>.

   [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.




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   *  REQ1: If the value of the GROUPNAME URI path and the group name in
      the access token scope (gname in Section 3.2) don't match, specify
      the mechanism to map the GROUPNAME value in the URI to the group
      name (REQ1) (see Section 4.1).

   *  REQ2: Specify the encoding and value of roles, for scope entries
      of 'scope' (see Section 3.1).

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

   *  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: 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).

   *  REQ8: Define what operations (e.g., CoAP methods) are allowed on
      each resource, for each role defined in REQ2 (see Section 3.3).

   *  REQ9: Specify the exact encoding of group identifier (see
      Section 4.1.1.1).

   *  REQ10: Specify the exact format of the 'key' value (see
      Section 4.1.2.1).

   *  REQ11: Specify the acceptable values of 'gkty' (see
      Section 4.1.2.1).

   *  REQ12: Specify the format of the identifiers of group members (see
      Section 4.1.2.1).

   *  REQ13: Specify the communication protocol the members of the group
      must use (e.g., multicast CoAP).

   *  REQ14: 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.





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   *  REQ15: Specify and register the application profile identifier
      (see Section 4.1.2.1).

   *  REQ16: Specify policies at the KDC to handle ids that are not
      included in 'get_pub_keys' (see Section 4.1.3.1).

   *  REQ17: If used, specify the format and content of 'group_policies'
      and its entries.  Specify the policies default values (see
      Section 4.1.2.1).

   *  REQ18: 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).

   *  REQ19: 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 4.3.

   *  REQ20: Specify the exact approaches used to compute and verify the
      PoP evidence to include in 'client_cred_verify', and which of
      those approaches is used in which case (see Section 4.1.2.1).

   *  REQ21: Specify how the nonce N_S is generated, if the token was
      not posted (e.g., if it is used directly to validate TLS instead).

   *  REQ22: Specify if 'mgt_key_material' used, and if yes specify its
      format and content (see Section 4.1.2.1).  If the usage of
      'mgt_key_material' is indicated and its format defined for a
      specific key management scheme, that format must explicitly
      indicate the key management scheme itself.  If a new rekeying
      scheme is defined to be used for an existing 'mgt_key_material' in
      an existing profile, then that profile will have to be updated
      accordingly, especially with respect to the usage of
      'mgt_key_material' related format and content.

   *  REQ23: Define the initial value of the 'num' parameter (see
      Section 4.1.2.1).

   *  REQ24: Specify and register the identifier of newly defined
      semantics for binary scopes (see Section 6).

   *  OPT1: Optionally, specify the negotiation of parameter values for
      signature algorithm and signature keys, if 'sign_info' is not used
      (see Section 3.3).

   *  OPT2: Optionally, specify the additional parameters used in the
      Token Post exchange (see Section 3.3).



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   *  OPT3: Optionally, specify the encoding of 'pub_keys_repos' if the
      default is not used (see Section 4.1.2.1).

   *  OPT4: Optionally, specify policies that instruct clients to retain
      messages and for how long, if they are unsuccessfully decrypted
      (see Section 4.4).  This makes it possible to decrypt such
      messages after getting updated keying material.

   *  OPT5: Optionally, specify possible or required payload formats for
      specific error cases.

   *  OPT6: Optionally, specify the behavior of the handler in case of
      failure to retrieve a public key for the specific node (see
      Section 4.1.2.1).

   *  OPT7: Optionally, specify CBOR values to use for abbreviating
      identifiers of roles in the group or topic (see Section 3.1).

   *  OPT8: 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 4.5).

   *  OPT9: Optionally, specify the functionalities implemented at the
      'control_uri' resource hosted at the Client, including message
      exchange encoding and other details (see Section 4.1.2.1).

   *  OPT10: Optionally, specify how the identifier of the sender's
      public key is included in the group request (see Section 4.7).

   *  OPT11: Optionally, specify additional identifiers of error types,
      as values of the 'error' field in an error response from the KDC.

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 Post 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.




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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].

      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 30: 'sign_info_entry' with general format

Appendix C.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

C.1.  Version -04 to -05

   *  Updated uppercase/lowercase URI segments for KDC resources.

   *  Supporting single Access Token for multiple groups/topics.

   *  Added 'control_uri' parameter in the Joining Request.

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




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   *  Clarification on stopping using owned keying material.

   *  Clarification on different reasons for processing failures,
      related policies, and requirement OPT4.

   *  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 OPT8.

   *  Editorial improvements.

C.2.  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.3.  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).

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

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




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   *  Added more profile requirements (Appendix A).

C.4.  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).

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

C.5.  Version -00 to -01





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   *  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, 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; by the H2020 project SIFIS-Home
   (Grant agreement 952652); and by the EIT-Digital High Impact
   Initiative ACTIVE.

Authors' Addresses





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   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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