Key Management for OSCORE Groups in ACE
draft-ietf-ace-key-groupcomm-oscore-10

Document Type Active Internet-Draft (ace WG)
Authors Marco Tiloca  , Jiye Park  , Francesca Palombini 
Last updated 2021-02-22
Replaces draft-tiloca-ace-oscoap-joining
Stream Internet Engineering Task Force (IETF)
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ACE Working Group                                              M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                                 J. Park
Expires: August 26, 2021                     Universitaet Duisburg-Essen
                                                            F. Palombini
                                                             Ericsson AB
                                                       February 22, 2021

                Key Management for OSCORE Groups in ACE
                 draft-ietf-ace-key-groupcomm-oscore-10

Abstract

   This specification defines an application profile of the ACE
   framework for Authentication and Authorization, to request and
   provision keying material in group communication scenarios that are
   based on CoAP and secured with Group Object Security for Constrained
   RESTful Environments (OSCORE).  This application profile delegates
   the authentication and authorization of Clients that join an OSCORE
   group through a Resource Server acting as Group Manager for that
   group.  This application profile leverages protocol-specific
   transport profiles of ACE to achieve communication security, server
   authentication and proof-of-possession for a key owned by the Client
   and bound to an OAuth 2.0 Access Token.

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
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   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 August 26, 2021.

Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Overview of the Joining Process . . . . . . . . . . . . .   6
     2.2.  Overview of the Group Rekeying Process  . . . . . . . . .   6
   3.  Format of Scope . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Joining Node to Authorization Server  . . . . . . . . . . . .   8
     4.1.  Authorization Request . . . . . . . . . . . . . . . . . .   9
     4.2.  Authorization Response  . . . . . . . . . . . . . . . . .   9
   5.  Interface at the Group Manager  . . . . . . . . . . . . . . .  10
     5.1.  ace-group/GROUPNAME/active  . . . . . . . . . . . . . . .  10
       5.1.1.  GET Handler . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Admitted Methods  . . . . . . . . . . . . . . . . . . . .  11
   6.  Token POST and Group Joining  . . . . . . . . . . . . . . . .  12
     6.1.  Token Post  . . . . . . . . . . . . . . . . . . . . . . .  13
       6.1.1.  'ecdh_info' Parameter . . . . . . . . . . . . . . . .  14
     6.2.  Sending the Joining Request . . . . . . . . . . . . . . .  16
       6.2.1.  Value of the N_S Challenge  . . . . . . . . . . . . .  17
     6.3.  Processing the Joining Request  . . . . . . . . . . . . .  18
     6.4.  Joining Response  . . . . . . . . . . . . . . . . . . . .  20
   7.  Public Keys of Joining Nodes  . . . . . . . . . . . . . . . .  24
   8.  Retrieval of Updated Keying Material  . . . . . . . . . . . .  26
     8.1.  Retrieval of Group Keying Material  . . . . . . . . . . .  26
     8.2.  Retrieval of Group Keying Material and Sender ID  . . . .  27
   9.  Requesting a Change of Keying Material  . . . . . . . . . . .  27
   10. Retrieval of Public Keys and Roles for Group Members  . . . .  29
   11. Update of Public Key  . . . . . . . . . . . . . . . . . . . .  29
   12. Retrieval of Group Policies . . . . . . . . . . . . . . . . .  30
   13. Retrieval of Keying Material Version  . . . . . . . . . . . .  31
   14. Retrieval of Group Status . . . . . . . . . . . . . . . . . .  31
   15. Retrieval of Group Names and URIs . . . . . . . . . . . . . .  32
   16. Request to Leave the Group  . . . . . . . . . . . . . . . . .  34
   17. Removal of a Group Member . . . . . . . . . . . . . . . . . .  34
   18. Group Rekeying Process  . . . . . . . . . . . . . . . . . . .  35
   19. Default Values for Group Configuration Parameters . . . . . .  37
   20. Security Considerations . . . . . . . . . . . . . . . . . . .  40

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     20.1.  Management of OSCORE Groups  . . . . . . . . . . . . . .  40
     20.2.  Size of Nonces for Signature Challenge . . . . . . . . .  41
     20.3.  Reusage of Nonces for Signature Challenge  . . . . . . .  42
   21. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  42
     21.1.  ACE Groupcomm Parameters Registry  . . . . . . . . . . .  43
     21.2.  ACE Groupcomm Key Registry . . . . . . . . . . . . . . .  43
     21.3.  ACE Groupcomm Profile Registry . . . . . . . . . . . . .  43
     21.4.  OSCORE Security Context Parameters Registry  . . . . . .  44
     21.5.  TLS Exporter Label Registry  . . . . . . . . . . . . . .  46
     21.6.  AIF Registry . . . . . . . . . . . . . . . . . . . . . .  46
     21.7.  Media Type Registrations . . . . . . . . . . . . . . . .  47
     21.8.  CoAP Content-Format Registry . . . . . . . . . . . . . .  48
     21.9.  Group OSCORE Roles Registry  . . . . . . . . . . . . . .  48
     21.10. CoRE Resource Type Registry  . . . . . . . . . . . . . .  49
     21.11. ACE Scope Semantics Registry . . . . . . . . . . . . . .  49
     21.12. ACE Groupcomm Errors . . . . . . . . . . . . . . . . . .  49
     21.13. Expert Review Instructions . . . . . . . . . . . . . . .  49
   22. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     22.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     22.2.  Informative References . . . . . . . . . . . . . . . . .  53
     22.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  54
   Appendix A.  Profile Requirements . . . . . . . . . . . . . . . .  54
   Appendix B.  Document Updates . . . . . . . . . . . . . . . . . .  58
     B.1.  Version -09 to -10  . . . . . . . . . . . . . . . . . . .  58
     B.2.  Version -08 to -09  . . . . . . . . . . . . . . . . . . .  58
     B.3.  Version -07 to -08  . . . . . . . . . . . . . . . . . . .  59
     B.4.  Version -06 to -07  . . . . . . . . . . . . . . . . . . .  59
     B.5.  Version -05 to -06  . . . . . . . . . . . . . . . . . . .  59
     B.6.  Version -04 to -05  . . . . . . . . . . . . . . . . . . .  60
     B.7.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  61
     B.8.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  61
     B.9.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  62
     B.10. Version -00 to -01  . . . . . . . . . . . . . . . . . . .  62
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  63
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  63

1.  Introduction

   Object Security for Constrained RESTful Environments (OSCORE)
   [RFC8613] is a method for application-layer protection of the
   Constrained Application Protocol (CoAP) [RFC7252], using CBOR Object
   Signing and Encryption (COSE)
   [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and
   enabling end-to-end security of CoAP payload and options.

   As described in [I-D.ietf-core-oscore-groupcomm], Group OSCORE is
   used to protect CoAP group communication over IP multicast
   [I-D.ietf-core-groupcomm-bis].  This relies on a Group Manager, which

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   is responsible for managing an OSCORE group and enables the group
   members to exchange CoAP messages secured with Group OSCORE.  The
   Group Manager can be responsible for multiple groups, coordinates the
   joining process of new group members, and is entrusted with the
   distribution and renewal of group keying material.

   This specification is an application profile of
   [I-D.ietf-ace-key-groupcomm], which itself builds on the ACE
   framework for Authentication and Authorization
   [I-D.ietf-ace-oauth-authz].  Message exchanges among the participants
   as well as message formats and processing follow what specified in
   [I-D.ietf-ace-key-groupcomm] for provisioning and renewing keying
   material in group communication scenarios, where Group OSCORE is used
   to protect CoAP group communication over IP multicast.

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:

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

   o  The terms and concept related to the message formats and
      processing specified in [I-D.ietf-ace-key-groupcomm], for
      provisioning and renewing keying material in group communication
      scenarios.

   o  The terms and concepts described in CBOR [RFC8949] and COSE
      [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs].

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

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   o  The terms and concepts for protection and processing of CoAP
      messages through OSCORE [RFC8613] and through Group OSCORE
      [I-D.ietf-core-oscore-groupcomm] in group communication scenarios.
      These include the concept of Group Manager, as the entity
      responsible for a set of groups where communications are secured
      with Group OSCORE.  In this specification, the Group Manager acts
      as Resource Server.

   Additionally, this document makes use of the following terminology.

   o  Requester: member of an OSCORE group that sends request messages
      to other members of the group.

   o  Responder: member of an OSCORE group that receives request
      messages from other members of the group.  A responder may reply
      back, by sending a response message to the requester which has
      sent the request message.

   o  Monitor: member of an OSCORE group that is configured as responder
      and never replies back to requesters after receiving request
      messages.  This corresponds to the term "silent server" used in
      [I-D.ietf-core-oscore-groupcomm].

   o  Signature verifier: entity external to the OSCORE group and
      intended to verify the countersignature of messages exchanged in
      the group.  An authorized signature verifier does not join the
      OSCORE group as an actual member, yet it can retrieve the public
      keys of the current group members from the Group Manager.

2.  Protocol Overview

   Group communication for CoAP over IP multicast has been enabled in
   [I-D.ietf-core-groupcomm-bis] and can be secured with Group Object
   Security for Constrained RESTful Environments (OSCORE) [RFC8613] as
   described in [I-D.ietf-core-oscore-groupcomm].  A network node joins
   an OSCORE group by interacting with the responsible Group Manager.
   Once registered in the group, the new node can securely exchange
   messages with other group members.

   This specification describes how to use [I-D.ietf-ace-key-groupcomm]
   and [I-D.ietf-ace-oauth-authz] to perform a number of authentication,
   authorization and key distribution actions, as defined in Section 2
   of [I-D.ietf-ace-key-groupcomm], for an OSCORE group.

   With reference to [I-D.ietf-ace-key-groupcomm]:

   o  The node wishing to join the OSCORE group, i.e. the joining node,
      is the Client.

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   o  The Group Manager is the Key Distribution Center (KDC), acting as
      a Resource Server.

   o  The Authorization Server associated to the Group Manager is the
      AS.

   All communications between the involved entities MUST be secured.

   In particular, communications between the Client and the Group
   Manager leverage protocol-specific transport profiles of ACE to
   achieve communication security, proof-of-possession and server
   authentication.  It is expected that, in the commonly referred base-
   case of this specification, the transport profile to use is pre-
   configured and well-known to nodes participating in constrained
   applications.

   Appendix A lists the specifications on this application profile of
   ACE, based on the requirements defined in Appendix A of
   [I-D.ietf-ace-key-groupcomm].

2.1.  Overview of the Joining Process

   A node performs the steps described in Section 4.3 of
   [I-D.ietf-ace-key-groupcomm] in order to join an OSCORE group.  The
   format and processing of messages exchanged among the participants
   are further specified in Section 4 and Section 6 of this document.

2.2.  Overview of the Group Rekeying Process

   If the application requires backward and forward security, the Group
   Manager MUST generate new keying material and distribute it to the
   group (rekeying) upon membership changes.

   That is, the group is rekeyed when a node joins the group as a new
   member, or after a current member leaves the group.  By doing so, a
   joining node cannot access communications in the group prior its
   joining, while a leaving node cannot access communications in the
   group after its leaving.

   The keying material distributed through a group rekeying MUST
   include:

   o  A new Group Identifier (Gid) for the group as introduced in
      [I-D.ietf-ace-key-groupcomm], used as ID Context parameter of the
      Group OSCORE Common Security Context of that group (see Section 2
      of [I-D.ietf-core-oscore-groupcomm]).

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      Note that the Gid differs from the group name also introduced in
      [I-D.ietf-ace-key-groupcomm], which is a plain, stable and
      invariant identifier, with no cryptographic relevance and meaning.

   o  A new value for the Master Secret parameter of the Group OSCORE
      Common Security Context of that group (see Section 2 of
      [I-D.ietf-core-oscore-groupcomm]).

   Also, the distributed keying material MAY include a new value for the
   Master Salt parameter of the Group OSCORE Common Security Context of
   that group.

   Upon generating the new group keying material and before starting its
   distribution, the Group Manager MUST increment the version number of
   the group keying material.  When rekeying a group, the Group Manager
   MUST preserve the current value of the Sender ID of each member in
   that group.

   The Group Manager MUST support the Group Rekeying Process described
   in Section 18.  Future application profiles may define alternative
   message formats and distribution schemes to perform group rekeying.

3.  Format of Scope

   Building on Section 3.1 of [I-D.ietf-ace-key-groupcomm], this section
   defines the exact format and encoding of scope to use.

   To this end, this profile uses the Authorization Information Format
   (AIF) [I-D.ietf-ace-aif], and defines the following AIF specific data
   model AIF-OSCORE-GROUPCOMM.

   With reference to the generic AIF model

      AIF-Generic<Toid, Tperm> = [* [Toid, Tperm]]

   the value of the CBOR byte string used as scope encodes the CBOR
   array [* [Toid, Tperm]], where each [Toid, Tperm] element corresponds
   to one scope entry.

   Then, for each scope entry:

   o  the object identifier ("Toid") is specialized as a CBOR text
      string, specifying the group name for the scope entry;

   o  the permission set ("Tperm") is specialized as a CBOR unsigned
      integer with value R, specifying the role(s) that the client
      wishes to take in the group (REQ2).  The value R is computed as
      follows:

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      *  each role in the permission set is converted into the
         corresponding numeric identifier X from the "Value" column of
         the table in Figure 1.

      *  the set of N numbers is converted into the single value R, by
         taking each numeric identifier X_1, X_2, ..., X_N to the power
         of two, and then computing the inclusive OR of the binary
         representations of all the power values.

 +-----------+-------+-------------------------------------------------+
 | Name      | Value | Description                                     |
 +===========+=======+=================================================+
 | Reserved  | 0     | This value is reserved                          |
 |-----------+-------+-------------------------------------------------+
 | Requester | 1     | Send requests; receive responses                |
 |-----------+-------+-------------------------------------------------+
 | Responder | 2     | Send responses; receive requests                |
 +-----------+-------+-------------------------------------------------+
 | Monitor   | 3     | Receive requests; never send requests/responses |
 |-----------+-------+-------------------------------------------------|
 | Verifier  | 4     | Verify countersignature of intercepted messages |
 +-----------+-------+-------------------------------------------------+

         Figure 1: Numeric identifier of roles in the OSCORE group

   The CDDL [RFC8610] definition of the AIF-OSCORE-GROUPCOMM data model
   is as follows:

      AIF-OSCORE-GROUPCOMM = AIF_Generic<path, permissions>

      path = tstr  ; Group name
      permissions = uint . bits roles
      roles = &(
         Requester: 1,
         Responder: 2,
         Monitor: 3,
         Verifier: 4
      )

   Future specifications that define new roles MUST register a
   corresponding numeric identifier in the "Group OSCORE Roles" Registry
   defined in Section 21.9 of this specification.

4.  Joining Node to Authorization Server

   This section describes how the joining node interacts with the AS in
   order to be authorized to join an OSCORE group under a given Group

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   Manager.  In particular, it considers a joining node that intends to
   contact that Group Manager for the first time.

   The message exchange between the joining node and the AS consists of
   the messages Authorization Request and Authorization Response defined
   in Section 3 of [I-D.ietf-ace-key-groupcomm].  Note that what is
   defined in [I-D.ietf-ace-key-groupcomm] applies, and only additions
   or modifications to that specification are defined here.

4.1.  Authorization Request

   The Authorization Request message is as defined in Section 3.1 of
   [I-D.ietf-ace-key-groupcomm], with the following additions.

   o  If the 'scope' parameter is present:

      *  The value of the CBOR byte string encodes a CBOR array, whose
         format MUST follow the data model AIF-OSCORE-GROUPCOMM defined
         in Section 3.  In particular, for each OSCORE group to join:

         +  The group name is encoded as a CBOR text string.

         +  The set of requested roles is expressed as a single CBOR
            unsigned integer, computed as defined in Section 3 (REQ2)
            from the numerical abbreviations defined in Figure 1 for
            each requested role (OPT8).

4.2.  Authorization Response

   The Authorization Response message is as defined in Section 3.2 of
   [I-D.ietf-ace-key-groupcomm], with the following additions:

   o  The AS MUST include the 'expires_in' parameter.  Other means for
      the AS to specify the lifetime of Access Tokens are out of the
      scope of this specification.

   o  The AS MUST include the 'scope' parameter, when the value included
      in the Access Token differs from the one specified by the joining
      node in the request.  In such a case, the second element of each
      scope entry MUST be present, and specifies the set of roles that
      the joining node is actually authorized to take in the OSCORE
      group for that scope entry, encoded as specified in Section 4.1.

   Furthermore, if the AS uses the extended format of scope defined in
   Section 6 of [I-D.ietf-ace-key-groupcomm] for the 'scope' claim of
   the Access Token, the first element of the CBOR sequence [RFC8742]
   MUST be the CBOR integer with value SEM_ID_TBD, defined in
   Section 21.11 of this specification (REQ23).  This indicates that the

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   second element of the CBOR sequence, as conveying the actual access
   control information, follows the scope semantics defined for this
   application profile in Section 3 of this specification.

5.  Interface at the Group Manager

   The Group Manager provides the interface defined in Section 4.1 of
   [I-D.ietf-ace-key-groupcomm], with one additional sub-resource
   defined in Section 5.1 of this specification.

   Furthermore, Section 5.2 provides a summary of the CoAP methods
   admitted to access different resources at the Group Manager, for
   nodes with different roles in the group or as non members (REQ8).

   The GROUPNAME segment of the URI path MUST match with the group name
   specified in the scope entry of the Access Token scope (i.e. 'gname'
   in Section 3.1 of [I-D.ietf-ace-key-groupcomm]) (REQ1).

   The Resource Type (rt=) Link Target Attribute value "core.osc.gm" is
   registered in Section 21.10 (REQ7), and can be used to describe
   group-membership resources and its sub-resources at a Group Manager,
   e.g. by using a link-format document [RFC6690].

   Applications can use this common resource type to discover links to
   group-membership resources for joining OSCORE groups, e.g. by using
   the approach described in [I-D.tiloca-core-oscore-discovery].

5.1.  ace-group/GROUPNAME/active

   This resource implements a GET handler.

5.1.1.  GET Handler

   The handler expects a GET request.

   The Group Manager verifies that the group name in the /ace-
   group/GROUPNAME/active path is a subset of the 'scope' stored in the
   Access Token associated to the requesting client.

   The Group Manager also verifies that the roles granted to the
   requesting client in the group allow it to perform this operation on
   this resource (REQ8).  If either verification fails, the Group
   Manager MUST respond with a 4.01 (Unauthorized) error message.

   Additionally, the handler verifies that the requesting client is a
   current member of the group.  If verification fails, the Group
   Manager MUST respond with a 4.01 (Unauthorized) 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 of
   [I-D.ietf-ace-key-groupcomm].  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 CBOR simple value True if the group is
   currently active, or the CBOR simple value False otherwise.  The
   group is considered active if it is set to allow new members to join,
   and if communication within the group is fine to happen.

   The method to set the current group status, i.e. active or inactive,
   is out of the scope of this specification, and is defined for the
   administrator interface of the Group Manager specified in
   [I-D.ietf-ace-oscore-gm-admin].

5.2.  Admitted Methods

   The table in Figure 2 summarizes the CoAP methods admitted to access
   different resources at the Group Manager, for (non-)members of a
   group with group name GROUPNAME, and considering different roles.
   The last two rows of the table apply to a node with node name
   NODENAME.

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    +------------------------------+--------+-------+-------+-------+
    | Resource                     | Type1  | Type2 | Type3 | Type4 |
    +------------------------------+--------+-------+-------+-------+
    | ace-group/                   | F      | F     | F     | -     |
    +------------------------------+--------+-------+-------+-------+
    | ace-group/GROUPNAME/         | G Po   | G Po  | Po *  | Po    |
    +------------------------------+--------+-------+-------+-------+
    | ace-group/GROUPNAME/active   | G      | G     | -     | -     |
    +------------------------------+--------+-------+-------+-------+
    | ace-group/GROUPNAME/pub-key  | G F    | G F   | G F   | -     |
    +------------------------------+--------+-------+-------+-------+
    | ace-group/GROUPNAME/policies | G      | G     | -     | -     |
    +------------------------------+--------+-------+-------+-------+
    | ace-group/GROUPNAME/num      | G      | G     | -     | -     |
    +------------------------------+--------+-------+-------+-------+
    | ace-group/GROUPNAME/nodes/   | G Pu D | G D   | -     | -     |
    |           NODENAME           |        |       |       |       |
    +------------------------------+--------+-------+-------+-------+
    | ace-group/GROUPNAME/nodes/   | Po     | -     | -     | -     |
    |           NODENAME/pub-key   |        |       |       |       |
    +------------------------------+--------+-------+-------+-------+

    Type1 = Member as Requester and/or Responder        |  G  = GET
    Type2 = Member as Monitor                           |  F  = FETCH
    Type3 = Non-member / Authorized to be Verifier      |  Po = POST
            (*) = cannot join the group as Verifier     |  Pu = PUT
    Type4 = Non-member / Not authorized to be Verifier  |  D  = DELETE

      Figure 2: Admitted CoAP Methods on the Group Manager Resources

6.  Token POST and Group Joining

   The rest of this section describes the interactions between the
   joining node and the Group Manager, i.e. the sending of the Access
   Token and the Request-Response exchange to join the OSCORE group.
   The message exchange between the joining node and the Group Manager
   consists of the messages defined in Section 3.3 and 4.3 of
   [I-D.ietf-ace-key-groupcomm].  Note that what is defined in
   [I-D.ietf-ace-key-groupcomm] applies, and only additions or
   modifications to that specification are defined here.

   A signature verifier provides the Group Manager with an Access Token,
   as described in Section 6.1, just as any another joining node does.
   However, unlike candidate group members, it does not join any OSCORE
   group, i.e. it does not perform the joining process defined in
   Section 6.2.  After successfully posting an Access Token, a signature
   verifier is authorized to perform only the operations specified in
   Section 10, to retrieve the public keys of group members, and only

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   for the OSCORE groups specified in the validated Access Token.  The
   Group Manager MUST respond with a 4.01 (Unauthorized) error message,
   in case a signature verifier attempts to access any other endpoint
   than /ace-group/GROUPNAME/pub-key at the Group Manager.

6.1.  Token Post

   The Token post exchange is defined in Section 3.3 of
   [I-D.ietf-ace-key-groupcomm].

   Additionally to what defined in [I-D.ietf-ace-key-groupcomm], the
   following applies.

   o  The CoAP POST request MAY additionally contain the following
      parameter, which, if included, MUST have the corresponding values:

      *  'ecdh_info' defined in Section 6.1.1, encoding the CBOR simple
         value Null to require information on the ECDH algorithm, the
         ECDH algorithm parameters, the ECDH key parameters and on the
         exact encoding of public keys used in the group, in case the
         joining node supports the pairwise mode of Group OSCORE
         [I-D.ietf-core-oscore-groupcomm].

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

   o  The 'kdcchallenge' parameter contains a dedicated nonce N_S
      generated by the Group Manager.  For the N_S value, it is
      RECOMMENDED to use a 8-byte long random nonce.  The joining node
      can use this nonce in order to prove the possession of its own
      private key, upon joining the group (see Section 6.2).

      The 'kdcchallenge' parameter MAY be omitted from the 2.01
      (Created) response, if the 'scope' of the Access Token specifies
      only the role "monitor" or only the role "verifier" or both of
      them, for each and every of the specified groups.

   o  If the 'sign_info' parameter is present in the response, the
      following applies for each element 'sign_info_entry'.

      *  'sign_alg' takes value from the "Value" column of the "COSE
         Algorithms" Registry [COSE.Algorithms].

      *  'sign_parameters' is a CBOR array including the following two
         elements:

         +  'sign_alg_capab', encoded as a CBOR array.  Its format and
            value are the same of the COSE capabilities for the

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            algorithm indicated in 'sign_alg', as specified for that
            algorithm in the "Capabilities" column of the "COSE
            Algorithms" Registry [COSE.Algorithms] (REQ4).

         +  'sign_key_type_capab', encoded as a CBOR array.  Its format
            and value are the same of the COSE capabilities for the COSE
            key type of the keys used with the algorithm indicated in
            'sign_alg', as specified for that key type in the
            "Capabilities" column of the "COSE Key Types" Registry
            [COSE.Key.Types] (REQ4).

      *  'sign_key_parameters' is a CBOR array.  Its format and value
         are the same of the COSE capabilities for the COSE key type of
         the keys used with the algorithm indicated in 'sign_alg', as
         specified for that key type in the "Capabilities" column of the
         "COSE Key Types" Registry [COSE.Key.Types] (REQ5).

      *  'pub_key_enc' takes value 1 ("COSE_Key") from the 'Confirmation
         Key' column of the "CWT Confirmation Method" Registry
         [CWT.Confirmation.Methods], so indicating that public keys in
         the OSCORE group are encoded as COSE Keys
         [I-D.ietf-cose-rfc8152bis-struct].  Future specifications may
         define additional values for this parameter.

   o  If 'ecdh_info' is included in the request, the Group Manager MAY
      include the 'ecdh_info' parameter defined in Section 6.1.1, with
      the same encoding.  Note that the field 'id' takes as value the
      group name, or array of group names, for which the corresponding
      'ecdh_info_entry' applies to.

   Note that, other than through the above parameters as defined in
   Section 3.3 of [I-D.ietf-ace-key-groupcomm], the joining node MAY
   have previously retrieved this information by other means, e.g. by
   using the approach described in [I-D.tiloca-core-oscore-discovery] to
   discover the OSCORE group and the link to the associated group-
   membership resource at the Group Manager (OPT2).

   Additionally, if allowed by the used transport profile of ACE, the
   joining node may instead provide the Access Token to the Group
   Manager by other means, e.g. during a secure session establishment
   (see Section 3.3.2 of [I-D.ietf-ace-dtls-authorize]).

6.1.1.  'ecdh_info' Parameter

   The 'ecdh_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].

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   This parameter is used to require and retrieve from the Group Manager
   information and parameters about the ECDH algorithm and about the
   public keys to be used in the OSCORE group to compute a static-static
   Diffie-Hellman shared secret [NIST-800-56A], in case the group
   supports the pairwise mode of Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].

   When used in the request, the 'ecdh_info' parameter encodes the CBOR
   simple value Null, to require information and parameters on the ECDH
   algorithm and on the public keys to be used to compute Diffie-Hellman
   shared secrets in the OSCORE group.

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

      ecdh_info_req = nil

   The 'ecdh_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 OSCORE groups the client has been authorized to join.

   Each element contains information about ECDH parameters and about
   public keys, for one or more OSCORE groups that support the pairwise
   mode of Group OSCORE and that the client has been authorized to join.
   Each element is formatted as follows.

   o  The first element 'id' is the group name of the OSCORE group or an
      array of group names for the OSCORE groups for which the specified
      information applies.

   o  The second element 'ecdh_alg' is an CBOR integer or a CBOR text
      string indicating the ECDH algorithm used in the OSCORE group
      identified by 'gname'.  Values are taken from the "Value" column
      of the "COSE Algorithms" Registry [COSE.Algorithms].

   o  The third element 'ecdh_parameters' is a CBOR array indicating the
      parameters of the ECDH algorithm used in the OSCORE group
      identified by 'gname'.  The CBOR array includes the following two
      elements, and its exact content depends on the value of the
      'ecdh_alg' element.

      *  'ecdh_alg_capab', encoded as a CBOR array.  Its format and
         value are the same of the COSE capabilities for the algorithm
         indicated in 'ecdh_alg', as specified for that algorithm in the
         "Capabilities" column of the "COSE Algorithms" Registry
         [COSE.Algorithms].

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      *  'ecdh_key_type_capab', encoded as a CBOR array.  Its format and
         value are the same of the COSE capabilities for the COSE key
         type of the keys used with the algorithm indicated in
         'ecdh_alg', as specified for that key type in the
         "Capabilities" column of the "COSE Key Types" Registry
         [COSE.Key.Types].

   o  The fourth element 'ecdh_key_parameters' is a CBOR array
      indicating the parameters of the keys used with the ECDH algorithm
      in the OSCORE group identified by 'gname'.  Its content depends on
      the value of 'ecdh_alg'.  In particular, its format and value are
      the same of the COSE capabilities for the COSE key type of the
      keys used with the algorithm indicated in 'ecdh_alg', as specified
      for that key type in the "Capabilities" column of the "COSE Key
      Types" Registry [COSE.Key.Types].

   o  The fifth element 'pub_key_enc' is CBOR integer indicating the
      encoding of public keys used in the OSCORE group identified by
      'gname'.  It takes value 1 ("COSE_Key") from the 'Confirmation
      Key' column of the "CWT Confirmation Method" Registry
      [CWT.Confirmation.Methods], so indicating that public keys in the
      OSCORE group are encoded as COSE Keys
      [I-D.ietf-cose-rfc8152bis-struct].  Future specifications may
      define additional values for this parameter.

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

      ecdh_info_res = [ + ecdh_info_entry ]

      ecdh_info_entry =
      [
        id : gname / [ + gname ],
        ecdh_alg : int / tstr,
        ecdh_parameters : [ any ],
        ecdh_key_parameters : [ any ],
        pub_key_enc = int
      ]

      gname = tstr

6.2.  Sending the Joining Request

   The joining node requests to join the OSCORE group by sending a
   Joining Request message to the related group-membership resource at
   the Group Manager, as per Section 4.3 of
   [I-D.ietf-ace-key-groupcomm].

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   Additionally to what defined in [I-D.ietf-ace-key-groupcomm], the
   following applies.

   o  The 'scope' parameter MUST be included.  Its value encodes one
      scope entry with the format defined in Section 3, indicating the
      group name and the role(s) that the joining node wants to take in
      the group.

   o  The 'get_pub_keys' parameter is present only if the joining node
      wants to retrieve the public keys of the group members from the
      Group Manager during the joining process (see Section 7).
      Otherwise, this parameter MUST NOT be present.

      If this parameter is present and its value is non-null, each
      element of the inner CBOR array 'role_filter' is encoded as a CBOR
      unsigned integer, with the same value of a permission set
      ("Tperm") indicating that role or combination of roles in a scope
      entry, as defined in Section 3.

   o  'cnonce' contains a dedicated nonce N_C generated by the joining
      node.  For the N_C value, it is RECOMMENDED to use a 8-byte long
      random nonce.

   o  The signature encoded in the 'client_cred_verify' parameter is
      computed by the joining node by using the same private key and
      countersignature algorithm it intends to use for signing messages
      in the OSCORE group.  Moreover, N_S is as defined in
      Section 6.2.1.

6.2.1.  Value of the N_S Challenge

   The value of the N_S challenge is determined as follows.

   1.  If the joining node has posted the Access Token to the /authz-
       info endpoint of the Group Manager as in Section 6.1, N_S takes
       the same value of the most recent 'kdcchallenge' parameter
       received by the joining node from the Group Manager.  This can be
       either the one specified in the 2.01 (Created) response to the
       Token POST, or the one possibly specified in a 4.00 (Bad Request)
       response to a following Joining Request (see Section 6.3).

   2.  If the Token posting has relied on the DTLS profile of ACE
       [I-D.ietf-ace-dtls-authorize] with the Access Token as content of
       the "psk_identity" field of the ClientKeyExchange message
       [RFC6347], N_S is an exporter value computed as defined in
       Section 7.5 of [RFC8446].  Specifically, N_S is exported from the
       DTLS session between the joining node and the Group Manager,
       using an empty 'context_value', 32 bytes as 'key_length', and the

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       exporter label "EXPORTER-ACE-Sign-Challenge-coap-group-oscore-
       app" defined in Section 21.5 of this specification.

   It is up to applications to define how N_S is computed in further
   alternative settings.

   Section 20.3 provides security considerations on the reusage of the
   N_S challenge.

6.3.  Processing the Joining Request

   The Group Manager processes the Joining Request as defined in
   Section 4.1.2.1 of [I-D.ietf-ace-key-groupcomm].  Additionally, the
   following applies.

   o  The Group Manager MUST return a 5.03 (Service Unavailable)
      response in case the OSCORE group that the joining node has been
      trying to join is currently inactive (see Section 5.1).  The
      response MUST have Content-Format set to application/ace-
      groupcomm+cbor and is formatted as defined in Section 4 of
      [I-D.ietf-ace-key-groupcomm].  The value of the 'error' field MUST
      be set to 7 ("Group currently not active").

   o  In case the joining node is not going to join the group
      exclusively as monitor and the Joining Request does not include
      the 'client_cred' parameter, the joining process fails if the
      Group Manager either: i) does not store a public key with an
      accepted format for the joining node; or ii) stores multiple
      public keys with an accepted format for the joining node.

   o  To compute the signature contained in 'client_cred_verify', the
      Group Manager considers:

      *  as signed value, the value of the 'scope' parameter from the
         Joining Request as a CBOR byte string, concatenated with N_S
         encoded as a CBOR byte string, concatenated with N_C encoded as
         a CBOR byte string.  In particular, N_S is determined as
         described in Section 6.2.1, while N_C is the nonce provided in
         the 'cnonce' parameter of the Joining Request;

      *  the countersignature algorithm used in the OSCORE group, and
         possible corresponding parameters;

      *  the public key of the joining node, either retrieved from the
         'client_cred' parameter, or already stored as acquired from
         previous interactions with the joining node.

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   o  A 4.00 (Bad Request) response from the Group Manager to the
      joining node MUST have content format application/ace+cbor.  The
      response payload is a CBOR map which MUST contain the 'sign_info'
      parameter, including a single element 'sign_info_entry' pertaining
      to the OSCORE group that the joining node has tried to join with
      the Joining Request.  If the group supports the pairwise mode of
      Group OSCORE, the CBOR map MUST contain also the 'ecdh_info'
      parameter, including a single element 'ecdh_info_entry' pertaining
      to the OSCORE group that the joining node has tried to join with
      the Joining Request.

   o  The Group Manager MUST return a 4.00 (Bad Request) response in
      case the 'scope' parameter is not present in the Joining Request,
      or if it is present and specifies any set of roles not included in
      the following list: "requester", "responder", "monitor",
      ("requester", "responder").  Future specifications that define a
      new role MUST define possible sets of roles including the new one
      and existing ones, that are acceptable to specify in the 'scope'
      parameter of a Joining Request.

   o  The Group Manager MUST return a 4.00 (Bad Request) response in
      case the Joining Request includes the 'client_cred' parameter but
      does not include both the 'cnonce' and 'client_cred_verify'
      parameters.

   o  The Group Manager MUST return a 4.00 (Bad Request) response in
      case it cannot retrieve a public key with an accepted format for
      the joining node, either from the 'client_cred' parameter or as
      already stored.

   o  When receiving a 4.00 Bad Request response, the joining node
      SHOULD send a new Joining Request to the Group Manager, where:

      *  The 'cnonce' parameter MUST include a new dedicated nonce N_C
         generated by the joining node.

      *  The 'client_cred' parameter MUST include a public key
         compatible with the encoding, countersignature algorithm and
         possible associated parameters indicated by the Group Manager.

      *  The 'client_cred_verify' parameter MUST include a signature
         computed as described in Section 6.2, by using the public key
         indicated in the current 'client_cred' parameter, with the
         countersignature algorithm and possible associated parameters
         indicated by the Group Manager.  If the error response from the
         Group Manager included the 'kdcchallenge' parameter, the
         joining node MUST use its content as new N_S challenge to
         compute the signature.

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6.4.  Joining Response

   If the processing of the Joining Request described in Section 6.3 is
   successful, the Group Manager updates the group membership by
   registering the joining node NODENAME as a new member of the OSCORE
   group GROUPNAME, as described in Section 4.1.2.1 of
   [I-D.ietf-ace-key-groupcomm].

   If the joining node has not taken exclusively the role of monitor,
   the Group Manager performs also the following actions.

   o  The Group Manager selects an available OSCORE Sender ID in the
      OSCORE group, and exclusively assigns it to the joining node.  If
      the joining node is recognized as a current group member, e.g.
      through the ongoing secure communication association, the Group
      Manager MUST assign a new Sender ID different than the one
      currently used by the joining node in the OSCORE group.

      Consistently with Section 3 of [I-D.ietf-core-oscore-groupcomm],
      the Group Manager MUST assign a Sender ID that has never been
      assigned before in the OSCORE group under the current Gid value.

      The Group Manager MUST NOT assign a Sender ID to the joining node
      if this joins the group exclusively with the role of monitor,
      according to what specified in the Access Token (see Section 4.2).

   o  The Group Manager stores the association between i) the public key
      of the joining node; and ii) the Group Identifier (Gid), i.e. the
      OSCORE ID Context, associated to the OSCORE group together with
      the OSCORE Sender ID assigned to the joining node in the group.
      The Group Manager MUST keep this association updated over time.

   Then, the Group Manager replies to the joining node, providing the
   updated security parameters and keying meterial necessary to
   participate in the group communication.  This success Joining
   Response is formatted as defined in Section 4.1.2.1 of
   [I-D.ietf-ace-key-groupcomm], with the following additions:

   o  The 'gkty' parameter identifies a key of type
      "Group_OSCORE_Input_Material object", defined in Section 21.2 of
      this specification.

   o  The 'key' parameter includes what the joining node needs in order
      to set up the Group OSCORE Security Context as per Section 2 of
      [I-D.ietf-core-oscore-groupcomm].

      This parameter has as value a Group_OSCORE_Input_Material object,
      which is defined in this specification and extends the

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      OSCORE_Input_Material object encoded in CBOR as defined in
      Section 3.2.1 of [I-D.ietf-ace-oscore-profile].  In particular, it
      contains the additional parameters 'group_senderId' 'cs_alg',
      'cs_params', 'cs_key_params', 'cs_key_enc', 'ecdh_alg',
      'ecdh_params' and 'ecdh_key_params' defined in Section 21.4 of
      this specification.

      More specifically, the 'key' parameter is composed as follows.

      *  The 'ms' parameter MUST be present and includes the OSCORE
         Master Secret value used in the OSCORE group.

      *  The 'hkdf' parameter, if present, has as value the KDF
         algorithm used in the OSCORE group.

      *  The 'alg' parameter, if present, has as value the AEAD
         algorithm used in the OSCORE group.

      *  The 'salt' parameter, if present, has as value the OSCORE
         Master Salt used in the OSCORE group.

      *  The 'contextId' parameter MUST be present and has as value the
         Group Identifier (Gid), i.e. the OSCORE ID Context of the
         OSCORE group.

      *  The 'group_senderId' parameter, if present, has as value the
         OSCORE Sender ID assigned to the joining node by the Group
         Manager, as described above.  This parameter is not present if
         the node joins the group exclusively with the role of monitor,
         according to what specified in the Access Token (see
         Section 4.2).  In any other case, this parameter MUST be
         present.

      *  The 'cs_alg' parameter MUST be present and specifies the
         algorithm used to countersign messages in the OSCORE group.
         This parameter takes values from the "Value" column of the
         "COSE Algorithms" Registry [COSE.Algorithms].

      *  The 'cs_params' parameter MAY be present and specifies the
         parameters for the counter signature algorithm.  This parameter
         is a CBOR array, which includes the following two elements:

         +  'sign_alg_capab', with the same encoding as defined in
            Section 6.1.  The value is the same as in the Token Post
            response where the 'sign_parameters' value was non-null.

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         +  'sign_key_type_capab', with the same encoding as defined in
            Section 6.1.  The value is the same as in the Token Post
            response where the 'sign_parameters' value was non-null.

      *  The 'cs_key_params' parameter MAY be present and specifies the
         parameters for the key used with the counter signature
         algorithm.  This parameter is a CBOR array, with the same non-
         null encoding and value as 'sign_key_parameters' of the
         Section 6.1.

      *  The 'cs_key_enc' parameter MAY be present and specifies the
         encoding of the public keys of the group members.  This
         parameter is a CBOR integer, whose value is 1 ("COSE_Key")
         taken from the 'Confirmation Key' column of the "CWT
         Confirmation Method" Registry [CWT.Confirmation.Methods], so
         indicating that public keys in the OSCORE group are encoded as
         COSE Keys [I-D.ietf-cose-rfc8152bis-struct].  Future
         specifications may define additional values for this parameter.
         If this parameter is not present, 1 ("COSE_Key") MUST be
         assumed as default value.

      *  The 'ecdh_alg' parameter, if present, specifies the ECDH
         algorithm used in the OSCORE group, if this supports the
         pairwise mode of Group OSCORE.  This parameter takes values
         from the "Value" column of the "COSE Algorithms" Registry
         [COSE.Algorithms].  This parameter MUST be present if the
         OSCORE group supports the pairwise mode of Group OSCORE, and
         MUST NOT be present otherwise.

      *  The 'ecdh_params' parameter, if present, specifies the
         parameters for the ECDH algorithm.  It MUST be present if the
         'ecdh_alg' parameter is present, and MUST NOT be present
         otherwise.  This parameter is a CBOR array, which includes the
         following two elements:

         +  'ecdh_alg_capab', with the same encoding as defined in
            Section 6.1.1.  The value is the same as in the Token Post
            response where the ecdh_parameters' value is non-null.

         +  'ecdh_key_type_capab', with the same encoding as defined in
            Section 6.1.1.  The value is the same as in the Token Post
            response where the 'ecdh_parameters' value is non-null.

      *  The 'ecdh_key_params' parameter, if present, specifies the
         parameters for the key used with the ECDH algorithm.  It MUST
         be present if the 'ecdh_alg' parameter is present, and MUST NOT
         be present otherwise.  This parameter is a CBOR array, with the

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         same non-null encoding and value of 'ecdh_key_parameters'
         defined in Section 6.1.1.

   o  The 'exp' parameter MUST be present.

   o  The 'ace-groupcomm-profile' parameter MUST be present and has
      value coap_group_oscore_app (TBD3), which is defined in
      Section 21.3 of this specification.

   o  The 'pub_keys' parameter, if present, includes the public keys
      requested by the joining node by means of the 'get_pub_keys'
      parameter in the Joining Request.  If public keys are encoded as
      COSE_Keys, each of them has as 'kid' the Sender ID that the
      corresponding owner has in the OSCORE group, thus used as group
      member identifier encoded as a CBOR byte string (REQ12).

      If the joining node has asked for the public keys of all the group
      members, i.e. 'get_pub_keys' had value Null in the Joining
      Request, then the Group Manager provides only the public keys of
      the group members that are relevant to the joining node.  That is,
      in such a case, 'pub_keys' includes only: i) the public keys of
      the responders currently in the OSCORE group, in case the joining
      node is configured (also) as requester; and ii) the public keys of
      the requesters currently in the OSCORE group, in case the joining
      node is configured (also) as responder or monitor.

   o  The 'group_policies' parameter SHOULD be present, and SHOULD
      include the following elements:

      *  "Key Update Check Interval" defined in Section 4.1.2.1 of
         [I-D.ietf-ace-key-groupcomm], with default value 3600;

      *  "Expiration Delta" defined in Section 4.1.2.1 of
         [I-D.ietf-ace-key-groupcomm], with default value 0.

   Finally, the joining node uses the information received in the
   Joining Response to set up the Group OSCORE Security Context, as
   described in Section 2 of [I-D.ietf-core-oscore-groupcomm].  In
   addition, the joining node maintains an association between each
   public key retrieved from the 'pub_keys' parameter and the role(s)
   that the corresponding group member has in the OSCORE group.

   From then on, the joining node can exchange group messages secured
   with Group OSCORE as described in [I-D.ietf-core-oscore-groupcomm].
   When doing so:

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   o  The joining node MUST NOT process an incoming request message, if
      protected by a group member whose public key is not associated to
      the role "Requester".

   o  The joining node MUST NOT process an incoming response message, if
      protected by a group member whose public key is not associated to
      the role "Responder".

   o  The joining node MUST NOT use the pairwise mode of Group OSCORE to
      process messages in the group, if the Joining Response did not
      include the 'ecdh_alg' parameter.

   If the application requires backward security, the Group Manager MUST
   generate updated security parameters and group keying material, and
   provide it to the current group members upon the new node's joining
   (see Section 18).  As a consequence, the joining node is not able to
   access secure communication in the OSCORE group occurred prior its
   joining.

7.  Public Keys of Joining Nodes

   Source authentication of a message sent within the group and
   protected with Group OSCORE is ensured by means of a digital counter
   signature embedded in the message (in group mode), or by integrity-
   protecting the message with pairwise keying material derived from the
   asymmetric keys of sender and recipient (in pairwise mode).

   Therefore, group members must be able to retrieve each other's public
   key from a trusted key repository, in order to verify source
   authenticity of incoming group messages.

   As also discussed in [I-D.ietf-core-oscore-groupcomm], the Group
   Manager acts as trusted repository of the public keys of the group
   members, and provides those public keys to group members if requested
   to.  Upon joining an OSCORE group, a joining node is thus expected to
   provide its own public key to the Group Manager.

   In particular, one of the following four cases can occur when a new
   node joins an OSCORE group.

   o  The joining node is going to join the group exclusively as
      monitor.  That is, it is not going to send messages to the group,
      and hence to produce signatures with its own private key.  In this
      case, the joining node is not required to provide its own public
      key to the Group Manager, which thus does not have to perform any
      check related to the public key encoding, or to a countersignature
      algorithm and possible associated parameters for that joining
      node.  In case that joining node still provides a public key in

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      the 'client_cred' parameter of the Joining Request (see
      Section 6.2), the Group Manager silently ignores that parameter,
      as well as the related parameters 'cnonce' and
      'client_cred_verify'.

   o  The Group Manager already acquired the public key of the joining
      node during a past joining process.  In this case, the joining
      node MAY choose not to provide again its own public key to the
      Group Manager, in order to limit the size of the Joining Request.
      The joining node MUST provide its own public key again if it has
      provided the Group Manager with multiple public keys during past
      joining processes, intended for different OSCORE groups.  If the
      joining node provides its own public key, the Group Manager
      performs consistency checks as per Section 6.3 and, in case of
      success, considers it as the public key associated to the joining
      node in the OSCORE group.

   o  The joining node and the Group Manager use an asymmetric proof-of-
      possession key to establish a secure communication association.
      Then, two cases can occur.

      1.  The proof-of-possession key is compatible with the encoding as
          well as with the counter signature algorithm and possible
          associated parameters used in the OSCORE group.  Then, the
          Group Manager considers the proof-of-possession key as the
          public key associated to the joining node in the OSCORE group.
          If the joining node is aware that the proof-of-possession key
          is also valid for the OSCORE group, it MAY not provide it
          again as its own public key to the Group Manager.  The joining
          node MUST provide its own public key again if it has provided
          the Group Manager with multiple public keys during past
          joining processes, intended for different OSCORE groups.  If
          the joining node provides its own public key in the
          'client_cred' parameter of the Joining Request (see
          Section 6.2), the Group Manager performs consistency checks as
          per Section 6.3 and, in case of success, considers it as the
          public key associated to the joining node in the OSCORE group.

      2.  The proof-of-possession key is not compatible with the
          encoding or with the counter signature algorithm and possible
          associated parameters used in the OSCORE group.  In this case,
          the joining node MUST provide a different compatible public
          key to the Group Manager in the 'client_cred' parameter of the
          Joining Request (see Section 6.2).  Then, the Group Manager
          performs consistency checks on this latest provided public key
          as per Section 6.3 and, in case of success, considers it as
          the public key associated to the joining node in the OSCORE
          group.

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   o  The joining node and the Group Manager use a symmetric proof-of-
      possession key to establish a secure communication association.
      In this case, upon performing a joining process with that Group
      Manager for the first time, the joining node specifies its own
      public key in the 'client_cred' parameter of the Joining Request
      targeting the group-membership endpoint (see Section 6.2).

8.  Retrieval of Updated Keying Material

   At some point, a group member considers the Group OSCORE Security
   Context invalid and to be renewed.  This happens, for instance, after
   a number of unsuccessful security processing of incoming messages
   from other group members, or when the Security Context expires as
   specified by the 'exp' parameter of the Joining Response.

   When this happens, the group member retrieves updated security
   parameters and group keying material.  This can occur in the two
   different ways described below.

8.1.  Retrieval of Group Keying Material

   If the group member wants to retrieve only the latest group keying
   material, it sends a Key Distribution Request to the Group Manager.

   In particular, it sends a CoAP GET request to the endpoint /ace-
   group/GROUPNAME at the Group Manager.

   The Group Manager processes the Key Distribution Request according to
   Section 4.1.2.2 of [I-D.ietf-ace-key-groupcomm].  The Key
   Distribution Response is formatted as defined in Section 4.1.2.2 of
   [I-D.ietf-ace-key-groupcomm].  In addition:

   o  The 'key' parameter is formatted as defined in Section 6.4 of this
      specification, with the difference that it does not include the
      'group_SenderId' parameter.

   o  The 'exp' parameter MUST be present.

   o  The 'ace-groupcomm-profile' parameter MUST be present and has
      value coap_group_oscore_app.

   Upon receiving the Key Distribution Response, the group member
   retrieves the updated security parameters and group keying material,
   and, if they differ from the current ones, uses them to set up the
   new Group OSCORE Security Context as described in Section 2 of
   [I-D.ietf-core-oscore-groupcomm].

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8.2.  Retrieval of Group Keying Material and Sender ID

   If the group member wants to retrieve the latest group keying
   material as well as the Sender ID that it has in the OSCORE group, it
   sends a Key Distribution Request to the Group Manager.

   In particular, it sends a CoAP GET request to the endpoint /ace-
   group/GROUPNAME/nodes/NODENAME at the Group Manager.

   The Group Manager processes the Key Distribution Request according to
   Section 4.1.6.2 of [I-D.ietf-ace-key-groupcomm].  The Key
   Distribution Response is formatted as defined in Section 4.1.6.2 of
   [I-D.ietf-ace-key-groupcomm].  In addition:

   o  The 'key' parameter is formatted as defined in Section 6.4 of this
      specification.  In particular, if the requesting group member has
      exclusively the role of monitor, no 'group_SenderId' is specified
      within the 'key' parameter.

      Note that, in any other case, the current Sender ID of the group
      member is not specified as a separate parameter, but rather
      specified as 'group_SenderId' within the 'key' parameter.

   o  The 'exp' parameter MUST be present.

   Upon receiving the Key Distribution Response, the group member
   retrieves the updated security parameters, group keying material and
   Sender ID, and, if they differ from the current ones, uses them to
   set up the new Group OSCORE Security Context as described in
   Section 2 of [I-D.ietf-core-oscore-groupcomm].

9.  Requesting a Change of Keying Material

   As discussed in Section 2.4.2 of [I-D.ietf-core-oscore-groupcomm], a
   group member may at some point exhaust its Sender Sequence Numbers in
   the OSCORE group.

   When this happens, the group member MUST send a Key Renewal Request
   message to the Group Manager, as per Section 4.5 of
   [I-D.ietf-ace-key-groupcomm].  In particular, it sends a CoAP PUT
   request to the endpoint /ace-group/GROUPNAME/nodes/NODENAME at the
   Group Manager.

   Upon receiving the Key Renewal Request, the Group Manager processes
   it as defined in Section 4.1.6.1 of [I-D.ietf-ace-key-groupcomm],
   with the following additions.

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   The Group Manager MUST return a 5.03 (Service Unavailable) response
   in case the OSCORE group identified by GROUPNAME is currently
   inactive (see Section 5.1).  The response MUST have Content-Format
   set to application/ace-groupcomm+cbor and is formatted as defined in
   Section 4 of [I-D.ietf-ace-key-groupcomm].  The value of the 'error'
   field MUST be set to 7 ("Group currently not active").

   Otherwise, the Group Manager performs one of the following actions.

   1.  If the requesting group member has exclusively the role of
       monitor, the Group Manager replies with a 4.00 (Unauthorized)
       error response.  The response MUST have Content-Format set to
       application/ace-groupcomm+cbor and is formatted as defined in
       Section 4 of [I-D.ietf-ace-key-groupcomm].  The value of the
       'error' field MUST be set to 1 ("Request inconsistent with the
       current roles").

   2.  Otherwise, the Group Manager takes one of the following actions.

       *  The Group Manager rekeys the OSCORE group.  That is, the Group
          Manager generates new group keying material for that group
          (see Section 18), and replies to the group member with a group
          rekeying message as defined in Section 18, providing the new
          group keying material.  Then, the Group Manager rekeys the
          rest of the OSCORE group, as discussed in Section 18.

          The Group Manager SHOULD perform a group rekeying only if
          already scheduled to occur shortly, e.g. according to an
          application-dependent rekeying period, or as a reaction to a
          recent change in the group membership.  In any other case, the
          Group Manager SHOULD NOT rekey the OSCORE group when receiving
          a Key Renewal Request (OPT9).

       *  The Group Manager determines and assigns a new Sender ID for
          that group member, and replies with a Key Renewal Response
          formatted as defined in Section 4.1.6.1 of
          [I-D.ietf-ace-key-groupcomm].  In particular, the CBOR Map in
          the response payload includes a single parameter
          'group_SenderId' defined in Section 21.1 of this document,
          specifying the new Sender ID of the group member encoded as a
          CBOR byte string.

          Consistently with Section 2.4.3.1 of
          [I-D.ietf-core-oscore-groupcomm], the Group Manager MUST
          assign a new Sender ID that has never been assigned before in
          the OSCORE group under the current Gid value.

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          The Group Manager MUST return a 5.03 (Service Unavailable)
          response in case there are currently no Sender IDs available
          to assign in the OSCORE group.  The response MUST have
          Content-Format set to application/ace-groupcomm+cbor and is
          formatted as defined in Section 4 of
          [I-D.ietf-ace-key-groupcomm].  The value of the 'error' field
          MUST be set to 4 ("No available node identifiers").

10.  Retrieval of Public Keys and Roles for Group Members

   A group member or a signature verifier may need to retrieve the
   public keys of (other) group members.  To this end, the group member
   or signature verifier sends a Public Key Request message to the Group
   Manager, as per Section 4.6 of [I-D.ietf-ace-key-groupcomm].  In
   particular, it sends the request to the endpoint /ace-
   group/GROUPNAME/pub-key at the Group Manager.

   If the Public Key Request uses the method FETCH, the Public Key
   Request is formatted as defined in Section 4.1.3.1 of
   [I-D.ietf-ace-key-groupcomm].  In particular:

   o  Each element (if any) of the inner CBOR array 'role_filter' is
      formatted as in the inner CBOR array 'role_filter' of the
      'get_pub_keys' parameter of the Joining Request when the parameter
      value is non-null (see Section 6.2).

   o  Each element (if any) of the inner CBOR array 'id_filter' is a
      CBOR byte string (REQ12), which encodes the Sender ID of the group
      member for which the associated public key is requested.

   Upon receiving the Public Key Request, the Group Manager processes it
   as per Section 4.1.3.1 or 4.1.3.2 of [I-D.ietf-ace-key-groupcomm],
   depending on the request method being FETCH or GET, respectively.
   Additionally, if the Public Key Request uses the method FETCH, the
   Group Manager silently ignores node identifiers included in the
   'get_pub_keys' parameter of the request that are not associated to
   any current group member.

   The success Public Key Response is formatted as defined in
   Section 4.1.3.1 or 4.1.3.2 of [I-D.ietf-ace-key-groupcomm], depending
   on the request method being FETCH or GET, respectively.

11.  Update of Public Key

   A group member may need to provide the Group Manager with its new
   public key to use in the group from then on, hence replacing the
   current one.  This can be the case, for instance, if the
   countersignature algorithm and possible associated parameters used in

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   the OSCORE group have been changed, and the current public key is not
   compatible with them.

   To this end, the group member sends a Public Key Update Request
   message to the Group Manager, as per Section 4.7 of
   [I-D.ietf-ace-key-groupcomm].  In particular, it sends a CoAP POST
   request to the endpoint /ace-group/GROUPNAME/nodes/NODENAME/pub-key
   at the Group Manager.

   Upon receiving the Public Key Update Request, the Group Manager
   processes it as per Section 4.1.7.1 of [I-D.ietf-ace-key-groupcomm],
   with the following additions.

   o  The Group Manager MUST return a 5.03 (Service Unavailable)
      response in case the OSCORE group identified by GROUPNAME is
      currently inactive (see Section 5.1).  The response MUST have
      Content-Format set to application/ace-groupcomm+cbor and is
      formatted as defined in Section 4 of [I-D.ietf-ace-key-groupcomm].
      The value of the 'error' field MUST be set to 7 ("Group currently
      not active").

   o  If the requesting group member has exclusively the role of
      monitor, the Group Manager replies with a 4.00 (Bad request) error
      response.  The response MUST have Content-Format set to
      application/ace-groupcomm+cbor and is formatted as defined in
      Section 4 of [I-D.ietf-ace-key-groupcomm].  The value of the
      'error' field MUST be set to 1 ("Request inconsistent with the
      current roles").

   o  The N_S signature challenge is computed as per point (1) in
      Section 6.2.1 (REQ20).

   o  If the request is successfully processed, the Group Manager stores
      the association between i) the new public key of the group member;
      and ii) the Group Identifier (Gid), i.e. the OSCORE ID Context,
      associated to the OSCORE group together with the OSCORE Sender ID
      assigned to the group member in the group.  The Group Manager MUST
      keep this association updated over time.

12.  Retrieval of Group Policies

   A group member may request the current policies used in the OSCORE
   group.  To this end, the group member sends a Policies Request, as
   per Section 4.8 of [I-D.ietf-ace-key-groupcomm].  In particular, it
   sends a CoAP GET request to the endpoint /ace-group/GROUPNAME/
   policies at the Group Manager, where GROUPNAME is the name of the
   OSCORE group.

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   Upon receiving the Policies Request, the Group Manager processes it
   as per Section 4.1.4.1 of [I-D.ietf-ace-key-groupcomm].  The success
   Policies Response is formatted as defined in Section 4.1.4.1 of
   [I-D.ietf-ace-key-groupcomm].

13.  Retrieval of Keying Material Version

   A group member may request the current version of the keying material
   used in the OSCORE group.  To this end, the group member sends a
   Version Request, as per Section 4.9 of [I-D.ietf-ace-key-groupcomm].
   In particular, it sends a CoAP GET request to the endpoint /ace-
   group/GROUPNAME/num at the Group Manager, where GROUPNAME is the name
   of the OSCORE group.

   Upon receiving the Version Request, the Group Manager processes it as
   per Section 4.1.5.1 of [I-D.ietf-ace-key-groupcomm].  The success
   Version Response is formatted as defined in Section 4.1.5.1 of
   [I-D.ietf-ace-key-groupcomm].

14.  Retrieval of Group Status

   A group member may request the current status of the the OSCORE
   group, i.e. active or inactive.  To this end, the group member sends
   a Group Status Request to the Group Manager.

   In particular, the group member sends a CoAP GET request to the
   endpoint /ace-group/GROUPNAME/active at the Group Manager defined in
   Section 5.1 of this specification, where GROUPNAME is the name of the
   OSCORE group.  The success Group Version Response is formatted as
   defined in Section 5.1 of this specification.

   Upon learning from a 2.05 (Content) response that the group is
   currently inactive, the group member SHOULD stop taking part in
   communications within the group, until it becomes active again.

   Upon learning from a 2.05 (Content) response that the group has
   become active again, the group member can resume taking part in
   communications within the group.

   Figure 3 gives an overview of the exchange described above.

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    Group                                                         Group
    Member                                                       Manager
      |                                                             |
      |--- Group Status Request: GET ace-group/GROUPNAME/active --->|
      |                                                             |
      |<---------- Group Status Response: 2.05 (Content) -----------|
      |                                                             |

          Figure 3: Message Flow of Group Status Request-Response

15.  Retrieval of Group Names and URIs

   A node may want to retrieve from the Group Manager the group name and
   the URI of the group-membership resource of a group.  This is
   relevant in the following cases.

   o  Before joining a group, a joining node may know only the current
      Group Identifier (Gid) of that group, but not the group name and
      the URI to the group-membership resource.

   o  As current group member in several groups, the node has missed a
      previous group rekeying in one of them (see Section 18).  Hence,
      it retains stale keying material and fails to decrypt received
      messages exchanged in that group.

      Such messages do not provide a direct hint to the correct group
      name, that the node would need in order to retrieve the latest
      keying material and public keys from the Group Manager (see
      Section 8.1, Section 8.2 and Section 10).  However, such messages
      may specify the current Gid of the group, as value of the
      'kid_context' field of the OSCORE CoAP option (see Section 6.1 of
      [RFC8613] and Section 4.2 of [I-D.ietf-core-oscore-groupcomm]).

   o  As signature verifier, the node also refers to a group name for
      retrieving the required public keys from the Group Manager (see
      Section 10).  As discussed above, intercepted messages do not
      provide a direct hint to the correct group name, while they may
      specify the current Gid of the group, as value of the
      'kid_context' field of the OSCORE CoAP option.  In such a case,
      upon intercepting a message in the group, the node requires to
      correctly map the Gid currently used in the group with the
      invariant group name.

      Furthermore, since it is not a group member, the node does not
      take part to a possible group rekeying.  Thus, following a group
      rekeying and the consequent change of Gid in a group, the node
      would retain the old Gid value and cannot correctly associate
      intercepted messages to the right group, especially if acting as

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      signature verifier in several groups.  This in turn prevents the
      efficient verification of counter signatures, and especially the
      retrieval of required, new public keys from the Group Manager.

   In either case, the node only knows the current Gid of the group, as
   learned from received or intercepted messages exchanged in the group.
   As detailed below, the node can contact the Group Manager, and
   request the group name and URI to the group-membership resource
   corresponding to that Gid. Then, it can use that information to
   either join the group as a candidate group member, get the latest
   keying material as a current group member, or retrieve public keys
   used in the group as a signature verifier.  To this end, the node
   sends a Group Name and URI Retrieval Request, as per Section 4.2 of
   [I-D.ietf-ace-key-groupcomm].

   In particular, the node sends a CoAP FETCH request to the endpoint
   /ace-group at the Group Manager formatted as defined in
   Section 4.1.1.1 of [I-D.ietf-ace-key-groupcomm].  Each element of the
   CBOR array 'gid' is a CBOR byte string (REQ9), which encodes the Gid
   of the group for which the group name and the URI to the group-
   membership resource are requested.

   Upon receiving the Group Name and URI Retrieval Request, the Group
   Manager processes it as per Section 4.1.1.1 of
   [I-D.ietf-ace-key-groupcomm].  The success Group Name and URI
   Retrieval Response is formatted as defined in Section 4.1.1.1 of
   [I-D.ietf-ace-key-groupcomm].  In particular, each element of the
   CBOR array 'gid' is a CBOR byte string (REQ9), which encodes the Gid
   of the group for which the group name and the URI to the group-
   membership resource are provided.

   For each of its groups, the Group Manager maintains an association
   between the group name and the URI to the group-membership resource
   on one hand, and only the current Gid for that group on the other
   hand.  That is, the Group Manager MUST NOT maintain an association
   between the former pair and any other Gid for that group than the
   current, most recent one.

   Figure 4 gives an overview of the exchanges described above.

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

      Figure 4: Message Flow of Group Name and URI Retrieval Request-
                                 Response

16.  Request to Leave the Group

   A group member may request to leave the OSCORE group.  To this end,
   the group member sends a Group Leaving Request, as per Section 4.10
   of [I-D.ietf-ace-key-groupcomm].  In particular, it sends a CoAP
   DELETE request to the endpoint /ace-group/GROUPNAME/nodes/NODENAME at
   the Group Manager.

   Upon receiving the Group Leaving Request, the Group Manager processes
   it as per Section 4.1.6.3 of [I-D.ietf-ace-key-groupcomm].

17.  Removal of a Group Member

   Other than after a spontaneous request to the Group Manager as
   described in Section 16, a node may be forcibly removed from the
   OSCORE group, e.g. due to expired or revoked authorization.

   If any of the two conditions below holds, the Group Manager MUST
   inform the leaving node of its eviction as follows.  If both
   conditions hold, the Group Manager MUST inform the leaving node only
   once, using either of the corresponding methods.

   o  If, upon joining the group (see Section 6.2), the leaving node
      specified a URI in the 'control_uri' parameter defined in
      Section 4.1.2.1 of [I-D.ietf-ace-key-groupcomm], the Group Manager
      sends a DELETE request targeting the URI specified in the
      'control_uri' parameter (OPT10).

   o  If the leaving node has been observing the associated resource at
      ace-group/GROUPNAME/nodes/NODENAME, the Group Manager sends an
      unsolicited 4.04 (Not Found) response to the leaving node, as
      specified in 4.1.6.2 of [I-D.ietf-ace-key-groupcomm].

   If the leaving node has not exclusively the role of monitor, the
   Group Manager performs the following actions.

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   o  The Group Manager frees the OSCORE Sender ID value of the leaving
      node.  However, this value MUST NOT become available for possible
      upcoming joining nodes in the same group.

   o  The Group Manager cancels the association between, on one hand,
      the public key of the leaving node and, on the other hand, the
      Group Identifier (Gid) associated to the OSCORE group together
      with the freed OSCORE Sender ID value.  The Group Manager deletes
      the public key of the leaving node, if that public key has no
      remaining association with any pair (Gid, Sender ID).

   If the application requires forward security, the Group Manager MUST
   generate updated security parameters and group keying material, and
   provide it to the remaining group members (see Section 18).  As a
   consequence, the leaving node is not able to acquire the new security
   parameters and group keying material distributed after its leaving.

   Same considerations in Section 5 of [I-D.ietf-ace-key-groupcomm]
   apply here as well, considering the Group Manager acting as KDC.

18.  Group Rekeying Process

   In order to rekey the OSCORE group, the Group Manager distributes a
   new Group Identifier (Gid), i.e. a new OSCORE ID Context; a new
   OSCORE Master Secret; and, optionally, a new OSCORE Master Salt for
   that group.  When doing so, the Group Manager MUST increment the
   version number of the group keying material, before starting its
   distribution.

   Consistently with Section 2.3 of [I-D.ietf-core-oscore-groupcomm],
   the Group Manager MUST assign a Gid that has never been assigned
   before to the OSCORE group.

   Furthermore, the Group Manager MUST preserve the same unchanged
   Sender IDs for all group members.  This avoids affecting the
   retrieval of public keys from the Group Manager as well as the
   verification of group messages.

   The Group Manager MUST support at least the following group rekeying
   scheme.  Future application profiles may define alternative message
   formats and distribution schemes.

   As group rekeying message, the Group Manager uses the same format of
   the Joining Response message in Section 6.4.  In particular:

   o  Only the parameters 'gkty', 'key', 'num', 'ace-groupcomm-profile'
      and 'exp' are present.

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   o  The 'ms' parameter of the 'key' parameter specifies the new OSCORE
      Master Secret value.

   o  The 'contextId' parameter of the 'key' parameter specifies the new
      Gid used as OSCORE ID Context value.

   The Group Manager separately sends a group rekeying message to each
   group member to be rekeyed.

   Each rekeying message MUST be secured with the pairwise secure
   communication channel between the Group Manager and the group member
   used during the joining process.  In particular, each rekeying
   message can target the 'control_uri' URI path defined in
   Section 4.1.2.1 of [I-D.ietf-ace-key-groupcomm] (OPT10), if provided
   by the intended recipient upon joining the group (see Section 6.2).

   It is RECOMMENDED that the Group Manager gets confirmation of
   successful distribution from the group members, and admits a maximum
   number of individual retransmissions to non-confirming group members.

   This approach requires group members to act (also) as servers, in
   order to correctly handle unsolicited group rekeying messages from
   the Group Manager.  In particular, if a group member and the Group
   Manager use OSCORE [RFC8613] to secure their pairwise communications,
   the group member MUST create a Replay Window in its own Recipient
   Context upon establishing the OSCORE Security Context with the Group
   Manager, e.g. by means of the OSCORE profile of ACE
   [I-D.ietf-ace-oscore-profile].

   Group members and the Group Manager SHOULD additionally support
   alternative rekeying approaches that do not require group members to
   act (also) as servers.  A number of such approaches are defined in
   Section 4.4 of [I-D.ietf-ace-key-groupcomm].  In particular, a group
   member may subscribe for updates to the group-membership resource of
   the group, at the endpoint /ace-group/GROUPNAME/ of the Group
   Manager.  This can rely on CoAP Observe [RFC7641] or on a full-
   fledged Pub-Sub model [I-D.ietf-core-coap-pubsub] with the Group
   Manager acting as Broker.

   In case the rekeying terminates and some group members have not
   received the new keying material, they will not be able to correctly
   process following secured messages exchanged in the group.  These
   group members will eventually contact the Group Manager, in order to
   retrieve the current keying material and its version.

   Some of these group members may be in multiple groups, each
   associated to a different Group Manager.  When failing to correctly
   process messages secured with the new keying material, these group

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   members may not have sufficient information to determine which exact
   Group Manager they should contact, in order to retrieve the current
   keying material they are missing.

   If the Gid is formatted as described in Appendix C of
   [I-D.ietf-core-oscore-groupcomm], the Group Prefix can be used as a
   hint to determine the right Group Manager, as long as no collisions
   among Group Prefixes are experienced.  Otherwise, a group member
   needs to contact the Group Manager of each group, e.g. by first
   requesting only the version of the current group keying material (see
   Section 13) and then possibly requesting the current keying material
   (see Section 8.1).

   Furthermore, some of these group members can be in multiple groups,
   all of which associated to the same Group Manager.  In this case,
   these group members may also not have sufficient information to
   determine which exact group they should refer to, when contacting the
   right Group Manager.  Hence, they need to contact a Group Manager
   multiple times, i.e. separately for each group they belong to and
   associated to that Group Manager.

19.  Default Values for Group Configuration Parameters

   This section defines the default values that the Group Manager
   assumes for the configuration parameters of an OSCORE group, unless
   differently specified when creating and configuring the group.  This
   can be achieved as specified in [I-D.ietf-ace-oscore-gm-admin].

   The Group Manager SHOULD use the same default values defined in
   Section 3.2 of [RFC8613] for both the HKDF algorithm and the AEAD
   algorithm used in the group.

   The Group Manager SHOULD use the following default values for the
   algorithm, algorithm parameters and key parameters used to
   countersign messages in the group, consistently with the "COSE
   Algorithms" Registry [COSE.Algorithms], the "COSE Key Types" Registry
   [COSE.Key.Types] and the "COSE Elliptic Curves" Registry
   [COSE.Elliptic.Curves].

   o  For the algorithm 'cs_alg' used to countersign messages in the
      group, the signature algorithm EdDSA [RFC8032].

   o  For the parameters 'cs_params' of the counter signature algorithm:

      *  The array [[OKP], [OKP, Ed25519]], indicating the elliptic
         curve Ed25519 [RFC8032], in case EdDSA is assumed or specified
         for 'cs_alg'.

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      *  The array [[EC2], [EC2, P-256]], indicating the elliptic curve
         P-256, in case ES256 [RFC6979] is specified for 'cs_alg'.

      *  The array [[EC2], [EC2, P-384]], indicating the elliptic curve
         P-384, in case ES384 [RFC6979] is specified for 'cs_alg'.

      *  The array [[EC2], [EC2, P-521]], indicating the elliptic curve
         P-521, in case ES512 [RFC6979] is specified for 'cs_alg'.

      *  The array [[RSA], [RSA]], in case PS256, PS384 or PS512
         [RFC8017] is specified for 'cs_alg'.

   o  For the parameters 'cs_key_params' of the key used with the
      counter signature algorithm:

      *  The array [OKP, Ed25519] as pair (key type, elliptic curve), in
         case EdDSA is assumed or specified for 'cs_alg' and Ed25519 is
         assumed or specified within the second array of 'cs_params'.

      *  The array [OKP, Ed448] as pair (key type, elliptic curve), in
         case EdDSA is assumed or specified for 'cs_alg' and the
         elliptic curve Ed448 [RFC8032] is specified within the second
         array of 'cs_params'.

      *  The array [EC2, P-256] as pair (key type, elliptic curve), in
         case ES256 [RFC6979] is specified for 'cs_alg' and the elliptic
         curve P-256 is assumed or specified within the second array of
         'cs_params'.

      *  The array [EC2, P-384] as pair (key type, elliptic curve), in
         case ES384 [RFC6979] is specified for 'cs_alg' and the elliptic
         curve P-384 is specified within the second array of
         'cs_params'.

      *  The array [EC2, P-521] as pair (key type, elliptic curve), in
         case ES512 [RFC6979] is specified for 'cs_alg' and the elliptic
         curve P-521 is specified within the second array of
         'cs_params'.

      *  The array [RSA] indicating RSA as key type, in case PS256,
         PS384 or PS512 [RFC8017] is specified for 'cs_alg'.

   o  For the 'cs_key_enc' encoding of the public keys of the group
      members, COSE_Key from the "CWT Confirmation Methods" Registry
      [CWT.Confirmation.Methods].

   If the group supports the pairwise mode of Group OSCORE, the Group
   Manager SHOULD use the following default values for the algorithm,

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   algorithm parameters and key parameters used to compute static-static
   Diffie-Hellman shared secrets, consistently with the "COSE
   Algorithms" Registry [COSE.Algorithms], the "COSE Key Types" Registry
   [COSE.Key.Types] and the "COSE Elliptic Curves" Registry
   [COSE.Elliptic.Curves].

   o  For the algorithm 'ecdh_alg' used to compute static-static Diffie-
      Hellman shared secrets, the ECDH algorithm ECDH-SS + HKDF-256
      specified in Section 6.3.1 of [I-D.ietf-cose-rfc8152bis-algs].

   o  For the parameters 'ecdh_params' of the ECDH algorithm:

      *  The array [[OKP], [OKP, X25519]], indicating the elliptic curve
         X25519 [RFC8032], in case EdDSA is assumed or specified for
         'cs_alg'.

      *  The array [[EC2], [EC2, P-256]], indicating the elliptic curve
         P-256, in case ES256 [RFC6979] is specified for 'cs_alg'.

      *  The array [[EC2], [EC2, P-384]], indicating the elliptic curve
         P-384, in case ES384 [RFC6979] is specified for 'cs_alg'.

      *  The array [[EC2], [EC2, P-521]], indicating the elliptic curve
         P-521, in case ES512 [RFC6979] is specified for 'cs_alg'.

   o  For the parameters 'ecdh_key_params' of the key used with the ECDH
      algorithm:

      *  The array [OKP, X25519] as pair (key type, elliptic curve), in
         case EdDSA is assumed or specified for 'cs_alg' and X25519 is
         assumed or specified within the second array of 'ecdh_params'.

      *  The array [OKP, X448] as pair (key type, elliptic curve), in
         case EdDSA is assumed or specified for 'cs_alg' and the
         elliptic curve X448 [RFC8032] is specified within the second
         array of 'ecdh_params'.

      *  The array [EC2, P-256] as pair (key type, elliptic curve), in
         case ES256 [RFC6979] is specified for 'cs_alg' and the elliptic
         curve P-256 is assumed or specified within the second array of
         'ecdh_params'.

      *  The array [EC2, P-384] as pair (key type, elliptic curve), in
         case ES384 [RFC6979] is specified for 'cs_alg' and the elliptic
         curve P-384 is specified within the second array of
         'ecdh_params'.

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      *  The array [EC2, P-521] as pair (key type, elliptic curve), in
         case ES512 [RFC6979] is specified for 'cs_alg' and the elliptic
         curve P-521 is specified within the second array of
         'ecdh_params'.

20.  Security Considerations

   Security considerations for this profile are inherited from
   [I-D.ietf-ace-key-groupcomm], the ACE framework for Authentication
   and Authorization [I-D.ietf-ace-oauth-authz], and the specific
   transport profile of ACE signalled by the AS, such as
   [I-D.ietf-ace-dtls-authorize] and [I-D.ietf-ace-oscore-profile].

   The following security considerations also apply for this profile.

20.1.  Management of OSCORE Groups

   This profile leverages the following management aspects related to
   OSCORE groups and discussed in the sections of
   [I-D.ietf-core-oscore-groupcomm] referred below.

   o  Management of group keying material (see Section 3.1 of
      [I-D.ietf-core-oscore-groupcomm]).  The Group Manager is
      responsible for the renewal and re-distribution of the keying
      material in the groups of its competence (rekeying).  According to
      the specific application requirements, this can include rekeying
      the group upon changes in its membership.  In particular, renewing
      the group keying material is required upon a new node's joining or
      a current node's leaving, in case backward security and forward
      security have to be preserved, respectively.

   o  Provisioning and retrieval of public keys (see Section 2 of
      [I-D.ietf-core-oscore-groupcomm]).  The Group Manager acts as key
      repository of public keys of group members, and provides them upon
      request.

   o  Synchronization of sequence numbers (see Section 6.2 of
      [I-D.ietf-core-oscore-groupcomm]).  This concerns how a responder
      node that has just joined an OSCORE group can synchronize with the
      sequence number of requesters in the same group.

   Before sending the Joining Response, the Group Manager MUST verify
   that the joining node actually owns the associated private key.  To
   this end, the Group Manager can rely on the proof-of-possession
   challenge-response defined in Section 6.  Alternatively, the joining
   node can use its own public key as asymmetric proof-of-possession key
   to establish a secure channel with the Group Manager, e.g. as in
   Section 3.2.2 of [I-D.ietf-ace-dtls-authorize].  However, this

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   requires such proof-of-possession key to be compatible with the
   encoding as well as with the countersignature algorithm and possible
   associated parameters used in the OSCORE group.

   A node may have joined multiple OSCORE groups under different non-
   synchronized Group Managers.  Therefore, it can happen that those
   OSCORE groups have the same Group Identifier (Gid).  It follows that,
   upon receiving a Group OSCORE message addressed to one of those
   groups, the node would have multiple Security Contexts matching with
   the Gid in the incoming message.  It is up to the application to
   decide how to handle such collisions of Group Identifiers, e.g. by
   trying to process the incoming message using one Security Context at
   the time until the right one is found.

20.2.  Size of Nonces for Signature Challenge

   With reference to the Joining Request message in Section 6.2, the
   proof-of-possession signature included in 'client_cred_verify' is
   computed over the challenge N_C | N_S, where | denotes concatenation.

   For the N_C challenge share, it is RECOMMENDED to use a 8-byte long
   random nonce.  Furthermore, N_C is always conveyed in the 'cnonce'
   parameter of the Joining Request, which is always sent over the
   secure communication channel between the joining node and the Group
   Manager.

   As defined in Section 6.2.1, the way the N_S value is computed
   depends on the particular way the joining node provides the Group
   Manager with the Access Token, as well as on following interactions
   between the two.

   o  If the Access Token is not explicitly posted to the /authz-info
      endpoint of the Group Manager, then N_S is computed as a 32-byte
      long challenge share.  For an example, see point (2) of
      Section 6.2.1.

   o  If the Access Token has been explicitly posted to the /authz-info
      endpoint of the Group Manager, N_S takes the most recent value
      provided to the client by the Group Manager in the 'kdcchallenge'
      parameter, as specified in point (1) of Section 6.2.1.  This is
      provided either in the 2.01 response to the Token Post (see
      Section 6.1), or in a 4.00 response to a following Joining Request
      (see Section 6.3).  In either case, it is RECOMMENDED to use a
      8-byte long random challenge as value for N_S.

   If we consider both N_C and N_S to take 8-byte long values, the
   following considerations hold.

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   o  Let us consider both N_C and N_S as taking random values, and the
      Group Manager to never change the value of the N_S provided to a
      Client during the lifetime of an Access Token.  Then, as per the
      birthday paradox, the average collision for N_S will happen after
      2^32 new posted Access Tokens, while the average collision for N_C
      will happen after 2^32 new Joining Requests.  This amounts to
      considerably more token provisionings than the expected new
      joinings of OSCORE groups under a same Group Manager, as well as
      to considerably more requests to join OSCORE groups from a same
      Client using a same Access Token under a same Group Manager.

   o  Section 7 of [I-D.ietf-ace-oscore-profile] as well Appendix B.2 of
      [RFC8613] recommend the use of 8-byte random values as well.
      Unlike in those cases, the values of N_C and N_S considered in
      this specification are not used for as sensitive operations as the
      derivation of a Security Context, and thus do not have possible
      implications in the security of AEAD ciphers.

20.3.  Reusage of Nonces for Signature Challenge

   As long as the Group Manager preserves the same N_S value currently
   associated to an Access Token, i.e. the latest value provided to a
   Client in a 'kdcchallenge' parameter, the Client is able to
   successfully reuse the same signature challenge for multiple Joining
   Requests to that Group Manager.

   In particular, the Client can reuse the same N_C value for every
   Joining Request to the Group Manager, and combine it with the same
   unchanged N_S value.  This results in reusing the same signature
   challenge for producing the signature to include in the
   'client_cred_verify' parameter of the Joining Requests.

   Unless the Group Manager maintains a list of N_C values already used
   by that Client since the latest update to the N_S value associated to
   the Access Token, the Group Manager can be forced to falsely believe
   that the Client possesses its own private key at that point in time,
   upon verifying the signature in the 'client_cred_verify' parameter.

21.  IANA Considerations

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

   This document has the following actions for IANA.

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21.1.  ACE Groupcomm Parameters Registry

   IANA is asked to register the following entry to the "ACE Groupcomm
   Parameters" Registry defined in Section 10.5 of
   [I-D.ietf-ace-key-groupcomm].

   o  Name: group_senderId

   o  CBOR Key: TBD1

   o  CBOR Type: Byte string

   o  Reference: [[This specification]] (Section 9)

21.2.  ACE Groupcomm Key Registry

   IANA is asked to register the following entry to the "ACE Groupcomm
   Key" Registry defined in Section 10.6 of
   [I-D.ietf-ace-key-groupcomm].

   o  Name: Group_OSCORE_Input_Material object

   o  Key Type Value: TBD2

   o  Profile: "coap_group_oscore_app", defined in Section 21.3 of this
      specification.

   o  Description: A Group_OSCORE_Input_Material object encoded as
      described in Section 6.4 of this specification.

   o  Reference: [[This specification]] (Section 6.4)

21.3.  ACE Groupcomm Profile Registry

   IANA is asked to register the following entry to the "ACE Groupcomm
   Profile" Registry defined in Section 10.7 of
   [I-D.ietf-ace-key-groupcomm].

   o  Name: coap_group_oscore_app

   o  Description: Application profile to provision keying material for
      participating in group communication protected with Group OSCORE
      as per [I-D.ietf-core-oscore-groupcomm].

   o  CBOR Value: TBD3

   o  Reference: [[This specification]] (Section 6.4)

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21.4.  OSCORE Security Context Parameters Registry

   IANA is asked to register the following entries in the "OSCORE
   Security Context Parameters" Registry defined in Section 9.4 of
   [I-D.ietf-ace-oscore-profile].

   o  Name: group_SenderId

   o  CBOR Label: TBD4

   o  CBOR Type: bstr

   o  Registry: -

   o  Description: OSCORE Sender ID assigned to a member of an OSCORE
      group

   o  Reference: [[This specification]] (Section 6.4)

   o  Name: cs_alg

   o  CBOR Label: TBD5

   o  CBOR Type: tstr / int

   o  Registry: COSE Algorithms

   o  Description: OSCORE Counter Signature Algorithm Value

   o  Reference: [[This specification]] (Section 6.4)

   o  Name: cs_params

   o  CBOR Label: TBD6

   o  CBOR Type: array

   o  Registry: COSE Algorithms, COSE Key Types, COSE Elliptic Curves

   o  Description: OSCORE Counter Signature Algorithm Additional
      Parameters

   o  Reference: [[This specification]] (Section 6.4)

   o  Name: cs_key_params

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   o  CBOR Label: TBD7

   o  CBOR Type: array

   o  Registry: COSE Algorithms, COSE Key Types, COSE Elliptic Curves

   o  Description: OSCORE Counter Signature Key Additional Parameters

   o  Reference: [[This specification]] (Section 6.4)

   o  Name: cs_key_enc

   o  CBOR Label: TBD8

   o  CBOR Type: integer

   o  Registry: CWT Confirmation Methods

   o  Description: Encoding of Public Keys to be used with the OSCORE
      Counter Signature Algorithm

   o  Reference: [[This specification]] (Section 6.4)

   o  Name: ecdh_alg

   o  CBOR Label: TBD9

   o  CBOR Type: tstr / int

   o  Registry: COSE Algorithms

   o  Description: OSCORE ECDH Algorithm Value

   o  Reference: [[This specification]] (Section 6.4)

   o  Name: ecdh_params

   o  CBOR Label: TBD10

   o  CBOR Type: array

   o  Registry: COSE Algorithms, COSE Key Types, COSE Elliptic Curves

   o  Description: OSCORE ECDH Algorithm Additional Parameters

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   o  Reference: [[This specification]] (Section 6.4)

   o  Name: ecdh_key_params

   o  CBOR Label: TBD11

   o  CBOR Type: array

   o  Registry: COSE Algorithms, COSE Key Types, COSE Elliptic Curves

   o  Description: OSCORE ECDH Key Additional Parameters

   o  Reference: [[This specification]] (Section 6.4)

21.5.  TLS Exporter Label Registry

   IANA is asked to register the following entry to the "TLS Exporter
   Label" Registry defined in Section 6 of [RFC5705] and updated in
   Section 12 of [RFC8447].

   o  Value: EXPORTER-ACE-Sign-Challenge-coap-group-oscore-app

   o  DTLS-OK: Y

   o  Recommended: N

   o  Reference: [[This specification]] (Section 6.2.1)

21.6.  AIF Registry

   IANA is asked to register the following entry to the "Toid" sub-
   registry of the "AIF" Registry defined in Section 5.2 of
   [I-D.ietf-ace-aif].

   o  Name: oscore-group-name

   o  Description/Specification: group name of the OSCORE group, as
      specified in [[This specification]].

   IANA is asked to register the following entry to the "Tperm" sub-
   Registry of the "AIF" Registry defined in Section 5.2 of
   [I-D.ietf-ace-aif].

   o  Name: oscore-group-roles

   o  Description/Specification: role(s) of the member of the OSCORE
      group, as specified in [[This specification]].

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21.7.  Media Type Registrations

   This specification registers the 'application/aif-groupcomm-
   oscore+cbor' media type for the AIF specific data model AIF-OSCORE-
   GROUPCOMM defined in Section 3 of [[This specification]].  This
   registration follows the procedures specified in [RFC6838].

   These media type has parameters for specifying the object identifier
   ("Toid") and set of permissions ("Tperm") defined for the AIF-generic
   model in [I-D.ietf-ace-aif]; default values are the values "oscore-
   group-name" for "Toid" and "oscore-group-roles" for "Tperm".

   Type name: application

   Subtype name: aif-groupcomm-oscore+cbor

   Required parameters: "Toid", "Tperm"

   Optional parameters: N/A

   Encoding considerations: Must be encoded as a CBOR array, each
   element of which is an array [Toid, Tperm] as defined in Section 3 of
   [[This specification]].

   Security considerations: See Section 20 of [[This specification]].

   Interoperability considerations: N/A

   Published specification: [[This specification]]

   Applications that use this media type: The type is used by
   applications that want to express authorization information about
   joining OSCORE groups, as specified in [[This specification]].

   Fragment identifier considerations: N/A

   Additional information: N/A

   Person & email address to contact for further information:
   iesg@ietf.org [1]

   Intended usage: COMMON

   Restrictions on usage: None

   Author: Marco Tiloca marco.tiloca@ri.se [2]

   Change controller: IESG

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21.8.  CoAP Content-Format Registry

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

   Media Type: application/aif-groupcomm-oscore+cbor;Toid="oscore-group-
   name",Tperm"oscore-group-roles"

   Encoding: -

   ID: TBD12

   Reference: [[This specification]]

21.9.  Group OSCORE Roles Registry

   This specification establishes the IANA "Group OSCORE Roles"
   Registry.  The Registry has been created to use the "Expert Review"
   registration procedure [RFC8126].  Expert review guidelines are
   provided in Section 21.13.

   This registry includes the possible roles that nodes can take in an
   OSCORE group, each in combination with a numeric identifier.  These
   numeric identifiers are used to express authorization information
   about joining OSCORE groups, as specified in Section 3 of [[This
   specification]].

   The columns of this registry are:

   o  Name: A value that can be used in documents for easier
      comprehension, to identify a possible role that nodes can take in
      an OSCORE group.

   o  Value: The numeric identifier for this role.  Integer values
      greater than 65535 are marked as "Private Use", all other values
      use the registration policy "Expert Review" [RFC8126].

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

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

   This registry will be initially populated by the values in Figure 1.

   The Reference column for all of these entries will be [[This
   specification]].

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21.10.  CoRE Resource Type Registry

   IANA is asked to register a new Resource Type (rt=) Link Target
   Attribute in the "Resource Type (rt=) Link Target Attribute Values"
   subregistry under the "Constrained Restful Environments (CoRE)
   Parameters" [CORE.Parameters] registry.

   o  Value: "core.osc.gm"

   o  Description: Group-membership resource of an OSCORE Group Manager.

   o  Reference: [[This specification]]

21.11.  ACE Scope Semantics Registry

   IANA is asked to register the following entry in the "ACE Scope
   Semantics" registry defined in Section 10.12 of
   [I-D.ietf-ace-key-groupcomm].

   o  Value: SEM_ID_TBD

   o  Description: Access to OSCORE groups through the ACE Group
      Manager.

   o  Reference: [[This specification]]

21.12.  ACE Groupcomm Errors

   IANA is asked to register the following entry in the "ACE Scope
   Semantics" registry defined in Section 10.13 of
   [I-D.ietf-ace-key-groupcomm].

   o  Value: 7

   o  Description: Group currently not active.

   o  Reference: [[This specification]]

21.13.  Expert Review Instructions

   The IANA Registry established in this document is 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:

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   o  Clarity and correctness of registrations.  Experts are expected to
      check the clarity of purpose and use of the requested entries.
      Experts should inspect the entry for the considered role, to
      verify the correctness of its description against the role as
      intended in the specification that defined it.  Expert should
      consider requesting an opinion on the correctness of registered
      parameters from the Authentication and Authorization for
      Constrained Environments (ACE) Working Group and the Constrained
      RESTful Environments (CoRE) Working Group.

      Entries that do not meet these objective of clarity and
      completeness should not be registered.

   o  Duplicated registration and 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.

   o  Experts should take into account the expected usage of roles when
      approving point assignment.  Given a 'Value' V as code point, the
      length of the encoding of (2^(V+1) - 1) should be weighed against
      the usage of the entry, considering the resources and capabilities
      of devices it will be used on.  Additionally, given a 'Value' V as
      code point, the length of the encoding of (2^(V+1) - 1) should be
      weighed against how many code points resulting in that encoding
      length are left, and the resources and capabilities of devices it
      will be used on.

   o  Specifications are recommended.  When specifications are not
      provided, the description provided needs to have sufficient
      information to verify the points above.

22.  References

22.1.  Normative References

   [CORE.Parameters]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters", <https://www.iana.org/assignments/core-
              parameters/core-parameters.xhtml>.

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

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   [COSE.Elliptic.Curves]
              IANA, "COSE Elliptic Curves",
              <https://www.iana.org/assignments/cose/
              cose.xhtml#elliptic-curves>.

   [COSE.Key.Types]
              IANA, "COSE Key Types",
              <https://www.iana.org/assignments/cose/cose.xhtml#key-
              type>.

   [CWT.Confirmation.Methods]
              IANA, "CWT Confirmation Methods",
              <https://www.iana.org/assignments/cwt/
              cwt.xhtml#confirmation-methods>.

   [I-D.ietf-ace-aif]
              Bormann, C., "An Authorization Information Format (AIF)
              for ACE", draft-ietf-ace-aif-02 (work in progress),
              February 2021.

   [I-D.ietf-ace-key-groupcomm]
              Palombini, F. and M. Tiloca, "Key Provisioning for Group
              Communication using ACE", draft-ietf-ace-key-groupcomm-11
              (work in progress), February 2021.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-37
              (work in progress), February 2021.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE Profile of the Authentication and Authorization
              for Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-16 (work in progress), January 2021.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J., and
              J. Park, "Group OSCORE - Secure Group Communication for
              CoAP", draft-ietf-core-oscore-groupcomm-11 (work in
              progress), February 2021.

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-12
              (work in progress), September 2020.

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   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", draft-ietf-cose-rfc8152bis-
              struct-15 (work in progress), February 2021.

   [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>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <https://www.rfc-editor.org/info/rfc5705>.

   [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>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <https://www.rfc-editor.org/info/rfc6979>.

   [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>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [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>.

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   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
              <https://www.rfc-editor.org/info/rfc8447>.

   [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>.

   [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>.

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

   [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>.

22.2.  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)", draft-ietf-ace-dtls-
              authorize-15 (work in progress), January 2021.

   [I-D.ietf-ace-oscore-gm-admin]
              Tiloca, M., Hoeglund, R., Stok, P., Palombini, F., and K.
              Hartke, "Admin Interface for the OSCORE Group Manager",
              draft-ietf-ace-oscore-gm-admin-02 (work in progress),
              February 2021.

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

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   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              ietf-core-groupcomm-bis-03 (work in progress), February
              2021.

   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsuess, C., and P. Stok, "Discovery of OSCORE
              Groups with the CoRE Resource Directory", draft-tiloca-
              core-oscore-discovery-08 (work in progress), February
              2021.

   [NIST-800-56A]
              Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
              Davis, "Recommendation for Pair-Wise Key-Establishment
              Schemes Using Discrete Logarithm Cryptography - NIST
              Special Publication 800-56A, Revision 3", April 2018,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-56Ar3.pdf>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [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>.

   [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>.

22.3.  URIs

   [1] mailto:iesg@ietf.org

   [2] mailto:marco.tiloca@ri.se

Appendix A.  Profile Requirements

   This appendix lists the specifications on this application profile of
   ACE, based on the requirements defined in Appendix A of
   [I-D.ietf-ace-key-groupcomm].

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   o  REQ1 - If the value of the GROUPNAME URI path and the group name
      in the Access Token scope (gname in Section 3.1 of
      [I-D.ietf-ace-key-groupcomm]) do not match, specify the mechanism
      to map the GROUPNAME value in the URI to the group name: not
      applicable, since a match is required.

   o  REQ2 - Specify the encoding and value of roles, for scope entries
      of 'scope': see Section 3 and Section 4.1.

   o  REQ3 - if used, specify the acceptable values for 'sign_alg':
      values from the "Value" column of the "COSE Algorithms" Registry
      [COSE.Algorithms].

   o  REQ4 - If used, specify the acceptable values for
      'sign_parameters': format and values from the COSE algorithm
      capabilities as specified in the "COSE Algorithms" Registry
      [COSE.Algorithms] and from the COSE key type capabilities as
      specified in the "COSE Key Types" Registry [COSE.Key.Types].

   o  REQ5 - If used, specify the acceptable values for
      'sign_key_parameters': format and values from the COSE key type
      capabilities as specified in the "COSE Key Types" Registry
      [COSE.Key.Types].

   o  REQ6 - If used, specify the acceptable values for 'pub_key_enc': 1
      ("COSE_Key") from the 'Confirmation Key' column of the "CWT
      Confirmation Method" Registry [CWT.Confirmation.Methods].  Future
      specifications may define additional values for this parameter.

   o  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 of [I-D.ietf-ace-key-groupcomm]): the Resource Type
      (rt=) Link Target Attribute value "core.osc.gm" is registered in
      Section 21.10.

   o  REQ8 - Define what operations (e.g.  CoAP methods) are allowed on
      each resource, for each role defined in REQ2: see Section 5.2.

   o  REQ9 - Specify the exact encoding of group identifier (see
      Section 4.1.1.1 of [I-D.ietf-ace-key-groupcomm]): CBOR byte string
      (see Section 15).

   o  REQ10 - Format of the 'key' value: see Section 6.4.

   o  REQ11 - Acceptable values of 'gkty': Group_OSCORE_Input_Material
      object (see Section 6.4).

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   o  REQ12 - Specify the format of the identifiers of group members:
      CBOR byte string (see Section 6.4 and Section 10).

   o  REQ13 - Specify the communication protocol that the members of the
      group must use: CoAP [RFC7252], possibly over IP multicast
      [I-D.ietf-core-groupcomm-bis].

   o  REQ14 - Specify the security protocols that the group members must
      use to protect their communication: Group OSCORE
      [I-D.ietf-core-oscore-groupcomm].

   o  REQ15 - Specify and register the application profile identifier:
      coap_group_oscore_app (see Section 21.3).

   o  REQ16 - Specify policies at the KDC to handle member ids that are
      not included in 'get_pub_keys': see Section 10.

   o  REQ17 - If used, specify the format and content of
      'group_policies' and its entries: see Section 6.4.

   o  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 9.

   o  REQ19 - Specify how the communication is secured between the
      Client and KDC: by means of any transport profile of ACE
      [I-D.ietf-ace-oauth-authz] between Client and Group Manager that
      complies with the requirements in Appendix C of
      [I-D.ietf-ace-oauth-authz].

   o  REQ20 - Specify how the nonce N_S is generated, if the token is
      not being posted (e.g. if it is used directly to validate TLS
      instead): see Section 6.2.1.

   o  REQ21 - Specify if 'mgt_key_material' is used, and if yes specify
      its format and content: not used in this version of the profile.

   o  REQ22 - Define the initial value of the 'num' parameter: The
      initial value MUST be set to 0 when creating the OSCORE group,
      e.g. as in [I-D.ietf-ace-oscore-gm-admin].

   o  REQ23 - Specify and register the identifier of newly defined
      semantics for binary scopes: see Section 21.11.

   o  OPT1 (Optional) - Specify the encoding of public keys, of
      'client_cred', and of 'pub_keys' if COSE_Keys are not used: no.

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   o  OPT2 (Optional) - Specify the negotiation of parameter values for
      signature algorithm and signature keys, if 'sign_info' is not
      used: possible early discovery by using the approach based on the
      CoRE Resource Directory described in
      [I-D.tiloca-core-oscore-discovery].

   o  OPT3 (Optional) - Specify additional parameters used in the Token
      Post exchange: 'ecdh_info', to negotiate the ECDH algorithm, ECDH
      algorithm parameters, ECDH key parameters and exact encoding of
      public keys used in the group, in case the joining node supports
      the pairwise mode of Group OSCORE.

   o  OPT4 (Optional) - Specify the encoding of 'pub_keys_repos' if the
      default is not used: no.

   o  OPT5 (Optional) - Specify policies that instruct clients to retain
      unsuccessfully decrypted messages and for how long, so that they
      can be decrypted after getting updated keying material: no.

   o  OPT6 (Optional) - Specify the behavior of the handler in case of
      failure to retrieve a public key for the specific node: send a
      4.00 Bad Request response to a Joining Request (see Section 6.3).

   o  OPT7 (Optional) - Specify possible or required payload formats for
      specific error cases: send a 4.00 Bad Request response to a
      Joining Request (see Section 6.3).

   o  OPT8 (Optional) - Specify CBOR values to use for abbreviating
      identifiers of roles in the group or topic: see Section 4.1.

   o  OPT9 (Optional) - Specify for the KDC to perform group rekeying
      (together or instead of renewing individual keying material) when
      receiving a Key Renewal Request: the Group Manager SHOULD NOT
      perform a group rekeying, unless already scheduled to occur
      shortly (see Section 9).

   o  OPT10 (Optional) - 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 of
      [I-D.ietf-ace-key-groupcomm]): see Section 17 for the eviction of
      a group member; see Section 18 for the group rekeying process.

   o  OPT11 (Optional) - Specify how the identifier of the sender's
      public key is included in the group request: no.

   o  OPT12 (Optional) - Specify additional identifiers of error types,
      as values of the 'error' field in an error response from the KDC:
      see Section 21.12.

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Appendix B.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

B.1.  Version -09 to -10

   o  Updated non-recycling policy of Sender IDs.

   o  Removed policies about Sender Sequence Number synchronization.

   o  'control_path' renamed to 'control_uri'.

   o  Format of 'get_pub_keys' aligned with draft-ietf-ace-key-
      groupcomm.

   o  Additional way to inform of group eviction.

   o  Registered semantics identifier for extended scope format.

   o  Extended error handling, with error type specified in some error
      responses.

   o  Renumbered requirements.

B.2.  Version -08 to -09

   o  The url-path "ace-group" is used.

   o  Added overview of admitted methods on the Group Manager resources.

   o  Added exchange of parameters relevant for the pairwise mode of
      Group OSCORE.

   o  The signed value for 'client_cred_verify' includes also the scope.

   o  Renamed the key material object as Group_OSCORE_Input_Material
      object.

   o  Replaced 'clientId' with 'group_SenderId'.

   o  Added message exchange for Group Names request-response.

   o  No reassignment of Sender ID and Gid in the same OSCORE group.

   o  Updates on group rekeying contextual with request of new Sender
      ID.

   o  Signature verifiers can also retrieve Group Names and URIs.

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   o  Removed group policy about supporting Group OSCORE in pairwise
      mode.

   o  Registration of the resource type rt="core.osc.gm".

   o  Update list of requirements.

   o  Clarifications and editorial revision.

B.3.  Version -07 to -08

   o  AIF specific data model to express scope entries.

   o  A set of roles is checked as valid when processing the Joining
      Request.

   o  Updated format of 'get_pub_keys' in the Joining Request.

   o  Payload format and default values of group policies in the Joining
      Response.

   o  Updated payload format of the FETCH request to retrieve public
      keys.

   o  Default values for group configuration parameters.

   o  IANA registrations to support the AIF specific data model.

B.4.  Version -06 to -07

   o  Alignments with draft-ietf-core-oscore-groupcomm.

   o  New format of 'sign_info', using the COSE capabilities.

   o  New format of Joining Response parameters, using the COSE
      capabilities.

   o  Considerations on group rekeying.

   o  Editorial revision.

B.5.  Version -05 to -06

   o  Added role of external signature verifier.

   o  Parameter 'rsnonce' renamed to 'kdcchallenge'.

   o  Parameter 'kdcchallenge' may be omitted in some cases.

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   o  Clarified difference between group name and OSCORE Gid.

   o  Removed the role combination ["requester", "monitor"].

   o  Admit implicit scope and audience in the Authorization Request.

   o  New format for the 'sign_info' parameter.

   o  Scope not mandatory to include in the Joining Request.

   o  Group policy about supporting Group OSCORE in pairwise mode.

   o  Possible individual rekeying of a single requesting node combined
      with a group rekeying.

   o  Security considerations on reusage of signature challenges.

   o  Addressing optional requirement OPT9 from draft-ietf-ace-key-
      groupcomm

   o  Editorial improvements.

B.6.  Version -04 to -05

   o  Nonce N_S also in error responses to the Joining Requests.

   o  Supporting single Access Token for multiple groups/topics.

   o  Supporting legal requesters/responders using the 'peer_roles'
      parameter.

   o  Registered and used dedicated label for TLS Exporter.

   o  Added method for uploading a new public key to the Group Manager.

   o  Added resource and method for retrieving the current group status.

   o  Fixed inconsistency in retrieving group keying material only.

   o  Clarified retrieval of keying material for monitor-only members.

   o  Clarification on incrementing version number when rekeying the
      group.

   o  Clarification on what is re-distributed with the group rekeying.

   o  Security considerations on the size of the nonces used for the
      signature challenge.

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   o  Added CBOR values to abbreviate role identifiers in the group.

B.7.  Version -03 to -04

   o  New abstract.

   o  Moved general content to draft-ietf-ace-key-groupcomm

   o  Terminology: node name; node resource.

   o  Creation and pointing at node resource.

   o  Updated Group Manager API (REST methods and offered services).

   o  Size of challenges 'cnonce' and 'rsnonce'.

   o  Value of 'rsnonce' for reused or non-traditionally-posted tokens.

   o  Removed reference to RFC 7390.

   o  New requirements from draft-ietf-ace-key-groupcomm

   o  Editorial improvements.

B.8.  Version -02 to -03

   o  New sections, aligned with the interface of ace-key-groupcomm .

   o  Exchange of information on the countersignature algorithm and
      related parameters, during the Token POST (Section 4.1).

   o  Nonce 'rsnonce' from the Group Manager to the Client
      (Section 4.1).

   o  Client PoP signature in the Key Distribution Request upon joining
      (Section 4.2).

   o  Local actions on the Group Manager, upon a new node's joining
      (Section 4.2).

   o  Local actions on the Group Manager, upon a node's leaving
      (Section 12).

   o  IANA registration in ACE Groupcomm Parameters Registry.

   o  More fulfilled profile requirements (Appendix A).

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B.9.  Version -01 to -02

   o  Editorial fixes.

   o  Changed: "listener" to "responder"; "pure listener" to "monitor".

   o  Changed profile name to "coap_group_oscore_app", to reflect it is
      an application profile.

   o  Added the 'type' parameter for all requests to a Join Resource.

   o  Added parameters to indicate the encoding of public keys.

   o  Challenge-response for proof-of-possession of signature keys
      (Section 4).

   o  Renamed 'key_info' parameter to 'sign_info'; updated its format;
      extended to include also parameters of the countersignature key
      (Section 4.1).

   o  Code 4.00 (Bad request), in responses to joining nodes providing
      an invalid public key (Section 4.3).

   o  Clarifications on provisioning and checking of public keys
      (Sections 4 and 6).

   o  Extended discussion on group rekeying and possible different
      approaches (Section 7).

   o  Extended security considerations: proof-of-possession of signature
      keys; collision of OSCORE Group Identifiers (Section 8).

   o  Registered three entries in the IANA Registry "Sequence Number
      Synchronization Method Registry" (Section 9).

   o  Registered one public key encoding in the "ACE Public Key
      Encoding" IANA Registry (Section 9).

B.10.  Version -00 to -01

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

   o  Added negotiation of countersignature algorithm/parameters between
      Client and Group Manager (Section 4).

   o  Updated format of the Key Distribution Response as a whole
      (Section 4.3).

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   o  Added parameter 'cs_params' in the 'key' parameter of the Key
      Distribution Response (Section 4.3).

   o  New IANA registrations in the "ACE Authorization Server Request
      Creation Hints" Registry, "ACE Groupcomm Key" Registry, "OSCORE
      Security Context Parameters" Registry and "ACE Groupcomm Profile"
      Registry (Section 9).

Acknowledgments

   The authors sincerely thank Santiago Aragon, Stefan Beck, Carsten
   Bormann, Martin Gunnarsson, Rikard Hoeglund, Daniel Migault, Jim
   Schaad, Ludwig Seitz, Goeran Selander and Peter van der Stok for
   their comments and feedback.

   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

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   Kista  SE-164 29 Stockholm
   Sweden

   Email: marco.tiloca@ri.se

   Jiye Park
   Universitaet Duisburg-Essen
   Schuetzenbahn 70
   Essen  45127
   Germany

   Email: ji-ye.park@uni-due.de

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

   Email: francesca.palombini@ericsson.com

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