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Key Management for OSCORE Groups in ACE
draft-ietf-ace-key-groupcomm-oscore-14

Document Type Active Internet-Draft (ace WG)
Authors Marco Tiloca , Jiye Park , Francesca Palombini
Last updated 2022-04-28
Replaces draft-tiloca-ace-oscoap-joining
Stream Internet Engineering Task Force (IETF)
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Sep 2021
Submission to the IESG of "Key Management for OSCORE Groups in ACE"
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draft-ietf-ace-key-groupcomm-oscore-14
ACE Working Group                                              M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                                 J. Park
Expires: 30 October 2022                     Universitaet Duisburg-Essen
                                                            F. Palombini
                                                             Ericsson AB
                                                           28 April 2022

                Key Management for OSCORE Groups in ACE
                 draft-ietf-ace-key-groupcomm-oscore-14

Abstract

   This document 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
   are secured with Group Object Security for Constrained RESTful
   Environments (Group 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on 30 October 2022.

Copyright Notice

   Copyright (c) 2022 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 to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Format of Scope . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Authentication Credentials  . . . . . . . . . . . . . . . . .  10
   5.  Authorization to Join a Group . . . . . . . . . . . . . . . .  12
     5.1.  Authorization Request . . . . . . . . . . . . . . . . . .  12
     5.2.  Authorization Response  . . . . . . . . . . . . . . . . .  13
     5.3.  Token Transferring  . . . . . . . . . . . . . . . . . . .  13
       5.3.1.  'ecdh_info' Parameter . . . . . . . . . . . . . . . .  16
       5.3.2.  'kdc_dh_creds' Parameter  . . . . . . . . . . . . . .  18
   6.  Group Joining . . . . . . . . . . . . . . . . . . . . . . . .  20
     6.1.  Send the Joining Request  . . . . . . . . . . . . . . . .  20
       6.1.1.  Value of the N_S Challenge  . . . . . . . . . . . . .  22
     6.2.  Receive the Joining Request . . . . . . . . . . . . . . .  22
       6.2.1.  Follow-up to a 4.00 (Bad Request) Error Response  . .  25
     6.3.  Send the Joining Response . . . . . . . . . . . . . . . .  26
     6.4.  Receive the Joining Response  . . . . . . . . . . . . . .  32
   7.  Overview of the Group Rekeying Process  . . . . . . . . . . .  34
     7.1.  Stale OSCORE Sender IDs . . . . . . . . . . . . . . . . .  35
   8.  Interface at the Group Manager  . . . . . . . . . . . . . . .  37
     8.1.  ace-group/GROUPNAME/active  . . . . . . . . . . . . . . .  37
       8.1.1.  GET Handler . . . . . . . . . . . . . . . . . . . . .  37
     8.2.  ace-group/GROUPNAME/verif-data  . . . . . . . . . . . . .  38
       8.2.1.  GET Handler . . . . . . . . . . . . . . . . . . . . .  38
     8.3.  ace-group/GROUPNAME/stale-sids  . . . . . . . . . . . . .  38
       8.3.1.  FETCH Handler . . . . . . . . . . . . . . . . . . . .  38
     8.4.  Admitted Methods  . . . . . . . . . . . . . . . . . . . .  39
       8.4.1.  Signature Verifiers . . . . . . . . . . . . . . . . .  40
     8.5.  Operations Supported by Clients . . . . . . . . . . . . .  41
   9.  Additional Interactions with the Group Manager  . . . . . . .  41
     9.1.  Retrieve Updated Keying Material  . . . . . . . . . . . .  42
       9.1.1.  Get Group Keying Material . . . . . . . . . . . . . .  42
       9.1.2.  Get Group Keying Material and OSCORE Sender ID  . . .  42
     9.2.  Request to Change Individual Keying Material  . . . . . .  43
     9.3.  Retrieve Authentication Credentials of Group Members  . .  45
     9.4.  Upload a New Authentication Credential  . . . . . . . . .  45

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     9.5.  Retrieve the Group Manager's Authentication Credential  .  47
     9.6.  Retrieve Signature Verification Data  . . . . . . . . . .  48
     9.7.  Retrieve the Group Policies . . . . . . . . . . . . . . .  50
     9.8.  Retrieve the Keying Material Version  . . . . . . . . . .  50
     9.9.  Retrieve the Group Status . . . . . . . . . . . . . . . .  50
     9.10. Retrieve Group Names  . . . . . . . . . . . . . . . . . .  51
     9.11. Leave the Group . . . . . . . . . . . . . . . . . . . . .  54
   10. Removal of a Group Member . . . . . . . . . . . . . . . . . .  54
   11. Group Rekeying Process  . . . . . . . . . . . . . . . . . . .  56
     11.1.  Sending Rekeying Messages  . . . . . . . . . . . . . . .  58
     11.2.  Receiving Rekeying Messages  . . . . . . . . . . . . . .  60
     11.3.  Missed Rekeying Instances  . . . . . . . . . . . . . . .  61
       11.3.1.  Retrieve Stale Sender IDs  . . . . . . . . . . . . .  63
   12. ACE Groupcomm Parameters  . . . . . . . . . . . . . . . . . .  65
   13. ACE Groupcomm Error Identifiers . . . . . . . . . . . . . . .  67
   14. Default Values for Group Configuration Parameters . . . . . .  68
     14.1.  Common . . . . . . . . . . . . . . . . . . . . . . . . .  68
     14.2.  Group Mode . . . . . . . . . . . . . . . . . . . . . . .  69
     14.3.  Pairwise Mode  . . . . . . . . . . . . . . . . . . . . .  70
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  71
     15.1.  Management of OSCORE Groups  . . . . . . . . . . . . . .  71
     15.2.  Size of Nonces as Proof-of-Possesion Challenge . . . . .  72
     15.3.  Reusage of Nonces for Proof-of-Possession Input  . . . .  73
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  74
     16.1.  OAuth Parameters . . . . . . . . . . . . . . . . . . . .  74
     16.2.  OAuth Parameters CBOR Mappings . . . . . . . . . . . . .  74
     16.3.  ACE Groupcomm Parameters . . . . . . . . . . . . . . . .  75
     16.4.  ACE Groupcomm Key Types  . . . . . . . . . . . . . . . .  76
     16.5.  ACE Groupcomm Profiles . . . . . . . . . . . . . . . . .  76
     16.6.  OSCORE Security Context Parameters . . . . . . . . . . .  76
     16.7.  TLS Exporter Labels  . . . . . . . . . . . . . . . . . .  78
     16.8.  AIF  . . . . . . . . . . . . . . . . . . . . . . . . . .  79
     16.9.  CoAP Content-Format  . . . . . . . . . . . . . . . . . .  79
     16.10. Group OSCORE Roles . . . . . . . . . . . . . . . . . . .  80
     16.11. CoRE Resource Type . . . . . . . . . . . . . . . . . . .  80
     16.12. ACE Scope Semantics  . . . . . . . . . . . . . . . . . .  81
     16.13. ACE Groupcomm Errors . . . . . . . . . . . . . . . . . .  81
     16.14. Expert Review Instructions . . . . . . . . . . . . . . .  82
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  82
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  82
     17.2.  Informative References . . . . . . . . . . . . . . . . .  86
   Appendix A.  Profile Requirements . . . . . . . . . . . . . . . .  88
     A.1.  Mandatory-to-Address Requirements . . . . . . . . . . . .  88
     A.2.  Optional-to-Address Requirements  . . . . . . . . . . . .  91
   Appendix B.  Extensibility for Future COSE Algorithms . . . . . .  92
     B.1.  Format of 'ecdh_info_entry' . . . . . . . . . . . . . . .  93
     B.2.  Format of 'key' . . . . . . . . . . . . . . . . . . . . .  94
   Appendix C.  Document Updates . . . . . . . . . . . . . . . . . .  95

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     C.1.  Version -13 to -14  . . . . . . . . . . . . . . . . . . .  95
     C.2.  Version -12 to -13  . . . . . . . . . . . . . . . . . . .  95
     C.3.  Version -11 to -12  . . . . . . . . . . . . . . . . . . .  95
     C.4.  Version -10 to -11  . . . . . . . . . . . . . . . . . . .  96
     C.5.  Version -09 to -10  . . . . . . . . . . . . . . . . . . .  97
     C.6.  Version -08 to -09  . . . . . . . . . . . . . . . . . . .  97
     C.7.  Version -07 to -08  . . . . . . . . . . . . . . . . . . .  98
     C.8.  Version -06 to -07  . . . . . . . . . . . . . . . . . . .  98
     C.9.  Version -05 to -06  . . . . . . . . . . . . . . . . . . .  99
     C.10. Version -04 to -05  . . . . . . . . . . . . . . . . . . .  99
     C.11. Version -03 to -04  . . . . . . . . . . . . . . . . . . . 100
     C.12. Version -02 to -03  . . . . . . . . . . . . . . . . . . . 100
     C.13. Version -01 to -02  . . . . . . . . . . . . . . . . . . . 101
     C.14. Version -00 to -01  . . . . . . . . . . . . . . . . . . . 102
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 102
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 102

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
   [I-D.ietf-core-groupcomm-bis], which can employ, for example, IP
   multicast as underlying data transport.  This relies on a Group
   Manager, which 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 document 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.

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1.1.  Terminology

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

   Readers are expected to be familiar with:

   *  The terms and concepts described in 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).

   *  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.

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

   *  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".

   *  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 especially include:

      -  Group Manager, as the entity responsible for a set of groups
         where communications are secured with Group OSCORE.  In this
         document, the Group Manager acts as Resource Server.

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      -  Authentication credential, as the set of information associated
         with an entity, including that entity's public key and
         parameters associated with the public key.  Examples of
         authentication credentials are CBOR Web Tokens (CWTs) and CWT
         Claims Sets (CCSs) [RFC8392], X.509 certificates [RFC7925] and
         C509 certificates [I-D.ietf-cose-cbor-encoded-cert].

   Additionally, this document makes use of the following terminology.

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

   *  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.

   *  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].

   *  Signature verifier: entity external to the OSCORE group and
      intended to verify the signature of messages exchanged in the
      group (see Sections 3.1 and 8.5 of
      [I-D.ietf-core-oscore-groupcomm]).  An authorized signature
      verifier does not join the OSCORE group as an actual member, yet
      it can retrieve the authentication credentials of the current
      group members from the Group Manager.

   *  Signature-only group: an OSCORE group that uses only the group
      mode (see Section 8 of [I-D.ietf-core-oscore-groupcomm]).

   *  Pairwise-only group: an OSCORE group that uses only the pairwise
      mode (see Section 9 of [I-D.ietf-core-oscore-groupcomm]).

2.  Protocol Overview

   Group communication for CoAP has been enabled in
   [I-D.ietf-core-groupcomm-bis] and can be secured with Group Object
   Security for Constrained RESTful Environments (Group OSCORE) as
   specified 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.

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   This document 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 overviewed in Section 2
   of [I-D.ietf-ace-key-groupcomm], when the considered group is
   specifically an OSCORE group.

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

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

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

   *  The Authorization Server associated with the Group Manager is the
      AS.

   A node performs the steps described in Sections 3 and 4.3.1.1 of
   [I-D.ietf-ace-key-groupcomm] in order to obtain an authorization for
   joining an OSCORE group and then to join that group.  The format and
   processing of messages exchanged during such steps are further
   specified in Section 5 and Section 6 of this document.

   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 document, the transport profile to use is pre-configured
   and well-known to nodes participating in constrained applications.

   With respect to what is defined in [I-D.ietf-ace-key-groupcomm]:

   *  The interface provided by the Group Manager extends the original
      interface defined in Section 4.1 of [I-D.ietf-ace-key-groupcomm]
      for the KDC, as specified in Section 8 of this document.

   *  In addition to those defined in Section 8 of
      [I-D.ietf-ace-key-groupcomm], additional parameters are defined in
      this document and summarized in Section 12.

   *  In addition to those defined in Section 9 of
      [I-D.ietf-ace-key-groupcomm], additional error identifiers are
      defined in this document and summarized in Section 13.

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   Finally, 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].

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 used in this profile.

   To this end, this profile uses the Authorization Information Format
   (AIF) [I-D.ietf-ace-aif].  In particular, 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.

   Furthermore, this document defines the new AIF specific data model
   AIF-OSCORE-GROUPCOMM, that this profile MUST use to format and encode
   scope entries.

   In particular, the following holds for each scope entry.

   *  The object identifier ("Toid") is specialized as a CBOR item
      specifying the name of the groups pertaining to the scope entry.

   *  The permission set ("Tperm") is specialized as a CBOR unsigned
      integer with value R, specifying the permissions that the Client
      wishes to have in the groups indicated by "Toid".

   More specifically, the following applies when, as defined in this
   document, a scope entry includes as set of permissions the set of
   roles to take in an OSCORE group.

   *  The object identifier ("Toid") is a CBOR text string, specifying
      the group name for the scope entry.

   *  The permission set ("Tperm") is a CBOR unsigned integer with value
      R, specifying the role(s) that the Client wishes to take in the
      group (REQ1).  The value R is computed as follows.

      -  Each role in the permission set is converted into the
         corresponding numeric identifier X from the "Value" column of
         the "Group OSCORE Roles" registry, for which this document
         defines the entries in Figure 1.

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      -  The set of N numbers is converted into the single value R, by
         taking two to the power of each numeric identifier X_1, X_2,
         ..., X_N, 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 signature of intercepted messages        |
 +-----------+-------+-------------------------------------------------+

        Figure 1: Numeric identifier of roles in an OSCORE group

   The following CDDL [RFC8610] notation defines a scope entry that uses
   the AIF-OSCORE-GROUPCOMM data model and expresses a set of Group
   OSCORE roles from those in Figure 1.

      AIF-OSCORE-GROUPCOMM = AIF-Generic<oscore-gname, oscore-gperm>

      oscore-gname = tstr  ; Group name
      oscore-gperm = uint . bits group-oscore-roles

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

      scope_entry = [oscore-gname, oscore-gperm]

   Future specifications that define new Group OSCORE roles MUST
   register a corresponding numeric identifier in the "Group OSCORE
   Roles" registry defined in Section 16.10 of this document.

   Note that the value 0 is not available to use as numeric identifier
   to specify a Group OSCORE role.  It follows that, when expressing
   Group OSCORE roles to take in a group as per this document, a scope
   entry has the least significant bit of "Tperm" always set to 0.

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   This is an explicit feature of the AIF-OSCORE-GROUPCOMM data model.
   That is, for each scope entry, the least significant bit of "Tperm"
   set to 0 explicitly identifies the scope entry as exactly expressing
   a set of Group OSCORE roles ("Tperm"), pertaining to a single group
   whose name is specified by the string literal in "Toid".

   Instead, by relying on the same AIF-OSCORE-GROUPCOMM data model,
   [I-D.ietf-ace-oscore-gm-admin] defines the format of scope entries
   for Administrator Clients that wish to access an admin interface at
   the Group Manager.  In such scope entries, the least significant bit
   of "Tperm" is always set to 1.

4.  Authentication Credentials

   Source authentication of a message sent within the group and
   protected with Group OSCORE is ensured by means of a digital
   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
   authentication credential from a trusted 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 authentication credentials
   of the group members, and provides those authentication credentials
   to group members if requested to.  Upon joining an OSCORE group, a
   joining node is thus expected to provide its own authentication
   credential to the Group Manager.

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

   *  The joining node is going to join the group exclusively as
      monitor, i.e., it is not going to send messages to the group.  In
      this case, the joining node is not required to provide its own
      authentication credential to the Group Manager, which thus does
      not have to perform any check related to the format of the
      authentication credential, to a signature or ECDH algorithm, and
      to possible parameters associated with the algorithm and the
      public key.  In case the joining node still provides an
      authentication credential in the 'client_cred' parameter of the
      Joining Request (see Section 6.1), the Group Manager silently
      ignores that parameter, as well as the related parameters 'cnonce'
      and 'client_cred_verify'.

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   *  The Group Manager already acquired the authentication credential
      of the joining node during a past joining process.  In this case,
      the joining node MAY choose not to provide again its own
      authentication credential to the Group Manager, in order to limit
      the size of the Joining Request.  The joining node MUST provide
      its own authentication credential again if it has provided the
      Group Manager with multiple authentication credentials during past
      joining processes, intended for different OSCORE groups.  If the
      joining node provides its own authentication credential, the Group
      Manager performs consistency checks as per Section 6.2 and, in
      case of success, considers it as the authentication credential
      associated with the joining node in the OSCORE group.

   *  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.  When establishing the secure communication association, the
          Group Manager obtained from the joining node the joining
          node's authentication credential, in the format used in the
          OSCORE group and including the asymmetric proof-of-possession
          key as public key.  Also, such authentication credential and
          the proof-of-possession key are compatible with the signature
          or ECDH algorithm, and possible associated parameters used in
          the OSCORE group.

          In this case, the Group Manager considers the authentication
          credential as the one associated with the joining node in the
          OSCORE group.  If the joining node is aware that the
          authentication credential and the public key included thereof
          are also valid for the OSCORE group, then the joining node MAY
          choose to not provide again its own authentication credential
          to the Group Manager.

          The joining node MUST provide again its own authentication
          credential if it has provided the Group Manager with multiple
          authentication credentials during past joining processes,
          intended for different OSCORE groups.  If the joining node
          provides its own authentication credential in the
          'client_cred' parameter of the Joining Request (see
          Section 6.1), the Group Manager performs consistency checks as
          per Section 6.2 and, in case of success, considers it as the
          authentication credential associated with the joining node in
          the OSCORE group.

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      2.  The authentication credential is not in the format used in the
          OSCORE group, or else the authentication credential and the
          proof-of-possession key included as public key are not
          compatible with the signature or ECDH algorithm, and possible
          associated parameters used in the OSCORE group.

          In this case, the joining node MUST provide a different
          compatible authentication credential and public key included
          thereof to the Group Manager in the 'client_cred' parameter of
          the Joining Request (see Section 6.1).  Then, the Group
          Manager performs consistency checks on this latest provided
          authentication credential as per Section 6.2 and, in case of
          success, considers it as the authentication credential
          associated with the joining node in the OSCORE group.

   *  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
      authentication credential in the 'client_cred' parameter of the
      Joining Request (see Section 6.1).

5.  Authorization to Join a Group

   This section builds on Section 3 of [I-D.ietf-ace-key-groupcomm] and
   is organized as follows.

   First, Section 5.1 and Section 5.2 describe how the joining node
   interacts with the AS, in order to be authorized to join an OSCORE
   group under a given Group Manager and to obtain an Access Token.
   Then, Section 5.3 describes how the joining node transfers the
   obtained Access Token to the Group Manager.  The following considers
   a joining node that intends to contact the Group Manager for the
   first time.

   Note that what is defined in Section 3 of
   [I-D.ietf-ace-key-groupcomm] applies, and only additions or
   modifications to that specification are defined in this document.

5.1.  Authorization Request

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

   *  If the 'scope' parameter is present:

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      -  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:

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

         o  The set of requested roles is expressed as a single CBOR
            unsigned integer.  This is computed as defined in Section 3,
            from the numerical abbreviations of each requested role
            defined in the "Group OSCORE Roles" registry, for which this
            document defines the entries in Figure 1 (REQ1).

5.2.  Authorization Response

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

   *  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 document.

   *  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 Authorization 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 5.1.

   Furthermore, if the AS uses the extended format of scope defined in
   Section 7 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 16.12 of this document (REQ28).  This indicates that the
   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 document.

5.3.  Token Transferring

   The exchange of Token Transfer Request and Token Transfer Response is
   defined in Section 3.3 of [I-D.ietf-ace-key-groupcomm].  In addition
   to that, the following applies.

   *  The Token Transfer Request MAY additionally contain the following
      parameters, which, if included, MUST have the corresponding values
      defined below (OPT2):

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      -  'ecdh_info' defined in Section 5.3.1 of this document, with
         value the CBOR simple value "null" (0xf6) to request
         information about the ECDH algorithm, the ECDH algorithm
         parameters, the ECDH key parameters and the exact format of
         authentication credentials used in the groups that the Client
         has been authorized to join.  This is relevant in case the
         joining node supports the pairwise mode of Group OSCORE
         [I-D.ietf-core-oscore-groupcomm].

      -  'kdc_dh_creds' defined in Section 5.3.2 of this document, with
         value the CBOR simple value "null" (0xf6) to request the
         Diffie-Hellman authentication credentials of the Group Manager
         for the groups that the Client has been authorized to join.
         That is, each of such authentication credentials includes a
         Diffie-Hellman public key of the Group Manager.  This is
         relevant 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.

   *  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 6.1).

      The 'kdcchallenge' parameter MAY be omitted from the Token
      Transfer Response, if the 'scope' of the Access Token specifies
      only the role "monitor" or only the role "verifier" or only the
      two roles combined, for each and every of the specified groups.

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

      -  'id' MUST NOT refer to OSCORE groups that are pairwise-only
         groups.

      -  'sign_alg' takes value from the "Value" column of the "COSE
         Algorithms" registry [COSE.Algorithms].

      -  'sign_parameters' is a CBOR array.  Its format and value are
         the same of the COSE capabilities array for the algorithm
         indicated in 'sign_alg', as specified for that algorithm in the
         "Capabilities" column of the "COSE Algorithms" registry
         [COSE.Algorithms] (REQ4).

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      -  'sign_key_parameters' is a CBOR array.  Its format and value
         are the same of the COSE capabilities array 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 from the "Label" column of the "COSE
         Header Parameters" registry [COSE.Header.Parameters] (REQ6).
         Consistently with Section 2.3 of
         [I-D.ietf-core-oscore-groupcomm], acceptable values denote a
         format of authentication credential that MUST explicitly
         provide the public key as well as the comprehensive set of
         information related to the public key algorithm, including,
         e.g., the used elliptic curve (when applicable).

         At the time of writing this specification, acceptable formats
         of authentication credentials are CBOR Web Tokens (CWTs) and
         CWT Claims Sets (CCSs) [RFC8392], X.509 certificates [RFC7925]
         and C509 certificates [I-D.ietf-cose-cbor-encoded-cert].
         Further formats may be available in the future, and would be
         acceptable to use as long as they comply with the criteria
         defined above.

         [ As to CWTs and CCSs, the COSE Header Parameters 'kcwt' and
         'kccs' are under pending registration requested by draft-ietf-
         lake-edhoc. ]

         [ As to C509 certificates, the COSE Header Parameters 'c5b' and
         'c5c' are under pending registration requested by draft-ietf-
         cose-cbor-encoded-cert. ]

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

   *  If 'ecdh_info' is included in the Token Transfer Request, the
      Group Manager SHOULD include the 'ecdh_info' parameter in the
      Token Transfer Response, as per the format defined in
      Section 5.3.1.  Note that the field 'id' of each 'ecdh_info_entry'
      specifies the name, or array of group names, for which that
      'ecdh_info_entry' applies to.

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      As an exception, the KDC MAY omit the 'ecdh_info' parameter in the
      Token Transfer Response even if 'ecdh_info' is included in the
      Token Transfer Request, in case all the groups that the Client is
      authorized to join are signature-only groups.

   *  If 'kdc_dh_creds' is included in the Token Transfer Request and
      any of the groups that the Client has been authorized to join is a
      pairwise-only group, then the Group Manager MUST include the
      'kdc_dh_creds' parameter in the Token Transfer Response, as per
      the format defined in Section 5.3.2.  Otherwise, if 'kdc_dh_creds'
      is included in the Token Transfer Request, the Group Manager MAY
      include the 'kdc_dh_creds' parameter in the Token Transfer
      Response.  Note that the field 'id' specifies the group name, or
      array of group names, for which the corresponding 'kdc_dh_creds'
      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 obtained such information by alternative means.  For example,
   information conveyed in the 'sign_info' and 'ecdh_info' parameters
   may have been pre-configured, or the joining node MAY early retrieve
   it 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 (OPT3).

5.3.1.  'ecdh_info' Parameter

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

   This parameter allows the Client and the RS to exchange information
   about an ECDH algorithm as well as about the authentication
   credentials and public keys to accordingly use for deriving Diffie-
   Hellman secrets.  Its exact semantics and content are application
   specific.

   In this application profile, this parameter is used to exchange
   information about the ECDH algorithm as well as about the
   authentication credentials and public keys to be used with it, in the
   groups indicated by the transferred Acces Token as per its 'scope'
   claim (see Section 3.2 of [I-D.ietf-ace-key-groupcomm]).

   When used in the Token Transfer Request sent to the Group Manager,
   the 'ecdh_info' parameter has value the CBOR simple value "null"
   (0xf6).  This is done to ask for information about the ECDH algorithm

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   as well as about the authentication credentials and public keys to be
   used to compute static-static Diffie-Hellman shared secrets
   [NIST-800-56A], in the OSCORE groups that the Client has been
   authorized to join and that use the pairwise mode of Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].

   When used in the following Token Transfer Response from the Group
   Manager, the 'ecdh_info' parameter is a CBOR array of one or more
   elements.  The number of elements is at most the number of OSCORE
   groups that the Client has been authorized to join.

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

   *  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.  In particular 'id' MUST NOT refer to OSCORE
      groups that are signature-only groups.

   *  The second element 'ecdh_alg' is a 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].

   *  The third element 'ecdh_parameters' is a CBOR array indicating the
      parameters of the ECDH algorithm used in the OSCORE group
      identified by 'gname'.  Its format and value are the same of the
      COSE capabilities array for the algorithm indicated in 'ecdh_alg',
      as specified for that algorithm in the "Capabilities" column of
      the "COSE Algorithms" registry [COSE.Algorithms].

   *  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 array 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].

   *  The fifth element 'cred_fmt' is a CBOR integer indicating the
      format of authentication credentials used in the OSCORE group
      identified by 'gname'.  It takes value from the "Label" column of
      the "COSE Header Parameters" registry [COSE.Header.Parameters]
      (REQ6).  Acceptable values denote a format that MUST provide the

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      public key as well as the comprehensive set of information related
      to the public key algorithm, including, e.g., the used elliptic
      curve (when applicable).  The same considerations and guidelines
      for the 'pub_key_enc' element of 'sign_info' apply (see
      Section 5.3).

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

   ecdh_info = ecdh_info_req / ecdh_info_resp

   ecdh_info_req = null                  ; in the Token Transfer
                                         ; Request to the
                                         ; Group Manager

   ecdh_info_res = [ + ecdh_info_entry ] ; in the Token Transfer
                                         ; Response from the
                                         ; Group Manager

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

   gname = tstr

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

5.3.2.  'kdc_dh_creds' Parameter

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

   This parameter allows the Client to request and retrieve the Diffie-
   Hellman authentication credentials of the RS, i.e., authentication
   credentials including a Diffie-Hellman public key of the RS.

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   In this application profile, this parameter is used to request and
   retrieve from the Group Manager its Diffie-Hellman authentication
   credentials to use, in the OSCORE groups that the Client has been
   authorized to join.  The Group Manager has specifically a Diffie-
   Hellman authentication credential in an OSCORE group, and thus a
   Diffie-Hellman public key in that group, if and only if the group is
   a pairwise-only group.  In this case, the early retrieval of the
   Group Manager's authentication credential is necessary in order for
   the joining node to prove the possession of its own private key, upon
   joining the group (see Section 6.1).

   When used in the Token Transfer Request sent to the Group Manager,
   the 'kdc_dh_creds' parameter has value the CBOR simple value "null"
   (0xf6).  This is done to ask for the Diffie-Hellman authentication
   credentials that the Group Manager uses in the OSCORE groups that the
   Client has been authorized to join.

   When used in the following Token Transfer Response from the Group
   Manager, the 'kdc_dh_creds' parameter is a CBOR array of one or more
   elements.  The number of elements is at most the number of OSCORE
   groups that the Client has been authorized to join.

   Each element 'kdc_dh_creds_entry' contains information about the
   Group Manager's Diffie-Hellman authentication credentials, for one or
   more OSCORE groups that are pairwise-only groups and that the Client
   has been authorized to join.  Each element is formatted as follows.

   *  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.  In particular 'id' MUST refer exclusively to
      OSCORE groups that are pairwise-only groups.

   *  The second element 'cred_fmt' is a CBOR integer indicating the
      format of authentication credentials used in the OSCORE group
      identified by 'gname'.  It takes value from the "Label" column of
      the "COSE Header Parameters" registry [COSE.Header.Parameters]
      (REQ6).  Acceptable values denote a format that MUST explicitly
      provide the public key as well as comprehensive set of information
      related to the public key algorithm, including, e.g., the used
      elliptic curve (when applicable).  The same considerations and
      guidelines for the 'pub_key_enc' element of 'sign_info' apply (see
      Section 5.3).

   *  The third element 'cred' is a CBOR byte string, which encodes the
      Group Manager's Diffie-Hellman authentication credential in its
      original binary representation made available to other endpoints
      in the group.  In particular, the original binary representation
      complies with the format specified by the 'cred_fmt' element.

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      Note that the authentication credential provides the comprehensive
      set of information related to its public key algorithm, i.e., the
      ECDH algorithm used in the OSCORE group as pairwise key agreement
      algorithm.

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

   kdc_dh_creds = kdc_dh_creds_req / kdc_dh_creds_resp

   kdc_dh_creds_req = null                     ; in the Token Transfer
                                               ; Request to the
                                               ; Group Manager

   kdc_dh_creds_res = [ + kdc_dh_creds_entry ] ; in the Token Transfer
                                               ; Response from the
                                               ; Group Manager

   kdc_dh_creds_entry =
   [
     id : gname / [ + gname ],
     cred_fmt = int,
     cred = bstr
   ]

   gname = tstr

6.  Group Joining

   This section describes the interactions between the joining node and
   the Group Manager to join an OSCORE group.  The message exchange
   between the joining node and the Group Manager consists of the
   messages defined in Section 4.3.1.1 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
   in this document.

6.1.  Send 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 4.3.1.1 of
   [I-D.ietf-ace-key-groupcomm].  Additionally to what is defined in
   Section 4.3.1 of [I-D.ietf-ace-key-groupcomm], the following applies.

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   *  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.

   *  The 'get_pub_keys' parameter is present only if the joining node
      wants to retrieve the authentication credentials of the group
      members from the Group Manager during the joining process (see
      Section 4).  Otherwise, this parameter MUST NOT be present.

      If this parameter is present and its value is not the CBOR simple
      value "null" (0xf6), 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.

   *  '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.

   *  The proof-of-possession (PoP) evidence included in
      'client_cred_verify' is computed as defined below (REQ14).  In
      either case, the N_S used to build the PoP input is as defined in
      Section 6.1.1.

      -  If the group is not a pairwise-only group, the PoP evidence
         MUST be a signature.  The joining node computes the signature
         by using the same private key and signature algorithm it
         intends to use for signing messages in the OSCORE group.

      -  If the group is a pairwise-only group, the PoP evidence MUST be
         a MAC computed as follows, by using the HKDF Algorithm HKDF
         SHA-256, which consists of composing the HKDF-Extract and HKDF-
         Expand steps [RFC5869].

         MAC = HKDF(salt, IKM, info, L)

         The input parameters of HKDF are as follows.

         o  salt takes as value the empty byte string.

         o  IKM is computed as a cofactor Diffie-Hellman shared secret,
            see Section 5.7.1.2 of [NIST-800-56A], using the ECDH
            algorithm used in the OSCORE group.  The joining node uses
            its own Diffie-Hellman private key and the Diffie-Hellman
            public key of the Group Manager.  For X25519 and X448, the
            procedure is described in Section 5 of [RFC7748].

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         o  info takes as value the PoP input.

         o  L is equal to 8, i.e., the size of the MAC, in bytes.

6.1.1.  Value of the N_S Challenge

   The value of the N_S challenge is determined as follows.

   1.  If the joining node has provided the Access Token to the Group
       Manager by means of a Token Transfer Request to the /authz-info
       endpoint as in Section 5.3, then 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 Token Transfer Response, or the one possibly specified in a
       4.00 (Bad Request) error response to a following Joining Request
       (see Section 6.2).

   2.  If the provisioning of the Access Token to the Group Manager 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], then 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 exporter label
       "EXPORTER-ACE-Sign-Challenge-coap-group-oscore-app" defined in
       Section 16.7 of this document.

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

   Section 15.3 provides security considerations on the reusage of the
   N_S challenge.

6.2.  Receive the Joining Request

   The Group Manager processes the Joining Request as defined in
   Section 4.3.1 of [I-D.ietf-ace-key-groupcomm], with the following
   additions.

   The Group Manager verifies the PoP evidence contained in
   'client_cred_verify' as follows:

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   *  As PoP input, the Group Manager uses 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 6.1.1, while N_C is the nonce
      provided in the 'cnonce' parameter of the Joining Request.

   *  As public key of the joining node, the Group Manager uses either
      the one included in the authentication credential retrieved from
      the 'client_cred' parameter of the Joining Request, or the one
      from the already stored authentication credential as acquired from
      previous interactions with the joining node (see Section 4).

   *  If the group is not a pairwise-only group, the PoP evidence is a
      signature.  The Group Manager verifies it by using the public key
      of the joining node, as well as the signature algorithm used in
      the OSCORE group and possible corresponding parameters.

   *  If the group is a pairwise-only group, the PoP evidence is a MAC.
      The Group Manager recomputes the MAC through the same process
      taken by the joining node when preparing the value of the
      'client_cred_verify' parameter for the Joining Request (see
      Section 6.1), with the difference that the Group Manager uses its
      own Diffie-Hellman private key and the Diffie-Hellman public key
      of the joining node.  The verification succeeds if and only if the
      recomputed MAC is equal to the MAC conveyed as PoP evidence in the
      Joining Request.

   The Group Manager MUST reply with a 5.03 (Service Unavailable) error
   response in the following cases:

   *  There are currently no OSCORE Sender IDs available to assign in
      the OSCORE group and, at the same time, the joining node is not
      going to join the group exclusively as monitor.  The response MUST
      have Content-Format set to application/ace-groupcomm+cbor and is
      formatted as defined in Section 4.1.2 of
      [I-D.ietf-ace-key-groupcomm].  The value of the 'error' field MUST
      be set to 4 ("No available node identifiers").

   *  The OSCORE group that the joining node has been trying to join is
      currently inactive (see Section 8.1).  The response MUST have
      Content-Format set to application/ace-groupcomm+cbor and is
      formatted as defined in Section 4.1.2 of
      [I-D.ietf-ace-key-groupcomm].  The value of the 'error' field MUST
      be set to 9 ("Group currently not active").

   The Group Manager MUST reply with a 4.00 (Bad Request) error response
   in the following cases:

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   *  The 'client_cred' parameter is present in the Joining Request and
      its value is not an eligible authentication credential (e.g., it
      is not of the format accepted in the group).

   *  The 'client_cred' parameter is not present in the Joining Request
      while the joining node is not going to join the group exclusively
      as monitor, and any of the following conditions holds:

      -  The Group Manager does not store an eligible authentication
         credential (e.g., of the format accepted in the group) for the
         joining node.

      -  The Group Manager stores multiple eligible authentication
         credentials (e.g., of the format accepted in the group) for the
         joining node.

   *  The 'scope' parameter is not present in the Joining Request, or 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 for
      members of OSCORE groups MUST define possible sets of roles
      (including the new role and existing roles) that are acceptable to
      specify in the 'scope' parameter of a Joining Request.

   *  The Joining Request includes the 'client_cred' parameter but does
      not include both the 'cnonce' and 'client_cred_verify' parameters.

   In order to prevent the acceptance of Ed25519 and Ed448 public keys
   that cannot be successfully converted to Montgomery coordinates, and
   thus cannot be used for the derivation of pairwise keys (see
   Section 2.4.1 of [I-D.ietf-core-oscore-groupcomm]), the Group Manager
   MAY reply with a 4.00 (Bad Request) error response in case all the
   following conditions hold:

   *  The OSCORE group uses the pairwise mode of Group OSCORE.

   *  The OSCORE group uses EdDSA public keys [RFC8032].

   *  The authentication credential of the joining node from the
      'client_cred' parameter includes a public key which:

      -  Is for the elliptic curve Ed25519 and has its Y coordinate
         equal to -1 or 1 (mod p), with p = (2^255 - 19), see
         Section 4.1 of [RFC7748]; or

      -  Is for the elliptic curve Ed448 and has its Y coordinate equal
         to -1 or 1 (mod p), with p = (2^448 - 2^224 - 1), see
         Section 4.2 of [RFC7748].

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   A 4.00 (Bad Request) error response from the Group Manager to the
   joining node MUST have content format application/ace-groupcomm+cbor.
   The response payload is a CBOR map formatted as follows:

   *  If the group uses (also) the group mode of Group OSCORE, the CBOR
      map MUST contain the 'sign_info' parameter, whose CBOR label is
      defined in Section 8 of [I-D.ietf-ace-key-groupcomm].  This
      parameter has the same format of 'sign_info_res' defined in
      Section 3.3.1 of [I-D.ietf-ace-key-groupcomm].  In particular, it
      includes 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 uses (also) the pairwise mode of Group OSCORE, the
      CBOR map MUST contain the 'ecdh_info' parameter, whose CBOR label
      is defined in Section 16.3.  This parameter has the same format of
      'ecdh_info_res' defined in Section 5.3.1.  In particular, it
      includes a single element 'ecdh_info_entry' pertaining to the
      OSCORE group that the joining node has tried to join with the
      Joining Request.

   *  If the group is a pairwise-only group, the CBOR map MUST contain
      the 'kdc_dh_creds' parameter, whose CBOR label is defined in
      Section 16.3.  This parameter has the same format of
      'kdc_dh_creds_res' defined in Section 5.3.2.  In particular, it
      includes a single element 'kdc_dh_creds_entry' pertaining to the
      OSCORE group that the joining node has tried to join with the
      Joining Request.

   *  The CBOR map MAY include the 'kdcchallenge' parameter, whose CBOR
      label is defined in Section 8 of [I-D.ietf-ace-key-groupcomm].  If
      present, this parameter is a CBOR byte string, which encodes a
      newly generated 'kdcchallenge' value that the Client can use when
      preparing a Joining Request (see Section 6.1).  In such a case the
      Group Manager MUST store the newly generated value as the
      'kdcchallenge' value associated with the joining node, possibly
      replacing the currently stored value.

6.2.1.  Follow-up to a 4.00 (Bad Request) Error Response

   When receiving a 4.00 (Bad Request) error response, the joining node
   MAY send a new Joining Request to the Group Manager.  In such a case:

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

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   *  The 'client_cred' parameter MUST include an authentication
      credential in the format indicated by the Group Manager.  Also,
      the authentication credential as well as the included public key
      MUST be compatible with the signature or ECDH algorithm, and
      possible associated parameters.

   *  The 'client_cred_verify' parameter MUST include a PoP evidence
      computed as described in Section 6.1, by using the private key
      associated with the authentication credential specified in the
      current 'client_cred' parameter, with the signature or ECDH
      algorithm, and possible associated parameters indicated by the
      Group Manager.  If the error response from the Group Manager
      includes the 'kdcchallenge' parameter, the joining node MUST use
      its content as new N_S challenge to compute the PoP evidence.

6.3.  Send the Joining Response

   If the processing of the Joining Request described in Section 6.2 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 4.3.1 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.

   *  The Group Manager selects an available OSCORE Sender ID in the
      OSCORE group, and exclusively assigns it to the joining node.  The
      Group Manager MUST NOT assign an OSCORE Sender ID to the joining
      node if this joins the group exclusively with the role of monitor,
      according to what is specified in the Access Token (see
      Section 5.2).

      Consistently with Section 3.2.1 of
      [I-D.ietf-core-oscore-groupcomm], the Group Manager MUST assign an
      OSCORE Sender ID that has not been used in the OSCORE group since
      the latest time when the current Gid value was assigned to the
      group.

      If the joining node is recognized as a current group member, e.g.,
      through the ongoing secure communication association, the
      following also applies.

      -  The Group Manager MUST assign a new OSCORE Sender ID different
         than the one currently used by the joining node in the OSCORE
         group.

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      -  The Group Manager MUST add the old, relinquished OSCORE Sender
         ID of the joining node to the most recent set of stale Sender
         IDs, in the collection associated with the group (see
         Section 7.1).

   *  The Group Manager stores the association between i) the
      authentication credential of the joining node; and ii) the Group
      Identifier (Gid), i.e., the OSCORE ID Context, associated with 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 4.3.1 of
   [I-D.ietf-ace-key-groupcomm], with the following additions:

   *  The 'gkty' parameter identifies a key of type
      "Group_OSCORE_Input_Material object", defined in Section 16.4 of
      this document.

   *  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 document and extends the
      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', 'cred_fmt',
      'sign_enc_alg', 'sign_alg', 'sign_params', 'ecdh_alg' and
      'ecdh_params' defined in Section 16.6 of this document.

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

      -  The 'hkdf' parameter, if present, specifies the HKDF Algorithm
         used in the OSCORE group.  The HKDF Algorithm is specified by
         the HMAC Algorithm value.  This parameter MAY be omitted, if
         the HKDF Algorithm used in the group is HKDF SHA-256.
         Otherwise, this parameter MUST be present.

      -  The 'salt' parameter, if present, has as value the OSCORE
         Master Salt used in the OSCORE group.  This parameter MAY be
         omitted, if the Master Salt used in the group is the empty byte
         string.  Otherwise, this parameter MUST be present.

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

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

      -  The 'group_senderId' parameter has as value the OSCORE Sender
         ID assigned to the joining node by the Group Manager, as
         described above.  This parameter MUST be present if and only if
         the node does not join the OSCORE group exclusively with the
         role of monitor, according to what is specified in the Access
         Token (see Section 5.2).

      -  The 'cred_fmt' parameter specifies the format of authentication
         credentials used in the OSCORE group.  This parameter MUST be
         present and it takes value from the "Label" column of the "COSE
         Header Parameters" registry [COSE.Header.Parameters] (REQ6).
         Consistently with Section 2.3 of
         [I-D.ietf-core-oscore-groupcomm], acceptable values denote a
         format that MUST explicitly provide the public key as well as
         the comprehensive set of information related to the public key
         algorithm, including, e.g., the used elliptic curve (when
         applicable).

         At the time of writing this specification, acceptable formats
         of authentication credentials are CBOR Web Tokens (CWTs) and
         CWT Claims Sets (CCSs) [RFC8392], X.509 certificates [RFC7925]
         and C509 certificates [I-D.ietf-cose-cbor-encoded-cert].
         Further formats may be available in the future, and would be
         acceptable to use as long as they comply with the criteria
         defined above.

         [ As to CWTs and CCSs, the COSE Header Parameters 'kcwt' and
         'kccs' are under pending registration requested by draft-ietf-
         lake-edhoc. ]

         [ As to C509 certificates, the COSE Header Parameters 'c5b' and
         'c5c' are under pending registration requested by draft-ietf-
         cose-cbor-encoded-cert. ]

      The 'key' parameter MUST also include the following parameters, if
      and only if the OSCORE group is not a pairwise-only group.

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      -  The 'sign_enc_alg' parameter, specifying the Signature
         Encryption Algorithm used in the OSCORE group to encrypt
         messages protected with the group mode.  This parameter takes
         values from the "Value" column of the "COSE Algorithms"
         registry [COSE.Algorithms].

      -  The 'sign_alg' parameter, specifying the Signature Algorithm
         used to sign messages in the OSCORE group.  This parameter
         takes values from the "Value" column of the "COSE Algorithms"
         registry [COSE.Algorithms].

      -  The 'sign_params' parameter, specifying the parameters of the
         Signature Algorithm.  This parameter is a CBOR array, which
         includes the following two elements:

         o  'sign_alg_capab': a CBOR array, with the same format and
            value of the COSE capabilities array for the Signature
            Algorithm indicated in 'sign_alg', as specified for that
            algorithm in the "Capabilities" column of the "COSE
            Algorithms" registry [COSE.Algorithms].

         o  'sign_key_type_capab': a CBOR array, with the same format
            and value of the COSE capabilities array for the COSE key
            type of the keys used with the Signature Algorithm indicated
            in 'sign_alg', as specified for that key type in the
            "Capabilities" column of the "COSE Key Types" registry
            [COSE.Key.Types].

      The 'key' parameter MUST also include the following parameters, if
      and only if the OSCORE group is not a signature-only group.

      -  The 'alg' parameter, specifying the AEAD Algorithm used in the
         OSCORE group to encrypt messages protected with the pairwise
         mode.

      -  The 'ecdh_alg' parameter, specifying the Pairwise Key Agreement
         Algorithm used in the OSCORE group.  This parameter takes
         values from the "Value" column of the "COSE Algorithms"
         registry [COSE.Algorithms].

      -  The 'ecdh_params' parameter, specifying the parameters of the
         Pairwise Key Agreement Algorithm.  This parameter is a CBOR
         array, which includes the following two elements:

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         o  'ecdh_alg_capab': a CBOR array, with the same format and
            value of the COSE capabilities array for the algorithm
            indicated in 'ecdh_alg', as specified for that algorithm in
            the "Capabilities" column of the "COSE Algorithms" registry
            [COSE.Algorithms].

         o  'ecdh_key_type_capab': a CBOR array, with the same format
            and value of the COSE capabilities array 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].

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

   Furthermore, the following applies.

   *  The 'exp' parameter MUST be present.

   *  The 'ace-groupcomm-profile' parameter MUST be present and has
      value coap_group_oscore_app (PROFILE_TBD), which is defined in
      Section 16.5 of this document.

   *  The 'pub_keys' parameter, if present, includes the authentication
      credentials requested by the joining node by means of the
      'get_pub_keys' parameter in the Joining Request.

      If the joining node has asked for the authentication credentials
      of all the group members, i.e., 'get_pub_keys' had value the CBOR
      simple value "null" (0xf6) in the Joining Request, then the Group
      Manager provides only the authentication credentials of the group
      members that are relevant to the joining node.  That is, in such a
      case, 'pub_keys' includes only: i) the authentication credentials
      of the responders currently in the OSCORE group, in case the
      joining node is configured (also) as requester; and ii) the
      authentication credentials of the requesters currently in the
      OSCORE group, in case the joining node is configured (also) as
      responder or monitor.

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   *  The 'peer_identifiers' parameter includes the OSCORE Sender ID of
      each group member whose authentication credential is specified in
      the 'pub_keys' parameter.  That is, a group member's Sender ID is
      used as identifier for that group member (REQ25).

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

      -  "Key Update Check Interval" defined in Section 4.3.1 of
         [I-D.ietf-ace-key-groupcomm], with default value 3600;

      -  "Expiration Delta" defined in Section 4.3.1 of
         [I-D.ietf-ace-key-groupcomm], with default value 0.

   *  The 'kdc_cred' parameter MUST be present, specifying the Group
      Manager's authentication credential in its original binary
      representation (REQ8).  The Group Manager's authentication
      credential MUST be in the format used in the OSCORE group.  Also,
      the authentication credential as well as the included public key
      MUST be compatible with the signature or ECDH algorithm, and
      possible associated parameters used in the OSCORE group.

   *  The 'kdc_nonce' parameter MUST be present, specifying the
      dedicated nonce N_KDC generated by the Group Manager.  For N_KDC,
      it is RECOMMENDED to use a 8-byte long random nonce.

   *  The 'kdc_cred_verify' parameter MUST be present, specifying the
      proof-of-possession (PoP) evidence computed by the Group Manager.
      The PoP evidence is computed over the nonce N_KDC, which is
      specified in the 'kdc_nonce' parameter and taken as PoP input.
      The PoP evidence is computed as defined below (REQ21).

      -  If the group is not a pairwise-only group, the PoP evidence
         MUST be a signature.  The Group Manager computes the signature
         by using the signature algorithm used in the OSCORE group, as
         well as its own private key associated with the authentication
         credential specified in the 'kdc_cred' parameter.

      -  If the group is a pairwise-only group, the PoP evidence MUST be
         a MAC computed as follows, by using the HKDF Algorithm HKDF
         SHA-256, which consists of composing the HKDF-Extract and HKDF-
         Expand steps [RFC5869].

         MAC = HKDF(salt, IKM, info, L)

         The input parameters of HKDF are as follows.

         o  salt takes as value the empty byte string.

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         o  IKM is computed as a cofactor Diffie-Hellman shared secret,
            see Section 5.7.1.2 of [NIST-800-56A], using the ECDH
            algorithm used in the OSCORE group.  The Group Manager uses
            its own Diffie-Hellman private key and the Diffie-Hellman
            public key of the joining node.  For X25519 and X448, the
            procedure is described in Section 5 of [RFC7748].

         o  info takes as value the PoP input.

         o  L is equal to 8, i.e., the size of the MAC, in bytes.

   *  The 'group_rekeying' parameter MAY be omitted, if the Group
      Manager uses the "Point-to-Point" group rekeying scheme registered
      in Section 11.14 of [I-D.ietf-ace-key-groupcomm] as rekeying
      scheme in the OSCORE group (OPT9).  Its detailed use for this
      profile is defined in Section 11 of this document.  In any other
      case, the 'group_rekeying' parameter MUST be included.

   As a last action, if the Group Manager reassigns Gid values during
   the group's lifetime (see Section 3.2.1.1 of
   [I-D.ietf-core-oscore-groupcomm]), then the Group Manager MUST store
   the Gid specified in the 'contextId' parameter of the 'key'
   parameter, as the Birth Gid of the joining node in the joined group
   (see Section 3 of [I-D.ietf-core-oscore-groupcomm]).  This applies
   also in case the joining node is in fact re-joining the group; in
   such a case, the newly determined Birth Gid overwrites the one
   currently stored.

6.4.  Receive the Joining Response

   Upon receiving the Joining Response, the joining node retrieves the
   Group Manager's authentication credential from the 'kdc_cred'
   parameter.  The joining node MUST verify the proof-of-possession
   (PoP) evidence specified in the 'kdc_cred_verify' parameter of the
   Joining Response as defined below (REQ21).

   *  If the group is not a pairwise-only group, the PoP evidence is a
      signature.  The joining node verifies it by using the public key
      of the Group Manager from the received authentication credential,
      as well as the signature algorithm used in the OSCORE group and
      possible corresponding parameters.

   *  If the group is a pairwise-only group, the PoP evidence is a MAC.
      The joining node recomputes the MAC through the same process taken
      by the Group Manager when computing the value of the
      'kdc_cred_verify' parameter (see Section 6.3), with the difference
      that the joining node uses its own Diffie-Hellman private key and
      the Diffie-Hellman public key of the Group Manager from the

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      received authentication credential.  The verification succeeds if
      and only if the recomputed MAC is equal to the MAC conveyed as PoP
      evidence in the Joining Response.

   In case of failed verification of the PoP evidence, the joining node
   MUST stop processing the Joining Response and MAY send a new Joining
   Request to the Group Manager (see Section 6.1).

   In case of successful verification of the PoP evidence, 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].  If the following parameters were
   not included in the 'key' parameter of the Joining Response, the
   joining node considers the default values specified below,
   consistently with Section 3.2 of [RFC8613].

   *  Absent the 'hkdf' parameter, the joining node considers HKDF
      SHA-256 as HKDF Algorithm to use in the OSCORE group.

   *  Absent the 'salt' parameter, the joining node considers the empty
      byte string as Master Salt to use in the OSCORE group.

   *  Absent the 'group_rekeying' parameter, the joining node considers
      the "Point-to-Point" group rekeying scheme registered in
      Section 11.14 of [I-D.ietf-ace-key-groupcomm] as the rekeying
      scheme used in the group (OPT9).  Its detailed use for this
      profile is defined in Section 11 of this document.

   In addition, the joining node maintains an association between each
   authentication credential 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:

   *  The joining node MUST NOT process an incoming request message, if
      protected by a group member whose authentication credential is not
      associated with the role "Requester".

   *  The joining node MUST NOT process an incoming response message, if
      protected by a group member whose authentication credential is not
      associated with the role "Responder".

   *  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.

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   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 11).  In such a case, the joining node is not able to
   access secure communication in the OSCORE group occurred prior its
   joining.

7.  Overview of the Group Rekeying Process

   In a number of cases, the Group Manager has to generate new keying
   material and distribute it to the group (rekeying), as also discussed
   in Section 3.2 of [I-D.ietf-core-oscore-groupcomm].

   To this end the Group Manager MUST support the Group Rekeying Process
   described in Section 11 of this document, as an instance of the
   "Point-to-Point" rekeying scheme defined in Section 6.1 of
   [I-D.ietf-ace-key-groupcomm] and registered in Section 11.14 of
   [I-D.ietf-ace-key-groupcomm].  Future documents may define the use of
   alternative group rekeying schemes for this application profile,
   together with the corresponding rekeying message formats.  The
   resulting group rekeying process MUST comply with the functional
   steps defined in Section 3.2 of [I-D.ietf-core-oscore-groupcomm].

   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 OSCORE Sender ID of each
   member in that group.

   The data distributed to a group through a rekeying MUST include:

   *  The new version number of the group keying material for the group.

   *  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]).

      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.

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

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   *  A set of stale Sender IDs, which allows each rekeyed node to purge
      authentication credentials and Recipient Contexts used in the
      group and associated with those Sender IDs.  This in turn allows
      every group member to rely on stored authentication credentials,
      in order to confidently assert the group membership of other
      sender nodes, when receiving protected messages in the group (see
      Section 3.2 of [I-D.ietf-core-oscore-groupcomm]).  More details on
      the maintenance of stale Sender IDs are provided in Section 7.1.

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

   The Group Manager MUST rekey the group in the following cases.

   *  The application requires backward security - In this case, the
      group is rekeyed when a node joins the group as a new member.
      Therefore, a joining node cannot access communications in the
      group prior its joining.

   *  One or more nodes leave the group - That is, the group is rekeyed
      when one or more current members spontaneously request to leave
      the group (see Section 9.11), or when the Group Manager forcibly
      evicts them from the group, e.g., due to expired or revoked
      authorization (see Section 10).  Therefore, a leaving node cannot
      access communications in the group after its leaving, thus
      ensuring forward security in the group.

      Due to the set of stale Sender IDs distributed through the
      rekeying, this ensures that a node owning the latest group keying
      material does not store the authentication credentials of former
      group members (see Sections 3.2 and 12.1 of
      [I-D.ietf-core-oscore-groupcomm]).

   *  Extension of group lifetime - That is, the group is rekeyed when
      the expiration time for the group keying material approaches or
      has passed, if it is appropriate to extend the group operation
      beyond that.

   The Group Manager MAY rekey the group for other reasons, e.g.,
   according to an application-specific rekeying period or scheduling.

7.1.  Stale OSCORE Sender IDs

   Throughout the lifetime of every group, the Group Manager MUST
   maintain a collection of stale Sender IDs for that group.

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   The collection associated with a group MUST include up to N > 1
   ordered sets of stale OSCORE Sender IDs.  It is up to the application
   to specify the value of N, possibly on a per-group basis.

   The N-th set includes the Sender IDs that have become "stale" under
   the current version V of the group keying material.  The (N - 1)-th
   set refers to the immediately previous version (V - 1) of the group
   keying material, and so on.

   In the following cases, the Group Manager MUST add a new element to
   the most recent set X, i.e., the set associated with the current
   version V of the group keying material.

   *  When a current group member obtains a new Sender ID, its old
      Sender ID is added to X.  This happens when the Group Manager
      assigns a new Sender ID upon request from the group member (see
      Section 9.2), or in case the group member re-joins the group (see
      Section 6.1 and Section 6.3), thus also obtaining a new Sender ID.

   *  When a current group member leaves the group, its current Sender
      ID is added to X.  This happens when a group member requests to
      leave the group (see Section 9.11) or is forcibly evicted from the
      group (see Section 10).

   The value of N can change throughout the lifetime of the group.  If
   the new value N' is smaller than N, the Group Manager MUST preserve
   the (up to) N' most recent sets in the collection and MUST delete any
   possible older set from the collection.

   Finally, the Group Manager MUST perform the following actions, when
   the group is rekeyed and the group shifts to the next version V' = (V
   + 1) of the group keying material.

   1.  The Group Manager rekeys the group.  This includes also
       distributing the set of stale Sender IDs X associated with the
       old group keying material with version V (see Section 7).

   2.  After completing the group rekeying, the Group Manager creates a
       new empty set X' associated with the new version V' of the newly
       established group keying material, i.e., V' = (V + 1).

   3.  If the current collection of stale Sender IDs has size N, the
       Group Manager deletes the oldest set in the collection.

   4.  The Group Manager adds the new set X' to the collection of stale
       Sender IDs, as the most recent set.

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8.  Interface at the Group Manager

   The Group Manager provides the interface defined in Section 4.1 of
   [I-D.ietf-ace-key-groupcomm], with the additional sub-resources
   defined from Section 8.1 to Section 8.3 of this document.

   Furthermore, Section 8.4 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 (REQ11).

   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]) (REQ7).

   The Resource Type (rt=) Link Target Attribute value "core.osc.gm" is
   registered in Section 16.11 (REQ10), 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].

8.1.  ace-group/GROUPNAME/active

   This resource implements a GET handler.

8.1.1.  GET Handler

   The handler expects a GET request.

   In addition to what is defined in Section 4.1.2 of
   [I-D.ietf-ace-key-groupcomm], the handler verifies that the
   requesting Client is a current member of the group.  If the
   verification fails, the KDC MUST reply with a 4.03 (Forbidden) error
   response.  The response MUST have Content-Format set to application/
   ace-groupcomm+cbor and is formatted as defined in Section 4.1.2 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 all verifications succeed, the handler replies with a 2.05
   (Content) response, specifying the current status of the group, i.e.,
   active or inactive.  The payload of the response is formatted as
   defined in Section 9.9.

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

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8.2.  ace-group/GROUPNAME/verif-data

   This resource implements a GET handler.

8.2.1.  GET Handler

   The handler expects a GET request.

   In addition to what is defined in Section 4.1.2 of
   [I-D.ietf-ace-key-groupcomm], the Group Manager performs the
   following checks.

   If the requesting Client is a current group member, the Group Manager
   MUST reply with a 4.03 (Forbidden) error response.  The response MUST
   have Content-Format set to application/ace-groupcomm+cbor and is
   formatted as defined in Section 4.1.2 of
   [I-D.ietf-ace-key-groupcomm].  The value of the 'error' field MUST be
   set to 8 ("Operation permitted only to signature verifiers").

   If GROUPNAME denotes a pairwise-only group, the Group Manager MUST
   reply with a 4.00 (Bad Request) error response.  The response MUST
   have Content-Format set to application/ace-groupcomm+cbor and is
   formatted as defined in Section 4.1.2 of
   [I-D.ietf-ace-key-groupcomm].  The value of the 'error' field MUST be
   set to 7 ("Signatures not used in the group").

   If all verifications succeed, the handler replies with a 2.05
   (Content) response, specifying data that allow also an external
   signature verifier to verify signatures of messages protected with
   the group mode and sent to the group (see Sections 3.1 and 8.5 of
   [I-D.ietf-core-oscore-groupcomm]).  The response MUST have Content-
   Format set to application/ace-groupcomm+cbor.  The payload of the
   response is a CBOR map, which is formatted as defined in Section 9.6.

8.3.  ace-group/GROUPNAME/stale-sids

   This resource implements a FETCH handler.

8.3.1.  FETCH Handler

   The handler expects a FETCH request, whose payload specifies a
   version number of the group keying material, encoded as an unsigned
   CBOR integer.

   In addition to what is defined in Section 4.1.2 of
   [I-D.ietf-ace-key-groupcomm], the handler verifies that the
   requesting Client is a current member of the group.  If the
   verification fails, the Group Manager MUST reply with a 4.03

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   (Forbidden) error response.  The response MUST have Content-Format
   set to application/ace-groupcomm+cbor and is formatted as defined in
   Section 4.1.2 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 all verifications succeed, the handler replies with a 2.05
   (Content) response, specifying data that allow the requesting Client
   to delete the Recipient Contexts and authentication credentials
   associated with former members of the group (see Section 3.2 of
   [I-D.ietf-core-oscore-groupcomm].  The payload of the response is
   formatted as defined in Section 11.3.1.

8.4.  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     | F     |
   +---------------------------------+--------+-------+-------+-------+
   | ace-group/GROUPNAME/            | G Po   | G Po  | Po *  | Po    |
   +---------------------------------+--------+-------+-------+-------+
   | ace-group/GROUPNAME/active      | G      | G     | -     | -     |
   +---------------------------------+--------+-------+-------+-------+
   | ace-group/GROUPNAME/verif-data  | -      | -     | G     | -     |
   +---------------------------------+--------+-------+-------+-------+
   | ace-group/GROUPNAME/pub-key     | G F    | G F   | G F   | -     |
   +---------------------------------+--------+-------+-------+-------+
   | ace-group/GROUPNAME/kdc-pub-key | G      | G     | G     | -     |
   +---------------------------------+--------+-------+-------+-------+
   | ace-group/GROUPNAME/stale-sids  | F      | 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      |        |       |       |       |
   +---------------------------------+--------+-------+-------+-------+

   CoAP methods: G = GET; F = FETCH; Po = POST; Pu = PUT; D = DELETE

   Type1 = Member as Requester and/or Responder
   Type2 = Member as Monitor
   Type3 = Non-member (authorized to be signature verifier)
           (*) = cannot join the group as signature verifier
   Type4 = Non-member (not authorized to be signature verifier)

       Figure 2: Admitted CoAP Methods on the Group Manager Resources

8.4.1.  Signature Verifiers

   Just like any candidate group member, a signature verifier provides
   the Group Manager with an Access Token, as described in Section 5.3.
   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.

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   After successfully transferring an Access Token to the Group Manager,
   a signature verifier is allowed to perform only some operations as
   non-member of a group, and only for the OSCORE groups specified in
   the validated Access Token.  These are the operations specified in
   Section 9.3, Section 9.5, Section 9.6 and Section 9.10.

   Consistently, in case a node is not a member of the group with group
   name GROUPNAME and is authorized to be only signature verifier for
   that group, the Group Manager MUST reply with a 4.03 (Forbidden)
   error response if that node attempts to access any other endpoint
   than: /ace-group; ace-group/GROUPNAME/verif-data; /ace-
   group/GROUPNAME/pub-key; and ace-group/GROUPNAME/kdc-pub-key.

8.5.  Operations Supported by Clients

   Building on what is defined in Section 4.1.1 of
   [I-D.ietf-ace-key-groupcomm], and with reference to the resources at
   the Group Manager newly defined earlier in Section 8 of this
   document, it is expected that a Client minimally supports also the
   following set of operations and corresponding interactions with the
   Group Manager (REQ12).

   *  GET request to ace-group/GROUPNAME/active, in order to check the
      current status of the group.

   *  GET request to ace-group/GROUPNAME/verif-data, in order for a
      signature verifier to retrieve data required to verify signatures
      of messages protected with the group mode of Group OSCORE and sent
      to a group (see Sections 3.1 and 8.5 of
      [I-D.ietf-core-oscore-groupcomm]).  Note that this operation is
      relevant to support only to signature verifiers.

   *  FETCH request to ace-group/GROUPNAME/stale-sids, in order to
      retrieve from the Group Manager the data required to delete some
      of the stored group members' authentication credentials and
      associated Recipient Contexts (see Section 8.3.1).  These data are
      provided as an aggregated set of stale Sender IDs, which are used
      as specified in Section 11.3.

9.  Additional Interactions with the Group Manager

   This section defines the possible interactions with the Group
   Manager, in addition to the group joining specified in Section 6.

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9.1.  Retrieve 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.

9.1.1.  Get 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 4.3.2 of [I-D.ietf-ace-key-groupcomm].  The Key Distribution
   Response is formatted as defined in Section 4.3.2 of
   [I-D.ietf-ace-key-groupcomm], with the following additions.

   *  The 'key' parameter is formatted as defined in Section 6.3 of this
      document, with the difference that it does not include the
      'group_SenderId' parameter.

   *  The 'exp' parameter MUST be present.

   *  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].

9.1.2.  Get Group Keying Material and OSCORE Sender ID

   If the group member wants to retrieve the latest group keying
   material as well as the OSCORE 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.

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   The Group Manager processes the Key Distribution Request according to
   Section 4.8.1 of [I-D.ietf-ace-key-groupcomm].  The Key Distribution
   Response is formatted as defined in Section 4.8.1 of
   [I-D.ietf-ace-key-groupcomm], with the following additions.

   *  The 'key' parameter is formatted as defined in Section 6.3 of this
      document.  In particular, if the requesting group member has
      exclusively the role of monitor, then the 'key' parameter does not
      include the 'group_SenderId'.

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

   *  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].

9.2.  Request to Change Individual Keying Material

   As discussed in Section 2.5.2 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 4.8.2.1 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 4.8.2 of [I-D.ietf-ace-key-groupcomm], with
   the following additions.

   The Group Manager MUST return a 5.03 (Service Unavailable) response
   in case the OSCORE group identified by GROUPNAME is currently
   inactive (see Section 8.1).  The response MUST have Content-Format
   set to application/ace-groupcomm+cbor and is formatted as defined in
   Section 4.1.2 of [I-D.ietf-ace-key-groupcomm].  The value of the
   'error' field MUST be set to 9 ("Group currently not active").

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

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   1.  If the requesting group member has exclusively the role of
       monitor, the Group Manager replies with a 4.03 (Forbidden) error
       response.  The response MUST have Content-Format set to
       application/ace-groupcomm+cbor and is formatted as defined in
       Section 4.1.2 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 11), and replies to the group member with a group
          rekeying message as defined in Section 11, providing the new
          group keying material.  Then, the Group Manager rekeys the
          rest of the OSCORE group, as discussed in Section 11.

          The Group Manager SHOULD perform a group rekeying only if
          already scheduled to occur shortly, e.g., according to an
          application-specific rekeying period or scheduling, 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 (OPT12).

       *  The Group Manager determines and assigns a new OSCORE Sender
          ID for that group member, and replies with a Key Renewal
          Response formatted as defined in Section 4.8.2 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 16.3 of this document,
          specifying the new Sender ID of the group member encoded as a
          CBOR byte string.

          Consistently with Section 2.5.3.1 of
          [I-D.ietf-core-oscore-groupcomm], the Group Manager MUST
          assign a new Sender ID that has not been used in the OSCORE
          group since the latest time when the current Gid value was
          assigned to the group.

          Furthermore, the Group Manager MUST add the old, relinquished
          Sender ID of the group member to the most recent set of stale
          Sender IDs, in the collection associated with the group (see
          Section 7.1).

          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

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          formatted as defined in Section 4.1.2 of
          [I-D.ietf-ace-key-groupcomm].  The value of the 'error' field
          MUST be set to 4 ("No available node identifiers").

9.3.  Retrieve Authentication Credentials of Group Members

   A group member or a signature verifier may need to retrieve the
   authentication credentials 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 Sections 4.4.1.1 and 4.4.2.1 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 4.4.1 of
   [I-D.ietf-ace-key-groupcomm].  In particular:

   *  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 not the CBOR simple value "null" (0xf6) (see
      Section 6.1).

   *  Each element (if any) of the inner CBOR array 'id_filter' is a
      CBOR byte string, which encodes the OSCORE Sender ID of the group
      member for which the associated authentication credential is
      requested (REQ25).

   Upon receiving the Public Key Request, the Group Manager processes it
   as per Section 4.4.1 or Section 4.4.2 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 with any current group member (REQ26).

   The success Public Key Response is formatted as defined in
   Section 4.4.1 or Section 4.4.2 of [I-D.ietf-ace-key-groupcomm],
   depending on the request method being FETCH or GET, respectively.

9.4.  Upload a New Authentication Credential

   A group member may need to provide the Group Manager with its new
   authentication credential to use in the group from then on, hence
   replacing the current one.  This can be the case, for instance, if
   the signature or ECDH algorithm and possible associated parameters
   used in the OSCORE group have been changed, and the current
   authentication credential is not compatible with them.

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   To this end, the group member sends a Public Key Update Request
   message to the Group Manager, as per Section 4.9.1.1 of
   [I-D.ietf-ace-key-groupcomm], with the following addition.

   *  The group member computes the proof-of-possession (PoP) evidence
      included in 'client_cred_verify' in the same way taken when
      preparing a Joining Request for the OSCORE group in question, as
      defined in Section 6.1 (REQ14).

   In particular, the group member 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 4.9.1 of [I-D.ietf-ace-key-groupcomm],
   with the following additions.

   *  The N_S challenge used to build the proof-of-possession input is
      computed as defined in Section 6.1.1 (REQ15).

   *  The Group Manager verifies the PoP challenge included in
      'client_cred_verify' in the same way taken when processing a
      Joining Request for the OSCORE group in question, as defined in
      Section 6.2 (REQ14).

   *  The Group Manager MUST return a 5.03 (Service Unavailable)
      response in case the OSCORE group identified by GROUPNAME is
      currently inactive (see Section 8.1).  The response MUST have
      Content-Format set to application/ace-groupcomm+cbor and is
      formatted as defined in Section 4.1.2 of
      [I-D.ietf-ace-key-groupcomm].  The value of the 'error' field MUST
      be set to 9 ("Group currently not active").

   *  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 4.1.2 of [I-D.ietf-ace-key-groupcomm].  The value of the
      'error' field MUST be set to 1 ("Request inconsistent with the
      current roles").

   *  If the request is successfully processed, the Group Manager stores
      the association between i) the new authentication credential of
      the group member; and ii) the Group Identifier (Gid), i.e., the
      OSCORE ID Context, associated with 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.

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9.5.  Retrieve the Group Manager's Authentication Credential

   A group member or a signature verifier may need to retrieve the
   authentication credential of the Group Manager.  To this end, the
   requesting Client sends a KDC Public Key Request message to the Group
   Manager.

   In particular, it sends a CoAP GET request to the endpoint /ace-
   group/GROUPNAME/kdc-pub-key at the Group Manager defined in
   Section 4.5.1.1 of [I-D.ietf-ace-key-groupcomm], where GROUPNAME is
   the name of the OSCORE group.

   In addition to what is defined in Section 4.5.1 of
   [I-D.ietf-ace-key-groupcomm], the Group Manager MUST respond with a
   4.00 (Bad Request) error response, if the requesting Client is not a
   current group member and GROUPNAME denotes a pairwise-only group.
   The response MUST have Content-Format set to application/ace-
   groupcomm+cbor and is formatted as defined in Section 4.1.2 of
   [I-D.ietf-ace-key-groupcomm].  The value of the 'error' field MUST be
   set to 7 ("Signatures not used in the group").

   The payload of the 2.05 (Content) KDC Public Key Response is a CBOR
   map, which is formatted as defined in Section 4.5.1 of
   [I-D.ietf-ace-key-groupcomm].  In particular, the Group Manager
   specifies the parameters 'kdc_cred', 'kdc_nonce' and 'kdc_challenge'
   as defined for the Joining Response in Section 6.3 of this document.
   This especially applies to the computing of the proof-of-possession
   (PoP) evidence included in 'kdc_cred_verify' (REQ21).

   Upon receiving a 2.05 (Content) KDC Public Key Response, the
   requesting Client retrieves the Group Manager's authentication
   credential from the 'kdc_cred' parameter, and proceeds as defined in
   Section 4.5.1.1 of [I-D.ietf-ace-key-groupcomm].  In particular, the
   requesting Client verifies the PoP evidence included in
   'kdc_cred_verify' by means of the same method used when processing
   the Joining Response, as defined in Section 6.3 of this document
   (REQ21).

   Note that a signature verifier would not receive a successful
   response from the Group Manager, in case GROUPNAME denotes a
   pairwise-only group.

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9.6.  Retrieve Signature Verification Data

   A signature verifier may need to retrieve data required to verify
   signatures of messages protected with the group mode and sent to a
   group (see Sections 3.1 and 8.5 of [I-D.ietf-core-oscore-groupcomm]).
   To this end, the signature verifier sends a Signature Verification
   Data Request message to the Group Manager.

   In particular, it sends a CoAP GET request to the endpoint /ace-
   group/GROUPNAME/verif-data at the Group Manager defined in
   Section 8.2 of this document, where GROUPNAME is the name of the
   OSCORE group.

   The payload of the 2.05 (Content) Signature Verification Data
   Response is a CBOR map, which has the format used for the Joining
   Response message in Section 6.3, with the following differences.

   *  From the Joining Response message, only the parameters 'gkty',
      'key', 'num', 'exp' and 'ace-groupcomm-profile' are present.  In
      particular, the 'key' parameter includes only the following data.

      -  The parameters 'hkdf', 'contextId', 'cred_fmt', 'sign_enc_alg',
         'sign_alg', 'sign_params'.  These parameters MUST be present.

      -  The parameters 'alg' and 'ecdh_alg'.  These parameter MUST NOT
         be present if the group is a signature-only group.  Otherwise,
         they MUST be present.

   *  The parameter 'group_enc_key' is also included, with CBOR label
      defined in Section 16.3.  This parameter specifies the Group
      Encryption Key of the OSCORE Group, encoded as a CBOR byte string.
      The Group Manager derives the Group Encryption Key from the group
      keying material, as per Section 2.1.6 of
      [I-D.ietf-core-oscore-groupcomm].  This parameter MUST be present.

   In order to verify signatures in the group (see Section 8.5 of
   [I-D.ietf-core-oscore-groupcomm]), the signature verifier relies on:
   the data retrieved from the 2.05 (Content) Signature Verification
   Data Response; the public keys of the group members signing the
   messages to verify, retrieved from those members' authentication
   credentials that can be obtained as defined in Section 9.3; and the
   public key of the Group Manager, retrieved from the Group Manager's
   authentication credential that can be obtained as defined in
   Section 9.5.

   Figure 3 gives an overview of the exchange described above, while
   Figure 4 shows an example.

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   Signature                                                     Group
   Verifier                                                     Manager
     |                                                             |
     |              Signature Verification Data Request            |
     |------------------------------------------------------------>|
     |              GET ace-group/GROUPNAME/verif-data             |
     |                                                             |
     |<--- Signature Verification Data Response: 2.05 (Content) ---|
     |                                                             |

       Figure 3: Message Flow of Signature Verification Data Request-
                                  Response

      Request:

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

      Response:

      Header: Content (Code=2.05)
      Content-Format: "application/ace-groupcomm+cbor"
      Payload (in CBOR diagnostic notation, with GROUPCOMM_KEY_TBD
               and PROFILE_TBD being CBOR integers, while GROUP_ENC_KEY
               being a CBOR byte string):
       {
         "gkty": GROUPCOMM_KEY_TBD,
         "key": {
           'hkdf': 5,                     ; HMAC 256/256
           'contextId': h'37fc',
           'cred_fmt': 33,                ; x5chain
           'sign_enc_alg': 10,            ; AES-CCM-16-64-128
           'sign_alg': -8,                ; EdDSA
           'sign_params': [[1], [1, 6]]   ; [[OKP], [OKP, Ed25519]]
         },
         "num": 12,
         "exp": 1609459200,
         "ace_groupcomm_profile": PROFILE_TBD,
         "group_enc_key": GROUP_ENC_KEY
       }

     Figure 4: Example of Signature Verification Data Request-Response

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9.7.  Retrieve the 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 4.6.1.1 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.

   Upon receiving the Policies Request, the Group Manager processes it
   as per Section 4.6.1 of [I-D.ietf-ace-key-groupcomm].  The success
   Policies Response is formatted as defined in Section 4.6.1 of
   [I-D.ietf-ace-key-groupcomm].

9.8.  Retrieve the 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 4.7.1.1 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 4.7.1 of [I-D.ietf-ace-key-groupcomm].  The success
   Version Response is formatted as defined in Section 4.7.1 of
   [I-D.ietf-ace-key-groupcomm].

9.9.  Retrieve the 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 8.1 of this document, where GROUPNAME is the name of the
   OSCORE group.

   The payload of the 2.05 (Content) Group Status Response includes the
   CBOR simple value "true" (0xf5) if the group is currently active, or
   the CBOR simple value "false" (0xf4) 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.

   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.

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   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 5 gives an overview of the exchange described above, while
   Figure 6 shows an example.

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

          Figure 5: Message Flow of Group Status Request-Response

      Request:

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

      Response:

      Header: Content (Code=2.05)
      Payload (in CBOR diagnostic notation):
        true

             Figure 6: Example of Group Status Request-Response

9.10.  Retrieve Group Names

   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.

   *  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.

   *  As current group member in several groups, the node has missed a
      previous group rekeying in one of them (see Section 11).  Hence,
      it retains stale keying material and fails to decrypt received
      messages exchanged in that group.

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      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 authentication credentials from the Group
      Manager (see Section 9.1.1, Section 9.1.2 and Section 9.3).
      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]).

   *  As signature verifier, the node also refers to a group name for
      retrieving the required authentication credentials from the Group
      Manager (see Section 9.3).  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
      signature verifier in several groups.  This in turn prevents the
      efficient verification of signatures, and especially the retrieval
      of required, new authentication credentials 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 authentication
   credentials used in the group as a signature verifier.  To this end,
   the node sends a Group Name and URI Retrieval Request, as per
   Section 4.2.1.1 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 4.2.1
   of [I-D.ietf-ace-key-groupcomm].  Each element of the CBOR array
   'gid' is a CBOR byte string (REQ13), which encodes the Gid of the
   group for which the group name and the URI to the group-membership
   resource are requested.

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   Upon receiving the Group Name and URI Retrieval Request, the Group
   Manager processes it as per Section 4.2.1 of
   [I-D.ietf-ace-key-groupcomm].  The success Group Name and URI
   Retrieval Response is formatted as defined in Section 4.2.1 of
   [I-D.ietf-ace-key-groupcomm].  In particular, each element of the
   CBOR array 'gid' is a CBOR byte string (REQ13), 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 does not maintain an association
   between the former pair and any other Gid for that group than the
   current, most recent one.

   Figure 7 gives an overview of the exchanges described above, while
   Figure 8 shows an example.

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

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

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      Request:

      Header: FETCH (Code=0.05)
      Uri-Host: "kdc.example.com"
      Uri-Path: "ace-group"
      Content-Format: "application/ace-groupcomm+cbor"
      Payload (in CBOR diagnostic notation):
        {
          "gid": [h'37fc', h'84bd']
        }

      Response:

      Header: Content (Code=2.05)
      Content-Format: "application/ace-groupcomm+cbor"
      Payload (in CBOR diagnostic notation):
        {
          "gid": [h'37fc', h'84bd'],
          "gname": ["g1", "g2"],
          "guri": ["ace-group/g1", "ace-group/g2"]
        }

     Figure 8: Example of Group Name and URI Retrieval Request-Response

9.11.  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 4.8.3.1 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 4.8.3 of [I-D.ietf-ace-key-groupcomm].  Then, the
   Group Manager performs the follow-up actions defined in Section 10 of
   this document.

10.  Removal of a Group Member

   Other than after a spontaneous request to the Group Manager as
   described in Section 9.11, a node may be forcibly removed from the
   OSCORE group, e.g., due to expired or revoked authorization.

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   In either case, if the Group Manager reassigns Gid values during the
   group's lifetime (see Section 3.2.1.1 of
   [I-D.ietf-core-oscore-groupcomm]), the Group Manager "forgets" the
   Birth Gid currently associated with the leaving node in the OSCORE
   group.  This was stored following the Joining Response sent to that
   node, after its latest (re-)joining of the OSCORE group (see
   Section 6.3).

   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 by
   using only the method corresponding to one of either conditions.

   *  If, upon joining the group (see Section 6.1), the leaving node
      specified a URI in the 'control_uri' parameter defined in
      Section 4.3.1 of [I-D.ietf-ace-key-groupcomm], the Group Manager
      sends a DELETE request targeting the URI specified in the
      'control_uri' parameter (OPT7).

   *  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) error response to the leaving node,
      as specified in Section 4.3.2 of [I-D.ietf-ace-key-groupcomm].

   Furthermore, the Group Manager might intend to evict all the current
   group members from the group at once.  In such a case, if the Joining
   Responses sent by the Group Manager to nodes joining the group (see
   Section 6.3) specify a URI in the 'control_group_uri' parameter
   defined in Section 4.3.1 of [I-D.ietf-ace-key-groupcomm], then the
   Group Manager MUST additionally send a DELETE request targeting the
   URI specified in the 'control_group_uri' parameter (OPT10).

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

   *  The Group Manager frees the OSCORE Sender ID value of the leaving
      node.  This value MUST NOT become available for possible upcoming
      joining nodes in the same group, until the group has been rekeyed
      and assigned a new Group Identifier (Gid).

   *  The Group Manager MUST add the relinquished Sender ID of the
      leaving node to the most recent set of stale Sender IDs, in the
      collection associated with the group (see Section 7.1).

   *  The Group Manager cancels the association between, on one hand,
      the authentication credential of the leaving node and, on the
      other hand, the Gid associated with the OSCORE group together with
      the freed Sender ID value.  The Group Manager deletes the

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      authentication credential of the leaving node, if that
      authentication credential has no remaining association with any
      pair (Gid, Sender ID).

   Then, the Group Manager MUST generate updated security parameters and
   group keying material, and provide it to the remaining group members
   (see Section 11).  As a consequence, the leaving node is not able to
   acquire the new security parameters and group keying material
   distributed after its leaving.

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

11.  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.

   As per Section 3.2.1.1 of [I-D.ietf-core-oscore-groupcomm], the Group
   Manager MAY reassign a Gid to the same group over that group's
   lifetime, e.g., once the whole space of Gid values has been used for
   the group in question.  If the Group Manager supports reassignment of
   Gid values and performs it in a group, then the Group Manager
   additionally takes the following actions.

   *  Before rekeying the group, the Group Manager MUST check if the new
      Gid to be distributed coincides with the Birth Gid of any of the
      current group members (see Section 6.3).

   *  If any of such "elder members" is found in the group, the Group
      Manager MUST evict them from the group.  That is, the Group
      Manager MUST terminate their membership and MUST rekey the group
      in such a way that the new keying material is not provided to
      those evicted elder members.  This also includes adding their
      relinquished Sender IDs to the most recent set of stale Sender
      IDs, in the collection associated with the group (see
      Section 7.1), before rekeying the group.

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      Until a further following group rekeying, the Group Manager MUST
      store the list of those latest-evicted elder members.  If any of
      those nodes re-joins the group before a further following group
      rekeying occurs, the Group Manager MUST NOT rekey the group upon
      their re-joining.  When one of those nodes re-joins the group, the
      Group Manager can rely, e.g., on the ongoing secure communication
      association to recognize the node as included in the stored list.

   Across the rekeying execution, the Group Manager MUST preserve the
   same unchanged OSCORE Sender IDs for all group members intended to
   remain in the group.  This avoids affecting the retrieval of
   authentication credentials from the Group Manager and the
   verification of group messages.

   The Group Manager MUST support the "Point-to-Point" group rekeying
   scheme registered in Section 11.14 of [I-D.ietf-ace-key-groupcomm],
   as per the detailed use defined in Section 11.1 of this document.
   Future specifications may define how this application profile can use
   alternative group rekeying schemes, which MUST comply with the
   functional steps defined in Section 3.2 of
   [I-D.ietf-core-oscore-groupcomm].  The Group Manager MUST indicate
   the use of such an alternative group rekeying scheme to joining
   nodes, by means of the 'group_rekeying' parameter included in Joining
   Response messages (see Section 6.3).

   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.
   Once completed the group rekeying process, the Group Manager creates
   a new empty set X' of stale Sender IDs associated with the version of
   the newly distributed group keying material.  Then, the Group Manager
   MUST add the set X' to the collection of stale Sender IDs associated
   with the group (see Section 7.1).

   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 with a different Group Manager.  When failing to correctly
   process messages secured with the new keying material, these group
   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.

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   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 9.8) and then possibly requesting the current keying material
   (see Section 9.1.1).

   Furthermore, some of these group members can be in multiple groups,
   all of which associated with 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 with that Group Manager.

   Section 11.2 defines the actions performed by a group member upon
   receiving the new group keying material.  Section 11.3 discusses how
   a group member can realize that it has missed one or more rekeying
   instances, and the actions it is accordingly required to take.

11.1.  Sending Rekeying Messages

   When using the "Point-to-Point" group rekeying scheme, the group
   rekeying messages MUST have Content-Format set to application/ace-
   groupcomm+cbor and have the same format used for the Joining Response
   message in Section 6.3, with the following differences.  Note that
   this extends the minimal content of a rekeying message as defined in
   Section 6 of [I-D.ietf-ace-key-groupcomm] (OPT14).

   *  From the Joining Response, only the parameters 'gkty', 'key',
      'num', 'exp', and 'ace-groupcomm-profile' are present.  In
      particular, the 'key' parameter includes only the following data.

      -  The 'ms' parameter, specifying the new OSCORE Master Secret
         value.  This parameter MUST be present.

      -  The 'contextId' parameter, specifying the new Gid to use as
         OSCORE ID Context value.  This parameter MUST be present.

      -  The 'salt' value, specifying the new OSCORE Master Salt value.
         This parameter MAY be present.

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   *  The parameter 'stale_node_ids' MUST also be included, with CBOR
      label defined in Section 16.3.  This parameter is encoded as a
      CBOR array, where each element is encoded as a CBOR byte string.
      The CBOR array has to be intended as a set, i.e., the order of its
      elements is irrelevant.  The parameter is populated as follows.

      -  The Group Manager creates an empty CBOR array ARRAY.

      -  The Group Manager considers the collection of stale Sender IDs
         associated with the group (see Section 7.1), and takes the most
         recent set X, i.e., the set associated with the current version
         of the group keying material about to be relinquished.

      -  For each Sender ID in X, the Group Manager encodes it as a CBOR
         byte string and adds the result to ARRAY.

      -  The parameter 'stale_node_ids' takes ARRAY as value.

   *  The parameters 'pub_keys', 'peer_roles' and 'peer_identifiers'
      SHOULD be present, if the group rekeying is performed due to one
      or multiple Clients that have requested to join the group.
      Following the same semantics used in the Joining Response message
      (see Section 6.3), the three parameters specify the authentication
      credential, roles in the group and node identifier of each of the
      Clients that have requested to join the group.  The Group Manager
      MUST NOT include a non-empty subset of these three parameters.

   The Group Manager separately sends a group rekeying message formatted
   as defined above to each group member to be rekeyed.

   Each rekeying message MUST be secured with the pairwise secure
   communication association 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 4.3.1 of [I-D.ietf-ace-key-groupcomm] (OPT7), if provided by
   the intended recipient upon joining the group (see Section 6.1).

   This distribution 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].

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   Group members and the Group Manager SHOULD additionally support
   alternative distribution approaches that do not require group members
   to act (also) as servers.  A number of such approaches are defined in
   Section 6 of [I-D.ietf-ace-key-groupcomm].  In particular, a group
   member may use CoAP Observe [RFC7641] and subscribe for updates to
   the group-membership resource of the group, at the endpoint /ace-
   group/GROUPNAME/ of the Group Manager (see Section 6.1 of
   [I-D.ietf-ace-key-groupcomm]).  Alternatively, a full-fledged Pub-Sub
   model can be considered [I-D.ietf-core-coap-pubsub], where the Group
   Manager publishes to a rekeying topic hosted at a Broker, while the
   group members subscribe to such topic (see Section 6.2 of
   [I-D.ietf-ace-key-groupcomm]).

11.2.  Receiving Rekeying Messages

   Once received the new group keying material, a group member proceeds
   as follows.  Unless otherwise specified, the following is independent
   of the specifically used group rekeying scheme.

   The group member considers the stale Sender IDs received from the
   Group Manager.  If the "Point-to-Point" group rekeying scheme as
   detailed in Section 11.1 is used, the stale Sender IDs are specified
   by the 'stale_node_ids' parameter.

   After that, as per Section 3.2 of [I-D.ietf-core-oscore-groupcomm],
   the group member MUST remove every authentication credential
   associated with a stale Sender ID from its list of group members'
   authentication credentials used in the group, and MUST delete each of
   its Recipient Contexts used in the group whose corresponding
   Recipient ID is a stale Sender ID.

   Then, the following cases can occur, based on the version number V'
   of the new group keying material distributed through the rekeying
   process.  If the "Point-to-Point" group rekeying scheme as detailed
   in Section 11.1 is used, this information is specified by the 'num'
   parameter.

   *  The group member has not missed any group rekeying.  That is, the
      old keying material stored by the group member has version number
      V, while the received new keying material has version number V' =
      (V + 1).  In such a case, the group member simply installs the new
      keying material and derives the corresponding new Security
      Context.

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   *  The group member has missed one or more group rekeying instances.
      That is, the old keying material stored by the group member has
      version number V, while the received new keying material has
      version number V' > (V + 1).  In such a case, the group member
      MUST proceed as defined in Section 11.3.

   *  The group member has received keying material not newer than the
      stored one.  That is, the old keying material stored by the group
      member has version number V, while the received keying material
      has version number V' < (V + 1).  In such a case, the group member
      MUST ignore the received rekeying messages and MUST NOT install
      the received keying material.

11.3.  Missed Rekeying Instances

   A group member can realize to have missed one or more rekeying
   instances in one of the ways discussed below.  In the following, V
   denotes the version number of the old keying material stored by the
   group member, while V' denotes the version number of the latest,
   possibly just distributed, keying material.

   a.  The group member has participated to a rekeying process that has
   distributed new keying material with version number V' > (V + 1), as
   discussed in Section 11.2.

   b.  The group member has obtained the latest keying material from the
   Group Manager, as a response to a Key Distribution Request (see
   Section 9.1.1) or to a Joining Request when re-joining the group (see
   Section 6.1).  In particular, V is different than V' specified by the
   'num' parameter in the response.

   c.  The group member has obtained the authentication credentials of
   other group members, through a Public Key Request-Response exchange
   with the Group Manager (see Section 9.3).  In particular, V is
   different than V' specified by the 'num' parameter in the response.

   d.  The group member has performed a Version Request-Response
   exchange with the Group Manager (see Section 9.8).  In particular, V
   is different than V' specified by the 'num' parameter in the
   response.

   In either case, the group member MUST delete the stored keying
   material with version number V.

   If case (a) or case (b) applies, the group member MUST perform the
   following actions.

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   1.  The group member MUST NOT install the latest keying material yet,
       in case that was already obtained.

   2.  The group member sends a Stale Sender IDs Request to the Group
       Manager (see Section 11.3.1), specifying the version number V as
       payload of the request.

       If the Stale Sender IDs Response from the Group Manager has no
       payload, the group member MUST remove all the authentication
       credentials from its list of group members' authentication
       credentials used in the group, and MUST delete all its Recipient
       Contexts used in the group.

       Otherwise, the group member considers the stale Sender IDs
       specified in the Stale Sender IDs Response from the Group
       Manager.  Then, the group member MUST remove every authentication
       credential associated with a stale Sender ID from its list of
       group members' authentication credentials used in the group, and
       MUST delete each of its Recipient Contexts used in the group
       whose corresponding Recipient ID is a stale Sender ID.

   3.  The group member installs the latest keying material with version
       number V' and derives the corresponding new Security Context.

   If case (c) or case (d) applies, the group member SHOULD perform the
   following actions.

   1.  The group member sends a Stale Sender IDs Request to the Group
       Manager (see Section 11.3.1), specifying the version number V as
       payload of the request.

       If the Stale Sender IDs Response from the Group Manager has no
       payload, the group member MUST remove all the authentication
       credentials from its list of group members' authentication
       credentials used in the group, and MUST delete all its Recipient
       Contexts used in the group.

       Otherwise, the group member considers the stale Sender IDs
       specified in the Stale Sender IDs Response from the Group
       Manager.  Then, the group member MUST remove every authentication
       credential associated with a stale Sender ID from its list of
       group members' authentication credentials used in the group, and
       MUST delete each of its Recipient Contexts used in the group
       whose corresponding Recipient ID is a stale Sender ID.

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   2.  The group member obtains the latest keying material with version
       number V' from the Group Manager.  This can happen by sending a
       Key Distribution Request to the Group Manager (see Section 9.1.1)
       and Section 9.1.2).

   3.  The group member installs the latest keying material with version
       number V' and derives the corresponding new Security Context.

   If case (c) or case (d) applies, the group member can alternatively
   perform the following actions.

   1.  The group member re-joins the group (see Section 6.1).  When
       doing so, the group member MUST re-join with the same roles it
       currently has in the group, and MUST request the Group Manager
       for the authentication credentials of all the current group
       members.  That is, the 'get_pub_keys' parameter of the Joining
       Request MUST be present and MUST be set to the CBOR simple value
       "null" (0xf6).

   2.  When receiving the Joining Response (see Section 6.4 and
       Section 6.4), the group member retrieves the set Z of
       authentication credentials specified in the 'pub_keys' parameter.

       Then, the group member MUST remove every authentication
       credential which is not in Z from its list of group members'
       authentication credentials used in the group, and MUST delete
       each of its Recipient Contexts used in the group that does not
       include any of the authentication credentials in Z.

   3.  The group member installs the latest keying material with version
       number V' and derives the corresponding new Security Context.

11.3.1.  Retrieve Stale Sender IDs

   When realizing to have missed one or more group rekeying instances
   (see Section 11.3), a node needs to retrieve from the Group Manager
   the data required to delete some of its stored group members'
   authentication credentials and Recipient Contexts (see
   Section 8.3.1).  These data are provided as an aggregated set of
   stale Sender IDs, which are used as specified in Section 11.3.

   In particular, the node sends a CoAP FETCH request to the endpoint
   /ace-group/GROUPNAME/stale-sids at the Group Manager defined in
   Section 8.3 of this document, where GROUPNAME is the name of the
   OSCORE group.

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   The payload of the Stale Sender IDs Request MUST include a CBOR
   unsigned integer.  This encodes the version number V of the most
   recent group keying material stored and installed by the requesting
   Client, which is older than the latest, possibly just distributed,
   keying material with version number V'.

   The handler MUST reply with a 4.00 (Bad Request) error response, if
   the request is not formatted correctly.  Also, the handler MUST
   respond with a 4.00 (Bad Request) error response, if the specified
   version number V is greater or equal than the version number V'
   associated with the latest keying material in the group, i.e., in
   case V >= V'.

   Otherwise, the handler responds with a 2.05 (Content) Stale Sender
   IDs Response.  The payload of the response is formatted as defined
   below, where SKEW = (V' - V + 1).

   *  The Group Manager considers ITEMS as the current number of sets
      stored in the collection of stale Sender IDs associated with the
      group (see Section 7.1).

   *  If SKEW > ITEMS, the Stale Sender IDs Response MUST NOT have a
      payload.

   *  Otherwise, the payload of the Stale Sender IDs Response MUST
      include a CBOR array, where each element is encoded as a CBOR byte
      string.  The CBOR array has to be intended as a set, i.e., the
      order of its elements is irrelevant.  The Group Manager populates
      the CBOR array as follows.

      -  The Group Manager creates an empty CBOR array ARRAY and an
         empty set X.

      -  The Group Manager considers the SKEW most recent sets stored in
         the collection of stale Sender IDs associated with the group.
         Note that the most recent set is the one associate to the
         latest version of the group keying material.

      -  The Group Manager copies all the Sender IDs from the selected
         sets into X.  When doing so, the Group Manager MUST discard
         duplicates.  That is, the same Sender ID MUST NOT be present
         more than once in the final content of X.

      -  For each Sender ID in X, the Group Manager encodes it as a CBOR
         byte string and adds the result to ARRAY.

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      -  Finally, ARRAY is specified as payload of the Stale Sender IDs
         Response.  Note that ARRAY might result in the empty CBOR
         array.

   Figure 9 gives an overview of the exchange described above, while
   Figure 10 shows an example.

                                                                 Group
   Node                                                         Manager
     |                                                             |
     |                   Stale Sender IDs Request                  |
     |------------------------------------------------------------>|
     |             FETCH ace-group/GROUPNAME/stale-sids            |
     |                                                             |
     |<---------- Stale Sender IDs Response: 2.05 (Content) -------|
     |                                                             |

        Figure 9: Message Flow of Stale Sender IDs Request-Response

      Request:

      Header: FETCH (Code=0.05)
      Uri-Host: "kdc.example.com"
      Uri-Path: "ace-group"
      Uri-Path: "g1"
      Uri-Path: "stale-sids"
      Payload (in CBOR diagnostic notation):
        42

      Response:

      Header: Content (Code=2.05)
      Payload (in CBOR diagnostic notation):
        [h'01', h'fc', h'12ab', h'de44', h'ff']

          Figure 10: Example of Stale Sender IDs Request-Response

12.  ACE Groupcomm Parameters

   In addition to those defined in Section 8 of
   [I-D.ietf-ace-key-groupcomm], this application profile defines
   additional parameters used during the second part of the message
   exchange with the Group Manager, i.e., after the exchange of Token
   Transfer Request and Response (see Section 5.3).  The table below
   summarizes them and specifies the CBOR key to use instead of the full
   descriptive name.

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

            +----------------+------+-------+-----------------+
            | Name           | CBOR | CBOR  | Reference       |
            |                | Key  | Type  |                 |
            +----------------+------+-------+-----------------+
            | group_senderId | TBD  | bstr  | [this document] |
            +----------------+------+-------+-----------------+
            | ecdh_info      | TBD  | array | [this document] |
            +----------------+------+-------+-----------------+
            | kdc_dh_creds   | TBD  | array | [this document] |
            +----------------+------+-------+-----------------+
            | group_enc_key  | TBD  | bstr  | [this document] |
            +----------------+------+-------+-----------------+
            | stale_node_ids | TBD  | array | [this document] |
            +----------------+------+-------+-----------------+

                    Figure 11: ACE Groupcomm Parameters

   The Group Manager is expected to support and understand all the
   parameters above.  Instead, a Client is required to support the new
   parameters defined in this application profile as specified below
   (REQ29).

   *  'group_senderId' MUST be supported by a Client that intends to
      join an OSCORE group with the role of Requester and/or Responder.

   *  'ecdh_info' MUST be supported by a Client that intends to join a
      group which uses the pairwise mode of Group OSCORE.

   *  'kdc_dh_creds' MUST be supported by a Client that intends to join
      a group which uses the pairwise mode of Group OSCORE and that does
      not plan to or cannot rely on an early retrieval of the Group
      Manager's Diffie-Hellman authentication credential.

   *  'group_enc_key' MUST be supported by a Client that intends to join
      a group which uses the group mode of Group OSCORE or to be
      signature verifier for that group.

   *  'stale_node_ids' MUST be supported.

   When the conditional parameters defined in Section 8 of
   [I-D.ietf-ace-key-groupcomm] are used with this application profile,
   a Client must, should or may support them as specified below (REQ30).

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   *  'client_cred', 'cnonce', 'client_cred_verify'.  A Client that has
      an own authentication credential to use in a group MUST support
      these parameters.

   *  'kdcchallenge'.  A Client that has an own authentication
      credential to use in a group and that provides the Access Token to
      the Group Manager through a Token Transfer Request (see
      Section 5.3) MUST support this parameter.

   *  'pub_keys_repo'.  This parameter is not relevant for this
      application profile, since the Group Manager always acts as
      repository of the group members' authentication credentials.

   *  'group_policies'.  A Client that is interested in the specific
      policies used in a group, but that does not know them or cannot
      become aware of them before joining that group, SHOULD support
      this parameter.

   *  'peer_roles'.  A Client MUST support this parameter, since in this
      application profile it is relevant for Clients to know the roles
      of the group member associated with each authentication
      credential.

   *  'kdc_nonce', 'kdc_cred' and 'kdc_cred_verify'.  A Client MUST
      support these parameters, since the Group Manager's authentication
      credential is required to process messages protected with Group
      OSCORE (see Section 4.3 of [I-D.ietf-core-oscore-groupcomm]).

   *  'mgt_key_material'.  A Client that supports an advanced rekeying
      scheme possibly used in the group, such as based on one-to-many
      rekeying messages sent by the Group Manager (e.g., over IP
      multicast), MUST support this parameter.

   *  'control_group_uri'.  A Client that supports the hosting of local
      resources each associated with a group (hence acting as CoAP
      server) and the reception of one-to-many requests sent to those
      resources by the Group Manager (e.g., over IP multicast) MUST
      support this parameter.

13.  ACE Groupcomm Error Identifiers

   In addition to those defined in Section 9 of
   [I-D.ietf-ace-key-groupcomm], this application profile defines new
   values that the Group Manager can include as error identifiers, in
   the 'error' field of an error response with Content-Format
   application/ace-groupcomm+cbor.

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        +-------+-------------------------------------------------+
        | Value |                   Description                   |
        +-------+-------------------------------------------------+
        |   7   | Signatures not used in the group                |
        +-------+-------------------------------------------------+
        |   8   | Operation permitted only to signature verifiers |
        +-------+-------------------------------------------------+
        |   9   | Group currently not active                      |
        +-------+-------------------------------------------------+

                 Figure 12: ACE Groupcomm Error Identifiers

   A Client supporting the 'error' parameter (see Sections 4.1.2 and 8
   of [I-D.ietf-ace-key-groupcomm]) and able to understand the specified
   error may use that information to determine what actions to take
   next.  If it is included in the error response and supported by the
   Client, the 'error_description' parameter may provide additional
   context.  In particular, the following guidelines apply.

   *  In case of error 7, the Client should stop sending the request in
      question to the Group Manager.  In this application profile, this
      error is relevant only for a signature verifier, in case it tries
      to access resources related to a pairwise-only group.

   *  In case of error 8, the Client should stop sending the request in
      question to the Group Manager.

   *  In case of error 9, the Client should wait for a certain (pre-
      configured) amount of time, before trying re-sending its request
      to the Group Manager.

14.  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].

14.1.  Common

   This section always applies, as related to common configuration
   parameters.

   *  For the HKDF Algorithm 'hkdf', the Group Manager SHOULD use HKDF
      SHA-256, defined as default in Section 3.2 of [RFC8613].  In the
      'hkdf' parameter, this HKDF Algorithm is specified by the HMAC
      Algorithm HMAC 256/256 (COSE algorithm encoding: 5).

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   *  For the format 'cred_fmt' used for the authentication credentials
      in the group, the Group Manager SHOULD use CBOR Web Token (CWT) or
      CWT Claims Set (CCS) [RFC8392], i.e., the COSE Header Parameter
      'kcwt' and 'kccs', respectively.

      [ These COSE Header Parameters are under pending registration
      requested by draft-ietf-lake-edhoc.  ]

   *  For 'max_stale_sets', the Group Manager SHOULD consider N = 3 as
      the maximum number of stored sets of stale Sender IDs in the
      collection associated with the group (see Section 7.1).

14.2.  Group Mode

   This section applies if the group uses (also) the group mode of Group
   OSCORE.

   *  For the Signature Encryption Algorithm 'sign_enc_alg' used to
      encrypt messages protected with the group mode, the Group Manager
      SHOULD use AES-CCM-16-64-128 (COSE algorithm encoding: 10) as
      default value.

   The Group Manager SHOULD use the following default values for the
   Signature Algorithm 'sign_alg' and related parameters 'sign_params',
   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].

   *  For the Signature Algorithm 'sign_alg' used to sign messages
      protected with the group mode, the signature algorithm EdDSA
      [RFC8032].

   *  For the parameters 'sign_params' of the Signature Algorithm:

      -  The array [[OKP], [OKP, Ed25519]], in case EdDSA is assumed or
         specified for 'sign_alg'.  In particular, this indicates to use
         the COSE key type OKP and the elliptic curve Ed25519 [RFC8032].

      -  The array [[EC2], [EC2, P-256]], in case ES256 [RFC6979] is
         specified for 'sign_alg'.  In particular, this indicates to use
         the COSE key type EC2 and the elliptic curve P-256.

      -  The array [[EC2], [EC2, P-384]], in case ES384 [RFC6979] is
         specified for 'sign_alg'.  In particular, this indicates to use
         the COSE key type EC2 and the elliptic curve P-384.

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      -  The array [[EC2], [EC2, P-521]], in case ES512 [RFC6979] is
         specified for 'sign_alg'.  In particular, this indicates to use
         the COSE key type EC2 and the elliptic curve P-521.

      -  The array [[RSA], [RSA]], in case PS256, PS384 or PS512
         [RFC8017] is specified for 'sign_alg'.  In particular, this
         indicates to use the COSE key type RSA.

14.3.  Pairwise Mode

   This section applies if the group uses (also) the pairwise mode of
   Group OSCORE.

   For the AEAD Algorithm 'alg' used to encrypt messages protected with
   the pairwise mode, the Group Manager SHOULD use the same default
   value defined in Section 3.2 of [RFC8613], i.e., AES-CCM-16-64-128
   (COSE algorithm encoding: 10).

   For the Pairwise Key Agreement Algorithm 'ecdh_alg' and related
   parameters 'ecdh_params', the Group Manager SHOULD use the following
   default values, 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].

   *  For the Pairwise Key Agreement 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].

   *  For the parameters 'ecdh_params' of the Pairwise Key Agreement
      Algorithm:

      -  The array [[OKP], [OKP, X25519]], in case EdDSA is assumed or
         specified for 'sign_alg', or in case the group is a pairwise-
         only group.  In particular, this indicates to use the COSE key
         type OKP and the elliptic curve X25519 [RFC8032].

      -  The array [[EC2], [EC2, P-256]], in case ES256 [RFC6979] is
         specified for 'sign_alg'.  In particular, this indicates to use
         the COSE key type EC2 and the elliptic curve P-256.

      -  The array [[EC2], [EC2, P-384]], in case ES384 [RFC6979] is
         specified for 'sign_alg'.  In particular, this indicates to use
         the COSE key type EC2 and the elliptic curve P-384.

      -  The array [[EC2], [EC2, P-521]], in case ES512 [RFC6979] is
         specified for 'sign_alg'.  In particular, this indicates to use
         the COSE key type EC2 and the elliptic curve P-521.

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15.  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.

15.1.  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.

   *  Management of group keying material (see Section 3.2 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).

      The Group Manager performs a rekeying when one ore more members
      leave the group, thus preserving forward security and ensuring
      that the security properties of Group OSCORE are fulfilled.
      According to the specific application requirements, the Group
      Manager can also rekey the group upon a new node's joining, in
      case backward security has also to be preserved.

   *  Provisioning and retrieval of authentication credentials (see
      Section 3 of [I-D.ietf-core-oscore-groupcomm]).  The Group Manager
      acts as repository of authentication credentials of group members,
      and provides them upon request.

   *  Synchronization of sequence numbers (see Section 6.3 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, when establishing a secure communication association
   with the Group Manager, the joining node can provide the Group
   Manager with its own authentication credential, and use the public
   key included thereof as asymmetric proof-of-possession key.  For
   example, this is the case when the joining node relies on

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   Section 3.2.2 of [I-D.ietf-ace-dtls-authorize] and authenticates
   itself during the DTLS handshake with the Group Manager.  However,
   this requires the authentication credential to be in the format used
   in the OSCORE group, and that both the authentication credential of
   the joining node and the included public key are compatible with the
   signature or ECDH 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.

15.2.  Size of Nonces as Proof-of-Possesion Challenge

   With reference to the Joining Request message in Section 6.1, the
   proof-of-possession (PoP) evidence included in 'client_cred_verify'
   is computed over an input including also N_C | N_S, where | denotes
   concatenation.

   For the N_C challenge, 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 association between the joining node and the Group
   Manager.

   As defined in Section 6.1.1, 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.

   *  If the Access Token has not been provided to the Group Manager by
      means of a Token Transfer Request to the /authz-info endpoint as
      in Section 5.3, then N_S is computed as a 32-byte long challenge.
      For an example, see point (2) of Section 6.1.1.

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   *  If the Access Token has been provided to the Group Manager by
      means of a Token Transfer Request to the /authz-info endpoint as
      in Section 5.3, then 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 6.1.1.  This value is
      provided either in the Token Transfer Response (see Section 5.3),
      or in a 4.00 (Bad Request) error response to a following Joining
      Request (see Section 6.2).  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.

   *  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 transferred 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.

   *  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 document 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.

15.3.  Reusage of Nonces for Proof-of-Possession Input

   As long as the Group Manager preserves the same N_S value currently
   associated with 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 proof-of-possession (PoP) input 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 PoP input for
   producing the PoP evidence 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
   with the Access Token, the Group Manager can be forced to falsely

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   believe that the Client possesses its own private key at that point
   in time, upon verifying the PoP evidence in the 'client_cred_verify'
   parameter.

16.  IANA Considerations

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

   This document has the following actions for IANA.

16.1.  OAuth Parameters

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

   *  Parameter name: ecdh_info

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

   *  Change Controller: IESG

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

   *  Parameter name: kdc_dh_creds

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

   *  Change Controller: IESG

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

16.2.  OAuth Parameters CBOR Mappings

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

   *  Name: ecdh_info

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

   *  Value Type: Simple value "null" / Array

   *  Reference: [[This document]]

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   *  Name: kdc_dh_creds

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

   *  Value Type: Simple value "null" / Array

   *  Reference: [[This document]]

16.3.  ACE Groupcomm Parameters

   IANA is asked to register the following entry to the "ACE Groupcomm
   Parameters" registry defined in Section 11.7 of
   [I-D.ietf-ace-key-groupcomm].

   *  Name: group_senderId

   *  CBOR Key: TBD

   *  CBOR Type: Byte string

   *  Reference: [[This document]] (Section 9.2)

   *  Name: ecdh_info

   *  CBOR Key: TBD

   *  CBOR Type: Array

   *  Reference: [[This document]] (Section 6.2)

   *  Name: kdc_dh_creds

   *  CBOR Key: TBD

   *  CBOR Type: Array

   *  Reference: [[This document]] (Section 6.2)

   *  Name: group_enc_key

   *  CBOR Key: TBD

   *  CBOR Type: Byte string

   *  Reference: [[This document]] (Section 8.2.1)

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   *  Name: stale_node_ids

   *  CBOR Key: TBD

   *  CBOR Type: Array

   *  Reference: [[This document]] (Section 11)

16.4.  ACE Groupcomm Key Types

   IANA is asked to register the following entry to the "ACE Groupcomm
   Key Types" registry defined in Section 11.8 of
   [I-D.ietf-ace-key-groupcomm].

   *  Name: Group_OSCORE_Input_Material object

   *  Key Type Value: GROUPCOMM_KEY_TBD

   *  Profile: "coap_group_oscore_app", defined in Section 16.5 of this
      document.

   *  Description: A Group_OSCORE_Input_Material object encoded as
      described in Section 6.3 of this document.

   *  Reference: [[This document]] (Section 6.3)

16.5.  ACE Groupcomm Profiles

   IANA is asked to register the following entry to the "ACE Groupcomm
   Profiles" registry defined in Section 11.9 of
   [I-D.ietf-ace-key-groupcomm].

   *  Name: coap_group_oscore_app

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

   *  CBOR Value: PROFILE_TBD

   *  Reference: [[This document]] (Section 6.3)

16.6.  OSCORE Security Context Parameters

   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].

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   *  Name: group_SenderId

   *  CBOR Label: TBD

   *  CBOR Type: Byte string

   *  Registry: -

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

   *  Reference: [[This document]] (Section 6.3)

   *  Name: cred_fmt

   *  CBOR Label: TBD

   *  CBOR Type: Integer

   *  Registry: COSE Header Parameters

   *  Description: Format of authentication credentials to be used in
      the OSCORE group

   *  Reference: [[This document]] (Section 6.3)

   *  Name: sign_enc_alg

   *  CBOR Label: TBD

   *  CBOR Type: Text string / Integer

   *  Registry: COSE Algorithms

   *  Description: OSCORE Signature Encryption Algorithm Value

   *  Reference: [[This document]] (Section 6.3)

   *  Name: sign_alg

   *  CBOR Label: TBD

   *  CBOR Type: Text string / Integer

   *  Registry: COSE Algorithms

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   *  Description: OSCORE Signature Algorithm Value

   *  Reference: [[This document]] (Section 6.3)

   *  Name: sign_params

   *  CBOR Label: TBD

   *  CBOR Type: Array

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

   *  Description: OSCORE Signature Algorithm Parameters

   *  Reference: [[This document]] (Section 6.3)

   *  Name: ecdh_alg

   *  CBOR Label: TBD

   *  CBOR Type: Text string / Integer

   *  Registry: COSE Algorithms

   *  Description: OSCORE Pairwise Key Agreement Algorithm Value

   *  Reference: [[This document]] (Section 6.3)

   *  Name: ecdh_params

   *  CBOR Label: TBD

   *  CBOR Type: Array

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

   *  Description: OSCORE Pairwise Key Agreement Algorithm Parameters

   *  Reference: [[This document]] (Section 6.3)

16.7.  TLS Exporter Labels

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

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

   *  DTLS-OK: Y

   *  Recommended: N

   *  Reference: [[This document]] (Section 6.1.1)

16.8.  AIF

   For the media-types application/aif+cbor and application/aif+json
   defined in Section 5.1 of [I-D.ietf-ace-aif], IANA is requested to
   register the following entries for the two media-type parameters Toid
   and Tperm, in the respective sub-registry defined in Section 5.2 of
   [I-D.ietf-ace-aif] within the "MIME Media Type Sub-Parameter"
   registry group.

   *  Name: oscore-gname

   *  Description/Specification: OSCORE group name

   *  Reference: [[This document]]

   *  Name: oscore-gperm

   *  Description/Specification: permissions pertaining OSCORE groups

   *  Reference: [[This document]]

16.9.  CoAP Content-Format

   IANA is asked to register the following entries to the "CoAP Content-
   Formats" registry within the "Constrained RESTful Environments (CoRE)
   Parameters" registry group.

   *  Media Type: application/aif+cbor;Toid="oscore-
      gname",Tperm="oscore-gperm"

   *  Encoding: -

   *  ID: TBD

   *  Reference: [[This document]]

   *  Media Type: application/aif+json;Toid="oscore-
      gname",Tperm="oscore-gperm"

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   *  Encoding: -

   *  ID: TBD

   *  Reference: [[This document]]

16.10.  Group OSCORE Roles

   This document 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 16.14.

   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
   document]].

   The columns of this registry are:

   *  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.

   *  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].

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

   *  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
   document]].

16.11.  CoRE Resource Type

   IANA is asked to register the following entry in the "Resource Type
   (rt=) Link Target Attribute Values" registry within the "Constrained
   Restful Environments (CoRE) Parameters" registry group.

   *  Value: "core.osc.gm"

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

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   *  Reference: [[This document]]

   Client applications can use this resource type to discover a group
   membership resource at an OSCORE Group Manager, where to send a
   request for joining the corresponding OSCORE group.

16.12.  ACE Scope Semantics

   IANA is asked to register the following entry in the "ACE Scope
   Semantics" registry defined in Section 11.12 of
   [I-D.ietf-ace-key-groupcomm].

   *  Value: SEM_ID_TBD

   *  Description: Membership and key management operations at the ACE
      Group Manager for Group OSCORE.

   *  Reference: [[This document]]

16.13.  ACE Groupcomm Errors

   IANA is asked to register the following entry in the "ACE Groupcomm
   Errors" registry defined in Section 11.13 of
   [I-D.ietf-ace-key-groupcomm].

   *  Value: 7

   *  Description: Signatures not used in the group.

   *  Reference: [[This document]]

   *  Value: 8

   *  Description: Operation permitted only to signature verifiers.

   *  Reference: [[This document]]

   *  Value: 9

   *  Description: Group currently not active.

   *  Reference: [[This document]]

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16.14.  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:

   *  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.  Experts 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.

   *  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.

   *  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.

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

17.  References

17.1.  Normative References

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

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

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

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

   [I-D.ietf-ace-aif]
              Bormann, C., "An Authorization Information Format (AIF)
              for ACE", Work in Progress, Internet-Draft, draft-ietf-
              ace-aif-07, 15 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-ace-aif-
              07.txt>.

   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE)", Work in Progress,
              Internet-Draft, draft-ietf-ace-dtls-authorize-18, 4 June
              2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
              dtls-authorize-18.txt>.

   [I-D.ietf-ace-key-groupcomm]
              Palombini, F. and M. Tiloca, "Key Provisioning for Group
              Communication using ACE", Work in Progress, Internet-
              Draft, draft-ietf-ace-key-groupcomm-15, 23 December 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-key-
              groupcomm-15.txt>.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,

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              draft-ietf-ace-oauth-authz-46, 8 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
              authz-46.txt>.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE Profile of the Authentication and Authorization
              for Constrained Environments Framework", Work in Progress,
              Internet-Draft, draft-ietf-ace-oscore-profile-19, 6 May
              2021, <https://www.ietf.org/archive/id/draft-ietf-ace-
              oscore-profile-19.txt>.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
              and J. Park, "Group OSCORE - Secure Group Communication
              for CoAP", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-groupcomm-14, 7 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-core-oscore-
              groupcomm-14.txt>.

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

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

   [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>.

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

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   [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>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [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>.

   [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>.

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   [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>.

17.2.  Informative References

   [I-D.ietf-ace-oscore-gm-admin]
              Tiloca, M., Höglund, R., Stok, P. V. D., and F. Palombini,
              "Admin Interface for the OSCORE Group Manager", Work in
              Progress, Internet-Draft, draft-ietf-ace-oscore-gm-admin-
              05, 7 March 2022, <https://www.ietf.org/archive/id/draft-
              ietf-ace-oscore-gm-admin-05.txt>.

   [I-D.ietf-core-coap-pubsub]
              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", Work in Progress, Internet-Draft, draft-ietf-
              core-coap-pubsub-09, 30 September 2019,
              <https://www.ietf.org/archive/id/draft-ietf-core-coap-
              pubsub-09.txt>.

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", Work in
              Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
              06, 7 March 2022, <https://www.ietf.org/archive/id/draft-
              ietf-core-groupcomm-bis-06.txt>.

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   [I-D.ietf-cose-cbor-encoded-cert]
              Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
              M. Furuhed, "CBOR Encoded X.509 Certificates (C509
              Certificates)", Work in Progress, Internet-Draft, draft-
              ietf-cose-cbor-encoded-cert-03, 10 January 2022,
              <https://www.ietf.org/archive/id/draft-ietf-cose-cbor-
              encoded-cert-03.txt>.

   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsuess, C., and P. V. D. Stok, "Discovery of
              OSCORE Groups with the CoRE Resource Directory", Work in
              Progress, Internet-Draft, draft-tiloca-core-oscore-
              discovery-11, 7 March 2022,
              <https://www.ietf.org/archive/id/draft-tiloca-core-oscore-
              discovery-11.txt>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [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>.

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016,
              <https://www.rfc-editor.org/info/rfc7925>.

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

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Appendix A.  Profile Requirements

   This section lists how this application profile of ACE addresses the
   requirements defined in Appendix A of [I-D.ietf-ace-key-groupcomm].

A.1.  Mandatory-to-Address Requirements

   *  REQ1 - Specify the format and encoding of 'scope'.  This includes
      defining the set of possible roles and their identifiers, as well
      as the corresponding encoding to use in the scope entries
      according to the used scope format: see Section 3 and Section 5.1.

   *  REQ2 - If the AIF format of 'scope' is used, register its specific
      instance of "Toid" and "Tperm" as Media Type parameters and a
      corresponding Content-Format, as per the guidelines in
      [I-D.ietf-ace-aif]: see Section 16.8 and Section 16.9.

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

   *  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].

   *  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].

   *  REQ6 - Specify the acceptable formats for authentication
      credentials and, if used, the acceptable values for 'pub_key_enc':
      acceptable formats explicitly provide the public key as well as
      the comprehensive set of information related to the public key
      algorithm (see Section 5.3 and Section 6.3).  Consistent
      acceptable values for 'pub_key_enc' are taken from the "Label"
      column of the "COSE Header Parameters" registry
      [COSE.Header.Parameters].

   *  REQ7 - 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]) are not required to coincide,
      specify the mechanism to map the GROUPNAME value in the URI to the
      group name: not applicable, since a perfect matching is required.

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   *  REQ8 - Define whether the KDC has an authentication credential and
      if this has to be provided through the 'kdc_cred' parameter, see
      Section 4.1 of [I-D.ietf-ace-key-groupcomm]: yes, as required by
      the Group OSCORE protocol [I-D.ietf-core-oscore-groupcomm], see
      Section 6.3 of this document.

   *  REQ9 - Specify if any part of the KDC interface as defined in
      Section 4.1 of [I-D.ietf-ace-key-groupcomm] is not supported by
      the KDC: not applicable.

   *  REQ10 - Register a Resource Type for the root url-path, which is
      used to discover the correct url to access at the KDC (see
      Section 4.1 of [I-D.ietf-ace-key-groupcomm]): the Resource Type
      (rt=) Link Target Attribute value "core.osc.gm" is registered in
      Section 16.11.

   *  REQ11 - Define what specific actions (e.g., CoAP methods) are
      allowed on each resource provided by the KDC interface, depending
      on whether the Client is a current group member; the roles that a
      Client is authorized to take as per the obtained access token; and
      the roles that the Client has as current group member: see
      Section 8.4.

   *  REQ12 - Categorize possible newly defined operations for Clients
      into primary operations expected to be minimally supported and
      secondary operations, and provide accompanying considerations: see
      Section 8.5.

   *  REQ13 - Specify the encoding of group identifier (see
      Section 4.2.1 of [I-D.ietf-ace-key-groupcomm]): CBOR byte string
      (see Section 9.10).

   *  REQ14 - Specify the approaches used to compute and verify the PoP
      evidence to include in 'client_cred_verify', and which of those
      approaches is used in which case: see Section 6.1 and Section 6.2.

   *  REQ15 - Specify how the nonce N_S is generated, if the token is
      not provided to the KDC through the Token Transfer Request to the
      authz-info endpoint (e.g., if it is used directly to validate TLS
      instead): see Section 6.1.1.

   *  REQ16 - 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].

   *  REQ17 - Specify the format of the 'key' parameter: see
      Section 6.3.

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   *  REQ18 - Specify acceptable values of the 'gkty' parameter:
      Group_OSCORE_Input_Material object (see Section 6.3).

   *  REQ19 - Specify and register the application profile identifier:
      coap_group_oscore_app (see Section 16.5).

   *  REQ20 - If used, specify the format and content of
      'group_policies' and its entries: see Section 6.3.

   *  REQ21 - Specify the approaches used to compute and verify the PoP
      evidence to include in 'kdc_cred_verify', and which of those
      approaches is used in which case: see Section 6.3, Section 6.4 and
      Section 9.5.

   *  REQ22 - Specify the communication protocol that the members of the
      group must use: CoAP [RFC7252], also for group communication
      [I-D.ietf-core-groupcomm-bis].

   *  REQ23 - Specify the security protocols that the group members must
      use to protect their communication: Group OSCORE
      [I-D.ietf-core-oscore-groupcomm].

   *  REQ24 - 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].

   *  REQ25 - Specify the format of the identifiers of group members:
      the Sender ID used in the OSCORE group (see Section 6.3 and
      Section 9.3).

   *  REQ26 - Specify policies at the KDC to handle member ids that are
      not included in 'get_pub_keys': see Section 9.3.

   *  REQ27 - Specify the format of newly-generated individual keying
      material for group members, or of the information to derive it,
      and corresponding CBOR label: see Section 9.2.

   *  REQ28 - Specify and register the identifier of newly defined
      semantics for binary scopes: see Section 16.12.

   *  REQ29 - Categorize newly defined parameters according to the same
      criteria of Section 8 of [I-D.ietf-ace-key-groupcomm]: see
      Section 12.

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   *  REQ30 - Define whether Clients must, should or may support the
      conditional parameters defined in Section 8 of
      [I-D.ietf-ace-key-groupcomm], and under which circumstances: see
      Section 12.

A.2.  Optional-to-Address Requirements

   *  OPT1 (Optional) - If the textual format of 'scope' is used,
      specify CBOR values to use for abbreviating the role identifiers
      in the group: not applicable.

   *  OPT2 (Optional) - Specify additional parameters used in the
      exchange of Token Transfer Request and Response:

      -  'ecdh_info', to negotiate the ECDH algorithm, ECDH algorithm
         parameters, ECDH key parameters and exact format of
         authentication credentials used in the group, in case the
         joining node supports the pairwise mode of Group OSCORE (see
         Section 5.3).

      -  'kdc_dh_creds', to ask for and retrieve the Group Manager's
         Diffie-Hellman authentication credentials, in case the joining
         node supports the pairwise mode of Group OSCORE and the Access
         Token authorizes to join parwise-only groups (see Section 5.3).

   *  OPT3 (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].

   *  OPT4 (Optional) - Specify possible or required payload formats for
      specific error cases: send a 4.00 (Bad Request) error response to
      a Joining Request (see Section 6.2).

   *  OPT5 (Optional) - Specify additional identifiers of error types,
      as values of the 'error' field in an error response from the KDC:
      see Section 16.13.

   *  OPT6 (Optional) - Specify the encoding of 'pub_keys_repos' if the
      default is not used: no.

   *  OPT7 (Optional) - Specify the functionalities implemented at the
      'control_uri' resource hosted at the Client, including message
      exchange encoding and other details (see Section 4.3.1 of
      [I-D.ietf-ace-key-groupcomm]): see Section 10 for the eviction of
      a group member; see Section 11 for the group rekeying process.

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   *  OPT8 (Optional) - Specify the behavior of the handler in case of
      failure to retrieve an authentication credential for the specific
      node: send a 4.00 (Bad Request) error response to a Joining
      Request (see Section 6.2).

   *  OPT9 (Optional) - Define a default group rekeying scheme, to refer
      to in case the 'rekeying_scheme' parameter is not included in the
      Joining Response (see Section 4.3.1.1 of
      [I-D.ietf-ace-key-groupcomm]): the "Point-to-Point" rekeying
      scheme registered in Section 11.14 of
      [I-D.ietf-ace-key-groupcomm], whose detailed use for this profile
      is defined in Section 11 of this document.

   *  OPT10 (Optional) - Specify the functionalities implemented at the
      'control_group_uri' resource hosted at the Client, including
      message exchange encoding and other details (see Section 4.3.1 of
      [I-D.ietf-ace-key-groupcomm]): see Section 10 for the eviction of
      multiple group members.

   *  OPT11 (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.

   *  OPT12 (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 9.2).

   *  OPT13 (Optional) - Specify how the identifier of a group members's
      authentication credential is included in requests sent to other
      group members: no.

   *  OPT14 (Optional) - Specify additional information to include in
      rekeying messages for the "Point-to-Point" group rekeying scheme
      (see Section 6.1 of [I-D.ietf-ace-key-groupcomm]): see
      Section 11.1.

   *  OPT15 (Optional) - Specify if Clients must or should support any
      of the parameters defined as optional in Section 8 of
      [I-D.ietf-ace-key-groupcomm]: no.

Appendix B.  Extensibility for Future COSE Algorithms

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

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   To enable the seamless use of such future registered algorithms, this
   section defines a general, agile format for:

   *  Each 'ecdh_info_entry' of the 'ecdh_info' parameter in the Token
      Transfer Response (see Section 5.3 and Section 5.3.1);

   *  The 'sign_params' and 'ecdh_params' parameters within the 'key'
      parameter (see Section 6.3), as part of the response payloads used
      in Section 6.3, Section 9.1.1, Section 9.1.2 and Section 11.

   Appendix B of [I-D.ietf-ace-key-groupcomm] describes the analogous
   general format for 'sign_info_entry' of the 'sign_info' parameter in
   the Token Transfer Response (see Section 5.3 of this document).

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

B.1.  Format of 'ecdh_info_entry'

   The format of each 'ecdh_info_entry' (see Section 5.3 and
   Section 5.3.1) is generalized as follows.  Given N the number of
   elements of the 'ecdh_parameters' array, i.e., the number of COSE
   capabilities of the ECDH algorithm, then:

   *  'ecdh_key_parameters' is replaced by N elements 'ecdh_capab_i',
      each of which is a CBOR array.

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

   ecdh_info_entry =
   [
     id : gname / [ + gname ],
     ecdh_alg : int / tstr,
     ecdh_parameters : [ alg_capab_1 : any,
                         alg_capab_2 : any,
                         ...,
                         alg_capab_N : any],
     ecdh_capab_1 : [ any ],
     ecdh_capab_2 : [ any ],
     ...,
     ecdh_capab_N : [ any ],
     cred_fmt = int / null
   ]

   gname = tstr

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              Figure 13: 'ecdh_info_entry' with general format

B.2.  Format of 'key'

   The format of 'key' (see Section 6.3) is generalized as follows.

   *  The 'sign_params' array includes N+1 elements, whose exact
      structure and value depend on the value of the signature algorithm
      specified in 'sign_alg'.

      -  The first element, i.e., 'sign_params'[0], is the array of the
         N COSE capabilities for the signature algorithm, as specified
         for that algorithm in the "Capabilities" column of the "COSE
         Algorithms" registry [COSE.Algorithms] (see Section 8.1 of
         [I-D.ietf-cose-rfc8152bis-algs]).

      -  Each following element 'sign_params'[i], i.e., with index i >
         0, is the array of COSE capabilities for the algorithm
         capability specified in 'sign_params'[0][i-1].

      For example, if 'sign_params'[0][0] specifies the key type as
      capability of the algorithm, then 'sign_params'[1] is the array of
      COSE capabilities for the COSE key type associated with the
      signature algorithm, as specified for that key type in the
      "Capabilities" column of the "COSE Key Types" registry
      [COSE.Key.Types] (see Section 8.2 of
      [I-D.ietf-cose-rfc8152bis-algs]).

   *  The 'ecdh_params' array includes M+1 elements, whose exact
      structure and value depend on the value of the ECDH algorithm
      specified in 'ecdh_alg'.

      -  The first element, i.e., 'ecdh_params'[0], is the array of the
         M COSE capabilities for the ECDH algorithm, as specified for
         that algorithm in the "Capabilities" column of the "COSE
         Algorithms" registry [COSE.Algorithms] (see Section 8.1 of
         [I-D.ietf-cose-rfc8152bis-algs]).

      -  Each following element 'ecdh_params'[i], i.e., with index i >
         0, is the array of COSE capabilities for the algorithm
         capability specified in 'ecdh_params'[0][i-1].

      For example, if 'ecdh_params'[0][0] specifies the key type as
      capability of the algorithm, then 'ecdh_params'[1] is the array of
      COSE capabilities for the COSE key type associated with the ECDH
      algorithm, as specified for that key type in the "Capabilities"
      column of the "COSE Key Types" registry [COSE.Key.Types] (see
      Section 8.2 of [I-D.ietf-cose-rfc8152bis-algs]).

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Appendix C.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

C.1.  Version -13 to -14

   *  Major reordering of the document sections.

   *  The HKDF Algorithm is specified by the HMAC Algorithm.

   *  Group communication does not necessarily use IP multicast.

   *  Generalized AIF data model, also for draft-ace-oscore-gm-admin.

   *  Clarifications and editorial improvements.

C.2.  Version -12 to -13

   *  Renamed parameters about authentication credentials.

   *  It is optional for the Group Manager to reassign Gids by tracking
      "Birth Gids".

   *  Distinction between authentication credentials and public keys.

   *  Updated IANA considerations related to AIF.

   *  Updated textual description of registered ACE Scope Semantics
      value.

C.3.  Version -11 to -12

   *  Clarified semantics of 'ecdh_info' and 'kdc_dh_creds'.

   *  Definition of /ace-group/GROUPNAME/kdc-pub-key moved to draft-
      ietf-ace-key-groupcomm.

   *  ace-group/ accessible also to non-members that are not Verifiers.

   *  Clarified what resources are accessible to Verifiers.

   *  Revised error handling for the newly defined resources.

   *  Revised use of CoAP error codes.

   *  Use of "Token Tranfer Request" and "Token Transfer Response".

   *  New parameter 'rekeying_scheme'.

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   *  Categorization of new parameters and inherited conditional
      parameters.

   *  Clarifications on what to do in case of enhanced error responses.

   *  Changed UCCS to CCS.

   *  Authentication credentials of just joined Clients can be in
      rekeying messages.

   *  Revised names of new IANA registries.

   *  Clarified meaning of registered CoRE resource type.

   *  Alignment to new requirements from draft-ietf-ace-key-groupcomm.

   *  Fixes and editorial improvements.

C.4.  Version -10 to -11

   *  Removed redundancy of key type capabilities, from 'sign_info',
      'ecdh_info' and 'key'.

   *  New resource to retrieve the Group Manager's authentication
      credential.

   *  New resource to retrieve material for Signature Verifiers.

   *  New parameter 'sign_enc_alg' related to the group mode.

   *  'cred_fmt' takes value from the COSE Header Parameters registry.

   *  Improved alignment of the Joining Response payload with the Group
      OSCORE Security Context parameters.

   *  Recycling Group IDs by tracking "Birth GIDs".

   *  Error handling in case of non available Sender IDs upon joining.

   *  Error handling in case EdDSA public keys with invalid Y coordinate
      when the pairwise mode of Group OSCORE is supported.

   *  Generalized proof-of-possession (PoP) for the joining node's
      private key; defined Diffie-Hellman based PoP for OSCORE groups
      using only the pairwise mode.

   *  Proof-of-possession of the Group Manager's private key in the
      Joining Response.

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   *  Always use 'peer_identifiers' to convey Sender IDs as node
      identifiers.

   *  Stale Sender IDs provided when rekeying the group.

   *  New resource for late retrieval of stale Sender IDs.

   *  Added examples of message exchanges.

   *  Revised default values of group configuration parameters.

   *  Fixes to IANA registrations.

   *  General format of parameters related to COSE capabilities,
      supporting future registered COSE algorithms (new Appendix).

C.5.  Version -09 to -10

   *  Updated non-recycling policy of Sender IDs.

   *  Removed policies about Sender Sequence Number synchronization.

   *  'control_path' renamed to 'control_uri'.

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

   *  Additional way to inform of group eviction.

   *  Registered semantics identifier for extended scope format.

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

   *  Renumbered requirements.

C.6.  Version -08 to -09

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

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

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

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

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   *  Renamed the key material object as Group_OSCORE_Input_Material
      object.

   *  Replaced 'clientId' with 'group_SenderId'.

   *  Added message exchange for Group Names request-response.

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

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

   *  Signature verifiers can also retrieve Group Names and URIs.

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

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

   *  Update list of requirements.

   *  Clarifications and editorial revision.

C.7.  Version -07 to -08

   *  AIF specific data model to express scope entries.

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

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

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

   *  Updated payload format of the FETCH request to retrieve
      authentication credentials.

   *  Default values for group configuration parameters.

   *  IANA registrations to support the AIF specific data model.

C.8.  Version -06 to -07

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

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

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

   *  Considerations on group rekeying.

   *  Editorial revision.

C.9.  Version -05 to -06

   *  Added role of external signature verifier.

   *  Parameter 'rsnonce' renamed to 'kdcchallenge'.

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

   *  Clarified difference between group name and OSCORE Gid.

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

   *  Admit implicit scope and audience in the Authorization Request.

   *  New format for the 'sign_info' parameter.

   *  Scope not mandatory to include in the Joining Request.

   *  Group policy about supporting Group OSCORE in pairwise mode.

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

   *  Security considerations on reusage of signature challenges.

   *  Addressing optional requirement OPT12 from draft-ietf-ace-key-
      groupcomm

   *  Editorial improvements.

C.10.  Version -04 to -05

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

   *  Supporting single Access Token for multiple groups/topics.

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

   *  Registered and used dedicated label for TLS Exporter.

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   *  Added method for uploading a new authentication credential to the
      Group Manager.

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

   *  Fixed inconsistency in retrieving group keying material only.

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

   *  Clarification on incrementing version number when rekeying the
      group.

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

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

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

C.11.  Version -03 to -04

   *  New abstract.

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

   *  Terminology: node name; node resource.

   *  Creation and pointing at node resource.

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

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

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

   *  Removed reference to RFC 7390.

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

   *  Editorial improvements.

C.12.  Version -02 to -03

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

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

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

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

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

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

   *  IANA registration in ACE Groupcomm Parameters registry.

   *  More fulfilled profile requirements (Appendix A).

C.13.  Version -01 to -02

   *  Editorial fixes.

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

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

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

   *  Added parameters to indicate the encoding of authentication
      credentials.

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

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

   *  Code 4.00 (Bad request), in responses to joining nodes providing
      an invalid authentication credential (Section 4.3).

   *  Clarifications on provisioning and checking of authentication
      credentials (Sections 4 and 6).

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

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

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

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

C.14.  Version -00 to -01

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

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

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

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

   *  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 Christian Amsuess, Santiago Aragon,
   Stefan Beck, Carsten Bormann, Martin Gunnarsson, Rikard Hoeglund,
   Watson Ladd, 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
   SE-164 29 Stockholm Kista
   Sweden
   Email: marco.tiloca@ri.se

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   Jiye Park
   Universitaet Duisburg-Essen
   Schuetzenbahn 70
   45127 Essen
   Germany
   Email: ji-ye.park@uni-due.de

   Francesca Palombini
   Ericsson AB
   Torshamnsgatan 23
   SE-16440 Stockholm Kista
   Sweden
   Email: francesca.palombini@ericsson.com

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