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Versions: (draft-tiloca-ace-oscoap-joining) 00 01 02 03 04 05 06 07 08

ACE Working Group                                              M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                                 J. Park
Expires: September 10, 2020                  Universitaet Duisburg-Essen
                                                            F. Palombini
                                                             Ericsson AB
                                                          March 09, 2020


                Key Management for OSCORE Groups in ACE
                 draft-ietf-ace-key-groupcomm-oscore-05

Abstract

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

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   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 September 10, 2020.

Copyright Notice

   Copyright (c) 2020 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Overview of the Joining Process . . . . . . . . . . . . .   5
     2.2.  Overview of the Group Rekeying Process  . . . . . . . . .   6
   3.  Joining Node to Authorization Server  . . . . . . . . . . . .   6
     3.1.  Authorization Request . . . . . . . . . . . . . . . . . .   6
     3.2.  Authorization Response  . . . . . . . . . . . . . . . . .   7
   4.  Interface at the Group Manager  . . . . . . . . . . . . . . .   7
     4.1.  GET Handler . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Joining a Group . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Token Post  . . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Sending the Joining Request . . . . . . . . . . . . . . .   9
       5.2.1.  Value of the N_S Challenge  . . . . . . . . . . . . .  10
     5.3.  Processing the Joining Request  . . . . . . . . . . . . .  11
     5.4.  Joining Response  . . . . . . . . . . . . . . . . . . . .  12
   6.  Public Keys of Joining Nodes  . . . . . . . . . . . . . . . .  15
   7.  Retrieval of Updated Keying Material  . . . . . . . . . . . .  17
     7.1.  Retrieval of Group Keying Material  . . . . . . . . . . .  17
     7.2.  Retrieval of Group Keying Material and Sender ID  . . . .  17
   8.  Retrieval of New Keying Material  . . . . . . . . . . . . . .  18
   9.  Retrieval of Public Keys of Group Members . . . . . . . . . .  19
   10. Update of Public Key  . . . . . . . . . . . . . . . . . . . .  19
   11. Retrieval of Group Policies . . . . . . . . . . . . . . . . .  20
   12. Retrieval of Keying Material Version  . . . . . . . . . . . .  20
   13. Retrieval of Group Status . . . . . . . . . . . . . . . . . .  21
   14. Request to Leave the Group  . . . . . . . . . . . . . . . . .  21
   15. Removal of a Group Member . . . . . . . . . . . . . . . . . .  21
   16. Group Rekeying Process  . . . . . . . . . . . . . . . . . . .  22
   17. Security Considerations . . . . . . . . . . . . . . . . . . .  23
     17.1.  Management of OSCORE Groups  . . . . . . . . . . . . . .  24
     17.2.  Size of Nonces for Signature Challenge . . . . . . . . .  25
   18. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     18.1.  ACE Groupcomm Profile Registry . . . . . . . . . . . . .  26
     18.2.  ACE Groupcomm Key Registry . . . . . . . . . . . . . . .  27
     18.3.  OSCORE Security Context Parameters Registry  . . . . . .  27



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     18.4.  Sequence Number Synchronization Method Registry  . . . .  28
     18.5.  ACE Groupcomm Parameters Registry  . . . . . . . . . . .  29
     18.6.  TLS Exporter Label Registry  . . . . . . . . . . . . . .  29
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     19.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     19.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Profile Requirements . . . . . . . . . . . . . . . .  32
   Appendix B.  Document Updates . . . . . . . . . . . . . . . . . .  34
     B.1.  Version -04 to -05  . . . . . . . . . . . . . . . . . . .  34
     B.2.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  35
     B.3.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  35
     B.4.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  36
     B.5.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  37
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

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) [RFC8152] and enabling end-to-end
   security of CoAP payload and options.

   As described in [I-D.ietf-core-oscore-groupcomm], Group OSCORE is
   used to protect CoAP group communication over IP multicast
   [I-D.dijk-core-groupcomm-bis].  This relies on a Group Manager, which
   is responsible for managing an OSCORE group, where members exchange
   CoAP messages secured with Group OSCORE.  The Group Manager can be
   responsible for multiple groups, coordinates the joining process of
   new group members, and is entrusted with the distribution and renewal
   of group keying material.

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

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP




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   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are expected to be familiar with:

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

   o  The terms and concepts related to the CoAP protocol described in
      [RFC7252][I-D.dijk-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".

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

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

   Additionally, this document makes use of the following terminology.

   o  Group name is used as a synonym for group identifier in
      [I-D.ietf-ace-key-groupcomm].

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

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

   o  Monitor: member of an OSCORE group that is configured as responder
      and never replies back to requesters after receiving request




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      messages.  This corresponds to the term "silent server" used in
      [I-D.ietf-core-oscore-groupcomm].

2.  Protocol Overview

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

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

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

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

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

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

   All communications between the involved entities rely on the CoAP
   protocol and 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.  Note that it is expected that in the commonly
   referred base-case of this specification, the transport profile to
   use is pre-configured and well-known to nodes participating in
   constrained applications.

2.1.  Overview of the Joining Process

   A node performs the steps described in Section 4.2 of
   [I-D.ietf-ace-key-groupcomm] in order to join an OSCORE group.  The
   format and processing of messages exchanged among the participants
   are further specified in Section 3 and Section 5 of this document.





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2.2.  Overview of the Group Rekeying Process

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

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

   The keying material distributed through a group rekeying MUST include
   a new Group Identifier (Gid) for the group and a new value for the
   Master Secret parameter of the OSCORE Common Security Context of that
   group (see Section 2 of [I-D.ietf-core-oscore-groupcomm]).  Also, it
   MAY include a new value for the Master Salt parameter of the OSCORE
   Common Security Context of that group.

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

   The Group Manager MUST support the Group Rekeying Process described
   in Section 16.  Future application profiles may define alternative
   message formats and distribution schemes to perform group rekeying.

3.  Joining Node to Authorization Server

   This section describes how the joining node interacts with the AS in
   order to be authorized to join an OSCORE group under a given Group
   Manager.  In particular, it considers a joining node that intends to
   contact that Group Manager for the first time.

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

3.1.  Authorization Request

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

   o  The 'scope' parameter MUST be present.



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      *  The group name of the OSCORE group to join under the Group
         Manager is encoded as a CBOR text string (REQ1).

      *  Accepted values for role identifiers in the OSCORE group to
         join are: "requester", "responder", and "monitor" (REQ2).
         Possible combinations are: ["requester" , "responder"];
         ["requester" , "monitor"].  Each role identifier MUST be
         encoded as a CBOR integer (REQ2), by using for abbreviation the
         values specified in Figure 1 (OPT7).

                        +-----------+------------+
                        | Name      | CBOR Value |
                        +-----------+------------+
                        | requester |    TBD8    |
                        | responder |    TBD9    |
                        | monitor   |    TBD10   |
                        +-----------+------------+

      Figure 1: CBOR Abbreviations for Role Identifiers in the Group

   o  The 'audience' parameter MUST be present.

3.2.  Authorization Response

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

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

   o  The AS MUST include the 'scope' parameter, when the value included
      in the Access Token differs from the one specified by the joining
      node in the request.  In such a case, the second element of each
      scope entry MUST be present, and includes the role or CBOR array
      of roles that the joining node is actually authorized to take in
      the OSCORE group for that scope entry, encoded as specified in
      Section 3.1 of this document.

4.  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 following additional resource:

   o  /group-manager/GROUPNAME/active: this sub-resource is fixed and
      supports the GET method, whose handler is defined in Section 4.1.





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4.1.  GET Handler

   The handler expects a GET request.

   The handler verifies that the group identifier of the /group-
   manager/GROUPNAME/active path is a subset of the 'scope' stored in
   the Access Token associated to the requesting client.  If
   verification fails, the Group Manager MUST respond with a 4.01
   (Unauthorized) error message.

   If verification succeeds, the handler returns a 2.05 (Content)
   message containing the CBOR simple value True if the group is
   currently active, or the CBOR simple value False otherwise.  The
   group is considered active if it is set to allow new members to join,
   and if communication within the group is expected.

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

5.  Joining a Group

   The following subsections describe the interactions between the
   joining node and the Group Manager, i.e. the sending of the Access
   Token and the Request-Response exchange to join the OSCORE group.
   The message exchange between the joining node and the KDC consists of
   the messages defined in Section 3.3 and 4.2 of
   [I-D.ietf-ace-key-groupcomm].  Note that what is defined in
   [I-D.ietf-ace-key-groupcomm] applies, and only additions or
   modifications to that specification are defined here.

5.1.  Token Post

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

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

   o  The 'rsnonce' 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 may use this nonce
      in order to prove the possession of its own private key, upon
      joining the group (see Section 5.2).

   o  If 'sign_info' is present in the response:




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      TODO: have 'sign_info' as an array of arrays, if 'scope' in the
      Access Token covers multiple groups/topics.

      *  'sign_alg' takes value from Tables 5 and 6 of [RFC8152].

      *  'sign_parameters' takes values from the "Counter Signature
         Parameters" Registry (see Section 11.1 of
         [I-D.ietf-core-oscore-groupcomm]).  Its structure depends on
         the value of 'sign_alg'.  If no parameters of the counter
         signature algorithm are specified, 'sign_parameters' MUST be
         encoding the CBOR simple value Null.

      *  'sign_key_parameters' takes values from the "Counter Signature
         Key Parameters" Registry (see Section 11.2 of
         [I-D.ietf-core-oscore-groupcomm]).  Its structure depends on
         the value of 'sign_alg'.  If no parameters of the key used with
         the counter signature algorithm are specified,
         'sign_key_parameters' MUST be encoding the CBOR simple value
         Null.

   TODO: have 'pub_key_enc' as an array of arrays, if 'scope' in the
   Access Token covers multiple groups/topics.

   o  If 'pub_key_enc' is present in the response, it takes value 1
      ("COSE_Key") from the 'Confirmation Key' column of the "CWT
      Confirmation Method" Registry defined in
      [I-D.ietf-ace-cwt-proof-of-possession], so indicating that public
      keys in the OSCORE group are encoded as COSE Keys [RFC8152].
      Future specifications may define additional values for this
      parameter.

   Note that, other than through the above parameters as defined in
   Section 3.3 of [I-D.ietf-ace-key-groupcomm], the joining node MAY
   have previously retrieved this information by other means, e.g. by
   using the approach described in [I-D.tiloca-core-oscore-discovery].

   Additionally, if allowed by the used transport profile of ACE, the
   joining node may instead provide the Access Token to the Group
   Manager by other means, e.g. during a secure session establishment
   (see Section 3.3.1 of [I-D.ietf-ace-dtls-authorize]).

5.2.  Sending the Joining Request

   The joining node requests to join the OSCORE group, by sending a
   Joining Request message to the related group-membership resource at
   the Group Manager, as per Section 4.2 of
   [I-D.ietf-ace-key-groupcomm].




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

   o  The string "group-oscore" is used instead of "ace-group" (see
      Section 4.1 of [I-D.ietf-ace-key-groupcomm]) as the top level path
      to the group-membership resource.  The url-path /group-oscore/ is
      a default name of this specifications: implementations are not
      required to use this name, and can define their own instead.

   o  The 'scope' parameter MUST be present.

   o  The 'get_pub_keys' parameter is present only if the joining node
      wants to retrieve the public keys of the group members from the
      Group Manager during the joining process (see Section 6).
      Otherwise, this parameter MUST NOT be present.

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

   o  The signature encoded in the 'client_cred_verify' parameter is
      computed by the joining node by using the same private key and
      countersignature algorithm it intends to use for signing messages
      in the OSCORE group.  Moreover, N_S is as defined in
      Section 5.2.1.

5.2.1.  Value of the N_S Challenge

   The N_S challenge takes one of the following values.

   1.  If the joining node has posted the Access Token to the /authz-
       info endpoint of the Group Manager as in Section 5.1, N_S takes
       the same value of the 'rsnonce' parameter in the 2.01 (Created)
       response to the Token POST.

   2.  If the Token posting has relied on the DTLS profile of ACE
       [I-D.ietf-ace-dtls-authorize] and the joining node included the
       Access Token as content of the "psk_identity" field of the
       ClientKeyExchange message [RFC6347], N_S is an exporter value
       computed as defined in Section 7.5 of [RFC8446].  Specifically,
       N_S is exported from the DTLS session between the joining node
       and the Group Manager, using an empty 'context_value', 32 bytes
       as 'key_length', and the exporter label "EXPORTER-ACE-Sign-
       Challenge-coap-group-oscore-app" defined in Section 18.6 of this
       specification.

   3.  If the joining node is in fact re-joining the group, without
       posting again the same and still valid Access Token:



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       *  If the joining node and the Group Manager communicates using
          DTLS, N_S is an exporter value, computed as described in point
          (2) above.

       *  If the joining node and the Group Manager communicates using
          OSCORE [RFC8613], the N_S is the output PRK of a HKDF-Extract
          step [RFC5869], i.e. PRK = HMAC-Hash(salt, IKM).  In
          particular, 'salt' takes (x1 | x2), where x1 is the ID Context
          of the OSCORE Security Context between the joining node and
          the Group Manager, x2 is the Sender ID of the joining node in
          that Context, and | denotes byte string concatenation.  Also,
          'IKM' is the OSCORE Master Secret of the OSCORE Security
          Context between the joining node and the Group Manager.  The
          HKDF MUST be one of the HMAC-based HKDF [RFC5869] algorithms
          defined for COSE [RFC8152].  HKDF SHA-256 is mandatory to
          implement.

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

5.3.  Processing the Joining Request

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

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

   o  To compute the signature contained in 'client_cred_verify', the GM
      considers: i) as signed value, N_S concatenated with N_C, where
      N_S is determined as described in Section 5.2.1, while N_C is the
      nonce provided in the 'cnonce' parameter of the Joining Request;
      ii) the countersignature algorithm used in the OSCORE group, and
      possible correponding parameters; and iii) the public key of the
      joining node, either retrieved from the 'client_cred' parameter,
      or already stored as acquired from previous interactions with the
      joining node.

   o  A 4.00 Bad Request response from the Group Manager to the joining
      node MUST have content format application/ace-group+cbor.  The
      response payload is a CBOR map which MUST contain the 'sign_info'
      and 'pub_key_enc' parameters.  The CBOR map SHOULD additionally
      contain the 'rsnonce' parameter, specifying a new dedicated 8-byte
      nonce generated by the Group Manager (see Section 5.1).



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

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

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

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

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

      *  The 'client_cred_verify' parameter MUST include a signature
         computed as described in Section 5.2, by using the public key
         indicated in the current 'client_cred' parameter, with the
         countersignature algorithm and possible associated parameters
         indicated by the Group Manager.  If the error response from the
         Group Manager included the 'rsnonce' parameter, the joining
         node MUST use its content as new N_S challenge to compute the
         signature.

5.4.  Joining Response

   If the processing of the Joining Request described in Section 5.3 is
   successful, the Group Manager updates the group membership by
   registering the joining node NODENAME as a new member of the OSCORE
   group GROUPNAME, as described in Section 4.1.2.1 of
   [I-D.ietf-ace-key-groupcomm].

   If the joining node is not exclusively configured as monitor, the
   Group Manager performs also the following actions.

   o  The Group Manager selects an available OSCORE Sender ID in the
      OSCORE group, and exclusively assigns it to the joining node.

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



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

   o  The 'gkty' parameter identifies a key of type
      "Group_OSCORE_Security_Context object", defined in Section 18.2 of
      this specification.

   o  The 'key' parameter includes what the joining node needs in order
      to set up the OSCORE Security Context as per Section 2 of
      [I-D.ietf-core-oscore-groupcomm].  This parameter has as value a
      Group_OSCORE_Security_Context object, which is defined in this
      specification and extends the OSCORE_Security_Context 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 'cs_alg', 'cs_params', 'cs_key_params' and
      'cs_key_enc' defined in Section 18.3 of this specification.  More
      specifically, the 'key' parameter is composed as follows.

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

      *  The 'clientId' parameter, if present, has as value the OSCORE
         Sender ID assigned to the joining node by the Group Manager, as
         described above.  This parameter is not present if the node
         joins the group exclusively as monitor, according to what
         specified in the Access Token (see Section 3.2).  In any other
         case, this parameter MUST be present.

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

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

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

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

      *  The 'cs_alg' parameter MUST be present and specifies the
         algorithm used to countersign messages in the group.  This
         parameter takes values from Tables 5 and 6 of [RFC8152].




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      *  The 'cs_params' parameter MAY be present and specifies the
         additional parameters for the counter signature algorithm.
         This parameter is a CBOR map whose content depends on the
         counter signature algorithm, as specified in Section 2 and
         Section 11.1 of [I-D.ietf-core-oscore-groupcomm].

      *  The 'cs_key_params' parameter MAY be present and specifies the
         additional parameters for the key used with the counter
         signature algorithm.  This parameter is a CBOR map whose
         content depends on the counter signature algorithm, as
         specified in Section 2 and Section 11.2 of
         [I-D.ietf-core-oscore-groupcomm].

      *  The 'cs_key_enc' parameter MAY be present and specifies the
         encoding of the public keys of the group members.  This
         parameter is a CBOR integer, whose value is 1 ("COSE_Key")
         taken from the 'Confirmation Key' column of the "CWT
         Confirmation Method" Registry defined in
         [I-D.ietf-ace-cwt-proof-of-possession], so indicating that
         public keys in the OSCORE group are encoded as COSE Keys
         [RFC8152].  Future specifications may define additional values
         for this parameter.  If this parameter is not present, 1
         ("COSE_Key") MUST be assumed as default value.

   o  The 'num' parameter MUST be present.

   o  The 'ace-groupcomm-profile' parameter MUST be present and has
      value coap_group_oscore_app (TBD1), which is defined in
      Section 18.1 of this specification.

   o  The 'exp' parameter MUST be present.

   o  The 'pub_keys' parameter, if present, includes the public keys of
      the group members that are relevant to the joining node.  That is,
      it includes: i) the public keys of the responders currently in the
      group, in case the joining node is configured (also) as requester;
      and ii) the public keys of the requesters currently in the group,
      in case the joining node is configured (also) as responder or
      monitor.  If public keys are encoded as COSE_Keys, each of them
      has as 'kid' the Sender ID that the corresponding owner has in the
      group, thus used as group member identifier.

   o  The 'group_policies' parameter SHOULD be present, and SHOULD
      include the elements "Sequence Number Synchronization Method" and
      "Key Update Check Interval" defined in Section 4.1.2. of
      [I-D.ietf-ace-key-groupcomm].





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

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

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

   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 16).  As a consequence, the joining node is not able to
   access secure communication in the group occurred prior its joining.

6.  Public Keys of Joining Nodes

   Source authentication of OSCORE messages exchanged within the group
   is ensured by means of digital counter signatures (see Sections 2 and
   4 of [I-D.ietf-core-oscore-groupcomm]).  Therefore, group members
   must be able to retrieve each other's public key from a trusted key
   repository, in order to verify source authenticity of incoming group
   messages.

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

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

   o  The joining node is going to join the group exclusively as
      monitor.  That is, it is not going to send messages to the group,
      and hence to produce signatures with its own private key.  In this
      case, the joining node is not required to provide its own public
      key to the Group Manager, which thus does not have to perform any



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      check related to the public key encoding, or to a countersignature
      algorithm and possible associated parameters for that joining
      node.

   o  The Group Manager already acquired the public key of the joining
      node during a past joining process.  In this case, the joining
      node MAY choose not to provide again its own public key to the
      Group Manager, in order to limit the size of the Joining Request.
      The joining node MUST provide its own public key again if it has
      provided the Group Manager with multiple public keys during past
      joining processes, intended for different OSCORE groups.  If the
      joining node provides its own public key, the Group Manager
      performs consistency checks as per Section 5.3 and, in case of
      success, considers it as the public key associated to the joining
      node in the OSCORE group.

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

      1.  The proof-of-possession key is compatible with the encoding as
          well as with the counter signature algorithm and possible
          associated parameters used in the OSCORE group.  Then, the
          Group Manager considers the proof-of-possession key as the
          public key associated to the joining node in the OSCORE group.
          If the joining node is aware that the proof-of-possession key
          is also valid for the OSCORE group, it MAY not provide it
          again as its own public key to the Group Manager.  The joining
          node MUST provide its own public key again if it has provided
          the Group Manager with multiple public keys during past
          joining processes, intended for different OSCORE groups.  If
          the joining node provides its own public key in the
          'client_cred' parameter of the Joining Request (see
          Section 5.2), the Group Manager performs consistency checks as
          per Section 5.3 and, in case of success, considers it as the
          public key associated to the joining node in the OSCORE group.

      2.  The proof-of-possession key is not compatible with the
          encoding or with the counter signature algorithm and possible
          associated parameters used in the OSCORE group.  In this case,
          the joining node MUST provide a different compatible public
          key to the Group Manager in the 'client_cred' parameter of the
          Joining Request (see Section 5.2).  Then, the Group Manager
          performs consistency checks on this latest provided public key
          as per Section 5.3 and, in case of success, considers it as
          the public key associated to the joining node in the OSCORE
          group.




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   o  The joining node and the Group Manager use a symmetric proof-of-
      possession key to establish a secure communication channel.  In
      this case, upon performing a joining process with that Group
      Manager for the first time, the joining node specifies its own
      public key in the 'client_cred' parameter of the Joining Request
      targeting the group-membership endpoint (see Section 5.2).

7.  Retrieval of Updated Keying Material

   At some point, a group member considers the 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.

7.1.  Retrieval of Group Keying Material

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

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

   The Group Manager processes the Key Distribution Request according to
   Section 4.1.2.2 of [I-D.ietf-ace-key-groupcomm].  The Key
   Distribution Response is formatted as defined in Section 4.1.2.2 of
   [I-D.ietf-ace-key-groupcomm].  In particular, the 'key' parameter is
   formatted as defined in Section 5.4 of this specification, with the
   difference that it does not include the 'clientId' parameter.

   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, use them to set up the new
   OSCORE Security Context as described in Section 2 of
   [I-D.ietf-core-oscore-groupcomm].

7.2.  Retrieval of Group Keying Material and Sender ID

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

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



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

   In particular, the 'key' parameter is formatted as defined in
   Section 5.4 of this specification, with the difference that if the
   requesting group member is configured exclusively as monitor, no
   'clientId' is specified within the 'key' parameter.  Note that, in
   any other case, the current Sender ID of the group member is not
   specified as a separate parameter, but rather specified as 'clientId'
   within the 'key' parameter.

   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, use them to set
   up the new OSCORE Security Context as described in Section 2 of
   [I-D.ietf-core-oscore-groupcomm].

8.  Retrieval of New Keying Material

   As discussed in Section 2.5 of [I-D.ietf-core-oscore-groupcomm], a
   group member may at some point experience a wrap-around of its own
   Sender Sequence Number in the group.

   When this happens, the group member MUST send a Key Renewal Request
   message to the Group Manager, as per Section 4.4 of
   [I-D.ietf-ace-key-groupcomm].  In particular, it sends a CoAP PUT
   request to the endpoint /group-oscore/GROUPNAME/nodes/NODENAME at the
   Group Manager.

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

   1.  If the requesting group member is configured exclusively as
       monitor, the Group Manager replies with a 4.00 (Bad Request)
       error response.

   2.  Otherwise, depending on the policies configured (OPT8):

       a.  Either the Group Manager replies to the group member with a
       4.00 (Bad Request) error response, and rekeys the whole OSCORE
       group as discussed in Section 16;

       b.  Or the Group Manager generates a new Sender ID for that group
       member and replies with a Key Renewal Response, formatted as
       defined in Section 4.1.6.1 of [I-D.ietf-ace-key-groupcomm].  In



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       particular, the CBOR Map in the response payload includes a
       single parameter 'clientId' defined in Section 18.5 of this
       document, specifying the new Sender ID of the group member
       encoded as a CBOR byte string.

9.  Retrieval of Public Keys of Group Members

   A group member may need to retrieve the public keys of other group
   members.  To this end, the group member sends a Public Key Request
   message to the Group Manager, as per Section 4.5 of
   [I-D.ietf-ace-key-groupcomm].  In particular, it sends the request to
   the endpoint /group-oscore/GROUPNAME/pub-key at the Group Manager.

   If the Public Key Request uses the method FETCH, the Public Key
   Request is formatted as defined in Section 4.1.3.1 of
   [I-D.ietf-ace-key-groupcomm].  In particular, each element of the
   'get_pub_keys' parameter is a CBOR byte string, which encodes the
   Sender ID of the group member for which the associated public key is
   requested.

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

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

10.  Update of Public Key

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

   To this end, the group member sends a Public Key Update Request
   message to the Group Manager, as per Section 4.6 of
   [I-D.ietf-ace-key-groupcomm].  In particular, it sends a CoAP POST
   request to the endpoint /group-oscore/GROUPNAME/nodes/NODENAME/pub-
   key at the Group Manager.





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

   o  If the requesting group member is configured exclusively as
      monitor, the Group Manager replies with a 4.00 (Bad request) error
      response.

   o  The N_S signature challenge is computed as per point (3) in
      Section 5.2.1 (REQ17).

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

11.  Retrieval of Group Policies

   A group member may request the current policies used in the OSCORE
   group.  To this end, the group member sends a Policies Request, as
   per Section 4.7 of [I-D.ietf-ace-key-groupcomm].  In particular, it
   sends a CoAP GET request to the endpoint /group-oscore/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 4.1.4.1 of [I-D.ietf-ace-key-groupcomm].  The success
   Policies Response is formatted as defined in Section 4.1.4.1 of
   [I-D.ietf-ace-key-groupcomm].

12.  Retrieval of Keying Material Version

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

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






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13.  Retrieval of Group Status

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

   In particular, the group member sends a CoAP GET request to the
   endpoint /group-oscore/GROUPNAME/active at the Group Manager defined
   in Section 4 of this specification, where GROUPNAME is the name of
   the OSCORE group.  The success Group Version Response is formatted as
   defined in Section 4 of this specification.

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

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

   Figure 2 gives an overview of the exchange described above.

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

          Figure 2: Message Flow of Group Status Request-Response

14.  Request to Leave the Group

   A group member may request to leave the OSCORE group.  To this end,
   the group member sends a Group Leaving Request, as per Section 4.9 of
   [I-D.ietf-ace-key-groupcomm].  In particular, it sends a CoAP DELETE
   request to the endpoint /group-oscore/GROUPNAME/nodes/NODENAME at the
   Group Manager.

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

15.  Removal of a Group Member

   Other than after a spontaneous request to the Group Manager as
   described in Section 14, 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 leaving node is not configured exclusively as
   monitor, the Group Manager performs the following actions.

   o  The Group Manager frees the OSCORE Sender ID value of the leaving
      node, which becomes available for possible upcoming joining nodes.

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

   If the application requires forward security, the Group Manager MUST
   generate updated security parameters and group keying material, and
   provide it to the remaining group members (see Section 16).  As a
   consequence, the leaving node is not able to acquire the new security
   parameters and group keying material distributed after its leaving.

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

16.  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.

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

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

   The Group Manager uses the same format of the Joining Response
   message in Section 5.4.  In particular:

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

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



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   o  The 'contextId' parameter of the 'key' parameter specifies the new
      Group ID.

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

   Each rekeying message MUST be secured with the pairwise secure
   communication channel between the Group Manager and the group member
   used during the joining process.  In particular, each rekeying
   message can target the 'control_path' URI path defined in
   Section 4.1.2.1 of [I-D.ietf-ace-key-groupcomm], if provided by the
   intended recipient upon joining the group (see Section 5.2).

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

   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.

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

   Group members and the Group Manager SHOULD additionally support
   alternative rekeying approaches that do not require group members to
   act (also) as servers.  A number of such approaches are defined in
   Section 4.3 of [I-D.ietf-ace-key-groupcomm].  In particular, a group
   member may subscribe for updates to the group-membership resource of
   the group, at the endpoint /group-oscore/GROUPNAME/nodes/NODENAME of
   the Group Manager.  This can rely on CoAP Observe [RFC7641] or on a
   full-fledged Pub-Sub model [I-D.ietf-core-coap-pubsub] with the Group
   Manager acting as Broker.

17.  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




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   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.

17.1.  Management of OSCORE Groups

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

   o  Management of group keying material (see Section 2.4 of
      [I-D.ietf-core-oscore-groupcomm]).  The Group Manager is
      responsible for the renewal and re-distribution of the keying
      material in the groups of its competence (rekeying).  According to
      the specific application requirements, this can include rekeying
      the group upon changes in its membership.  In particular, renewing
      the group keying material is required upon a new node's joining or
      a current node's leaving, in case backward security and forward
      security have to be preserved, respectively.

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

   o  Synchronization of sequence numbers (see Section 6.1 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 5.  Alternatively, the joining
   node can use its own public key as asymmetric proof-of-possession key
   to establish a secure channel with the Group Manager, e.g. as in
   Section 3.2 of [I-D.ietf-ace-dtls-authorize].  However, this requires
   such proof-of-possession key to be compatible with the encoding as
   well as with the countersignature algorithm and possible associated
   parameters used in the OSCORE group.

   A node may have joined multiple OSCORE groups under different non-
   synchronized Group Managers.  Therefore, it can happen that those
   OSCORE groups have the same Group Identifier (Gid).  It follows that,
   upon receiving a Group OSCORE message addressed to one of those
   groups, the node would have multiple Security Contexts matching with
   the Gid in the incoming message.  It is up to the application to



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   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.

17.2.  Size of Nonces for Signature Challenge

   With reference to the Joining Request message in Section 5.2, the
   proof-of-possession signature included in 'client_cred_verify' is
   computed over the challenge N_C | N_S, where | denotes concatenation.

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

   As defined in Section 5.2.1, the way the N_S value is computed
   depends on the particular interaction between the joining node and
   the Group Manager.

   o  If the Access Token is not explicitly posted to the /authz-info
      endpoint of the Group Manager, or if the joining node re-joins
      without re-posting the same still valid Access Token, then N_S is
      computed as a 32-byte long nonce (see points (2) and (3) of
      Section 5.2.1).

   o  If the Access Token has been explicitly posted to the /authz-info
      endpoint of the Group Manager, N_S takes the value conveyed in the
      'rsnonce' parameter of the 2.01 response to the Token Post (see
      Section 5.1).  Similarly, if a Joining Request is not successfully
      processed by the Group Manager, the returned error response should
      also include the 'rsnonce' parameter specifying a new nonce N_S
      (see Section 5.3).  In either case, it is RECOMMENDED to use a
      8-byte long random nonce as value for N_S.

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

   o  If both N_C and N_S are random nonces, the average collision for
      each nonce will happen after 2^32 messages, as per the birthday
      paradox and as also discussed in Section 7 of
      [I-D.ietf-ace-oscore-profile].  This amounts to considerably more
      token provisionings than the expected new joinings of OSCORE
      groups under a same Group Manager.

   o  If N_C and N_S are not generated randomly, e.g. by using a
      counter, the joining node and the Group Manager need to guarantee
      that reboot and loss of state on either node does not provoke re-
      use.  If that is not guaranteed, a joining node may repeatedly



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      post a valid Access Token to the /authz-info endpoint of the Group
      Manager, until it gets back an exact, re-used value N_S* to use as
      nonce.  Then, the joining node can send a Joining Request,
      conveying a reused N_C* nonce in 'cnonce' and an old stored
      signature in 'client_cred_verify', computed over N_C* | N_S*. By
      verifying the signature, the Group Manager would falsely believe
      that the joining node possesses its own private key at that point
      in time.

   o  Since N_C is always conveyed in a secured Joining Request, it is
      practically infeasible for an on-path attacker to replay Joining
      Requests from a joining node to the Group Manager, in order to
      cause that joining node to use an arbitrary nonce N_S.

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

18.  IANA Considerations

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

   This document has the following actions for IANA.

18.1.  ACE Groupcomm Profile Registry

   IANA is asked to register the following entry in the "ACE Groupcomm
   Profile" Registry defined in Section 9.6 of
   [I-D.ietf-ace-key-groupcomm].

   o  Name: coap_group_oscore_app

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

   o  CBOR Value: TBD1

   o  Reference: [[This specification]] (Section 5.4)







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18.2.  ACE Groupcomm Key Registry

   IANA is asked to register the following entry in the "ACE Groupcomm
   Key" Registry defined in Section 9.5 of [I-D.ietf-ace-key-groupcomm].

   o  Name: Group_OSCORE_Security_Context object

   o  Key Type Value: TBD2

   o  Profile: "coap_group_oscore_app", defined in Section 18.1 of this
      specification.

   o  Description: A Group_OSCORE_Security_Context object encoded as
      described in Section 5.4 of this specification.

   o  Reference: [[This specification]] (Section 5.4)

18.3.  OSCORE Security Context Parameters Registry

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

   o  Name: cs_alg

   o  CBOR Label: TBD3

   o  CBOR Type: tstr / int

   o  Registry: COSE Algorithm Values (ECDSA, EdDSA)

   o  Description: OSCORE Counter Signature Algorithm Value

   o  Reference: [[This specification]] (Section 5.4)

   o  Name: cs_params

   o  CBOR Label: TBD4

   o  CBOR Type: map

   o  Registry: Counter Signatures Parameters

   o  Description: OSCORE Counter Signature Algorithm Additional
      Parameters

   o  Reference: [[This specification]] (Section 5.4)




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   o  Name: cs_key_params

   o  CBOR Label: TBD5

   o  CBOR Type: map

   o  Registry: Counter Signatures Key Parameters

   o  Description: OSCORE Counter Signature Key Additional Parameters

   o  Reference: [[This specification]] (Section 5.4)

   o  Name: cs_key_enc

   o  CBOR Label: TBD6

   o  CBOR Type: integer

   o  Registry: ACE Public Key Encoding

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

   o  Reference: [[This specification]] (Section 5.4)

18.4.  Sequence Number Synchronization Method Registry

   IANA is asked to register the following entries in the "Sequence
   Number Synchronization Method" Registry defined in Section 9.8 of
   [I-D.ietf-ace-key-groupcomm].

   o  Name: Best effort

   o  Value: 1

   o  Description: No action is taken.

   o  Reference: [I-D.ietf-core-oscore-groupcomm] (Appendix E.1)

   o  Name: Baseline

   o  Value: 2

   o  Description: The first received request sets the baseline
      reference point, and is discarded with no delivery to the
      application.

   o  Reference: [I-D.ietf-core-oscore-groupcomm] (Appendix E.2)



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   o  Name: Echo challenge-response

   o  Value: 3

   o  Description: Challenge response using the Echo Option for CoAP
      from [I-D.ietf-core-echo-request-tag].

   o  Reference: [I-D.ietf-core-oscore-groupcomm] (Appendix E.3)

18.5.  ACE Groupcomm Parameters Registry

   IANA is asked to register the following entry in the "ACE Groupcomm
   Parameters" Registry defined in Section 9.4 of
   [I-D.ietf-ace-key-groupcomm].

   o  Name: clientId

   o  CBOR Key: TBD7

   o  CBOR Type: Byte string

   o  Reference: [[This specification]] (Section 8)

18.6.  TLS Exporter Label Registry

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

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

   o  DTLS-OK: Y

   o  Recommended: N

   o  Reference: [[This specification]] (Section 5.2.1)

19.  References

19.1.  Normative References

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





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   [I-D.ietf-ace-key-groupcomm]
              Palombini, F. and M. Tiloca, "Key Provisioning for Group
              Communication using ACE", draft-ietf-ace-key-groupcomm-05
              (work in progress), March 2020.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-33
              (work in progress), February 2020.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE profile of the Authentication and Authorization
              for Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-10 (work in progress), March 2020.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Group OSCORE - Secure Group Communication for CoAP",
              draft-ietf-core-oscore-groupcomm-07 (work in progress),
              March 2020.

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

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

   [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>.

   [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>.

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





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   [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>.

   [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>.

   [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>.

19.2.  Informative References

   [I-D.dijk-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              dijk-core-groupcomm-bis-03 (work in progress), March
              2020.

   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-dtls-
              authorize-09 (work in progress), December 2019.

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

   [I-D.ietf-core-echo-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo,
              Request-Tag, and Token Processing", draft-ietf-core-echo-
              request-tag-09 (work in progress), March 2020.

   [I-D.tiloca-ace-oscore-gm-admin]
              Tiloca, M., Hoeglund, R., Stok, P., and F. Palombini,
              "Admin Interface for the OSCORE Group Manager", draft-
              tiloca-ace-oscore-gm-admin-01 (work in progress), March
              2020.



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   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsuess, C., and P. Stok, "Discovery of OSCORE
              Groups with the CoRE Resource Directory", draft-tiloca-
              core-oscore-discovery-05 (work in progress), March
              2020.

   [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>.

   [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>.

Appendix A.  Profile Requirements

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

   o  REQ1 - Specify the encoding and value of the identifier of group,
      for scope entries of 'scope': see Section 3.1.

   o  REQ2 - Specify the encoding and value of roles, for scope entries
      of 'scope': see Section 3.1.

   o  REQ3 - if used, specify the acceptable values for 'sign_alg':
      values from Tables 5 and 6 of [RFC8152].

   o  REQ4 - If used, specify the acceptable values for
      'sign_parameters': values from the "Counter Signature Parameters"
      Registry (see Section 11.1 of [I-D.ietf-core-oscore-groupcomm]).

   o  REQ5 - If used, specify the acceptable values for
      'sign_key_parameters': values from the "Counter Signature Key
      Parameters" Registry (see Section 11.2 of
      [I-D.ietf-core-oscore-groupcomm]).

   o  REQ6 - If used, specify the acceptable values for 'pub_key_enc': 1
      ("COSE_Key") from the 'Confirmation Key' column of the "CWT
      Confirmation Method" Registry defined in
      [I-D.ietf-ace-cwt-proof-of-possession].  Future specifications may
      define additional values for this parameter.



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   o  REQ7 - Format of the 'key' value: see Section 5.4.

   o  REQ8 - Acceptable values of 'gkty': Group_OSCORE_Security_Context
      object (see Section 5.4).

   o  REQ9: Specify the format of the identifiers of group members: see
      Section 5.4 and Section 9.

   o  REQ10 - Specify the communication protocol that the members of the
      group must use: CoAP, possibly over IP multicast.

   o  REQ11 - Specify the security protocols that the group members must
      use to protect their communication: Group OSCORE.

   o  REQ12 - Specify and register the application profile identifier:
      coap_group_oscore_app (see Section 18.1).

   o  REQ13 - Specify policies at the KDC to handle member ids that are
      not included in 'get_pub_keys': see Section 9.

   o  REQ14 - If used, specify the format and content of
      'group_policies' and its entries: see Section 5.4, and the three
      values defined and registered, as content of the entry "Sequence
      Number Synchronization Method" (see Section 18.4).

   o  REQ15 - 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 8.

   o  REQ16 - Specify how the communication is secured between the
      Client and KDC: by means of any transport profile of ACE
      [I-D.ietf-ace-oauth-authz] between Client and Group Manager that
      complies with the requirements in Appendix C of
      [I-D.ietf-ace-oauth-authz].

   o  REQ17: Specify how the nonce N_S is generated, if the token is not
      being posted (e.g. if it is used directly to validate TLS
      instead): see Section 5.2.1.

   o  REQ18: Specify if 'mgt_key_material' used, and if yes specify its
      format and content: not used in this version of the profile.

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

   o  OPT2 (Optional) - Specify the negotiation of parameter values for
      signature algorithm and signature keys, if 'sign_info' and
      'pub_key_enc' are not used: possible early discovery by using the



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      approach based on the CoRE Resource Directory described in
      [I-D.tiloca-core-oscore-discovery].

   o  OPT3 (Optional) - Specify the encoding of 'pub_keys_repos' if the
      default is not used: no.

   o  OPT4 (Optional) - Specify policies that instruct clients to retain
      unsuccessfully decrypted messages and for how long, so that they
      can be decrypted after getting updated keying material: no.

   o  OPT5 (Optional) - Specify the behavior of the handler in case of
      failure to retrieve a public key for the specific node: send a
      4.00 Bad Request response to a Joining Request (see Section 5.3).

   o  OPT6 (Optional) - Specify possible or required payload formats for
      specific error cases: send a 4.00 Bad Request response to a
      Joining Request (see Section 5.3).

   o  OPT7 (Optional) - Specify CBOR values to use for abbreviating
      identifiers of roles in the group or topic (see Section 3.1).

   o  OPT8 (Optional) - Specify policies for the KDC to perform group
      rekeying after receiving a Key Renewal Request: no.

Appendix B.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

B.1.  Version -04 to -05

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

   o  Supporting single Access Token for multiple groups/topics.

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

   o  Registered and used dedicated label for TLS Exporter.

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

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

   o  Fixed inconsistency in retrieving group keying material only.

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





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   o  Clarification on incrementing version number when rekeying the
      group.

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

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

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

B.2.  Version -03 to -04

   o  New abstract.

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

   o  Terminology: node name; node resource.

   o  Creation and pointing at node resource.

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

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

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

   o  Removed reference to RFC 7390.

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

   o  Editorial improvements.

B.3.  Version -02 to -03

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

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

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

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

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




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

   o  IANA registration in ACE Groupcomm Parameters Registry.

   o  More fulfilled profile requirements (Appendix A).

B.4.  Version -01 to -02

   o  Editorial fixes.

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

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

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

   o  Added parameters to indicate the encoding of public keys.

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

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

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

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

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

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

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

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







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B.5.  Version -00 to -01

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

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

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

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

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

Acknowledgments

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

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

Authors' Addresses

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

   Email: marco.tiloca@ri.se


   Jiye Park
   Universitaet Duisburg-Essen
   Schuetzenbahn 70
   Essen  45127
   Germany

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



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Internet-Draft   Key Management for OSCORE Groups in ACE      March 2020


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

   Email: francesca.palombini@ericsson.com












































Tiloca, et al.         Expires September 10, 2020              [Page 38]


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