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Versions: (draft-tiloca-core-multicast-oscoap) 00 01 02 03 04 05 06 07 08 09 10

CoRE Working Group                                             M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                             G. Selander
Expires: May 6, 2021                                        F. Palombini
                                                             Ericsson AB
                                                                 J. Park
                                             Universitaet Duisburg-Essen
                                                       November 02, 2020


           Group OSCORE - Secure Group Communication for CoAP
                   draft-ietf-core-oscore-groupcomm-10

Abstract

   This document defines Group Object Security for Constrained RESTful
   Environments (Group OSCORE), providing end-to-end security of CoAP
   messages exchanged between members of a group, e.g. sent over IP
   multicast.  In particular, the described approach defines how OSCORE
   is used in a group communication setting to provide source
   authentication for CoAP group requests, sent by a client to multiple
   servers, and for protection of the corresponding CoAP responses.

Status of This Memo

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

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

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

   This Internet-Draft will expire on May 6, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Security Context  . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Common Context  . . . . . . . . . . . . . . . . . . . . .   9
       2.1.1.  ID Context  . . . . . . . . . . . . . . . . . . . . .   9
       2.1.2.  Counter Signature Algorithm . . . . . . . . . . . . .   9
       2.1.3.  Counter Signature Parameters  . . . . . . . . . . . .   9
       2.1.4.  Secret Derivation Algorithm . . . . . . . . . . . . .  10
       2.1.5.  Secret Derivation Parameters  . . . . . . . . . . . .  10
     2.2.  Sender Context and Recipient Context  . . . . . . . . . .  11
     2.3.  Pairwise Keys . . . . . . . . . . . . . . . . . . . . . .  12
       2.3.1.  Derivation of Pairwise Keys . . . . . . . . . . . . .  12
       2.3.2.  Usage of Sequence Numbers . . . . . . . . . . . . . .  13
       2.3.3.  Security Context for Pairwise Mode  . . . . . . . . .  13
     2.4.  Update of Security Context  . . . . . . . . . . . . . . .  14
       2.4.1.  Loss of Mutable Security Context  . . . . . . . . . .  14
       2.4.2.  Exhaustion of Sender Sequence Number  . . . . . . . .  15
       2.4.3.  Retrieving New Security Context Parameters  . . . . .  16
   3.  The Group Manager . . . . . . . . . . . . . . . . . . . . . .  18
     3.1.  Management of Group Keying Material . . . . . . . . . . .  19
     3.2.  Responsibilities of the Group Manager . . . . . . . . . .  20
   4.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  21
     4.1.  Counter Signature . . . . . . . . . . . . . . . . . . . .  21
     4.2.  The 'kid' and 'kid context' parameters  . . . . . . . . .  21
     4.3.  external_aad  . . . . . . . . . . . . . . . . . . . . . .  22
       4.3.1.  external_aad for Encryption . . . . . . . . . . . . .  22
       4.3.2.  external_aad for Signing  . . . . . . . . . . . . . .  23
   5.  OSCORE Header Compression . . . . . . . . . . . . . . . . . .  24
     5.1.  Examples of Compressed COSE Objects . . . . . . . . . . .  25
       5.1.1.  Examples in Group Mode  . . . . . . . . . . . . . . .  25
       5.1.2.  Examples in Pairwise Mode . . . . . . . . . . . . . .  26
   6.  Message Binding, Sequence Numbers, Freshness and Replay
       Protection  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     6.1.  Update of Replay Window . . . . . . . . . . . . . . . . .  27
   7.  Message Reception . . . . . . . . . . . . . . . . . . . . . .  28
   8.  Message Processing in Group Mode  . . . . . . . . . . . . . .  29
     8.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  29
       8.1.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  30
     8.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  31



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       8.2.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  32
     8.3.  Protecting the Response . . . . . . . . . . . . . . . . .  32
       8.3.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  33
     8.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  34
       8.4.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  34
   9.  Message Processing in Pairwise Mode . . . . . . . . . . . . .  35
     9.1.  Pre-Conditions  . . . . . . . . . . . . . . . . . . . . .  36
     9.2.  Protecting the Request  . . . . . . . . . . . . . . . . .  36
     9.3.  Verifying the Request . . . . . . . . . . . . . . . . . .  37
     9.4.  Protecting the Response . . . . . . . . . . . . . . . . .  37
     9.5.  Verifying the Response  . . . . . . . . . . . . . . . . .  38
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  38
     10.1.  Group-level Security . . . . . . . . . . . . . . . . . .  39
     10.2.  Uniqueness of (key, nonce) . . . . . . . . . . . . . . .  40
     10.3.  Management of Group Keying Material  . . . . . . . . . .  40
     10.4.  Update of Security Context and Key Rotation  . . . . . .  41
       10.4.1.  Late Update on the Sender  . . . . . . . . . . . . .  41
       10.4.2.  Late Update on the Recipient . . . . . . . . . . . .  42
     10.5.  Collision of Group Identifiers . . . . . . . . . . . . .  42
     10.6.  Cross-group Message Injection  . . . . . . . . . . . . .  43
       10.6.1.  Attack Description . . . . . . . . . . . . . . . . .  43
       10.6.2.  Attack Prevention in Group Mode  . . . . . . . . . .  44
     10.7.  Group OSCORE for Unicast Requests  . . . . . . . . . . .  45
     10.8.  End-to-end Protection  . . . . . . . . . . . . . . . . .  46
     10.9.  Master Secret  . . . . . . . . . . . . . . . . . . . . .  46
     10.10. Replay Protection  . . . . . . . . . . . . . . . . . . .  47
     10.11. Client Aliveness . . . . . . . . . . . . . . . . . . . .  48
     10.12. Cryptographic Considerations . . . . . . . . . . . . . .  48
     10.13. Message Segmentation . . . . . . . . . . . . . . . . . .  49
     10.14. Privacy Considerations . . . . . . . . . . . . . . . . .  49
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  50
     11.1.  OSCORE Flag Bits Registry  . . . . . . . . . . . . . . .  50
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     12.2.  Informative References . . . . . . . . . . . . . . . . .  52
   Appendix A.  Assumptions and Security Objectives  . . . . . . . .  54
     A.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .  55
     A.2.  Security Objectives . . . . . . . . . . . . . . . . . . .  56
   Appendix B.  List of Use Cases  . . . . . . . . . . . . . . . . .  57
   Appendix C.  Example of Group Identifier Format . . . . . . . . .  60
   Appendix D.  Set-up of New Endpoints  . . . . . . . . . . . . . .  60
   Appendix E.  Examples of Synchronization Approaches . . . . . . .  61
     E.1.  Best-Effort Synchronization . . . . . . . . . . . . . . .  61
     E.2.  Baseline Synchronization  . . . . . . . . . . . . . . . .  62
     E.3.  Challenge-Response Synchronization  . . . . . . . . . . .  62
   Appendix F.  No Verification of Signatures in Group Mode  . . . .  65
   Appendix G.  Example Values with COSE Capabilities  . . . . . . .  66
   Appendix H.  Document Updates . . . . . . . . . . . . . . . . . .  67



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     H.1.  Version -09 to -10  . . . . . . . . . . . . . . . . . . .  67
     H.2.  Version -08 to -09  . . . . . . . . . . . . . . . . . . .  68
     H.3.  Version -07 to -08  . . . . . . . . . . . . . . . . . . .  69
     H.4.  Version -06 to -07  . . . . . . . . . . . . . . . . . . .  70
     H.5.  Version -05 to -06  . . . . . . . . . . . . . . . . . . .  71
     H.6.  Version -04 to -05  . . . . . . . . . . . . . . . . . . .  72
     H.7.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  72
     H.8.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  73
     H.9.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  74
     H.10. Version -00 to -01  . . . . . . . . . . . . . . . . . . .  74
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  75
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  75

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is a web
   transfer protocol specifically designed for constrained devices and
   networks [RFC7228].  Group communication for CoAP
   [I-D.ietf-core-groupcomm-bis] addresses use cases where deployed
   devices benefit from a group communication model, for example to
   reduce latencies, improve performance and reduce bandwidth
   utilization.  Use cases include lighting control, integrated building
   control, software and firmware updates, parameter and configuration
   updates, commissioning of constrained networks, and emergency
   multicast (see Appendix B).  This specification defines the security
   protocol for Group communication for CoAP
   [I-D.ietf-core-groupcomm-bis].

   Object Security for Constrained RESTful Environments (OSCORE)
   [RFC8613] describes a security protocol based on the exchange of
   protected CoAP messages.  OSCORE builds on CBOR Object Signing and
   Encryption (COSE)
   [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and
   provides end-to-end encryption, integrity, replay protection and
   binding of response to request between a sender and a recipient,
   independent of transport also in the presence of intermediaries.  To
   this end, a CoAP message is protected by including its payload (if
   any), certain options, and header fields in a COSE object, which
   replaces the authenticated and encrypted fields in the protected
   message.

   This document defines Group OSCORE, providing the same end-to-end
   security properties as OSCORE in the case where CoAP requests have
   multiple recipients.  In particular, the described approach defines
   how OSCORE is used in a group communication setting to provide source
   authentication for CoAP group requests, sent by a client to multiple
   servers, and for protection of the corresponding CoAP responses.




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   Just like OSCORE, Group OSCORE is independent of transport layer and
   works wherever CoAP does.  Group communication for CoAP
   [I-D.ietf-core-groupcomm-bis] uses UDP/IP multicast as the underlying
   data transport.

   As with OSCORE, it is possible to combine Group OSCORE with
   communication security on other layers.  One example is the use of
   transport layer security, such as DTLS
   [RFC6347][I-D.ietf-tls-dtls13], between one client and one proxy (and
   vice versa), or between one proxy and one server (and vice versa), in
   order to protect the routing information of packets from observers.
   Note that DTLS does not define how to secure messages sent over IP
   multicast.

   Group OSCORE defines two modes of operation:

   o  In the group mode, Group OSCORE requests and responses are
      digitally signed with the private key of the sender and the
      signature is embedded in the protected CoAP message.  The group
      mode supports all COSE algorithms as well as signature
      verification by intermediaries.  This mode is defined in Section 8
      and MUST be supported.

   o  In the pairwise mode, two group members exchange Group OSCORE
      requests and responses over unicast, and the messages are
      protected with symmetric keys.  These symmetric keys are derived
      from Diffie-Hellman shared secrets, calculated with the asymmetric
      keys of the sender and recipient, allowing for shorter integrity
      tags and therefore lower message overhead.  This mode is defined
      in Section 9 and is OPTIONAL to support.

   Both modes provide source authentication of CoAP messages.  The
   application decides what mode to use, potentially on a per-message
   basis.  Such decisions can be based, for instance, on pre-configured
   policies or dynamic assessing of the target recipient and/or
   resource, among other things.  One important case is when requests
   are protected with the group mode, and responses with the pairwise
   mode.  Since such responses convey shorter integrity tags instead of
   bigger, full-fledged signatures, this significantly reduces the
   message overhead in case of many responses to one request.

   A special deployment of Group OSCORE is to use pairwise mode only.
   For example, consider the case of a constrained-node network
   [RFC7228] with a large number of CoAP endpoints and the objective to
   establish secure communication between any pair of endpoints with a
   small provisioning effort and message overhead.  Since the total
   number of security associations that needs to be established grows
   with the square of the number of nodes, it is desirable to restrict



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   the provisioned keying material.  Moreover, a key establishment
   protocol would need to be executed for each security association.
   One solution to this is to deploy Group OSCORE, with the endpoints
   being part of a group, and use the pairwise mode.  This solution
   assumes a trusted third party called Group Manager (see Section 3),
   but has the benefit of restricting the symmetric keying material
   while distributing only the public key of each group member.  After
   that, a CoAP endpoint can locally derive the OSCORE Security Context
   for the other endpoint in the group, and protect CoAP communications
   with very low overhead [I-D.ietf-lwig-security-protocol-comparison].

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 CoAP [RFC7252] including "endpoint", "client", "server",
   "sender" and "recipient"; group communication for CoAP
   [I-D.ietf-core-groupcomm-bis]; CBOR [I-D.ietf-cbor-7049bis]; COSE
   [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and
   related counter signatures [I-D.ietf-cose-countersign].

   Readers are also expected to be familiar with the terms and concepts
   for protection and processing of CoAP messages through OSCORE, such
   as "Security Context" and "Master Secret", defined in [RFC8613].

   Terminology for constrained environments, such as "constrained
   device" and "constrained-node network", is defined in [RFC7228].

   This document refers also to the following terminology.

   o  Keying material: data that is necessary to establish and maintain
      secure communication among endpoints.  This includes, for
      instance, keys and IVs [RFC4949].

   o  Group: a set of endpoints that share group keying material and
      security parameters (Common Context, see Section 2).  Unless
      specified otherwise, the term group used in this specification
      refers thus to a "security group" (see Section 2.1 of
      [I-D.ietf-core-groupcomm-bis]), not to be confused with "CoAP
      group" or "application group".

   o  Group Manager: entity responsible for a group.  Each endpoint in a
      group communicates securely with the respective Group Manager,



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      which is neither required to be an actual group member nor to take
      part in the group communication.  The full list of
      responsibilities of the Group Manager is provided in Section 3.2.

   o  Silent server: member of a group that never sends protected
      responses in reply to requests.  For CoAP group communications,
      requests are normally sent without necessarily expecting a
      response.  A silent server may send unprotected responses, as
      error responses reporting an OSCORE error.  Note that an endpoint
      can implement both a silent server and a client, i.e. the two
      roles are independent.  An endpoint acting only as a silent server
      performs only Group OSCORE processing on incoming requests.
      Silent servers maintain less keying material and in particular do
      not have a Sender Context for the group.  Since silent servers do
      not have a Sender ID, they cannot support the pairwise mode.

   o  Group Identifier (Gid): identifier assigned to the group, unique
      within the set of groups of a given Group Manager.

   o  Group request: CoAP request message sent by a client in the group
      to all servers in that group.

   o  Source authentication: evidence that a received message in the
      group originated from a specific identified group member.  This
      also provides assurance that the message was not tampered with by
      anyone, be it a different legitimate group member or an endpoint
      which is not a group member.

2.  Security Context

   This specification refers to a group as a set of endpoints sharing
   keying material and security parameters for executing the Group
   OSCORE protocol (see Section 1.1).  Each endpoint which is member of
   a group maintains a Security Context as defined in Section 3 of
   [RFC8613], extended as follows (see Figure 1):

   o  One Common Context, shared by all the endpoints in the group.  Two
      new parameters are included in the Common Context, namely Counter
      Signature Algorithm and Counter Signature Parameters.  These
      relate to the computation of counter signatures, when messages are
      protected using the group mode (see Section 8).

      If the pairwise mode is supported, the Common Context is further
      extended with two new parameters, namely Secret Derivation
      Algorithm and Secret Derivation Parameters.  These relate to the
      derivation of a static-static Diffie-Hellman shared secret, from
      which pairwise keys are derived (see Section 2.3.1) to protect
      messages with the pairwise mode (see Section 9).



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   o  One Sender Context, extended with the endpoint's private key.  The
      private key is used to sign the message in group mode, and for
      deriving the pairwise keys in pairwise mode (see Section 2.3).  If
      the pairwise mode is supported, the Sender Context is also
      extended with the Pairwise Sender Keys associated to the other
      endpoints (see Section 2.3).  The Sender Context is omitted if the
      endpoint is configured exclusively as silent server.

   o  One Recipient Context for each endpoint from which messages are
      received.  It is not necessary to maintain Recipient Contexts
      associated to endpoints from which messages are not (expected to
      be) received.  The Recipient Context is extended with the public
      key of the associated endpoint, used to verify the signature in
      group mode and for deriving the pairwise keys in pairwise mode
      (see Section 2.3).  If the pairwise mode is supported, then the
      Recipient Context is also extended with the Pairwise Recipient Key
      associated to the other endpoint (see Section 2.3).

   +-------------------+-----------------------------------------------+
   | Context Component | New Information Elements                      |
   +-------------------+-----------------------------------------------+
   | Common Context    | Counter Signature Algorithm                   |
   |                   | Counter Signature Parameters                  |
   |                   | *Secret Derivation Algorithm                  |
   |                   | *Secret Derivation Parameters                 |
   +-------------------+-----------------------------------------------+
   | Sender Context    | Endpoint's own private key                    |
   |                   | *Pairwise Sender Keys for the other endpoints |
   +-------------------+-----------------------------------------------+
   | Each              | Public key of the other endpoint              |
   | Recipient Context | *Pairwise Recipient Key of the other endpoint |
   +-------------------+-----------------------------------------------+

       Figure 1: Additions to the OSCORE Security Context.  Optional
                  additions are labeled with an asterisk.

   Further details about the Security Context of Group OSCORE are
   provided in the remainder of this section.  How the Security Context
   is established by the group members is out of scope for this
   specification, but if there is more than one Security Context
   applicable to a message, then the endpoints MUST be able to tell
   which Security Context was latest established.

   The default setting for how to manage information about the group is
   described in terms of a Group Manager (see Section 3).






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2.1.  Common Context

   The Common Context may be acquired from the Group Manager (see
   Section 3).  The following sections define how the Common Context is
   extended, compared to [RFC8613].

2.1.1.  ID Context

   The ID Context parameter (see Sections 3.3 and 5.1 of [RFC8613]) in
   the Common Context SHALL contain the Group Identifier (Gid) of the
   group.  The choice of the Gid format is application specific.  An
   example of specific formatting of the Gid is given in Appendix C.
   The application needs to specify how to handle potential collisions
   between Gids (see Section 10.5).

2.1.2.  Counter Signature Algorithm

   Counter Signature Algorithm identifies the digital signature
   algorithm used to compute a counter signature on the COSE object (see
   Sections 3.2 and 3.3 of [I-D.ietf-cose-countersign]), when messages
   are protected using the group mode (see Section 8).

   This parameter is immutable once the Common Context is established.
   Counter Signature Algorithm MUST take value from the "Value" column
   of the "COSE Algorithms" Registry [COSE.Algorithms].  The value is
   associated to a COSE key type, as specified in the "Capabilities"
   column of the "COSE Algorithms" Registry [COSE.Algorithms].  COSE
   capabilities for algorithms are defined in Section 8 of
   [I-D.ietf-cose-rfc8152bis-algs].

   The EdDSA signature algorithm and the elliptic curve Ed25519
   [RFC8032] are mandatory to implement.  If elliptic curve signatures
   are used, it is RECOMMENDED to implement deterministic signatures
   with additional randomness as specified in
   [I-D.mattsson-cfrg-det-sigs-with-noise].

2.1.3.  Counter Signature Parameters

   Counter Signature Parameters identifies the parameters associated to
   the digital signature algorithm specified in Counter Signature
   Algorithm.  This parameter is immutable once the Common Context is
   established.

   This parameter is a CBOR array including the following two elements,
   whose exact structure and value depend on the value of Counter
   Signature Algorithm:





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   o  The first element is the array of COSE capabilities for Counter
      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]).

   o  The second element is the array of COSE capabilities for the COSE
      key type associated to Counter 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]).

   Examples of Counter Signature Parameters are in Appendix G.

2.1.4.  Secret Derivation Algorithm

   Secret Derivation Algorithm identifies the elliptic curve Diffie-
   Hellman algorithm used to derive a static-static Diffie-Hellman
   shared secret, from which pairwise keys are derived (see
   Section 2.3.1) to protect messages with the pairwise mode (see
   Section 9).

   This parameter is immutable once the Common Context is established.
   Secret Derivation Algorithm MUST take value from the "Value" column
   of the "COSE Algorithms" Registry [COSE.Algorithms].  The value is
   associated to a COSE key type, as specified in the "Capabilities"
   column of the "COSE Algorithms" Registry [COSE.Algorithms].  COSE
   capabilities for algorithms are defined in Section 8 of
   [I-D.ietf-cose-rfc8152bis-algs].

   For endpoints that support the pairwise mode, the ECDH-SS + HKDF-256
   algorithm specified in Section 6.3.1 of
   [I-D.ietf-cose-rfc8152bis-algs] and the X25519 curve [RFC7748] are
   mandatory to implement.

2.1.5.  Secret Derivation Parameters

   Secret Derivation Parameters identifies the parameters associated to
   the elliptic curve Diffie-Hellman algorithm specified in Secret
   Derivation Algorithm.  This parameter is immutable once the Common
   Context is established.

   This parameter is a CBOR array including the following two elements,
   whose exact structure and value depend on the value of Secret
   Derivation Algorithm:

   o  The first element is the array of COSE capabilities for Secret
      Derivation Algorithm, as specified for that algorithm in the



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      "Capabilities" column of the "COSE Algorithms" Registry
      [COSE.Algorithms] (see Section 8.1 of
      [I-D.ietf-cose-rfc8152bis-algs]).

   o  The second element is the array of COSE capabilities for the COSE
      key type associated to Secret Derivation 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]).

   Examples of Secret Derivation Parameters are in Appendix G.

2.2.  Sender Context and Recipient Context

   OSCORE specifies the derivation of Sender Context and Recipient
   Context, specifically of Sender/Recipient Keys and Common IV, from a
   set of input parameters (see Section 3.2 of [RFC8613]).  This
   derivation applies also to Group OSCORE, and the mandatory-to-
   implement HKDF and AEAD algorithms are the same as in [RFC8613].  The
   Sender ID SHALL be unique for each endpoint in a group with a fixed
   Master Secret, Master Salt and Group Identifier (see Section 3.3 of
   [RFC8613]).

   For Group OSCORE, the Sender Context and Recipient Context
   additionally contain asymmetric keys, as described previously in
   Section 2.  The private/public key pair of the sender can, for
   example, be generated by the endpoint or provisioned during
   manufacturing.

   With the exception of the public key of the sender endpoint, a
   receiver endpoint can derive a complete Security Context from a
   received Group OSCORE message and the Common Context.  The public
   keys in the Recipient Contexts can be retrieved from the Group
   Manager (see Section 3) upon joining the group.  A public key can
   alternatively be acquired from the Group Manager at a later time, for
   example the first time a message is received from a particular
   endpoint in the group (see Section 8.2 and Section 8.4).

   For severely constrained devices, it may be not feasible to
   simultaneously handle the ongoing processing of a recently received
   message in parallel with the retrieval of the sender endpoint's
   public key.  Such devices can be configured to drop a received
   message for which there is no (complete) Recipient Context, and
   retrieve the sender endpoint's public key in order to have it
   available to verify subsequent messages from that endpoint.

   Furthermore, sufficiently large replay windows should be considered,
   to handle Partial IV values moving forward fast.  This can happen



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   when a client engages in frequent or long sequences of one-to-one
   exchanges with servers in the group, such as a large number of block-
   wise transfers to a single server.  When receiving following group
   requests from that client, other servers in the group may believe to
   have lost synchronization with the client's Sender Sequence Number.
   If these servers use an Echo exchange to re-gain synchronization (see
   Appendix E.3), this in itself may consume a considerable amount of
   client's Sender Sequence Numbers, hence later resulting in the
   servers possibly performing a new Echo exchange.

2.3.  Pairwise Keys

   Certain signature schemes, such as EdDSA and ECDSA, support a secure
   combined signature and encryption scheme.  This section specifies the
   derivation of "pairwise keys", for use in the pairwise mode of Group
   OSCORE defined in Section 9.

2.3.1.  Derivation of Pairwise Keys

   Using the Group OSCORE Security Context (see Section 2), a group
   member can derive AEAD keys to protect point-to-point communication
   between itself and any other endpoint in the group.  The same AEAD
   algorithm as in the group mode is used.  The key derivation of these
   so-called pairwise keys follows the same construction as in
   Section 3.2.1 of [RFC8613]:

   Pairwise Recipient Key = HKDF(Recipient Key, Shared Secret, info, L)
   Pairwise Sender Key    = HKDF(Sender Key, Shared Secret, info, L)

   where:

   o  The Pairwise Recipient Key is the AEAD key for processing incoming
      messages from endpoint X.

   o  The Pairwise Sender Key is the AEAD key for processing outgoing
      messages addressed to endpoint X.

   o  HKDF is the HKDF algorithm specified by Secret Derivation
      Algorithm from the Common Context (see Section 2.1.4).

   o  The Shared Secret is computed as a static-static Diffie-Hellman
      shared secret [NIST-800-56A], where the endpoint uses its private
      key and the public key of the other endpoint X.

   o  The Recipient Key and the public key are from the Recipient
      Context associated to endpoint X.

   o  The Sender Key and private key are from the Sender Context.



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   o  info and L are defined as in Section 3.2.1 of [RFC8613].

   If EdDSA asymmetric keys are used, the Edward coordinates are mapped
   to Montgomery coordinates using the maps defined in Sections 4.1 and
   4.2 of [RFC7748], before using the X25519 and X448 functions defined
   in Section 5 of [RFC7748].

   After establishing a partially or completely new Security Context
   (see Section 3.1 and Section 2.4), the old pairwise keys MUST be
   deleted.  Since new Sender/Recipient Keys are derived from the new
   group keying material (see Section 2.2), every group member MUST use
   the new Sender/Recipient Keys when deriving new pairwise keys.

   As long as any two group members preserve the same asymmetric keys,
   their Diffie-Hellman shared secret does not change across updates of
   the group keying material.

2.3.2.  Usage of Sequence Numbers

   When using any of its Pairwise Sender Keys, a sender endpoint
   including the 'Partial IV' parameter in the protected message MUST
   use the current fresh value of the Sender Sequence Number from its
   Sender Context (see Section 2.2).  That is, the same Sender Sequence
   Number space is used for all outgoing messages protected with Group
   OSCORE, thus limiting both storage and complexity.

   On the other hand, when combining group and pairwise communication
   modes, this may result in the Partial IV values moving forward more
   often.  This can happen when a client engages in frequent or long
   sequences of one-to-one exchanges with servers in the group, by
   sending requests over unicast.

2.3.3.  Security Context for Pairwise Mode

   If the pairwise mode is supported, the Security Context additionally
   includes Secret Derivation Algorithm, Secret Derivation Parameters
   and the pairwise keys, as described at the beginning of Section 2.

   The pairwise keys as well as the shared secrets used in their
   derivation (see Section 2.3.1) may be stored in memory or recomputed
   every time they are needed.  The shared secret changes only when a
   public/private key pair used for its derivation changes, which
   results in the pairwise keys also changing.  Additionally, the
   pairwise keys change if the Sender ID changes or if a new Security
   Context is established for the group (see Section 2.4.3).  In order
   to optimize protocol performance, an endpoint may store the derived
   pairwise keys for easy retrieval.




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   In the pairwise mode, the Sender Context includes the Pairwise Sender
   Keys to use with the other endpoints (see Figure 1).  In order to
   identify the right key to use, the Pairwise Sender Key for endpoint X
   may be associated to the Recipient ID of endpoint X, as defined in
   the Recipient Context (i.e. the Sender ID from the point of view of
   endpoint X).  In this way, the Recipient ID can be used to lookup for
   the right Pairwise Sender Key. This association may be implemented in
   different ways, e.g. by storing the pair (Recipient ID, Pairwise
   Sender Key) or linking a Pairwise Sender Key to a Recipient Context.

2.4.  Update of Security Context

   It is RECOMMENDED that the immutable part of the Security Context is
   stored in non-volatile memory, or that it can otherwise be reliably
   accessed throughout the operation of the group, e.g. after a device
   reboots.  However, also immutable parts of the Security Context may
   need to be updated, for example due to scheduled key renewal, new or
   re-joining members in the group, or the fact that the endpoint
   changes Sender ID (see Section 2.4.3).

   On the other hand, the mutable parts of the Security Context are
   updated by the endpoint when executing the security protocol, but may
   nevertheless become outdated, e.g. due to loss of the mutable
   Security Context (see Section 2.4.1) or exhaustion of Sender Sequence
   Numbers (see Section 2.4.2).

   If it is not feasible or practically possible to store and maintain
   up-to-date the mutable part in non-volatile memory (e.g., due to
   limited number of write operations), the endpoint MUST be able to
   detect a loss of the mutable Security Context.

   When a loss of mutable Security Context is detected (e.g., following
   a reboot), the endpoint MUST NOT protect further messages using this
   Security Context to avoid reusing a nonce with the same AEAD key, and
   SHOULD instead retrieve new security parameters from the Group
   Manager (see Section 2.4.1).

2.4.1.  Loss of Mutable Security Context

   An endpoint that has lost its mutable Security Context, e.g. due to a
   reboot, needs to prevent the re-use of a nonce with the same AEAD
   key, and to handle incoming replayed messages.

   To this end, after a loss of mutable Security Context, the endpoint
   SHOULD inform the Group Manager, retrieve new Security Context
   parameters from the Group Manager (see Section 2.4.3), and use them
   to derive a new Sender Context (see Section 2.2).  In particular,
   regardless the exact actions taken by the Group Manager, the endpoint



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   resets its Sender Sequence Number to 0, and derives a new Sender Key.
   This is in turn used to possibly derive new Pairwise Sender Keys.

   From then on, the endpoint MUST use its latest installed Sender
   Context to protect outgoing messages.

   If an endpoint is not able to establish an updated Sender Context,
   e.g. because of lack of connectivity with the Group Manager, the
   endpoint MUST NOT protect further messages using the current Security
   Context.

   In order to handle the update of Replay Window in Recipient Contexts,
   three approaches are discussed in Appendix E.  In particular, the
   approach specified in Appendix E.3 and based on the Echo Option
   [I-D.ietf-core-echo-request-tag] is a variant of the approach defined
   in Appendix B.1.2 of [RFC8613] as applicable to Group OSCORE.

2.4.2.  Exhaustion of Sender Sequence Number

   An endpoint can eventually exhaust the Sender Sequence Number, which
   is incremented for each new outgoing message including a Partial IV.
   This is the case for group requests, Observe notifications [RFC7641]
   and, optionally, any other response.

   Implementations MUST be able to detect an exhaustion of Sender
   Sequence Number, after the endpoint has consumed the largest usable
   value.  If an implementation's integers support wrapping addition,
   the implementation MUST treat Sender Sequence Number as exhausted
   when a wrap-around is detected.

   Upon exhausting the Sender Sequence Numbers, the endpoint MUST NOT
   use this Security Context to protect further messages including a
   Partial IV.

   The endpoint SHOULD inform the Group Manager, retrieve new Security
   Context parameters from the Group Manager (see Section 2.4.3), and
   use them to derive a new Sender Context (see Section 2.2).  In
   particular, regardless the exact actions taken by the Group Manager,
   the endpoint resets its Sender Sequence Number to 0, and derives a
   new Sender Key. This is in turn used to possibly derive new Pairwise
   Sender Keys.

   From then on, the endpoint MUST use its latest installed Sender
   Context to protect outgoing messages.







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2.4.3.  Retrieving New Security Context Parameters

   The Group Manager can assist an endpoint with an incomplete Sender
   Context to retrieve missing data of the Security Context and thereby
   become fully operational in the group again.  The two main options
   for the Group Manager are described in this section: i) assignment of
   a new Sender ID to the endpoint (see Section 2.4.3.1); and ii)
   establishment of a new Security Context for the group (see
   Section 2.4.3.2).  Update of Replay Window in Recipient Contexts is
   discussed in Section 6.1.

   As group membership changes, or as group members get new Sender IDs
   (see Section 2.4.3.1) so do the relevant Recipient IDs that the other
   endpoints need to keep track of.  As a consequence, group members may
   end up retaining stale Recipient Contexts, that are no longer useful
   to verify incoming secure messages.

   The Recipient ID ('kid') SHOULD NOT be considered as a persistent and
   reliable indicator of a group member.  Such an indication can be
   achieved only by using that member's public key, when verifying
   countersignatures of received messages (in group mode), or when
   verifying messages integrity-protected with pairwise keying material
   derived from asymmetric keys (in pairwise mode).

   Furthermore, applications MAY define policies to: i) delete
   (long-)unused Recipient Contexts and reduce the impact on storage
   space; as well as ii) check with the Group Manager that a public key
   is currently the one associated to a 'kid' value, after a number of
   consecutive failed verifications.

2.4.3.1.  New Sender ID for the Endpoint

   The Group Manager may assign a new Sender ID to an endpoint, while
   leaving the Gid, Master Secret and Master Salt unchanged in the
   group.  In this case, the Group Manager MUST assign a Sender ID that
   has never been assigned before in the group.

   Having retrieved the new Sender ID, and potentially other missing
   data of the immutable Security Context, the endpoint can derive a new
   Sender Context (see Section 2.2).  When doing so, the endpoint re-
   initilizes the Sender Sequence Number in its Sender Context to 0.

   From then on, the endpoint MUST use its latest installed Sender
   Context to protect outgoing messages.

   The assignment of a new Sender ID may be the result of different
   processes.  The endpoint may request a new Sender ID, e.g. because of
   exhaustion of Sender Sequence Numbers (see Section 2.4.2).  An



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   endpoint may request to re-join the group, e.g. because of losing its
   mutable Security Context (see Section 2.4.1), and receive as response
   a new Sender ID together with the latest immutable Security Context.

   For the other group members, the Recipient Context corresponding to
   the old Sender ID becomes stale (see Section 3.1).

2.4.3.2.  New Security Context for the Group

   The Group Manager may establish a new Security Context for the group
   (see Section 3.1).  The Group Manager does not necessarily establish
   a new Security Context for the group if one member has an outdated
   Security Context (see Section 2.4.3.1), unless that was already
   planned or required for other reasons.

   All the group members need to acquire new Security Context parameters
   from the Group Manager.  Once having acquired new Security Context
   parameters, each group member performs the following actions.

   o  From then on, it MUST NOT use the current Security Context to
      start processing new messages for the considered group.

   o  It completes any ongoing message processing for the considered
      group.

   o  It derives and install a new Security Context.  In particular:

      *  It re-derives the keying material stored in its Sender Context
         and Recipient Contexts (see Section 2.2).  The Master Salt used
         for the re-derivations is the updated Master Salt parameter if
         provided by the Group Manager, or the empty byte string
         otherwise.

      *  It resets to 0 its Sender Sequence Number in its Sender
         Context.

      *  It re-initializes the Replay Window of each Recipient Context.

      *  It resets to 0 the sequence number of each ongoing observation
         where it is an observer client and that it wants to keep
         active.

   From then on, it can resume processing new messages for the
   considered group.  In particular:

   o  It MUST use its latest installed Sender Context to protect
      outgoing messages.




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   o  It SHOULD use its latest installed Recipient Contexts to process
      incoming messages, unless application policies admit to
      temporarily retain and use the old, recent, Security Context (see
      Section 10.4.1).

   The distribution of a new Gid and Master Secret may result in
   temporarily misaligned Security Contexts among group members.  In
   particular, this may result in a group member not being able to
   process messages received right after a new Gid and Master Secret
   have been distributed.  A discussion on practical consequences and
   possible ways to address them, as well as on how to handle the old
   Security Context, is provided in Section 10.4.

3.  The Group Manager

   As with OSCORE, endpoints communicating with Group OSCORE need to
   establish the relevant Security Context.  Group OSCORE endpoints need
   to acquire OSCORE input parameters, information about the group(s)
   and about other endpoints in the group(s).  This specification is
   based on the existence of an entity called Group Manager which is
   responsible for the group, but does not mandate how the Group Manager
   interacts with the group members.  The responsibilities of the Group
   Manager are compiled in Section 3.2.

   It is RECOMMENDED to use a Group Manager as described in
   [I-D.ietf-ace-key-groupcomm-oscore], where the join process is based
   on the ACE framework for authentication and authorization in
   constrained environments [I-D.ietf-ace-oauth-authz].

   The Group Manager assigns unique Group Identifiers (Gids) to
   different groups under its control, as well as unique Sender IDs (and
   thereby Recipient IDs) to the members of those groups.  The Group
   Manager MUST NOT reassign a Sender ID within the same group, and MUST
   NOT reassign a Gid value to the same group.  According to a
   hierarchical approach, the Gid value assigned to a group is
   associated to a dedicated space for the values of Sender ID and
   Recipient ID of the members of that group.

   In addition, the Group Manager maintains records of the public keys
   of endpoints in a group, and provides information about the group and
   its members to other members and selected roles.  Upon nodes'
   joining, the Group Manager collects such public keys and MUST verify
   proof-of-possession of the respective private key.

   An endpoint acquires group data such as the Gid and OSCORE input
   parameters including its own Sender ID from the Group Manager, and
   provides information about its public key to the Group Manager, for
   example upon joining the group.



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   A group member can retrieve from the Group Manager the public key and
   other information associated to another group member, with which it
   can generate the corresponding Recipient Context.  An application can
   configure a group member to asynchronously retrieve information about
   Recipient Contexts, e.g. by Observing [RFC7641] a resource at the
   Group Manager to get updates on the group membership.

   The Group Manager MAY serve additional entities acting as signature
   checkers, e.g. intermediary gateways.  These entities do not join a
   group as members, but can retrieve public keys of group members from
   the Group Manager, in order to verify counter signatures of group
   messages.  A signature checker MUST be authorized for retrieving
   public keys of members in a specific group from the Group Manager.
   To this end, the same method mentioned above based on the ACE
   framework [I-D.ietf-ace-oauth-authz] can be used.

3.1.  Management of Group Keying Material

   In order to establish a new Security Context for a group, a new Group
   Identifier (Gid) for that group and a new value for the Master Secret
   parameter MUST be generated.  When distributing the new Gid and
   Master Secret, the Group Manager MAY distribute also a new value for
   the Master Salt parameter, and SHOULD preserve the current value of
   the Sender ID of each group member.

   The Group Manager MUST NOT reassign a Gid value to the same group.
   That is, each group can have a given Gid at most once during its
   lifetime.  An example of Gid format supporting this operation is
   provided in Appendix C.

   The Group Manager MUST NOT reassign a previously used Sender ID
   ('kid') with the same Gid, Master Secret and Master Salt.  Even if
   Gid and Master Secret are renewed as described in this section, the
   Group Manager MUST NOT reassign an endpoint's Sender ID ('kid')
   within a same group (see Section 2.4.3.1).

   If required by the application (see Appendix A.1), it is RECOMMENDED
   to adopt a group key management scheme, and securely distribute a new
   value for the Gid and for the Master Secret parameter of the group's
   Security Context, before a new joining endpoint is added to the group
   or after a currently present endpoint leaves the group.  This is
   necessary to preserve backward security and forward security in the
   group, if the application requires it.

   The specific approach used to distribute new group data is out of the
   scope of this document.  However, it is RECOMMENDED that the Group
   Manager supports the distribution of the new Gid and Master Secret




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   parameter to the group according to the Group Rekeying Process
   described in [I-D.ietf-ace-key-groupcomm-oscore].

3.2.  Responsibilities of the Group Manager

   The Group Manager is responsible for performing the following tasks:

   1.   Creating and managing OSCORE groups.  This includes the
        assignment of a Gid to every newly created group, as well as
        ensuring uniqueness of Gids within the set of its OSCORE groups.

   2.   Defining policies for authorizing the joining of its OSCORE
        groups.

   3.   Handling the join process to add new endpoints as group members.

   4.   Establishing the Common Context part of the Security Context,
        and providing it to authorized group members during the join
        process, together with the corresponding Sender Context.

   5.   Generating and managing Sender IDs within its OSCORE groups, as
        well as assigning and providing them to new endpoints during the
        join process, or to current group members upon request of
        renewal.  This includes ensuring that each Sender ID is unique
        within each of the OSCORE groups, and that it is not reassigned
        within the same group.

   6.   Defining communication policies for each of its OSCORE groups,
        and signalling them to new endpoints during the join process.

   7.   Renewing the Security Context of an OSCORE group upon membership
        change, by revoking and renewing common security parameters and
        keying material (rekeying).

   8.   Providing the management keying material that a new endpoint
        requires to participate in the rekeying process, consistently
        with the key management scheme used in the group joined by the
        new endpoint.

   9.   Updating the Gid of its OSCORE groups, upon renewing the
        respective Security Context.  This includes ensuring that the
        same Gid value is not reassigned to the same group.

   10.  Acting as key repository, in order to handle the public keys of
        the members of its OSCORE groups, and providing such public keys
        to other members of the same group upon request.  The actual
        storage of public keys may be entrusted to a separate secure
        storage device or service.



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   11.  Validating that the format and parameters of public keys of
        group members are consistent with the countersignature algorithm
        and related parameters used in the respective OSCORE group.

   The Group Manager described in [I-D.ietf-ace-key-groupcomm-oscore]
   provides these functionalities.

4.  The COSE Object

   Building on Section 5 of [RFC8613], this section defines how to use
   COSE [I-D.ietf-cose-rfc8152bis-struct] to wrap and protect data in
   the original message.  OSCORE uses the untagged COSE_Encrypt0
   structure with an Authenticated Encryption with Associated Data
   (AEAD) algorithm.  Unless otherwise specified, the following
   modifications apply for both the group mode and the pairwise mode of
   Group OSCORE.

4.1.  Counter Signature

   For the group mode only, the 'unprotected' field MUST additionally
   include the following parameter:

   o  COSE_CounterSignature0: its value is set to the counter signature
      of the COSE object, computed by the sender as described in
      Sections 3.2 and 3.3 of [I-D.ietf-cose-countersign], by using its
      private key and according to the Counter Signature Algorithm and
      Counter Signature Parameters in the Security Context.

      In particular, the Countersign_structure contains the context text
      string "CounterSignature0", the external_aad as defined in
      Section 4.3.2 of this specification, and the ciphertext of the
      COSE object as payload.

4.2.  The 'kid' and 'kid context' parameters

   The value of the 'kid' parameter in the 'unprotected' field of
   response messages MUST be set to the Sender ID of the endpoint
   transmitting the message.  That is, unlike in [RFC8613], the 'kid'
   parameter is always present in all messages, both requests and
   responses.

   The value of the 'kid context' parameter in the 'unprotected' field
   of requests messages MUST be set to the ID Context, i.e. the Group
   Identifier value (Gid) of the group.  That is, unlike in [RFC8613],
   the 'kid context' parameter is always present in requests.






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4.3.  external_aad

   The external_aad of the Additional Authenticated Data (AAD) is
   different compared to OSCORE.  In particular, there is one
   external_aad used for encryption (both in group mode and pairwise
   mode), and another external_aad used for signing (only in group
   mode).

4.3.1.  external_aad for Encryption

   The external_aad for encryption (see Section 4.3 of
   [I-D.ietf-cose-rfc8152bis-struct]), used both in group mode and
   pairwise mode, includes also the counter signature algorithm and
   related signature parameters, as well as the value of the 'kid
   context' in the COSE object of the request (see Figure 2).

    external_aad = bstr .cbor aad_array

    aad_array = [
       oscore_version : uint,
       algorithms : [alg_aead : int / tstr,
                     alg_countersign : int / tstr,
                     par_countersign : [countersign_alg_capab,
                                        countersign_key_type_capab],
                     par_countersign_key : countersign_key_type_capab],
       request_kid : bstr,
       request_piv : bstr,
       options : bstr,
       request_kid_context : bstr
    ]

                   Figure 2: external_aad for Encryption

   Compared with Section 5.4 of [RFC8613], the aad_array has the
   following differences.

   o  The 'algorithms' array in the aad_array additionally includes:

      *  'alg_countersign', which specifies Counter Signature Algorithm
         from the Common Context (see Section 2.1.2).  This parameter
         MUST encode the value of Counter Signature Algorithm as a CBOR
         integer or text string, consistently with the "Value" field in
         the "COSE Algorithms" Registry for this counter signature
         algorithm.

      *  'par_countersign', which specifies the CBOR array Counter
         Signature Parameters from the Common Context (see
         Section 2.1.3).  In particular:



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         +  'countersign_alg_capab' is the array of COSE capabilities
            for the countersignature algorithm indicated in
            'alg_countersign'.  This is the first element of the CBOR
            array Counter Signature Parameters from the Common Context.

         +  'countersign_key_type_capab' is the array of COSE
            capabilities for the COSE key type used by the
            countersignature algorithm indicated in 'alg_countersign'.
            This is the second element of the CBOR array Counter
            Signature Parameters from the Common Context.

      *  'par_countersign_key', which specifies the parameters
         associated to the keys used with the countersignature algorithm
         indicated in 'alg_countersign'.  These parameters are encoded
         as a CBOR array 'countersign_key_type_capab', whose exact
         structure and value depend on the value of 'alg_countersign'.

         In particular, 'countersign_key_type_capab' is the array of
         COSE capabilities for the COSE key type of the keys used with
         the countersignature algorithm.  This is the second element of
         the CBOR array Counter Signature Parameters from the Common
         Context.

         Examples of 'par_countersign_key' are in Appendix G.

   o  The new element 'request_kid_context' contains the value of the
      'kid context' in the COSE object of the request (see Section 4.2).

4.3.2.  external_aad for Signing

   The external_aad for signing (see Section 4.3 of
   [I-D.ietf-cose-rfc8152bis-struct]) used in group mode is identical to
   the external_aad for encryption (see Section 4.3.1) with the addition
   of the OSCORE option (see Figure 3).

















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    external_aad = bstr .cbor aad_array

    aad_array = [
       oscore_version : uint,
       algorithms : [alg_aead : int / tstr,
                     alg_countersign : int / tstr,
                     par_countersign : [countersign_alg_capab,
                                        countersign_key_type_capab],
                     par_countersign_key : countersign_key_type_capab],
       request_kid : bstr,
       request_piv : bstr,
       options : bstr,
       request_kid_context : bstr,
       OSCORE_option: bstr
    ]

                    Figure 3: external_aad for Signing

   Compared with Section 5.4 of [RFC8613] the aad_array additionally
   includes:

   o  the 'algorithms' array, as defined in the external_aad for
      encryption (see Section 4.3.1);

   o  the 'request_kid_context' element, as defined in the external_aad
      for encryption (see Section 4.3.1);

   o  the value of the OSCORE Option present in the protected message,
      encoded as a binary string.

   Note for implementation: this construction requires the OSCORE option
   of the message to be generated before calculating the signature.
   Also, the aad_array needs to be large enough to contain the largest
   possible OSCORE option.

5.  OSCORE Header Compression

   The OSCORE header compression defined in Section 6 of [RFC8613] is
   used, with the following differences.

   o  The payload of the OSCORE message SHALL encode the ciphertext of
      the COSE_Encrypt0 object.  In the group mode, the ciphertext above
      is concatenated with the value of the COSE_CounterSignature0 of
      the COSE object, computed as described in Section 4.1.

   o  This specification defines the usage of the sixth least
      significant bit, called the "Group Flag", in the first byte of the




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      OSCORE option containing the OSCORE flag bits.  This flag bit is
      specified in Section 11.1.

   o  The Group Flag MUST be set to 1 if the OSCORE message is protected
      using the group mode (see Section 8).

   o  The Group Flag MUST be set to 0 if the OSCORE message is protected
      using the pairwise mode (see Section 9).  The Group Flag MUST also
      be set to 0 for ordinary OSCORE messages processed according to
      [RFC8613].

5.1.  Examples of Compressed COSE Objects

   This section covers a list of OSCORE Header Compression examples of
   Group OSCORE used in group mode (see Section 5.1.1) or in pairwise
   mode (see Section 5.1.2).

   The examples assume that the COSE_Encrypt0 object is set (which means
   the CoAP message and cryptographic material is known).  Note that the
   examples do not include the full CoAP unprotected message or the full
   Security Context, but only the input necessary to the compression
   mechanism, i.e. the COSE_Encrypt0 object.  The output is the
   compressed COSE object as defined in Section 5 and divided into two
   parts, since the object is transported in two CoAP fields: OSCORE
   option and payload.

   The examples assume that the plaintext (see Section 5.3 of [RFC8613])
   is 6 bytes long, and that the AEAD tag is 8 bytes long, hence
   resulting in a ciphertext which is 14 bytes long.  When using the
   group mode, COUNTERSIGN denotes the COSE_CounterSignature0 byte
   string as described in Section 4, and is 64 bytes long.

5.1.1.  Examples in Group Mode

   o  Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
      0x25, Partial IV = 5 and kid context = 0x44616c

   Before compression (96 bytes):

   [
   h'',
   { 4:h'25', 6:h'05', 10:h'44616c', 11:COUNTERSIGN },
   h'aea0155667924dff8a24e4cb35b9'
   ]







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   After compression (85 bytes):

   Flag byte: 0b00111001 = 0x39

   Option Value: 39 05 03 44 61 6c 25 (7 bytes)

   Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 COUNTERSIGN
   (14 bytes + size of COUNTERSIGN)


   o  Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid =
      0x52 and no Partial IV.

   Before compression (88 bytes):

   [
   h'',
   { 4:h'52', 11:COUNTERSIGN },
   h'60b035059d9ef5667c5a0710823b'
   ]

   After compression (80 bytes):

   Flag byte: 0b00101000 = 0x28

   Option Value: 28 52 (2 bytes)

   Payload: 60 b0 35 05 9d 9e f5 66 7c 5a 07 10 82 3b COUNTERSIGN
   (14 bytes + size of COUNTERSIGN)

5.1.2.  Examples in Pairwise Mode

   o  Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
      0x25, Partial IV = 5 and kid context = 0x44616c

   Before compression (32 bytes):

   [
   h'',
   { 4:h'25', 6:h'05', 10:h'44616c' },
   h'aea0155667924dff8a24e4cb35b9'
   ]









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   After compression (21 bytes):

   Flag byte: 0b00011001 = 0x19

   Option Value: 19 05 03 44 61 6c 25 (7 bytes)

   Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)


   o  Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid =
      0x52 and no Partial IV.

   Before compression (24 bytes):

   [
   h'',
   { 4:h'52'},
   h'60b035059d9ef5667c5a0710823b'
   ]

   After compression (16 bytes):

   Flag byte: 0b00001000 = 0x08

   Option Value: 08 52 (2 bytes)

   Payload: 60 b0 35 05 9d 9e f5 66 7c 5a 07 10 82 3b (14 bytes)

6.  Message Binding, Sequence Numbers, Freshness and Replay Protection

   The requirements and properties described in Section 7 of [RFC8613]
   also apply to OSCORE used in group communication.  In particular,
   Group OSCORE provides message binding of responses to requests, which
   enables absolute freshness of responses that are not notifications,
   relative freshness of requests and notification responses, and replay
   protection of requests.

6.1.  Update of Replay Window

   A new server joining a group may not be aware of the current Partial
   IVs (Sender Sequence Numbers of the clients).  Hence, when receiving
   a request from a particular client for the first time, the new server
   is not able to verify if that request is a replay.  The same holds
   when a server loses its mutable Security Context (see Section 2.4.1),
   for instance after a device reboot.

   The exact way to address this issue is application specific, and
   depends on the particular use case and its replay requirements.  The



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   list of methods to handle the update of a Replay Window is part of
   the group communication policy, and different servers can use
   different methods.  Appendix E describes three possible approaches
   that can be considered to address the issue discussed above.

   Furthermore, when the Group Manager establishes a new Security
   Context for the group (see Section 2.4.3.2), every server re-
   initializes the Replay Window in each of its Recipient Contexts.

7.  Message Reception

   Upon receiving a protected message, a recipient endpoint retrieves a
   Security Context as in [RFC8613].  An endpoint MUST be able to
   distinguish between a Security Context to process OSCORE messages as
   in [RFC8613] and a Security Context to process Group OSCORE messages
   as defined in this specification.

   To this end, an endpoint can take into account the different
   structure of the Security Context defined in Section 2, for example
   based on the presence of Counter Signature Algorithm in the Common
   Context.  Alternatively implementations can use an additional
   parameter in the Security Context, to explicitly signal that it is
   intended for processing Group OSCORE messages.

   If either of the following two conditions holds, a recipient endpoint
   MUST discard the incoming protected message:

   o  The Group Flag is set to 1, and the recipient endpoint can not
      retrieve a Security Context which is both valid to process the
      message and also associated to an OSCORE group.

   o  The Group Flag is set to 0, and the recipient endpoint retrieves a
      Security Context which is both valid to process the message and
      also associated to an OSCORE group, but the endpoint does not
      support the pairwise mode.

   Otherwise, if a Security Context associated to an OSCORE group and
   valid to process the message is retrieved, the recipient endpoint
   processes the message with Group OSCORE, using the group mode (see
   Section 8) if the Group Flag is set to 1, or the pairwise mode (see
   Section 9) if the Group Flag is set to 0.

   Note that, if the Group Flag is set to 0, and the recipient endpoint
   retrieves a Security Context which is valid to process the message
   but is not associated to an OSCORE group, then the message is
   processed according to [RFC8613].





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8.  Message Processing in Group Mode

   When using the group mode, messages are protected and processed as
   specified in [RFC8613], with the modifications described in this
   section.  The security objectives of the group mode are discussed in
   Appendix A.2.  The group mode MUST be supported.

   During all the steps of the message processing, an endpoint MUST use
   the same Security Context for the considered group.  That is, an
   endpoint MUST NOT install a new Security Context for that group (see
   Section 2.4.3.2) until the message processing is completed.

   The group mode MUST be used to protect group requests intended for
   multiple recipients or for the whole group.  This includes both
   requests directly addressed to multiple recipients, e.g. sent by the
   client over multicast, as well as requests sent by the client over
   unicast to a proxy, that forwards them to the intended recipients
   over multicast [I-D.ietf-core-groupcomm-bis].

   As per [RFC7252][I-D.ietf-core-groupcomm-bis], group requests sent
   over multicast MUST be Non-Confirmable, and thus are not
   retransmitted by the CoAP messaging layer.  Instead, applications
   should store such outgoing messages for a pre-defined, sufficient
   amount of time, in order to correctly perform possible
   retransmissions at the application layer.  According to Section 5.2.3
   of [RFC7252], responses to Non-Confirmable group requests SHOULD also
   be Non-Confirmable, but endpoints MUST be prepared to receive
   Confirmable responses in reply to a Non-Confirmable group request.
   Confirmable group requests are acknowledged in non-multicast
   environments, as specified in [RFC7252].

   Furthermore, endpoints in the group locally perform error handling
   and processing of invalid messages according to the same principles
   adopted in [RFC8613].  However, a recipient MUST stop processing and
   silently reject any message which is malformed and does not follow
   the format specified in Section 4, or which is not cryptographically
   validated in a successful way.  In either case, it is RECOMMENDED
   that the recipient does not send back any error message.  This
   prevents servers from replying with multiple error messages to a
   client sending a group request, so avoiding the risk of flooding and
   possibly congesting the network.

8.1.  Protecting the Request

   A client transmits a secure group request as described in Section 8.1
   of [RFC8613], with the following modifications.





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   o  In step 2, the Additional Authenticated Data is modified as
      described in Section 4 of this document.

   o  In step 4, the encryption of the COSE object is modified as
      described in Section 4 of this document.  The encoding of the
      compressed COSE object is modified as described in Section 5 of
      this document.  In particular, the Group Flag MUST be set to 1.

   o  In step 5, the counter signature is computed and the format of the
      OSCORE message is modified as described in Section 4 and Section 5
      of this document.  In particular, the payload of the OSCORE
      message includes also the counter signature.

8.1.1.  Supporting Observe

   If Observe [RFC7641] is supported, the following holds for each newly
   started observation.

   o  If the client intends to keep the observation active beyond a
      possible change of Sender ID, the client MUST store the value of
      the 'kid' parameter from the original Observe request, and retain
      it for the whole duration of the observation.  Even in case the
      client is individually rekeyed and receives a new Sender ID from
      the Group Manager (see Section 2.4.3.1), the client MUST NOT
      update the stored value associated to a particular Observe
      request.

   o  If the client intends to keep the observation active beyond a
      possible change of ID Context following a group rekeying (see
      Section 3.1), then the following applies.

      *  The client MUST store the value of the 'kid context' parameter
         from the original Observe request, and retain it for the whole
         duration of the observation.  Upon establishing a new Security
         Context with a new ID Context as Gid (see Section 2.4.3.2), the
         client MUST NOT update the stored value associated to a
         particular Observe request.

      *  The client MUST store an invariant identifier of the group,
         which is immutable even in case the Security Context of the
         group is re-established.  For example, this invariant
         identifier can be the "group name" in
         [I-D.ietf-ace-key-groupcomm-oscore], where it is used for
         joining the group and retrieving the current group keying
         material from the Group Manager.

         After a group rekeying, such an invariant information makes it
         simpler for the observer client to retrieve the current group



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         keying material from the Group Manager, in case the client has
         missed both the rekeying messages and the first observe
         notification protected with the new Security Context (see
         Section 8.3.1).

8.2.  Verifying the Request

   Upon receiving a secure group request with the Group Flag set to 1,
   following the procedure in Section 7, a server proceeds as described
   in Section 8.2 of [RFC8613], with the following modifications.

   o  In step 2, the decoding of the compressed COSE object follows
      Section 5 of this document.  In particular:

      *  If the server discards the request due to not retrieving a
         Security Context associated to the OSCORE group, the server MAY
         respond with a 4.02 (Bad Option) error.  When doing so, the
         server MAY set an Outer Max-Age option with value zero, and MAY
         include a descriptive string as diagnostic payload.

      *  If the received 'kid context' matches an existing ID Context
         (Gid) but the received 'kid' does not match any Recipient ID in
         this Security Context, then the server MAY create a new
         Recipient Context for this Recipient ID and initialize it
         according to Section 3 of [RFC8613], and also retrieve the
         associated public key.  Such a configuration is application
         specific.  If the application does not specify dynamic
         derivation of new Recipient Contexts, then the server SHALL
         stop processing the request.

   o  In step 4, the Additional Authenticated Data is modified as
      described in Section 4 of this document.

   o  In step 6, the server also verifies the counter signature using
      the public key of the client from the associated Recipient
      Context.  In particular:

      *  If the server does not have the public key of the client yet,
         the server MUST stop processing the request and MAY respond
         with a 5.03 (Service Unavailable) response.  The response MAY
         include a Max-Age Option, indicating to the client the number
         of seconds after which to retry.  If the Max-Age Option is not
         present, a retry time of 60 seconds will be assumed by the
         client, as default value defined in Section 5.10.5 of
         [RFC7252].

      *  If the signature verification fails, the server SHALL stop
         processing the request and MAY respond with a 4.00 (Bad



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         Request) response.  If the verification fails, the same steps
         are taken as if the decryption had failed.  In particular, the
         Replay Window is only updated if both the signature
         verification and the decryption succeed.

   o  Additionally, if the used Recipient Context was created upon
      receiving this group request and the message is not verified
      successfully, the server MAY delete that Recipient Context.  Such
      a configuration, which is specified by the application, mitigates
      attacks that aim at overloading the server's storage.

   A server SHOULD NOT process a request if the received Recipient ID
   ('kid') is equal to its own Sender ID in its own Sender Context.  For
   an example where this is not fulfilled, see Section 6.2.1 in
   [I-D.tiloca-core-observe-multicast-notifications].

8.2.1.  Supporting Observe

   If Observe [RFC7641] is supported, the following holds for each newly
   started observation.

   o  The server MUST store the value of the 'kid' parameter from the
      original Observe request, and retain it for the whole duration of
      the observation.  The server MUST NOT update the stored value of a
      'kid' parameter associated to a particular Observe request, even
      in case the observer client is individually rekeyed and starts
      using a new Sender ID received from the Group Manager (see
      Section 2.4.3.1).

   o  The server MUST store the value of the 'kid context' parameter
      from the original Observe request, and retain it for the whole
      duration of the observation, beyond a possible change of ID
      Context following a group rekeying (see Section 3.1).  That is,
      upon establishing a new Security Context with a new ID Context as
      Gid (see Section 2.4.3.2), the server MUST NOT update the stored
      value associated to the ongoing observation.

8.3.  Protecting the Response

   If a server generates a CoAP message in response to a Group OSCORE
   request, then the server SHALL follow the description in Section 8.3
   of [RFC8613], with the modifications described in this section.

   Note that the server always protects a response with the Sender
   Context from its latest Security Context, and that establishing a new
   Security Context resets the Sender Sequence Number to 0 (see
   Section 3.1).




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   o  In step 2, the Additional Authenticated Data is modified as
      described in Section 4 of this document.

   o  In step 3, if the server is using a different Security Context for
      the response compared to what was used to verify the request (see
      Section 3.1), then the server MUST include its Sender Sequence
      Number as Partial IV in the response and use it to build the AEAD
      nonce to protect the response.  This prevents the AEAD nonce from
      the request from being reused.

   o  In step 4, the encryption of the COSE object is modified as
      described in Section 4 of this document.  The encoding of the
      compressed COSE object is modified as described in Section 5 of
      this document.  In particular, the Group Flag MUST be set to 1.
      If the server is using a different ID Context (Gid) for the
      response compared to what was used to verify the request (see
      Section 3.1), then the new ID Context MUST be included in the 'kid
      context' parameter of the response.

   o  In step 5, the counter signature is computed and the format of the
      OSCORE message is modified as described in Section 5 of this
      document.  In particular, the payload of the OSCORE message
      includes also the counter signature.

8.3.1.  Supporting Observe

   If Observe [RFC7641] is supported, the following holds when
   protecting notifications for an ongoing observation.

   o  The server MUST use the stored value of the 'kid' parameter from
      the original Observe request (see Section 8.2.1), as value for the
      'request_kid' parameter in the two external_aad structures (see
      Section 4.3.1 and Section 4.3.2).

   o  The server MUST use the stored value of the 'kid context'
      parameter from the original Observe request (see Section 8.2.1),
      as value for the 'request_kid_context' parameter in the two
      external_aad structures (see Section 4.3.1 and Section 4.3.2).

   Furthermore, the server may have ongoing observations started by
   Observe requests protected with an old Security Context.  After
   completing the establishment of a new Security Context, the server
   MUST protect the following notifications with the Sender Context of
   the new Security Context.

   For each ongoing observation, the server MUST include in the first
   notification protected with the new Security Context also the 'kid
   context' parameter, which is set to the ID Context (Gid) of the new



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   Security Context.  It is OPTIONAL for the server to include the ID
   Context (Gid) in the 'kid context' parameter also in further
   following notifications for those observations.

8.4.  Verifying the Response

   Upon receiving a secure response message with the Group Flag set to
   1, following the procedure in Section 7, the client proceeds as
   described in Section 8.4 of [RFC8613], with the following
   modifications.

   Note that a client may receive a response protected with a Security
   Context different from the one used to protect the corresponding
   group request, and that, upon the establishment of a new Security
   Context, the client re-initializes its replay windows in its
   Recipient Contexts (see Section 3.1).

   o  In step 2, the decoding of the compressed COSE object is modified
      as described in Section 5 of this document.  If the received 'kid
      context' matches an existing ID Context (Gid) but the received
      'kid' does not match any Recipient ID in this Security Context,
      then the client MAY create a new Recipient Context for this
      Recipient ID and initialize it according to Section 3 of
      [RFC8613], and also retrieve the associated public key.  If the
      application does not specify dynamic derivation of new Recipient
      Contexts, then the client SHALL stop processing the response.

   o  In step 3, the Additional Authenticated Data is modified as
      described in Section 4 of this document.

   o  In step 5, the client also verifies the counter signature using
      the public key of the server from the associated Recipient
      Context.  If the verification fails, the same steps are taken as
      if the decryption had failed.

   o  Additionally, if the used Recipient Context was created upon
      receiving this response and the message is not verified
      successfully, the client MAY delete that Recipient Context.  Such
      a configuration, which is specified by the application, mitigates
      attacks that aim at overloading the client's storage.

8.4.1.  Supporting Observe

   If Observe [RFC7641] is supported, the following holds when verifying
   notifications for an ongoing observation.

   o  The client MUST use the stored value of the 'kid' parameter from
      the original Observe request (see Section 8.1.1), as value for the



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      'request_kid' parameter in the two external_aad structures (see
      Section 4.3.1 and Section 4.3.2).

   o  The client MUST use the stored value of the 'kid context'
      parameter from the original Observe request (see Section 8.1.1),
      as value for the 'request_kid_context' parameter in the two
      external_aad structures (see Section 4.3.1 and Section 4.3.2).

   This ensures that the client can correctly verify notifications, even
   in case it is individually rekeyed and starts using a new Sender ID
   received from the Group Manager (see Section 2.4.3.1), as well as
   when it establishes a new Security Context with a new ID Context
   (Gid) following a group rekeying (see Section 3.1).

9.  Message Processing in Pairwise Mode

   When using the pairwise mode of Group OSCORE, messages are protected
   and processed as in Section 8, with the modifications described in
   this section.  The security objectives of the pairwise mode are
   discussed in Appendix A.2.

   The pairwise mode takes advantage of an existing Security Context for
   the group mode to establish a Security Context shared exclusively
   with any other member.  In order to use the pairwise mode, the
   signature scheme of the group mode MUST support a combined signature
   and encryption scheme.  This can be, for example, signature using
   ECDSA, and encryption using AES-CCM with a key derived with ECDH.

   The pairwise mode does not support the use of additional entities
   acting as verifiers of source authentication and integrity of group
   messages, such as intermediary gateways (see Section 3).

   The pairwise mode MAY be supported.  An endpoint implementing only a
   silent server does not support the pairwise mode.

   If the signature algorithm used in the group supports ECDH (e.g.,
   ECDSA, EdDSA), the pairwise mode MUST be supported by endpoints that
   use the CoAP Echo Option [I-D.ietf-core-echo-request-tag] and/or
   block-wise transfers [RFC7959], for instance for responses after the
   first block-wise request, which possibly targets all servers in the
   group and includes the CoAP Block2 option (see Section 2.3.6 of
   [I-D.ietf-core-groupcomm-bis]).  This prevents the attack described
   in Section 10.7, which leverages requests sent over unicast to a
   single group member and protected with the group mode.

   The pairwise mode protects messages between two members of a group,
   essentially following [RFC8613], but with the following notable
   differences:



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   o  The 'kid' and 'kid context' parameters of the COSE object are used
      as defined in Section 4.2 of this document.

   o  The external_aad defined in Section 4.3.1 of this document is used
      for the encryption process.

   o  The Pairwise Sender/Recipient Keys used as Sender/Recipient keys
      are derived as defined in Section 2.3 of this document.

   Senders MUST NOT use the pairwise mode to protect a message intended
   for multiple recipients.  The pairwise mode is defined only between
   two endpoints and the keying material is thus only available to one
   recipient.

   The Group Manager MAY indicate that the group uses also the pairwise
   mode, as part of the group data provided to candidate group members
   when joining the group.

9.1.  Pre-Conditions

   In order to protect an outgoing message in pairwise mode, the sender
   needs to know the public key and the Recipient ID for the recipient
   endpoint, as stored in the Recipient Context associated to that
   endpoint (see Pairwise Sender Context of Section 2.3.3).

   Furthermore, the sender needs to know the individual address of the
   recipient endpoint.  This information may not be known at any given
   point in time.  For instance, right after having joined the group, a
   client may know the public key and Recipient ID for a given server,
   but not the addressing information required to reach it with an
   individual, one-to-one request.

   To make addressing information of individual endpoints available,
   servers in the group MAY expose a resource to which a client can send
   a group request targeting a server or a set of servers, identified by
   their 'kid' value(s).  The specified set may be empty, hence
   identifying all the servers in the group.  Further details of such an
   interface are out of scope for this document.

9.2.  Protecting the Request

   When using the pairwise mode, the request is protected as defined in
   Section 8.1, with the following differences.

   o  The Group Flag MUST be set to 0.

   o  The used Sender Key is the Pairwise Sender Key (see Section 2.3).




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   o  The counter signature is not computed and therefore not included
      in the message.  The payload of the protected request thus
      terminates with the encoded ciphertext of the COSE object, just
      like in [RFC8613].

   Note that, like in the group mode, the external_aad for encryption is
   generated as in Section 4.3.1, and the Partial IV is the current
   fresh value of the client's Sender Sequence Number (see
   Section 2.3.2).

9.3.  Verifying the Request

   Upon receiving a request with the Group Flag set to 0, following the
   procedure in Section 7, the server MUST process it as defined in
   Section 8.2, with the following differences.

   o  If the server discards the request due to not retrieving a
      Security Context associated to the OSCORE group or to not
      supporting the pairwise mode, the server MAY respond with a 4.02
      (Bad Option) error.  When doing so, the server MAY set an Outer
      Max-Age option with value zero, and MAY include a descriptive
      string as diagnostic payload.

   o  If a new Recipient Context is created for this Recipient ID, new
      Pairwise Sender/Recipient Keys are also derived (see
      Section 2.3.1).  The new Pairwise Sender/Recipient Keys are
      deleted if the Recipient Context is deleted as a result of the
      message not being successfully verified.

   o  The used Recipient Key is the Pairwise Recipient Key (see
      Section 2.3).

   o  No verification of counter signature occurs, as there is none
      included in the message.

9.4.  Protecting the Response

   When using the pairwise mode, a response is protected as defined in
   Section 8.3, with the following differences.

   o  The Group Flag MUST be set to 0.

   o  The used Sender Key is the Pairwise Sender Key (see Section 2.3).

   o  The counter signature is not computed and therefore not included
      in the message.  The payload of the protected response thus
      terminates with the encoded ciphertext of the COSE object, just
      like in [RFC8613].



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9.5.  Verifying the Response

   Upon receiving a response with the Group Flag set to 0, following the
   procedure in Section 7, the client MUST process it as defined in
   Section 8.4, with the following differences.

   o  If a new Recipient Context is created for this Recipient ID, new
      Pairwise Sender/Recipient Keys are also derived (see
      Section 2.3.1).  The new Pairwise Sender/Recipient Keys are
      deleted if the Recipient Context is deleted as a result of the
      message not being successfully verified.

   o  The used Recipient Key is the Pairwise Recipient Key (see
      Section 2.3).

   o  No verification of counter signature occurs, as there is none
      included in the message.

10.  Security Considerations

   The same threat model discussed for OSCORE in Appendix D.1 of
   [RFC8613] holds for Group OSCORE.  In addition, when using the group
   mode, source authentication of messages is explicitly ensured by
   means of counter signatures, as discussed in Section 10.1.

   The same considerations on supporting Proxy operations discussed for
   OSCORE in Appendix D.2 of [RFC8613] hold for Group OSCORE.

   The same considerations on protected message fields for OSCORE
   discussed in Appendix D.3 of [RFC8613] hold for Group OSCORE.

   The same considerations on uniqueness of (key, nonce) pairs for
   OSCORE discussed in Appendix D.4 of [RFC8613] hold for Group OSCORE.
   This is further discussed in Section 10.2 of this document.

   The same considerations on unprotected message fields for OSCORE
   discussed in Appendix D.5 of [RFC8613] hold for Group OSCORE, with
   the following difference.  The counter signature included in a Group
   OSCORE message protected in group mode is computed also over the
   value of the OSCORE option, which is part of the Additional
   Authenticated Data used in the signing process.  This is further
   discussed in Section 10.6 of this document.

   As discussed in Section 6.2.3 of [I-D.ietf-core-groupcomm-bis], Group
   OSCORE addresses security attacks against CoAP listed in Sections
   11.2-11.6 of [RFC7252], especially when run over IP multicast.





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   The rest of this section first discusses security aspects to be taken
   into account when using Group OSCORE.  Then it goes through aspects
   covered in the security considerations of OSCORE (see Section 12 of
   [RFC8613]), and discusses how they hold when Group OSCORE is used.

10.1.  Group-level Security

   The group mode described in Section 8 relies on commonly shared group
   keying material to protect communication within a group.  This has
   the following implications.

   o  Messages are encrypted at a group level (group-level data
      confidentiality), i.e. they can be decrypted by any member of the
      group, but not by an external adversary or other external
      entities.

   o  The AEAD algorithm provides only group authentication, i.e. it
      ensures that a message sent to a group has been sent by a member
      of that group, but not necessarily by the alleged sender.  This is
      why source authentication of messages sent to a group is ensured
      through a counter signature, which is computed by the sender using
      its own private key and then appended to the message payload.

   Instead, the pairwise mode described in Section 9 protects messages
   by using pairwise symmetric keys, derived from the static-static
   Diffie-Hellman shared secret computed from the asymmetric keys of the
   sender and recipient endpoint (see Section 2.3).  Therefore, in the
   parwise mode, the AEAD algorithm provides both pairwise data-
   confidentiality and source authentication of messages, without using
   counter signatures.

   The long-term storing of the Diffie-Hellman shared secret is a
   potential security issue.  In fact, if the shared secret of two group
   members is leaked, a third group member can exploit it to impersonate
   any of those two group members, by deriving and using their pairwise
   key.  The possibility of such leakage should be contemplated, as more
   likely to happen than the leakage of a private key, which could be
   rather protected at a significantly higher level than generic memory,
   e.g. by using a Trusted Platform Module.  Therefore, there is a
   trade-off between the maximum amount of time a same shared secret is
   stored and the frequency of its re-computing.

   Note that, even if an endpoint is authorized to be a group member and
   to take part in group communications, there is a risk that it behaves
   inappropriately.  For instance, it can forward the content of
   messages in the group to unauthorized entities.  However, in many use
   cases, the devices in the group belong to a common authority and are
   configured by a commissioner (see Appendix B), which results in a



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   practically limited risk and enables a prompt detection/reaction in
   case of misbehaving.

10.2.  Uniqueness of (key, nonce)

   The proof for uniqueness of (key, nonce) pairs in Appendix D.4 of
   [RFC8613] is also valid in group communication scenarios.  That is,
   given an OSCORE group:

   o  Uniqueness of Sender IDs within the group is enforced by the Group
      Manager, which never reassigns the same Sender ID within the same
      group.

   o  The case A in Appendix D.4 of [RFC8613] concerns all group
      requests and responses including a Partial IV (e.g.  Observe
      notifications).  In this case, same considerations from [RFC8613]
      apply here as well.

   o  The case B in Appendix D.4 of [RFC8613] concerns responses not
      including a Partial IV (e.g. single response to a group request).
      In this case, same considerations from [RFC8613] apply here as
      well.

   As a consequence, each message encrypted/decrypted with the same
   Sender Key is processed by using a different (ID_PIV, PIV) pair.
   This means that nonces used by any fixed encrypting endpoint are
   unique.  Thus, each message is processed with a different (key,
   nonce) pair.

10.3.  Management of Group Keying Material

   The approach described in this specification should take into account
   the risk of compromise of group members.  In particular, this
   document specifies that a key management scheme for secure revocation
   and renewal of Security Contexts and group keying material should be
   adopted.

   [I-D.ietf-ace-key-groupcomm-oscore] provides a simple rekeying scheme
   for renewing the Security Context in a group.

   Alternative rekeying schemes which are more scalable with the group
   size may be needed in dynamic, large-scale groups where endpoints can
   join and leave at any time, in order to limit the impact on
   performance due to the Security Context and keying material update.







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10.4.  Update of Security Context and Key Rotation

   A group member can receive a message shortly after the group has been
   rekeyed, and new security parameters and keying material have been
   distributed by the Group Manager.

   This may result in a client using an old Security Context to protect
   a group request, and a server using a different new Security Context
   to protect a corresponding response.  As a consequence, clients may
   receive a response protected with a Security Context different from
   the one used to protect the corresponding group request.

   In particular, a server may first get a group request protected with
   the old Security Context, then install the new Security Context, and
   only after that produce a response to send back to the client.  In
   such a case, as specified in Section 8.3, the server MUST protect the
   potential response using the new Security Context.  Specifically, the
   server MUST include its Sender Sequence Number as Partial IV in the
   response and use it to build the AEAD nonce to protect the response.
   This prevents the AEAD nonce from the request from being reused with
   the new Security Context.

   The client will process that response using the new Security Context,
   provided that it has installed the new security parameters and keying
   material before the message processing.

   In case block-wise transfer [RFC7959] is used, the same
   considerations from Section 7.2 of [I-D.ietf-ace-key-groupcomm] hold.

   Furthermore, as described below, a group rekeying may temporarily
   result in misaligned Security Contexts between the sender and
   recipient of a same message.

10.4.1.  Late Update on the Sender

   In this case, the sender protects a message using the old Security
   Context, i.e. before having installed the new Security Context.
   However, the recipient receives the message after having installed
   the new Security Context, and is thus unable to correctly process it.

   A possible way to ameliorate this issue is to preserve the old,
   recent, Security Context for a maximum amount of time defined by the
   application.  By doing so, the recipient can still try to process the
   received message using the old retained Security Context as second
   attempt.  This makes particular sense when the recipient is a client,
   that would hence be able to process incoming responses protected with
   the old, recent, Security Context used to protect the associated
   group request.  Instead, a recipient server would better and more



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   simply discard an incoming group request which is not successfully
   processed with the new Security Context.

   This tolerance preserves the processing of secure messages throughout
   a long-lasting key rotation, as group rekeying processes may likely
   take a long time to complete, especially in large scale groups.  On
   the other hand, a former (compromised) group member can abusively
   take advantage of this, and send messages protected with the old
   retained Security Context.  Therefore, a conservative application
   policy should not admit the retention of old Security Contexts.

10.4.2.  Late Update on the Recipient

   In this case, the sender protects a message using the new Security
   Context, but the recipient receives that message before having
   installed the new Security Context.  Therefore, the recipient would
   not be able to correctly process the message and hence discards it.

   If the recipient installs the new Security Context shortly after that
   and the sender endpoint retransmits the message, the former will
   still be able to receive and correctly process the message.

   In any case, the recipient should actively ask the Group Manager for
   an updated Security Context according to an application-defined
   policy, for instance after a given number of unsuccessfully decrypted
   incoming messages.

10.5.  Collision of Group Identifiers

   In case endpoints are deployed in multiple groups managed by
   different non-synchronized Group Managers, it is possible for Group
   Identifiers of different groups to coincide.

   This does not impair the security of the AEAD algorithm.  In fact, as
   long as the Master Secret is different for different groups and this
   condition holds over time, AEAD keys are different among different
   groups.

   The entity assigning an IP multicast address may help limiting the
   chances to experience such collisions of Group Identifiers.  In
   particular, it may allow the Group Managers of groups using the same
   IP multicast address to share their respective list of assigned Group
   Identifiers currently in use.








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10.6.  Cross-group Message Injection

   A same endpoint is allowed to and would likely use the same public/
   private key pair in multiple OSCORE groups, possibly administered by
   different Group Managers.

   When a sender endpoint sends a message protected in pairwise mode to
   a recipient endpoint in an OSCORE group, a malicious group member may
   attempt to inject the message to a different OSCORE group also
   including the same endpoints (see Section 10.6.1).

   This practically relies on altering the content of the OSCORE option,
   and having the same MAC in the ciphertext still correctly validating,
   which has a success probability depending on the size of the MAC.

   As discussed in Section 10.6.2, the attack is practically infeasible
   if the message is protected in group mode, since the counter
   signature is bound also to the OSCORE option, through the Additional
   Authenticated Data used in the signing process (see Section 4.3.2).

10.6.1.  Attack Description

   Let us consider:

   o  Two OSCORE groups G1 and G2, with ID Context (Group ID) Gid1 and
      Gid2, respectively.  Both G1 and G2 use the AEAD cipher AES-CCM-
      16-64-128, i.e. the MAC of the ciphertext is 8 bytes in size.

   o  A sender endpoint X which is member of both G1 and G2, and uses
      the same public/private key pair in both groups.  The endpoint X
      has Sender ID Sid1 in G1 and Sender ID Sid2 in G2.  The pairs
      (Sid1, Gid1) and (Sid2, Gid2) identify the same public key of X in
      G1 and G2, respectively.

   o  A recipient endpoint Y which is member of both G1 and G2, and uses
      the same public/private key pair in both groups.  The endpoint Y
      has Sender ID Sid3 in G1 and Sender ID Sid4 in G2.  The pairs
      (Sid3, Gid1) and (Sid4, Gid2) identify the same public key of Y in
      G1 and G2, respectively.

   o  A malicious endpoint Z is also member of both G1 and G2.  Hence, Z
      is able to derive the symmetric keys associated to X in G1 and G2.

   When X sends a message M1 addressed to Y in G1 and protected in
   pairwise mode, Z can intercept M1, and forge a valid message M2 to be
   injected in G2, making it appear as still sent by X to Y and valid to
   be accepted.




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   More in detail, Z intercepts and stops message M1, and forges a
   message M2 by changing the value of the OSCORE option from M1 as
   follows: the 'kid context' is changed from G1 to G2; and the 'kid' is
   changed from Sid1 to Sid2.  Then, Z injects message M2 as addressed
   to Y in G2.

   Upon receiving M2, there is a probability equal to 2^-64 that Y
   successfully verifies the same unchanged MAC by using Sid2 as
   'request_kid' and using the Pairwise Recipient Key associated to X in
   G2.

   Note that Z does not know the pairwise keys of X and Y, since it does
   not know and is not able to compute their shared Diffie-Hellman
   secret.  Therefore, Z is not able to check offline if a performed
   forgery is actually valid, before sending the forged message to G2.

10.6.2.  Attack Prevention in Group Mode

   When a Group OSCORE message is protected with the group mode, the
   counter signature is computed also over the value of the OSCORE
   option, which is part of the Additional Authenticated Data used in
   the signing process (see Section 4.3.2).

   That is, the countersignature is computed also over: the ID Context
   (Group ID) and the Partial IV, which are always present in group
   requests; as well as the Sender ID of the message originator, which
   is always present in all group requests and responses.

   Since the signing process takes as input also the ciphertext of the
   COSE_Encrypt0 object, the countersignature is bound not only to the
   intended OSCORE group, hence to the triplet (Master Secret, Master
   Salt, ID Context), but also to a specific Sender ID in that group and
   to its specific symmetric key used for AEAD encryption, hence to the
   quartet (Master Secret, Master Salt, ID Context, Sender ID).

   This makes it practically infeasible to perform the attack described
   in Section 10.6.1, since it would require the adversary to
   additionally forge a valid countersignature that replaces the
   original one in the forged message M2.

   If the countersignature did not cover the OSCORE option, the attack
   would be possible also in group mode, since the same unchanged
   countersignature from messsage M1 would be also valid in message M2.
   Also, the following attack simplifications would hold, since Z is
   able to derive the Sender/Recipient Keys of X and Y in G1 and G2.

   o  If M2 is used as a request, Z can check offline if a performed
      forgery is actually valid before sending the forged message to G2.



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      That is, this attack would have a complexity of 2^64 offline
      calculations.

   o  If M2 is used as a response, Z can also change the response
      Partial IV, until the same unchanged MAC is successfully verified
      by using Sid2 as 'request_kid' and the symmetric key associated to
      X in G2.  Since the Partial IV is 5 bytes in size, this requires
      2^40 operations to test all the Partial IVs, which can be done in
      real-time.  Also, the probability that a single given message M1
      can be used to forge a response M2 for a given request would be
      equal to 2^-24, since there are more MAC values (8 bytes in size)
      than Partial IV values (5 bytes in size).

      Note that, by changing the Partial IV as discussed above, any
      member of G1 would also be able to forge a valid signed response
      message M2 to be injected in G1.

10.7.  Group OSCORE for Unicast Requests

   If a request is intended to be sent over unicast as addressed to a
   single group member, it is NOT RECOMMENDED for the client to protect
   the request by using the group mode as defined in Section 8.1.

   This does not include the case where the client sends a request over
   unicast to a proxy, to be forwarded to multiple intended recipients
   over multicast [I-D.ietf-core-groupcomm-bis].  In this case, the
   client MUST protect the request with the group mode, even though it
   is sent to the proxy over unicast (see Section 8).

   If the client uses the group mode with its own Sender Key to protect
   a unicast request to a group member, an on-path adversary can, right
   then or later on, redirect that request to one/many different group
   member(s) over unicast, or to the whole OSCORE group over multicast.
   By doing so, the adversary can induce the target group member(s) to
   perform actions intended for one group member only.  Note that the
   adversary can be external, i.e. (s)he does not need to also be a
   member of the OSCORE group.

   This is due to the fact that the client is not able to indicate the
   single intended recipient in a way which is secure and possible to
   process for Group OSCORE on the server side.  In particular, Group
   OSCORE does not protect network addressing information such as the IP
   address of the intended recipient server.  It follows that the
   server(s) receiving the redirected request cannot assert whether that
   was the original intention of the client, and would thus simply
   assume so.





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   The impact of such an attack depends especially on the REST method of
   the request, i.e. the Inner CoAP Code of the OSCORE request message.
   In particular, safe methods such as GET and FETCH would trigger
   (several) unintended responses from the targeted server(s), while not
   resulting in destructive behavior.  On the other hand, non safe
   methods such as PUT, POST and PATCH/iPATCH would result in the target
   server(s) taking active actions on their resources and possible
   cyber-physical environment, with the risk of destructive consequences
   and possible implications for safety.

   A client can instead use the pairwise mode as defined in Section 9.2,
   in order to protect a request sent to a single group member by using
   pairwise keying material (see Section 2.3).  This prevents the attack
   discussed above by construction, as only the intended server is able
   to derive the pairwise keying material used by the client to protect
   the request.  A client supporting the pairwise mode SHOULD use it to
   protect requests sent to a single group member over unicast, instead
   of using the group mode.  For an example where this is not fulfilled,
   see Section 6.2.1 in
   [I-D.tiloca-core-observe-multicast-notifications].

   With particular reference to block-wise transfers [RFC7959],
   Section 2.3.6 of [I-D.ietf-core-groupcomm-bis] points out that, while
   an initial request including the CoAP Block2 option can be sent over
   multicast, any other request in a transfer has to occur over unicast,
   individually addressing the servers in the group.

   Additional considerations are discussed in Appendix E.3, with respect
   to requests including a CoAP Echo Option
   [I-D.ietf-core-echo-request-tag] that has to be sent over unicast, as
   a challenge-response method for servers to achieve synchronization of
   clients' Sender Sequence Number.

10.8.  End-to-end Protection

   The same considerations from Section 12.1 of [RFC8613] hold for Group
   OSCORE.

   Additionally, (D)TLS and Group OSCORE can be combined for protecting
   message exchanges occurring over unicast.  However, it is not
   possible to combine (D)TLS and Group OSCORE for protecting message
   exchanges where messages are (also) sent over multicast.

10.9.  Master Secret

   Group OSCORE derives the Security Context using the same construction
   as OSCORE, and by using the Group Identifier of a group as the
   related ID Context.  Hence, the same required properties of the



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   Security Context parameters discussed in Section 3.3 of [RFC8613]
   hold for this document.

   With particular reference to the OSCORE Master Secret, it has to be
   kept secret among the members of the respective OSCORE group and the
   Group Manager responsible for that group.  Also, the Master Secret
   must have a good amount of randomness, and the Group Manager can
   generate it offline using a good random number generator.  This
   includes the case where the Group Manager rekeys the group by
   generating and distributing a new Master Secret.  Randomness
   requirements for security are described in [RFC4086].

10.10.  Replay Protection

   As in OSCORE, also Group OSCORE relies on sender sequence numbers
   included in the COSE message field 'Partial IV' and used to build
   AEAD nonces.

   Note that the Partial IV of an endpoint does not necessarily grow
   monotonically.  For instance, upon exhaustion of the endpoint Sender
   Sequence Number, the Partial IV also gets exhausted.  As discussed in
   Section 2.4.3, this results either in the endpoint being individually
   rekeyed and getting a new Sender ID, or in the establishment of a new
   Security Context in the group.  Therefore, uniqueness of (key, nonce)
   pairs (see Section 10.2) is preserved also when a new Security
   Context is established.

   As discussed in Section 6.1, an endpoint that has just joined a group
   is exposed to replay attack, as it is not aware of the Sender
   Sequence Numbers currently used by other group members.  Appendix E
   describes how endpoints can synchronize with others' Sender Sequence
   Number.

   Unless exchanges in a group rely only on unicast messages, Group
   OSCORE cannot be used with reliable transport.  Thus, unless only
   unicast messages are sent in the group, it cannot be defined that
   only messages with sequence numbers that are equal to the previous
   sequence number + 1 are accepted.

   The processing of response messages described in Section 2.3.1 of
   [I-D.ietf-core-groupcomm-bis] also ensures that a client accepts a
   single valid response to a given request from each replying server,
   unless CoAP observation is used.








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10.11.  Client Aliveness

   As discussed in Section 12.5 of [RFC8613], a server may use the CoAP
   Echo Option [I-D.ietf-core-echo-request-tag] to verify the aliveness
   of the client that originated a received request.  This would also
   allow the server to (re-)synchronize with the client's Sender
   Sequence Number, as well as to ensure that the request is fresh and
   has not been replayed or (purposely) delayed, if it is the first one
   received from that client after having joined the group or rebooted
   (see Appendix E.3).

10.12.  Cryptographic Considerations

   The same considerations from Section 12.6 of [RFC8613] about the
   maximum Sender Sequence Number hold for Group OSCORE.

   As discussed in Section 2.4.2, an endpoint that experiences an
   exhaustion of its own Sender Sequence Numbers MUST NOT protect
   further messages including a Partial IV, until it has derived a new
   Sender Context.  This prevents the endpoint to reuse the same AEAD
   nonces with the same Sender Key.

   In order to renew its own Sender Context, the endpoint SHOULD inform
   the Group Manager, which can either renew the whole Security Context
   by means of group rekeying, or provide only that endpoint with a new
   Sender ID value.  In either case, the endpoint derives a new Sender
   Context, and in particular a new Sender Key.

   Additionally, the same considerations from Section 12.6 of [RFC8613]
   hold for Group OSCORE, about building the AEAD nonce and the secrecy
   of the Security Context parameters.

   The EdDSA signature algorithm and the elliptic curve Ed25519
   [RFC8032] are mandatory to implement.  For endpoints that support the
   pairwise mode, the ECDH-SS + HKDF-256 algorithm specified in
   Section 6.3.1 of [I-D.ietf-cose-rfc8152bis-algs] and the X25519 curve
   [RFC7748] are also mandatory to implement.

   Constrained IoT devices may alternatively represent Montgomery curves
   and (twisted) Edwards curves [RFC7748] in the short-Weierstrass form
   Wei25519, with which the algorithms ECDSA25519 and ECDH25519 can be
   used for signature operations and Diffie-Hellman secret calculation,
   respectively [I-D.ietf-lwig-curve-representations].

   For many constrained IoT devices, it is problematic to support more
   than one signature algorithm or multiple whole cipher suites.  As a
   consequence, some deployments using, for instance, ECDSA with NIST




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   P-256 may not support the mandatory signature algorithm but that
   should not be an issue for local deployments.

   The derivation of pairwise keys defined in Section 2.3.1 is
   compatible with ECDSA and EdDSA asymmetric keys, but is not
   compatible with RSA asymmetric keys.  The security of using the same
   key pair for Diffie-Hellman and for signing is demonstrated in
   [Degabriele].

10.13.  Message Segmentation

   The same considerations from Section 12.7 of [RFC8613] hold for Group
   OSCORE.

10.14.  Privacy Considerations

   Group OSCORE ensures end-to-end integrity protection and encryption
   of the message payload and all options that are not used for proxy
   operations.  In particular, options are processed according to the
   same class U/I/E that they have for OSCORE.  Therefore, the same
   privacy considerations from Section 12.8 of [RFC8613] hold for Group
   OSCORE.

   Furthermore, the following privacy considerations hold, about the
   OSCORE option that may reveal information on the communicating
   endpoints.

   o  The 'kid' parameter, which is intended to help a recipient
      endpoint to find the right Recipient Context, may reveal
      information about the Sender Endpoint.  Since both requests and
      responses always include the 'kid' parameter, this may reveal
      information about both a client sending a group request and all
      the possibly replying servers sending their own individual
      response.

   o  The 'kid context' parameter, which is intended to help a recipient
      endpoint to find the right Security Context, reveals information
      about the sender endpoint.  In particular, it reveals that the
      sender endpoint is a member of a particular OSCORE group, whose
      current Group ID is indicated in the 'kid context' parameter.

   When receiving a group request, each of the recipient endpoints can
   reply with a response that includes its Sender ID as 'kid' parameter.
   All these responses will be matchable with the request through the
   Token.  Thus, even if these responses do not include a 'kid context'
   parameter, it becomes possible to understand that the responder
   endpoints are in the same group of the requester endpoint.




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   Furthermore, using the mechanisms described in Appendix E.3 to
   achieve sequence number synchronization with a client may reveal when
   a server device goes through a reboot.  This can be mitigated by the
   server device storing the precise state of the replay window of each
   known client on a clean shutdown.

   Finally, the mechanism described in Section 10.5 to prevent
   collisions of Group Identifiers from different Group Managers may
   reveal information about events in the respective OSCORE groups.  In
   particular, a Group Identifier changes when the corresponding group
   is rekeyed.  Thus, Group Managers might use the shared list of Group
   Identifiers to infer the rate and patterns of group membership
   changes triggering a group rekeying, e.g. due to newly joined members
   or evicted (compromised) members.  In order to alleviate this privacy
   concern, it should be hidden from the Group Managers which exact
   Group Manager has currently assigned which Group Identifiers in its
   OSCORE groups.

11.  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.

11.1.  OSCORE Flag Bits Registry

   IANA is asked to add the following value entry to the "OSCORE Flag
   Bits" subregistry defined in Section 13.7 of [RFC8613] as part of the
   "CoRE Parameters" registry.

  +--------------+------------+----------------------------+-----------+
  | Bit Position |    Name    |         Description        | Reference |
  +--------------+------------+----------------------------+-----------+
  |       2      | Group Flag | Set to 1 if the message is | [This     |
  |              |            | protected with the group   | Document] |
  |              |            | mode of Group OSCORE       |           |
  +--------------+------------+----------------------------+-----------+

12.  References

12.1.  Normative References

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



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

   [I-D.ietf-cbor-7049bis]
              Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", draft-ietf-cbor-7049bis-16 (work
              in progress), September 2020.

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              ietf-core-groupcomm-bis-02 (work in progress), November
              2020.

   [I-D.ietf-cose-countersign]
              Schaad, J. and R. Housley, "CBOR Object Signing and
              Encryption (COSE): Countersignatures", draft-ietf-cose-
              countersign-01 (work in progress), October 2020.

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

   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", draft-ietf-cose-rfc8152bis-
              struct-14 (work in progress), September 2020.

   [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>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.



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   [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>.

   [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>.

   [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>.

   [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>.

12.2.  Informative References

   [Degabriele]
              Degabriele, J., Lehmann, A., Paterson, K., Smart, N., and
              M. Strefler, "On the Joint Security of Encryption and
              Signature in EMV", December 2011,
              <https://eprint.iacr.org/2011/615>.

   [I-D.ietf-ace-key-groupcomm]
              Palombini, F. and M. Tiloca, "Key Provisioning for Group
              Communication using ACE", draft-ietf-ace-key-groupcomm-10
              (work in progress), November 2020.

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", draft-ietf-ace-key-groupcomm-
              oscore-09 (work in progress), November 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-35
              (work in progress), June 2020.




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   [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-10 (work in progress), July 2020.

   [I-D.ietf-lwig-curve-representations]
              Struik, R., "Alternative Elliptic Curve Representations",
              draft-ietf-lwig-curve-representations-12 (work in
              progress), August 2020.

   [I-D.ietf-lwig-security-protocol-comparison]
              Mattsson, J., Palombini, F., and M. Vucinic, "Comparison
              of CoAP Security Protocols", draft-ietf-lwig-security-
              protocol-comparison-04 (work in progress), March 2020.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-38 (work in progress), May
              2020.

   [I-D.mattsson-cfrg-det-sigs-with-noise]
              Mattsson, J., Thormarker, E., and S. Ruohomaa,
              "Deterministic ECDSA and EdDSA Signatures with Additional
              Randomness", draft-mattsson-cfrg-det-sigs-with-noise-02
              (work in progress), March 2020.

   [I-D.somaraju-ace-multicast]
              Somaraju, A., Kumar, S., Tschofenig, H., and W. Werner,
              "Security for Low-Latency Group Communication", draft-
              somaraju-ace-multicast-02 (work in progress), October
              2016.

   [I-D.tiloca-core-observe-multicast-notifications]
              Tiloca, M., Hoeglund, R., Amsuess, C., and F. Palombini,
              "Observe Notifications as CoAP Multicast Responses",
              draft-tiloca-core-observe-multicast-notifications-04 (work
              in progress), November 2020.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.




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   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <https://www.rfc-editor.org/info/rfc6282>.

   [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>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

Appendix A.  Assumptions and Security Objectives

   This section presents a set of assumptions and security objectives
   for the approach described in this document.  The rest of this
   section refers to three types of groups:

   o  Application group, i.e. a set of CoAP endpoints that share a
      common pool of resources.

   o  Security group, as defined in Section 1.1 of this specification.
      There can be a one-to-one or a one-to-many relation between
      security groups and application groups, and vice versa.

   o  CoAP group, as defined in [I-D.ietf-core-groupcomm-bis] i.e. a set
      of CoAP endpoints, where each endpoint is configured to receive
      CoAP multicast requests that are sent to the group's associated IP
      multicast address and UDP port.  An endpoint may be a member of
      multiple CoAP groups.  There can be a one-to-one or a one-to-many
      relation between application groups and CoAP groups.  Note that a
      device sending a CoAP request to a CoAP group is not necessarily
      itself a member of that group: it is a member only if it also has
      a CoAP server endpoint listening to requests for this CoAP group,
      sent to the associated IP multicast address and port.  In order to
      provide secure group communication, all members of a CoAP group as



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      well as all further endpoints configured only as clients sending
      CoAP (multicast) requests to the CoAP group have to be member of a
      security group.  There can be a one-to-one or a one-to-many
      relation between security groups and CoAP groups, and vice versa.

A.1.  Assumptions

   The following assumptions are assumed to be already addressed and are
   out of the scope of this document.

   o  Multicast communication topology: this document considers both
      1-to-N (one sender and multiple recipients) and M-to-N (multiple
      senders and multiple recipients) communication topologies.  The
      1-to-N communication topology is the simplest group communication
      scenario that would serve the needs of a typical Low-power and
      Lossy Network (LLN).  Examples of use cases that benefit from
      secure group communication are provided in Appendix B.

      In a 1-to-N communication model, only a single client transmits
      data to the CoAP group, in the form of request messages; in an
      M-to-N communication model (where M and N do not necessarily have
      the same value), M clients transmit data to the CoAP group.
      According to [I-D.ietf-core-groupcomm-bis], any possible proxy
      entity is supposed to know about the clients and to not perform
      aggregation of response messages from multiple servers.  Also,
      every client expects and is able to handle multiple response
      messages associated to a same request sent to the CoAP group.

   o  Group size: security solutions for group communication should be
      able to adequately support different and possibly large security
      groups.  The group size is the current number of members in a
      security group.  In the use cases mentioned in this document, the
      number of clients (normally the controlling devices) is expected
      to be much smaller than the number of servers (i.e. the controlled
      devices).  A security solution for group communication that
      supports 1 to 50 clients would be able to properly cover the group
      sizes required for most use cases that are relevant for this
      document.  The maximum group size is expected to be in the range
      of 2 to 100 devices.  Security groups larger than that should be
      divided into smaller independent groups.

   o  Communication with the Group Manager: an endpoint must use a
      secure dedicated channel when communicating with the Group
      Manager, also when not registered as a member of the security
      group.

   o  Provisioning and management of Security Contexts: a Security
      Context must be established among the members of the security



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      group.  A secure mechanism must be used to generate, revoke and
      (re-)distribute keying material, communication policies and
      security parameters in the security group.  The actual
      provisioning and management of the Security Context is out of the
      scope of this document.

   o  Multicast data security ciphersuite: all members of a security
      group must agree on a ciphersuite to provide authenticity,
      integrity and confidentiality of messages in the group.  The
      ciphersuite is specified as part of the Security Context.

   o  Backward security: a new device joining the security group should
      not have access to any old Security Contexts used before its
      joining.  This ensures that a new member of the security group is
      not able to decrypt confidential data sent before it has joined
      the security group.  The adopted key management scheme should
      ensure that the Security Context is updated to ensure backward
      confidentiality.  The actual mechanism to update the Security
      Context and renew the group keying material in the security group
      upon a new member's joining has to be defined as part of the group
      key management scheme.

   o  Forward security: entities that leave the security group should
      not have access to any future Security Contexts or message
      exchanged within the security group after their leaving.  This
      ensures that a former member of the security group is not able to
      decrypt confidential data sent within the security group anymore.
      Also, it ensures that a former member is not able to send
      protected messages to the security group anymore.  The actual
      mechanism to update the Security Context and renew the group
      keying material in the security group upon a member's leaving has
      to be defined as part of the group key management scheme.

A.2.  Security Objectives

   The approach described in this document aims at fulfilling the
   following security objectives:

   o  Data replay protection: group request messages or response
      messages replayed within the security group must be detected.

   o  Data confidentiality: messages sent within the security group
      shall be encrypted.

   o  Group-level data confidentiality: the group mode provides group-
      level data confidentiality since messages are encrypted at a group
      level, i.e. in such a way that they can be decrypted by any member




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      of the security group, but not by an external adversary or other
      external entities.

   o  Pairwise data confidentiality: the pairwise mode especially
      provides pairwise data confidentiality, since messages are
      encrypted using pairwise keying material shared between any two
      group members, hence they can be decrypted only by the intended
      single recipient.

   o  Source message authentication: messages sent within the security
      group shall be authenticated.  That is, it is essential to ensure
      that a message is originated by a member of the security group in
      the first place, and in particular by a specific, identifiable
      member of the security group.

   o  Message integrity: messages sent within the security group shall
      be integrity protected.  That is, it is essential to ensure that a
      message has not been tampered with, either by a group member, or
      by an external adversary or other external entities which are not
      members of the security group.

   o  Message ordering: it must be possible to determine the ordering of
      messages coming from a single sender.  In accordance with OSCORE
      [RFC8613], this results in providing absolute freshness of
      responses that are not notifications, as well as relative
      freshness of group requests and notification responses.  It is not
      required to determine ordering of messages from different senders.

Appendix B.  List of Use Cases

   Group Communication for CoAP [I-D.ietf-core-groupcomm-bis] provides
   the necessary background for multicast-based CoAP communication, with
   particular reference to low-power and lossy networks (LLNs) and
   resource constrained environments.  The interested reader is
   encouraged to first read [I-D.ietf-core-groupcomm-bis] to understand
   the non-security related details.  This section discusses a number of
   use cases that benefit from secure group communication, and refers to
   the three types of groups from Appendix A.  Specific security
   requirements for these use cases are discussed in Appendix A.

   o  Lighting control: consider a building equipped with IP-connected
      lighting devices, switches, and border routers.  The lighting
      devices acting as servers are organized into application groups
      and CoAP groups, according to their physical location in the
      building.  For instance, lighting devices in a room or corridor
      can be configured as members of a single application group and
      corresponding CoAP group.  Those ligthing devices together with
      the switches acting as clients in the same room or corridor can be



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      configured as members of the corresponding security group.
      Switches are then used to control the lighting devices by sending
      on/off/dimming commands to all lighting devices in the CoAP group,
      while border routers connected to an IP network backbone (which is
      also multicast-enabled) can be used to interconnect routers in the
      building.  Consequently, this would also enable logical groups to
      be formed even if devices with a role in the lighting application
      may be physically in different subnets (e.g. on wired and wireless
      networks).  Connectivity between lighting devices may be realized,
      for instance, by means of IPv6 and (border) routers supporting
      6LoWPAN [RFC4944][RFC6282].  Group communication enables
      synchronous operation of a set of connected lights, ensuring that
      the light preset (e.g. dimming level or color) of a large set of
      luminaires are changed at the same perceived time.  This is
      especially useful for providing a visual synchronicity of light
      effects to the user.  As a practical guideline, events within a
      200 ms interval are perceived as simultaneous by humans, which is
      necessary to ensure in many setups.  Devices may reply back to the
      switches that issue on/off/dimming commands, in order to report
      about the execution of the requested operation (e.g.  OK, failure,
      error) and their current operational status.  In a typical
      lighting control scenario, a single switch is the only entity
      responsible for sending commands to a set of lighting devices.  In
      more advanced lighting control use cases, a M-to-N communication
      topology would be required, for instance in case multiple sensors
      (presence or day-light) are responsible to trigger events to a set
      of lighting devices.  Especially in professional lighting
      scenarios, the roles of client and server are configured by the
      lighting commissioner, and devices strictly follow those roles.

   o  Integrated building control: enabling Building Automation and
      Control Systems (BACSs) to control multiple heating, ventilation
      and air-conditioning units to pre-defined presets.  Controlled
      units can be organized into application groups and CoAP groups in
      order to reflect their physical position in the building, e.g.
      devices in the same room can be configured as members of a single
      application group and corresponding CoAP group.  As a practical
      guideline, events within intervals of seconds are typically
      acceptable.  Controlled units are expected to possibly reply back
      to the BACS issuing control commands, in order to report about the
      execution of the requested operation (e.g.  OK, failure, error)
      and their current operational status.

   o  Software and firmware updates: software and firmware updates often
      comprise quite a large amount of data.  This can overload a Low-
      power and Lossy Network (LLN) that is otherwise typically used to
      deal with only small amounts of data, on an infrequent base.
      Rather than sending software and firmware updates as unicast



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      messages to each individual device, multicasting such updated data
      to a larger set of devices at once displays a number of benefits.
      For instance, it can significantly reduce the network load and
      decrease the overall time latency for propagating this data to all
      devices.  Even if the complete whole update process itself is
      secured, securing the individual messages is important, in case
      updates consist of relatively large amounts of data.  In fact,
      checking individual received data piecemeal for tampering avoids
      that devices store large amounts of partially corrupted data and
      that they detect tampering hereof only after all data has been
      received.  Devices receiving software and firmware updates are
      expected to possibly reply back, in order to provide a feedback
      about the execution of the update operation (e.g.  OK, failure,
      error) and their current operational status.

   o  Parameter and configuration update: by means of multicast
      communication, it is possible to update the settings of a set of
      similar devices, both simultaneously and efficiently.  Possible
      parameters are related, for instance, to network load management
      or network access controls.  Devices receiving parameter and
      configuration updates are expected to possibly reply back, to
      provide a feedback about the execution of the update operation
      (e.g.  OK, failure, error) and their current operational status.

   o  Commissioning of Low-power and Lossy Network (LLN) systems: a
      commissioning device is responsible for querying all devices in
      the local network or a selected subset of them, in order to
      discover their presence, and be aware of their capabilities,
      default configuration, and operating conditions.  Queried devices
      displaying similarities in their capabilities and features, or
      sharing a common physical location can be configured as members of
      a single application group and corresponding CoAP group.  Queried
      devices are expected to reply back to the commissioning device, in
      order to notify their presence, and provide the requested
      information and their current operational status.

   o  Emergency multicast: a particular emergency related information
      (e.g. natural disaster) is generated and multicast by an emergency
      notifier, and relayed to multiple devices.  The latter may reply
      back to the emergency notifier, in order to provide their feedback
      and local information related to the ongoing emergency.  This kind
      of setups should additionally rely on a fault tolerance multicast
      algorithm, such as Multicast Protocol for Low-Power and Lossy
      Networks (MPL).







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Appendix C.  Example of Group Identifier Format

   This section provides an example of how the Group Identifier (Gid)
   can be specifically formatted.  That is, the Gid can be composed of
   two parts, namely a Group Prefix and a Group Epoch.

   For each group, the Group Prefix is constant over time and is
   uniquely defined in the set of all the groups associated to the same
   Group Manager.  The choice of the Group Prefix for a given group's
   Security Context is application specific.  The size of the Group
   Prefix directly impact on the maximum number of distinct groups under
   the same Group Manager.

   The Group Epoch is set to 0 upon the group's initialization, and is
   incremented by 1 each time new keying material, together with a new
   Gid, is distributed to the group in order to establish a new Security
   Context (see Section 3.1).

   As an example, a 3-byte Gid can be composed of: i) a 1-byte Group
   Prefix '0xb1' interpreted as a raw byte string; and ii) a 2-byte
   Group Epoch interpreted as an unsigned integer ranging from 0 to
   65535.  Then, after having established the Common Context 61532 times
   in the group, its Gid will assume value '0xb1f05c'.

   Using an immutable Group Prefix for a group assumes that enough time
   elapses before all possible Group Epoch values are used, since the
   Group Manager does not reassign the same Gid to the same group.
   Thus, the expected highest rate for addition/removal of group members
   and consequent group rekeying should be taken into account for a
   proper dimensioning of the Group Epoch size.

   As discussed in Section 10.5, if endpoints are deployed in multiple
   groups managed by different non-synchronized Group Managers, it is
   possible that Group Identifiers of different groups coincide at some
   point in time.  In this case, a recipient has to handle coinciding
   Group Identifiers, and has to try using different Security Contexts
   to process an incoming message, until the right one is found and the
   message is correctly verified.  Therefore, it is favourable that
   Group Identifiers from different Group Managers have a size that
   result in a small probability of collision.  How small this
   probability should be is up to system designers.

Appendix D.  Set-up of New Endpoints

   An endpoint joins a group by explicitly interacting with the
   responsible Group Manager.  When becoming members of a group,
   endpoints are not required to know how many and what endpoints are in
   the same group.



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   Communications between a joining endpoint and the Group Manager rely
   on the CoAP protocol and must be secured.  Specific details on how to
   secure communications between joining endpoints and a Group Manager
   are out of the scope of this document.

   The Group Manager must verify that the joining endpoint is authorized
   to join the group.  To this end, the Group Manager can directly
   authorize the joining endpoint, or expect it to provide authorization
   evidence previously obtained from a trusted entity.  Further details
   about the authorization of joining endpoints are out of scope.

   In case of successful authorization check, the Group Manager
   generates a Sender ID assigned to the joining endpoint, before
   proceeding with the rest of the join process.  That is, the Group
   Manager provides the joining endpoint with the keying material and
   parameters to initialize the Security Context (see Section 2).  The
   actual provisioning of keying material and parameters to the joining
   endpoint is out of the scope of this document.

   It is RECOMMENDED that the join process adopts the approach described
   in [I-D.ietf-ace-key-groupcomm-oscore] and based on the ACE framework
   for Authentication and Authorization in constrained environments
   [I-D.ietf-ace-oauth-authz].

Appendix E.  Examples of Synchronization Approaches

   This section describes three possible approaches that can be
   considered by server endpoints to synchronize with Sender Sequence
   Numbers of client endpoints sending group requests.

   The Group Manager MAY indicate which of such approaches are used in
   the group, as part of the group communication policies signalled to
   candidate group members upon their group joining.

   If a server has recently lost the mutable Security Context, e.g. due
   to a reboot, the server has also to establish an updated Security
   Context before resuming to send protected messages to the group (see
   Section 2.4.1).  Since this results in deriving a new Sender Key for
   its Sender Context, the server does not reuse the same pair (key,
   nonce), even when using the Partial IV of (old re-injected) requests
   to build the AEAD nonce for protecting the corresponding responses.

E.1.  Best-Effort Synchronization

   Upon receiving a group request from a client, a server does not take
   any action to synchronize with the Sender Sequence Number of that
   client.  This provides no assurance at all as to message freshness,
   which can be acceptable in non-critical use cases.



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   With the notable exception of Observe notifications and responses
   following a group rekeying, it is optional for the server to use its
   Sender Sequence Number as Partial IV when protecting a response.
   Instead, for efficiency reasons, the server may rather use the
   request's Partial IV when protecting a response to that request.

E.2.  Baseline Synchronization

   Upon receiving a group request from a given client for the first
   time, a server initializes the last-seen Sender Sequence Number
   associated to that client in its corresponding Recipient Context.
   The server may also drop the group request without delivering it to
   the application.  This method provides a reference point to identify
   if future group requests from the same client are fresher than the
   last one received.

   A replay time interval exists, between when a possibly replayed or
   delayed message is originally transmitted by a given client and the
   first authentic fresh message from that same client is received.
   This can be acceptable for use cases where servers admit such a
   trade-off between performance and assurance of message freshness.

   With the notable exception of Observe notifications and responses
   following a group rekeying, it is optional for the server to use its
   Sender Sequence Number as Partial IV when protecting a response.
   Instead, for efficiency reasons, the server may rather use the
   request's Partial IV when protecting a response to that request.

E.3.  Challenge-Response Synchronization

   A server performs a challenge-response exchange with a client, by
   using the Echo Option for CoAP described in Section 2 of
   [I-D.ietf-core-echo-request-tag] and according to Appendix B.1.2 of
   [RFC8613].

   That is, upon receiving a group request from a particular client for
   the first time, the server processes the message as described in this
   specification, but, even if valid, does not deliver it to the
   application.  Instead, the server replies to the client with an
   OSCORE protected 4.01 (Unauthorized) response message, including only
   the Echo Option and no diagnostic payload.  The server MUST NOT set
   the Echo Option to a value which is both predictable and reusable.
   Since this response is protected with the Security Context used in
   the group, the client will consider the response valid upon
   successfully decrypting and verifying it.

   The server stores the Echo Option value included therein, together
   with the pair (gid,kid), where 'gid' is the Group Identifier of the



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   OSCORE group and 'kid' is the Sender ID of the client in the group,
   as specified in the 'kid context' and 'kid' fields of the OSCORE
   Option of the group request, respectively.  After a group rekeying
   has been completed and a new Security Context has been established in
   the group, which results also in a new Group Identifier (see
   Section 3.1), the server MUST delete all the stored Echo values
   associated to members of that group.

   Upon receiving a 4.01 (Unauthorized) response that includes an Echo
   Option and originates from a verified group member, a client sends a
   request as a unicast message addressed to the same server, echoing
   the Echo Option value.  The client MUST NOT send the request
   including the Echo Option over multicast.

   In particular, the client does not necessarily resend the same group
   request, but can instead send a more recent one, if the application
   permits it.  This makes it possible for the client to not retain
   previously sent group requests for full retransmission, unless the
   application explicitly requires otherwise.  In either case, the
   client uses a fresh Sender Sequence Number value from its own Sender
   Context.  If the client stores group requests for possible
   retransmission with the Echo Option, it should not store a given
   request for longer than a pre-configured time interval.  Note that
   the unicast request echoing the Echo Option is correctly treated and
   processed as a message, since the 'kid context' field including the
   Group Identifier of the OSCORE group is still present in the OSCORE
   Option as part of the COSE object (see Section 4).

   Upon receiving the unicast request including the Echo Option, the
   server performs the following verifications.

   o  If the server does not store an Echo Option value for the pair
      (gid,kid), it considers: i) the time t1 when it has established
      the Security Context used to protect the received request; and ii)
      the time t2 when the request has been received.  Since a valid
      request cannot be older than the Security Context used to protect
      it, the server verifies that (t2 - t1) is less than the largest
      amount of time acceptable to consider the request fresh.

   o  If the server stores an Echo Option value for the pair (gid,kid)
      associated to that same client in the same group, the server
      verifies that the option value equals that same stored value
      previously sent to that client.

   If the verifications above fail, the server MUST NOT process the
   request further and MAY send a 4.01 (Unauthorized) response including
   an Echo Option.




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   In case of positive verification, the request is further processed
   and verified.  Finally, the server updates the Recipient Context
   associated to that client, by setting the Replay Window according to
   the Sender Sequence Number from the unicast request conveying the
   Echo Option.  The server either delivers the request to the
   application if it is an actual retransmission of the original one, or
   discards it otherwise.  Mechanisms to signal whether the resent
   request is a full retransmission of the original one are out of the
   scope of this specification.

   A server should not deliver group requests from a given client to the
   application until one valid request from that same client has been
   verified as fresh, as conveying an echoed Echo Option
   [I-D.ietf-core-echo-request-tag].  Also, a server may perform the
   challenge-response described above at any time, if synchronization
   with Sender Sequence Numbers of clients is (believed to be) lost, for
   instance after a device reboot.  A client has to be always ready to
   perform the challenge-response based on the Echo Option in case a
   server starts it.

   It is the role of the server application to define under what
   circumstances Sender Sequence Numbers lose synchronization.  This can
   include experiencing a "large enough" gap D = (SN2 - SN1), between
   the Sender Sequence Number SN1 of the latest accepted group request
   from a client and the Sender Sequence Number SN2 of a group request
   just received from that client.  However, a client may send several
   unicast requests to different group members as protected with the
   pairwise mode (see Section 9.2), which may result in the server
   experiencing the gap D in a relatively short time.  This would induce
   the server to perform more challenge-response exchanges than actually
   needed.

   To ameliorate this, the server may rather rely on a trade-off between
   the Sender Sequence Number gap D and a time gap T = (t2 - t1), where
   t1 is the time when the latest group request from a client was
   accepted and t2 is the time when the latest group request from that
   client has been received, respectively.  Then, the server can start a
   challenge-response when experiencing a time gap T larger than a
   given, pre-configured threshold.  Also, the server can start a
   challenge-response when experiencing a Sender Sequence Number gap D
   greater than a different threshold, computed as a monotonically
   increasing function of the currently experienced time gap T.

   The challenge-response approach described in this appendix provides
   an assurance of absolute message freshness.  However, it can result
   in an impact on performance which is undesirable or unbearable,
   especially in large groups where many endpoints at the same time
   might join as new members or lose synchronization.



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   Note that endpoints configured as silent servers are not able to
   perform the challenge-response described above, as they do not store
   a Sender Context to secure the 4.01 (Unauthorized) response to the
   client.  Therefore, silent servers should adopt alternative
   approaches to achieve and maintain synchronization with sender
   sequence numbers of clients.

   Since requests including the Echo Option are sent over unicast, a
   server can be a victim of the attack discussed in Section 10.7, when
   such requests are protected with the group mode of Group OSCORE, as
   described in Section 8.1.

   Instead, protecting requests with the Echo Option by using the
   pairwise mode of Group OSCORE as described in Section 9.2 prevents
   the attack in Section 10.7.  In fact, only the exact server involved
   in the Echo exchange is able to derive the correct pairwise key used
   by the client to protect the request including the Echo Option.

   In either case, an internal on-path adversary would not be able to
   mix up the Echo Option value of two different unicast requests, sent
   by a same client to any two different servers in the group.  In fact,
   if the group mode was used, this would require the adversary to forge
   the client's counter signature in both such requests.  As a
   consequence, each of the two servers remains able to selectively
   accept a request with the Echo Option only if it is waiting for that
   exact integrity-protected Echo Option value, and is thus the intended
   recipient.

Appendix F.  No Verification of Signatures in Group Mode

   There are some application scenarios using group communication that
   have particularly strict requirements.  One example of this is the
   requirement of low message latency in non-emergency lighting
   applications [I-D.somaraju-ace-multicast].  For those applications
   which have tight performance constraints and relaxed security
   requirements, it can be inconvenient for some endpoints to verify
   digital signatures in order to assert source authenticity of received
   messages protected with the group mode.  In other cases, the
   signature verification can be deferred or only checked for specific
   actions.  For instance, a command to turn a bulb on where the bulb is
   already on does not need the signature to be checked.  In such
   situations, the counter signature needs to be included anyway as part
   of a message protected with the group mode, so that an endpoint that
   needs to validate the signature for any reason has the ability to do
   so.

   In this specification, it is NOT RECOMMENDED that endpoints do not
   verify the counter signature of received messages protected with the



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   group mode.  However, it is recognized that there may be situations
   where it is not always required.  The consequence of not doing the
   signature validation in messages protected with the group mode is
   that security in the group is based only on the group-authenticity of
   the shared keying material used for encryption.  That is, endpoints
   in the group would have evidence that the received message has been
   originated by a group member, although not specifically identifiable
   in a secure way.  This can violate a number of security requirements,
   as the compromise of any element in the group means that the attacker
   has the ability to control the entire group.  Even worse, the group
   may not be limited in scope, and hence the same keying material might
   be used not only for light bulbs but for locks as well.  Therefore,
   extreme care must be taken in situations where the security
   requirements are relaxed, so that deployment of the system will
   always be done safely.

Appendix G.  Example Values with COSE Capabilities

   The table below provides examples of values for Counter Signature
   Parameters in the Common Context (see Section 2.1.3), for different
   values of Counter Signature Algorithm.

    +-------------------+---------------------------------------------+
    | Counter Signature | Example Values for Counter                  |
    | Algorithm         | Signature Parameters                        |
    +-------------------+---------------------------------------------+
    |  (-8)   // EdDSA  | [1], [1, 6]  // 1: OKP ; 1: OKP, 6: Ed25519 |
    |  (-8)   // EdDSA  | [1], [1, 6]  // 1: OKP ; 1: OKP, 7: Ed448   |
    |  (-7)   // ES256  | [2], [2, 1]  // 2: EC2 ; 2: EC2, 1: P-256   |
    |  (-35)  // ES384  | [2], [2, 2]  // 2: EC2 ; 2: EC2, 2: P-384   |
    |  (-36)  // ES512  | [2], [2, 3]  // 2: EC2 ; 2: EC2, 3: P-512   |
    |  (-37)  // PS256  | [], [3]      // empty  ; 3: RSA             |
    |  (-38)  // PS384  | [], [3]      // empty  ; 3: RSA             |
    |  (-39)  // PS512  | [], [3]      // empty  ; 3: RSA             |
    +-------------------+---------------------------------------------+

            Figure 4: Examples of Counter Signature Parameters

   The table below provides examples of values for Secret Derivation
   Parameters in the Common Context (see Section 2.1.5), for different
   values of Secret Derivation Algorithm.










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  +-----------------------+--------------------------------------------+
  | Secret Derivation     | Example Values for Secret                  |
  | Algorithm             | Derivation Parameters                      |
  +-----------------------+--------------------------------------------+
  |  (-27)  // ECDH-SS    | [1], [1, 6]  // 1: OKP ; 1: OKP, 4: X25519 |
  |         // + HKDF-256 |                                            |
  |  (-27)  // ECDH-SS    | [1], [1, 6]  // 1: OKP ; 1: OKP, 5: X448   |
  |         // + HKDF-256 |                                            |
  |  (-27)  // ECDH-SS    | [2], [2, 1]  // 2: EC2 ; 2: EC2, 1: P-256  |
  |         // + HKDF-256 |                                            |
  |  (-27)  // ECDH-SS    | [2], [2, 2]  // 2: EC2 ; 2: EC2, 2: P-384  |
  |         // + HKDF-256 |                                            |
  |  (-27)  // ECDH-SS    | [2], [2, 3]  // 2: EC2 ; 2: EC2, 3: P-512  |
  |         // + HKDF-256 |                                            |
  +-----------------------+--------------------------------------------+

            Figure 5: Examples of Secret Derivation Parameters

   The table below provides examples of values for the
   'par_countersign_key' element of the 'algorithms' array used in the
   two external_aad structures (see Section 4.3.1 and Section 4.3.2),
   for different values of Counter Signature Algorithm.

          +-------------------+---------------------------------+
          | Counter Signature | Example Values for              |
          | Algorithm         | 'par_countersign_key'           |
          +-------------------+---------------------------------+
          | (-8)    // EdDSA  | [1, 6]   // 1: OKP , 6: Ed25519 |
          | (-8)    // EdDSA  | [1, 6]   // 1: OKP , 7: Ed448   |
          | (-7)    // ES256  | [2, 1]   // 2: EC2 , 1: P-256   |
          | (-35)   // ES384  | [2, 2]   // 2: EC2 , 2: P-384   |
          | (-36)   // ES512  | [2, 3]   // 2: EC2 , 3: P-512   |
          | (-37)   // PS256  | [3]      // 3: RSA              |
          | (-38)   // PS384  | [3]      // 3: RSA              |
          | (-39)   // PS512  | [3]      // 3: RSA              |
          +-------------------+---------------------------------+

                Figure 6: Examples of 'par_countersign_key'

Appendix H.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

H.1.  Version -09 to -10

   o  Removed 'Counter Signature Key Parameters' from the Common
      Context.




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   o  New parameters in the Common Context covering the DH secret
      derivation.

   o  New counter signature header parameter from draft-ietf-cose-
      countersign.

   o  Stronger policies non non-recycling of Sender IDs and Gid.

   o  The Sender Sequence Number is reset when establishing a new
      Security Context.

   o  Added 'request_kid_context' in the aad_array.

   o  The server can respond with 5.03 if the client's public key is not
      available.

   o  The observer client stores an invariant identifier of the group.

   o  Relaxed storing of original 'kid' for observer clients.

   o  Both client and server store the 'kid_context' of the original
      observation request.

   o  The server uses a fresh PIV if protecting the response with a
      Security Context different from the one used to protect the
      request.

   o  Clarifications on MTI algorithms and curves.

   o  Removed optimized requests.

   o  Overall clarifications and editorial revision.

H.2.  Version -08 to -09

   o  Pairwise keys are discarded after group rekeying.

   o  Signature mode renamed to group mode.

   o  The parameters for countersignatures use the updated COSE
      registries.  Newly defined IANA registries have been removed.

   o  Pairwise Flag bit renamed as Group Flag bit, set to 1 in group
      mode and set to 0 in pairwise mode.

   o  Dedicated section on updating the Security Context.





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   o  By default, sender sequence numbers and replay windows are not
      reset upon group rekeying.

   o  An endpoint implementing only a silent server does not support the
      pairwise mode.

   o  Separate section on general message reception.

   o  Pairwise mode moved to the document body.

   o  Considerations on using the pairwise mode in non-multicast
      settings.

   o  Optimized requests are moved as an appendix.

   o  Normative support for the signature and pairwise mode.

   o  Revised methods for synchronization with clients' sender sequence
      number.

   o  Appendix with example values of parameters for countersignatures.

   o  Clarifications and editorial improvements.

H.3.  Version -07 to -08

   o  Clarified relation between pairwise mode and group communication
      (Section 1).

   o  Improved definition of "silent server" (Section 1.1).

   o  Clarified when a Recipient Context is needed (Section 2).

   o  Signature checkers as entities supported by the Group Manager
      (Section 2.3).

   o  Clarified that the Group Manager is under exclusive control of Gid
      and Sender ID values in a group, with Sender ID values under each
      Gid value (Section 2.3).

   o  Mitigation policies in case of recycled 'kid' values
      (Section 2.4).

   o  More generic exhaustion (not necessarily wrap-around) of sender
      sequence numbers (Sections 2.5 and 10.11).

   o  Pairwise key considerations, as to group rekeying and Sender
      Sequence Numbers (Section 3).



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   o  Added reference to static-static Diffie-Hellman shared secret
      (Section 3).

   o  Note for implementation about the external_aad for signing
      (Sectino 4.3.2).

   o  Retransmission by the application for group requests over
      multicast as Non-Confirmable (Section 7).

   o  A server MUST use its own Partial IV in a response, if protecting
      it with a different context than the one used for the request
      (Section 7.3).

   o  Security considerations: encryption of pairwise mode as
      alternative to group-level security (Section 10.1).

   o  Security considerations: added approach to reduce the chance of
      global collisions of Gid values from different Group Managers
      (Section 10.5).

   o  Security considerations: added implications for block-wise
      transfers when using the signature mode for requests over unicast
      (Section 10.7).

   o  Security considerations: (multiple) supported signature algorithms
      (Section 10.13).

   o  Security considerations: added privacy considerations on the
      approach for reducing global collisions of Gid values
      (Section 10.15).

   o  Updates to the methods for synchronizing with clients' sequence
      number (Appendix E).

   o  Simplified text on discovery services supporting the pairwise mode
      (Appendix G.1).

   o  Editorial improvements.

H.4.  Version -06 to -07

   o  Updated abstract and introduction.

   o  Clarifications of what pertains a group rekeying.

   o  Derivation of pairwise keying material.





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   o  Content re-organization for COSE Object and OSCORE header
      compression.

   o  Defined the Pairwise Flag bit for the OSCORE option.

   o  Supporting CoAP Observe for group requests and responses.

   o  Considerations on message protection across switching to new
      keying material.

   o  New optimized mode based on pairwise keying material.

   o  More considerations on replay protection and Security Contexts
      upon key renewal.

   o  Security considerations on Group OSCORE for unicast requests, also
      as affecting the usage of the Echo option.

   o  Clarification on different types of groups considered
      (application/security/CoAP).

   o  New pairwise mode, using pairwise keying material for both
      requests and responses.

H.5.  Version -05 to -06

   o  Group IDs mandated to be unique under the same Group Manager.

   o  Clarifications on parameter update upon group rekeying.

   o  Updated external_aad structures.

   o  Dynamic derivation of Recipient Contexts made optional and
      application specific.

   o  Optional 4.00 response for failed signature verification on the
      server.

   o  Removed client handling of duplicated responses to multicast
      requests.

   o  Additional considerations on public key retrieval and group
      rekeying.

   o  Added Group Manager responsibility on validating public keys.

   o  Updates IANA registries.




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   o  Reference to RFC 8613.

   o  Editorial improvements.

H.6.  Version -04 to -05

   o  Added references to draft-dijk-core-groupcomm-bis.

   o  New parameter Counter Signature Key Parameters (Section 2).

   o  Clarification about Recipient Contexts (Section 2).

   o  Two different external_aad for encrypting and signing
      (Section 3.1).

   o  Updated response verification to handle Observe notifications
      (Section 6.4).

   o  Extended Security Considerations (Section 8).

   o  New "Counter Signature Key Parameters" IANA Registry
      (Section 9.2).

H.7.  Version -03 to -04

   o  Added the new "Counter Signature Parameters" in the Common Context
      (see Section 2).

   o  Added recommendation on using "deterministic ECDSA" if ECDSA is
      used as counter signature algorithm (see Section 2).

   o  Clarified possible asynchronous retrieval of keying material from
      the Group Manager, in order to process incoming messages (see
      Section 2).

   o  Structured Section 3 into subsections.

   o  Added the new 'par_countersign' to the aad_array of the
      external_aad (see Section 3.1).

   o  Clarified non reliability of 'kid' as identity indicator for a
      group member (see Section 2.1).

   o  Described possible provisioning of new Sender ID in case of
      Partial IV wrap-around (see Section 2.2).

   o  The former signature bit in the Flag Byte of the OSCORE option
      value is reverted to reserved (see Section 4.1).



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   o  Updated examples of compressed COSE object, now with the sixth
      less significant bit in the Flag Byte of the OSCORE option value
      set to 0 (see Section 4.3).

   o  Relaxed statements on sending error messages (see Section 6).

   o  Added explicit step on computing the counter signature for
      outgoing messages (see Setions 6.1 and 6.3).

   o  Handling of just created Recipient Contexts in case of
      unsuccessful message verification (see Sections 6.2 and 6.4).

   o  Handling of replied/repeated responses on the client (see
      Section 6.4).

   o  New IANA Registry "Counter Signature Parameters" (see
      Section 9.1).

H.8.  Version -02 to -03

   o  Revised structure and phrasing for improved readability and better
      alignment with draft-ietf-core-object-security.

   o  Added discussion on wrap-Around of Partial IVs (see Section 2.2).

   o  Separate sections for the COSE Object (Section 3) and the OSCORE
      Header Compression (Section 4).

   o  The countersignature is now appended to the encrypted payload of
      the OSCORE message, rather than included in the OSCORE Option (see
      Section 4).

   o  Extended scope of Section 5, now titled " Message Binding,
      Sequence Numbers, Freshness and Replay Protection".

   o  Clarifications about Non-Confirmable messages in Section 5.1
      "Synchronization of Sender Sequence Numbers".

   o  Clarifications about error handling in Section 6 "Message
      Processing".

   o  Compacted list of responsibilities of the Group Manager in
      Section 7.

   o  Revised and extended security considerations in Section 8.

   o  Added IANA considerations for the OSCORE Flag Bits Registry in
      Section 9.



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   o  Revised Appendix D, now giving a short high-level description of a
      new endpoint set-up.

H.9.  Version -01 to -02

   o  Terminology has been made more aligned with RFC7252 and draft-
      ietf-core-object-security: i) "client" and "server" replace the
      old "multicaster" and "listener", respectively; ii) "silent
      server" replaces the old "pure listener".

   o  Section 2 has been updated to have the Group Identifier stored in
      the 'ID Context' parameter defined in draft-ietf-core-object-
      security.

   o  Section 3 has been updated with the new format of the Additional
      Authenticated Data.

   o  Major rewriting of Section 4 to better highlight the differences
      with the message processing in draft-ietf-core-object-security.

   o  Added Sections 7.2 and 7.3 discussing security considerations
      about uniqueness of (key, nonce) and collision of group
      identifiers, respectively.

   o  Minor updates to Appendix A.1 about assumptions on multicast
      communication topology and group size.

   o  Updated Appendix C on format of group identifiers, with practical
      implications of possible collisions of group identifiers.

   o  Updated Appendix D.2, adding a pointer to draft-palombini-ace-key-
      groupcomm about retrieval of nodes' public keys through the Group
      Manager.

   o  Minor updates to Appendix E.3 about Challenge-Response
      synchronization of sequence numbers based on the Echo option from
      draft-ietf-core-echo-request-tag.

H.10.  Version -00 to -01

   o  Section 1.1 has been updated with the definition of group as
      "security group".

   o  Section 2 has been updated with:

      *  Clarifications on etablishment/derivation of Security Contexts.





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      *  A table summarizing the the additional context elements
         compared to OSCORE.

   o  Section 3 has been updated with:

      *  Examples of request and response messages.

      *  Use of CounterSignature0 rather than CounterSignature.

      *  Additional Authenticated Data including also the signature
         algorithm, while not including the Group Identifier any longer.

   o  Added Section 6, listing the responsibilities of the Group
      Manager.

   o  Added Appendix A (former section), including assumptions and
      security objectives.

   o  Appendix B has been updated with more details on the use cases.

   o  Added Appendix C, providing an example of Group Identifier format.

   o  Appendix D has been updated to be aligned with draft-palombini-
      ace-key-groupcomm.

Acknowledgments

   The authors sincerely thank Christian Amsuess, Stefan Beck, Rolf
   Blom, Carsten Bormann, Esko Dijk, Klaus Hartke, Rikard Hoeglund,
   Richard Kelsey, John Mattsson, Dave Robin, Jim Schaad, Ludwig Seitz,
   Peter van der Stok and Erik Thormarker for their feedback and
   comments.

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC; the H2020 project SIFIS-Home (Grant
   agreement 952652); the SSF project SEC4Factory under the grant
   RIT17-0032; and the EIT-Digital High Impact Initiative ACTIVE.

Authors' Addresses

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

   Email: marco.tiloca@ri.se




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   Goeran Selander
   Ericsson AB
   Torshamnsgatan 23
   Kista  SE-16440 Stockholm
   Sweden

   Email: goran.selander@ericsson.com


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

   Email: francesca.palombini@ericsson.com


   Jiye Park
   Universitaet Duisburg-Essen
   Schuetzenbahn 70
   Essen  45127
   Germany

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


























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