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Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-14

Document Type Active Internet-Draft (core WG)
Authors Marco Tiloca , Göran Selander , Francesca Palombini , John Preuß Mattsson , Jiye Park
Last updated 2022-04-02 (Latest revision 2022-03-07)
Replaces draft-tiloca-core-multicast-oscoap
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draft-ietf-core-oscore-groupcomm-14
CoRE Working Group                                             M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                             G. Selander
Expires: 8 September 2022                                   F. Palombini
                                                             J. Mattsson
                                                             Ericsson AB
                                                                 J. Park
                                             Universitaet Duisburg-Essen
                                                            7 March 2022

           Group OSCORE - Secure Group Communication for CoAP
                  draft-ietf-core-oscore-groupcomm-14

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.
   Group OSCORE also defines a pairwise mode where each member of the
   group can efficiently derive a symmetric pairwise key with any other
   member of the group for pairwise OSCORE communication.

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 8 September 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Security Context  . . . . . . . . . . . . . . . . . . . . . .   8
     2.1.  Common Context  . . . . . . . . . . . . . . . . . . . . .  11
       2.1.1.  AEAD Algorithm  . . . . . . . . . . . . . . . . . . .  11
       2.1.2.  ID Context  . . . . . . . . . . . . . . . . . . . . .  11
       2.1.3.  Group Manager Authentication Credential . . . . . . .  12
       2.1.4.  Signature Encryption Algorithm  . . . . . . . . . . .  12
       2.1.5.  Signature Algorithm . . . . . . . . . . . . . . . . .  12
       2.1.6.  Group Encryption Key  . . . . . . . . . . . . . . . .  12
       2.1.7.  Pairwise Key Agreement Algorithm  . . . . . . . . . .  13
     2.2.  Sender Context and Recipient Context  . . . . . . . . . .  13
     2.3.  Authentication Credentials  . . . . . . . . . . . . . . .  14
     2.4.  Pairwise Keys . . . . . . . . . . . . . . . . . . . . . .  16
       2.4.1.  Derivation of Pairwise Keys . . . . . . . . . . . . .  16
       2.4.2.  ECDH with Montgomery Coordinates  . . . . . . . . . .  18
       2.4.3.  Usage of Sequence Numbers . . . . . . . . . . . . . .  19
       2.4.4.  Security Context for Pairwise Mode  . . . . . . . . .  20
     2.5.  Update of Security Context  . . . . . . . . . . . . . . .  20
       2.5.1.  Loss of Mutable Security Context  . . . . . . . . . .  21
       2.5.2.  Exhaustion of Sender Sequence Number  . . . . . . . .  22
       2.5.3.  Retrieving New Security Context Parameters  . . . . .  23
   3.  The Group Manager . . . . . . . . . . . . . . . . . . . . . .  25
     3.1.  Support for Additional Entities . . . . . . . . . . . . .  26
     3.2.  Management of Group Keying Material . . . . . . . . . . .  27
       3.2.1.  Recycling of Identifiers  . . . . . . . . . . . . . .  30
     3.3.  Responsibilities of the Group Manager . . . . . . . . . .  32
   4.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  34
     4.1.  Countersignature  . . . . . . . . . . . . . . . . . . . .  34
       4.1.1.  Keystream Derivation  . . . . . . . . . . . . . . . .  35
       4.1.2.  Clarifications on Using a Countersignature  . . . . .  36
     4.2.  The 'kid' and 'kid context' parameters  . . . . . . . . .  36
     4.3.  external_aad  . . . . . . . . . . . . . . . . . . . . . .  36
   5.  OSCORE Header Compression . . . . . . . . . . . . . . . . . .  39
     5.1.  Examples of Compressed COSE Objects . . . . . . . . . . .  40
       5.1.1.  Examples in Group Mode  . . . . . . . . . . . . . . .  40
       5.1.2.  Examples in Pairwise Mode . . . . . . . . . . . . . .  41

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   6.  Message Binding, Sequence Numbers, Freshness and Replay
           Protection  . . . . . . . . . . . . . . . . . . . . . . .  42
     6.1.  Supporting Observe  . . . . . . . . . . . . . . . . . . .  42
     6.2.  Update of Replay Window . . . . . . . . . . . . . . . . .  42
     6.3.  Message Freshness . . . . . . . . . . . . . . . . . . . .  43
   7.  Message Reception . . . . . . . . . . . . . . . . . . . . . .  43
   8.  Message Processing in Group Mode  . . . . . . . . . . . . . .  44
     8.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  46
       8.1.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  46
     8.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  47
       8.2.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  49
     8.3.  Protecting the Response . . . . . . . . . . . . . . . . .  49
       8.3.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  50
     8.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  51
       8.4.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  53
     8.5.  External Signature Checkers . . . . . . . . . . . . . . .  54
   9.  Message Processing in Pairwise Mode . . . . . . . . . . . . .  55
     9.1.  Pre-Conditions  . . . . . . . . . . . . . . . . . . . . .  56
     9.2.  Main Differences from OSCORE  . . . . . . . . . . . . . .  56
     9.3.  Protecting the Request  . . . . . . . . . . . . . . . . .  57
     9.4.  Verifying the Request . . . . . . . . . . . . . . . . . .  57
     9.5.  Protecting the Response . . . . . . . . . . . . . . . . .  57
     9.6.  Verifying the Response  . . . . . . . . . . . . . . . . .  58
   10. Challenge-Response Synchronization  . . . . . . . . . . . . .  59
   11. Implementation Compliance . . . . . . . . . . . . . . . . . .  62
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  63
     12.1.  Security of the Group Mode . . . . . . . . . . . . . . .  64
     12.2.  Security of the Pairwise Mode  . . . . . . . . . . . . .  66
     12.3.  Uniqueness of (key, nonce) . . . . . . . . . . . . . . .  67
     12.4.  Management of Group Keying Material  . . . . . . . . . .  67
     12.5.  Update of Security Context and Key Rotation  . . . . . .  68
       12.5.1.  Late Update on the Sender  . . . . . . . . . . . . .  68
       12.5.2.  Late Update on the Recipient . . . . . . . . . . . .  69
     12.6.  Collision of Group Identifiers . . . . . . . . . . . . .  69
     12.7.  Cross-group Message Injection  . . . . . . . . . . . . .  70
       12.7.1.  Attack Description . . . . . . . . . . . . . . . . .  70
       12.7.2.  Attack Prevention in Group Mode  . . . . . . . . . .  71
     12.8.  Prevention of Group Cloning Attack . . . . . . . . . . .  72
     12.9.  Group OSCORE for Unicast Requests  . . . . . . . . . . .  73
     12.10. End-to-end Protection  . . . . . . . . . . . . . . . . .  74
     12.11. Master Secret  . . . . . . . . . . . . . . . . . . . . .  74
     12.12. Replay Protection  . . . . . . . . . . . . . . . . . . .  75
     12.13. Message Freshness  . . . . . . . . . . . . . . . . . . .  75
     12.14. Client Aliveness . . . . . . . . . . . . . . . . . . . .  75
     12.15. Cryptographic Considerations . . . . . . . . . . . . . .  76
     12.16. Message Segmentation . . . . . . . . . . . . . . . . . .  77
     12.17. Privacy Considerations . . . . . . . . . . . . . . . . .  78
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  79

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     13.1.  OSCORE Flag Bits Registry  . . . . . . . . . . . . . . .  79
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  79
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  79
     14.2.  Informative References . . . . . . . . . . . . . . . . .  81
   Appendix A.  Assumptions and Security Objectives  . . . . . . . .  84
     A.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .  85
     A.2.  Security Objectives . . . . . . . . . . . . . . . . . . .  86
   Appendix B.  List of Use Cases  . . . . . . . . . . . . . . . . .  87
   Appendix C.  Example of Group Identifier Format . . . . . . . . .  90
   Appendix D.  Set-up of New Endpoints  . . . . . . . . . . . . . .  91
   Appendix E.  Document Updates . . . . . . . . . . . . . . . . . .  91
     E.1.  Version -13 to -14  . . . . . . . . . . . . . . . . . . .  91
     E.2.  Version -12 to -13  . . . . . . . . . . . . . . . . . . .  92
     E.3.  Version -11 to -12  . . . . . . . . . . . . . . . . . . .  92
     E.4.  Version -10 to -11  . . . . . . . . . . . . . . . . . . .  93
     E.5.  Version -09 to -10  . . . . . . . . . . . . . . . . . . .  94
     E.6.  Version -08 to -09  . . . . . . . . . . . . . . . . . . .  95
     E.7.  Version -07 to -08  . . . . . . . . . . . . . . . . . . .  95
     E.8.  Version -06 to -07  . . . . . . . . . . . . . . . . . . .  97
     E.9.  Version -05 to -06  . . . . . . . . . . . . . . . . . . .  97
     E.10. Version -04 to -05  . . . . . . . . . . . . . . . . . . .  98
     E.11. Version -03 to -04  . . . . . . . . . . . . . . . . . . .  98
     E.12. Version -02 to -03  . . . . . . . . . . . . . . . . . . .  99
     E.13. Version -01 to -02  . . . . . . . . . . . . . . . . . . . 100
     E.14. Version -00 to -01  . . . . . . . . . . . . . . . . . . . 101
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 102
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 102

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).  Group communication for CoAP
   [I-D.ietf-core-groupcomm-bis] mainly uses UDP/IP multicast as the
   underlying data transport.

   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

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   provides end-to-end encryption, integrity, replay protection and
   binding of response to request between a sender and a recipient,
   independent of the transport layer 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, a security protocol for Group
   communication for CoAP [I-D.ietf-core-groupcomm-bis], 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.  Group OSCORE also defines a pairwise mode where each
   member of the group can efficiently derive a symmetric pairwise key
   with any other member of the group for pairwise OSCORE communication.
   Just like OSCORE, Group OSCORE is independent of the transport layer
   and works wherever CoAP does.

   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).
   This prevents observers from accessing addressing information
   conveyed in CoAP options that would not be protected by Group OSCORE,
   but would be protected by DTLS.  These options include Uri-Host, Uri-
   Port and Proxy-Uri. Note that DTLS does not define how to secure
   messages sent over IP multicast.

   Group OSCORE defines two modes of operation, that can be used
   independently or together:

   *  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 signature algorithms as well as signature
      verification by intermediaries.  This mode is defined in
      Section 8.

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   *  In the pairwise mode, two group members exchange OSCORE requests
      and responses (typically) 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.

   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 endpoints, it is desirable to
   restrict the amount of secret keying material provided to each
   endpoint.  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).  However, it has the benefit of
   providing a single shared secret, while distributing only the public
   keys of group members or a subset of those.  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

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   [I-D.ietf-core-groupcomm-bis]; CBOR [RFC8949]; COSE
   [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] and
   related countersignatures [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.

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

   *  Authentication credential: set of information associated with an
      entity, including that entity's public key and parameters
      associated with the public key.  Examples of authentication
      credentials are CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs)
      [RFC8392], X.509 certificates [RFC7925] and C509 certificates
      [I-D.ietf-cose-cbor-encoded-cert].  Further details about
      authentication credentials are provided in Section 2.3.

   *  Group: a set of endpoints that share group keying material and
      security parameters (Common Context, see Section 2).  That is,
      unless otherwise specified, the term group used in this document
      refers 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".

   *  Group Manager: entity responsible for a group.  Each endpoint in a
      group communicates securely with the respective Group Manager,
      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 3.3.

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   *  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.

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

   *  Birth Gid: with respect to a group member, the Gid obtained by
      that group member upon (re-)joining the group.

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

   *  Key Generation Number: an integer value identifying the current
      version of the keying material used in a group.

   *  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

   As per the terminology in Section 1.1, this document refers to a
   group as a set of endpoints sharing keying material and security
   parameters for executing the Group OSCORE protocol.  Each endpoint of
   a group is aware of whether the group uses the group mode, or the
   pairwise mode, or both.  Then, an endpoint can use any mode it
   supports if also used in the group.

   All members of a group maintain a Security Context as defined in
   Section 3 of [RFC8613] and extended as defined in this section.  How
   the Security Context is established by the group members is out of
   scope for this document, 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.

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   The default setting for how to manage information about the group,
   including the Security Context, is described in terms of a Group
   Manager (see Section 3).  In particular, the Group Manager indicates
   whether the group uses the group mode, the pairwise mode, or both of
   them, as part of the group data provided to candidate group members
   when joining the group.

   The remainder of this section provides further details about the
   Security Context of Group OSCORE.  In particular, 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).

   *  One Common Context, shared by all the endpoints in the group.
      Several new parameters are included in the Common Context.

      If a Group Manager is used for maintaining the group, the Common
      Context is extended with the authentication credential of the
      Group Manager, including the Group Manager's public key.  When
      processing messages, the authentication credential of the Group
      Manager is included in the external additional authenticated data
      (see Section 4.3).

      If the group uses the group mode, the Common context is extended
      with the following new parameters.

      -  Signature Encryption Algorithm and Signature Algorithm.  These
         relate to the encryption/decryption operations and to the
         computation/verification of countersignatures, respectively,
         when a message is protected with the group mode (see
         Section 8).

      -  Group Encryption Key, used to perform encryption/decryption of
         countersignatures, when a message is protected with the group
         mode (see Section 8).

      If the group uses the pairwise mode, the Common Context is
      extended with a Pairwise Key Agreement Algorithm used for
      agreement on a static-static Diffie-Hellman shared secret, from
      which pairwise keys are derived (see Section 2.4.1) to protect
      messages with the pairwise mode (see Section 9).

   *  One Sender Context, extended with the endpoint's private key and
      authentication credential including the endpoint's public key.

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      The private key is used to sign messages protected with the group
      mode, or for deriving pairwise keys in pairwise mode (see
      Section 2.4).  The authentication credential is used for deriving
      pairwise keys in pairwise mode, and is included in the external
      additional authenticated data when processing outgoing messages
      (see Section 9).

      If the endpoint supports the pairwise mode, the Sender Context is
      also extended with the Pairwise Sender Keys associated with the
      other endpoints (see Section 2.4).

      The Sender Context is omitted if the endpoint is configured
      exclusively as silent server.

   *  One Recipient Context for each other endpoint from which messages
      are received.  It is not necessary to maintain Recipient Contexts
      associated with endpoints from which messages are not (expected to
      be) received.  The Recipient Context is extended with the
      authentication credential of the other endpoint, including that
      endpoint's public key.

      The public key is used to verify the signature of messages
      protected with the group mode from the other endpoint and for
      deriving the pairwise keys in pairwise mode (see Section 2.4).
      The authentication credential is used for deriving pairwise keys
      in pairwise mode, and is included in the external additional
      authenticated data when processing incoming messages from the
      other endpoint (see Section 9).

      If the endpoint supports the pairwise mode, then the Recipient
      Context is also extended with the Pairwise Recipient Key
      associated with the other endpoint (see Section 2.4).

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 +-------------------+-------------------------------------------------+
 | Context Component | New Information Elements                        |
 +-------------------+-------------------------------------------------+
 | Common Context    |   Group Manager Authentication Credential       |
 |                   | * Signature Encryption Algorithm                |
 |                   | * Signature Algorithm                           |
 |                   | * Group Encryption Key                          |
 |                   | ^ Pairwise Key Agreement Algorithm              |
 +-------------------+-------------------------------------------------+
 | Sender Context    |   Endpoint's own private key                    |
 |                   |   Endpoint's own authentication credential      |
 |                   | ^ Pairwise Sender Keys for the other endpoints  |
 +-------------------+-------------------------------------------------+
 | Each              |   Other endpoint's authentication credential    |
 | Recipient Context | ^ Pairwise Recipient Key for the other endpoint |
 +-------------------+-------------------------------------------------+

   Figure 1: Additions to the OSCORE Security Context.  The optional
     elements labeled with * (with ^) are present only if the group
                uses the group mode (the pairwise mode).

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.  AEAD Algorithm

   AEAD Algorithm identifies the COSE AEAD algorithm to use for
   encryption, when messages are protected using the pairwise mode (see
   Section 9).  This algorithm MUST provide integrity protection.  This
   parameter is immutable once the Common Context is established, and it
   is not relevant if the group uses only the group mode.

2.1.2.  ID Context

   The ID Context parameter (see Sections 3.1 and 3.3 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 12.6).

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2.1.3.  Group Manager Authentication Credential

   Group Manager Authentication Credential specifies the authentication
   credential of the Group Manager, including the Group Manager's public
   key.  This is included in the external additional authenticated data
   when processing messages (see Section 4.3).

   Each group member MUST obtain the authentication credential of the
   Group Manager with a valid proof-of-possession of the corresponding
   private key, for instance from the Group Manager itself when joining
   the group.  Further details on the provisioning of the Group
   Manager's authentication credential to the group members are out of
   the scope of this document.

2.1.4.  Signature Encryption Algorithm

   Signature Encryption Algorithm identifies the algorithm to use for
   encryption, when messages are protected using the group mode (see
   Section 8).  This algorithm MAY provide integrity protection.  This
   parameter is immutable once the Common Context is established.

   This algorithm is not used to encrypt the countersignature in
   messages protected using the group mode, for which the method defined
   in Section 4.1 is used.

2.1.5.  Signature Algorithm

   Signature Algorithm identifies the digital signature algorithm used
   to compute a countersignature 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.

2.1.6.  Group Encryption Key

   Group Encryption Key specifies the encryption key for deriving a
   keystream to encrypt/decrypt a countersignature, when a message is
   protected with the group mode (see Section 8).

   The Group Encryption Key is derived as defined for Sender/Recipient
   Keys in Section 3.2.1 of [RFC8613], with the following differences.

   *  The 'id' element of the 'info' array is the empty byte string.

   *  The 'alg_aead' element of the 'info' array takes the value of
      Signature Encryption Algorithm from the Common Context (see
      Section 2.1.5).

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   *  The 'type' element of the 'info' array is "Group Encryption Key".
      The label is an ASCII string and does not include a trailing NUL
      byte.

   *  L and the 'L' element of the 'info' array are the size of the key
      for the Signature Encryption Algorithm from the Common Context
      (see Section 2.1.5), in bytes.

2.1.7.  Pairwise Key Agreement Algorithm

   Pairwise Key Agreement 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 2.4.1) to protect messages with the pairwise mode (see
   Section 9).  This parameter is immutable once the Common Context is
   established.

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]).

   The derivation of Sender/Recipient Keys and Common IV defined in
   OSCORE applies also to Group OSCORE, with the following extensions
   compared to Section 3.2.1 of [RFC8613].

   *  If the group uses (also) the group mode, the 'alg_aead' element of
      the 'info' array takes the value of Signature Encryption Algorithm
      from the Common Context (see Section 2.1.5).

   *  If the group uses only the pairwise mode, the 'alg_aead' element
      of the 'info' array takes the value of AEAD Algorithm from the
      Common Context (see Section 2.1.1).

   The Sender ID SHALL be unique for each endpoint in a group with a
   certain tuple (Master Secret, Master Salt, 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 key of the sender and the authentication
   credential including the corresponding public key can, for example,
   be generated by the endpoint or provisioned during manufacturing.

   With the exception of the authentication credential of the sender
   endpoint and the possibly associated pairwise keys, a receiver
   endpoint can derive a complete Security Context from a received Group

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   OSCORE message and the Common Context.  The authentication
   credentials in the Recipient Contexts can be retrieved from the Group
   Manager (see Section 3) upon joining the group.  An authentication
   credential 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
   authentication credential.  Such devices can be configured to drop a
   received message for which there is no (complete) Recipient Context,
   and retrieve the sender endpoint's authentication credential in order
   to have it available to verify subsequent messages from that
   endpoint.

   An endpoint admits a maximum amount of Recipient Contexts for a same
   Security Context, e.g., due to memory limitations.  After reaching
   that limit, the creation of a new Recipient Context results in an
   overflow.  When this happens, the endpoint has to delete a current
   Recipient Context to install the new one.  It is up to the
   application to define policies for selecting the current Recipient
   Context to delete.  If the new Recipient Context has been installed
   after the endpoint has experienced the overflow above, then the
   Recipient Context is initialized with an invalid Replay Window, and
   accordingly requires the endpoint to take appropriate actions (see
   Section 2.5.1.2).

2.3.  Authentication Credentials

   In a group, the following MUST hold for the authentication credential
   of each endpoint as well as for the authentication credential of the
   Group Manager.

   *  All authentication credentials MUST be encoded according to the
      same format used in the group.  The used format MUST provide the
      public key as well as the comprehensive set of information related
      to the public key algorithm, including, e.g., the used elliptic
      curve (when applicable).

   *  All authentication credentials and the public key specified
      therein MUST be for the public key algorithm used in the group and
      aligned with the possible associated parameters used in the group,
      e.g., the used elliptic curve (when applicable).

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      If the group uses (also) the group mode, the public key algorithm
      is the Signature Algorithm used in the group.  If the group uses
      only the pairwise mode, the public key algorithm is the Pairwise
      Key Agreement Algorithm used in the group.

      If the authentication credentials are X.509 certificates [RFC7925]
      or C509 certificates [I-D.ietf-cose-cbor-encoded-cert], the public
      key algorithm is fully described by the "algorithm" field of the
      "SubjectPublicKeyInfo" structure, and by the
      "subjectPublicKeyAlgorithm" element, respectively.

      If authentication credentials are CBOR Web Tokens (CWTs) or CWT
      Claims Sets (CCSs) [RFC8392], the public key algorithm is fully
      described by a COSE key type and its "kty" and "crv" parameters.

   Authentication credentials are used to derive pairwise keys (see
   Section 2.4.1) and are included in the external additional
   authenticated data when processing messages (see Section 4.3).  In
   both these cases, an endpoint in a group MUST treat authentication
   credentials as opaque data, i.e., by considering the same binary
   representation made available to other endpoints in the group,
   possibly through a designated trusted source (e.g., the Group
   Manager).

   For example, an X.509 certificate is provided as its direct binary
   serialization.  If C509 certificates or CWTs are used as
   authentication credentials, each is provided as the binary
   serialization of a (possibly tagged) CBOR array.  If CCSs are used as
   authentication credentials, each is provided as the binary
   serialization of a CBOR map.

   If authentication credentials are CWTs, then the untagged CWT
   associated with an entity is stored in the Security Context and used
   as authentication credential for that entity.

   If authentication credentials are X.509 / C509 certificates or CWTs
   and the authentication credential associated with an entity is
   provided within a chain or a bag, then only the end-entity
   certificate or end-entity untagged CWT is stored in the Security
   Context and used as authentication credential for that entity.

   Storing whole authentication credentials rather than only a subset of
   those may result in a non-negligible storage overhead.  On the other
   hand, it also ensures that authentication credentials are correctly
   used in a simple, flexible and non-error-prone way, also taking into
   account future credential formats as entirely new or extending
   existing ones.  In particular, it is ensured that:

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   *  When used to derive pairwise keys and when included in the
      external additional authenticated data, authentication credentials
      can also specify possible metadata and parameters related to the
      included public key.  Besides the public key algorithm, these
      comprise other relevant pieces of information such as key usage,
      expiration time, issuer and subject.

   *  All endpoints using another endpoint's authentication credential
      use exactly the same binary serialization, as obtained and
      distributed by the credential provider (e.g., the Group Manager)
      and as originally crafted by the credential issuer.  In turn, this
      does not require to define and maintain canonical subsets of
      authentication credentials and their corresponding encoding, and
      spares endpoints from error-prone re-encoding operations.

   Depending on the particular deployment and the intended group size,
   limiting the storage overhead of endpoints in a group can be an
   incentive for system/network administrators to prefer using a compact
   format of authentication credentials in the first place.

2.4.  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 defined
   in Section 9.

   Group OSCORE keys used for both signature and encryption MUST be used
   only for purposes related to Group OSCORE.  These include the
   processing of messages with Group OSCORE, as well as performing
   proof-of-possession of private keys, e.g., upon joining a group
   through the Group Manager (see Section 3).

2.4.1.  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 X in the group by means of the
   AEAD Algorithm from the Common Context (see Section 2.1.1).  The key
   derivation of these so-called pairwise keys follows the same
   construction as in Section 3.2.1 of [RFC8613]:

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  Pairwise Sender Key    = HKDF(Sender Key, IKM-Sender, info, L)
  Pairwise Recipient Key = HKDF(Recipient Key, IKM-Recipient, info, L)

  with

  IKM-Sender    = Sender Auth Cred | Recipient Auth Cred | Shared Secret
  IKM-Recipient = Recipient Auth Cred | Sender Auth Cred | Shared Secret

   where:

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

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

   *  HKDF is the OSCORE HKDF algorithm [RFC8613] from the Common
      Context.

   *  The Sender Key from the Sender Context is used as salt in the
      HKDF, when deriving the Pairwise Sender Key.

   *  The Recipient Key from the Recipient Context associated with
      endpoint X is used as salt in the HKDF, when deriving the Pairwise
      Recipient Key.

   *  Sender Auth Cred is the endpoint's own authentication credential
      from the Sender Context.

   *  Recipient Auth Cred is the endpoint X's authentication credential
      from the Recipient Context associated with the endpoint X.

   *  The Shared Secret is computed as a cofactor Diffie-Hellman shared
      secret, see Section 5.7.1.2 of [NIST-800-56A], using the Pairwise
      Key Agreement Algorithm.  The endpoint uses its private key from
      the Sender Context and the other endpoint's public key included in
      Recipient Auth Cred.  Note the requirement of validation of public
      keys in Section 12.15.  For X25519 and X448, the procedure is
      described in Section 5 of [RFC7748] using public keys mapped to
      Montgomery coordinates, see Section 2.4.2.

   *  IKM-Sender is the Input Keying Material (IKM) used in the HKDF for
      the derivation of the Pairwise Sender Key. IKM-Sender is the byte
      string concatenation of Sender Auth Cred, Recipient Auth Cred and
      the Shared Secret.  The authentication credentials Sender Auth
      Cred and Recipient Auth Cred are binary encoded as defined in
      Section 2.3.

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   *  IKM-Recipient is the Input Keying Material (IKM) used in the HKDF
      for the derivation of the Pairwise Recipient Key. IKM-Recipient is
      the byte string concatenation of Recipient Auth Cred, Sender Auth
      Cred and the Shared Secret.  The authentication credentials
      Recipient Auth Cred and Sender Auth Cred are binary encoded as
      defined in Section 2.3.

   *  info and L are as defined in Section 3.2.1 of [RFC8613].  That is:

      -  The 'alg_aead' element of the 'info' array takes the value of
         AEAD Algorithm from the Common Context (see Section 2.1.1).

      -  L and the 'L' element of the 'info' array are the size of the
         key for the AEAD Algorithm from the Common Context (see
         Section 2.1.1), in bytes.

   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].  For further details, see Section 2.4.2.
   ECC asymmetric keys in Montgomery or Weirstrass form are used
   directly in the key agreement algorithm without coordinate mapping.

   After establishing a partially or completely new Security Context
   (see Section 2.5 and Section 3.2), 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.

2.4.2.  ECDH with Montgomery Coordinates

2.4.2.1.  Curve25519

   The y-coordinate of the other endpoint's Ed25519 public key is
   decoded as specified in Section 5.1.3 of [RFC8032].  The Curve25519
   u-coordinate is recovered as u = (1 + y) / (1 - y) (mod p) following
   the map in Section 4.1 of [RFC7748].  Note that the mapping is not
   defined for y = 1, and that y = -1 maps to u = 0 which corresponds to
   the neutral group element and thus will result in a degenerate shared
   secret.  Therefore implementations MUST abort if the y-coordinate of
   the other endpoint's Ed25519 public key is 1 or -1 (mod p).

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   The private signing key byte strings (= the lower 32 bytes used for
   generating the public key, see step 1 of Section 5.1.5 of [RFC8032])
   are decoded the same way for signing in Ed25519 and scalar
   multiplication in X25519.  Hence, to compute the shared secret the
   endpoint applies the X25519 function to the Ed25519 private signing
   key byte string and the encoded u-coordinate byte string as specified
   in Section 5 of [RFC7748].

2.4.2.2.  Curve448

   The y-coordinate of the other endpoint's Ed448 public key is decoded
   as specified in Section 5.2.3. of [RFC8032].  The Curve448
   u-coordinate is recovered as u = y^2 * (d * y^2 - 1) / (y^2 - 1) (mod
   p) following the map from "edwards448" in Section 4.2 of [RFC7748],
   and also using the relation x^2 = (y^2 - 1)/(d * y^2 - 1) from the
   curve equation.  Note that the mapping is not defined for y = 1 or
   -1.  Therefore implementations MUST abort if the y-coordinate of the
   peer endpoint's Ed448 public key is 1 or -1 (mod p).

   The private signing key byte strings (= the lower 57 bytes used for
   generating the public key, see step 1 of Section 5.2.5 of [RFC8032])
   are decoded the same way for signing in Ed448 and scalar
   multiplication in X448.  Hence, to compute the shared secret the
   endpoint applies the X448 function to the Ed448 private signing key
   byte string and the encoded u-coordinate byte string as specified in
   Section 5 of [RFC7748].

2.4.3.  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.  In turn, this contributes to a sooner
   exhaustion of the Sender Sequence Number space of the client, which
   would then require to take actions for deriving a new Sender Context
   before resuming communications in the group (see Section 2.5.2).

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2.4.4.  Security Context for Pairwise Mode

   If the pairwise mode is supported, the Security Context additionally
   includes Pairwise Key Agreement Algorithm 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 2.4.1) 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 2.5.3).  In order
   to optimize protocol performance, an endpoint may store the derived
   pairwise keys for easy retrieval.

   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 with 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.

2.5.  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 2.5.3).

   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 2.5.1) or exhaustion of Sender Sequence
   Numbers (see Section 2.5.2).

   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 and MUST accordingly
   take the actions defined in Section 2.5.1.

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2.5.1.  Loss of Mutable Security Context

   An endpoint may lose its mutable Security Context, e.g., due to a
   reboot (see Section 2.5.1.1) or to an overflow of Recipient Contexts
   (see Section 2.5.1.2).

   In such a case, the endpoint needs to prevent the re-use of a nonce
   with the same AEAD key, and to handle incoming replayed messages.

2.5.1.1.  Reboot and Total Loss

   In case a loss of the Sender Context and/or of the Recipient Contexts
   is detected (e.g., following a reboot), the endpoint MUST NOT protect
   further messages using this Security Context to avoid reusing an AEAD
   nonce with the same AEAD key.

   In particular, before resuming its operations in the group, the
   endpoint MUST retrieve new Security Context parameters from the Group
   Manager (see Section 2.5.3) and use them to derive a new Sender
   Context (see Section 2.2).  Since this includes a newly derived
   Sender Key, a server will 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.

   From then on, the endpoint MUST use the latest installed Sender
   Context to protect outgoing messages.  Also, newly created Recipient
   Contexts will have a Replay Window which is initialized as valid.

   If 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 and MUST
   NOT accept incoming messages from other group members, as currently
   unable to detect possible replays.

   An adversary may leverage the above to perform a Denial of Service
   attack and prevent some group members from communicating altogether.
   That is, the adversary can first block the communication path between
   the Group Manager and some individual group members.  This can be
   achieved, for instance, by injecting fake responses to DNS queries
   for the Group Manager hostname, or by removing a network link used
   for routing traffic towards the Group Manager.  Then, the adversary
   can induce a reboot for some endpoints in the group, e.g., by
   triggering a short power outage.  After that, such endpoints that
   have lost their Sender Context and/or Recipient Contexts following
   the reboot would not be able to obtain new Security Context
   parameters from the Group Manager, as specified above.  Thus, they
   would not be able to further communicate in the group until
   connectivity with the Group Manager is restored.

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2.5.1.2.  Overflow of Recipient Contexts

   After reaching the maximum amount of Recipient Contexts, an endpoint
   will experience an overflow when installing a new Recipient Context,
   as it requires to first delete an existing one (see Section 2.2).

   Every time this happens, the Replay Window of the new Recipient
   Context is initialized as not valid.  Therefore, the endpoint MUST
   take the following actions, before accepting request messages from
   the client associated with the new Recipient Context.

   If it is not configured as silent server, the endpoint MUST either:

   *  Retrieve new Security Context parameters from the Group Manager
      and derive a new Sender Context, as defined in Section 2.5.1.1; or

   *  When receiving a first request to process with the new Recipient
      Context, use the approach specified in Section 10 and based on the
      Echo Option for CoAP [RFC9175], if supported.  In particular, the
      endpoint MUST use its Partial IV when generating the AEAD nonce
      and MUST include the Partial IV in the response message conveying
      the Echo Option.  If the endpoint supports the CoAP Echo Option,
      it is RECOMMENDED to take this approach.

   If it is configured exclusively as silent server, the endpoint MUST
   wait for the next group rekeying to occur, in order to derive a new
   Security Context and re-initialize the Replay Window of each
   Recipient Contexts as valid.

2.5.2.  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.

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   The endpoint SHOULD inform the Group Manager, retrieve new Security
   Context parameters from the Group Manager (see Section 2.5.3), and
   use them to derive a new Sender Context (see Section 2.2).

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

2.5.3.  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 2.5.3.1); and ii)
   establishment of a new Security Context for the group (see
   Section 2.5.3.2).  The update of the Replay Window in each of the
   Recipient Contexts is discussed in Section 6.2.

   As group membership changes, or as group members get new Sender IDs
   (see Section 2.5.3.1) 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 identifier 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 authentication credentials and associated 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 an
   authentication credential with the public key included therein is
   currently the one associated with a 'kid' value, after a number of
   consecutive failed verifications.

2.5.3.1.  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 not been used in the group since the latest time when the current
   Gid value was assigned to the group (see Section 3.2).

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   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 resets
   the Sender Sequence Number in its Sender Context 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.

   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 2.5.2).  An
   endpoint may request to re-join the group, e.g., because of losing
   its mutable Security Context (see Section 2.5.1), and is provided
   with 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 3.2).

2.5.3.2.  New Security Context for the Group

   The Group Manager may establish a new Security Context for the group
   (see Section 3.2).  The Group Manager does not necessarily establish
   a new Security Context for the group if one member has an outdated
   Security Context (see Section 2.5.3.1), 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.

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

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

   *  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.

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

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

      -  For each ongoing observation where it is an observer client and
         that it wants to keep active, it resets to 0 the Notification
         Number of each associated server (see Section 6.1).

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

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

   *  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 12.5.1).

   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 12.5.

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 document is based on
   the existence of an entity called Group Manager and responsible for
   the group, but it does not mandate how the Group Manager interacts
   with the group members.  The list of responsibilities of the Group
   Manager is compiled in Section 3.3.

   A possible Group Manager to use is specified 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 an integer Key Generation Number to each of
   its groups, identifying the current version of the keying material
   used in that group.  The first Key Generation Number assigned to
   every group MUST be 0.  Separately for each group, the value of the

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   Key Generation Number increases strictly monotonically, each time the
   Group Manager distributes new keying material to that group (see
   Section 3.2).  That is, if the current Key Generation Number for a
   group is X, then X+1 will denote the keying material distributed and
   used in that group immediately after the current one.

   The Group Manager assigns unique Group Identifiers (Gids) to the
   groups under its control.  Also, for each group, the Group Manager
   assigns unique Sender IDs (and thus Recipient IDs) to the respective
   group members.  According to a hierarchical approach, the Gid value
   assigned to a group is associated with a dedicated space for the
   values of Sender ID and Recipient ID of the members of that group.
   When an endpoint (re-)joins a group, it is provided also with the
   current Gid to use in the group.

   The Group Manager maintains records of the authentication credentials
   of endpoints in a group, and provides information about the group and
   its members to other group members and to external entities with
   selected roles (see Section 3.1).  Upon endpoints' joining, the Group
   Manager collects such authentication credentials 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 authentication credential to the Group
   Manager, for example upon joining the group.

   Furthermore, when joining the group or later on as a group member, an
   endpoint can retrieve from the Group Manager the authentication
   credential of the Group Manager as well as the authentication
   credential and other information associated with other members of the
   group, with which it can derive the corresponding Recipient Context.
   Together with the requested authentication credentials, the Group
   Manager MUST provide the Sender ID of the associated group members
   and the current Key Generation Number in the group.  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.

3.1.  Support for Additional Entities

   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 authentication credentials of
   group members and other selected group data from the Group Manager,
   in order to solely verify countersignatures of messages protected in
   group mode (see Section 8.5).

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   In order to verify countersignatures of messages in a group, a
   signature checker needs to retrieve the following information about
   that group from the Group Manager.

   *  The current ID Context (Gid) used in the group.

   *  The authentication credentials of the group members and the
      authentication credential of the Group Manager.

      If the signature checker is provided with a CWT for a given
      entity, then the authentication credential associated with that
      entity that the signature checker stores and uses is the untagged
      CWT.

      If the signature checker is provided with a chain or a bag of
      X.509 / C509 certificates or of CWTs for a given entity, then the
      authentication credential associated with that entity that the
      signature checker stores and uses is just the end-entity
      certificate or end-entity untagged CWT.

   *  The current Group Encryption Key (see Section 2.1.6).

   *  The identifiers of the algorithms used in the group (see
      Section 2), i.e.: i) Signature Encryption Algorithm and Signature
      Algorithm; and ii) AEAD Algorithm and Pairwise Key Agreement
      Algorithm, if the group uses also the pairwise mode.

   A signature checker MUST be authorized before it can retrieve such
   information.  To this end, the same method mentioned above based on
   the ACE framework [I-D.ietf-ace-oauth-authz] can be used.

3.2.  Management of Group Keying Material

   In order to establish a new Security Context for a group, the Group
   Manager MUST generate and assign to the group a new Group Identifier
   (Gid) and a new value for the Master Secret parameter.  When doing
   so, a new value for the Master Salt parameter MAY also be generated
   and assigned to the group.  When establishing the new Security
   Context, the Group Manager should preserve the current value of the
   Sender ID of each group member.

   The specific group key management scheme used to distribute new
   keying material is out of the scope of this document.  A simple group
   key management scheme is defined in
   [I-D.ietf-ace-key-groupcomm-oscore].  When possible, the delivery of
   rekeying messages should use a reliable transport, in order to reduce
   chances of group members missing a rekeying instance.

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   The set of group members should not be assumed as fixed, i.e., the
   group membership is subject to changes, possibly on a frequent basis.

   The Group Manager MUST rekey the group when one or more endpoints
   leave the group.  An endpoint may leave the group at own initiative,
   or may be evicted from the group by the Group Manager, e.g., in case
   an endpoint is compromised, or is suspected to be compromised.  In
   either case, rekeying the group excludes such endpoints from future
   communications in the group, and thus preserves forward security.  If
   a network node is compromised or suspected to be compromised, the
   Group Manager MUST evict from the group all the endpoints hosted by
   that node that are member of the group and rekey the group
   accordingly.

   If required by the application, the Group Manager MUST rekey the
   group also before one or more new joining endpoints are added to the
   group, thus preserving backward security.

   The establishment of the new Security Context for the group takes the
   following steps.

   1.  The Group Manager MUST increment the Key Generation Number for
       the group by 1.

   2.  The Group Manager MUST build a set of stale Sender IDs including:

       *  The Sender IDs that, during the current Gid, were both
          assigned to an endpoint and subsequently relinquished (see
          Section 2.5.3.1).

       *  The current Sender IDs of the group members that the upcoming
          group rekeying aims to exclude from future group
          communications, if any.

   3.  The Group Manager rekeys the group, by distributing:

       *  The new keying material, i.e., the new Master Secret, the new
          Gid and (optionally) the new Master Salt.

       *  The new Key Generation Number from step 1.

       *  The set of stale Sender IDs from step 2.

       Further information may be distributed, depending on the specific
       group key management scheme used in the group.

   When receiving the new group keying materal, a group member considers
   the received stale Sender IDs and performs the following actions.

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   *  The group member MUST remove every authentication credential
      associated with a stale Sender ID from its list of group members'
      authentication credentials used in the group.

   *  The group member MUST delete each of its Recipient Contexts used
      in the group whose corresponding Recipient ID is a stale Sender
      ID.

   After that, the group member installs the new keying material and
   derives the corresponding new Security Context.

   A group member might miss one group rekeying or more consecutive
   instances.  As a result, the group member will retain old group
   keying material with Key Generation Number GEN_OLD.  Eventually, the
   group member can notice the discrepancy, e.g., by repeatedly failing
   to verify incoming messages, or by explicitly querying the Group
   Manager for the current Key Generation Number.  Once the group member
   gains knowledge of having missed a group rekeying, it MUST delete the
   old keying material it stores.

   Then, the group member proceeds according to the following steps.

   1.  The group member retrieves from the Group Manager the current
       group keying material, together with the current Key Generation
       Number GEN_NEW.  The group member MUST NOT install the obtained
       group keying material yet.

   2.  The group member asks the Group Manager for the set of stale
       Sender IDs.

   3.  If no exact indication can be obtained from the Group Manager,
       the group member MUST remove all the authentication credentials
       from its list of group members' authentication credentials used
       in the group and MUST delete all its Recipient Contexts used in
       the group.

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

   4.  The group member installs the current group keying material, and
       derives the corresponding new Security Context.

   Alternatively, the group member can re-join the group.  In such a
   case, the group member MUST take one of the following two actions.

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   *  The group member performs steps 2 and 3 above.  Then, the group
      member re-joins the group.

   *  The group member re-joins the group with the same roles it
      currently has in the group, and, during the re-joining process, it
      asks the Group Manager for the authentication credentials of all
      the current group members.

      Then, given Z the set of authentication credentials received from
      the Group Manager, the group member removes every authentication
      credential which is not in Z from its list of group members'
      authentication credentials used in the group, and deletes each of
      its Recipient Contexts used in the group that does not include any
      of the authentication credentials in Z.

   By removing authentication credentials and deleting Recipient
   Contexts associated with stale Sender IDs, it is ensured that a
   recipient endpoint storing the latest group keying material does not
   store the authentication credentials of sender endpoints that are not
   current group members.  This in turn allows group members to rely on
   stored authentication credentials to confidently assert the group
   membership of sender endpoints, when receiving incoming messages
   protected in group mode (see Section 8).

3.2.1.  Recycling of Identifiers

   This section specifies how the Group Manager handles and possibly
   reassigns Gid values and Sender ID values in a group.

3.2.1.1.  Recycling of Group Identifiers

   Since the Gid value changes every time a group is rekeyed, it can
   happen that, after several rekeying instances, the whole space of Gid
   values has been used for the group in question.  When this happens,
   the Group Manager has no available Gid values to use that have never
   been assigned to the group during the group's lifetime.

   The occurrence of such an event and how long it would take to occur
   depend on the format and encoding of Gid values used in the group
   (see, e.g., Appendix C), as well as on the frequency of rekeying
   instances yielding a change of Gid value.  Independently for each
   group under its control, the Group Manager can take one of the two
   following approaches.

   *  The Group Manager does not reassign Gid values.  That is, once the
      whole space of Gid values has been used for a group, the Group
      Manager terminates the group and may re-establish a new group.

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   *  While the Gid value changes every time a group is rekeyed, the
      Group Manager can reassign Gid values previously used during a
      group's lifetime.  By doing so, the group can continue to exist
      even once the whole space of Gid values has been used.

      The Group Manager MAY support and use this approach.  In such a
      case, the Group Manager MUST take additional actions when handling
      Gid values and rekeying the group, as specified below.

      When a node (re-)joins the group and it is provided with the
      current Gid to use in the group, the Group Manager considers such
      a Gid as the Birth Gid of that endpoint for that group.  For each
      group member, the Group Manager MUST store the latest
      corresponding Birth Gid until that member leaves the group.  In
      case the endpoint has in fact re-joined the group, the newly
      determined Birth Gid overwrites the one currently stored.

      When establishing a new Security Context for the group, the Group
      Manager takes the additional following step between steps 1 and 2
      of Section 3.2.

      A.  The Group Manager MUST check if the new Gid to be distributed
      is equal to the Birth Gid of any of the current group members.  If
      any of such "elder members" is found in the group, then:

      -  The Group Manager MUST evict the elder members from the group.
         That is, the Group Manager MUST terminate their membership and
         MUST rekey the group in such a way that the new keying material
         is not provided to those evicted elder members.

         This ensures that an Observe notification [RFC7641] can never
         successfully match against the Observe requests of two
         different observations.  In fact, the excluded elder members
         would eventually re-join the group, thus terminating any of
         their ongoing (long-lasting) observations (see Section 6.1).
         Therefore, it is ensured by construction that no observer
         client can have two different ongoing observations such that
         the two respective Observe requests were protected using the
         same Partial IV, Gid and Sender ID.

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

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3.2.1.2.  Recycling of Sender IDs

   From the moment when a Gid is assigned to a group until the moment a
   new Gid is assigned to that same group, the Group Manager MUST NOT
   reassign a Sender ID within the group.  This prevents to reuse a
   Sender ID ('kid') with the same Gid, Master Secret and Master Salt.
   Within this restriction, the Group Manager can assign a Sender ID
   used under an old Gid value (including under a same, recycled Gid
   value), thus avoiding Sender ID values to irrecoverably grow in size.

   Even when an endpoint joining a group is recognized as a current
   member of that group, e.g., through the ongoing secure communication
   association, the Group Manager MUST assign a new Sender ID different
   than the one currently used by the endpoint in the group, unless the
   group is rekeyed first and a new Gid value is established.

3.2.1.3.  Relation between Identifiers and Keying Material

   Figure 2 overviews the different identifiers and keying material
   components, considering their relation and possible reuse across
   group rekeying.

 Components changed in lockstep
     upon a group rekeying
 +----------------------------+            * Changing a kid does not
 |                            |              need changing the Group ID
 | Master               Group |<--> kid1
 | Secret <---> o <--->  ID   |            * A kid is not reassigned
 |              ^             |<--> kid2     under the ongoing usage of
 |              |             |              the current Group ID
 |              |             |<--> kid3
 |              v             |            * Upon changing the Group ID,
 |         Master Salt        | ... ...      every current kid should
 |         (optional)         |              be preserved for efficient
 |                            |              key rollover
 | The Key Generation Number  |
 | is incremented by 1        |            * After changing Group ID, an
 |                            |              unused kid can be assigned,
 +----------------------------+              even if it was used before
                                             the Group ID change

         Figure 2: Relations among keying material components.

3.3.  Responsibilities of the Group Manager

   The Group Manager is responsible for performing the following tasks:

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

   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.   Updating the Key Generation Number and the Gid of its OSCORE
        groups, upon renewing the respective Security Context.

   6.   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 or re-joining.  This includes ensuring that:

        *  Each Sender ID is unique within each of the OSCORE groups;

        *  Each Sender ID is not reassigned within the same group since
           the latest time when the current Gid value was assigned to
           the group.  That is, the Sender ID is not reassigned even to
           a current group member re-joining the same group, without a
           rekeying happening first.

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

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

   9.   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.

   10.  Assisting a group member that has missed a group rekeying
        instance to understand which authentication credentials and
        Recipient Contexts to delete, as associated with former group
        members.

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

   12.  Validating that the format and parameters of authentication
        credentials of group members are consistent with the public key
        algorithm and related parameters used in the respective OSCORE
        group.

   The Group Manager specified 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.  Countersignature

   When protecting a message in group mode, the 'unprotected' field MUST
   additionally include the following parameter:

   *  COSE_CounterSignature0: its value is set to the encrypted
      countersignature of the COSE object, namely ENC_SIGNATURE.  That
      is:

      -  The countersignature of the COSE object, namely SIGNATURE, is
         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 Signature Algorithm in the Security Context.

         In particular, the Countersign_structure contains the context
         text string "CounterSignature0", the external_aad as defined in
         Section 4.3 of this document, and the ciphertext of the COSE
         object as payload.

      -  The encrypted countersignature, namely ENC_SIGNATURE, is
         computed as

         ENC_SIGNATURE = SIGNATURE XOR KEYSTREAM

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         where KEYSTREAM is derived as per Section 4.1.1.

4.1.1.  Keystream Derivation

   The following defines how an endpoint derives the keystream
   KEYSTREAM, used to encrypt/decrypt the countersignature of an
   outgoing/incoming message M protected in group mode.

   The keystream SHALL be derived as follows, by using the HKDF
   Algorithm from the Common Context (see Section 3.2 of [RFC8613]),
   which consists of composing the HKDF-Extract and HKDF-Expand steps
   [RFC5869].

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

   The input parameters of HKDF are as follows.

   *  salt takes as value the Partial IV (PIV) used to protect M.  Note
      that, if M is a response, salt takes as value either: i) the fresh
      Partial IV generated by the server and included in the response;
      or ii) the same Partial IV of the request generated by the client
      and not included in the response.

   *  IKM is the Group Encryption Key from the Common Context (see
      Section 2.1.6).

   *  info is the serialization of a CBOR array consisting of (the
      notation follows [RFC8610]):

   info = [
     id : bstr,
     id_context : bstr,
     type : bool,
     L: uint
   ]

   where:

   *  id is the Sender ID of the endpoint that generated PIV.

   *  id_context is the ID Context (Gid) used when protecting M.

      Note that, in case of group rekeying, a server might use a
      different Gid when protecting a response, compared to the Gid that
      it used to verify (that the client used to protect) the request,
      see Section 8.3.

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   *  type is the CBOR simple value "true" (0xf5) if M is a request, or
      the CBOR simple value "false" (0xf4) otherwise.

   *  L is the size of the countersignature, as per Signature Algorithm
      from the Common Context (see Section 2.1.5), in bytes.

4.1.2.  Clarifications on Using a Countersignature

   Note that the literature commonly refers to a countersignature as a
   signature computed by an entity A over a document already protected
   by a different entity B.

   However, the COSE_Countersignature0 structure belongs to the set of
   abbreviated countersignatures defined in Sections 3.2 and 3.3 of
   [I-D.ietf-cose-countersign], which were designed primarily to deal
   with the problem of encrypted group messaging, but where it is
   required to know who originated the message.

   Since the parameters for computing or verifying the abbreviated
   countersignature generated by A are provided by the same context used
   to describe the security processing performed by B and to be
   countersigned, these structures are applicable also when the two
   entities A and B are actually the same one, like the sender of a
   Group OSCORE message protected in group mode.

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, if the request was protected in group mode.
   That is, unlike in [RFC8613], the 'kid' parameter is always present
   in responses to a request that was protected in group mode.

   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.

4.3.  external_aad

   The external_aad of the Additional Authenticated Data (AAD) is
   different compared to OSCORE [RFC8613], and is defined in this
   section.

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   The same external_aad structure is used in group mode and pairwise
   mode for authenticated encryption/decryption (see Section 5.3 of
   [I-D.ietf-cose-rfc8152bis-struct]), as well as in group mode for
   computing and verifying the countersignature (see Section 4.4 of
   [I-D.ietf-cose-rfc8152bis-struct]).

   In particular, the external_aad includes also the Signature
   Algorithm, the Signature Encryption Algorithm, the Pairwise Key
   Agreement Algorithm, the value of the 'kid context' in the COSE
   object of the request, the OSCORE option of the protected message,
   the sender's authentication credential, and the Group Manager's
   authentication credential.

   The external_aad SHALL be a CBOR array wrapped in a bstr object as
   defined below, following the notation of [RFC8610]:

     external_aad = bstr .cbor aad_array

     aad_array = [
        oscore_version : uint,
        algorithms : [alg_aead : int / tstr / null,
                      alg_signature_enc : int / tstr / null,
                      alg_signature : int / tstr / null,
                      alg_pairwise_key_agreement : int / tstr / null],
        request_kid : bstr,
        request_piv : bstr,
        options : bstr,
        request_kid_context : bstr,
        OSCORE_option: bstr,
        sender_cred: bstr,
        gm_cred: bstr / null
     ]

                           Figure 3: external_aad

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

   *  The 'algorithms' array is extended as follows.

      The parameter 'alg_aead' MUST be set to the CBOR simple value
      "null" (0xf6) if the group does not use the pairwise mode,
      regardless whether the endpoint supports the pairwise mode or not.
      Otherwise, this parameter MUST encode the value of AEAD Algorithm
      from the Common Context (see Section 2.1.1), as per Section 5.4 of
      [RFC8613].

      Furthermore, the 'algorithms' array additionally includes:

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      -  'alg_signature_enc', which specifies Signature Encryption
         Algorithm from the Common Context (see Section 2.1.5).  This
         parameter MUST be set to the CBOR simple value "null" (0xf6) if
         the group does not use the group mode, regardless whether the
         endpoint supports the group mode or not.  Otherwise, this
         parameter MUST encode the value of Signature Encryption
         Algorithm as a CBOR integer or text string, consistently with
         the "Value" field in the "COSE Algorithms" Registry for this
         AEAD algorithm.

      -  'alg_signature', which specifies Signature Algorithm from the
         Common Context (see Section 2.1.5).  This parameter MUST be set
         to the CBOR simple value "null" (0xf6) if the group does not
         use the group mode, regardless whether the endpoint supports
         the group mode or not.  Otherwise, this parameter MUST encode
         the value of Signature Algorithm as a CBOR integer or text
         string, consistently with the "Value" field in the "COSE
         Algorithms" Registry for this signature algorithm.

      -  'alg_pairwise_key_agreement', which specifies Pairwise Key
         Agreement Algorithm from the Common Context (see
         Section 2.1.5).  This parameter MUST be set to the CBOR simple
         value "null" (0xf6) if the group does not use the pairwise
         mode, regardless whether the endpoint supports the pairwise
         mode or not.  Otherwise, this parameter MUST encode the value
         of Pairwise Key Agreement Algorithm as a CBOR integer or text
         string, consistently with the "Value" field in the "COSE
         Algorithms" Registry for this HKDF algorithm.

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

      In case Observe [RFC7641] is used, this enables endpoints to
      safely keep an observation active beyond a possible change of Gid
      (i.e., of ID Context), following a group rekeying (see
      Section 3.2).  In fact, it ensures that every notification
      cryptographically matches with only one observation request,
      rather than with multiple ones that were protected with different
      keying material but share the same 'request_kid' and 'request_piv'
      values.

   *  The new element 'OSCORE_option', containing the value of the
      OSCORE Option present in the protected message, encoded as a
      binary string.  This prevents the attack described in Section 12.7
      when using the group mode, as further explained in Section 12.7.2.

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      Note for implementation: this construction requires the OSCORE
      option of the message to be generated and finalized before
      computing the ciphertext of the COSE_Encrypt0 object (when using
      the group mode or the pairwise mode) and before calculating the
      countersignature (when using the group mode).  Also, the aad_array
      needs to be large enough to contain the largest possible OSCORE
      option.

   *  The new element 'sender_cred', containing the sender's
      authentication credential.  This parameter MUST be set to a CBOR
      byte string, which encodes the sender's authentication credential
      in its original binary representation made available to other
      endpoints in the group (see Section 2.3).

   *  The new element 'gm_cred', containing the Group Manager's
      authentication credential.  If no Group Manager maintains the
      group, this parameter MUST encode the CBOR simple value "null"
      (0xf6).  Otherwise, this parameter MUST be set to a CBOR byte
      string, which encodes the Group Manager's authentication
      credential in its original binary representation made available to
      other endpoints in the group (see Section 2.3).  This prevents the
      attack described in Section 12.8.

5.  OSCORE Header Compression

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

   *  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.

   *  This document defines the usage of the sixth least significant
      bit, called "Group Flag", in the first byte of the OSCORE option
      containing the OSCORE flag bits.  This flag bit is specified in
      Section 13.1.

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

   *  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].

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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, the COSE_CounterSignature0 byte string as described in
   Section 4 is assumed to be 64 bytes long.

5.1.1.  Examples in Group Mode

   *  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:h'de9e ... f1' },
         h'aea0155667924dff8a24e4cb35b9'
         ]

      * After compression (85 bytes):

         Flag byte: 0b00111001 = 0x39 (1 byte)

         Option Value: 0x39 05 03 44 61 6c 25 (7 bytes)

         Payload: 0xaea0155667924dff8a24e4cb35b9 de9e ... f1
         (14 bytes + size of the encrypted countersignature)

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

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      * Before compression (88 bytes):

         [
         h'',
         { 4:h'52', 11:h'ca1e ... b3' },
         h'60b035059d9ef5667c5a0710823b'
         ]

      * After compression (80 bytes):

         Flag byte: 0b00101000 = 0x28 (1 byte)

         Option Value: 0x28 52 (2 bytes)

         Payload: 0x60b035059d9ef5667c5a0710823b ca1e ... b3
         (14 bytes + size of the encrypted countersignature)

5.1.2.  Examples in Pairwise Mode

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

      * Before compression (29 bytes):

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

      * After compression (21 bytes):

         Flag byte: 0b00011001 = 0x19 (1 byte)

         Option Value: 0x19 05 03 44 61 6c 25 (7 bytes)

         Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes)

   *  Response with ciphertext = 0x60b035059d9ef5667c5a0710823b and no
      Partial IV.

      * Before compression (18 bytes):

         [
         h'',
         {},
         h'60b035059d9ef5667c5a0710823b'
         ]

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      * After compression (14 bytes):

         Flag byte: 0b00000000 = 0x00 (1 byte)

         Option Value: 0x (0 bytes)

         Payload: 0x60b035059d9ef5667c5a0710823b (14 bytes)

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

   The requirements and properties described in Section 7 of [RFC8613]
   also apply to Group OSCORE.  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.  In addition, the following holds for Group OSCORE.

6.1.  Supporting Observe

   When Observe [RFC7641] is used, a client maintains for each ongoing
   observation one Notification Number for each different server.  Then,
   separately for each server, the client uses the associated
   Notification Number to perform ordering and replay protection of
   notifications received from that server (see Section 8.4.1).

   Group OSCORE allows to preserve an observation active indefinitely,
   even in case the group is rekeyed, with consequent change of ID
   Context, or in case the observer client obtains a new Sender ID.

   As defined in Section 8 when discussing support for Observe, this is
   achieved by the client and server(s) storing the 'kid' and 'kid
   context' used in the original Observe request, throughout the whole
   duration of the observation.

   Upon leaving the group or before re-joining the group, a group member
   MUST terminate all the ongoing observations that it has started in
   the group as observer client.

6.2.  Update of Replay Window

   Sender Sequence Numbers seen by a server as Partial IV values in
   request messages can spontaneously increase at a fast pace, for
   example when a client exchanges unicast messages with other servers
   using the Group OSCORE Security Context.  As in OSCORE [RFC8613], a
   server always needs to accept such increases and accordingly updates
   the Replay Window in each of its Recipient Contexts.

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   As discussed in Section 2.5.1, a newly created Recipient Context
   would have an invalid Replay Window, if its installation has required
   to delete another Recipient Context.  Hence, the server is not able
   to verify if a request from the client associated with the new
   Recipient Context is a replay.  When this happens, the server MUST
   validate the Replay Window of the new Recipient Context, before
   accepting messages from the associated client (see Section 2.5.1).

   Furthermore, when the Group Manager establishes a new Security
   Context for the group (see Section 2.5.3.2), every server re-
   initializes the Replay Window in each of its Recipient Contexts.

6.3.  Message Freshness

   When receiving a request from a client for the first time, the server
   is not synchronized with the client's Sender Sequence Number, i.e.,
   it is not able to verify if that request is fresh.  This applies to a
   server that has just joined the group, with respect to already
   present clients, and recurs as new clients are added as group
   members.

   During its operations in the group, the server may also lose
   synchronization with a client's Sender Sequence Number.  This can
   happen, for instance, if the server has rebooted or has deleted its
   previously synchronized version of the Recipient Context for that
   client (see Section 2.5.1).

   If the application requires message freshness, e.g., according to
   time- or event-based policies, the server has to (re-)synchronize
   with a client's Sender Sequence Number before delivering request
   messages from that client to the application.  To this end, the
   server can use the approach in Section 10 based on the Echo Option
   for CoAP [RFC9175], as a variant of the approach defined in
   Appendix B.1.2 of [RFC8613] applicable to Group OSCORE.

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 Group OSCORE Security Context to process Group
   OSCORE messages as defined in this document.

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   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 Signature Algorithm and/or Pairwise Key
   Agreement 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 conditions holds, a recipient endpoint
   MUST discard the incoming protected message:

   *  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 with an OSCORE group, but the endpoint does not
      support the pairwise mode.

   *  The Group Flag is set to 1, and the recipient endpoint retrieves a
      Security Context which is both valid to process the message and
      also associated with an OSCORE group, but the endpoint does not
      support the group mode.

   *  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 with an OSCORE group.

      As per Section 6.1 of [RFC8613], this holds also when retrieving a
      Security Context which is valid but not associated with an OSCORE
      group.  Future specifications may define how to process incoming
      messages protected with a Security Contexts as in [RFC8613], when
      the Group Flag bit is set to 1.

   Otherwise, if a Security Context associated with 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 with an OSCORE group, then the message is
   processed according to [RFC8613].

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.

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   The Group Manager indicates that the group uses (also) the group
   mode, as part of the group data provided to candidate group members
   when joining the group.

   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 2.5.3.2) 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].  For encryption and
   decryption operations, the Signature Encryption Algorithm from the
   Common Context is used.

   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 predefined, sufficient
   amount of time, in order to correctly perform potential
   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 when sent over non-
   multicast transports, 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
   reject any message which is malformed and does not follow the format
   specified in Section 4 of this document, or which is not
   cryptographically validated in a successful way.

   In either case, it is RECOMMENDED that a server does not send back
   any error message in reply to a received request, if any of the two
   following conditions holds:

   *  The server is not able to identify the received request as a group
      request, i.e., as sent to all servers in the group.

   *  The server identifies the received request as a group request.

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   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.

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

   *  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.
      The Signature Encryption Algorithm from the Common Context MUST be
      used.

   *  In step 5, the countersignature 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 encrypted countersignature (see Section 4.1).

8.1.1.  Supporting Observe

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

   *  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 2.5.3.1), the client MUST NOT
      update the stored value associated with a particular Observe
      request.

   *  If the client intends to keep the observation active beyond a
      possible change of ID Context following a group rekeying (see
      Section 3.2), 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 Gid as ID Context (see Section 2.5.3.2), the
         client MUST NOT update the stored value associated with a
         particular Observe request.

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      -  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
         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.

   *  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 with the OSCORE group, the server
         MAY respond with a 4.01 (Unauthorized) error message.  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
         authentication credential associated with the Recipient ID to
         be stored in the new Recipient Context.  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.

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

   *  In step 6, the server also verifies the countersignature, by using
      the public key from the client's authentication credential stored
      in the associated Recipient Context.  In particular:

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      -  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].

      -  The server MUST perform signature verification before
         decrypting the COSE object, as defined below.  Implementations
         that cannot perform the two steps in this order MUST ensure
         that no access to the plaintext is possible before a successful
         signature verification and MUST prevent any possible leak of
         time-related information that can yield side-channel attacks.

      -  The server retrieves the encrypted countersignature
         ENC_SIGNATURE from the message payload, and computes the
         original countersignature SIGNATURE as

         SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM

         where KEYSTREAM is derived as per Section 4.1.1.

         The server verifies the original countersignature SIGNATURE.

      -  If the signature verification fails, the server SHALL stop
         processing the request, SHALL NOT update the Replay Window, and
         MAY respond with a 4.00 (Bad Request) response.  The server MAY
         set an Outer Max-Age option with value zero.  The diagnostic
         payload MAY contain a string, which, if present, MUST be
         "Decryption failed" as if the decryption had failed.

      -  When decrypting the COSE object using the Recipient Key, the
         Signature Encryption Algorithm from the Common Context MUST be
         used.

   *  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 7.2.1 of
   [I-D.ietf-core-observe-multicast-notifications].

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8.2.1.  Supporting Observe

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

   *  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 with 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 2.5.3.1).

   *  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 3.2).  That is,
      upon establishing a new Security Context with a new Gid as ID
      Context (see Section 2.5.3.2), the server MUST NOT update the
      stored value associated with 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 3.2).

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

   *  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 3.2), 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.

   *  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.
      The Signature Encryption Algorithm from the Common Context MUST be
      used.

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      If the server is using a different ID Context (Gid) for the
      response compared to what was used to verify the request (see
      Section 3.2), then the new ID Context MUST be included in the 'kid
      context' parameter of the response.

      The server can obtain a new Sender ID from the Group Manager, when
      individually rekeyed (see Section 2.5.3.1) or when re-joining the
      group.  In such a case, the server can help the client to
      synchronize, by including the 'kid' parameter in a response
      protected in group mode, even when the request was protected in
      pairwise mode (see Section 9.3).

      That is, when responding to a request protected in pairwise mode,
      the server SHOULD include the 'kid' parameter in a response
      protected in group mode, if it is replying to that client for the
      first time since the assignment of its new Sender ID.

   *  In step 5, the countersignature 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 encrypted countersignature (see Section 4.1).

8.3.1.  Supporting Observe

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

   *  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 external_aad structure (see
      Section 4.3).

   *  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
      external_aad structure (see Section 4.3).

   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 can help the client to
   synchronize, by including also the 'kid context' parameter in
   notifications following a group rekeying, with value set to the ID
   Context (Gid) of the new Security Context.

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   If there is a known upper limit to the duration of a group rekeying,
   the server SHOULD include the 'kid context' parameter during that
   time.  Otherwise, the server SHOULD include it until the Max-Age has
   expired for the last notification sent before the installation of the
   new Security Context.

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
   request, and that, upon the establishment of a new Security Context,
   the client re-initializes its Replay Windows in its Recipient
   Contexts (see Section 3.2).

   *  In step 2, the decoding of the compressed COSE object is modified
      as described in Section 5 of this document.  In particular, a
      'kid' may not be present, if the response is a reply to a request
      protected in pairwise mode.  In such a case, the client assumes
      the response 'kid' to be the Recipient ID for the server to which
      the request protected in pairwise mode was intended for.

      If the response 'kid context' matches an existing ID Context (Gid)
      but the received/assumed '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 authentication
      credential associated with the Recipient ID to be stored in the
      new Recipient Context.  If the application does not specify
      dynamic derivation of new Recipient Contexts, then the client
      SHALL stop processing the response.

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

   *  In step 5, the client also verifies the countersignature, by using
      the public key from the server's authentication credential stored
      in the associated Recipient Context.  In particular:

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      -  The client MUST perform signature verification before
         decrypting the COSE object, as defined below.  Implementations
         that cannot perform the two steps in this order MUST ensure
         that no access to the plaintext is possible before a successful
         signature verification and MUST prevent any possible leak of
         time-related information that can yield side-channel attacks.

      -  The client retrieves the encrypted countersignature
         ENC_SIGNATURE from the message payload, and computes the
         original countersignature SIGNATURE as

         SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM

         where KEYSTREAM is derived as per Section 4.1.1.

         The client verifies the original countersignature SIGNATURE.

      -  If the verification of the countersignature fails, the server
         SHALL stop processing the response, and SHALL NOT update the
         Notification Number associated with the server if the response
         is an Observe notification [RFC7641].

      -  After a successful verification of the countersignature, the
         client performs also the following actions if the response is
         not an Observe notification.

         o  In case the request was protected in pairwise mode and the
            'kid' parameter is present in the response, the client
            checks whether this received 'kid' is equal to the expected
            'kid', i.e., the known Recipient ID for the server to which
            the request was intended for.

         o  In case the request was protected in pairwise mode and the
            'kid' parameter is not present in the response, the client
            checks whether the server that has sent the response is the
            same one to which the request was intended for.  This can be
            done by checking that the public key used to verify the
            countersignature of the response is equal to the public key
            included in the authentication credential Recipient Auth
            Cred, which was taken as input to derive the Pairwise Sender
            Key used for protecting the request (see Section 2.4.1).

         In either case, if the client determines that the response has
         come from a different server than the expected one, then the
         client SHALL discard the response and SHALL NOT deliver it to
         the application.  Otherwise, the client hereafter considers the
         received 'kid' as the current Recipient ID for the server.

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      -  When decrypting the COSE object using the Recipient Key, the
         Signature Encryption Algorithm from the Common Context MUST be
         used.

   *  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.

   *  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
      'request_kid' parameter in the external_aad structure (see
      Section 4.3).

   *  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
      external_aad structure (see Section 4.3).

   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 2.5.3.1), as well as
   when it installs a new Security Context with a new ID Context (Gid)
   following a group rekeying (see Section 3.2).

   *  The ordering and the replay protection of notifications received
      from a server are performed as per Sections 4.1.3.5.2 and 7.4.1 of
      [RFC8613], by using the Notification Number associated with that
      server for the observation in question.  In addition, the client
      performs the following actions for each ongoing observation.

      -  When receiving the first valid notification from a server, the
         client MUST store the current kid "kid1" of that server for the
         observation in question.  If the 'kid' field is included in the
         OSCORE option of the notification, its value specifies "kid1".
         If the Observe request was protected in pairwise mode (see
         Section 9.3), the 'kid' field may not be present in the OSCORE
         option of the notification (see Section 4.2).  In this case,
         the client assumes "kid1" to be the Recipient ID for the server
         to which the Observe request was intended for.

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      -  When receiving another valid notification from the same server
         - which can be identified and recognized through the same
         public key used to verify the countersignature and included in
         the server's authentication credential - the client determines
         the current kid "kid2" of the server as above for "kid1", and
         MUST check whether "kid2" is equal to the stored "kid1".  If
         "kid1" and "kid2" are different, the client MUST cancel or re-
         register the observation in question.

         Note that, if "kid2" is different from "kid1" and the 'kid'
         field is omitted from the notification - which is possible if
         the Observe request was protected in pairwise mode - then the
         client will compute a wrong keystream to decrypt the
         countersignature (i.e., by using "kid1" rather than "kid2" in
         the 'id' field of the 'info' array in Section 4.1.1), thus
         subsequently failing to verify the countersignature and
         discarding the notification.

   This ensures that the client remains able to correctly perform the
   ordering and replay protection of notifications, even in case a
   server legitimately starts using a new Sender ID, as received from
   the Group Manager when individually rekeyed (see Section 2.5.3.1) or
   when re-joining the group.

8.5.  External Signature Checkers

   When receiving a message protected in group mode, a signature checker
   (see Section 3.1) proceeds as follows.

   *  The signature checker retrieves the encrypted countersignature
      ENC_SIGNATURE from the message payload, and computes the original
      countersignature SIGNATURE as

      SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM

      where KEYSTREAM is derived as per Section 4.1.1.

   *  The signature checker verifies the original countersignature
      SIGNATURE, by using the public key of the sender endpoint as
      included in that endpoint's authentication credential.  The
      signature checker determines the right authentication credential
      based on the ID Context (Gid) and the Sender ID of the sender
      endpoint.

   Note that the following applies when attempting to verify the
   countersignature of a response message.

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   *  The response may not include a Partial IV and/or an ID Context.
      In such a case, the signature checker considers the same values
      from the corresponding request, i.e., the request matching with
      the response by CoAP Token value.

   *  The response may not include a Sender ID.  This can happen when
      the response protected in group mode matches a request protected
      in pairwise mode (see Section 9.1), with a case in point provided
      by [I-D.amsuess-core-cachable-oscore].  In such a case, the
      signature checker needs to use other means (e.g., source
      addressing information of the server endpoint) to identify the
      correct authentication credential including the public key to use
      for verifying the countersignature of the response.

   The particular actions following a successful or unsuccessful
   verification of the countersignature are application specific and out
   of the scope of this document.

9.  Message Processing in Pairwise Mode

   When using the pairwise mode of Group OSCORE, messages are protected
   and processed as in [RFC8613], 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 in a group
   that uses also the group 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.  For encryption and decryption
   operations, the AEAD Algorithm from the Common Context is used (see
   Section 2.1.1).

   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).

   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 [RFC9175] 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 3.8 of

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   [I-D.ietf-core-groupcomm-bis]).  This prevents the attack described
   in Section 12.9, which leverages requests sent over unicast to a
   single group member and protected with the group mode.

   Senders cannot use the pairwise mode to protect a message intended
   for multiple recipients.  In fact, the pairwise mode is defined only
   between two endpoints and the keying material is thus only available
   to one recipient.

   However, a sender can use the pairwise mode to protect a message sent
   to (but not intended for) multiple recipients, if interested in a
   response from only one of them.  For instance, this is useful to
   support the address discovery service defined in Section 9.1, when a
   single 'kid' value is indicated in the payload of a request sent to
   multiple recipients, e.g., over multicast.

   The Group Manager indicates 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 authentication credential and the Recipient ID for
   the recipient endpoint, as stored in the Recipient Context associated
   with that endpoint (see Section 2.4.4).

   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 authentication credential 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 set of servers, identified by their 'kid'
   values specified in the request payload.  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.  Main Differences from OSCORE

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

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

   *  The external_aad defined in Section 4.3 of this document is used
      for the encryption process.

   *  The Pairwise Sender/Recipient Keys used as Sender/Recipient keys
      are derived as defined in Section 2.4 of this document.

9.3.  Protecting the Request

   When using the pairwise mode, the request is protected as defined in
   Section 8.1 of [RFC8613], with the differences summarized in
   Section 9.2 of this document.  The following difference also applies.

   *  If Observe [RFC7641] is supported, what is defined in
      Section 8.1.1 of this document holds.

9.4.  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 of [RFC8613], with the differences summarized in
   Section 9.2 of this document.  The following differences also apply.

   *  If the server discards the request due to not retrieving a
      Security Context associated with the OSCORE group or to not
      supporting the pairwise mode, the server MAY respond with a 4.01
      (Unauthorized) error message or a 4.02 (Bad Option) error message,
      respectively.  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 a new Recipient Context is created for this Recipient ID, new
      Pairwise Sender/Recipient Keys are also derived (see
      Section 2.4.1).  The new Pairwise Sender/Recipient Keys are
      deleted if the Recipient Context is deleted as a result of the
      message not being successfully verified.

   *  If Observe [RFC7641] is supported, what is defined in
      Section 8.2.1 of this document holds.

9.5.  Protecting the Response

   When using the pairwise mode, a response is protected as defined in
   Section 8.3 of [RFC8613], with the differences summarized in
   Section 9.2 of this document.  The following differences also apply.

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   *  If the server is using a different Security Context for the
      response compared to what was used to verify the request (see
      Section 3.2), 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.

   *  If the server is using a different ID Context (Gid) for the
      response compared to what was used to verify the request (see
      Section 3.2), then the new ID Context MUST be included in the 'kid
      context' parameter of the response.

   *  The server can obtain a new Sender ID from the Group Manager, when
      individually rekeyed (see Section 2.5.3.1) or when re-joining the
      group.  In such a case, the server can help the client to
      synchronize, by including the 'kid' parameter in a response
      protected in pairwise mode, even when the request was also
      protected in pairwise mode.

      That is, when responding to a request protected in pairwise mode,
      the server SHOULD include the 'kid' parameter in a response
      protected in pairwise mode, if it is replying to that client for
      the first time since the assignment of its new Sender ID.

   *  If Observe [RFC7641] is supported, what is defined in
      Section 8.3.1 of this document holds.

9.6.  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 of [RFC8613], with the differences summarized in
   Section 9.2 of this document.  The following differences also apply.

   *  The client may receive a response protected with a Security
      Context different from the one used to protect the corresponding
      request.  Also, upon the establishment of a new Security Context,
      the client re-initializes its Replay Windows in its Recipient
      Contexts (see Section 2.2).

   *  The same as described in Section 8.4 holds with respect to
      handling the 'kid' parameter of the response, when received as a
      reply to a request protected in pairwise mode.  The client can
      also in this case check whether the replying server is the
      expected one, by relying on the server's public key.  However,
      since the response is protected in pairwise mode, the public key
      is not used for verifying a countersignature as in Section 8.4.
      Instead, the expected server's authentication credential - namely

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      Recipient Auth Cred and including the server's public key - was
      taken as input to derive the Pairwise Recipient Key used to
      decrypt and verify the response (see Section 2.4.1).

   *  If a new Recipient Context is created for this Recipient ID, new
      Pairwise Sender/Recipient Keys are also derived (see
      Section 2.4.1).  The new Pairwise Sender/Recipient Keys are
      deleted if the Recipient Context is deleted as a result of the
      message not being successfully verified.

   *  If Observe [RFC7641] is supported, what is defined in
      Section 8.4.1 of this document holds.  The client can also in this
      case identify a server to be the same one across a change of
      Sender ID, by relying on the server's public key.  As to the
      expected server's authentication credential, the same holds as
      specified above for non-notification responses.

10.  Challenge-Response Synchronization

   This section describes how a server endpoint can synchronize with
   Sender Sequence Numbers of client endpoints in the group.  Similarly
   to what is defined in Appendix B.1.2 of [RFC8613], the server
   performs a challenge-response exchange with a client, by using the
   Echo Option for CoAP specified in Section 2 of [RFC9175].

   Upon receiving a request from a particular client for the first time,
   the server processes the message as described in this document, 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 Echo option value SHOULD NOT be reused;
   when it is reused, it MUST be highly unlikely to have been recently
   used with this client.  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 in the response
   together with the pair (gid,kid), where 'gid' is the Group Identifier
   of the OSCORE group and 'kid' is the Sender ID of the client in the
   group.  These are specified in the 'kid context' and 'kid' fields of
   the OSCORE Option of the 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 3.2), the server MUST delete all the stored
   Echo values associated with members of the group.

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   Upon receiving a 4.01 (Unauthorized) response that includes an Echo
   Option and originates from a verified group member, the 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.

   If the group uses also the group mode and the used Signature
   Algorithm supports ECDH (e.g., ECDSA, EdDSA), the client MUST use the
   pairwise mode to protect the request, as per Section 9.3.  Note that,
   as defined in Section 9, endpoints that are members of such a group
   and that use the Echo Option MUST support the pairwise mode.

   The client does not necessarily resend the same group request, but
   can instead send a more recent one, if the application permits it.
   This allows 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 preconfigured time
   interval.  Note that the unicast request echoing the Echo Option is
   correctly treated and processed, 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.

   *  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.

   *  If the server stores an Echo Option value for the pair (gid,kid)
      associated with 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, hence performing a new challenge-response exchange.

   If the verifications above are successful, the server proceeds as
   follows.  In case the Replay Window in the Recipient Context
   associated with the client has not been set yet, the server updates

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   the Replay Window to mark the current Sender Sequence Number from the
   latest received request as seen (but all newer ones as new), and
   delivers the message as fresh to the application.  Otherwise, the
   server discards the verification result and treats the message as
   fresh or as a replay, according to the existing Replay Window.

   A server should not deliver 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.  A server may
   perform the challenge-response described above at any time, if
   synchronization with Sender Sequence Numbers of clients is lost,
   e.g., after a device reboot.  A client has to be ready to perform the
   challenge-response based on the Echo Option if 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, 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.

   In order to ameliorate this, the server may 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, preconfigured 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 section 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.

   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.
   Thus, silent servers should adopt alternative approaches to achieve
   and maintain synchronization with Sender Sequence Numbers of clients.

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   Since requests including the Echo Option are sent over unicast, a
   server can be victim of the attack discussed in Section 12.9, in case
   such requests are protected with the group mode.  Instead, protecting
   those requests with the pairwise mode prevents the attack above.  In
   fact, only the exact server involved in the challenge-response
   exchange is able to derive the 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,
   even if the group mode was used, this would require the adversary to
   forge the countersignature of both 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.

11.  Implementation Compliance

   Like in [RFC8613], HKDF SHA-256 is the mandatory to implement HKDF.

   An endpoint may support only the group mode, or only the pairwise
   mode, or both.

   For endpoints that support the group mode, the following applies.

   *  For endpoints that use authenticated encryption, the AEAD
      algorithm AES-CCM-16-64-128 defined in Section 4.2 of
      [I-D.ietf-cose-rfc8152bis-algs] is mandatory to implement as
      Signature Encryption Algorithm (see Section 2.1.4).

   *  For many constrained IoT devices it is problematic to support more
      than one signature algorithm.  Existing devices can be expected to
      support either EdDSA or ECDSA.  In order to enable as much
      interoperability as we can reasonably achieve, the following
      applies with respect to the Signature Algorithm (see
      Section 2.1.5).

      Less constrained endpoints SHOULD implement both: the EdDSA
      signature algorithm together with the elliptic curve Ed25519
      [RFC8032]; and the ECDSA signature algorithm together with the
      elliptic curve P-256.

      Constrained endpoints SHOULD implement: the EdDSA signature
      algorithm together with the elliptic curve Ed25519 [RFC8032]; or
      the ECDSA signature algorithm together with the elliptic curve
      P-256.

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   *  Endpoints that implement the ECDSA signature algorithm MAY use
      "deterministic ECDSA" as specified in [RFC6979].  Pure
      deterministic elliptic-curve signature algorithms such as
      deterministic ECDSA and EdDSA have the advantage of not requiring
      access to a source of high-quality randomness.  However, these
      signature algorithms have been shown vulnerable to some side-
      channel and fault injection attacks due to their determinism,
      which can result in extracting a device's private key.  As
      suggested in Section 2.1.1 of [I-D.ietf-cose-rfc8152bis-algs],
      this can be addressed by combining both randomness and determinism
      [I-D.mattsson-cfrg-det-sigs-with-noise].

   For endpoints that support the pairwise mode, the following applies.

   *  The AEAD algorithm AES-CCM-16-64-128 defined in Section 4.2 of
      [I-D.ietf-cose-rfc8152bis-algs] is mandatory to implement as AEAD
      Algorithm (see Section 2.1.1).

   *  The ECDH-SS + HKDF-256 algorithm specified in Section 6.3.1 of
      [I-D.ietf-cose-rfc8152bis-algs] is mandatory to implement as
      Pairwise Key Agreement Algorithm (see Section 2.1.7).

   *  In order to enable as much interoperability as we can reasonably
      achieve in the presence of constrained devices (see above), the
      following applies.

      Less constrained endpoints SHOULD implement both the X25519 curve
      [RFC7748] and the P-256 curve as ECDH curves.

      Constrained endpoints SHOULD implement the X25519 curve [RFC7748]
      or the P-256 curve as ECDH curve.

   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].

12.  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 countersignatures, as discussed in Section 12.1.

   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

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

   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 12.3 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 differences.  First, the 'kid context' of request
   messages is part of the Additional Authenticated Data, thus safely
   enabling to keep observations active beyond a possible change of ID
   Context (Gid), following a group rekeying (see Section 4.3).  Second,
   the countersignature included in a Group OSCORE message protected in
   group mode is computed also over the value of the OSCORE option,
   which is also part of the Additional Authenticated Data used in the
   signing process.  This is further discussed in Section 12.7 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.

   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.

12.1.  Security of the Group Mode

   The group mode defined in Section 8 relies on commonly shared group
   keying material to protect communication within a group.  Using the
   group mode has the implications discussed below.  The following
   refers to group members as the endpoints in the group storing the
   latest version of the group keying material.

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   *  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.

   *  If the used encryption algorithm provides integrity protection,
      then it also ensures group authentication and proof of group
      membership, but not source authentication.  That is, 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.  In fact, any
      group member is able to derive the Sender Key used by the actual
      sender endpoint, and thus can compute a valid authentication tag.
      Therefore, the message content could originate from any of the
      current group members.

      Furthermore, if the used encryption algorithm does not provide
      integrity protection, then it does not ensure any level of message
      authentication or proof of group membership.

      On the other hand, proof of group membership is always ensured by
      construction through the strict management of the group keying
      material (see Section 3.2).  That is, the group is rekeyed in case
      of members' leaving, and the current group members are informed of
      former group members.  Thus, a current group member storing the
      latest group keying material does not store the authentication
      credential of any former group member.

      This allows a recipient endpoint to rely on the stored
      authentication credentials and public keys included therin, in
      order to always confidently assert the group membership of a
      sender endpoint when processing an incoming message, i.e., to
      assert that the sender endpoint was a group member when it signed
      the message.  In turn, this prevents a former group member to
      possibly re-sign and inject in the group a stored message that was
      protected with old keying material.

   *  Source authentication of messages sent to a group is ensured
      through a countersignature, which is computed by the sender using
      its own private key and then appended to the message payload.
      Also, the countersignature is encrypted by using a keystream
      derived from the group keying material (see Section 4.1).  This
      ensures group privacy, i.e., an attacker cannot track an endpoint
      over two groups by linking messages between the two groups, unless
      being also a member of those groups.

   The security properties of the group mode are summarized below.

   1.  Asymmetric source authentication, by means of a countersignature.

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   2.  Symmetric group authentication, by means of an authentication tag
       (only for encryption algorithms providing integrity protection).

   3.  Symmetric group confidentiality, by means of symmetric
       encryption.

   4.  Proof of group membership, by strictly managing the group keying
       material, as well as by means of integrity tags when using an
       encryption algorithm that provides also integrity protection.

   5.  Group privacy, by encrypting the countersignature.

   The group mode fulfills the security properties above while also
   displaying the following benefits.  First, the use of an encryption
   algorithm that does not provide integrity protection results in a
   minimal communication overhead, by limiting the message payload to
   the ciphertext and the encrypted countersignature.  Second, it is
   possible to deploy semi-trusted entities such as signature checkers
   (see Section 3.1), which can break property 5, but cannot break
   properties 1, 2 and 3.

12.2.  Security of the Pairwise Mode

   The pairwise mode defined 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 2.4).

   The used encryption algorithm MUST provide integrity protection.
   Therefore, the pairwise mode ensures both pairwise data-
   confidentiality and source authentication of messages, without using
   countersignatures.  Furthermore, the recipient endpoint achieves
   proof of group membership for the sender endpoint, since only current
   group members have the required keying material to derive a valid
   Pairwise Sender/Recipient Key.

   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.

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12.3.  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:

   *  Uniqueness of Sender IDs within the group is enforced by the Group
      Manager.  In fact, from the moment when a Gid is assigned to a
      group until the moment a new Gid is assigned to that same group,
      the Group Manager does not reassign a Sender ID within the group
      (see Section 3.2).

   *  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.

   *  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.

12.4.  Management of Group Keying Material

   The approach described in this document 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 MUST be
   adopted.

   [I-D.ietf-ace-key-groupcomm-oscore] specifies 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 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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12.5.  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 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 request.

   In particular, a server may first get a 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 10.3 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.

12.5.1.  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 a 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

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   group request.  Instead, a recipient server would better and more
   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 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.

12.5.2.  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.

12.6.  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.

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

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12.7.  Cross-group Message Injection

   A same endpoint is allowed to and would likely use the same pair
   (private key, authentication credential) 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 12.7.1).

   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 12.7.2, the attack is practically infeasible
   if the message is protected in group mode, thanks to the
   countersignature also bound to the OSCORE option through the
   Additional Authenticated Data used in the signing process (see
   Section 4.3).

12.7.1.  Attack Description

   Let us consider:

   *  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.

   *  A sender endpoint X which is member of both G1 and G2, and uses
      the same pair (private key, authentication credential) 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 authentication credential of X in G1 and G2, respectively.

   *  A recipient endpoint Y which is member of both G1 and G2, and uses
      the same pair (private key, authentication credential) 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 authentication credential of Y in G1 and G2, respectively.

   *  A malicious endpoint Z is also member of both G1 and G2.  Hence, Z
      is able to derive the Sender Keys used by 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 attempt to 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 set to G2 (rather than G1); and the
   'kid' is set to Sid2 (rather than Sid1).  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 the Pairwise
   Recipient Key associated with 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.

12.7.2.  Attack Prevention in Group Mode

   When a Group OSCORE message is protected with the group mode, the
   countersignature 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).

   That is, other than over the ciphertext, the countersignature is
   computed over: the ID Context (Gid) 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 group requests as well
   as in responses to requests protected in group mode.

   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 12.7.1, since it would require the adversary to
   additionally forge a valid countersignature that replaces the
   original one in the forged message M2.

   If, hypothetically, the countersignature did not cover the OSCORE
   option:

   *  The attack described in Section 12.7.1 would still be possible
      against response messages protected in group mode, since the same
      unchanged countersignature from message M1 would be also valid in
      message M2.

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   *  A simplification would also be possible in performing the attack,
      since Z is able to derive the Sender/Recipient Keys of X and Y in
      G1 and G2.  That is, Z can also set a convenient Partial IV in the
      response, until the same unchanged MAC is successfully verified by
      using G2 as 'request_kid_context', Sid2 as 'request_kid', and the
      symmetric key associated with 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.  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 the same group G1.

12.8.  Prevention of Group Cloning Attack

   Both when using the group mode and the pairwise mode, the message
   protection covers also the Group Manager's authentication credential.
   This is included in the Additional Authenticated Data used in the
   signing process and/or in the integrity-protected encryption process
   (see Section 4.3).

   By doing so, an endpoint X member of a group G1 cannot perform the
   following attack.

   1.  X sets up a group G2 where it acts as Group Manager.

   2.  X makes G2 a "clone" of G1, i.e., G1 and G2 use the same
       algorithms and have the same Master Secret, Master Salt and ID
       Context.

   3.  X collects a message M sent to G1 and injects it in G2.

   4.  Members of G2 accept M and believe it to be originated in G2.

   The attack above is effectively prevented, since message M is
   protected by including the authentication credential of G1's Group
   Manager in the Additional Authenticated Data.  Therefore, members of
   G2 do not successfully verify and decrypt M, since they correctly use
   the authentication credential of X as Group Manager of G2 when
   attempting to.

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12.9.  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.

   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.

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   A client can instead use the pairwise mode as defined in Section 9.3,
   in order to protect a request sent to a single group member by using
   pairwise keying material (see Section 2.4).  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 7.2.1 of [I-D.ietf-core-observe-multicast-notifications].

   With particular reference to block-wise transfers [RFC7959],
   Section 3.8 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 Section 10, with respect
   to requests including a CoAP Echo Option [RFC9175] that have to be
   sent over unicast, as a challenge-response method for servers to
   achieve synchronization of clients' Sender Sequence Number.

12.10.  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.

12.11.  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
   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].

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12.12.  Replay Protection

   As in OSCORE [RFC8613], 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 2.5.3, 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 12.3) is preserved also when a new Security
   Context is established.

   Since one-to-many communication such as multicast usually involves
   unreliable transports, the simplification of the Replay Window to a
   size of 1 suggested in Section 7.4 of [RFC8613] is not viable with
   Group OSCORE, unless exchanges in the group rely only on unicast
   messages.

   As discussed in Section 6.2, a Replay Window may be initialized as
   not valid, following the loss of mutable Security Context
   Section 2.5.1.  In particular, Section 2.5.1.1 and Section 2.5.1.2
   define measures that endpoints need to take in such a situation,
   before resuming to accept incoming messages from other group members.

12.13.  Message Freshness

   As discussed in Section 6.3, a server may not be able to assert
   whether an incoming request is fresh, in case it does not have or has
   lost synchronization with the client's Sender Sequence Number.

   If freshness is relevant for the application, the server may
   (re-)synchronize with the client's Sender Sequence Number at any
   time, by using the approach described in Section 10 and based on the
   CoAP Echo Option [RFC9175], as a variant of the approach defined in
   Appendix B.1.2 of [RFC8613] applicable to Group OSCORE.

12.14.  Client Aliveness

   Building on Section 12.5 of [RFC8613], a server may use the CoAP Echo
   Option [RFC9175] to verify the aliveness of the client that
   originated a received request, by using the approach described in
   Section 10 of this document.

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12.15.  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 2.5.2, 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 group mode uses the "encrypt-then-sign" construction, i.e., the
   countersignature is computed over the COSE_Encrypt0 object (see
   Section 4.1).  This is motivated by enabling additional entities
   acting as signature checkers (see Section 3.1), which do not join a
   group as members but are allowed to verify countersignatures of
   messages protected in group mode without being able to decrypt them
   (see Section 8.5).

   If the encryption algorithm used in group mode provides integrity
   protection, countersignatures of COSE_Encrypt0 with short
   authentication tags do not provide the security properties associated
   with the same algorithm used in COSE_Sign (see Section 6 of
   [I-D.ietf-cose-countersign]).  To provide 128-bit security against
   collision attacks, the tag length MUST be at least 256-bits.  A
   countersignature of a COSE_Encrypt0 with AES-CCM-16-64-128 provides
   at most 32 bits of integrity protection.

   The derivation of pairwise keys defined in Section 2.4.1 is
   compatible with ECDSA and EdDSA asymmetric keys, but is not
   compatible with RSA asymmetric keys.

   For the public key translation from Ed25519 (Ed448) to X25519 (X448)
   specified in Section 2.4.1, variable time methods can be used since
   the translation operates on public information.  Any byte string of
   appropriate length is accepted as a public key for X25519 (X448) in
   [RFC7748].  It is therefore not necessary for security to validate
   the translated public key (assuming the translation was successful).

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   The security of using the same key pair for Diffie-Hellman and for
   signing (by considering the ECDH procedure in Section 2.4 as a Key
   Encapsulation Mechanism (KEM)) is demonstrated in [Degabriele] and
   [Thormarker].

   Applications using ECDH (except X25519 and X448) based KEM in
   Section 2.4 are assumed to verify that a peer endpoint's public key
   is on the expected curve and that the shared secret is not the point
   at infinity.  The KEM in [Degabriele] checks that the shared secret
   is different from the point at infinity, as does the procedure in
   Section 5.7.1.2 of [NIST-800-56A] which is referenced in Section 2.4.

   Extending Theorem 2 of [Degabriele], [Thormarker] shows that the same
   key pair can be used with X25519 and Ed25519 (X448 and Ed448) for the
   KEM specified in Section 2.4.  By symmetry in the KEM used in this
   document, both endpoints can consider themselves to have the
   recipient role in the KEM - as discussed in Section 7 of [Thormarker]
   - and rely on the mentioned proofs for the security of their key
   pairs.

   Theorem 3 in [Degabriele] shows that the same key pair can be used
   for an ECDH based KEM and ECDSA.  The KEM uses a different KDF than
   in Section 2.4, but the proof only depends on that the KDF has
   certain required properties, which are the typical assumptions about
   HKDF, e.g., that output keys are pseudorandom.  In order to comply
   with the assumptions of Theorem 3, received public keys MUST be
   successfully validated, see Section 5.6.2.3.4 of [NIST-800-56A].  The
   validation MAY be performed by a trusted Group Manager.  For
   [Degabriele] to apply as it is written, public keys need to be in the
   expected subgroup.  For this we rely on cofactor DH, Section 5.7.1.2
   of [NIST-800-56A] which is referenced in Section 2.4.

   HashEdDSA variants of Ed25519 and Ed448 are not used by COSE, see
   Section 2.2 of [I-D.ietf-cose-rfc8152bis-algs], and are not covered
   by the analysis in [Thormarker].  Hence, they MUST NOT be used with
   the public keys used to derive pairwise keys as specified in this
   document.

12.16.  Message Segmentation

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

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12.17.  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, with the following addition.

   *  When protecting a message in group mode, the countersignature is
      encrypted by using a keystream derived from the group keying
      material (see Section 4.1 and Section 4.1.1).  This ensures group
      privacy.  That is, an attacker cannot track an endpoint over two
      groups by linking messages between the two groups, unless being
      also a member of those groups.

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

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

   *  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.

   Furthermore, using the approach described in Section 10 to achieve
   Sender 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.

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   Finally, the approach described in Section 12.6 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.

13.  IANA Considerations

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

   This document has the following actions for IANA.

13.1.  OSCORE Flag Bits Registry

   IANA is asked to add the following value entry to the "OSCORE Flag
   Bits" registry within the "Constrained RESTful Environments (CoRE)
   Parameters" registry group.

 +--------------+------------+-----------------------------+-----------+
 | Bit Position |    Name    |         Description         | Reference |
 +--------------+------------+-----------------------------+-----------+
 |       2      | Group Flag | For using a Group OSCORE    | [This     |
 |              |            | Security Context, set to 1  | Document] |
 |              |            | if the message is protected |           |
 |              |            | with the group mode         |           |
 +--------------+------------+-----------------------------+-----------+

14.  References

14.1.  Normative References

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

   [I-D.ietf-cose-countersign]
              Schaad, J. and R. Housley, "CBOR Object Signing and
              Encryption (COSE): Countersignatures", Work in Progress,

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              Internet-Draft, draft-ietf-cose-countersign-05, 23 June
              2021, <https://www.ietf.org/archive/id/draft-ietf-cose-
              countersign-05.txt>.

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

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

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

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

   [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>.

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

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

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   [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>.

   [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>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [RFC9175]  Amsüss, C., Preuß Mattsson, J., and G. Selander,
              "Constrained Application Protocol (CoAP): Echo, Request-
              Tag, and Token Processing", RFC 9175,
              DOI 10.17487/RFC9175, February 2022,
              <https://www.rfc-editor.org/info/rfc9175>.

14.2.  Informative References

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

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   [I-D.amsuess-core-cachable-oscore]
              Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
              Progress, Internet-Draft, draft-amsuess-core-cachable-
              oscore-04, 6 March 2022, <https://www.ietf.org/archive/id/
              draft-amsuess-core-cachable-oscore-04.txt>.

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

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", Work in Progress, Internet-
              Draft, draft-ietf-ace-key-groupcomm-oscore-13, 7 March
              2022, <https://www.ietf.org/archive/id/draft-ietf-ace-key-
              groupcomm-oscore-13.txt>.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
              draft-ietf-ace-oauth-authz-46, 8 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
              authz-46.txt>.

   [I-D.ietf-core-observe-multicast-notifications]
              Tiloca, M., Höglund, R., Amsüss, C., and F. Palombini,
              "Observe Notifications as CoAP Multicast Responses", Work
              in Progress, Internet-Draft, draft-ietf-core-observe-
              multicast-notifications-03, 7 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-core-observe-
              multicast-notifications-03.txt>.

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

   [I-D.ietf-lwig-curve-representations]
              Struik, R., "Alternative Elliptic Curve Representations",
              Work in Progress, Internet-Draft, draft-ietf-lwig-curve-

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              representations-23, 21 January 2022,
              <https://www.ietf.org/archive/id/draft-ietf-lwig-curve-
              representations-23.txt>.

   [I-D.ietf-lwig-security-protocol-comparison]
              Mattsson, J. P., Palombini, F., and M. Vucinic,
              "Comparison of CoAP Security Protocols", Work in Progress,
              Internet-Draft, draft-ietf-lwig-security-protocol-
              comparison-05, 2 November 2020,
              <https://www.ietf.org/archive/id/draft-ietf-lwig-security-
              protocol-comparison-05.txt>.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-43, 30 April 2021, <https://www.ietf.org/internet-
              drafts/draft-ietf-tls-dtls13-43.txt>.

   [I-D.mattsson-cfrg-det-sigs-with-noise]
              Mattsson, J. P., Thormarker, E., and S. Ruohomaa,
              "Deterministic ECDSA and EdDSA Signatures with Additional
              Randomness", Work in Progress, Internet-Draft, draft-
              mattsson-cfrg-det-sigs-with-noise-04, 15 February 2022,
              <https://www.ietf.org/archive/id/draft-mattsson-cfrg-det-
              sigs-with-noise-04.txt>.

   [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>.

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

   [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>.

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   [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>.

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

   [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>.

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

   [Thormarker]
              Thormarker, E., "On using the same key pair for Ed25519
              and an X25519 based KEM", April 2021,
              <https://eprint.iacr.org/2021/509>.

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:

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

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

   *  CoAP group, i.e., a set of CoAP endpoints where each endpoint is
      configured to receive one-to-many CoAP requests, e.g., sent to the
      group's associated IP multicast address and UDP port as defined in
      [I-D.ietf-core-groupcomm-bis].  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

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      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
      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 points are assumed to be already addressed and are out
   of the scope of this document.

   *  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.  Also, every client
      expects and is able to handle multiple response messages
      associated with a same request sent to the CoAP group.

   *  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.  One should not assume
      that the set of members of a security group remains fixed.  That
      is, the group membership is subject to changes, possibly on a
      frequent basis.

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   *  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.

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

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

   *  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.

   *  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:

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

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

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

   *  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.

   *  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.

   *  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.

   *  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.

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   *  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 lighting devices together with
      the switches acting as clients in the same room or corridor can be
      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.

   *  Integrated building control: enabling Building Automation and
      Control Systems (BACSs) to control multiple heating, ventilation
      and air-conditioning units to predefined 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

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      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.

   *  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
      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.

   *  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.

   *  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.

   *  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

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      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-tolerant multicast algorithm, such as Multicast Protocol for
      Low-Power and Lossy Networks (MPL).

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 with 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 3.2).

   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, i.e., before
   the Group Manager terminates the group or starts reassigning Gid
   values to the group (see Section 3.2).  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 12.6, 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 favorable 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.

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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.

   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, including its own
   authentication credential (see Section 2).  The actual provisioning
   of keying material and parameters to the joining endpoint is out of
   the scope of this document.

   As mentioned in Section 3, the Group Manager and the join process can
   be as specified in [I-D.ietf-ace-key-groupcomm-oscore].

Appendix E.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

E.1.  Version -13 to -14

   *  Replaced "node" with "endpoint" where appropriate.

   *  Replaced "owning" with "storing" (of keying material).

   *  Distinction between "authentication credential" and "public key".

   *  Considerations on storing whole authentication credentials.

   *  Considerations on Denial of Service.

   *  Recycling of Group IDs by tracking the "Birth Gid" of each group
      member is now optional to support and use for the Group Manager.

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   *  Fine-grained suppression of error responses.

   *  Changed section title "Mandatory-to-Implement Compliance
      Requirements" to "Implementation Compliance".

   *  "Challenge-Response Synchronization" moved to the document body.

   *  RFC 7641 and draft-ietf-core-echo-request-tag as normative
      references.

   *  Clarifications and editorial improvements.

E.2.  Version -12 to -13

   *  Fixes in the derivation of the Group Encryption Key.

   *  Added Mandatory-to-Implement compliance requirements.

   *  Changed UCCS to CCS.

E.3.  Version -11 to -12

   *  No mode of operation is mandatory to support.

   *  Revised parameters of the Security Context, COSE object and
      external_aad.

   *  Revised management of keying material for the Group Manager.

   *  Informing of former members when rekeying the group.

   *  Admit encryption-only algorithms in group mode.

   *  Encrypted countersignature through a keystream.

   *  Added public key of the Group Manager as key material and
      protected data.

   *  Clarifications about message processing, especially notifications.

   *  Guidance for message processing of external signature checkers.

   *  Updated derivation of pairwise keys, with more security
      considerations.

   *  Termination of ongoing observations as client, upon leaving or
      before re-joining the group.

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   *  Recycling Group IDs by tracking the "Birth Gid" of each group
      member.

   *  Expanded security and privacy considerations about the group mode.

   *  Removed appendices on skipping signature verification and on COSE
      capabilities.

   *  Fixes and editorial improvements.

E.4.  Version -10 to -11

   *  Loss of Recipient Contexts due to their overflow.

   *  Added diagram on keying material components and their relation.

   *  Distinction between anti-replay and freshness.

   *  Preservation of Sender IDs over rekeying.

   *  Clearer cause-effect about reset of SSN.

   *  The GM provides public keys of group members with associated
      Sender IDs.

   *  Removed 'par_countersign_key' from the external_aad.

   *  One single format for the external_aad, both for encryption and
      signing.

   *  Presence of 'kid' in responses to requests protected with the
      pairwise mode.

   *  Inclusion of 'kid_context' in notifications following a group
      rekeying.

   *  Pairwise mode presented with OSCORE as baseline.

   *  Revised examples with signature values.

   *  Decoupled growth of clients' Sender Sequence Numbers and loss of
      synchronization for server.

   *  Sender IDs not recycled in the group under the same Gid.

   *  Processing and description of the Group Flag bit in the OSCORE
      option.

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   *  Usage of the pairwise mode for multicast requests.

   *  Clarifications on synchronization using the Echo option.

   *  General format of context parameters and external_aad elements,
      supporting future registered COSE algorithms (new Appendix).

   *  Fixes and editorial improvements.

E.5.  Version -09 to -10

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

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

   *  New countersignature header parameter from draft-ietf-cose-
      countersign.

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

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

   *  Added 'request_kid_context' in the aad_array.

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

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

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

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

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

   *  Clarifications on MTI algorithms and curves.

   *  Removed optimized requests.

   *  Overall clarifications and editorial revision.

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E.6.  Version -08 to -09

   *  Pairwise keys are discarded after group rekeying.

   *  Signature mode renamed to group mode.

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

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

   *  Dedicated section on updating the Security Context.

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

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

   *  Separate section on general message reception.

   *  Pairwise mode moved to the document body.

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

   *  Optimized requests are moved as an appendix.

   *  Normative support for the signature and pairwise mode.

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

   *  Appendix with example values of parameters for countersignatures.

   *  Clarifications and editorial improvements.

E.7.  Version -07 to -08

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

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

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

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

   *  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).

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

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

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

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

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

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

   *  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).

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

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

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

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

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

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

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

   *  Editorial improvements.

E.8.  Version -06 to -07

   *  Updated abstract and introduction.

   *  Clarifications of what pertains a group rekeying.

   *  Derivation of pairwise keying material.

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

   *  Defined the Pairwise Flag bit for the OSCORE option.

   *  Supporting CoAP Observe for group requests and responses.

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

   *  New optimized mode based on pairwise keying material.

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

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

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

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

E.9.  Version -05 to -06

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

   *  Clarifications on parameter update upon group rekeying.

   *  Updated external_aad structures.

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

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

   *  Removed client handling of duplicated responses to multicast
      requests.

   *  Additional considerations on public key retrieval and group
      rekeying.

   *  Added Group Manager responsibility on validating public keys.

   *  Updates IANA registries.

   *  Reference to RFC 8613.

   *  Editorial improvements.

E.10.  Version -04 to -05

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

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

   *  Clarification about Recipient Contexts (Section 2).

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

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

   *  Extended Security Considerations (Section 8).

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

E.11.  Version -03 to -04

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

   *  Added recommendation on using "deterministic ECDSA" if ECDSA is
      used as countersignature algorithm (see Section 2).

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

   *  Structured Section 3 into subsections.

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

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

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

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

   *  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).

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

   *  Added explicit step on computing the countersignature for outgoing
      messages (see Sections 6.1 and 6.3).

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

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

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

E.12.  Version -02 to -03

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

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

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

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

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

   *  Clarifications about Non-confirmable messages in Section 5.1
      "Synchronization of Sender Sequence Numbers".

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

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

   *  Revised and extended security considerations in Section 8.

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

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

E.13.  Version -01 to -02

   *  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".

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

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

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

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

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

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

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

   *  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.

E.14.  Version -00 to -01

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

   *  Section 2 has been updated with:

      -  Clarifications on establishment/derivation of Security
         Contexts.

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

   *  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.

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

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

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

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

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

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Acknowledgments

   The authors sincerely thank Christian Amsuess, Stefan Beck, Rolf
   Blom, Carsten Bormann, Esko Dijk, Martin Gunnarsson, Klaus Hartke,
   Rikard Hoeglund, Richard Kelsey, 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
   SE-16440 Stockholm Kista
   Sweden
   Email: marco.tiloca@ri.se

   Göran Selander
   Ericsson AB
   Torshamnsgatan 23
   SE-16440 Stockholm Kista
   Sweden
   Email: goran.selander@ericsson.com

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

   John Preuss Mattsson
   Ericsson AB
   Torshamnsgatan 23
   SE-16440 Stockholm Kista
   Sweden
   Email: john.mattsson@ericsson.com

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

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