Group OSCORE - Secure Group Communication for CoAP
draft-ietf-core-oscore-groupcomm-08

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Replaces draft-tiloca-core-multicast-oscoap
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CoRE Working Group                                             M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                             G. Selander
Expires: October 8, 2020                                    F. Palombini
                                                             Ericsson AB
                                                                 J. Park
                                             Universitaet Duisburg-Essen
                                                          April 06, 2020

           Group OSCORE - Secure Group Communication for CoAP
                   draft-ietf-core-oscore-groupcomm-08

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. using IP
   multicast.  In particular, the described approach defines how OSCORE
   should be used in a group communication setting to provide source
   authentication for CoAP group requests, sent by a client to multiple
   servers, and the corresponding CoAP responses.

Status of This Memo

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

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   This Internet-Draft will expire on October 8, 2020.

Copyright Notice

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

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

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Security Context  . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Common Context  . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Sender Context and Recipient Context  . . . . . . . . . .   8
     2.3.  The Group Manager . . . . . . . . . . . . . . . . . . . .   9
     2.4.  Management of Group Keying Material . . . . . . . . . . .  10
     2.5.  Exhaustion of Partial IV Values . . . . . . . . . . . . .  11
   3.  Pairwise Keys . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.1.  Key Derivation  . . . . . . . . . . . . . . . . . . . . .  12
     3.2.  Usage of Sequence Numbers . . . . . . . . . . . . . . . .  13
     3.3.  Note on Implementation  . . . . . . . . . . . . . . . . .  13
   4.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  14
     4.1.  Counter Signature . . . . . . . . . . . . . . . . . . . .  14
     4.2.  The 'kid' and 'kid context' parameters  . . . . . . . . .  14
     4.3.  external_aad  . . . . . . . . . . . . . . . . . . . . . .  15
       4.3.1.  external_aad for Encryption . . . . . . . . . . . . .  15
       4.3.2.  external_aad for Signing  . . . . . . . . . . . . . .  16
   5.  OSCORE Header Compression . . . . . . . . . . . . . . . . . .  17
     5.1.  Examples of Compressed COSE Objects . . . . . . . . . . .  17
   6.  Message Binding, Sequence Numbers, Freshness and Replay
       Protection  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Synchronization of Sender Sequence Numbers  . . . . . . .  19
   7.  Message Processing  . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  20
       7.1.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  21
     7.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  21
       7.2.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  22
     7.3.  Protecting the Response . . . . . . . . . . . . . . . . .  22
       7.3.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  23
     7.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  23
       7.4.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  24
   8.  Responsibilities of the Group Manager . . . . . . . . . . . .  24
   9.  Optimized Mode  . . . . . . . . . . . . . . . . . . . . . . .  25
     9.1.  Optimized Request . . . . . . . . . . . . . . . . . . . .  25
       9.1.1.  Optimized Compressed Request  . . . . . . . . . . . .  25
     9.2.  Optimized Response  . . . . . . . . . . . . . . . . . . .  26
       9.2.1.  Optimized Compressed Response . . . . . . . . . . . .  26
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  26

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     10.1.  Group-level Security . . . . . . . . . . . . . . . . . .  27
     10.2.  Uniqueness of (key, nonce) . . . . . . . . . . . . . . .  28
     10.3.  Management of Group Keying Material  . . . . . . . . . .  28
     10.4.  Update of Security Context and Key Rotation  . . . . . .  29
       10.4.1.  Late Update on the Sender  . . . . . . . . . . . . .  29
       10.4.2.  Late Update on the Recipient . . . . . . . . . . . .  30
     10.5.  Collision of Group Identifiers . . . . . . . . . . . . .  30
     10.6.  Cross-group Message Injection  . . . . . . . . . . . . .  30
     10.7.  Group OSCORE for Unicast Requests  . . . . . . . . . . .  32
     10.8.  End-to-end Protection  . . . . . . . . . . . . . . . . .  33
     10.9.  Security Context Establishment . . . . . . . . . . . . .  33
     10.10. Master Secret  . . . . . . . . . . . . . . . . . . . . .  34
     10.11. Replay Protection  . . . . . . . . . . . . . . . . . . .  34
     10.12. Client Aliveness . . . . . . . . . . . . . . . . . . . .  35
     10.13. Cryptographic Considerations . . . . . . . . . . . . . .  35
     10.14. Message Segmentation . . . . . . . . . . . . . . . . . .  36
     10.15. Privacy Considerations . . . . . . . . . . . . . . . . .  36
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
     11.1.  Counter Signature Parameters Registry  . . . . . . . . .  37
     11.2.  Counter Signature Key Parameters Registry  . . . . . . .  40
     11.3.  OSCORE Flag Bits Registry  . . . . . . . . . . . . . . .  42
     11.4.  Expert Review Instructions . . . . . . . . . . . . . . .  42
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  43
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  43
     12.2.  Informative References . . . . . . . . . . . . . . . . .  44
   Appendix A.  Assumptions and Security Objectives  . . . . . . . .  46
     A.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .  47
     A.2.  Security Objectives . . . . . . . . . . . . . . . . . . .  48
   Appendix B.  List of Use Cases  . . . . . . . . . . . . . . . . .  49
   Appendix C.  Example of Group Identifier Format . . . . . . . . .  51
   Appendix D.  Set-up of New Endpoints  . . . . . . . . . . . . . .  52
   Appendix E.  Examples of Synchronization Approaches . . . . . . .  53
     E.1.  Best-Effort Synchronization . . . . . . . . . . . . . . .  53
     E.2.  Baseline Synchronization  . . . . . . . . . . . . . . . .  53
     E.3.  Challenge-Response Synchronization  . . . . . . . . . . .  54
   Appendix F.  No Verification of Signatures  . . . . . . . . . . .  56
   Appendix G.  Pairwise Mode  . . . . . . . . . . . . . . . . . . .  57
     G.1.  Pre-Requirements  . . . . . . . . . . . . . . . . . . . .  57
     G.2.  Pairwise Protected Request  . . . . . . . . . . . . . . .  57
     G.3.  Pairwise Protected Response . . . . . . . . . . . . . . .  58
   Appendix H.  Document Updates . . . . . . . . . . . . . . . . . .  58
     H.1.  Version -07 to -08  . . . . . . . . . . . . . . . . . . .  58
     H.2.  Version -06 to -07  . . . . . . . . . . . . . . . . . . .  60
     H.3.  Version -05 to -06  . . . . . . . . . . . . . . . . . . .  60
     H.4.  Version -04 to -05  . . . . . . . . . . . . . . . . . . .  61
     H.5.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  61
     H.6.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  62
     H.7.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  63

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     H.8.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  64
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  64
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  65

1.  Introduction

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

   Object Security for Constrained RESTful Environments (OSCORE)
   [RFC8613] describes a security protocol based on the exchange of
   protected CoAP messages.  OSCORE builds on CBOR Object Signing and
   Encryption (COSE) [RFC8152] and provides end-to-end encryption,
   integrity, replay protection and binding of response to request
   between a sender and a recipient, independent of transport also in
   the presence of intermediaries.  To this end, a CoAP message is
   protected by including its payload (if any), certain options, and
   header fields in a COSE object, which replaces the authenticated and
   encrypted fields in the protected message.

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

   Group OSCORE provides source authentication of CoAP messages, by
   means of two possible methods.  The first method relies on a digital
   signature produced with the private key of the sender and embedded in
   the protected CoAP message.  The second method relies on a symmetric
   key, which is derived from a pairwise shared secred computed from the
   asymmetric keys of the message sender and recipient.

   The second method is intended for one-to-one messages sent in the
   group.  These include all responses, as individually sent by servers,
   as well as requests addressed to an individual server.  For instance,
   such requests are sent as part of an exchange using the CoAP Echo

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   option [I-D.ietf-core-echo-request-tag], or as part of a block-wise
   transfer [RFC7959] in the group, after the first block-wise request
   possibly targeting all servers in the group and including the CoAP
   Block2 option (see Section 2.3.6 of [I-D.ietf-core-groupcomm-bis]).

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

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

   Group OSCORE defines different modes of operation:

   o  In the signature mode, Group OSCORE requests and responses are
      digitally signed.  The signature mode supports all COSE algorithms
      as well as signature verification by intermediaries.

   o  The pairwise mode allows two group members to exchange unicast
      OSCORE requests and responses protected with symmetric keys.
      These symmetric keys are derived from Diffie-Hellman shared
      secrets, calculated with the asymmetric keys of the two group
      members.  This allows for shorter integrity tags and therefore
      lower message overhead.

   o  In the (hybrid) optimized mode, the CoAP requests are digitally
      signed as in the signature mode, and the CoAP responses are
      integrity protected with the symmetric key of the pairwise mode.

   The signature and optimized modes are detailed in the body of this
   document.  The pairwise mode is detailed in Appendix G.  Unless
   otherwise stated, this specification refers to the signature mode.

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.

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   Readers are expected to be familiar with the terms and concepts
   described in CoAP [RFC7252] including "endpoint", "client", "server",
   "sender" and "recipient"; group communication for CoAP
   [I-D.ietf-core-groupcomm-bis]; COSE and counter signatures [RFC8152].

   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", "constrained-node network", is defined in [RFC7228].

   This document refers also to the following terminology.

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

   o  Group: a set of endpoints that share group keying material and
      security parameters (Common Context, see Section 2).  Unless
      specified otherwise, the term group used in this specification
      refers thus to a "security group", not to be confused with
      CoAP/network/multicast group or application group.

   o  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 8.

   o  Silent server: member of a group that never responds to requests.
      Given that, for CoAP group communications, messages are normally
      sent without requesting a confirmation, the idea of a server
      silently acting on a message is not unreasonable.  Note that an
      endpoint can implement both a silent server and a client, the two
      roles are independent.  An endpoint implementing only a silent
      server processes only incoming requests, and, in case it supports
      only the signature mode, it maintains less keying material and
      especially does not have a Sender Context for the group.

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

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

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

2.  Security Context

   Each endpoint registered as member of a group maintains a Security
   Context as defined in Section 3 of [RFC8613], extended as follows
   (see Figure 1):

   o  One Common Context, shared by all the endpoints in the group.
      Three new parameters are included in the Common Context: Counter
      Signature Algorithm, Counter Signature Parameters and Counter
      Signature Key Parameters.

   o  One Sender Context, extended with the endpoint's private key.  The
      Sender Context is omitted if the endpoint is configured
      exclusively as silent server.

   o  One Recipient Context for each endpoint from which messages are
      received.  No Recipient Contexts are maintained as associated to
      endpoints from which messages are not (expected to be) received.
      The Recipient Context is extended with the public key of the
      associated endpoint.

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

            Figure 1: Additions to the OSCORE Security Context

2.1.  Common Context

   The ID Context parameter (see Sections 3.3 and 5.1 of [RFC8613]) in
   the Common Context SHALL contain the Group Identifier (Gid) of the
   group.  The choice of the Gid is application specific.  An example of
   specific formatting of the Gid is given in Appendix C.  The

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   application needs to specify how to handle possible collisions
   between Gids, see Section 10.5.

   The Counter Signature Algorithm identifies the digital signature
   algorithm used to compute a counter signature on the COSE object (see
   Section 4.5 of [RFC8152]).  Its value is immutable once the Common
   Context is established.  The used Counter Signature Algorithm MUST be
   selected among the signing ones defined in the COSE Algorithms
   Registry (see section 16.4 of [RFC8152]).  The EdDSA signature
   algorithm Ed25519 [RFC8032] is mandatory to implement.  If Elliptic
   Curve Digital Signature Algorithm (ECDSA) is used, it is RECOMMENDED
   that implementations implement "deterministic ECDSA" as specified in
   [RFC6979].

   The Counter Signature Parameters identifies the parameters associated
   to the digital signature algorithm specified in the Counter Signature
   Algorithm.  This parameter MAY be empty and is immutable once the
   Common Context is established.  The exact structure of this parameter
   depends on the value of Counter Signature Algorithm, and is defined
   in the Counter Signature Parameters Registry (see Section 11.1),
   where each entry indicates a specified structure of the Counter
   Signature Parameters.

   The Counter Signature Key Parameters identifies the parameters
   associated to the keys used with the digital signature algorithm
   specified in the Counter Signature Algorithm.  This parameter MAY be
   empty and is immutable once the Common Context is established.  The
   exact structure of this parameter depends on the value of Counter
   Signature Algorithm, and is defined in the Counter Signature Key
   Parameters Registry (see Section 11.2), where each entry indicates a
   specified structure of the Counter Signature Key Parameters.

2.2.  Sender Context and Recipient Context

   OSCORE specifies the derivation of Sender Context and Recipient
   Context, specifically Sender/Recipient Keys and Common IV, from a set
   of input parameters (see Section 3.2 of [RFC8613]).  This derivation
   applies also to Group OSCORE, and the mandatory-to-implement HKDF and
   AEAD algorithms are the same as in [RFC8613].  However, for Group
   OSCORE the Sender Context and Recipient Context additionally contain
   asymmetric keys.

   The Sender Context needs to be configured with the private key of the
   endpoint.  The private key is used to generate a signature (see
   Section 4) included in the sent OSCORE message.  How the private key
   is established is out of scope for this specification.

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   Each Recipient Context needs to be configured with the public key of
   the associated endpoint.  The public key is used to verify a
   signature (see Section 4) included in the received OSCORE message.

   The input parameters for deriving the Recipient Context parameters
   and the public key of the associated endpoint may be provided to the
   recipient endpoint upon joining the group.  These parameters may
   alternatively be acquired at a later time, for example the first time
   a message is received from this particular endpoint in the group (see
   Section 7.2 and Section 7.4).  The received message together with the
   Common Context contains the necessary information to derive a
   security context for verifying a message, except for the public key
   of the associated endpoint.

   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 associated endpoint's
   public key.  Such devices can be configured to drop a received
   message for which there is no Recipient Context, and retrieve the
   public key in order to have it available to verify subsequent
   messages from that endpoint.

2.3.  The Group Manager

   Endpoints communicating with Group OSCORE need, in addition to the
   OSCORE input parameters, also to be provisioned with information
   about the group(s) and other endpoints in the group(s).

   The Group Manager is an entity responsible for the group, including
   the Group Identifier (Gid) used as ID Context, as well as the Sender
   ID and Recipient ID of the group members (see Section 8).

   The Group Manager is exclusively in control of the Gid values
   uniquely assigned to the different groups under its control, as well
   as of the Sender ID and Recipient ID values uniquely assigned to the
   members of each of those groups.  According to a hierarchical
   approach, the Gid value assigned to a group is associated to a
   dedicated space for the values of Sender ID and Recipient ID of the
   members of that group.  In addition, the Group Manager records the
   public keys of endpoints joining a group, and provides information
   about the group and its members to other members.

   An endpoint receives the Group Identifier and OSCORE input
   parameters, including its own Sender ID, from the Group Manager upon
   joining the group.  That Sender ID is valid only within that group,
   and is unique within the group.  Endpoints which are configured only
   as silent servers do not have a Sender ID.

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

   According to this specification, it is RECOMMENDED to use a Group
   Manager as described in [I-D.ietf-ace-key-groupcomm-oscore], where
   the join process is based on the ACE framework for authentication and
   authorization in constrained environments [I-D.ietf-ace-oauth-authz].
   Further details about how public keys can be handled and retrieved in
   the group is out of the scope of this document.

   The Group Manager may serve additional entities acting as signature
   checkers, e.g. intermediary gateways.  These entities do not join a
   group as members, but can retrieve public keys of group members from
   the Group Manager, in order to verify counter signatures of group
   messages.  A signature checker is required to be authorized for
   retrieving public keys of members in a specific group from the Group
   Manager.  To this end, the same method mentioned above and based on
   the ACE framework can be used.

2.4.  Management of Group Keying Material

   In order to establish a new Security Context in a group, a new Group
   Identifier (Gid) for that group and a new value for the Master Secret
   parameter MUST be distributed.  An example of Gid format supporting
   this operation is provided in Appendix C.  When distributing the new
   Gid and Master Secret, the Group Manager MAY distribute also a new
   value for the Master Salt parameter, and SHOULD preserve the current
   value of the Sender ID of each group member.

   Then, each group member re-derives the keying material stored in its
   own Sender Context and Recipient Contexts as described in Section 2,
   using the updated Gid and Master Secret parameter.  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.
   From then on, each group member MUST use its latest installed Sender
   Context to protect outgoing messages.

   After a new Gid has been distributed, a same Recipient ID ('kid')
   should not be considered as a persistent and reliable indicator of
   the same group member.  Such an indication can be actually achieved
   only by using that members's public key.  This occurs when verifying
   countersignatures of received messages (in signature mode), or when
   verifying messages integrity-protected with pairwise keying material
   derived from asymmetric keys (in pairwise mode).  As a consequence,

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   group members may end up retaining stale Recipient Contexts, that are
   no longer useful to verify incoming secure messages.

   In order to alleviate this issue, the Group Manager SHOULD NOT
   recycle 'kid' values within a same group, especially in the short
   term.  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 owned
   public key is currently the one associated to a 'kid' value, after a
   number of consecutive failed verifications.

   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 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 is provided in Section 10.4.

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

   The specific approach used to distribute the new Gid and Master
   Secret parameter to the group is out of the scope of this document.
   However, it is RECOMMENDED that the Group Manager supports the
   distribution of the new Gid and Master Secret parameter to the group
   according to the Group Rekeying Process described in
   [I-D.ietf-ace-key-groupcomm-oscore].

2.5.  Exhaustion of Partial IV Values

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

   If an implementation's integers only support wrapping addition, the
   implementation MUST detect a wrap-around of the Sender Sequence
   Number value and treat that as exhausted instead.

   Upon exhausting its own Sender Sequence Number, the endpoint MUST NOT
   transmit further messages for that group until it has derived a new
   Sender Context, in order to avoid reusing nonces with the same keys.

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   Furthermore, the endpoint SHOULD inform the Group Manager, that can
   take one of the following actions:

   o  The Group Manager renews the Security Context in the group (see
      Section 2.4).

   o  The Group Manager provides a new Sender ID value to the endpoint
      that has experienced the exhaustion.  Then, the endpoint derives a
      new Sender Context using the new Sender ID, as described in
      Section 3.2 of [RFC8613].

   In either case, same considerations from Section 2.4 hold about
   possible retaining of stale Recipient Contexts.

3.  Pairwise Keys

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

3.1.  Key Derivation

   Two group members can derive a symmetric pairwise key, from their
   Sender/Recipient Key and a static-static Diffe-Hellman shared secret
   [NIST-800-56A].  The key derivation is as follows, and uses the same
   construction used in Section 3.2.1 of [RFC8613].

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

   where:

   o  The Sender/Recipient key is the Sender Key of the sender, i.e. the
      Recipient Key that the recipient stores in its own Recipient
      Context corresponding to the sender.

   o  The Shared Secret is computed as a static-static Diffie-Hellman
      shared secret [NIST-800-56A], where the sender uses its own
      private key and the recipient's public key, while the recipient
      uses its own private key and the senders's public key.  The Shared
      Secret may be stored in memory, rather than recomputed each time
      it is needed.

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

   The security of using the same key pair for Diffie-Hellman and for
   signing is proven in [Degabriele].  The derivation of pairwise keys

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   defined above is compatible with ECDSA and EdDSA asymmetric keys, but
   is not compatible with RSA asymmetric keys.

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

   After completing the establishment of a new Security Context (see
   Section 2.4), every group member MUST delete all its pairwise keys.
   Since new Sender/Recipient keys are derived from the new group keying
   material (see Section 2.2), every group member MUST use such new
   Sender/Recipient keys when possibly deriving new pairwise keys.

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

3.2.  Usage of Sequence Numbers

   When using any of its pairwise keys, a sender endpoint MUST use the
   current fresh value of its own Sender Sequence Number, from its own
   Sender Context (see Section 2.2).  That is, the same Sender Sequence
   Number space is used for all outgoing messages sent to the group and
   protected with Group OSCORE, thus limiting both storage and
   complexity.

   On the other hand, when combining one-to-many and one-to-one
   communication in the group, this may result in the Partial IV values
   moving forward more often.  Fundamentally, this is due to the fact
   that not all the recipients receive all messages from a given sender.
   For instance, requests sent over multicast (in signature mode) are
   addressed to the whole group, while requests sent over unicast (in
   signature mode or pairwise mode) are not.

   As a consequence, replay checks may be invoked more often on the
   recipient side, where larger replay windows should be considered.

3.3.  Note on Implementation

   In order to optimize performance, an endpoint A may derive a pairwise
   key used with an endpoint B in the OSCORE group only once, and then
   store it in its own Security Context for future retrieval.  This can
   work as follows.

   Endpoint A can have a Pairwise Sender Context associated to B, within
   its own Sender Context.  This Pairwise Sender Context includes:

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   o  The Recipient ID of B for A, i.e. the Sender ID of B.

   o  The Pairwise Key derived as defined in Section 3, with A acting as
      sender and B acting as recipient.

   More generally, A has one of such Paiwise Sender Contexts within its
   own Sender Context, for each different intended recipient.

   Furthermore, A can additionally store in its own Recipient Context
   associated to B the Pairwise Key to use for incoming traffic from B.
   That is, this Pairwise Key is derived as defined in Section 3, with A
   acting as recipient and B acting as sender.

4.  The COSE Object

   Building on Section 5 of [RFC8613], this section defines how to use
   COSE [RFC8152] 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.  For
   the signature mode of Group OSCORE the following modifications apply.

4.1.  Counter Signature

   The 'unprotected' field MUST additionally include the following
   parameter:

   o  CounterSignature0 : its value is set to the counter signature of
      the COSE object, computed by the sender as described in
      Appendix A.2 of [RFC8152], by using its own private key and
      according to the Counter Signature Algorithm and Counter Signature
      Parameters in the Security Context.  In particular, the
      Sig_structure contains the external_aad as defined in
      Section 4.3.2 and the ciphertext of the COSE_Encrypt0 object as
      payload.

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

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

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

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4.3.  external_aad

   The external_aad of the Additional Authenticated Data (AAD) is built
   differently.  In particular, it has one structure used for the
   encryption process producing the ciphertext, and a second structure
   used for the signing process producing the counter signature.

4.3.1.  external_aad for Encryption

   The first external_aad structure used for the encryption process
   producing the ciphertext (see Section 5.3 of [RFC8152]) includes also
   the counter signature algorithm and related parameters used to sign
   messages.  In particular, compared with Section 5.4 of [RFC8613], the
   'algorithms' array in the aad_array MUST also include:

   o  'alg_countersign', which contains the Counter Signature Algorithm
      from the Common Context (see Section 2).  This parameter has the
      value specified in the "Value" field of the Counter Signature
      Parameters Registry (see Section 11.1) for this counter signature
      algorithm.

   o  'par_countersign', which contains the Counter Signature Parameters
      from the Common Context (see Section 2).  This parameter contains
      the counter signature parameters encoded as specified in the
      "Parameters" field of the Counter Signature Parameters Registry
      (see Section 11.1), for the used counter signature algorithm.  If
      the Counter Signature Parameters in the Common Context is empty,
      'par_countersign' MUST be encoding the CBOR simple value Null.

   o  'par_countersign_key', which contains the Counter Signature Key
      Parameters from the Common Context (see Section 2).  This
      parameter contains the counter signature key parameters encoded as
      specified in the "Parameters" field of the Counter Signature Key
      Parameters Registry (see Section 11.2), for the used counter
      signature algorithm.  If the Counter Signature Key Parameters in
      the Common Context is empty, 'par_countersign_key' MUST be
      encoding the CBOR simple value Null.

   Thus, the following external_aad structure is used for the encryption
   process producing the ciphertext (see Section 5.3 of [RFC8152]).

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   external_aad = bstr .cbor aad_array

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

4.3.2.  external_aad for Signing

   The second external_aad structure used for the signing process
   producing the counter signature as defined below includes also:

   o  the counter signature algorithm and related parameters used to
      sign messages, encoded as in the external_aad structure defined in
      Section 4.3.1;

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

   Thus, the following external_aad structure is used for the signing
   process producing the counter signature, as defined below.

   external_aad = bstr .cbor aad_array

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

   Note for implementation: this requires the value of the OSCORE option
   to be fully ready, before starting the signing process.  Also, this
   requires that the aad_array is long enough to contain the longest
   possible OSCORE option.

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5.  OSCORE Header Compression

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

   o  The payload of the OSCORE message SHALL encode the ciphertext of
      the COSE object.  In the signature mode as well as in the
      optimized compressed requests of the optimized mode (see
      Section 9.1.1), the ciphertext above is concatenated with the
      value of the CounterSignature0 of the COSE object, computed as
      described in Section 4.1.

   o  This specification defines the usage of the sixth least
      significant bit, namely the Pairwise Flag bit, in the first byte
      of the OSCORE option containing the OSCORE flag bits.  This flag
      bit is registered in Section 11.3 of this specification.

   o  The Pairwise Flag bit is set to 1 if the OSCORE message is
      protected using pairwise keying material, which is shared with a
      single group member as single intended recipient and derived as
      defined in Section 3.  This is used, for instance, to send
      responses with the optimized mode defined in Section 9.  In any
      other case, especially when the OSCORE message is protected as per
      Section 7.1 and Section 7.3, this bit MUST be set to 0.

      If any of the following conditions holds, a recipient MUST discard
      an incoming OSCORE message with the Pairwise Flag bit set to 1:

      *  The recipient does not support this feature, i.e. it is not
         capable or willing to process OSCORE messages protected using
         pairwise keying material.

      *  The recipient can not retrieve a Security Context which is both
         valid to process the message and also associated to an OSCORE
         group.

5.1.  Examples of Compressed COSE Objects

   This section covers a list of OSCORE Header Compression examples for
   group requests and responses, with Group OSCORE used in signature
   mode.

   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

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   parts, since the object is transported in two CoAP fields: OSCORE
   option and payload.

   The examples assume that the label for the new kid context defined in
   [RFC8613] has value 10.  The examples also 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.  Finally, COUNTERSIGN is the CounterSignature0 byte string as
   described in Section 4 and is 64 bytes long.

   1.  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', 9:COUNTERSIGN },
   h'aea0155667924dff8a24e4cb35b9'
   ]

   After compression (85 bytes):

   Flag byte: 0b00011001 = 0x19

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

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

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

   Before compression (88 bytes):

   [
   h'',
   { 4:h'52', 9:COUNTERSIGN },
   h'60b035059d9ef5667c5a0710823b'
   ]

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   After compression (80 bytes):

   Flag byte: 0b00001000 = 0x08

   Option Value: 08 52 (2 bytes)

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

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

   The requirements and properties described in Section 7 of [RFC8613]
   also apply to OSCORE used in group communication.  In particular,
   group OSCORE provides message binding of responses to requests, which
   provides relative freshness of responses, and replay protection of
   requests.

6.1.  Synchronization of Sender Sequence Numbers

   Upon joining the group, new servers are not aware of the Sender
   Sequence Number values currently used by different clients to
   transmit group requests.  This means that, when such servers receive
   a secure group request from a given client for the first time, they
   are not able to verify if that request is fresh and has not been
   replayed or (purposely) delayed.  The same holds when a server loses
   synchronization with Sender Sequence Numbers of clients, for instance
   after a device reboot.

   The exact way to address this issue is application specific, and
   depends on the particular use case and its synchronization
   requirements.  The list of methods to handle synchronization of
   Sender Sequence Numbers is part of the group communication policy,
   and different servers can use different methods.

   Appendix E describes three possible approaches that can be considered
   for synchronization of sequence numbers.

7.  Message Processing

   Each request message and response message is protected and processed
   as specified in [RFC8613], with the modifications described in the
   following sections.  In particular, the following sections refer to
   the signature mode of Group OSCORE, while the optimized mode and the
   pairwise mode are described in Section 9 and Appendix G,
   respectively.

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   The following security objectives are fulfilled, as further discussed
   in Appendix A.2: data replay protection, group-level data
   confidentiality, source authentication and message integrity.

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

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

7.1.  Protecting the Request

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

   o  In step 2, the 'algorithms' array in the Additional Authenticated
      Data is modified as described in Section 4 of this specification.

   o  In step 4, the encryption of the COSE object is modified as
      described in Section 4 of this specification.  The encoding of the
      compressed COSE object is modified as described in Section 5 of
      this specification.

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

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7.1.1.  Supporting Observe

   If Observe [RFC7641] is supported, for each newly started
   observation, the client MUST store the value of the 'kid' parameter
   from the original Observe request.

   The client MUST NOT update the stored value, even in case it is
   individually rekeyed and receives a new Sender ID from the Group
   Manager (see Section 2.5).

7.2.  Verifying the Request

   Upon receiving a secure group request, a server proceeds as described
   in Section 8.2 of [RFC8613], with the following modifications.

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

      *  If the Pairwise Flag bit is set to 1, and the server discards
         the request due to not supporting this feature or not
         retrieving a Security Context associated to the OSCORE group,
         the server MAY respond with a 4.02 (Bad Option) error.  When
         doing so, the server MAY set an Outer Max-Age option with value
         zero, and MAY include a descriptive string as diagnostic
         payload.

      *  If the received Recipient ID ('kid') does not match with any
         Recipient Context for the retrieved Gid ('kid context'), then
         the server MAY create a new Recipient Context and initializes
         it according to Section 3 of [RFC8613], also retrieving the
         client's public key.  Such a configuration is application
         specific.  If the application does not specify dynamic
         derivation of new Recipient Contexts, then the server SHALL
         stop processing the request.

   o  In step 4, the 'algorithms' array in the Additional Authenticated
      Data is modified as described in Section 4 of this specification.

   o  In step 6, the server also verifies the counter signature using
      the public key of the client from the associated Recipient
      Context.  If the signature verification fails, the server MAY
      reply with a 4.00 (Bad Request) response.

   o  Additionally, if the used Recipient Context was created upon
      receiving this group request and the message is not verified
      successfully, the server MAY delete that Recipient Context.  Such
      a configuration, which is specified by the application, would

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      prevent attackers from overloading the server's storage and
      creating processing overhead on the server.

   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.

7.2.1.  Supporting Observe

   If Observe [RFC7641] is supported, for each newly started
   observation, the server MUST store the value of the 'kid' parameter
   from the original Observe request.

   The server MUST NOT update the stored value, 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).

7.3.  Protecting the Response

   A server that has received a secure group request may reply with a
   secure response, which is protected as described in Section 8.3 of
   [RFC8613], with the following modifications.

   o  In step 2, the 'algorithms' array in the Additional Authenticated
      Data is modified as described in Section 4 of this specification.

   o  In step 4, the encryption of the COSE object is modified as
      described in Section 4 of this specification.  The encoding of the
      compressed COSE object is modified as described in Section 5 of
      this specification.

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

   Note that the server MUST always protect a response by using its own
   Sender Context from the latest owned Security Context.

   Consistently, upon the establishment of a new Security Context, the
   server may end up protecting a response by using a Security Context
   different from the one used to protect the group request (see
   Section 10.4).  In such a case:

   o  The server MUST encode the Partial IV (Sender Sequence Number in
      network byte order), which is set to the Sender Sequence Number of
      the server; increment the Sender Sequence Number by one; compute
      the AEAD nonce from the Sender ID, Common IV, and Partial IV.

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   o  The server MUST include in the response the 'Partial IV'
      parameter, which is set to the encoded Partial IV value above.

   o  The server SHOULD include in the response the 'kid context'
      parameter, which is set to the ID Context of the new Security
      Context, i.e. the new Group Identifier (Gid).

7.3.1.  Supporting Observe

   If Observe [RFC7641] is supported, 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 its own Sender
   Context from the new Security Context.

   For each ongoing observation, the server SHOULD include in the first
   notification protected with the new Security Context also the 'kid
   context' parameter, which is set to the ID Context of the new
   Security Context, i.e. the new Group Identifier (Gid).  It is
   OPTIONAL for the server to include the 'kid context' parameter, as
   set to the new Gid, also in further following notifications for those
   observations.

   Furthermore, for each ongoing observation, the server MUST use the
   stored value of the 'kid' parameter from the original Observe
   request, as value for the 'request_kid' parameter in the two
   external_aad structures (see Section 4.3.1 and Section 4.3.2), when
   protecting notifications for that observation.

7.4.  Verifying the Response

   Upon receiving a secure response message, the client proceeds as
   described in Section 8.4 of [RFC8613], with the following
   modifications.

   o  In step 2, the decoding of the compressed COSE object is modified
      as described in Section 4 of this specification.  If the received
      Recipient ID ('kid') does not match with any Recipient Context for
      the retrieved Gid ('kid context'), then the client MAY create a
      new Recipient Context and initializes it according to Section 3 of
      [RFC8613], also retrieving the server's public key.  If the
      application does not specify dynamic derivation of new Recipient
      Contexts, then the client SHALL stop processing the response.

   o  In step 3, the 'algorithms' array in the Additional Authenticated
      Data is modified as described in Section 4 of this specification.

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   o  In step 5, the client also verifies the counter signature using
      the public key of the server from the associated Recipient
      Context.

   o  Additionally, if the used Recipient Context was created upon
      receiving this response and the message is not verified
      successfully, the client MAY delete that Recipient Context.  Such
      a configuration, which is specified by the application, would
      prevent attackers from overloading the client's storage and
      creating processing overhead on the client.

   Note that, as discussed in Section 10.4, a client may receive a
   response protected with a Security Context different from the one
   used to protect the corresponding group request.

7.4.1.  Supporting Observe

   If Observe [RFC7641] is supported, for each ongoing observation, the
   client MUST use the stored value of the 'kid' parameter from the
   original Observe request, as value for the 'request_kid' parameter in
   the two external_aad structures (see Section 4.3.1 and
   Section 4.3.2), when verifying notifications for that observation.

   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).

8.  Responsibilities of the Group Manager

   The Group Manager is responsible for performing the following tasks:

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

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

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

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

   5.   Generating and managing Sender IDs within its OSCORE groups, as
        well as assigning and providing them to new endpoints during the
        join process.  This includes ensuring uniqueness of Sender IDs
        within each of its OSCORE groups.

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

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

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

   9.   Updating the Gid of its OSCORE groups, upon renewing the
        respective Security Context.

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

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

9.  Optimized Mode

   For use cases that do not require an intermediary performing
   signature verification and that use a compatible signature algorithm,
   the optimized mode defined in this section provides significant
   smaller message sizes and increases the security by making responses
   confidential to other group members than the intended recipient.

9.1.  Optimized Request

   No changes.

9.1.1.  Optimized Compressed Request

   The OSCORE header compression defined in Section 5 is used, with the
   following difference: the payload of the OSCORE message SHALL encode
   the ciphertext without the tag, concatenated with the value of the
   CounterSignature0 of the COSE object computed as described in
   Section 4.1.

   The optimized compressed request is compatible with all AEAD
   algorithms defined in [RFC8152], but would not be compatible with
   AEAD algorithms that do not have a well-defined tag.

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9.2.  Optimized Response

   An optimized response is protected as defined in Section 7.3, with
   the following differences.

   o  The server MUST set to 1 the sixth least significant bit of the
      OSCORE flag bits in the OSCORE option, i.e. the Pairwise Flag.

   o  The COSE_Encrypt0 object included in the optimized response is
      encrypted using a symmetric pairwise key K, that the server
      derives as defined in Section 3.  In particular, the Sender/
      Recipient Key is the Sender Key of the server from its own Sender
      Context, i.e. the Recipient Key that the client stores in its own
      Recipient Context corresponding to the server.

   o  The Counter Signature is not computed.  That is, unlike defined in
      Section 5, the payload of the OSCORE message terminates with the
      encoded ciphertext of the COSE object.

   Note that no changes are made to the AEAD nonce and AAD.

   Upon receiving a response with the Pairwise Flag set to 1, the client
   MUST process it as defined in Section 7.4, with the following
   differences.

   o  No countersignature to verify is included.

   o  The COSE_Encrypt0 object included in the optimized response is
      decrypted and verified using the same symmetric pairwise key K,
      that the client derives as described above for the server side and
      as defined in Section 3.

9.2.1.  Optimized Compressed Response

   No changes.

10.  Security Considerations

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

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

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

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

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

   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 mounted 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 (Section 12 of
   [RFC8613]), and discusses how they hold when Group OSCORE is used.

10.1.  Group-level Security

   The signature mode described in Section 7 relies on commonly shared
   group keying material to protect communication within a group.  This
   has the following implications.

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

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

   Instead, the pairwise mode described in Appendix G 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 3).  Therefore, in the
   parwise mode, the AEAD algorithm provides both pairwise data-
   confidentiality and source authentication of messages, without using
   counter signatures.

   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

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   inappropriately.  For instance, it can forward the content of
   messages in the group to unauthorized entities.  However, in many use
   cases, the devices in the group belong to a common authority and are
   configured by a commissioner (see Appendix B), which results in a
   practically limited risk and enables a prompt detection/reaction in
   case of misbehaving.

10.2.  Uniqueness of (key, nonce)

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

   o  Uniqueness of Sender IDs within the group is enforced by the Group
      Manager.

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

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

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

10.3.  Management of Group Keying Material

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

   Especially in dynamic, large-scale, groups where endpoints can join
   and leave at any time, it is important that the considered group key
   management scheme is efficient and highly scalable with the group
   size, in order to limit the impact on performance due to the Security
   Context and keying material update.

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10.4.  Update of Security Context and Key Rotation

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

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

   In particular, a server may first get a group request protected with
   the old Security Context, then install the new Security Context, and
   only after that produce a response to send back to the client.  Since
   a sender always protects an outgoing message using the latest owned
   Security Context, the server discussed above protects the possible
   response using the new Security Context.  Then, 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 reception.

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

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

10.4.1.  Late Update on the Sender

   In this case, the sender protects a message using the old Security
   Context, i.e. before having installed the new Security Context.
   However, the recipient receives the message after having installed
   the new Security Context, hence not being able to correctly process
   it.

   A possible way to ameliorate this issue is to preserve the old,
   recent, Security Context for a maximum amount of time defined by the
   application.  By doing so, the recipient can still try to process the
   received message using the old retained Security Context as second
   attempt.  This makes particular sense when the recipient is a client,
   that would hence be able to process incoming responses protected with
   the old, recent, Security Context used to protect the associated
   group request.  Instead, a recipient server would better and more
   simply discard an incoming group request which is not successfully
   processed with the new Security Context.

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

10.4.2.  Late Update on the Recipient

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

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

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

10.5.  Collision of Group Identifiers

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

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

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

10.6.  Cross-group Message Injection

   A same endpoint is allowed to and would likely use the same signature
   key in multiple OSCORE groups, possibly administered by different
   Group Managers.  Also, the same endpoint can register several times
   in the same group, getting multiple unique Sender IDs.  This requires
   that, when a sender endpoint sends a message to an OSCORE group using

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   a Sender ID, the countersignature included in the message is
   explicitly bound also to that group and to the used Sender ID.

   To this end, the countersignature of each message protected with
   Group OSCORE is computed also over the value of the OSCORE option,
   which is part of the Additional Authenticated Data used in the
   signing process (see Section 4.3.2).  That is, the countersignature
   is computed also over: the ID Context (Group ID) and the Partial IV,
   which are always present in group requests; as well as the Sender ID
   of the message originator, which is always present in all group
   requests and responses.

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

   This makes it practically infeasible to perform the attack described
   below, where a malicious group member injects forged messages to a
   different OSCORE group than the originally intended one.  Let us
   consider:

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

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

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

   If countersignatures were not computed also over the value of the
   OSCORE option as discussed above, Z can intercept a group message M1
   sent by V to G1, and forge a valid signed message M2 to be injected
   in G2, making it appear as sent by V and valid to be accepted.

   More in detail, Z first intercepts a message M1 sent by V in G1, and
   tries to forge a message M2, by changing the value of the OSCORE
   option from M1 as follows: the 'kid context' is changed from G1 to
   G2; and the 'kid' is changed from Sid1 to Sid2.

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   If M2 is used as a request message, there is a probability equal to
   2^-64 that the same unchanged MAC is successfully verified by using
   Sid2 as 'request_kid' and the symmetric key associated to V in G2.
   In such a case, the same unchanged signature would be also valid.
   Note that Z can check offline if a performed forgery is actually
   valid before sending the forged message to G2.  That is, this attack
   has a complexity of 2^64 offline calculations.

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

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

10.7.  Group OSCORE for Unicast Requests

   With reference to the processing defined in Section 7.1 for the
   signature mode and in Section 9.1.1 for the optimized mode, it is NOT
   RECOMMENDED for a client to use Group OSCORE for securing a request
   sent to a single group member over unicast.

   If the client uses 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 to one group member only.  Note that the adversary
   can be external, i.e. (s)he does not need to also be a member of the
   OSCORE group.

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

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

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

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

   A client may instead use the pairwise mode defined in Appendix G.2,
   in order to protect a request sent to a single group member by using
   pairwise keying material (see Section 3).  This prevents the attack
   discussed above by construction, as only the intended server is able
   to derive the pairwise keying material used by the client to protect
   the request.  A client supporting the pairwise mode SHOULD use it to
   protect requests sent to a single group member over unicast, instead
   of using the signature mode.

10.8.  End-to-end Protection

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

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

10.9.  Security Context Establishment

   The use of COSE_Encrypt0 and AEAD to protect messages as specified in
   this document requires an endpoint to be a member of an OSCORE group.

   That is, upon joining the group, the endpoint securely receives from
   the Group Manager the necessary input parameters, which are used to

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   derive the Common Context and the Sender Context (see Section 2).
   The Group Manager ensures uniqueness of Sender IDs in the same group.

   Each different Recipient Context for decrypting messages from a
   particular sender can be derived at runtime, at the latest upon
   receiving a message from that sender for the first time.

   Countersignatures of group messages are verified by means of the
   public key of the respective sender endpoint.  Upon nodes' joining,
   the Group Manager collects such public keys and MUST verify proof-of-
   possession of the respective private key.  Later on, a group member
   can request from the Group Manager the public keys of other group
   members.

   The joining process can occur, for instance, as defined in
   [I-D.ietf-ace-key-groupcomm-oscore].

10.10.  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].

10.11.  Replay Protection

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

   Note that the Partial IV of an endpoint does not necessarily grow
   monotonically.  For instance, upon exhaustion of the endpoint Sender
   Sequence Number, the Partial IV also gets exhausted.  As discussed in
   Section 2.5, this results either in the endpoint being individually
   rekeyed and getting a new Sender ID, or in the establishment of a new
   Security Context in the group.  Therefore, uniqueness of (key, nonce)
   pairs (see Section 10.2) is preserved also when a new Security
   Context is established.

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

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

   The processing of response messages described in Section 7.4 also
   ensures that a client accepts a single valid response to a given
   request from each replying server, unless CoAP observation is used.

10.12.  Client Aliveness

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

10.13.  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, an endpoint that experiences a
   exhaustion of its own Sender Sequence Number MUST NOT transmit
   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.

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   The EdDSA signature algorithm Ed25519 [RFC8032] is mandatory to
   implement.  For many constrained IoT devices, it is problematic to
   support more than one signature algorithm or multiple whole cipher
   suites.  This means that some deployments using, for instance, ECDSA
   with NIST P-256 may not support the mandatory signature algorithm.
   However, this is not a problem for local deployments.

10.14.  Message Segmentation

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

10.15.  Privacy Considerations

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

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

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

   o  The 'kid context' parameter, which is intended to help a recipient
      endpoint to find the right Recipient 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.
      Moreover, this parameter explicitly relates two or more
      communicating endpoints, as members of the same OSCORE group.

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

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   Finally, the mechanism described in Section 10.5 to prevent
   collisions of Group Identifiers from different Group Managers may
   reveal information about events in the respective OSCORE groups.  In
   particular, a Group Idenfier changes when the corresponding group is
   rekeyed.  Thus, changes in the shared list of Group Identifiers may
   be used to infer about 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 such privacy
   concerns, it should be hidden from the Group Managers which exact
   Group Manager has currently assigned which Group Identifiers in its
   OSCORE groups.

11.  IANA Considerations

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

   This document has the following actions for IANA.

11.1.  Counter Signature Parameters Registry

   This specification establishes the IANA "Counter Signature
   Parameters" Registry.  The Registry has been created to use the
   "Expert Review Required" registration procedure [RFC8126].  Expert
   review guidelines are provided in Section 11.4.

   This registry specifies the parameters of each admitted
   countersignature algorithm, as well as the possible structure they
   are organized into.  This information is used to populate the
   parameter Counter Signature Parameters of the Common Context (see
   Section 2).

   The columns of this table are:

   o  Name: A value that can be used to identify an algorithm in
      documents for easier comprehension.  Its value is taken from the
      'Name' column of the "COSE Algorithms" Registry.

   o  Value: The value to be used to identify this algorithm.  Its
      content is taken from the 'Value' column of the "COSE Algorithms"
      Registry.  The value MUST be the same one used in the "COSE
      Algorithms" Registry for the entry with the same 'Name' field.

   o  Parameters: This indicates the CBOR encoding of the parameters (if
      any) for the counter signature algorithm indicated by the 'Value'
      field.

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   o  Description: A short description of the parameters encoded in the
      'Parameters' field (if any).

   o  Reference: This contains a pointer to the public specification for
      the field, if one exists.

   Initial entries in the registry are as follows.

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   +-------------+-------+--------------+-----------------+-----------+
   |    Name     | Value |  Parameters  |   Description   | Reference |
   +-------------+-------+--------------+-----------------+-----------+
   |             |       |              |                 |           |
   |    EdDSA    |  -8   |  crv : int   | crv value taken | [This     |
   |             |       |              | from the COSE   | Document] |
   |             |       |              | Elliptic Curve  |           |
   |             |       |              | Registry        |           |
   |             |       |              |                 |           |
   +-------------+-------+--------------+-----------------+-----------+
   |             |       |              |                 |           |
   |    ES256    |  -7   |  crv : int   | crv value taken | [This     |
   |             |       |              | from the COSE   | Document] |
   |             |       |              | Elliptic Curve  |           |
   |             |       |              | Registry        |           |
   |             |       |              |                 |           |
   +-------------+-------+--------------+-----------------+-----------+
   |             |       |              |                 |           |
   |    ES384    |  -35  |  crv : int   | crv value taken | [This     |
   |             |       |              | from the COSE   | Document] |
   |             |       |              | Elliptic Curve  |           |
   |             |       |              | Registry        |           |
   |             |       |              |                 |           |
   +-------------+-------+--------------+-----------------+-----------+
   |             |       |              |                 |           |
   |    ES512    |  -36  |  crv : int   | crv value taken | [This     |
   |             |       |              | from the COSE   | Document] |
   |             |       |              | Elliptic Curve  |           |
   |             |       |              | Registry        |           |
   |             |       |              |                 |           |
   +-------------+-------+--------------+-----------------+-----------+
   |             |       |              |                 |           |
   |    PS256    |  -37  |              | Parameters not  | [This     |
   |             |       |              | present         | Document] |
   |             |       |              |                 |           |
   +-------------+-------+--------------+-----------------+-----------+
   |             |       |              |                 |           |
   |    PS384    |  -38  |              | Parameters not  | [This     |
   |             |       |              | present         | Document] |
   |             |       |              |                 |           |
   +-------------+-------+--------------+-----------------+-----------+
   |             |       |              |                 |           |
   |    PS512    |  -39  |              | Parameters not  | [This     |
   |             |       |              | present         | Document] |
   |             |       |              |                 |           |
   +-------------+-------+--------------+-----------------+-----------+

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11.2.  Counter Signature Key Parameters Registry

   This specification establishes the IANA "Counter Signature Key
   Parameters" Registry.  The Registry has been created to use the
   "Expert Review Required" registration procedure [RFC8126].  Expert
   review guidelines are provided in Section 11.4.

   This registry specifies the parameters of countersignature keys for
   each admitted countersignature algorithm, as well as the possible
   structure they are organized into.  This information is used to
   populate the parameter Counter Signature Key Parameters of the Common
   Context (see Section 2).

   The columns of this table are:

   o  Name: A value that can be used to identify an algorithm in
      documents for easier comprehension.  Its value is taken from the
      'Name' column of the "COSE Algorithms" Registry.

   o  Value: The value to be used to identify this algorithm.  Its
      content is taken from the 'Value' column of the "COSE Algorithms"
      Registry.  The value MUST be the same one used in the "COSE
      Algorithms" Registry for the entry with the same 'Name' field.

   o  Parameters: This indicates the CBOR encoding of the key parameters
      (if any) for the counter signature algorithm indicated by the
      'Value' field.

   o  Description: A short description of the parameters encoded in the
      'Parameters' field (if any).

   o  Reference: This contains a pointer to the public specification for
      the field, if one exists.

   Initial entries in the registry are as follows.

  +-------------+-------+--------------+-------------------+-----------+
  |    Name     | Value |  Parameters  |   Description     | Reference |
  +-------------+-------+--------------+-------------------+-----------+
  |             |       |              |                   |           |
  |    EdDSA    |  -8   | [kty : int , | kty value is 1,   | [This     |
  |             |       |              | as Key Type "OKP" | Document] |
  |             |       |              | from the COSE Key |           |
  |             |       |              | Types Registry    |           |
  |             |       |              |                   |           |
  |             |       |              |                   |           |
  |             |       |  crv : int]  | crv value taken   |           |
  |             |       |              | from the COSE     |           |

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  |             |       |              | Elliptic Curve    |           |
  |             |       |              | Registry          |           |
  |             |       |              |                   |           |
  +-------------+-------+--------------+-------------------+-----------+
  |             |       |              |                   |           |
  |    ES256    |  -7   | [kty : int , | kty value is 2,   | [This     |
  |             |       |              | as Key Type "EC2" | Document] |
  |             |       |              | from the COSE Key |           |
  |             |       |              | Types Registry    |           |
  |             |       |              |                   |           |
  |             |       |              |                   |           |
  |             |       |  crv : int]  | crv value taken   |           |
  |             |       |              | from the COSE     |           |
  |             |       |              | Elliptic Curve    |           |
  |             |       |              | Registry          |           |
  |             |       |              |                   |           |
  +-------------+-------+--------------+-------------------+-----------+
  |             |       |              |                   |           |
  |    ES384    |  -35  | [kty : int , | kty value is 2,   | [This     |
  |             |       |              | as Key Type "EC2" | Document] |
  |             |       |              | from the COSE Key |           |
  |             |       |              | Types Registry    |           |
  |             |       |              |                   |           |
  |             |       |  crv : int]  | crv value taken   |           |
  |             |       |              | from the COSE     |           |
  |             |       |              | Elliptic Curve    |           |
  |             |       |              | Registry          |           |
  |             |       |              |                   |           |
  +-------------+-------+--------------+-------------------+-----------+
  |             |       |              |                   |           |
  |    ES512    |  -36  | [kty : int , | kty value is 2,   | [This     |
  |             |       |              | as Key Type "EC2" | Document] |
  |             |       |              | from the COSE Key |           |
  |             |       |              | Types Registry    |           |
  |             |       |              |                   |           |
  |             |       |  crv : int]  | crv value taken   |           |
  |             |       |              | from the COSE     |           |
  |             |       |              | Elliptic Curve    |           |
  |             |       |              | Registry          |           |
  |             |       |              |                   |           |
  +-------------+-------+--------------+-------------------+-----------+
  |             |       |              |                   |           |
  |    PS256    |  -37  |  kty : int   | kty value is 3,   | [This     |
  |             |       |              | as Key Type "RSA" | Document] |
  |             |       |              | from the COSE Key |           |
  |             |       |              | Types Registry    |           |
  |             |       |              |                   |           |
  +-------------+-------+--------------+-------------------+-----------+

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  |             |       |              |                   |           |
  |    PS384    |  -38  |  kty : int   | kty value is 3,   | [This     |
  |             |       |              | as Key Type "RSA" | Document] |
  |             |       |              | from the COSE Key |           |
  |             |       |              | Types Registry    |           |
  |             |       |              |                   |           |
  +-------------+-------+--------------+-------------------+-----------+
  |             |       |              |                   |           |
  |    PS512    |  -39  |  kty : int   | kty value is 3,   | [This     |
  |             |       |              | as Key Type "RSA" | Document] |
  |             |       |              | from the COSE Key |           |
  |             |       |              | Types Registry    |           |
  |             |       |              |                   |           |
  +-------------+-------+--------------+-------------------+-----------+

11.3.  OSCORE Flag Bits Registry

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

   +--------------+-------------+--------------------------+-----------+
   | Bit Position |     Name    |        Description       | Reference |
   +--------------+-------------+--------------------------+-----------+
   |       2      | Pairwise    | Set to 1 if the message  | [This     |
   |              | Protection  | is protected with        | Document] |
   |              | Flag        | pairwise keying material |           |
   +--------------+-------------+--------------------------+-----------+

11.4.  Expert Review Instructions

   The IANA Registries established in this document are defined as
   "Expert Review".  This section gives some general guidelines for what
   the experts should be looking for, but they are being designated as
   experts for a reason so they should be given substantial latitude.

   Expert reviewers should take into consideration the following points:

   o  Clarity and correctness of registrations.  Experts are expected to
      check the clarity of purpose and use of the requested entries.
      Experts should inspect the entry for the algorithm considered, to
      verify the conformity of the encoding proposed against the
      theoretical algorithm, including completeness of the 'Parameters'
      column.  Expert needs to make sure values are taken from the right
      registry, when that's required.  Expert should consider requesting
      an opinion on the correctness of registered parameters from the
      CBOR Object Signing and Encryption Working Group (COSE).

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

   o  Duplicated registration and point squatting should be discouraged.
      Reviewers are encouraged to get sufficient information for
      registration requests to ensure that the usage is not going to
      duplicate one that is already registered and that the point is
      likely to be used in deployments.

   o  Experts should take into account the expected usage of fields when
      approving point assignment.  The length of the 'Parameters'
      encoding should be weighed against the usage of the entry,
      considering the size of device it will be used on.  Additionally,
      the length of the encoded value should be weighed against how many
      code points of that length are left, the size of device it will be
      used on, and the number of code points left that encode to that
      size.

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

12.  References

12.1.  Normative References

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              ietf-core-groupcomm-bis-00 (work in progress), March 2020.

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

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

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

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

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

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

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

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

12.2.  Informative References

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

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

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", draft-ietf-ace-key-groupcomm-
              oscore-05 (work in progress), March 2020.

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

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

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

   [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>.

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

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

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

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

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

Appendix A.  Assumptions and Security Objectives

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

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

   o  Security group, as defined in Section 1.1 of this specification.
      There can be a one-to-one or a one-to-many relation between
      security groups and application groups.  Any two application
      groups associated to the same security group do not share any same
      resource.

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

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A.1.  Assumptions

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

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

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

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

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

   o  Provisioning and management of Security Contexts: a Security
      Context must be established among the members of the security
      group.  A secure mechanism must be used to generate, revoke and
      (re-)distribute keying material, multicast security 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.

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

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

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

A.2.  Security Objectives

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

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

   o  Group-level data confidentiality: messages sent within the
      security group shall be encrypted if privacy sensitive data is
      exchanged within the security group.  This document considers
      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.

   o  Source 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

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      first place, and in particular by a specific member of the
      security group.

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

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

Appendix B.  List of Use Cases

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

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

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      luminaires are changed at the same perceived time.  This is
      especially useful for providing a visual synchronicity of light
      effects to the user.  As a practical guideline, events within a
      200 ms interval are perceived as simultaneous by humans, which is
      necessary to ensure in many setups.  Devices may reply back to the
      switches that issue on/off/dimming commands, in order to report
      about the execution of the requested operation (e.g.  OK, failure,
      error) and their current operational status.  In a typical
      lighting control scenario, a single switch is the only entity
      responsible for sending commands to a set of lighting devices.  In
      more advanced lighting control use cases, a M-to-N communication
      topology would be required, for instance in case multiple sensors
      (presence or day-light) are responsible to trigger events to a set
      of lighting devices.  Especially in professional lighting
      scenarios, the roles of client and server are configured by the
      lighting commissioner, and devices strictly follow those roles.

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

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

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      about the execution of the update operation (e.g.  OK, failure,
      error) and their current operational status.

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

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

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

Appendix C.  Example of Group Identifier Format

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

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

   The Group Epoch is set to 0 upon the group's initialization, and is
   incremented by 1 upon completing each renewal of the Security Context

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   and keying material in the group (see Section 2.4).  In particular,
   once a new Master Secret has been distributed to the group, all the
   group members increment by 1 the Group Epoch in the Group Identifier
   of that group.

   As an example, a 3-byte Group Identifier 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 Group Identifier will assume value
   '0xb1f05c'.

   Using an immutable Group Prefix for a group assumes that enough time
   elapses between two consecutive usages of the same Group Epoch value
   in that group.  This ensures that the Gid value is temporally unique
   during the lifetime of a given message.  Thus, the expected highest
   rate for addition/removal of group members and consequent group
   rekeying should be taken into account for a proper dimensioning of
   the Group Epoch size.

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

Appendix D.  Set-up of New Endpoints

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

   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.

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

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

Appendix E.  Examples of Synchronization Approaches

   This section describes three possible approaches that can be
   considered by server endpoints to synchronize with sender sequence
   numbers of client endpoints sending group requests.

E.1.  Best-Effort Synchronization

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

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

   Since it provides no assurance as to freshness of requests, it is
   thus RECOMMENDED that a server using this synchronization approach
   always uses its own sender sequence number as Partial IV when
   protecting a response.

E.2.  Baseline Synchronization

   Upon receiving a group request from a given client for the first
   time, a server initializes its last-seen sender sequence number in
   its Recipient Context associated to that client.  This provides a
   reference point to identify if future group requests from the same
   client are fresher than the last one received.

   A replay time interval exists, between when a possibly replayed or
   delayed message is originally transmitted by a given client and the
   first authentic fresh message from that same client is received.

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   This can be acceptable for use cases where servers admit such a
   trade-off between performance and assurance of message freshness.

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

   In case the baseline synchronization state related to a client is
   lost, it is RECOMMENDED that the server uses its own sender sequence
   number as Partial IV when protecting a response to that client, until
   a new baseline synchronization state for that client is established.

E.3.  Challenge-Response Synchronization

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

   That is, upon receiving a group request from a particular client for
   the first time, the server processes the message as described in this
   specification, but, even if valid, does not deliver it to the
   application.  Instead, the server replies to the client with an
   OSCORE protected 4.01 (Unauthorized) response message, including only
   the Echo Option and no diagnostic payload.  The server stores the
   option value included therein.

   Upon receiving a 4.01 (Unauthorized) response that includes an Echo
   Option and originates from a verified group member, a client sends a
   request as a unicast message addressed to the same server, echoing
   the Echo Option value.  In particular, the client does not
   necessarily resend the same group request, but can instead send a
   more recent one, if the application permits it.  This makes it
   possible for the client to not retain previously sent group requests
   for full retransmission, unless the application explicitly requires
   otherwise.  In either case, the client uses the sender sequence
   number value currently stored in its own Sender Context.  If the
   client stores group requests for possible retransmission with the
   Echo Option, it should not store a given request for longer than a
   pre-configured time interval.  Note that the unicast request echoing
   the Echo Option is correctly treated and processed as a message,
   since the 'kid context' field including the Group Identifier of the
   OSCORE group is still present in the OSCORE Option as part of the
   COSE object (see Section 4).

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   Upon receiving the unicast request including the Echo Option, the
   server verifies that the option value equals the stored and
   previously sent value.  If not, the server MUST NOT process the
   request further and MAY send a 4.01 (Unauthorized) response including
   an Echo option.

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

   A server should not deliver group requests from a given client to the
   application until one valid request from that same client has been
   verified as fresh, as conveying an echoed Echo Option
   [I-D.ietf-core-echo-request-tag].  Also, a server may perform the
   challenge-response described above at any time, if synchronization
   with sender sequence numbers of clients is (believed to be) lost, for
   instance after a device reboot.  It is the role of the application to
   define under what circumstances sender sequence numbers lose
   synchronization.  This can include a minimum gap between the sender
   sequence number of the latest accepted group request from a client
   and the sender sequence number of a group request just received from
   the same client.  A client has to be always ready to perform the
   challenge-response based on the Echo Option in case a server starts
   it.

   This approach 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.

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

   Since requests including the Echo Option are sent over unicast, a
   server can be a victim of the attack discussed in Section 10.7, when
   such requests are protected with the signature mode of Group OSCORE,
   as described in Section 7.1.

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   Instead, protecting requests with the Echo Option by using the
   pairwise mode of Group OSCORE as described in Appendix G.2 prevents
   the attack in Section 10.7.  In fact, only the exact server involved
   in the Echo exchange is able to derive the correct pairwise key used
   by the client to protect the request including the Echo Option.

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

Appendix F.  No Verification of Signatures

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

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

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Appendix G.  Pairwise Mode

   For use cases that do not require an intermediary performing
   signature verification and that use a compatible signature algorithm,
   the pairwise mode defined in this section can be used for unicast
   communication.

   This mode uses the derivation process defined in Section 3, and
   allows two group members to protect requests and responses exchanged
   with each other using pairwise keying material.

   Senders MUST NOT use the pairwise mode to protect a message addressed
   to multiple recipients or to the whole group.  This prevents a client
   that wants to address one specific server from protecting a request
   with the pairwise key associated to that server, and then send the
   request over multicast.

   The pairwise mode results in the same performance and security
   improvements displayed by optimized responses (see Section 9.2).

G.1.  Pre-Requirements

   In order to protect an outgoing message in pairwise mode, a sender
   needs to know the public key and the Recipient ID for the message
   recipient, as stored in its own Recipient Context associated to that
   recipient.

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

   To make this information available, servers MAY provide a resource to
   which a client can send a request for a server identified by its
   'kid' value, or a set thereof.  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.

G.2.  Pairwise Protected Request

   A request in pairwise mode is protected as defined in Section 7.1,
   with the following differences.

   o  The client MUST set to 1 the sixth least significant bit of the
      OSCORE flag bits in the OSCORE option, i.e. the Pairwise Flag.

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   o  The COSE_Encrypt0 object included in the request is encrypted
      using a symmetric pairwise key K, that the client derives as
      defined in Section 3.  In particular, the Sender/Recipient Key is
      the Sender Key of the client from its own Sender Context, i.e. the
      Recipient Key that the server stores in its own Recipient Context
      corresponding to the client.

   o  The Counter Signature is not computed.  That is, unlike defined in
      Section 5, the payload of the OSCORE message terminates with the
      encoded ciphertext of the COSE object.

   Note that no changes are made to the AEAD nonce and AAD.

   Upon receiving a request with the Pairwise Flag set to 1, the server
   MUST process it as defined in Section 7.2, with the following
   differences.

   o  No countersignature to verify is included.

   o  The COSE_Encrypt0 object included in the request is decrypted and
      verified using the same symmetric pairwise key K, that the server
      derives as described above for the client side and as defined in
      Section 3.

G.3.  Pairwise Protected Response

   When using the pairwise mode, the processing of a response occurs as
   described in Section 9.2 for an optimized response.

Appendix H.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

H.1.  Version -07 to -08

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   o  Editorial improvements.

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H.2.  Version -06 to -07

   o  Updated abstract and introduction.

   o  Clarifications of what pertains a group rekeying.

   o  Derivation of pairwise keying material.

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

   o  Defined the Pairwise Flag bit for the OSCORE option.

   o  Supporting CoAP Observe for group requests and responses.

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

   o  New optimized mode based on pairwise keying material.

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

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

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

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

H.3.  Version -05 to -06

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

   o  Clarifications on parameter update upon group rekeying.

   o  Updated external_aad structures.

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

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

   o  Removed client handling of duplicated responses to multicast
      requests.

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   o  Additional considerations on public key retrieval and group
      rekeying.

   o  Added Group Manager responsibility on validating public keys.

   o  Updates IANA registries.

   o  Reference to RFC 8613.

   o  Editorial improvements.

H.4.  Version -04 to -05

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

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

   o  Clarification about Recipient Contexts (Section 2).

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

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

   o  Extended Security Considerations (Section 8).

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

H.5.  Version -03 to -04

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

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

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

   o  Structured Section 3 into subsections.

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

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

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

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

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

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

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

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

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

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

H.6.  Version -02 to -03

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

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

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

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

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

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

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

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

   o  Revised and extended security considerations in Section 8.

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

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

H.7.  Version -01 to -02

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

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

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

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

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

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

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

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

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

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H.8.  Version -00 to -01

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

   o  Section 2 has been updated with:

      *  Clarifications on etablishment/derivation of Security Contexts.

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

   o  Section 3 has been updated with:

      *  Examples of request and response messages.

      *  Use of CounterSignature0 rather than CounterSignature.

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

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

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

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

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

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

Acknowledgments

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

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

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Authors' Addresses

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

   Email: marco.tiloca@ri.se

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

   Email: goran.selander@ericsson.com

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

   Email: francesca.palombini@ericsson.com

   Jiye Park
   Universitaet Duisburg-Essen
   Schuetzenbahn 70
   Essen  45127
   Germany

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

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