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

CoRE Working Group                                             M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                             G. Selander
Expires: May 7, 2020                                        F. Palombini
                                                             Ericsson AB
                                                                 J. Park
                                             Universitaet Duisburg-Essen
                                                       November 04, 2019


           Group OSCORE - Secure Group Communication for CoAP
                  draft-ietf-core-oscore-groupcomm-06

Abstract

   This document describes a mode for protecting group communication
   over the Constrained Application Protocol (CoAP).  The proposed mode
   relies on Object Security for Constrained RESTful Environments
   (OSCORE) and the CBOR Object Signing and Encryption (COSE) format.
   In particular, it defines how OSCORE is used in a group communication
   setting, while fulfilling the same security requirements for group
   requests and responses.  Source authentication of all messages
   exchanged within the group is provided by means of digital signatures
   produced by the sender and embedded in the protected CoAP messages.

Status of This Memo

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

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

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

   This Internet-Draft will expire on May 7, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  OSCORE Security Context . . . . . . . . . . . . . . . . . . .   5
     2.1.  Management of Group Keying Material . . . . . . . . . . .   9
     2.2.  Wrap-Around of Partial IVs  . . . . . . . . . . . . . . .   9
   3.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Updated external_aad  . . . . . . . . . . . . . . . . . .  10
       3.1.1.  Updated external_aad for Encryption . . . . . . . . .  10
       3.1.2.  Updated external_aad for Signing  . . . . . . . . . .  11
     3.2.  Use of the 'kid' Parameter  . . . . . . . . . . . . . . .  12
     3.3.  Updated 'unprotected' Field . . . . . . . . . . . . . . .  12
   4.  OSCORE Header Compression . . . . . . . . . . . . . . . . . .  12
     4.1.  Encoding of the OSCORE Option Value . . . . . . . . . . .  12
     4.2.  Encoding of the OSCORE Payload  . . . . . . . . . . . . .  13
     4.3.  Examples of Compressed COSE Objects . . . . . . . . . . .  14
   5.  Message Binding, Sequence Numbers, Freshness and Replay
       Protection  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Synchronization of Sender Sequence Numbers  . . . . . . .  15
   6.  Message Processing  . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  16
     6.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  16
     6.3.  Protecting the Response . . . . . . . . . . . . . . . . .  17
     6.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  17
   7.  Responsibilities of the Group Manager . . . . . . . . . . . .  18
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     8.1.  Group-level Security  . . . . . . . . . . . . . . . . . .  20
     8.2.  Uniqueness of (key, nonce)  . . . . . . . . . . . . . . .  20
     8.3.  Management of Group Keying Material . . . . . . . . . . .  21
     8.4.  Update of Security Context and Key Rotation . . . . . . .  21
     8.5.  Collision of Group Identifiers  . . . . . . . . . . . . .  22
     8.6.  Cross-group Message Injection . . . . . . . . . . . . . .  22
     8.7.  End-to-end Protection . . . . . . . . . . . . . . . . . .  24
     8.8.  Security Context Establishment  . . . . . . . . . . . . .  24
     8.9.  Master Secret . . . . . . . . . . . . . . . . . . . . . .  24
     8.10. Replay Protection . . . . . . . . . . . . . . . . . . . .  25
     8.11. Client Aliveness  . . . . . . . . . . . . . . . . . . . .  25



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     8.12. Cryptographic Considerations  . . . . . . . . . . . . . .  25
     8.13. Message Segmentation  . . . . . . . . . . . . . . . . . .  26
     8.14. Privacy Considerations  . . . . . . . . . . . . . . . . .  26
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     9.1.  Counter Signature Parameters Registry . . . . . . . . . .  27
     9.2.  Counter Signature Key Parameters Registry . . . . . . . .  29
     9.3.  Expert Review Instructions  . . . . . . . . . . . . . . .  31
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     10.2.  Informative References . . . . . . . . . . . . . . . . .  33
   Appendix A.  Assumptions and Security Objectives  . . . . . . . .  34
     A.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .  34
     A.2.  Security Objectives . . . . . . . . . . . . . . . . . . .  36
   Appendix B.  List of Use Cases  . . . . . . . . . . . . . . . . .  36
   Appendix C.  Example of Group Identifier Format . . . . . . . . .  39
   Appendix D.  Set-up of New Endpoints  . . . . . . . . . . . . . .  40
   Appendix E.  Examples of Synchronization Approaches . . . . . . .  40
     E.1.  Best-Effort Synchronization . . . . . . . . . . . . . . .  40
     E.2.  Baseline Synchronization  . . . . . . . . . . . . . . . .  41
     E.3.  Challenge-Response Synchronization  . . . . . . . . . . .  41
   Appendix F.  No Verification of Signatures  . . . . . . . . . . .  43
   Appendix G.  Document Updates . . . . . . . . . . . . . . . . . .  43
     G.1.  Version -05 to -06  . . . . . . . . . . . . . . . . . . .  43
     G.2.  Version -04 to -05  . . . . . . . . . . . . . . . . . . .  44
     G.3.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  44
     G.4.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  45
     G.5.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  46
     G.6.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  47
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  47
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  48

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 [RFC7390][I-D.dijk-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 utilisation.  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).
   Furthermore, [RFC7390] recognizes the importance to introduce a
   secure mode for CoAP group communication.  This specification defines
   such a mode.




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   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 receipient, 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 end-to-end security of
   CoAP messages exchanged between members of a group, and preserving
   independence of transport layer.  In particular, the described
   approach defines how OSCORE should be used in a group communication
   setting, so that end-to-end security is assured in the same way as
   OSCORE for unicast communication.  That is, end-to-end security is
   provided for CoAP multicast requests sent by a client to the group,
   and for related CoAP responses sent by multiple servers.  Group
   OSCORE provides source authentication of all CoAP messages exchanged
   within the group, by means of digital signatures produced through
   private keys of sender devices and embedded in the protected CoAP
   messages.

   As defined in the latest [I-D.dijk-core-groupcomm-bis], Group OSCORE
   is the security protocol to use for applications that rely on CoAP
   group communication.  As in OSCORE, it is still possible to
   simultaneously rely on DTLS [RFC6347] to protect hop-by-hop
   communication between a sender and a proxy (and vice versa), and
   between a proxy and a recipient (and vice versa).  Note that DTLS
   cannot be used to secure messages sent over multicast.

1.1.  Terminology

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

   Readers are expected to be familiar with the terms and concepts
   described in CoAP [RFC7252] including "endpoint", "client", "server",
   "sender" and "recipient"; group communication for CoAP
   [RFC7390][I-D.dijk-core-groupcomm-bis]; COSE and counter signatures
   [RFC8152].






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   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).  The term
      group used in this specification refers thus to a "security
      group", not to be confused with 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 7.

   o  Silent server: member of a group that never responds to requests.
      Note that a silent server can act as a client, the two roles are
      independent.

   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.

   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.  OSCORE Security Context

   To support group communication secured with OSCORE, each endpoint
   registered as member of a group maintains a Security Context as
   defined in Section 3 of [RFC8613], extended as defined below.  Each
   endpoint in a group makes use of:



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   1.  one Common Context, shared by all the endpoints in a given group.
       In particular:

       *  The ID Context parameter contains the Gid of the group, which
          is used to retrieve the Security Context for processing
          messages intended to the endpoints of the group (see
          Section 6).  The choice of the Gid is application specific.
          An example of specific formatting of the Gid is given in
          Appendix C.  The application needs to specify how to handle
          possible collisions between Gids, see Section 8.5.

       *  A new parameter Counter Signature Algorithm is included.  Its
          value identifies the digital signature algorithm used to
          compute a counter signature on the COSE object (see
          Section 4.5 of [RFC8152]) which provides source authentication
          within the group.  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].

       *  A new parameter Counter Signature Parameters is included.
          This parameter 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 9.1), where each entry indicates a
          specified structure of the Counter Signature Parameters.

       *  A new parameter Counter Signature Key Parameters is included.
          This parameter 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 9.2), where
          each entry indicates a specified structure of the Counter
          Signature Key Parameters.

   2.  one Sender Context, unless the endpoint is configured exclusively
       as silent server.  The Sender Context is used to secure outgoing
       messages and is initialized according to Section 3 of [RFC8613],



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       once the endpoint has joined the group.  The Sender Context of a
       given endpoint matches the corresponding Recipient Context in all
       the endpoints receiving a protected message from that endpoint.
       Besides, in addition to what is defined in [RFC8613], the Sender
       Context stores also the endpoint's private key.

   3.  one Recipient Context for each distinct endpoint from which
       messages are received, used to process incoming messages.  The
       recipient may generate a Recipient Context whenever in possession
       of all the required pieces of information on the corresponding
       endpoint, e.g. they may be provided to the recipient upon joining
       the group.  Alternatively, the recipient may generate a Recipient
       Context upon receiving an incoming message from another endpoint
       in the group for the first time (see Section 6.2 and
       Section 6.4).  Each Recipient Context matches the Sender Context
       of the endpoint from which protected messages are received.
       Besides, in addition to what is defined in [RFC8613], each
       Recipient Context stores also the public key of the associated
       other endpoint from which messages are received.  Note that each
       Recipient Context includes a Replay Window, unless the recipient
       acts only as client and hence processes only responses as
       incoming messages.

   The table in Figure 1 overviews the new information included in the
   OSCORE Security Context, with respect to what defined in Section 3 of
   [RFC8613].

         +---------------------------+------------------------------+
         |      Context portion      |       New information        |
         +---------------------------+------------------------------+
         |                           |                              |
         |      Common Context       | Counter signature algorithm  |
         |                           |                              |
         |      Common Context       | Counter signature parameters |
         |                           |                              |
         |      Sender Context       | Endpoint's own private key   |
         |                           |                              |
         |  Each Recipient Context   | Public key of the            |
         |                           | associated other endpoint    |
         |                           |                              |
         +---------------------------+------------------------------+

            Figure 1: Additions to the OSCORE Security Context

   Upon receiving a secure CoAP message, a recipient uses the sender's
   public key, in order to verify the counter signature of the COSE
   Object (see Section 3).




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   If not already stored in the Recipient Context associated to the
   sender, the recipient retrieves the sender's public key from the
   Group Manager, which collects public keys upon endpoints' joining the
   group, acts as trusted key repository and ensures the correct
   association between the public key and the identifier of the sender,
   for instance by means of public key certificates.

   For very constrained devices, it may be not feasible to
   simultaneously handle the ongoing processing of a just received
   message and the parallel retrieval of the sender's public key.  Such
   devices can be configured to drop that received message altogether,
   switch to the retrieval of the sender's public key, and thus have it
   available to verify following messages from that sender.

   Note that a group member can retrieve public keys from the Group
   Manager and generate the Recipient Context associated to another
   group member at any point in time, as long as this is done before
   verifying a received secure CoAP message.  The exact configuration is
   application dependent.  For example, an application can configure a
   group member to retrieve all the required information and to create
   the Recipient Context exactly upon receiving a message from another
   group member for the first time.  As an alternative, the application
   can configure a group member to asynchronously retrieve the required
   information and update its list of Recipient Contexts well before
   receiving any message, e.g. by Observing [RFC7641] the Group Manager
   to get updates on the group membership.

   It is RECOMMENDED that the Group Manager collects public keys and
   provides them to group members upon request 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.

   An endpoint receives 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.  An endpoint uses its own Sender ID
   (together with other data) to generate unique AEAD nonces for
   outgoing messages, as in [RFC8613].  Endpoints which are configured
   only as silent servers do not have a Sender ID.

   The Sender/Recipient Keys and the Common IV are derived according to
   the same scheme defined in Section 3.2 of [RFC8613].  The mandatory-
   to-implement HKDF and AEAD algorithms for Group OSCORE are the same
   as in [RFC8613].





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2.1.  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.  When doing so, a new value for the
   Master Salt parameter MAY also be distributed, and the Group Manager
   SHOULD preserve the current value of the Sender ID of each group
   member.  An example of Gid format supporting this operation is
   provided in Appendix C.  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.

   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 verifying countersignatures of received messages.

   As a consequence, group members may end up retaining stale Recipient
   Contexts, that are no longer useful to verify incoming secure
   messages.  Applications may define policies to delete (long-)unused
   Recipient Contexts and reduce the impact on storage space.

   If the application requires so (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].

2.2.  Wrap-Around of Partial IVs

   An endpoint can eventually experience a wrap-around of 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.

   When a wrap-around happens, the endpoint MUST NOT transmit further
   messages including a Partial IV 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 OSCORE Security Context in the group
      (see Section 2.1).

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

   Either case, same considerations from Section 2.1 hold about possible
   retaining of stale Recipient Contexts.

3.  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
   Group OSCORE, the following modifications apply.

3.1.  Updated external_aad

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

3.1.1.  Updated 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 9.1) for this counter signature
      algorithm.

   The 'algorithms' array in the aad_array MAY also include:

   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



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      "Parameters" field of the Counter Signature Parameters Registry
      (see Section 9.1), 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 9.2), 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]).

   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
   ]

3.1.2.  Updated 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 3.1.1;

   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.







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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,
      OSCORE_option: bstr
   ]

   Note for implementation: this requires the value of the OSCORE option
   to be fully ready, before starting the signing process.

3.2.  Use of the 'kid' Parameter

   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.

3.3.  Updated 'unprotected' Field

   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 using its own private key
      as described in Appendix A.2 of [RFC8152].  In particular, the
      Sig_structure contains the external_aad as defined in
      Section 3.1.2 and the ciphertext of the COSE_Encrypt0 object as
      payload.

4.  OSCORE Header Compression

   The OSCORE compression defined in Section 6 of [RFC8613] is used,
   with the following additions for the encoding of the OSCORE Option
   and the OSCORE Payload.

4.1.  Encoding of the OSCORE Option Value

   Analogously to [RFC8613], the value of the OSCORE option SHALL
   contain the OSCORE flag bits, the Partial IV parameter, the kid




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   context parameter (length and value), and the kid parameter, with the
   following modifications:

   o  The first byte, containing the OSCORE flag bits, has the following
      encoding modifications:

      *  The fourth least significant bit MUST be set to 1 in every
         message, to indicate the presence of the 'kid' parameter for
         all group requests and responses.  That is, unlike in
         [RFC8613], the 'kid' parameter is always present in all
         messages.

      *  The fifth least significant bit MUST be set to 1 for group
         requests, to indicate the presence of the 'kid context'
         parameter in the compressed COSE object.  The 'kid context' MAY
         be present in responses if the application requires it.  In
         such a case, the kid context flag MUST be set to 1.

   The flag bits are registered in the OSCORE Flag Bits registry
   specified in Section 13.7 of [RFC8613].

   o  The 'kid context' value encodes the Group Identifier value (Gid)
      of the group's Security Context.

   o  The remaining bytes in the OSCORE Option value encode the value of
      the 'kid' parameter, which is always present both in group
      requests and in responses.

            0 1 2 3 4 5 6 7 <------------ n bytes ------------>
           +-+-+-+-+-+-+-+-+-----------------------------------+
           |0 0|0|h|1|  n  |       Partial IV (if any)         |
           +-+-+-+-+-+-+-+-+-----------------------------------+

            <-- 1 byte ---> <------ s bytes ------>
           +---------------+-----------------------+-----------+
           |  s (if any)   |   kid context = Gid   |    kid    |
           +---------------+-----------------------+-----------+

                       Figure 2: OSCORE Option Value

4.2.  Encoding of the OSCORE Payload

   The payload of the OSCORE message SHALL encode the ciphertext of the
   COSE object concatenated with the value of the CounterSignature0 of
   the COSE object, computed as in Appendix A.2 of [RFC8152] according
   to the Counter Signature Algorithm and Counter Signature Parameters
   in the Security Context.




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4.3.  Examples of Compressed COSE Objects

   This section covers a list of OSCORE Header Compression examples for
   group requests and responses.  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 4 and divided into two 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.  COUNTERSIGN is the CounterSignature0 byte
   string as described in Section 3 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 = 60b035059d9ef5667c5a0710823b, 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)

5.  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.

5.1.  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.

6.  Message Processing

   Each request message and response message is protected and processed
   as specified in [RFC8613], with the modifications described in the
   following sections.  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][RFC7390][I-D.dijk-core-groupcomm-bis], group
   requests sent over multicast MUST be Non-Confirmable.  Thus, senders



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   should store their outgoing messages for an amount of time defined by
   the application and sufficient to correctly handle possible
   retransmissions.  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 3, or which is not cryptographically
   validated in a successful way.  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.

6.1.  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 3 of this specification.

   o  In step 4, the encryption of the COSE object is modified as
      described in Section 3 of this specification.  The encoding of the
      compressed COSE object is modified as described in Section 4 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 4.2 of this
      specification.  In particular, the payload of the OSCORE message
      includes also the counter signature.

6.2.  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 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 server MAY create a new
      Recipient Context and initializes it according to Section 3 of



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      [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 3 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
      prevent attackers from overloading the server's storage and
      creating processing overhead on the server.

6.3.  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 3 of this specification.

   o  In step 4, the encryption of the COSE object is modified as
      described in Section 3 of this specification.  The encoding of the
      compressed COSE object is modified as described in Section 4 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 4.2 of this
      specification.  In particular, the payload of the OSCORE message
      includes also the counter signature.

6.4.  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 3 of this specification.  If the received
      Recipient ID ('kid') does not match with any Recipient Context for



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      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 3 of this specification.

   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.

7.  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.

   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).



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   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.

8.  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
   further discussed in Section 8.1.

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

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

   The same considerations on uniqueness of (key, nonce) pairs for
   OSCORE discussed in Appendix D.4 of [RFC8613] hold for Group OSCORE.
   This is further discussed in Section 8.2.

   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 8.6.

   As discussed in Section 6.2.3 of [I-D.dijk-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



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   covered in the security considerations of OSCORE (Section 12 of
   [RFC8613]), and discusses how they hold when Group OSCORE is used.

8.1.  Group-level Security

   The approach described in this document 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.

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

8.2.  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.



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   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.

8.3.  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.

8.4.  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.  In the following two cases, this
   may result in misaligned Security Contexts between the sender and the
   recipient.

   In the first 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 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.

   In the second case, the sender protects a message using the new
   Security Context, but the recipient receives that request before
   having installed the new Security Context.  Therefore, the recipient



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   would not be able to correctly process the request and hence discards
   it.  If the recipient receives 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.

8.5.  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.

   However, 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.

8.6.  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
   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 3.1.2).  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



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   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.

   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.



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8.7.  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.

8.8.  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
   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].

8.9.  Master Secret

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



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   generating and distributing a new Master Secret.  Randomness
   requirements for security are described in [RFC4086].

8.10.  Replay Protection

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

   As discussed in Section 5.1, 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 6.4 also
   ensures that a client accepts a single valid response to a given
   request from each replying server, unless CoAP observation is used.

8.11.  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).

8.12.  Cryptographic Considerations

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

   As discussed in Section 2.2, an endpoint that experiences a wrap-
   around 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



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   by means of group rekeying, or provide only that endpoint with a new
   Sender ID value.  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.

8.13.  Message Segmentation

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

8.14.  Privacy Considerations

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

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

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

   o  The 'kid context' parameter, which is intended to help a recipient
      endpoint to find the right 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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9.  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.

9.1.  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 9.3.

   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.

   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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9.2.  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 9.3.

   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    |           |
  |             |       |              |                   |           |
  +-------------+-------+--------------+-------------------+-----------+

9.3.  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).
      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



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      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.

10.  References

10.1.  Normative References

   [I-D.dijk-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              dijk-core-groupcomm-bis-01 (work in progress), July 2019.

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

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

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

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

   [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>.






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   [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>.

10.2.  Informative References

   [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-03 (work in progress), November 2019.

   [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-25
              (work in progress), October 2019.

   [I-D.ietf-core-echo-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo,
              Request-Tag, and Token Processing", draft-ietf-core-echo-
              request-tag-08 (work in progress), November 2019.

   [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>.





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

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

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

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <https://www.rfc-editor.org/info/rfc7390>.

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

Appendix A.  Assumptions and Security Objectives

   This section presents a set of assumptions and security objectives
   for the approach described in this document.

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 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 group members are clients.  According to [RFC7390],
      any possible proxy entity is supposed to know about the clients in
      the group and to not perform aggregation of response messages from



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

   o  Group size: security solutions for group communication should be
      able to adequately support different and possibly large groups.
      The group size is the current number of members in a 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.  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 group member.

   o  Provisioning and management of Security Contexts: an OSCORE
      Security Context must be established among the group members.  A
      secure mechanism must be used to generate, revoke and
      (re-)distribute keying material, multicast security policies and
      security parameters in the group.  The actual provisioning and
      management of the Security Context is out of the scope of this
      document.

   o  Multicast data security ciphersuite: all group members 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 group should not have
      access to any old Security Contexts used before its joining.  This
      ensures that a new group member is not able to decrypt
      confidential data sent before it has joined the 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 upon a group member's joining has to be defined as
      part of the group key management scheme.

   o  Forward security: entities that leave the group should not have
      access to any future Security Contexts or message exchanged within
      the group after their leaving.  This ensures that a former group
      member is not able to decrypt confidential data sent within the



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      group anymore.  Also, it ensures that a former member is not able
      to send encrypted and/or integrity protected messages to the group
      anymore.  The actual mechanism to update the Security Context and
      renew the group keying material upon a group 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: replayed group request messages or
      response messages must be detected.

   o  Group-level data confidentiality: messages sent within the group
      shall be encrypted if privacy sensitive data is exchanged within
      the 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
      group, but not by an external adversary or other external
      entities.

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

   o  Message integrity: messages sent within the 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 group members.

   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 [RFC7390][I-D.dijk-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
   [RFC7390][I-D.dijk-core-groupcomm-bis] to understand the non-security
   related details.  This section discusses a number of use cases that




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   benefit from secure group communication.  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 devices are
      organized into groups according to their physical location in the
      building.  For instance, lighting devices and switches in a room
      or corridor can be configured as members of a single group.
      Switches are then used to control the lighting devices by sending
      on/off/dimming commands to all lighting devices in a 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 in the lighting group 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 group of connected lights, ensuring that the light
      preset (e.g. dimming level or color) of a large group of
      luminaires are changed at the same perceived time.  This is
      especially useful for providing a visual synchronicity of light
      effects to the user.  As a practical guideline, events within a
      200 ms interval are perceived as simultaneous by humans, which is
      necessary to ensure in many setups.  Devices may reply back to the
      switches that issue on/off/dimming commands, in order to report
      about the execution of the requested operation (e.g.  OK, failure,
      error) and their current operational status.  In a typical
      lighting control scenario, a single switch is the only entity
      responsible for sending commands to a group 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 group 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 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 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




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

   o  Parameter and configuration update: by means of multicast
      communication, it is possible to update the settings of a group 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 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



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      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
   and keying material in the group (see Section 2.1).  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 8.5, 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



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   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.

   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.





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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.  However, the server
   drops the group request without delivering it to the application
   layer.  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.
   This can be acceptable for use cases where servers admit such a
   trade-off between performance and assurance of message freshness.

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
   Section 6.2 of 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.  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 3).




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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; otherwise, the request is silently discarded.
   Then, the server verifies that the unicast request has been received
   within a pre-configured time interval, as described in
   [I-D.ietf-core-echo-request-tag].  In such a case, the request is
   further processed and verified; otherwise, it is silently discarded.
   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.

   In case it does not receive a valid unicast request including the
   Echo Option within the configured time interval, the server endpoint
   should perform the same challenge-response upon receiving the next
   group request from that same client.

   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.

   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.

   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.



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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.

Appendix G.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

G.1.  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.



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

   o  Removed client handling of duplicated responses to multicast
      requests.

   o  Additional considerations on public key retrieval and group
      rekeying.

   o  Added Group Manager responsibility on validating public keys.

   o  Updates IANA registries.

   o  Reference to RFC 8613.

   o  Editorial improvements.

G.2.  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).

G.3.  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 key material from the
      Group Manager, in order to process incoming messages (see
      Section 2).




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   o  Structured Section 3 into subsections.

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

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

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

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

   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).

G.4.  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".



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

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

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

   o  Revised and extended security considerations in Section 8.

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

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

G.5.  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.




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   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.

G.6.  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 Stefan Beck, Rolf Blom, Carsten Bormann,
   Esko Dijk, Klaus Hartke, Rikard Hoeglund, Richard Kelsey, John
   Mattsson, Dave Robin, Jim Schaad, Ludwig Seitz and Peter van der Stok
   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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