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

CoRE Working Group                                             M. Tiloca
Internet-Draft                                              RISE SICS AB
Intended status: Standards Track                             G. Selander
Expires: September 6, 2018                                  F. Palombini
                                                             Ericsson AB
                                                                 J. Park
                                             Universitaet Duisburg-Essen
                                                          March 05, 2018


                  Secure group communication for CoAP
                  draft-ietf-core-oscore-groupcomm-01

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 is defined how OSCORE should be used in a group
   communication setting, while fulfilling the same security
   requirements for request messages and related response messages.
   Source authentication of all messages exchanged within the group is
   ensured, by means of digital signatures produced through private keys
   of sender endpoints 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 September 6, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  OSCORE Security Context . . . . . . . . . . . . . . . . . . .   5
     2.1.  Management of Group Keying Material . . . . . . . . . . .   7
   3.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Example: Request  . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Example: Response . . . . . . . . . . . . . . . . . . . .  10
   4.  Message Processing  . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  11
     4.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  12
     4.3.  Protecting the Response . . . . . . . . . . . . . . . . .  12
     4.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  12
   5.  Synchronization of Sequence Numbers . . . . . . . . . . . . .  13
   6.  Responsibilities of the Group Manager . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     7.1.  Group-level Security  . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Assumptions and Security Objectives  . . . . . . . .  19
     A.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .  19
     A.2.  Security Objectives . . . . . . . . . . . . . . . . . . .  20
   Appendix B.  List of Use Cases  . . . . . . . . . . . . . . . . .  21
   Appendix C.  Example of Group Identifier Format . . . . . . . . .  23
   Appendix D.  Set-up of New Endpoints  . . . . . . . . . . . . . .  24
     D.1.  Join Process  . . . . . . . . . . . . . . . . . . . . . .  24
     D.2.  Provisioning and Retrieval of Public Keys . . . . . . . .  27
     D.3.  Group Joining Based on the ACE Framework  . . . . . . . .  29
   Appendix E.  Examples of Synchronization Approaches . . . . . . .  29
     E.1.  Best-Effort Synchronization . . . . . . . . . . . . . . .  30
     E.2.  Baseline Synchronization  . . . . . . . . . . . . . . . .  30
     E.3.  Challenge-Response Synchronization  . . . . . . . . . . .  30
   Appendix F.  No Verification of Signatures  . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32



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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] addresses use cases where
   deployed devices benefit from a group communication model, for
   example to reduce latencies and improve performance.  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.

   Object Security for Constrained RESTful Environments
   (OSCORE)[I-D.ietf-core-object-security] 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, and replay protection between a sending
   endpoint and a receiving endpoint possibly involving intermediary
   endpoints.  To this end, a CoAP message is protected by including its
   payload (if any), certain options, and header fields in a COSE
   object, which finally replaces the authenticated and encrypted fields
   in the protected message.

   This document describes group OSCORE, providing end-to-end security
   of CoAP messages exchanged between members of a group.  In
   particular, the described approach defines how OSCORE should be used
   in a group communication setting, so that end-to-end security is
   assured by using the same security method.  That is, end-to-end
   security is assured for multicast CoAP requests sent by multicaster
   endpoints to the group and for related CoAP responses sent as reply
   by multiple listener endpoints.  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 in OSCORE,
   it is still possible to simultaneously rely on DTLS to protect hop-
   by-hop communication between a multicaster endpoint and a proxy (and
   vice versa), and between a proxy and a listener endpoint (and vice
   versa).

1.1.  Terminology

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



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

   Readers are expected to be familiar with the terms and concepts
   described in CoAP [RFC7252] including "endpoint", "sender" and
   "recipient"; group communication for CoAP [RFC7390]; COSE and counter
   signatures [RFC8152].

   Readers are also expected to be familiar with the terms and concepts
   for protection and processing of CoAP messages through OSCORE, such
   as "Security Context", "Master Secret" and "Master Salt", defined in
   [I-D.ietf-core-object-security].

   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
      parameters (Common Context of the group's Security Context, see
      Section 2).  That is, the term group used in this specification
      refers to a "security group", not to be confused with network/
      multicast groups or application groups.

   o  Group Manager (GM): entity responsible for a set of OSCORE groups.
      Each endpoint in a group securely communicates with the respective
      GM, which is not required to be an actual group member and to take
      part in the group communication.  The full list of
      responsibilities of the Group Manager is provided in Section 6.

   o  Multicaster: member of a group that sends multicast CoAP request
      messages intended for all members of the group.  In a 1-to-N
      communication model, only a single multicaster transmits data to
      the group; in an M-to-N communication model (where M and N do not
      necessarily have the same value), M group members are
      multicasters.  According to [RFC7390], any possible proxy entity
      is supposed to know about the multicasters in the group and to not
      perform aggregation of response messages.  Also, every multicaster
      expects and is able to handle multiple response messages
      associated to a given multicast request message that it has
      previously sent to the group.

   o  Listener: member of a group that receives multicast CoAP request
      messages when listening to the multicast IP address associated to



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      the group.  A listener may reply back, by sending a response
      message to the multicaster which has sent the request message.

   o  Pure listener: member of a group that is configured as listener
      and never replies back to multicasters after receiving request
      messages.

   o  Group ID: group identifier assigned to the group.  Group IDs are
      unique within the set of groups of a same Group Manager.

   o  Endpoint ID: Sender ID of the endpoint, as defined in
      [I-D.ietf-core-object-security].  An Endpoint ID is provided to an
      endpoint upon joining a group, is valid only within that group,
      and is unique within the same group.  Endpoints which are
      configured only as pure listeners do not have an Endpoint ID.

   o  Group request: multicast CoAP request message sent by a
      multicaster in the group to all listeners in the group through
      multicast IP, unless otherwise specified.

   o  Source authentication: evidence that a received message in the
      group originated from a specifically identified group member.
      This also provides assurances that the message was not tampered
      with by a different group member or by a non-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 [I-D.ietf-core-object-security].  In
   particular, each endpoint in a group stores:

   1.  one Common Context, shared by all the endpoints in the group.
       All the endpoints in the group agree on the same COSE AEAD
       algorithm.  In addition to what is defined in Section 3 of
       [I-D.ietf-core-object-security], the Common Context includes the
       following information.

       *  Group Identifier (Gid).  Variable length byte string
          identifying the Security Context.  A Gid MUST have a random
          component and be long enough, in order to achieve a negligible
          probability of collisions between Group Identifiers from
          different Group Managers.  A Group ID is used i) alone or
          together with other parameters, such as the multicast IP
          address of the group, to retrieve the OSCORE Security Context
          of the associated group (see Section 4); and ii) as OSCORE
          Master Salt (see Section 3.1 of
          [I-D.ietf-core-object-security]).  The choice of the Gid for a



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          given group's Security Context is application specific.  It is
          the role of the application to specify how to handle possible
          collisions.  An example of specific formatting of the Group
          Identifier that would follow this specification is given in
          Appendix C.

       *  Counter Signature Algorithm.  Value identifying the algorithm
          used for source authenticating messages sent within the group,
          by means of a counter signature (see Section 4.5 of
          [RFC8152]).  Its value is immutable once the Common Context is
          established.  All the endpoints in the group agree on the same
          counter signature algorithm.  The list of supported signature
          algorithms is part of the group communication policy and MUST
          include the EdDSA signature algorithm ed25519 [RFC8032].

   2.  one Sender Context, unless the endpoint is configured exclusively
       as pure listener.  The Sender Context is used to secure outgoing
       group messages and is initialized according to Section 3 of
       [I-D.ietf-core-object-security], once the endpoint has joined the
       group.  In practice, the symmetric keying material in the Sender
       Context of the sender endpoint is shared with all the recipient
       endpoints that have received group messages from that same sender
       endpoint.  Besides, in addition to what is defined in
       [I-D.ietf-core-object-security], the Sender Context stores also
       the endpoint's public-private key pair.

   3.  one Recipient Context for each distinct endpoint from which group
       messages are received, used to process such incoming messages.
       The recipient endpoint creates a new Recipient Context upon
       receiving an incoming message from another endpoint in the group
       for the first time (see Section 4.2 and Section 4.4).  In
       practice, the symmetric keying material in a given Recipient
       Context of the recipient endpoint is shared with the associated
       sender endpoint from which group messages are received.  Besides,
       in addition to what is defined in
       [I-D.ietf-core-object-security], each Recipient Context stores
       also the public key of the associated other endpoint from which
       group messages are received.

   The table in Figure 1 overviews the new information included in the
   OSCORE Security Context, with respect to what defined in Section 3 of
   [I-D.ietf-core-object-security].









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         +---------------------------+-----------------------------+
         |      Context portion      |       New information       |
         +---------------------------+-----------------------------+
         |                           |                             |
         |      Common Context       | Group Identifier (Gid)      |
         |                           |                             |
         |      Common Context       | Counter signature algorithm |
         |                           |                             |
         |      Sender Context       | Endpoint's private key      |
         |                           |                             |
         |      Sender Context       | Endpoint's public 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 endpoint relies on
   the sender endpoint's public key, in order to verify the counter
   signature conveyed in the COSE Object.

   If not already stored in the Recipient Context associated to the
   sender endpoint, the recipient endpoint retrieves the public key from
   a trusted key repository.  In such a case, the correct binding
   between the sender endpoint and the retrieved public key must be
   assured, for instance by means of public key certificates.  Further
   discussion about how public keys can be handled and retrieved in the
   group is provided in Appendix D.2.

   The Sender Key/IV stored in the Sender Context and the Recipient
   Keys/IVs stored in the Recipient Contexts are derived according to
   the same scheme defined in Section 3.2 of
   [I-D.ietf-core-object-security].

2.1.  Management of Group Keying Material

   The approach described in this specification should take into account
   the risk of compromise of group members.  In particular, the adoption
   of key management schemes for secure revocation and renewal of
   Security Contexts and group keying material should be considered.

   Consistently with the security assumptions in Appendix A.1, it is
   RECOMMENDED to adopt a group key management scheme, and securely
   distribute a new value 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



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   necessary in order to preserve backward security and forward security
   in the group.

   In particular, a new Group Identifier (Gid) for that group and a new
   value for the Master Secret parameter must also be distributed.  An
   example of Group Identifier 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 Group
   Identifier.

   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.

3.  The COSE Object

   When creating a protected CoAP message, an endpoint in the group
   computes the COSE object using the untagged COSE_Encrypt0 structure
   [RFC8152] as defined in Section 5 of [I-D.ietf-core-object-security],
   with the following modifications.

   o  The value of the "kid" parameter in the "unprotected" field of
      response messagess SHALL be set to the Endpoint ID of the endpoint
      transmitting the message, i.e. the Sender ID.

   o  The "unprotected" field of the "Headers" field SHALL additionally
      include the following parameter:

      *  CounterSignature0 : its value is set to the counter signature
         of the COSE object, computed by the endpoint by means of its
         own private key as described in Section 4.5 of [RFC8152].  The
         presence of this parameter is explicitly signaled, by using the
         reserved sixth least significant bit of the first byte of flag
         bits in the value of the Object-Security option (see
         Section 6.1 of [I-D.ietf-core-object-security]).

   o  The Additional Authenticated Data (AAD) considered to compute the
      COSE object is extended, by adding the countersignature algorithm
      used to protect group messages.  In particular, the "external_aad"
      defined in Section 5.4 of [I-D.ietf-core-object-security] SHALL
      also include "alg_countersign", which contains the Counter
      Signature Algorithm from the Common Context (see Section 2).






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   external_aad = [
      oscore_version : uint,
      [alg_aead : int / tstr , alg_countersign : int / tstr],
      request_kid : bstr,
      request_piv : bstr,
      options : bstr
   ]

   o  The OSCORE compression defined in Section 6 of
      [I-D.ietf-core-object-security] is used, with the following
      additions for the encoding of the Object-Security option.

      *  The fourth least significant bit of the first byte of flag bits
         SHALL be set to 1, to indicate the presence of the "kid"
         parameter for both group requests and responses.

      *  The fifth least significant bit of the first byte of flag bits
         MUST be set to 1 for group requests, to indicate the presence
         of the kid context in the OSCORE payload.  The kid context flag
         MAY be set to 1 for responses.

      *  The sixth least significant bit of the first byte of flag bits
         is originally marked as reserved in
         [I-D.ietf-core-object-security] and its usage is defined in
         this specification.  This bit is set to 1 if the
         "CounterSignature0" parameter is present, or to 0 otherwise.
         In order to ensure source authentication of group messages as
         described in this specification, this bit SHALL be set to 1.

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

      *  The following q bytes (q given by the Counter Signature
         Algorithm specified in the Security Context) encode the value
         of the "CounterSignature0" parameter including the counter
         signature of the COSE object.

      *  The remaining bytes in the Object-Security value encode the
         value of the "kid" parameter, which is always present both in
         group requests and in responses.











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

    <------ s bytes ------> <--------- q bytes --------->
    -----------------------+-----------------------------+-----------+
       kid context = Gid   |      CounterSignature0      |    kid    |
    -----------------------+-----------------------------+-----------+

                      Figure 2: Object-Security Value

3.1.  Example: Request

   Request with kid = 0x25, Partial IV = 5 and kid context = 0x44616c,
   assuming the label for the new kid context defined in
   [I-D.ietf-core-object-security] has value 10.  COUNTERSIGN is the
   CounterSignature0 byte string as described in Section 3 and is 64
   bytes long in this example.  The ciphertext in this example is 14
   bytes long.

   Before compression (96 bytes):

   [
   h'',
   { 4:h'25', 6:h'05', 10:h'44616c', 9:COUNTERSIGN },
   h'aea0155667924dff8a24e4cb35b9'
   ]

   After compression (85 bytes):

   Flag byte: 0b00111001 = 0x39

   Option Value: 39 05 03 44 61 6c COUNTERSIGN 25 (7 bytes + size of
    COUNTERSIGN)

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

3.2.  Example: Response

   Response with kid = 0x52.  COUNTERSIGN is the CounterSignature0 byte
   string as described in Section 3 and is 64 bytes long in this
   example.  The ciphertext in this example is 14 bytes long.

   Before compression (88 bytes):






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

   After compression (80 bytes):

   Flag byte: 0b00101000 = 0x28

   Option Value: 28 COUNTERSIGN 52 (2 bytes + size of COUNTERSIGN)

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

4.  Message Processing

   Each request message and response message is protected and processed
   as specified in [I-D.ietf-core-object-security], 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, message integrity, and
   message ordering.

   Furthermore, endpoints in the group locally perform error handling
   and processing of invalid messages according to the same principles
   adopted in [I-D.ietf-core-object-security].  However, a receiver
   endpoint MUST stop processing and silently reject any message which
   is malformed and does not follow the format specified in Section 3,
   without sending back any error message.  This prevents listener
   endpoints from sending multiple error messages to a multicaster
   endpoint, so avoiding the risk of flooding and possibly congesting
   the group.

4.1.  Protecting the Request

   A multicaster endpoint transmits a secure group request as described
   in Section 8.1 of [I-D.ietf-core-object-security], with the following
   modifications.

   1.  The multicaster endpoint stores the association Token - Group
       Identifier.  That is, it SHALL be able to find the correct
       Security Context used to protect the group request and verify the
       response(s) by using the CoAP Token used in the message exchange.

   2.  The multicaster computes the COSE object as defined in Section 3
       of this specification.




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4.2.  Verifying the Request

   Upon receiving a secure group request, a listener endpoint proceeds
   as described in Section 8.2 of [I-D.ietf-core-object-security], with
   the following modifications.

   1.  The listener endpoint retrieves the Group Identifier from the
       'kid context' parameter of the received COSE object.  Then, it
       uses the Group Identifier together with the destination IP
       address of the group request to identify the correct group's
       Security Context.

   2.  The listener endpoint retrieves the Sender ID from the "kid"
       parameter of the received COSE object.  Then, the Sender ID is
       used to retrieve the correct Recipient Context associated to the
       multicaster endpoint and used to process the group request.  When
       receiving a secure group request message from that multicaster
       endpoint for the first time, the listener endpoint creates a new
       Recipient Context, initializes it according to Section 3 of
       [I-D.ietf-core-object-security], and includes the multicaster
       endpoint's public key.

   3.  The listener endpoint retrieves the corresponding public key of
       the multicaster endpoint from the associated Recipient Context.
       Then, it verifies the counter signature and decrypts the group
       request.

4.3.  Protecting the Response

   A listener endpoint that has received a secure group request may
   reply with a secure response, which is protected as described in
   Section 8.3 of [I-D.ietf-core-object-security], with the following
   modifications.

   1.  The listener endpoint computes the COSE object as defined in
       Section 3 of this specification.

4.4.  Verifying the Response

   Upon receiving a secure response message, a multicaster endpoint
   proceeds as described in Section 8.4 of
   [I-D.ietf-core-object-security], with the following modifications.

   1.  The multicaster endpoint retrieves the Security Context by using
       the Token of the received response message.

   2.  The multicaster endpoint retrieves the Sender ID from the "kid"
       parameter of the received COSE object.  Then, the Sender ID is



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       used to retrieve the correct Recipient Context associated to the
       listener endpoint and used to process the response message.  When
       receiving a secure response message from that listener endpoint
       for the first time, the multicaster endpoint creates a new
       Recipient Context, initializes it according to Section 3 of
       [I-D.ietf-core-object-security], and includes the listener
       endpoint's public key.

   3.  The multicaster endpoint retrieves the corresponding public key
       of the listener endpoint from the associated Recipient Context.
       Then, it verifies the counter signature and decrypts the response
       message.

   The mapping between response messages from listener endpoints and the
   associated group request from a multicaster endpoint relies on the
   pair (Sender ID, Partial IV) associated to the secure group request.
   This is used by listener endpoints as part of the Additional
   Authenticated Data when protecting their own response message, as
   described in Section 3.

5.  Synchronization of Sequence Numbers

   Upon joining the group, new listeners are not aware of the sequence
   number values currently used by different multicasters to transmit
   group requests.  This means that, when such listeners receive a
   secure group request from a given multicaster for the first time,
   they are not able to verify if that request is fresh and has not been
   replayed.  The same holds when a listener endpoint loses
   synchronization with sequence numbers of multicasters, for instance
   after a device reboot.

   The exact way to address this issue depends on the specific use case
   and its synchronization requirements.  The list of methods to handle
   synchronization of sequence numbers is part of the group
   communication policy, and different listener endpoints can use
   different methods.  Appendix E describes three possible approaches
   that can be considered.

6.  Responsibilities of the Group Manager

   The Group Manager is responsible for performing the following tasks:

   o  Creating and managing OSCORE groups.  This includes the assignment
      of a Group ID to every newly created group, as well as ensuring
      uniqueness of Group IDs within the set of its OSCORE groups.

   o  Defining policies for authorizing the joining of its OSCORE
      groups.  Such policies can be enforced by a third party, which is



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      in a trust relation with the Group Manager and enforces join
      policies on behalf of the Group Manager.

   o  Driving the join process to add new endpoints as group members.

   o  Establishing Security Common Contexts and providing them to
      authorized group members during the join process, together with a
      corresponding Security Sender Context.

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

   o  Defining a set of supported signature algorithms as part of the
      communication policy of each of its OSCORE groups, and signalling
      it to new endpoints during the join process.

   o  Defining the methods to handle loss of synchronization with
      sequence numbers as part of the communication policy of each of
      its OSCORE groups, and signaling the one(s) to use to new
      endpoints during the join process.

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

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

   o  Updating the Group ID of its OSCORE groups, upon renewing the
      respective Security Context.

   The Group Manager may additionally be responsible for the following
   tasks:

   o  Acting as trusted key repository, in order to store the public
      keys of the members of its OSCORE groups, and provide such public
      keys to other members of the same group upon request.  This
      specification recommends that the Group Manager is entrusted to
      perform this task.

   o  Acting as network router device where endpoints register to
      correctly receive group messages sent to the multicast IP address
      of that group.




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   o  Autonomously and locally enforcing access policies to authorize
      new endpoints to join its OSCORE groups.

7.  Security Considerations

   The same security considerations from OSCORE (Section 11 of
   [I-D.ietf-core-object-security]) apply to this specification.
   Additional security aspects to be taken into account are discussed
   below.

7.1.  Group-level Security

   The approach described in this document relies on commonly shared
   group keying material to protect communication within a group.  This
   means that 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.

   In addition, it is required that all group members are trusted, i.e.
   they do not forward the content of group messages 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).

8.  IANA Considerations

   This document has no actions for IANA.

9.  Acknowledgments

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

   The work on this document has been partly supported by the EIT-
   Digital High Impact Initiative ACTIVE.

10.  References

10.1.  Normative References

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-08 (work in
              progress), January 2018.





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   [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>.

   [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>.

   [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>.

10.2.  Informative References

   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profiles for Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-dtls-
              authorize-02 (work in progress), October 2017.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-oauth-
              authz-10 (work in progress), February 2018.

   [I-D.ietf-ace-oscore-profile]
              Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE
              profile of the Authentication and Authorization for
              Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-00 (work in progress), December 2017.

   [I-D.ietf-core-echo-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "Echo and
              Request-Tag", draft-ietf-core-echo-request-tag-00 (work in
              progress), October 2017.



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   [I-D.palombini-ace-key-groupcomm]
              Palombini, F. and M. Tiloca, "Key Provisioning for Group
              Communication using ACE", draft-palombini-ace-key-
              groupcomm-00 (work in progress), March 2018.

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

   [I-D.tiloca-ace-oscoap-joining]
              Tiloca, M. and J. Park, "Joining of OSCORE multicast
              groups in ACE", draft-tiloca-ace-oscoap-joining-02 (work
              in progress), October 2017.

   [RFC2093]  Harney, H. and C. Muckenhirn, "Group Key Management
              Protocol (GKMP) Specification", RFC 2093,
              DOI 10.17487/RFC2093, July 1997,
              <https://www.rfc-editor.org/info/rfc2093>.

   [RFC2094]  Harney, H. and C. Muckenhirn, "Group Key Management
              Protocol (GKMP) Architecture", RFC 2094,
              DOI 10.17487/RFC2094, July 1997,
              <https://www.rfc-editor.org/info/rfc2094>.

   [RFC2627]  Wallner, D., Harder, E., and R. Agee, "Key Management for
              Multicast: Issues and Architectures", RFC 2627,
              DOI 10.17487/RFC2627, June 1999,
              <https://www.rfc-editor.org/info/rfc2627>.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
              <https://www.rfc-editor.org/info/rfc3376>.

   [RFC3740]  Hardjono, T. and B. Weis, "The Multicast Group Security
              Architecture", RFC 3740, DOI 10.17487/RFC3740, March 2004,
              <https://www.rfc-editor.org/info/rfc3740>.

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,
              <https://www.rfc-editor.org/info/rfc3810>.







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   [RFC4046]  Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
              "Multicast Security (MSEC) Group Key Management
              Architecture", RFC 4046, DOI 10.17487/RFC4046, April 2005,
              <https://www.rfc-editor.org/info/rfc4046>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
              "GSAKMP: Group Secure Association Key Management
              Protocol", RFC 4535, DOI 10.17487/RFC4535, June 2006,
              <https://www.rfc-editor.org/info/rfc4535>.

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

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

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

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

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [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>.






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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 multicaster and multiple listeners) and M-to-N
      (multiple multicasters and multiple listeners) 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.

   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
      multicasters (normally the controlling devices) is expected to be
      much smaller than the number of listeners (i.e. the controlled
      devices).  A security solution for group communication that
      supports 1 to 50 multicasters 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, e.g. by grouping lights in a
      building on a per floor basis.

   o  Communication with the Group Manager: an endpoint must use a
      secure dedicated channel when communicating with the Group
      Manager, even when not registered as group member.  In particular,
      communications with the Group Manager occuring during the join
      process to become a group member must also be secured.

   o  Establishment and management of Security Contexts: an OSCORE
      Security Context must be established among the group members.  In
      particular, a Common Context must be provided to a new joining
      endpoint together with a corresponding Sender Context.  On the
      other hand, Recipient Contexts are locally and individually
      derived by each group member.  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 establishment and management of the Security Context is out



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      of the scope of this document, and it is anticipated that an
      activity in IETF dedicated to the design of a generic key
      management scheme will include this feature, preferably based on
      [RFC3740][RFC4046][RFC4535].

   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
      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



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      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 endpoint.  In accordance with
      OSCORE [I-D.ietf-core-object-security], 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 sender endpoints.

Appendix B.  List of Use Cases

   Group Communication for CoAP [RFC7390] 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] to understand the non-security related details.
   This section discusses a number of use cases that 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



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      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 multicaster and
      listener 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
      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 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



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      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 LLNs 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 latters may reply
      back to the emergency notifier, in order to provide their feedback
      and local information related to the ongoing emergency.  This kind
      of setups should additionally rely on a fault tolerance multicast
      algorithm, such as 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.

   The Group Prefix 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.  A
   Group Prefix is random, constant over time, and long enough to
   achieve a negligible probability of collisions between Group
   Identifiers from different Group Managers.  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.





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   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 Security Common
   Context 61532 times in the group, its Group Identifier will assume
   value '0xb1f05c'.

Appendix D.  Set-up of New Endpoints

   An endpoint joins a group by explicitly interacting with the
   responsible Group Manager.  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 scope.

   In order to receive multicast messages sent to the group, a joining
   endpoint has to register with a network router device
   [RFC3376][RFC3810], signaling its intent to receive packets sent to
   the multicast IP address of that group.  As a particular case, the
   Group Manager can also act as such a network router device.  Upon
   joining the group, endpoints are not required to know how many and
   what endpoints are active in the same group.

   Furthermore, in order to participate in the secure group
   communication, an endpoint needs to be properly initialized upon
   joining the group.  In particular, the Group Manager provides keying
   material and parameters to a joining endpoint, which can then
   initialize its own Security Context (see Section 2).

   The following Appendix D.1 provides an example describing how such
   information can be provided to an endpoint upon joining a group
   through the responsible Group Manager.  Then, Appendix D.2 discusses
   how public keys of group members can be handled and made available to
   group members.  Finally, Appendix D.3 overviews how the ACE framework
   for Authentication and Authorization in constrained environments
   [I-D.ietf-ace-oauth-authz] can be possibly used to support such a
   join process.

D.1.  Join Process

   An endpoint requests to join a group by sending a confirmable CoAP
   POST request to the Group Manager responsible for that group.  This
   join request can reflect the format of the Key Distribution Request
   message defined in Section 4.1 of [I-D.palombini-ace-key-groupcomm].
   Besides, it can be addressed to a CoAP resource associated to that
   group and carries the following information.





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   o  Group identifier: the Group Identifier (Gid) of the group, as
      known to the joining endpoint at this point in time.  This may not
      fully coincide with the Gid currently associated to the group,
      e.g. if it includes a dynamic component.  This information can be
      mapped to the first element of the "scope" parameter of the Key
      Distribution Request message defined in Section 4.1 of
      [I-D.palombini-ace-key-groupcomm].

   o  Role: the exact role of the joining endpoint in the group.
      Possible values are: "multicaster", "listener", "pure listener",
      "multicaster and listener", or "multicaster and pure listener".
      This information can be mapped to the second element of the
      "scope" parameter of the Key Distribution Request message defined
      in Section 4.1 of [I-D.palombini-ace-key-groupcomm].

   o  Retrieval flag: indication of interest to receive the public keys
      of the endpoints currently in the group, as included in the
      following join response.  This flag must not be present if the
      Group Manager is not configured to store the public keys of group
      members, or if the joining endpoint is configured exclusively as
      pure listener for the group to join.  This information can be
      mapped to the "get_pub_keys" parameter of the Key Distribution
      Request message defined in Section 4.1 of
      [I-D.palombini-ace-key-groupcomm].

   o  Identity credentials: information elements to enforce source
      authentication of group messages from the joining endpoint, such
      as its public key.  The exact content depends on whether the Group
      Manager is configured to store the public keys of group members.
      If this is the case, this information is omitted if it has been
      provided to the same Group Manager upon previously joining the
      same or a different group under its control.  This information is
      also omitted if the joining endpoint is configured exclusively as
      pure listener for the joined group.  Appendix D.2 discusses
      additional details on provisioning of public keys and other
      information to enforce source authentication of joining
      endpoints's messages.  This information can be mapped to the
      "client_cred" parameter of the Key Distribution Request message
      defined in Section 4.1 of [I-D.palombini-ace-key-groupcomm].

   The Group Manager must be able to verify that the joining endpoint is
   authorized to become a member of 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.  Appendix D.3 describes how this can be achieved by
   leveraging the ACE framework for Authentication and Authorization in
   constrained environments [I-D.ietf-ace-oauth-authz].




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   In case of successful authorization check, the Group Manager
   generates an Endpoint ID assigned to the joining endpoint, before
   proceeding with the rest of the join process.  Instead, in case the
   authorization check fails, the Group Manager aborts the join process.
   Further details about the authorization of joining endpoint are out
   of scope.

   As discussed in Section 2.1, it is recommended that the Security
   Context is renewed before the joining endpoint receives the group
   keying material and becomes a new active member of the group.  This
   is achieved by securely distributing a new Master Secret and a new
   Group Identifier to the endpoints currently present in the same
   group.

   Once renewed the Security Context in the group, the Group Manager
   replies to the joining endpoint with a CoAP response carrying the
   following information.  This join response can reflect the format of
   the Key Distribution Response message defined in Section 4.2 of
   [I-D.palombini-ace-key-groupcomm].

   o  Security Common Context: the OSCORE Security Common Context
      associated to the joined group (see Section 2).  This information
      can be mapped to the "key" parameter of the Key Distribution
      Response message defined in Section 4.2 of
      [I-D.palombini-ace-key-groupcomm].

   o  Endpoint ID: the Endpoint ID associated to the joining endpoint.
      This information is not included in case "Role" in the join
      request is equal to "pure listener".  This information can be
      mapped to the "clientID" parameter within the "key" parameter of
      the Key Distribution Response message defined in Section 4.2 of
      [I-D.palombini-ace-key-groupcomm].

   o  Member public keys: the public keys of the endpoints currently
      present in the group.  This includes: the public keys of the non-
      pure listeners currently in the group, if the joining endpoint is
      configured (also) as multicaster; and the public keys of the
      multicasters currently in the group, if the joining endpoint is
      configured (also) as listener or pure listener.  This information
      is omitted in case the Group Manager is not configured to store
      the public keys of group members or if the "Retrieval flag" was
      not present in the join request.  Appendix D.2 discusses
      additional details on provisioning public keys upon joining the
      group and on retrieving public keys of group members.  This
      information can be mapped to the "pub_keys" parameter of the Key
      Distribution Response message defined in Section 4.2 of
      [I-D.palombini-ace-key-groupcomm].




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   o  Group policies: a list of key words indicating the particular
      policies enforced in the group.  This includes, for instance, the
      list of supported signature algorithms and the method to achieve
      synchronization of sequence numbers among group members (see
      Appendix E).  This information can be mapped to the
      "group_policies" parameter of the Key Distribution Response
      message defined in Section 4.2 of
      [I-D.palombini-ace-key-groupcomm].

   o  Management keying material: the set of administrative keying
      material used to participate in the group rekeying process run by
      the Group Manager (see Section 2.1).  The specific elements of
      this management keying material depend on the group rekeying
      protocol used in the group.  For instance, this can simply consist
      in a group key encryption key and a pairwise symmetric key shared
      between the joining endpoint and the Group Manager, in case GKMP
      [RFC2093][RFC2094] is used.  Instead, if key-tree based rekeying
      protocols like LKH [RFC2627] are used, it can consist in the set
      of symmetric keys associated to the key-tree leaf representing the
      group member up to the key-tree root representing the group key
      encryption key.  This information can be mapped to the
      "mgt_key_material" parameter of the Key Distribution Response
      message defined in Section 4.2 of
      [I-D.palombini-ace-key-groupcomm].

D.2.  Provisioning and Retrieval of Public Keys

   As mentioned in Section 6, it is recommended that the Group Manager
   acts as trusted key repository, so storing public keys of group
   members and providing them to other members of the same group upon
   request.  In such a case, a joining endpoint provides its own public
   key to the Group Manager, as "Identity credentials" of the join
   request, when joining the group (see Appendix D.1).

   After that, the Group Manager should verify that the joining endpoint
   actually owns the associated private key, for instance by performing
   a proof-of-possession challenge-response, whose details are out of
   scope.  In case of failure, the Group Manager performs up to a pre-
   defined maximum number of retries, after which it aborts the join
   process.

   In case of successful challenge-response, the Group Manager stores
   the received public key as associated to the joining endpoint and its
   Endpoint ID.  From then on, that public key will be available for
   secure and trusted delivery to other endpoints in the group.
   Finally, the Group Manager sends the join response to the joining
   endpoint, as described in Appendix D.1.




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   The joining endpoint does not have to provide its own public key if
   that already occurred upon previously joining the same or a different
   group under the same Group Manager.  However, separately for each
   group under its control, the Group Manager maintains an updated list
   of active Endpoint IDs associated to the respective endpoint's public
   key.

   Instead, in case the Group Manager does not act as trusted key
   repository, the following exchange with the Group Manager can occur
   during the join process.

   1.  The joining endpoint signs its own certificate by using its own
       private key.  The certificate includes also the identifier of the
       issuer Certification Authority (CA).  There is no restriction on
       the Certificate Subject included in the joining endpoint's
       certificate.

   2.  The joining endpoint specifies the signed certificate as
       "Identity credentials" in the join request (Appendix D.1).  The
       joining endpoint can optionally specify also a list of public key
       repositories storing its own certificate.  In such a case, this
       information can be mapped to the "pub_keys_repos" parameter of
       the Key Distribution Request message defined in Section 4.1 of
       [I-D.palombini-ace-key-groupcomm].

   3.  When processing the join request, the Group Manager first
       validates the certificate by verifying the signature of the
       issuer CA, and then verifies the signature of the joining
       endpoint.

   4.  The Group Manager stores the association between the Certificate
       Subject of the joining endpoint's certificate and the pair {Group
       ID, Endpoint ID of the joining endpoint}. If received from the
       joining endpoint, the Group Manager also stores the list of
       public key repositories storing the certificate of the joining
       endpoint.

   When a group member X wants to retrieve the public key of another
   group member Y in the same group, the endpoint X proceeds as follows.

   1.  The endpoint X contacts the Group Manager, specifying the pair
       {Group ID, Endpoint ID of the endpoint Y}.

   2.  The Group Manager provides the endpoint X with the Certificate
       Subject CS from the certificate of endpoint Y.  If available, the
       Group Manager provides the endpoint X also with the list of
       public key repositories storing the certificate of the endpoint
       Y.



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   3.  The endpoint X retrieves the certificate of the endpoint X from a
       key repository storing it, by using the Certificate Subject CS.

D.3.  Group Joining Based on the ACE Framework

   The join process to register an endpoint as a new member of a group
   can be based on the ACE framework for Authentication and
   Authorization in constrained environments [I-D.ietf-ace-oauth-authz],
   built on re-use of OAuth 2.0 [RFC6749].

   In particular, the approach described in
   [I-D.tiloca-ace-oscoap-joining] uses the ACE framework to delegate
   the authentication and authorization of joining endpoints to an
   Authorization Server in a trust relation with the Group Manager.  At
   the same time, it allows a joining endpoint to establish a secure
   channel with the Group Manager, by leveraging protocol-specific
   profiles of ACE, such as [I-D.ietf-ace-oscore-profile] and
   [I-D.ietf-ace-dtls-authorize], to achieve communication security,
   proof-of-possession and server authentication.

   More specifically and with reference to the terminology defined in
   OAuth 2.0:

   o  The joining endpoint acts as Client;

   o  The Group Manager acts as Resource Server, with different CoAP
      resources for different groups it is responsible for;

   o  An Authorization Server enables and enforces authorized access of
      the joining endpoint to the Group Manager and its CoAP resources
      paired with groups to join.

   Messages exchanged among the participants follow the formats defined
   in [I-D.palombini-ace-key-groupcomm].  Both the joining endpoint and
   the Group Manager have to adopt secure communication also for any
   message exchange with the Authorization Server.  To this end,
   different alternatives are possible, such as OSCORE, DTLS [RFC6347]
   or IPsec [RFC4301].

Appendix E.  Examples of Synchronization Approaches

   This section describes three possible approaches that can be
   considered by listener endpoints to synchronize with sequence numbers
   of multicasters.







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E.1.  Best-Effort Synchronization

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

E.2.  Baseline Synchronization

   Upon receiving a multicast request from a given multicaster for the
   first time, a listener endpoint initializes its last-seen sequence
   number in its Recipient Context associated to that multicaster.
   However, the listener drops the multicast request without delivering
   it to the application layer.  This provides a reference point to
   identify if future group requests from the same multicaster are
   fresher than the last one received.

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

E.3.  Challenge-Response Synchronization

   A listener endpoint performs a challenge-response exchange with a
   multicaster, by using the Repeat Option for CoAP described in
   Section 2 of [I-D.ietf-core-echo-request-tag].

   That is, upon receiving a group request from a particular multicaster
   for the first time, the listener processes the message as described
   in Section 4.2 of this specification, but, even if valid, does not
   deliver it to the application.  Instead, the listener replies to the
   multicaster with a 4.03 Forbidden response message including a Repeat
   Option, and stores the option value included therein.

   Upon receiving a 4.03 Forbidden response that includes a Repeat
   Option and originates from a verified group member, a multicaster
   sends a request as a unicast message addressed to the same listener,
   echoing the Repeat Option value.  In particular, the multicaster 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 multicaster to not retain previously sent group
   requests for full retransmission, unless the application explicitly
   requires otherwise.  In either case, the multicaster uses the
   sequence number value currently stored in its own Sender Context.  If
   the multicaster stores group requests for possible retransmission



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   with the Repeat Option, it should not store a given request for
   longer than a pre-configured time interval.  Note that the unicast
   request echoing the Repeat Option is correctly treated and processed
   as a group message, since the 'kid context' field including the Group
   Identifier of the OSCORE group is still present in the Object-
   Security Option as part of the COSE object (see Section 3).

   Upon receiving the unicast request including the Repeat Option, the
   listener verifies that the option value equals the stored and
   previously sent value; otherwise, the request is silently discarded.
   Then, the listener 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 listener updates the Recipient Context associated to
   that multicaster, by setting the Replay Window according to the
   Sequence Number from the unicast request conveying the Repeat Option.
   The listener 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
   Repeat Option within the configured time interval, the listener
   endpoint should perform the same challenge-response upon receiving
   the next multicast request from that same multicaster.

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

   Note that endpoints configured as pure listeners are not able to
   perform the challenge-response described above, as they do not store
   a Sender Context to secure the 4.03 Forbidden response to the
   multicaster.  Therefore, pure listeners should adopt alternative




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   approaches to achieve and maintain synchronization with sequence
   numbers of multicasters.

   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.

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
   group 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 group 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 group 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.

Authors' Addresses







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   Marco Tiloca
   RISE SICS 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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