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CoRE Working Group                                             M. Tiloca
Internet-Draft                                              RISE SICS AB
Intended status: Standards Track                             G. Selander
Expires: April 30, 2018                                     F. Palombini
                                                             Ericsson AB
                                                                 J. Park
                                             Universitaet Duisburg-Essen
                                                        October 27, 2017


                  Secure group communication for CoAP
                 draft-tiloca-core-multicast-oscoap-04

Abstract

   This document describes a method for protecting group communication
   over the Constrained Application Protocol (CoAP).  The proposed
   approach relies on Object Security for Constrained RESTful
   Environments (OSCORE) and the CBOR Object Signing and Encryption
   (COSE) format.  All security requirements fulfilled by OSCORE are
   maintained for multicast OSCORE request messages and related OSCORE
   response messages.  Source authentication of all messages exchanged
   within the group is ensured, by means of digital signatures produced
   through private keys of sender devices 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 http://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 April 30, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (http://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.  Assumptions and Security Objectives . . . . . . . . . . . . .   5
     2.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Security Objectives . . . . . . . . . . . . . . . . . . .   7
   3.  OSCORE Security Context . . . . . . . . . . . . . . . . . . .   7
     3.1.  Management of Group Keying Material . . . . . . . . . . .   9
   4.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Message Processing  . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  12
     5.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  13
     5.3.  Protecting the Response . . . . . . . . . . . . . . . . .  13
     5.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  13
   6.  Synchronization of Sequence Numbers . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     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.  List of Use Cases  . . . . . . . . . . . . . . . . .  18
   Appendix B.  Example of Group Identifier Format . . . . . . . . .  20
   Appendix C.  Set-up of New Endpoints  . . . . . . . . . . . . . .  21
     C.1.  Join Process  . . . . . . . . . . . . . . . . . . . . . .  21
     C.2.  Provisioning and Retrieval of Public Keys . . . . . . . .  23
     C.3.  Group Joining Based on the ACE Framework  . . . . . . . .  24
   Appendix D.  Examples of Synchronization Approaches . . . . . . .  25
     D.1.  Best-Effort Synchronization . . . . . . . . . . . . . . .  25
     D.2.  Baseline Synchronization  . . . . . . . . . . . . . . . .  25
     D.3.  Challenge-Response Synchronization  . . . . . . . . . . .  26
   Appendix E.  No Verification of Signatures  . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28






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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 A).
   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 across intermediary nodes.  To this
   end, a CoAP message is protected by including 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 multicast OSCORE, providing end-to-end
   security of CoAP messages exchanged between members of a multicast
   group.  In particular, the described approach defines how OSCORE
   should be used in a group communication context, while fulfilling the
   same security requirements.  That is, end-to-end security is assured
   for multicast CoAP requests sent by multicaster nodes to the group
   and for related CoAP responses sent as reply by multiple listener
   nodes.  Multicast 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 node and a proxy (and vice versa), and between
   a proxy and a listener node (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]; 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 member of a multicast group.  This
      includes, for instance, keys and IVs [RFC4949].

   o  Group Manager (GM): entity responsible for creating a multicast
      group, establishing and provisioning Security Contexts among
      authorized group members, as well as managing the joining of new
      group members and the leaving of current group members.  A GM can
      be responsible for multiple multicast groups.  Besides, a GM is
      not required to be an actual group member and to take part in the
      group communication.  The GM is also responsible for renewing/
      updating Security Contexts and related keying material in the
      multicast groups of its competence.  Each endpoint in a multicast
      group securely communicates with the respective GM.

   o  Multicaster: member of a multicast group that sends multicast CoAP
      messages intended for all members of the group.  In a 1-to-N
      multicast group, only a single multicaster transmits data to the
      group; in an M-to-N multicast group (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 multicast group that receives multicast CoAP
      messages when listening to the multicast IP address associated to
      the multicast group.  A listener may reply back, by sending a




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      response message to the multicaster which has sent the multicast
      message.

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

   o  Endpoint ID: identifier assigned by the Group Manager to an
      endpoint upon joining the group as a new member, unless configured
      exclusively as pure listener.  The Group Manager generates and
      manages Endpoint IDs in order to ensure their uniqueness within a
      same multicast group.  That is, within a single multicast group,
      the same Endpoint ID cannot be associated to more endpoints at the
      same time.  Endpoint IDs are not necessarily related to any
      protocol-relevant identifiers, such as IP addresses.

   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 either by a different group member or by a non-group member.

2.  Assumptions and Security Objectives

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

2.1.  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).  For instance, in a
      typical lighting control use case, 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.




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   o  Multicast group size: security solutions for group communication
      should be able to adequately support different, possibly large,
      group sizes.  Group size is the combination of the number of
      multicasters and listeners in a multicast group, with possible
      overlap (i.e. a multicaster may also be a listener at the same
      time).  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 total number of group members is expected to be in
      the range of 2 to 100 devices.  Groups larger than that should be
      divided into smaller independent multicast groups, e.g. by
      grouping lights in a building on a per floor basis.

   o  Establishment and management of Security Contexts: a Security
      Context must be established among the group members by the Group
      Manager which manages the multicast group.  A secure mechanism
      must be used to generate, revoke and (re-)distribute keying
      material, multicast security policies and security parameters in
      the multicast group.  The actual establishment and management of
      the Security Context is out 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 multicast group.  The
      ciphersuite is specified as part of the Security Context.

   o  Backward security: a new device joining the multicast 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 multicast 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



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      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.

2.2.  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
      multicast group SHALL be encrypted if privacy sensitive data is
      exchanged within the group.  In fact, some control commands and/or
      associated responses could pose unforeseen security and privacy
      risks to the system users, when sent as plaintext.  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 multicast group, but not by an
      external adversary or other external entities.

   o  Source authentication: messages sent within the multicast 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
      (group authentication), and in particular by a specific member of
      the group (source authentication).

   o  Message integrity: messages sent within the multicast 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.

3.  OSCORE Security Context

   To support multicast communication secured with OSCORE, each endpoint
   registered as member of a multicast 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:




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   1.  one Common Context, received from the Group Manager upon joining
       the multicast group and 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 and used as Master Salt
          parameter in the derivation of keying material.  The Gid is
          used together with the multicast IP address of the group to
          retrieve the Security Context, upon receiving a secure
          multicast request message (see Section 5.2).  The Gid
          associated to a multicast group is determined by the
          responsible Group Manager.  The choice of the Gid for a given
          group's Security Context is application specific.  However, a
          Gid MUST be random as well as long enough, in order to achieve
          a negligible probability of collisions between Group
          Identifiers from different Group Managers.  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 B.

       *  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 Security Context
          is established.  All the endpoints in the group agree on the
          same counter signature algorithm.  The Group Manager MUST
          define a list of supported signature algorithms as part of the
          group communication policy.  Such a list 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
       messages and is initialized according to Section 3 of
       [I-D.ietf-core-object-security], once the endpoint has joined the
       multicast group.  In practice, the sender endpoint shares the
       same symmetric keying material stored in the Sender Context with
       all the recipient endpoints receiving its outgoing OSCORE
       messages.  The Sender ID in the Sender Context coincides with the
       Endpoint ID received upon joining the group.  It is
       responsibility of the Group Manager to assign Endpoint IDs to new
       joining endpoints in such a way that uniquess is ensured within
       the multicast group.  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.




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   3.  one Recipient Context for each distinct endpoint from which
       messages are received, used to process such incoming secure
       messages.  The endpoint creates a new Recipient Context upon
       receiving an incoming message from another endpoint in the group
       for the first time.  In practice, the recipient endpoint shares
       the symmetric keying material stored in the Recipient Context
       with the associated other endpoint from which secure 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
       secure messages are received.

   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.

   It is RECOMMENDED that the Group Manager acts as trusted key
   repository, and hence is configured to store public keys of group
   members and provide them to other members of the same group upon
   request.  Possible approaches to provision public keys upon joining
   the group and to retrieve public keys of group members are discussed
   in Appendix C.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].

3.1.  Management of Group Keying Material

   The approach described in this specification should take into account
   the risk of compromise of group members.  Such a risk is reduced when
   multicast groups are deployed in physically secured locations, like
   lighting inside office buildings.  Nevertheless, 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 Section 2, 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 multicast group.  The Group Manager responsible for the group
   is entrusted with such a task.

   In particular, the Group Manager MUST distribute also a new Group
   Identifier (Gid) for that group, together with a new value for the
   Master Secret parameter.  An example of how this can be done is
   provided in Appendix B.  Then, each group member re-derives the
   keying material stored in its own Sender Context and Recipient
   Contexts as described in Section 3, using the updated Group
   Identifier.

   Especially in dynamic, large-scale, multicast 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.

4.  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
      responses SHALL be set to the Sender ID of the endpoint
      transmitting the group message.

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

      *  gid : its value is set to the Group Identifier (Gid) of the
         group's Security Context.  This parameter MAY be omitted if the
         message is a CoAP response.

      *  countersign : its value is set to the counter signature of the
         COSE object (Appendix C.3.3 of [RFC8152]), computed by the
         endpoint by means of its own private key as described in
         Section 4.5 of [RFC8152].

   In particular, "gid" is included as COSE header parameter as defined
   in Figure 1.








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    +------+-------+------------+----------------+-------------------+
    | name | label | value type | value registry | description       |
    +------+-------+------------+----------------+-------------------+
    | gid  | TBD   | bstr       |                | Identifies the    |
    |      |       |            |                | OSCORE group      |
    |      |       |            |                | Security Context  |
    +------+-------+------------+----------------+-------------------+

     Figure 1: Additional common header parameter for the COSE object

   o  The Additional Authenticated Data (AAD) considered to compute the
      COSE object is extended, in order to include also the Group
      Identifier (Gid) of the Security Context used to protect the
      request message.  In particular, the "external_aad" in Section 5.3
      of [I-D.ietf-core-object-security] SHALL include also gid as
      follows:

   external_aad = [
      version : uint,
      alg : int,
      request_kid : bstr,
      request_piv : bstr,
      gid : bstr,
      options : bstr
   ]

   o  The OSCORE compression defined in Section 8 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 the
         object-security option value SHALL be set to 1, to indicate the
         presence of the "kid" parameter for both multicast requests and
         responses.

      *  The fifth least significant bit of the first byte MUST be set
         to 1 for multicast requests, to indicate the presence of the
         Context Hint in the OSCORE payload.  The Context Hint flag MAY
         be set to 1 for responses.

      *  The sixth least significant bit of the first byte is set to 1
         if the "countersign" 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 Context Hint value encodes the Group Identifier value (Gid)
         of the group's Security Context.




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      *  The following q bytes (q given by the counter signature
         algorithm specified in the Security Context) encode the value
         of the "countersign" 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
         multicast requests and in responses.

     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 --------->
    -----------------------+-----------------------------+-----------+
          Gid (if any)     |         countersign         |    kid    |
    -----------------------+-----------------------------+-----------+

                      Figure 2: Object-Security Value

5.  Message Processing

   Each multicast 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.

   Furthermore, endpoints in the multicast 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 4, 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 the multicast
   group.

5.1.  Protecting the Request

   A multicaster endpoint transmits a secure multicast request message
   as described in Section 7.1 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 multicast request and verify
       the response(s) by using the CoAP Token used in the message
       exchange.



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   2.  The multicaster computes the COSE object as defined in Section 4
       of this specification.

5.2.  Verifying the Request

   Upon receiving a secure multicast request message, a listener
   endpoint proceeds as described in Section 7.2 of
   [I-D.ietf-core-object-security], with the following modifications.

   1.  The listener endpoint retrieves the Group Identifier from the
       "gid" parameter of the received COSE object.  Then, it uses the
       Group Identifier together with the destination IP address of the
       multicast request message 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 request message.
       When receiving a secure multicast CoAP 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 request
       message.

5.3.  Protecting the Response

   A listener endpoint that has received a multicast request message may
   reply with a secure response message, which is protected as described
   in Section 7.3 of [I-D.ietf-core-object-security], with the following
   modifications.

   1.  The listener endpoint computes the COSE object as defined in
       Section 4 of this specification.

5.4.  Verifying the Response

   Upon receiving a secure response message, a multicaster endpoint
   proceeds as described in Section 7.4 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.



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   2.  The multicaster 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
       listener endpoint and used to process the response message.  When
       receiving a secure CoAP 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 multicast request message from a multicaster endpoint
   relies on the 3-tuple (Group ID, Sender ID, Partial IV) associated to
   the secure multicast request message.  This is used by listener
   endpoints as part of the Additional Authenticated Data when
   protecting their own response message, as described in Section 4.

6.  Synchronization of Sequence Numbers

   Upon joining the multicast group, new listeners are not aware of the
   sequence number values currently used by different multicasters to
   transmit multicast request messages.  This means that, when such
   listeners receive a secure multicast 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 applies 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 Group Manager should
   define also how to handle synchronization of sequence numbers, as
   part of the policies enforced in the multicast group.  In particular,
   the Group Manager can suggest to single specific listener endpoints
   how they can exceptionally behave in order to synchronize with
   sequence numbers of multicasters.  Appendix D describes three
   possible approaches that can be considered.

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.



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7.1.  Group-level Security

   The approach described in this document relies on commonly shared
   group keying material to protect communication within a multicast
   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 multicast 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 multicast
   group belong to a common authority and are configured by a
   commissioner.  For instance, in a professional lighting scenario, the
   roles of multicaster and listener are configured by the lighting
   commissioner, and devices strictly follow those roles.

8.  IANA Considerations

   TBD.  Header parameter 'gid'.

9.  Acknowledgments

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

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-06 (work in
              progress), October 2017.

   [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>.





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   [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.amsuess-core-repeat-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "Repeat And
              Request-Tag", draft-amsuess-core-repeat-request-tag-00
              (work in progress), July 2017.

   [I-D.aragon-ace-ipsec-profile]
              Aragon, S., Tiloca, M., and S. Raza, "IPsec profile of
              ACE", draft-aragon-ace-ipsec-profile-00 (work in
              progress), July 2017.

   [I-D.ietf-ace-dtls-authorize]
              Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-dtls-
              authorize-01 (work in progress), July 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-08 (work in progress), October 2017.

   [I-D.seitz-ace-oscoap-profile]
              Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE
              profile of the Authentication and Authorization for
              Constrained Environments Framework", draft-seitz-ace-
              oscoap-profile-06 (work in progress), October 2017.








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   [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 OSCOAP multicast
              groups in ACE", draft-tiloca-ace-oscoap-joining-00 (work
              in progress), July 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>.

   [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>.



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   [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>.

Appendix A.  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 Section 2.




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   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 multicast
      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 multicast 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 ligthing
      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.  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.

   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 multicast 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 multicast group.
      Furthermore, 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



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      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 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 multicast 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.

Appendix B.  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 multicast
   groups associated to the same Group Manager.  The choice of the Group
   Prefix for a given group's Security Context is application specific.
   Group Prefixes are random as well as long enough, in order to achieve
   a negligible probability of collisions between Group Identifiers from
   different Group Managers.

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



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   and keying material in the group (see Section 3.1).  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 (see Section 3).

Appendix C.  Set-up of New Endpoints

   An endpoint joins a multicast group by explicitly interacting with
   the responsible Group Manager.  All 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
   specification.

   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 maintain a number of information
   elements stored in its own Security Context (see Section 3).  The
   following Appendix C.1 describes which of this information is
   provided to an endpoint upon joining a multicast group through the
   responsible Group Manager.

C.1.  Join Process

   An endpoint requests to join a multicast group by sending a
   confirmable CoAP POST request to the Group Manager responsible for
   that group.  The join request is addressed to a CoAP resource
   associated to that group and carries the following information.

   o  Role: the exact role of the joining endpoint in the multicast
      group.  Possible values are: "multicaster", "listener", "pure
      listener", "multicaster and listener", or "multicaster and pure
      listener".

   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 multicast group under its control.  This



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      information is also omitted if the joining endpoint is configured
      exclusively as pure listener for the joined group.  Appendix C.2
      discusses additional details on provisioning of public keys and
      other information to enforce source authentication of joining
      node's messages.

   o  Retrieval flag: indication of interest to receive the public keys
      of the endpoints currently in the multicast group, as included in
      the following join response.  This flag MUST be set to false 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 joined group.

   The Group Manager MUST be able to verify that the joining enpoint is
   authorized to become a member of the multicast 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 C.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].

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

   As discussed in Section 3.1, it is then RECOMMENDED that the Security
   Context is renewed before the joining endpoint becomes a new active
   member of the multicast 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 multicast group, the Group
   Manager replies to the joining endpoint with a CoAP response carrying
   the following information.

   o  Security Common Context: the OSCORE Security Common Context
      associated to the joined multicast group (see Section 3).

   o  Endpoint ID: the Endpoint ID associated to the joining node.  This
      information is not included in case "Role" in the join request is
      equal to "pure listener".

   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 3.1).  The specific elements of



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      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 node 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.

   o  Member public keys: the public keys of the endpoints currently
      present in the multicast 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 set to false in the join request.  Appendix C.2
      discusses additional details on provisioning public keys upon
      joining the group and on retrieving public keys of group members.

C.2.  Provisioning and Retrieval of Public Keys

   As mentioned in Section 3, it is RECOMMENDED that the Group Manager
   acts as trusted key repository, stores public keys of group members
   and provide 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 multicast group (see Appendix C.1).

   After that, the Group Manager MUST verify that the joining endpoint
   actually owns the associated private key, for instance by performing
   a proof-of-possession challenge-response.  In case of success, the
   Group Manager stores the received public key as associated to the
   joining endpoint and its Endpoint ID, before sending the join
   response and continuing with the rest of the join process.  From then
   on, that public key will be available for secure and trusted delivery
   to other endpoints in the multicast group.

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





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   Instead, in case the Group Manager does not act as trusted key
   repository, the following information is exchanged with the Group
   Manager during the join process.

   1.  The joining endpoint signs its own certificate by using its own
       private key.  There is no restriction on the Certificate Subject
       included in the joining node's certificate.

   2.  The joining endpoint includes the following information as
       "Identity credentials" in the join request (Appendix C.1): the
       signed certificate; and the identifier of the Certification
       Authority that issued the certificate.  The joining endpoint can
       optionally specify also a list of public key repositories storing
       its own certificate.

   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 node.

   4.  The Group Manager stores the association between the Certificate
       Subject of the joining node's certificate and the pair {Group ID,
       Endpoint ID of the joining node}. 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 multicast 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.

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

C.3.  Group Joining Based on the ACE Framework

   The join process to register an endpoint as a new member of a
   multicast 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].




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   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 [I-D.seitz-ace-oscoap-profile][I-D.ietf-ace-dtls-auth
   orize][I-D.aragon-ace-ipsec-profile] 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 multicast 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 multicast groups to join.

   Both the joining endpoint and the Group Manager MUST 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 D.  Examples of Synchronization Approaches

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

D.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.

D.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




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   identify if future multicast 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.

D.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.amsuess-core-repeat-request-tag].

   That is, upon receiving a multicast request from a particular
   multicaster for the first time, the listener processes the message as
   described in Section 5.2 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
   MUST send a group 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
   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 "gid" 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 4).

   Upon receiving the unicast group 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 group request has been
   received within a pre-configured time interval, as described in
   [I-D.amsuess-core-repeat-request-tag].  In such a case, the request



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   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 group 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 discard 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 group request including the
   Repeat Option within the configured time interval, the listener node
   SHOULD perform the same challenge-response upon receiving the next
   multicast request from that same multicaster.

   A listener SHOULD NOT deliver group request messages from a given
   multicaster to the application until one valid group request from
   that same multicaster has been verified as fresh, as conveying an
   echoed Repeat Option [I-D.amsuess-core-repeat-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 MUST always be 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
   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 multicast groups where
   many nodes at the same time might join as new members or lose
   synchronization.

Appendix E.  No Verification of Signatures

   There are some application scenarios using group communications that
   have particularly strict requirements.  One example of this is the
   requirement of low message latency in non-emergency lighting



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   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 multicast 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

   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




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   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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