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CoRE Working Group                                             M. Tiloca
Internet-Draft                                              RISE SICS AB
Intended status: Standards Track                             G. Selander
Expires: September 14, 2017                                 F. Palombini
                                                             Ericsson AB
                                                          March 13, 2017


                  Secure group communication for CoAP
                 draft-tiloca-core-multicast-oscoap-01

Abstract

   This document describes a method for application layer protection of
   messages exchanged with the Constrained Application Protocol (CoAP)
   in a group communication context.  The proposed approach relies on
   Object Security of CoAP (OSCOAP) and the CBOR Object Signing and
   Encryption (COSE) format.  All security requirements fulfilled by
   OSCOAP are maintained for multicast CoAP request messages and related
   unicast CoAP response messages.  Source authentication of all
   messages exchanged within the group is ensured, by means of digital
   signatures produced through asymmetric 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 September 14, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Scope Description . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Security Context  . . . . . . . . . . . . . . . . . . . . . .   8
   5.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Message Processing  . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  10
     6.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  10
     6.3.  Protecting the Response . . . . . . . . . . . . . . . . .  11
     6.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     7.1.  Group-level Security  . . . . . . . . . . . . . . . . . .  12
     7.2.  Management of Group Keying Material . . . . . . . . . . .  12
     7.3.  Late Joining Endpoints  . . . . . . . . . . . . . . . . .  13
     7.4.  Provisioning of Public Keys . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     10.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Group Joining Based on the ACE Framework . . . . . .  16
   Appendix B.  List of Use Cases  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is a web
   transfer protocol specifically designed for constrained devices and
   networks.

   [RFC7390] enables group communication for CoAP, addressing use cases
   where deployed devices benefit from a group communication model for
   example to limit 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.  [RFC7390]




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   recognizes the importance to introduce a secure mode for CoAP group
   communication.  This specification defines such a mode.

   Object Security of CoAP (OSCOAP)[I-D.ietf-core-object-security]
   describes a security protocol based on the exchange of protected CoAP
   messages.  OSCOAP builds on CBOR Object Signing and Encryption (COSE)
   [I-D.ietf-cose-msg] and provides end-to-end encryption, integrity,
   and replay protection across intermediate modes.  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 OSCOAP, providing end-to-end
   security of CoAP messages exchanged between members of a multicast
   group.  In particular, the described approach defines how OSCOAP
   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 unicast CoAP responses sent as reply by multiple
   listener nodes.  Multicast OSCOAP provides source authentication of
   all CoAP messages exchanged within the group, by means of digital
   signatures produced through asymmetric private keys of sender devices
   and embedded in the protected CoAP messages.  As in OSCOAP, 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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  These
   words may also appear in this document in lowercase, absent their
   normative meanings.

   Readers are expected to be familiar with the terms and concepts
   described in [RFC7252], [RFC7390] and [RFC7641].

   Terminology for constrained environments, such as "constrained
   device", "constrained-node network", is defined in [RFC7228].

   Terminology for protection and processing of CoAP messages through
   OSCOAP, such as "Security Context", "Master Secret", "Master Salt",
   is defined in [I-D.ietf-core-object-security].

   This document refers also to the following terminology.





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   o  Keying material: data that is necessary to establish and maintain
      secure communication among member of a multicast group.  This
      includes, for instance, keys, key pairs, and IVs [RFC4949].

   o  Group Manager (GM): entity responsible for creating a multicast
      group, establishing and provisioning security contexts among
      authorized group members, and managing the joining of new group
      members.  A GM can be responsible for multiple multicast groups,
      while it is not required to be an actual group member and to take
      part in the group communication.  The GM may also be responsible
      for renewing/updating security contexts and related keying
      material.  Any message exchange with the GM MUST be secured and
      based on different secure channels for different endpoints.

   o  Multicaster: member of a multicast group that sends multicast CoAP
      messagges 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.

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

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

   o  Group response: unicast CoAP response message sent back by a
      listener in the group as a response to a group request received
      from a multicaster.

   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 any other group member or an adversary outside the group.

2.  Requirements

   The following security requirements are out of the scope of this
   document and are assumed to be already fulfilled.

   o  Establishment and management of a security context: a security
      context must be established among the group members by the Group
      Manager which manages the multicast group.  A secure mechanism



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      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
      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.

   The following security requirements need to be fulfilled by the
   approach described in 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 LLN.  For instance, in the lighting control use case,
      switches are the only entities 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  Data replay protection: it MUST NOT be possible to replay a group
      request message or a group response message, which would disrupt
      the correct communication in the group and the activity of group
      members.

   o  Group-level data confidentiality: messages sent within the
      multicast group SHALL be encrypted.  In fact, some control
      commands and/or associated responses could pose unforeseen
      security and privacy risks to the system users, when sent as
      plaintext.  In particular, data confidentiality MAY be required if
      privacy sensitive data is exchanged in 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 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).  The approach proposed in this
      document provides both group authentication and source
      authentication, both for group requests originated by multicasters
      and group responses originated by listeners.  In order to provide
      source authentication, outgoing messages are signed by the
      respective originator group member by means of its own asymmetric
      private key.  The resulting signature is included in the COSE
      object.

   o  Message integrity: messages sent within the multicast group SHOULD
      be integrity protected.  That is, it is essential to ensure that a
      message has not been tampered with by an external adversary or



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      other external entities which are not group members.  Message
      integrity is provided through the same means used to provide
      source authentication.

3.  Scope Description

   An endpoint joins a multicast group by explicitly interacting with
   the responsible Group Manager.  The actual join process MAY be based
   on the ACE framework [I-D.ietf-ace-oauth-authz] and the OSCOAP
   profile of ACE [I-D.seitz-ace-oscoap-profile], as discussed in
   Appendix A.

   An endpoint registered as member of a group can behave as a
   multicaster and/or as a listener.  As a multicaster, it can transmit
   multicast request messages to the group.  As a listener, it receives
   multicast request messages from any multicaster in the group, and
   possibly replies by transmitting unicast response messages.  A number
   of use cases that benefit from secure group communication are
   discussed in Appendix B.  Upon joining the group, endpoints are not
   required to know how many and what endpoints are active in the same
   group.

   An endpoint which is registered as member of a group is identified by
   an endpoint ID, which is not necessarily related to any protocol-
   relevant identifiers, such as IP addresses.  The Group Manager
   generates and manages endpoint IDs in order to ensure their
   uniqueness within a same multicast group.  That is, there cannot be
   multiple endpoints that belong to the same group and are associated
   to a same endpoint ID.

   In order to participate in the secure group communication, an
   endpoint needs to maintain additional information elements, stored in
   its own security context.  Those include keying material used to
   protect and verify group messages, as well as the public keys of
   other endpoints in the groups, in order to verify digital signatures
   of secure messages and ensure their source authenticity.  These
   pieces of information are provided by the Group Manager through out-
   of-band means or other pre-established secure channels.  Further
   details about establishment, revocation and renewal of the security
   context and keying material are out of the scope of this document.

   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 unicast response messages associated to a given
   multicast request message.





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4.  Security Context

   To support multicast communication secured with OSCOAP, 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:

   1.  one Common Context, received from the Group Manager upon joining
       the multicast group and shared by all the endpoints in the group.
       The Common Context contains the COSE AEAD algorithm, the Master
       Secret and, optionally, the Master Salt used to derive endpoint-
       based keying material (see Section 3.2 of
       [I-D.ietf-core-object-security]).  All the endpoints in the group
       agree on the same COSE AEAD algorithm.  Besides, in addition to
       what is defined in [I-D.ietf-core-object-security], the Common
       Context stores the following parameters:

       *  Context Identifier (Cid).  Variable length byte string that
          identifies the Security Context.  The Cid used in a multicast
          group is determined by the responsible Group Manager and does
          not change over time.  A Cid MUST be unique in the sets of all
          the multicast groups associated to the same Group Manager.
          The choice of the Cid for a given group's Security Context is
          application specific, but it is RECOMMENDED to use 64-bit long
          pseudo-random Cids, in order to have globally unique Context
          Identifiers.  It is the role of the application to specify how
          to handle possible collisions.

       *  Counter signature algorithm.  Value that identifies the
          algorithm used for source authenticating messages sent within
          the group.  Its value is immutable once the security context
          is established.  All the endpoints in the group agree on the
          same counter signature algorithm.

   2.  one Sender Context, used to secure outgoing messages.  In
       particular, the Sender Context is initialized according to
       Section 3 of [I-D.ietf-core-object-security], once the endpoint
       has joined 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 asymmetric public-private key pair;

   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.  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



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       secure messages are received.  Possible approaches to provision
       and retrieve public keys of group members are discussed in
       Section 7.4.

   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].

   The 3-tuple (Cid, Sender ID, Partial IV) is called Transaction
   Identifier (Tid), and SHALL be unique for each Master Secret.  The
   Tid is used as a unique challenge in the COSE object of the protected
   CoAP request.  The Tid is part of the Additional Authenticated Data
   (AAD, see Section of 5.2 of [I-D.ietf-core-object-security]) of the
   protected CoAP response message, which is how unicast responses are
   bound to multicast requests.

5.  The COSE Object

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

   1.  The value of the "Partial IV" parameter in the "protected" field
       is set to the Sequence Number and SHALL be present in both
       multicast requests and unicast responses.  Specifically, a
       multicaster endpoint sets the value of "Partial IV" to the
       Sequence Number from its own Sender Context, upon sending a
       multicast request message.  Similarly, a listener endpoint sets
       the value of "Partial IV" to the Sequence Number from its own
       Sender Context, upon sending a unicast response message.

   2.  The value of the "kid" parameter in the "protected" field is set
       to the Sender ID of the endpoint and SHALL be present in both
       multicast requests and unicast responses.

   3.  The "protected" field of the "Headers" field SHALL include also
       the following parameter:

       *  gid : its value is set to the Context Identifier (Cid) of the
          group's Security Context.  This parameter is optional if the
          message is a CoAP response.

   4.  The Additional Authenticated Data (AAD) considered to compute the
       COSE object is extended.  In particular, the "external_aad"
       considered for secure response messages SHALL include also the
       following parameter:




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       *  cid : bstr, contains the Context Idenfier (Cid) of the
          Security Context considered to protect the request message
          (which is same as the Cid considered to protect the response
          message).

   5.  Before transmitting any secure CoAP message, the sender endpoint
       uses its own private key to create a counter signature of the
       COSE_Encrypt0 object (Appendix C.4 of [I-D.ietf-cose-msg]).
       Then, the counter signature is included in the Header of the COSE
       object in its "unprotected" field.

6.  Message Processing

   Each multicast request message and unicast response message is
   protected and processed as specified in
   [I-D.ietf-core-object-security], with the modifications described in
   the following sections.  Furthermore, error handling and processing
   of invalid messages are performed according to the same principles
   adopted in [I-D.ietf-core-object-security].

6.1.  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 - Cid. That
       is, it SHALL be able to find the correct Security Context used to
       protect the multicast request and verify the unicast response(s)
       by using the CoAP Token considered in the message exchange.

   2.  The multicaster endpoint computes the COSE object as defined in
       Section 5 of this specification.

6.2.  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 Context Identifier from the
       "gid" parameter of the received COSE object, and uses it to
       identify the correct group's Security Context.

   2.  The listener endpoint retrieves the Sender ID from the header of
       the 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



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       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
       and uses it to verify the counter signature, before proceeding
       with the verification and decryption of the secure request
       message.

6.3.  Protecting the Response

   A listener endpoint that has received a multicast request message MAY
   reply with a secure unicast 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 considers the Transaction Identifier (Tid)
       as defined in Section 4 of this specification.

   2.  The listener endpoint computes the COSE object as defined in
       Section 5 of this specification.

6.4.  Verifying the Response

   Upon receiving a secure unicast 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 considers the Security Context
       identified by the Token of the received response message.

   2.  The multicaster endpoint retrieves the Sender ID from the header
       of the 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
       and uses it to verify the counter signature, before proceeding




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       with the verification and decryption of the secure response
       message.

   The mapping between unicast response messages from listener endpoints
   and the associated multicast request message from a multicaster
   endpoint relies on the same principle adopted in
   [I-D.ietf-core-object-security].  That is, it is based on the
   Transaction Identifier (Tid) associated to the secure multicast
   request message, which is considered by listener endpoints as part of
   the Additional Authenticated Data when protecting their own response
   message.

7.  Security Considerations

   Specific 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 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.

   Furthermore, the presented approach SHOULD take into consideration
   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.  The adoption of key management
   schemes for secure revocation and renewal of security contexts group
   keying material SHOULD be considered.

7.2.  Management of Group Keying Material

   As stated in Section 2, it is important to adopt a group key
   management scheme that SHOULD update the security context and keying
   material in the group, before a new endpoint joins the group or after
   a currently present endpoint leaves the group.  This is necessary in




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   order to preserve backward confidentiality and forward
   confidentiality in the multicast group.

   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.

7.3.  Late Joining Endpoints

   Upon joining the multicast group when the system is fully operative,
   listeners are not aware of the current sequence number values used by
   different multicasters to transmit multicast request messages.  This
   means that, when such listeners receive a secure multicast message
   from a multicaster, they are not able to verify if that message is
   fresh and has not been replayed.

   In order to address this issue, upon receiving a multicast message
   from a particular multicaster for the first time, late joining
   listeners can initialize their last-seen sequence number in their
   Recipient Context associated to that multicaster.  However, after
   that they drop the message, without delivering it to the application
   layer.  This provides a reference point to identify if future
   multicast messages from the same multicaster are fresher than the
   last one seen.  As an alternative, a late joining listener can
   directly contact the multicaster, and explicitly request a
   confirmation of the sequence number in the first received multicast
   message.

   A possible different approach considers the GM as an additional
   listener in the multicast group.  Then, the GM can maintain the
   sequence number values of each multicaster in the group.  When late
   joiners send a request to the GM to join the group, the GM can
   provide them with the list of sequence number values to be stored in
   the Recipient Contexts associated to the appropriate multicasters.

7.4.  Provisioning of Public Keys

   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



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   details about how this requirement can be fulfilled are out of the
   scope of this document.

   Alternatively, the Group Manager can be configured to store public
   keys of group members and provide them upon request.  In such a case,
   upon joining a multicast group, an endpoint provides the Group
   Manager with its own public key, by means of the same secure channel
   used to carry out the join procedure.  After that, the Group Manager
   MUST perform a proof-of-possession challenge-response with the
   joining endpoint, in order to verify that it actually owns the
   associated private key.  In case of success, 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 multicast
   group.

   Note that in simple, less dynamic, multicast groups, it can be
   convenient for the Group Manager to provide an endpoint upon its
   joining with the public keys associated to all endpoints currently
   present in the group.

8.  IANA Considerations

   This document has no actions for IANA.

9.  Acknowledgments

   The authors sincerely thank Rolf Blom, Carsten Bormann, John Mattsson
   and Jim Schaad 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 of CoAP (OSCOAP)", draft-ietf-core-
              object-security-01 (work in progress), December 2016.

   [I-D.ietf-cose-msg]
              Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              draft-ietf-cose-msg-24 (work in progress), November 2016.

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




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

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

10.2.  Informative References

   [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-05 (work in progress), February 2017.

   [I-D.seitz-ace-oscoap-profile]
              Seitz, L. and F. Palombini, "OSCOAP profile of ACE",
              draft-seitz-ace-oscoap-profile-01 (work in progress),
              October 2016.

   [I-D.selander-ace-cose-ecdhe]
              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
              cose-ecdhe-04 (work in progress), October 2016.

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

   [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,
              <http://www.rfc-editor.org/info/rfc4046>.

   [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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc4944>.





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   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <http://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,
              <http://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, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7390>.

Appendix A.  Group Joining Based on the ACE Framework

   The join process to register an endpoint as a new member of a
   multicast group MAY be based on the ACE framework
   [I-D.ietf-ace-oauth-authz] and the OSCOAP profile of ACE
   [I-D.seitz-ace-oscoap-profile].  With reference to the terminology
   defined in OAuth 2.0 [RFC6749]:

   o  The joining endpoint acts as Client;

   o  The Group Manager acts as Resource Server, exporting one join-
      resource for each multicast group it is responsible for;

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

   Then, in accordance with [I-D.seitz-ace-oscoap-profile], the joining
   endpoint and the Group Manager rely on OSCOAP
   [I-D.ietf-core-object-security] for secure communication and consider
   Ephemeral Diffie-Hellman Over COSE (EDHOC)




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   [I-D.selander-ace-cose-ecdhe] as a possible method to establish key
   material.

   The joining endpoint sends to the Group Manager an OSCOAP request to
   access the join-resource associated to the multicast group to join.
   The Group Manager replies with an OSCOAP response including the
   Common Context associated to that group (see Section 4).  In case the
   Group Manager is configured to store the public keys of group
   members, the joining endpoint additionally provides the Group Manager
   with its own public key, and MAY request from the Group Manager the
   public keys of the endpoints currently present in the group (see
   Section 7.4).

   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, including
   OSCOAP and DTLS [RFC6347].

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 Section 2.

   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



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      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.  Therefore, it 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
      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



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      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.

Authors' Addresses

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

   Email: marco.tiloca@ri.se


   Goeran Selander
   Ericsson AB
   Farogatan 6
   Kista  SE-16480 Stockholm
   Sweden

   Email: goran.selander@ericsson.com


   Francesca Palombini
   Ericsson AB
   Farogatan 6
   Kista  SE-16480 Stockholm
   Sweden

   Email: francesca.palombini@ericsson.com










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