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DICE Working Group                                               S. Keoh
Internet-Draft                           University of Glasgow Singapore
Intended status: Standards Track                           S. Kumar, Ed.
Expires: August 9, 2014                                O. Garcia-Morchon
                                                                 E. Dijk
                                                        Philips Research
                                                               A. Rahman
                                                            InterDigital
                                                        February 5, 2014


 DTLS-based Multicast Security for Low-Power and Lossy Networks (LLNs)
                 draft-keoh-dice-multicast-security-04

Abstract

   The CoAP and 6LoWPAN standards are fast emerging as key protocols in
   the area of resource-constrained devices.  Such IP-based systems are
   foreseen to be used for building and lighting control systems where
   wireless devices interconnect with each other, forming low-power and
   lossy networks (LLNs).  Both multicast and its security are key needs
   in these networks.  This draft presents a method for securing IPv6
   multicast communication in LLNs based on the DTLS which is already
   supported for unicast communication for CoAP devices.  This draft
   deals with the adaptation of the DTLS record layer to protect
   multicast group communication, assuming that all group members
   already have the group security association parameters in their
   possession.  The adapted DTLS record layer provides message
   confidentiality, integrity and replay protection to group messages
   using the group keying material before sending the message via IPv6
   multicast to the group.

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 August 9, 2014.



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Copyright Notice

   Copyright (c) 2014 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
   (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 . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Outline . . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Use Cases and Requirements  . . . . . . . . . . . . . . . . .   5
     2.1.  Group Communication Use Cases . . . . . . . . . . . . . .   5
     2.2.  Security Requirements . . . . . . . . . . . . . . . . . .   6
   3.  Overview of DTLS-based Secure Multicast . . . . . . . . . . .   9
     3.1.  IP Multicast  . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Securing Multicast in LLNs  . . . . . . . . . . . . . . .  10
   4.  Multicast Data Security . . . . . . . . . . . . . . . . . . .  11
     4.1.  SecurityParameter derivation  . . . . . . . . . . . . . .  11
     4.2.  Record layer adaptation . . . . . . . . . . . . . . . . .  12
     4.3.  Sending Secure Multicast Messages . . . . . . . . . . . .  14
     4.4.  Receiving Secure Multicast Messages . . . . . . . . . . .  14
     4.5.  Proxy Operation . . . . . . . . . . . . . . . . . . . . .  15
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
     6.1.  Late joiners  . . . . . . . . . . . . . . . . . . . . . .  16
     6.2.  Uniqueness of SenderIDs . . . . . . . . . . . . . . . . .  16
     6.3.  Reduced sequence number space . . . . . . . . . . . . . .  16
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20








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1.  Introduction

   There is an increased use of wireless control networks in
   environmental monitoring, industrial automation, lighting controls
   and building management systems.  This is mainly driven by the fact
   that the independence from physical control wires allows for freedom
   of placement, portability and for reducing the cost of installation
   as less cable placement and drilling are required.  Consequently,
   there is an ever growing number of electronic devices, sensors and
   actuators that have become Internet connected, thus creating a trend
   towards the Internet-of-Things (IoT).  These connected devices are
   equipped with communication capability that enables them to interact
   with each other as well as with the wider Internet services.
   However, the devices in such wireless control networks are
   characterized by power constraints (as these are usually battery-
   operated), have limited computational resources (low CPU clock, small
   RAM and flash storage) and often, the communication bandwidth is
   limited and unreliable (e.g., IEEE 802.15.4 radio).  Hence, such
   wireless control networks are also known as Low-power and Lossy
   Networks (LLNs).

   In addition to the usual device-to-device unicast communication that
   allow devices to directly interact with each other, group
   communication is an important feature in LLNs.  It is more effective
   in LLNs to convey messages to a group of devices without requiring
   the sender to perform multiple time and energy consuming unicast
   transmissions to reach each individual group member.  For example, in
   a building and lighting control system, the heating, ventilation,
   air-conditioning and lighting devices are often grouped according to
   the layout of the building, and control commands are issued
   simultaneously to a group of devices.  Group communication for LLNs
   is based on the Constrained Application Protocol (CoAP)
   [I-D.ietf-core-coap]  sent over IP- multicast
   [I-D.ietf-core-groupcomm].

   Currently, CoAP messages are protected using Datagram Transport Layer
   Security (DTLS) [RFC6347].  However, DTLS is currently used to secure
   a connection between two endpoints and it cannot be used to protect
   multicast group communication.  Group communication in LLNs is
   equally important and should be secured as it is also vulnerable to
   the usual attacks over the air (eavesdropping, tampering, message
   forgery, replay, etc).  There have been a lot of previous efforts in
   IETF to standardize mechanisms to secure multicast communication such
   as [RFC3830], [RFC4082], [RFC3740], [RFC4046], and [RFC4535].
   However, these approaches are not necessarily suitable for LLNs which
   have much more limited bandwidth and resources.  For example, the
   MIKEY Architecture [RFC3830] is mainly designed to facilitate
   multimedia distribution, while TESLA [RFC4082] is proposed as a



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   protocol for broadcast authentication of the source and not for
   protecting the confidentiality of multicast messages.  [RFC3740] and
   [RFC4046] provide reference architectures for multicast security.
   [RFC4535] describes Group Secure Association Key Management Protocol
   (GSAKMP), a security framework for creating and managing
   cryptographic groups on a network which can be reused for key
   management in our context with any needed adaptation for LLNs.

   This draft describes an approach to use DTLS as mandated in CoAP
   unicast to also support multicast security.  We will assume that all
   devices in the group already have a group security association
   parameters based on a key management mechanism which is outside the
   scope of this draft.  This draft focuses primarily on the adaptation
   of the DTLS record layer to protect multicast messages to be sent to
   the group, and thus providing confidentiality, integrity and replay
   protection to the CoAP group messages.

   Lastly, even though this draft is written from the perspective of
   securing CoAP based group communication, it is important to note that
   DTLS is a powerful and flexible security protocol.  Thus use of DTLS-
   based multicast for application layer protocols other than CoAP are
   possible as long as they follow the approach outlined in this draft.

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 RFC 2119 [RFC2119].

   This specification uses the following terminology:

   o  Group Controller: The entity that is responsible for creating a
      multicast group and establishing security associations among
      authorized group members.  It is also responsible for renewing/
      updating the multicast group keys.

   o  Sender: The Sender is an entity that sends data to the multicast
      group.  In a 1-to-N multicast group only a single sender is
      authorized to transmit data to the group.  In an M-to-N multicast
      group (where M and N are not necessarily the same value), M group
      members are authorized to be senders.

   o  Listener: The entity that receives multicast messages when
      listening to a multicast IP address.

   o  Security Association (SA): A set of policy and cryptographic keys
      that provide security services to network traffic that matches




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      that policy [RFC3740].  A Security Association usually contains
      the following attributes:

      *  selectors, such as source and destination transport addresses.

      *  properties, such as identities.

      *  cryptographic policy, such as the algorithms, modes, key
         lifetimes, and key lengths used for authentication or
         confidentiality.

      *  keying material for authentication, encryption and signing.

   o  Group Security Association (GSA): A bundling of security
      associations (SAs) that together define how a group communicates
      securely.  [RFC3740]

   o  Keying material: Data that is specified as part of the SA which is
      needed to establish and maintain a cryptographic security
      association, such as keys, key pairs, and IVs [RFC4949].

1.2.  Outline

   This draft is structured as follows: Section 2 motivates the proposed
   solution with group communication use cases in LLNs and derives a set
   of requirements.  Section 3 provides an overview of the proposed
   DTLS-based multicast security assuming that all devices in the group
   already have a group security association parameters in their
   possession.  In Section 4, we describe the details of the adaptation
   of DTLS record layer for confidentiality and integrity protection of
   the multicast messages.  Section 6 presents the security
   considerations.

2.  Use Cases and Requirements

   This section defines the use cases for group communication in LLNs
   and specifies a set of security requirements for these use cases.

2.1.  Group Communication Use Cases

   The "Group Communication for CoAP" draft [I-D.ietf-core-groupcomm]
   provides the necessary background for multicast based CoAP
   communication in LLNs and the interested reader is encouraged to
   first read this document to understand the non-security related
   details.  This document also lists a few multicast group
   communication uses cases with detailed descriptions and some are
   listed here briefly:




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   a.  Lighting control: enabling synchronous operation of a group of
       6LoWPAN [RFC4944] [RFC6282] connected lights in a room/floor/
       building.  This ensures that the light preset of a large group of
       luminaries are changed at the same time, hence providing a visual
       synchronicity of light effects to the user.

   b.  Firmware update: firmware of devices in a building control system
       are updated simultaneously, avoiding an excessive load on the LLN
       due to unicast firmware updates.

   c.  Parameter update: settings of a group of similar devices are
       updated simultaneously and efficiently.

   d.  Commissioning of above systems: information about the devices in
       the local network and their capabilities can be queried and
       requested, e.g. by a commissioning device.

   Elaborating on one of the main use cases, Lighting control, consider
   a building equipped with 6LoWPAN IP-connected lighting devices,
   switches, and 6LoWPAN border routers; the devices are organized in
   groups according to their physical location in the building, e.g.,
   lighting devices and switches in a room/floor can be configured as a
   single multicast group.  The switches are then used to control the
   lighting devices in the group by sending on/off/dimming commands to
   all lighting devices in the group. 6LoWPAN border routers that are
   connected to an IPv6 network backbone (which is also multicast
   enabled) are used to interconnect 6LoWPANs in the building.
   Consequently, this would also enable multicast groups to be formed
   across different physical subnets in the entire building.

2.2.  Security Requirements

   The "Miscellaneous CoAP Group Communication Topics" draft
   [I-D.dijk-core-groupcomm-misc] already defines a set of security
   requirements for group communication in LLNs.  We re-iterate and
   further describe those security requirements in this section with
   respect to the use cases:

   a.  Multicast communication topology: We consider both 1-to-N (one
       sender with multiple listeners) and M-to-N (multiple senders with
       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
       example, in the simple lighting control use case, the switch is
       the only entity that is responsible for sending control commands
       to a group of lighting devices.  In more advanced lighting
       control use cases, a N-to-M communication topology would be




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       required, for example if multiple sensors (presence or day-light)
       are responsible to trigger events to a group of lighting devices.

   b.  Multicast group size: The security solutions should support the
       typical group sizes that "Group Communication for CoAP" draft
       [I-D.ietf-core-groupcomm] intends to support.  Group size is the
       combination of the number of Senders and Listeners in a group
       with possible overlap (a Sender can also be a Listener but need
       not be always).  In typical LLN use cases, the number of Senders
       (normally the controlling devices) is much smaller than the
       number of Listeners (the controlled devices).  A security
       solution that supports 1 to 255 Senders would cover the group
       sizes required for most use cases that are relevant for this
       document.  The number of Listeners can be larger in the range of
       2 to 5,000 devices.

   c.  Establishment of a GSA: A secure mechanism must be used to
       distribute keying materials, multicast security policies and
       security parameters to members of a multicast group.  A GSA must
       be established by the group controller (which manages the
       multicast group) among the group members.  The 6LoWPAN border
       router, a device in the 6LoWPAN, or a remote server outside the
       6LoWPAN could play the role of the group controller.  However,
       GSA establishment is outside the scope of this draft, and it is
       anticipated that an activity in IETF dedicated to the design of a
       generic key management scheme for the LLN will include this
       feature preferably based on [RFC3740], [RFC4046] and [RFC4535].

   d.  Multicast data confidentiality: Multicast message should be
       encrypted, as some control commands when sent in the clear could
       pose privacy risks to the users.

   e.  Multicast data replay protection: It must not be possible to
       replay a multicast message as this would disrupt the operation of
       the group communication.

   f.  Multicast data group authentication and integrity: It is
       essential to ensure that a multicast message originated from a
       member of the group and that messages have not been tampered with
       by attackers who are not members.  The multicast group key which
       is known to all group members is used to provide authenticity to
       the multicast messages (e.g., using a Message Authentication
       Code, MAC).  This assumes that all other group members are
       trusted not to tamper with the multicast message.

   g.  Multicast data security ciphersuite: All group members must use
       the same ciphersuite to protect the authenticity, integrity and
       confidentiality of multicast messages.  The ciphersuite is part



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       of the GSA.  Typically authenticity is more important than
       confidentiality in LLNs.  Therefore the proposed multicast data
       security protocol must support at least ciphersuites with MAC
       only (NULL encryption) and AEAD [RFC5116] ciphersuites.  Other
       ciphersuites that are defined for data record security in DTLS
       should also be preferably supported.

   h.  Multicast data source authentication: Source authenticity is
       required if the group members are assumed to be untrusted and can
       tamper with the multicast messages.  Source authenticity is not a
       critical feature to be always enabled in every LLN use case.  If
       source authenticity is required for a specific use case, then it
       can be typically provided using public-key cryptography in which
       every multicast message is additionally signed by each sender.
       This requires much higher computational resources on both the
       sender and the receivers, thus incurring too much overhead and
       computational requirements on devices in LLNs.  Alternatively, a
       lightweight broadcast authentication, i.e., TESLA [RFC4082] can
       be deployed, however it requires devices in the multicast group
       to have a trusted clock and have the ability to loosely
       synchronize their clocks with the sender.  Consequently, given
       that the targeted devices have limited resources, and the need
       for source authenticity is not critical in every use case, source
       authenticity is not performed by default as part of the proposed
       data security protocol.  However, it can be supported using
       additional mechanisms which are not specified in this version of
       the draft.  It is important to note that for use cases demanding
       source authenticity, additional security mechanism is needed to
       provide such guarantee.

   i.  Forward security: Devices that leave the group should not have
       access to any future GSAs.  This ensures that a past member
       device cannot continue to decrypt confidential data that is sent
       in the group.  It also ensures that this device cannot send
       encrypted and/or integrity protected data after it leaves the
       group.  The GSA update mechanism has to be defined as part of the
       key management scheme.

   j.  Backward confidentiality: A new device joining the group should
       not have access to any old GSAs.  This ensures that a new member
       device cannot decrypt data sent before it joins the group.  The
       key management scheme should ensure that the GSA is updated to
       ensure backward confidentiality.








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3.  Overview of DTLS-based Secure Multicast

   The goal of this draft is to secure CoAP Group communication over
   6LoWPAN networks, by extending the use of the DTLS security protocol
   to allow for the use of DTLS record layer with minimal adaptation.
   The IETF CoRE WG has selected DTLS [RFC6347] as the default must-
   implement security protocol for securing CoAP, therefore it is
   desirable that DTLS be extended to facilitate CoAP-based group
   communication.  Reusing DTLS for different purposes while
   guaranteeing the required security properties can avoid the need to
   implement multiple security protocols and this is especially
   beneficial when the target deployment consists of resource-
   constrained embedded devices.  This section first describes group
   communication based on IP multicast, and subsequently sketches a
   solution for securing group communication using DTLS.

3.1.  IP Multicast

   Devices in the LLN are categorized into two roles, (1) sender and (2)
   listener.  Any node in the LLN may have one of these roles, or both
   roles.  The application(s) running on a device basically determine
   these roles by the function calls they execute on the IP stack of the
   device.

   In principle, a sender or listener does not require any prior access
   procedures or authentication to send or listen to a multicast message
   [RFC5374].  A sender to an IPv6 multicast group sets the destination
   of the packet to an IPv6 address that has been allocated for IPv6
   multicast.  A device becomes a listener by "joining" to the specific
   IPv6 multicast group by registering with a network routing device,
   signaling its intent to receive packets sent to that particular IPv6
   multicast group.  Figure 1 depicts a 1-to-N multicast communication
   and the roles of the nodes.  Any device can in principle decide to
   listen to any IPv6 multicast address.  This also means applications
   on the other devices do not know, or do not get notified, when new
   listeners join the LLN.  More details on the IPv6 multicast and CoAP
   group communication can be found in [I-D.ietf-core-groupcomm].  This
   draft does not intend to modify any of the underlying group
   communication or multicast routing protocols.












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                                       ++++
                                       |. |
                                     --| ++++
                            ++++    /  ++|. |
                            |A |---------| ++++
                            |  |    \    ++|B |
                            ++++     \-----|  |
                           Sender          ++++
                                         Listeners


     Figure 1: The roles of nodes in a 1-to-N multicast communication
                                 topology

3.2.  Securing Multicast in LLNs

   A group controller in an LLN creates a multicast group.  The group
   controller may be hosted by a remote server, or a border router that
   creates a new group over the network.  In some cases, devices may be
   configured using a commissioning tool that mediates the communication
   between the devices and the group controller.  The controller in the
   network can be discovered by the devices using various methods
   defined in [I-D.vanderstok-core-dna] such as DNS-SD [RFC6763] and
   Resource Directory [I-D.ietf-core-resource-directory].  The group
   controller communicates with individual device to add them to the new
   group.  Additionally it distributes the GSA consisting of keying
   material, security policies security parameters and ciphersuites
   using a standardized key management for LLN which is outside the
   scope of this draft.  Additional ciphersuites may need to be defined
   to convey the bulk cipher algorithm, MAC algorithm and key lengths
   within the key management protocol.  We provide two examples of
   ciphersuites (based on the security requirements) that could be
   defined as part of a future key management mechanism:


       Ciphersuite MTS_WITH_AES_128_CCM_8 = {TBD1, TBD2}
       Ciphersuite MTS_WITH_NULL_SHA256   = {TBD3, TBD4}

   Ciphersuite MTS_WITH_AES_128_CCM_8 is used to provide
   confidentiality, integrity and authenticity to the multicast messages
   where the encryption algorithm is AES [FIPS.197.2001], key length is
   128-bit, and the authentication function is CCM [RFC6655] with a
   Message Authentication Code (MAC) length of 8 octets.  Similar to
   [RFC4785], the ciphersuite MTS_WITH_NULL_SHA is used when
   confidentiality of multicast messages is not required, it only
   provides integrity and authenticity protection to the multicast
   message.  When this ciphersuite is used, the message is not encrypted
   but the MAC must be included in which it is computed using a HMAC



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   [RFC2104] that is based on Secure Hash Function SHA256
   [FIPS.180-2.2002].  Depending on the future needs, other ciphersuites
   with different cipher algorithms and MAC length may be supported.

   Senders in the group can encrypt and authenticate the CoAP group
   messages from the application using the keying material into the DTLS
   record.  The authenticated encrypted message is passed down to the
   lower layer of the IPv6 protocol stack for transmission to the
   multicast address as depicted in Figure 2.  The listeners when
   receiving the message, use the multicast IPv6 destination address and
   port (i.e., Multicast identifier) to look up the GSA needed for that
   group connection.  The received message is then decrypted and the
   authenticity is verified using the keying material for that
   connection.


       +--------+-------------------------------------------------+
       |        | +--------+------------------------------------+ |
       |        | |        | +-------------+------------------+ | |
       |        | |        | |             | +--------------+ | | |
       |   IP   | |   UDP  | | DTLS Record | |   multicast  | | | |
       | header | | header | |    Header   | |    message   | | | |
       |        | |        | |             | +--------------+ | | |
       |        | |        | +-------------+------------------+ | |
       |        | +--------+------------------------------------+ |
       +--------+-------------------------------------------------+


     Figure 2: Sending a multicast message protected using DTLS Record
                                   Layer

4.  Multicast Data Security

   This section describes in detail the use of DTLS record layer to
   secure multicast messages.  This assumes that group membership has
   been configured by the group controller, and all member devices in
   the group have the GSA.

4.1.  SecurityParameter derivation

   The GSA is used to derive the same "SecurityParameters" structure as
   defined in [RFC5246] for all devices.

   The SecurityParameters.ConnectionEnd should be set to "server" for
   senders and "client" for listeners.  The current read and write
   states can be derived from SecurityParameters by generating the six
   keying materials:




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       client write MAC key
       server write MAC key
       client write encryption key
       server write encryption key
       client write IV
       server write IV


   This requires that the client_random and server_random within the
   SecurityParameters are also set to the same value for all devices as
   part of the GSA to derive the same keying material for all devices in
   the group with the PRF function defined in Section 6.3 of [RFC5246] .
   Alternatively, the GSA could directly include the above six keying
   material when being configured in all group devices.

   The current read and write states are instantiated for all group
   members based on the keying material and according to their roles:
   senders use "server write" parameters for the write state and
   listeners use "server write" parameters for the read state.
   Additionally each connection state contains the sequence number which
   is incremented for each record sent; the first record sent has the
   sequence number 0.

4.2.  Record layer adaptation

   In this section, we describe in detail the adaptation of the DTLS
   Record layer to enable multiple senders in the group to securely send
   information using a common group key, while preserving the
   confidentiality, integrity and freshness of the messages.

   The following Figure 3 illustrates the structure of the DTLS record
   layer header, the epoch and seq_number are used to ensure message
   freshness and to detect message replays.


   +---------+---------+--------+--------+--------+------------+-------+
   | 1 Byte  | 2 Byte  | 2 Byte | 6 Byte | 2 Byte |            |       |
   +---------+---------+--------+--------+--------+------------+-------+
   | Content | Version | epoch  |  seq_  | Length | Ciphertext |  MAC  |
   |   Type  | Ma | Mi |        | number |        |   (Enc)    | (Enc) |
   +---------+---------+--------+--------+--------+------------+-------+


      Figure 3: The DTLS record layer header and optionally encrypted
                              payload and MAC

   The epoch is fixed by the DTLS handshake and the seq_number is
   initialized to 0.  The seq_number is increased by one whenever a



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   sender sends a new record message.  This is the mechanism of DTLS to
   detect message replay.  Finally, the message is protected (encrypted
   and authenticated with a MAC) using the session keys in the "server
   write" parameters.

   One of the problems with supporting multiple senders is that, the
   seq_number used by senders need to be synchronized to avoid their
   reuse, otherwise packets sent by different senders may get discarded
   as replayed packets.  Further, the bigger problem is using a single
   key in a multiple sender scenario leads to nonce reuse in AEAD cipher
   suites like AES-CCM [RFC6655] and AES-GCM [RFC5288] as defined in
   DTLS.  Nonce reuse can completely break the security of these cipher
   suites.

   According to the AES-CCM for TLS, Section 3 [RFC6655], the CCMNonce
   is a combination of a salt value and the sequence number.


                         struct {
                             opaque salt[4];
                             opaque nonce_explicit[8];
                         } CCMNonce;

   The salt is the "client write IV" (when the client is sending) or the
   "server write IV" (when the server is sending) as defined in the
   "SecurityParameters".  Further [RFC6655] requires that the value of
   the nonce_explicit MUST be distinct for each distinct invocation of
   the CCM encrypt function for any fixed key.  When the nonce_explicit
   is equal to the sequence number of the TLS packets, the CCMNonce has
   the structure as below:


              struct {
                  uint32 client_write_IV; // low order 32-bits
                  uint64 seq_num;         // TLS sequence number
              } CCMClientNonce.

              struct {
                  uint32 server_write_IV; // low order 32-bits
                  uint64 seq_num;         // TLS sequence number
              } CCMServerNonce.

   In DTLS, the 64-bit sequence number is the 16-bit epoch concatenated
   with the 48-bit seq_number.  Therefore to prevent that the CCMNonce
   is reused, either all senders need to synchronize or separate non-
   overlapping sequence number spaces need to be created for each
   sender.  Synchronization between senders is especially hard in LLN
   and therefore we go for the second approach of separating the



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   sequence number spaces by embedding a unique sender identifier in the
   sequence number as suggested in [RFC5288].

   Thus in addition to configuring each device in the group with the
   GSA, the controller needs to assign a unique SenderID to each device
   which has the sender role in the group.  The size of the SenderID is
   1-octet based on the requirement for the supported group size
   mentioned in Section 2.2.  The list of SenderIDs are then distributed
   to all the group members by the controller.

   The existing DTLS record layer header is adapted such that the
   6-octet seq_number field is split into a 1-octet SenderID field and a
   5-octet "truncated" trunc_seq_number field.  Figure 4 illustrates the
   adapted DTLS record layer header.


         +---------+---------+--------+--------+-----------+--------+
         | 1 Byte  | 2 Byte  | 2 Byte | 1 Byte | 5 Byte    | 2 Byte |
         +---------+---------+--------+--------+-----------+--------+
         | Content | Version | Epoch  | Sender | trunc_seq_| Length |
         |   Type  | Ma | Mi |        |   ID   | number    |        |
         +---------+---------+--------+--------+-----------+--------+


              Figure 4: The adapted DTLS record layer header

4.3.  Sending Secure Multicast Messages

   Senders in the multicast group when sending a CoAP group message from
   the application, create the adapted DTLS record payload based on the
   "server write" parameters.  Each sender in the group uses its own
   unique SenderID in the DTLS record layer header.  It also manages its
   own epoch and trunc_seq_number in the "server write" connection
   state; the first record sent has the trunc_seq_number 0.  After
   creating the DTLS record, the trun_seq_number is incremented in the
   "server write" connection state.  The adapted DTLS record is then
   passed down to UDP and IPv6 layer for transmission on the multicast
   IPv6 destination address and port.

4.4.  Receiving Secure Multicast Messages

   When a listeners receives a protected multicast message from the
   sender, it looks up the corresponding "client read" connection state
   based on the multicast IP destination and port of the packet.  This
   is fundamentally different from standard DTLS logic in that the
   current "client read" connection state is bound to the source IP
   address and port.




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   Listener devices in a multiple senders multicast group, need to store
   multiple "client read" connection states for the different senders
   linked to the SenderIDs.  The keying material is same for all senders
   however the epoch and the trunc_seq_number of the last received
   packets needs to be kept different for different senders.

   The listeners first perform a "server write" keys lookup by using the
   multicast IPv6 destination address and port of the packet.  By
   knowing the keys, the listeners decrypt and check the MAC of the
   message.  This guarantees that no one has spoofed the SenderID, as it
   is protected by the MAC.  Subsequently, by authenticating the
   SenderID field, the listeners retrieve the "client read" connection
   state which contains the last stored epoch and trunc_seq_number of
   the sender, which is used to check the freshness of the message
   received.  The listeners must ensure that the epoch is the same and
   trunc_seq_number in the message received is higher than the stored
   value, otherwise the message is discarded.  Alternatively a windowing
   mechanism can be used to accept genuine out-of-order packets.  Once
   the authenticity and freshness of the message have been checked, the
   listeners can pass the message to the higher layer protocols.  The
   epoch and the trunc_seq_number in the corresponding "client read"
   connection state are updated as well.

4.5.  Proxy Operation

   CoAP allows a client to designate a (forward) proxy to process its
   CoAP request for both unicast and multicast scenarios as described in
   Section 2.10 of [I-D.ietf-core-groupcomm].  In this case, the proxy
   (and not the client) appears as the originating point to the
   destination server for the CoAP request.

   As mentioned in Section 11.2 of [I-D.ietf-core-coap], proxies are by
   their nature men-in-the-middle and break DTLS protection of CoAP
   message exchanges.  Therefore, in a DTLS-based multicast scenario
   involving a proxy, a two-step approach is required.  First, the
   client will send a unicast DTLS request to the proxy.  The proxy will
   then receive and decrypt the unicast message.  The proxy will then
   take the contents of the received message and create a new multicast
   message and secure it using DTLS-based multicast before sending it
   out to the group.  For this approach to work properly, the client
   needs to be able to designate the proxy as an authorized sender.  The
   mechanism for this authorization is outside the scope of this draft.

5.  IANA Considerations

   This memo includes no request to IANA.





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6.  Security Considerations

   Some of the security issues that should be taken into consideration
   are discussed below.

6.1.  Late joiners

   Listeners who are late joiners to a multicast group, do not know the
   current epoch and trun_seq_number being used by different senders.
   When they receive a packet from a sender with a random
   trunc_seq_number in it, it is impossible for the listener to verify
   if the packet is fresh and has not been replayed by an attacker.  To
   overcome this late joiner security issue, we can use the techniques
   similar to AERO [I-D.mcgrew-aero] where the late joining listener on
   receiving the first packet from a particular sender, initialize its
   last seen epoch and trunc_seq_number in the "client read" state for
   that sender, however does not pass this packet to the application
   layer and instead drops it.  This provides a reference point to
   identify if future packets are fresher than the last seen packet.
   Alternatively, the group controller which can act as a listener in
   the multicast group can maintain the epoch and trunc_seq_number of
   each sender.  When late joiners send a request to the group
   controller to join the multicast group, the group controller can send
   the list of epoch and trunc_seq_numbers as part of the GSA.

6.2.  Uniqueness of SenderIDs

   It is important that SenderIDs are unique to maintain the security
   properties of the DTLS record layer messages.  However in the event
   that two or more senders are configured with the same SenderID, a
   mechanism needs to be present to avoid a security weakness and
   recover from the situation.  One such mechanism is that all senders
   of the multicast group are also listeners.  This allows a sender
   which receives a packet from a different device with its own SenderID
   in the DTLS header to become aware of a clash.  Once aware, the
   sender can inform the controller on a secure channel about the clash
   along with the source IP address.  The controller can then provide a
   different SenderID to either device or both.

6.3.  Reduced sequence number space

   The DTLS record layer seq_number is truncated from 6 octets to 5
   octets.  This reduction of the seq_number space should be taken into
   account to ensure that epoch is incremented before the
   trunc_seq_number wraps over.  The sender or the controller can
   increase the epoch number by sending a ChangeCipherSpec message
   whenever the trunc_seq_number has been exhausted.  This should be




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   done as part of the key management mechanism which is not defined in
   this draft.

7.  Acknowledgements

   The authors greatly acknowledge discussion, comments and feedback
   from Dee Denteneer, Peter van der Stok, Zach Shelby and Michael
   StJohns.  Additionally thank David McGrew for suggesting options for
   recovering from a SenderID clash, and John Foley for the extensive
   review and pointing us to the AERO draft.  We also appreciate
   prototyping and implementation efforts by Pedro Moreno Sanchez who
   worked as an intern at Philips Research.

8.  References

8.1.  Normative References

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

   [I-D.ietf-core-groupcomm]
              Rahman, A. and E. Dijk, "Group Communication for CoAP",
              draft-ietf-core-groupcomm-18 (work in progress), December
              2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              August 2008.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655, July 2012.






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8.2.  Informative References

   [FIPS.180-2.2002]
              National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-2, August 2002,
              <http://csrc.nist.gov/publications/fips/fips180-2/
              fips180-2.pdf>.

   [FIPS.197.2001]
              National Institute of Standards and Technology, "Advanced
              Encryption Standard (AES)", FIPS PUB 197, November 2001,
              <http://csrc.nist.gov/publications/fips/fips197/
              fips-197.pdf>.

   [I-D.dijk-core-groupcomm-misc]
              Dijk, E. and A. Rahman, "Miscellaneous CoAP Group
              Communication Topics", draft-dijk-core-groupcomm-misc-05
              (work in progress), December 2013.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Bormann, C., and S. Krco, "CoRE Resource
              Directory", draft-ietf-core-resource-directory-01 (work in
              progress), December 2013.

   [I-D.mcgrew-aero]
              McGrew, D. and J. Foley, "Authenticated Encryption with
              Replay prOtection (AERO)", draft-mcgrew-aero-00 (work in
              progress), October 2013.

   [I-D.vanderstok-core-dna]
              Stok, P., Lynn, K., and A. Brandt, "CoRE Discovery,
              Naming, and Addressing", draft-vanderstok-core-dna-02
              (work in progress), July 2012.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

   [RFC3740]  Hardjono, T. and B. Weis, "The Multicast Group Security
              Architecture", RFC 3740, March 2004.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.

   [RFC4046]  Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
              "Multicast Security (MSEC) Group Key Management
              Architecture", RFC 4046, April 2005.



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   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
              Briscoe, "Timed Efficient Stream Loss-Tolerant
              Authentication (TESLA): Multicast Source Authentication
              Transform Introduction", RFC 4082, June 2005.

   [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
              "GSAKMP: Group Secure Association Key Management
              Protocol", RFC 4535, June 2006.

   [RFC4785]  Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK)
              Ciphersuites with NULL Encryption for Transport Layer
              Security (TLS)", RFC 4785, January 2007.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007.

   [RFC5374]  Weis, B., Gross, G., and D. Ignjatic, "Multicast
              Extensions to the Security Architecture for the Internet
              Protocol", RFC 5374, November 2008.

   [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              September 2011.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, February 2013.

Appendix A.  Change Log

   (To be removed by RFC editor before publication.)

   Changes from keoh-03 to keoh-04:

   o  Added description of Proxy operation in a DTLS-based multicast
      scenario in Section 4.5 (Proxy Operation).

   o  Corrected text in Section 2.2 (Security Requirements), item "h",
      to indicate that multicast source authentication is not specified
      in this version of the draft.

   o  Clarified that draft is written primarily for securing of CoAP
      based group communication, but that other protocols may also be
      supported if they have similar characteristics.  See Section 1
      (Introduction).



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   o  Ran IETF spell checker and ID-Nits tools and corrected various
      issues throughout the document.

   o  Various editorial updates.

Authors' Addresses

   Sye Loong Keoh
   University of Glasgow Singapore
   Republic PolyTechnic, 9 Woodlands Ave 9
   Singapore  838964
   SG

   Email: SyeLoong.Keoh@glasgow.ac.uk


   Sandeep S. Kumar (editor)
   Philips Research
   High Tech Campus 34
   Eindhoven  5656 AE
   NL

   Email: sandeep.kumar@philips.com


   Oscar Garcia-Morchon
   Philips Research
   High Tech Campus 34
   Eindhoven  5656 AE
   NL

   Email: oscar.garcia@philips.com


   Esko Dijk
   Philips Research
   High Tech Campus 34
   Eindhoven  5656 AE
   NL

   Email: esko.dijk@philips.com










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   Akbar Rahman
   InterDigital
   1000 Sherbrooke Street West
   Montreal  H3A 3G4
   CA

   Email: akbar.rahman@interdigital.com












































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