One document matched: draft-keoh-dice-multicast-security-08.xml
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<rfc category="std" docName="draft-keoh-dice-multicast-security-08"
ipr="trust200902" obsoletes="" submissionType="IETF" updates=""
xml:lang="en">
<front>
<title abbrev="DTLS-based Multicast Security">DTLS-based Multicast
Security in Constrained Environments</title>
<author fullname="Sye Loong Keoh" initials="S." surname="Keoh">
<organization>University of Glasgow Singapore</organization>
<address>
<postal>
<street>Republic PolyTechnic, 9 Woodlands Ave 9</street>
<city>Singapore</city>
<region/>
<code>838964</code>
<country>SG</country>
</postal>
<email>SyeLoong.Keoh@glasgow.ac.uk</email>
</address>
</author>
<author fullname="Sandeep S. Kumar" initials="S.S." surname="Kumar" role="editor">
<organization>Philips Research</organization>
<address>
<postal>
<street>High Tech Campus 34</street>
<city>Eindhoven</city>
<region/>
<code>5656 AE</code>
<country>NL</country>
</postal>
<email>sandeep.kumar@philips.com</email>
</address>
</author>
<author fullname="Oscar Garcia-Morchon" initials="O."
surname="Garcia-Morchon">
<organization>Philips Research</organization>
<address>
<postal>
<street>High Tech Campus 34</street>
<city>Eindhoven</city>
<region/>
<code>5656 AE</code>
<country>NL</country>
</postal>
<email>oscar.garcia@philips.com</email>
</address>
</author>
<author fullname="Esko Dijk" initials="E." surname="Dijk">
<organization>Philips Research</organization>
<address>
<postal>
<street>High Tech Campus 34</street>
<city>Eindhoven</city>
<region/>
<code>5656 AE</code>
<country>NL</country>
</postal>
<email>esko.dijk@philips.com</email>
</address>
</author>
<author fullname="Akbar Rahman" initials="A." surname="Rahman">
<organization>InterDigital</organization>
<address>
<postal>
<street>1000 Sherbrooke Street West</street>
<city>Montreal</city>
<region/>
<code>H3A 3G4</code>
<country>CA</country>
</postal>
<email>akbar.rahman@interdigital.com</email>
</address>
</author>
<date day="03" month="July" year="2014"/>
<area>Security</area>
<workgroup>DICE Working Group</workgroup>
<abstract>
<t>The CoAP standard is fast emerging as a key
protocol in the area of resource-constrained devices. Such IP-based
systems are foreseen to be used for building and lighting automation
systems where devices interconnect with each other, forming, for example,
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 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.</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction" toc="default">
<t>There is an increased use of wireless networks in
lighting and
building management systems. This is mainly driven by the fact that the
independence from physical 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 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 networks are also known as
Low-power and Lossy Networks (LLNs).</t>
<t>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 constrained environments. It is more effective in
constrained environments 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 automation
system, the heating, ventilation, air-conditioning and lighting devices
are often grouped according to the layout of the building, and
commands are issued simultaneously to a group of devices. Group
communication is based on the Constrained Application Protocol
(CoAP) <xref target="I-D.ietf-core-coap" /> sent over IP- multicast
<xref target="I-D.ietf-core-groupcomm" />.</t>
<t>Currently, CoAP messages are protected using Datagram Transport Layer
Security (DTLS) <xref target="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 constrained environments 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
<xref target="RFC3830" />, <xref target="RFC4082" />, <xref target="RFC3740" />,
<xref target="RFC4046" />, and <xref target="RFC4535" />. However, these approaches are not necessarily
suitable for constrained environments which have much more limited bandwidth and resources.
For example, the MIKEY Architecture <xref target="RFC3830" /> is mainly designed to
facilitate multimedia distribution, while TESLA <xref target="RFC4082" /> is proposed as
a protocol for broadcast authentication of the source and not for
protecting the confidentiality of multicast messages. <xref target="RFC3740" />
and <xref target="RFC4046" /> provide reference architectures for multicast security.
<xref target="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 constrained networks.</t>
<t>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.</t>
<t>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.</t>
<section anchor="term" title="Terminology" toc="default">
<t>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
<xref target="RFC2119">RFC 2119</xref>.</t>
<t>This specification uses the following terminology: <list
style="symbols">
<t>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.</t>
<t>Sender: The Sender is an entity that sends data to the
multicast group. In a 1-to-N multicast group only a single sender
transmits 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 senders.</t>
<t>Listener: The entity that receives multicast messages when
listening to a specific multicast IP address.</t>
<t>Security Association (SA): A set of policy and cryptographic
keys that provide security services to network traffic that
matches that policy <xref target="RFC3740" />. A Security Association usually
contains the following attributes: <list style="symbols">
<t>selectors, such as source and destination identifiers.</t>
<t>cryptographic policy, such as the algorithms, modes, key
lifetimes, and key lengths used for authentication or
confidentiality.</t>
<t>keying material for authentication, encryption and
signing.</t>
</list></t>
<t>Group Security Association (GSA): A bundling of security associations
(SAs) that together define how a group communicates securely.
<xref target="RFC3740" /></t>
<t>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 <xref target="RFC4949" />. </t>
</list></t>
</section>
<section anchor="outline" title="Outline" toc="default">
<t>This draft is structured as follows: <xref format="default"
pageno="false" target="ucreq"/> motivates the proposed solution with
group communication use cases in LLNs and derives a set of
requirements. <xref format="default" pageno="false"
target="DTLSMulticast"/> 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 <xref format="default" pageno="false"
target="MultDataSec"/>, we describe the details of the adaptation of
DTLS record layer for confidentiality and integrity protection of the
multicast messages. <xref format="default" pageno="false"
target="Sec"/> presents the security considerations.</t>
</section>
</section>
<section anchor="ucreq" title="Use Cases and Requirements" toc="default">
<t>This section defines the use cases for group communication in LLNs
and specifies a set of security requirements for these use cases.</t>
<section anchor="uc" title="Group Communication Use Cases" toc="default">
<t>The "Group Communication for CoAP" draft <xref format="default"
pageno="false" target="I-D.ietf-core-groupcomm"/> provides the
necessary background for multicast based CoAP communication in constrained environments (e.g. 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:</t>
<t><list style="letters">
<t>Lighting automation: enabling synchronous operation of a group of
6LoWPAN <xref target="RFC4944" /> <xref target="RFC6282" /> connected
lights in a room/floor/building. This ensures
that the light preset like on/off/dim-level of a large group of luminaries are changed
at the same time, hence providing a visual synchronicity of light
effects to the user.</t>
<t>Parameter update: configuration settings of a group of similar devices are
updated simultaneously and efficiently.</t>
<t>Device and Service discovery: information about the devices
in the local network and their capabilities can be queried and
requested using multicast, e.g. by a commissioning device.
The responses are sent back in unicast.</t>
</list></t>
<t>Elaborating on one of the main use cases that this document addresses, Lighting automation,
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
automate 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 (which may be individually protected with L2 security). In such a multicast group,
group messages can traverse from one physical subnet to another physical subnet through a IPv6 backbone
which may not be protected. Additionally, other non-lighting devices (like window blind automation) may share the physical subnet for networking.</t>
</section>
<section anchor="secreq" title="Security Requirements" toc="default">
<t>The "Miscellaneous CoAP Group Communication Topics" draft <xref
format="default" pageno="false"
target="I-D.dijk-core-groupcomm-misc"/> already defines a set of
security requirements for CoAP group communications. We re-iterate
and further describe those security requirements in this section with
respect to the use cases. The security requirements are classified into those that are assumed to be fulfilled
and those that need to be fulfilled by the solution in this draft.</t>
<t>The security requirements which are out-of-scope of this draft and assumed to be already fulfilled:</t>
<t><list style="letters">
<t>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
<xref target="RFC3740" />, <xref target="RFC4046" /> and <xref target="RFC4535" />.</t>
<t>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 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 <xref target="RFC5116" /> ciphersuites. Other ciphersuites that are defined for data record security in DTLS should also be
preferably supported.</t>
<t>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.</t>
<t>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. </t>
</list></t>
<t>The security requirements which need to be fulfilled by the solution described in this draft:</t>
<t><list style="letters">
<t>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 automation use case, the switch is the only
entity that is responsible for sending commands to a group
of lighting devices. In more advanced lighting automation use cases,
a N-to-M communication topology would be required, for example if
multiple sensors (presence or day-light) are responsible to
trigger events to a group of lighting devices.</t>
<t>Multicast group size: The security solutions should support the
typical group sizes that "Group Communication for CoAP" draft
<xref format="default" pageno="false"
target="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 LLN use cases mentioned in the document, 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 50 Senders would cover
the group sizes required for most use cases that are relevant for this
document. The total number of group devices must be in the range of 2
to 100 devices. Groups larger than these should be divided into smaller
independent multicast groups such as grouping lights of a building per floor.</t>
<t>Multicast data confidentiality: Multicast message should be
encrypted, as some control commands when sent in the clear could
pose unforeseen privacy risks to the users of the system.</t>
<t>Multicast data replay protection: It must not be possible to
replay a multicast message as this would disrupt the operation of
the group communication.</t>
<t>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.</t>
</list></t>
</section>
</section>
<section anchor="DTLSMulticast"
title="Overview of DTLS-based Secure Multicast" toc="default">
<t>The goal of this draft is to secure CoAP Group communication
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 <xref target="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.</t>
<section anchor="IPMulticast" title="IP Multicast" toc="default">
<t>Devices in the network (e.g. LLN) are categorized into two roles, (1) sender and
(2) listener. Any node 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.</t>
<t>In principle, a sender or listener does not require any prior
access procedures or authentication to send or listen to a multicast
message <xref target="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. <xref format="default" pageno="false"
target="OneManyMulticast"/> 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 network. More details on the IPv6 multicast and CoAP
group communication can be found in <xref target="I-D.ietf-core-groupcomm" />. This
draft does not intend to modify any of the underlying group
communication or multicast routing protocols.</t>
<figure align="center" alt="" anchor="OneManyMulticast" height=""
suppress-title="false"
title="The roles of nodes in a 1-to-N multicast communication topology"
width="">
<artwork align="center" alt="" height="" name="" type="" width=""
xml:space="preserve"><![CDATA[
++++
|. |
--| ++++
++++ / ++|. |
|A |---------| ++++
| | \ ++|B |
++++ \-----| |
Sender ++++
Listeners
]]></artwork>
</figure>
</section>
<section anchor="SecMulticast" title="Securing Multicast in Constrained Networks"
toc="default">
<t>A group controller in a constrained network 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 <xref target="I-D.vanderstok-core-dna" /> such as DNS-SD <xref target="RFC6763" /> and Resource
Directory <xref target="I-D.ietf-core-resource-directory" />. The group controller
communicates with individual devices 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 constrained networks 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.</t>
<t>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 <xref format="default" pageno="false" target="DTLSPacket"/>.
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.</t>
<figure align="center" alt="" anchor="DTLSPacket" height=""
suppress-title="false"
title="Sending a multicast message protected using DTLS Record Layer"
width="">
<artwork align="center" alt="" height="" name="" type="" width=""
xml:space="preserve"><![CDATA[
+--------+-------------------------------------------------+
| | +--------+------------------------------------+ |
| | | | +-------------+------------------+ | |
| | | | | | +--------------+ | | |
| IP | | UDP | | DTLS Record | | multicast | | | |
| header | | header | | Header | | message | | | |
| | | | | | +--------------+ | | |
| | | | +-------------+------------------+ | |
| | +--------+------------------------------------+ |
+--------+-------------------------------------------------+
]]></artwork>
</figure>
</section>
</section>
<section anchor="MultDataSec" title="Multicast Data Security"
toc="default">
<t>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. </t>
<section anchor="secparam" title="SecurityParameter derivation"
toc="default">
<t>The GSA is used to derive the same "SecurityParameters"
structure as defined in <xref target="RFC5246" /> for all devices.</t>
<t>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:</t>
<figure align="left" alt="" height="" suppress-title="false" title=""
width="">
<artwork align="left" alt="" height="" name="" type="" width=""
xml:space="preserve"><![CDATA[
client write MAC key
server write MAC key
client write encryption key
server write encryption key
client write IV
server write IV
]]></artwork>
</figure>
<t>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 <xref target="RFC5246" /> .
Alternatively, the GSA could directly include the above six keying
material when being configured in all group devices.</t>
<t>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.</t>
</section>
<section anchor="recordadapt" title="Record layer adaptation"
toc="default">
<t>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.</t>
<t>The following <xref format="default" pageno="false"
target="DTLSHeader"/> 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.</t>
<figure align="center" alt="" anchor="DTLSHeader" height=""
suppress-title="false"
title="The DTLS record layer header and optionally encrypted payload and MAC"
width="">
<artwork align="center" alt="" height="" name="" type="" width=""
xml:space="preserve"><![CDATA[
+---------+---------+--------+--------+--------+------------+-------+
| 1 Byte | 2 Byte | 2 Byte | 6 Byte | 2 Byte | | |
+---------+---------+--------+--------+--------+------------+-------+
| Content | Version | epoch | seq_ | Length | Ciphertext | MAC |
| Type | Ma | Mi | | number | | (Enc) | (Enc) |
+---------+---------+--------+--------+--------+------------+-------+
]]></artwork>
</figure>
<t>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 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.</t>
<t>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 <xref target="RFC6655" /> and AES-GCM <xref target="RFC5288" /> as defined in DTLS. Nonce reuse can
completely break the security of these cipher suites. </t>
<t>According to the AES-CCM for TLS, Section 3 <xref target="RFC6655" />, the CCMNonce is a
combination of a salt value and the sequence number.</t>
<figure align="left" alt="" height="" suppress-title="false"
title="" width="">
<artwork align="center" alt="" height="" name="" type="" width=""
xml:space="preserve"><![CDATA[
struct {
opaque salt[4];
opaque nonce_explicit[8];
} CCMNonce;
]]></artwork>
</figure>
<t>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 <xref target="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:</t>
<figure align="left" alt="" height="" suppress-title="false"
title="" width="">
<artwork align="center" alt="" height="" name="" type="" width=""
xml:space="preserve"><![CDATA[
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.
]]></artwork>
</figure>
<t>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 constrained networks and therefore we go for
the second approach of separating the sequence number spaces by
embedding a unique sender identifier in the sequence number as
suggested in <xref target="RFC5288" />.</t>
<t>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 <xref target="secreq" />. The list of
SenderIDs are then distributed to all the group members by the controller.</t>
<t>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. <xref target="adapDTLSHeader" />
illustrates the adapted DTLS record layer header.</t>
<figure align="center" alt="" anchor="adapDTLSHeader" height=""
suppress-title="false"
title="The adapted DTLS record layer header" width="">
<artwork align="center" alt="" height="" name="" type="" width=""
xml:space="preserve"><![CDATA[
+---------+---------+--------+--------+-----------+--------+
| 1 Byte | 2 Byte | 2 Byte | 1 Byte | 5 Byte | 2 Byte |
+---------+---------+--------+--------+-----------+--------+
| Content | Version | Epoch | Sender | trunc_seq_| Length |
| Type | Ma | Mi | | ID | number | |
+---------+---------+--------+--------+-----------+--------+
]]></artwork>
</figure>
</section>
<section anchor="SendMult" title="Sending Secure Multicast Messages"
toc="default">
<t>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.</t>
</section>
<section anchor="RecMult" title="Receiving Secure Multicast Messages"
toc="default">
<t>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.</t>
<t>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.</t>
<t>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 outside the group 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.</t>
</section>
<section anchor="UnicastResp" title="Unicast Responses to Multicast Messages"
toc="default">
<t>In CoAP, responses to multicast messages are always sent back as unicast. That is,
the group members that receive a multicast message may individually decide to send
(or suppress) a unicast response as described in Section 2.5 of <xref target="I-D.ietf-core-groupcomm" />.
The unicast responses to a DTLS-based multicast message MUST be secured. Specifically, the unicast response
may be sent back in a unicast DTLS message as described in Section 9.1 of <xref target="I-D.ietf-core-coap" />.
This requires that a unicast DTLS session is already established between the multicast sender and the listener.</t>
<t>Either the multicast message sender or listener may initiate the unicast DTLS handshake to establish the DTLS session.
If the DTLS handshake was initiated by the multicast message sender, it requires that the sender be aware of the
membership of the multicast group. This can be accomplished, for example, as described in Section 2.6 of
<xref target="I-D.ietf-core-groupcomm" />. If the listener initiated the DTLS handshake, it may have done so, for example,
after receiving a multicast message from a specific sender for the first time.</t>
<t>In the extreme scenario, a multicast sender may attempt to initiate the unicast DTLS handshake with all,
or a subset of, known listeners just before or just after it sends out the DTLS-based multicast message. This may result
in the multicast sender having to process unicast DTLS handshake messages from multiple multicast listeners in a short period.</t>
<t>For matching a CoAP response to its corresponding CoAP multicast request, the matching rules
for multicast CoAP in Section 8.2 of <xref target="I-D.ietf-core-coap" /> are used: only the Token value MUST match.
Note in particular that the matching rules for unicast DTLS of Section 9.1.2 of <xref target="I-D.ietf-core-coap" />
do not apply in the multicast request case.
</t>
<t>Note: There is an obvious timing and processing load issue for the multicast sender in all scenarios
where multiple DTLS sessions are established just before or just after the sender sends out
the DTLS-based multicast message.
In the case that the DTLS handshake initiation is left to the listeners, the processing load in the
multicast sender (i.e. unicast DTLS client) is reduced somewhat by the fact that CoAP requires
a randomized back-off delay before responding to a multicast request. The delay is determined by the
Leisure mechanism as described in Section 8.2 of <xref target="I-D.ietf-core-coap" />.</t>
</section>
<section anchor="ProxyOp" title="Proxy Operation"
toc="default">
<t>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
<xref target="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.</t>
<t>As mentioned in Section 11.2 of <xref target="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.</t>
</section>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>This memo includes no request to IANA.</t>
</section>
<section anchor="Sec" title="Security Considerations" toc="default">
<t>Some of the security issues that should be taken into consideration
are discussed below.</t>
<section anchor="groupsecurlity" title="Group level security" toc="default">
<t> This proposal uses a single group key to protect communication within the group. This
requires that all group members are trusted, for e.g. they do not forge messages as a different
sender in the group. In many use case, the devices in a group belong to a common authority and are
configured by a commissioner. In a professional lighting scenario, the roles of the senders and
listeners are configured by the lighting commissioner and devices follow those roles.</t>
<t> The use of the protocol should take into consideration the risk of compromise of a group
device in a deployment scenario. Therefore the group size should be limited to 100 devices unless
additional source authenticity mechanisms are implemented at the application layer. Further, the damage
due to a compromised key can be limited by increasing the frequency of re-keying based on the unique
unicast key-pair shared by each device with the controller. Additionally the risk of compromise is
reduced when deployments are in physically secured locations, like lighting inside office buildings. </t>
</section>
<section anchor="latejoiner" title="Late joiners" toc="default">
<t>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, 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. </t>
</section>
<section anchor="uniqueSenderID" title="Uniqueness of SenderIDs" toc="default">
<t> 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.</t>
</section>
<section anchor="seqSpace" title="Reduced sequence number space" toc="default">
<t>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 done as part of the key management mechanism
which is not defined in this draft.</t>
</section>
</section>
<section anchor="Ack" title="Acknowledgements" toc="default">
<t>The authors greatly acknowledge discussion, comments and feedback
from Dee Denteneer, Peter van der Stok, Zach Shelby, Michael StJohns and Marco Tiloca. Additionally
thanks to 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.</t>
</section>
</middle>
<back>
<references title="Normative References">
&RFC2119;
&RFC5116;
&RFC5246;
&RFC5288;
&RFC6347;
&RFC6655;
&I-D.ietf-core-coap;
&I-D.ietf-core-groupcomm;
</references>
<references title="Informative References">
&RFC2104;
&RFC3740;
&RFC3830;
&RFC4046;
&RFC4082;
&RFC4535;
&RFC4785;
&RFC4944;
&RFC4949;
&RFC5374;
&RFC6282;
&RFC6763;
&FIPS.197.2001;
&FIPS.180-2.2002;
&I-D.ietf-core-resource-directory;
&I-D.vanderstok-core-dna;
&I-D.dijk-core-groupcomm-misc;
&I-D.mcgrew-aero;
</references>
<section title="Change Log">
<t>
(To be removed by RFC editor before publication.)
</t>
<t>Changes from keoh-03 to keoh-04:
<list style="symbols">
<t>Added description of Proxy operation in a DTLS-based multicast scenario in Section 4.5 (Proxy Operation).</t>
<t>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.</t>
<t>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).</t>
<t>Ran IETF spell checker and ID-Nits tools and corrected various issues throughout the document.</t>
<t>Various editorial updates.</t>
</list>
</t>
<t>Changes from keoh-04 to keoh-05:
<list style="symbols">
<t>In section 2.1, removed the firmware upgrade usecase and clarified the commissioning use case. The lighting use-case expanded with shared and multiple subnets issues. </t>
<t>In Section 2.2, (b) reduced the group size to 100; (h) clarified data source authenticity </t>
<t>Added new Section 6.1 (Group level security) in security considerations to make clear the risks of the single group key.</t>
</list>
</t>
<t>Changes from keoh-05 to keoh-06:
<list style="symbols">
<t>Added description of protection of unicast responses to multicast request in new Section 4.5 (Unicast Responses to Multicast Messages).</t>
<t>Clarified that CoAP may be run over either LLNs or regular networks. This also included changing the title of the I-D.</t>
<t>Various editorial updates.</t>
</list>
</t>
<t>Changes from keoh-06 to keoh-07:
<list style="symbols">
<t>Clarified that either the sender or receiver may initiate the unicast DTLS handshake (for the protected unicast response) in Section 4.5.</t>
<t>Various editorial updates.</t>
</list>
</t>
<t>Changes from keoh-07 to keoh-08:
<list style="symbols">
<t>Changed focus of usage of the DTLS-multicast solution from "control applications" to a "Lighting automation" theme.</t>
<t>Added request/response matching rules for DTLS-multicast.</t>
<t>Various editorial updates for better clarity.</t>
</list>
</t>
</section>
</back>
</rfc>
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