DTLS-based Multicast Security for Low-Power and Lossy Networks (LLNs)
draft-keoh-dice-multicast-security-01
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| Authors | Sye Loong Keoh , Sandeep Kumar , Oscar Garcia-Morchon , Esko Dijk | ||
| Last updated | 2013-11-04 | ||
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draft-keoh-dice-multicast-security-01
DICE Working Group S. Keoh
Internet-Draft University of Glasgow
Intended status: Standards Track S.S. Kumar, Ed.
Expires: May 8, 2014 O. Garcia-Morchon
E. Dijk
Philips Research
November 4, 2013
DTLS-based Multicast Security for Low-Power and Lossy Networks (LLNs)
draft-keoh-dice-multicast-security-01
Abstract
Wireless IP-based systems will be increasingly used for building
control systems in the future where wireless devices interconnect
with each other, forming low-power and lossy networks (LLNs). The
CoAP and 6LoWPAN standards are emerging as the de-facto protocols in
this area for resource-constrained devices. Both multicast and
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 available in CoAP devices. This draft deals
with the adaptation of the DTLS record layer to protect CoAP group
communication, assuming that all group member devices are already
configured with the group security association. The DTLS record
layer is used to provide authentication and encrypt multicast
messages using the group keying material before sending the message
via IPv6 multicast to the group.
Requirements Language
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].
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 8, 2014.
Copyright Notice
Copyright (c) 2013 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
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Use Cases and Requirements . . . . . . . . . . . . . . . . . . 4
2.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Security Requirements . . . . . . . . . . . . . . . . . . 5
3. Overview of DTLS-based Secure Multicast . . . . . . . . . . . 7
3.1. IP Multicast in LLN . . . . . . . . . . . . . . . . . . . 7
3.2. Securing Multicast in LLNs . . . . . . . . . . . . . . . . 8
4. Multicast Data Security . . . . . . . . . . . . . . . . . . . 9
4.1. Sending Secure Multicast Messages . . . . . . . . . . . . 10
4.1.1. One Sender, Multiple Listeners Multicast Group . . . . 11
4.1.2. Multiple Senders, Multiple Listeners Multicast Group . 12
4.2. Receiving Secure Multicast Messages . . . . . . . . . . . 13
4.2.1. One Sender, Multiple Listeners Multicast Group . . . . 13
4.2.2. Multiple Senders, Multiple Listeners Multicast Group . 14
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
There is an increased use of wireless control networks in city
infrastructure, environmental monitoring, industrial automation, 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 Internet of Things (IoT). These connected devices are
equipped with communication capability that enables them to interact
with each other as well as with Internet services at anytime and
anyplace. However, the devices in such wireless control networks are
usually battery-operated or powered by scavenged energy, they have
limited computational resources (low CPU clock, small RAM and flash
storage) and often, the communication bandwidth is limited (e.g.,
IEEE 802.15.4 radio), and also the transmission is unreliable. 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
would allow devices to interact with each other, group communication
is an important feature in LLNs that can be effectively used to
convey messages to a group of devices without requiring the sender to
perform time- and energy-consuming multiple unicast transmissions to
reach group members. For example, in a building control management
system, Heating, Ventilation and Air-Conditioning (HVAC) and lighting
devices can be grouped according to the layout of the building, and
control commands can be issued to a group of devices. Group
communication for LLNs has been made possible using the Constrained
Application Protocol (CoAP) [I-D.ietf-core-coap] based on IP-
multicast.
Currently, CoAP can be protected using Datagram Transport Layer
Security (DTLS) [RFC6347]. However, DTLS is mainly used to secure a
connection between two endpoints and it cannot be used to protect
multicast group communication. We believe that 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). Although there have been a
lot of efforts in IETF to standardize mechanisms to secure multicast
communication, they 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 protocol for
broadcast authentication of the source and not for protecting the
confidentiality of multicast messages.
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This draft describes an approach to use DTLS as mandated in CoAP to
support multicast security. It assumes that all devices in the group
share a security parameters and keying material, for e.g., it can be
distributed by a controller in the network through a DTLS unicast
secure channel to each device in the group. This draft focuses only
on the use of DTLS record layer to protect multicast messages to be
sent to the group, and thus providing integrity, confidentiality and
authenticity to the IP multicast messages in the LLN.
1.1. Terminology
This specification defines the following terminology:
Controller: The entity that is responsible for creating a multicast
group, adding members, and distributing keying material to members of
the group. It is also responsible for renewing/updating the
multicast group keying material. It is not necessarily the sender in
the multicast group.
Sender: The entity that sends multicast messages to the multicast
group.
Listener: The entity that receives multicast messages when listening
to a multicast IP address.
1.2. Outline
This draft is structured as follows: Section 2 motivates the proposed
solution with multicast use cases in LLNs and derives a set of
requirements. Section 3 provides an overview of the DTLS-based
multicast security. In Section 4, we describe the use of DTLS record
layer to encrypt and integrity protect multicast messages assuming
that all devices in the group already have a security parameters and
group keying material in possession. Section 5 and Section 6 describe
Security and IANA considerations.
2. Use Cases and Requirements
This section defines the use cases for multicast and specifies a set
of security requirements for these use cases.
2.1. Use Cases
As stated in the Group Communication for CoAP Internet Draft
[I-D.ietf-core-groupcomm] in the IETF CoRE WG, multicast is essential
in several application use cases. Consider a building equipped with
6LoWPAN [RFC4944] IP-connected lighting devices, switches, and
6LoWPAN border routers; the devices are organized as groups according
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to their location in the building, e.g., lighting devices and
switches in a room/floor can be configured as a multicast group, the
switches are then used to control the lighting devices in the group
by sending on/off/dimming commands to 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 subnets in the entire building. The following lists
a few multicast group communication uses cases in a building
management system; a detailed description of each use case can be
found in Group Communication for CoAP Internet Draft
[I-D.ietf-core-groupcomm].
a. Lighting control: enabling synchronous operation of a group of
6LoWPAN 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 or a campus
control application are updated simultaneously, avoiding an
excessive load on the LLN due to unicast firmware updates.
c. Parameter update: settings of 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.
2.2. Security Requirements
The Miscellaneous CoAP Group Communication Topics Internet Draft
[I-D.dijk-core-groupcomm-misc] has defined 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 as presented in Section 2.1:
a. Multicast communication topology: We consider both one-to-many
and many-to-many communication topologies in this draft. The
one-to-many communication topology is the simplest group
communication scenario that would serve the needs of a typical
LLN. For example, in the lighting control use case, the switch
is the only entity that is responsible for sending control
commands to a group of lighting devices. These lighting devices
are actuators that do not issue commands to each other. In other
use cases, a many-to-many multicast communication topology would
be required, in particular multiple sensors and actuators are
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part of a multicast group and these sensors will trigger events
to the group in order to notify the interested parties. Devices
in the group could also send commands in order to trigger some
actions on other devices in the group.
b. Establishment of a Group Security Association (GSA) [RFC3740]: A
secure channel must be used to distribute keying material,
multicast security policy and security parameters to members of a
multicast group. A GSA must be established between the
controller (which manages the multicast group and may be a
different device than the sender) and the group members. The
6LoWPAN border router, a device in the 6LoWPAN, or a remote
server outside the 6LoWPAN could play the role of controller for
distributing keying materials. Since the keying material is used
to derive subsequent group keys to protect multicast messages, it
is important that it is encrypted, integrity protected and
authenticated when it is distributed. However, this is out of
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 be started in the future.
c. Multicast security policy: All group members must use the same
ciphersuite to protect the authenticity, integrity and
confidentiality of multicast messages. The ciphersuite can
either be negotiated or set by the controller and then
distributed to the group members. It is generally very complex
and difficult to require all devices to negotiate and agree with
each other on the ciphersuite to be used, it is therefore more
effective that the multicast security policy is set by the
controller.
d. Multicast data group authentication: It is essential to ensure
that a multicast message is originated from a member of the
group. 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 only the sender of the multicast group is sending the
message, and that all other group members are trusted not to
tamper with the multicast message.
e. Multicast data source authentication: Source authenticity is
optional. It can typically be provided using public-key
cryptography in which every multicast message is signed by the
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
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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,
it is advocated that source authenticity is made optional.
f. Multicast data integrity: A group level integrity is required to
ensure that messages have not been tampered with by attackers who
are not members of the multicast group.
g. Multicast data confidentiality: Multicast message may be
encrypted, as some control commands when sent in the clear could
pose privacy risks to the users.
h. Multicast data replay protection: It must not be possible to
replay a multicast message as this would disrupt the operation of
the group communication.
i. Multicast key management: Group keys used to protect the
multicast communication must be renewed periodically. When
members have left the multicast group, the group keys might be
leaked; and when a device is detected to have been compromised,
this also implies that the group keys could have been compromised
too. In these situations, the controller must perform a re-key
protocol to renew the group keys. This work will be addressed as
part of the key management for LLN in the future based on
[RFC3740] and [RFC4046].
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 to provide protection to
multicast messages. The IETF CoRE WG has selected DTLS [RFC6347] as
the default must-implement security protocol for securing CoAP,
therefore it is conceivable that DTLS can 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 in LLN
Devices in the LLN are categorized into two roles, (1) sender and (2)
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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. 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, of new senders or
listeners in 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.
++++
|. |
--| ++++
++++ / ++|. |
|A |---------| ++++
| | \ ++|B |
++++ \-----| |
Sender ++++
Listeners
Figure 3.1: The roles of nodes in a one-to-many multicast
communication topology
3.2. Securing Multicast in LLNs
A controller in an LLN creates a multicast group. The 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 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 controller
communicates with individual device to add them to the new group.
Additionally, the controller can distribute a Group Security
Association (GSA) consisting of keying material, security policies
and security parameters to use, to all the member devices in the
group, e.g., by establishing a secure DTLS channel with each device.
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As mentioned previously, a standardized way of performing key
management for LLN is out of scope of this draft, and we assumes that
each device in the group has been configured with a GSA using a.
Senders in the group can encrypt and authenticate application
messages using the keying material in the DTLS record layer before it
is sent using IP multicast. For example, a CoAP message addressed to
a multicast group is protected using DTLS record layer and then sent
to a multicast group. The listeners when receiving the message, use
the multicast IP destination address (i.e., Multicast identifier) to
look up the GSA needed for that connection. The received message is
decrypted and the authenticity is verified using the keying material
for that connection.
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 controller, and all devices in the group have
been configured with the GSA. Since the exact details of the group
key management are outside the scope of this draft, we assume that
the GSA can be used to derive the same SecurityParameters structure
as defined in [RFC5246] for all devices. 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
such examples of ciphersuites 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 [AES], key length is 128-bit,
and the authentication function is CCM [RFC6655] with a Message
Authentication Code (MAC) length of 8 bytes. Similar to RFC4785
[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
[RFC2104] that is based on Secure Hash Function SHA256 [SHA].
Depending on the future needs, other ciphersuites with different
cipher algorithms and MAC length may be supported.
Apart from existing ciphersuites defined for (D)TLS, new ciphersuites
based on AERO[ID.mcgrew-aero] which are particularly designed to
support multiple senders, may be more suitable. More work needs to be
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done in future to identify the usage of AERO with the DTLS record
layer protection for group communication specified in this draft .
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 key
material items:
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 set same for all devices as part of the key
management protocol 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 key management protocol could directly
provide the above six key material to all group devices as part of
the GSA.
The current read and write states are instantiated for all group
members based on the keying material; 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.
For the optional multicast data source authentication, the sender can
sign the message using public key cryptography at the application
layer and send it as the multicast message in the DTLS record
payload. This option is independent of the DTLS layer and outside the
scope of this draft.
4.1. Sending Secure Multicast Messages
All messages addressed to the multicast group must be secured using
"server write" parameters. Using the DTLS record layer, multicast
messages are encrypted and protected using a Message Authentication
Code (MAC) according to the chosen ciphersuite. The authenticated
encrypted message is passed down to the lower layer of the IP
protocol stack for transmission to the multicast address.
As described in the previous section, the example ciphersuite
MTS_WITH_AES_128_CCM_8 defines that the multicast message must be
encrypted using AES with a 128-bit "server write encryption key".
Since the CCM mode of operation is used for authenticated encryption,
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the same key is used to compute the MAC. As for the ciphersuite
example MTS_WITH_NULL_SHA, the multicast message must not be
encrypted, but a MAC must be computed using the "server write MAC
key".
+--------+-------------------------------------------------+
| | +--------+------------------------------------+ |
| | | | +-------------+------------------+ | |
| | | | | | +--------------+ | | |
| IP | | UDP | | DTLS Record | | multicast | | | |
| header | | header | | Header | | message | | | |
| | | | | | +--------------+ | | |
| | | | +-------------+------------------+ | |
| | +--------+------------------------------------+ |
+--------+-------------------------------------------------+
Figure 4.1: Sending a multicast message protected using DTLS Record
Layer
4.1.1. One Sender, Multiple Listeners Multicast Group
This section describes the use of DTLS record layer to protect a one-
sender, multiple-listeners multicast group communication. In this
setting, it is the responsibility of the controller which configures
the group membership to ensure that there is only one sender in a
multicast group and other devices never send multicast messages to
the same group in order to ensure the security properties of the
multicast messages. This is especially a concern in AEAD cipher
suites if multiple senders reuse the same nonce for encryption as
described in Section 5.1.1 in [RFC5116].
The following illustrates the structure of the DTLS record layer
header, the epoch and sequence number are used to ensure message
freshness and to detect message replays. As there is only one sender
in the multicast group, the sender is responsible for maintaining and
manipulating the epoch and sequence number when sending multicast
messages. The receivers in the group are "trusted" not to tamper
with these parameters.
+---------+---------+--------+--------+--------+------------+-------+
| 1 Byte | 2 Byte | 2 Byte | 6 Byte | 2 Byte | | |
+---------+---------+--------+--------+--------+------------+-------+
| Content | Version | Epoch | Seq | Length | Ciphertext | MAC |
| Type | Ma | Mi | | Number | | (Enc) | |
+---------+---------+--------+--------+--------+------------+-------+
Figure 4.2: The DTLS record layer header and optionally encrypted
payload and MAC
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The sequence number is initialized to 0, and it is increased by one
whenever the sender sends a new multicast record message. This is
the standard behavior of the current DTLS in order to detect message
replay. The sender or the controller can increase the epoch number
by sending a ChangeCipherSpec message whenever the sequence number
has been exhausted, or whenever the ciphersuite has been changed in
order to reset the sequence number. Finally, the multicast message
is protected (encrypted if needed, and authenticated with a MAC)
using the "server write" parameters.
4.1.2. Multiple Senders, Multiple Listeners Multicast Group
There is a need to support multi-senders in group communication. In
particular, in a lighting network there are multiple presence sensors
that would be assigned the sender role as they are responsible for
multicasting the presence information to the luminaries in the group.
In this section, we outline an approach to enable all senders in the
group to securely send information using a common group key, while
preserving the freshness and integrity of the messages.
One of the main problems with supporting multiple senders using a
single key is that it leads to nonce reuse AEAD cipher suites like
AES-CCM[RFC6655] and AES-GCM[RFC5288]. 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.
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In DTLS, the 64-bit sequence number is the 16-bit epoch concatenated
with the 48-bit seq_number.
Therefore to prevent that CCMNonce is reused, either all senders need
to synchronize or seperate 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
by creating different sequence number spaces by embedding unique
sender identifiers in the sequence number as suggested in [RFC5288].
Therefore in addition to configuring each device in the group with
the GSA, the controller needs to assign a unique SenderID
(represented as two octets) to each device which has the sender role
in the group. The list of SenderIDs are then distributed to all the
group members by the controller. Alternatively, this setup procedure
can be eliminated by allowing senders to derive their SenderIDs
themselves based on the device's IPv6 or MAC address, or even
randomly. The specific method to be used is not defined here, except
care should be taken that it would lead to a high probability of
unique SenderIDs for all senders within the specific multicast group.
To overcome potential clash in SenderIDs, a back-off mechanism is
defined in the Security Considerations section.
The existing DTLS record layer header is adapted such that the 6-byte
sequence number field is split into a 2-byte SenderID field and a 4-
byte "truncated" sequence number field. Each sender in the group uses
its own unique SenderID in the DTLS record layer header when sending
a multicast message to the group. It also manages its own epoch and
"truncated" sequence number in the "server write" connection state,
hence they do not need to synchronize them with other senders in the
group. Figure 4.3 illustrates the adapted DTLS record layer header.
+---------+---------+--------+--------+----------+--------+
| 1 Byte | 2 Byte | 2 Byte | 2 Byte | 4 Byte | 2 Byte |
+---------+---------+--------+--------+----------+--------+
| Content | Version | Epoch | Sender | "T." Seq | Length |
| Type | Ma | Mi | | ID | Number | |
+---------+---------+--------+--------+----------+--------+
Figure 4.3: The adapted DTLS record layer header
4.2. Receiving Secure Multicast Messages
4.2.1. One Sender, Multiple Listeners Multicast Group
When a listeners receives a protected multicast message from the
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sender, it looks up the corresponding "client read" connection state
based on the multicast IP destination 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.
However, given that this is a one sender- multiple listeners
communication topology, it is possible to bind the current "client
read" connection state to the source IP address if it is already
known to all listeners. Therefore a lookup based on the source IP
address is also possible in this case.
The listeners authenticate and decrypt the multicast message using
the "server write" keys. The verification of MAC ensures that the
payload and the DTLS Record Layer header have not been tampered with.
As there is only one sender, and all other group members are
"trusted", only the sender is able to manipulate the epoch and the
sequence number, hence once the DTLS header has been authenticated,
the epoch and the sequence number can be sufficiently trusted to
detect any message replay.
4.2.2. Multiple Senders, Multiple Listeners Multicast Group
Listener devices in a multi-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 "truncated" sequence 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 IP destination address 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 "truncated" sequence 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 "truncated"
sequence number in the message received is higher than the stored
value, otherwise the message is discarded. As each sender manages
its own epoch and sequence number, receivers are confident that these
values are reliable. 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 sequence number in the
corresponding "client read" connection state are updated as well.
Listeners who are late joiners to a multicast group, do not know the
current epoch and sequence number being used by different senders.
When they receive a packet from a sender with a random sequence
numbered 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
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this late joiner security issue, we can use the techniques similar
AERO [ID.mcgrew-aero] where the late joining listener on receiving
the first packet from a particular sender, initialize its last seen
epoch and sequence number in the "client read" state, however does
not pass it to the application and drops this packet. This provides a
reference point to identify if future packets are fresher than the
last seen packet.
5. IANA Considerations
tbd
Note to RFC Editor: this section may be removed on publication as an
RFC.
6. Security Considerations
This document discusses various design aspects for multicast security
in LLNs. As such this document, in entirety, concerns security.
Section 4.1.2 with multiple senders require 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 mutlicast 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 be aware
of a clash in SenderID. 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.
Section 4.1.2 additionally truncates the sequence number from 6
octets to 4 octets. This reduction of the sequence number space
should be taken into account to ensure that epoch is incremented
before the "truncated" sequence number wraps over. This should be
done with an appropriate 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 and Zach Shelby. 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.
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8. References
8.1. Normative References
[AES] National Institute of Standards and Technology, ,
"Specification for the Advanced Encryption Statndard
(AES)", FIPS 197, Nov 2001.
[SHA] National Institute of Standards and Technology, , "Secure
Hash Standard", FIPS 180-2, Aug 2002.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, 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.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
8.2. Informative References
[I-D.mcgrew-aero]
McGrew, D. and Foley, J., "Authenticated Encryption with
Replay prOtection (AERO)", draft-mcgrew-aero-00
(work in progress), October 2013.
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[I-D.dijk-core-groupcomm-misc]
Dijk, E. and A. Rahman, "Miscellaneous CoAP Group
Communication Topics", draft-dijk-core-groupcomm-misc-04
(work in progress), June 2013.
[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-16 (work in progress),
October 2013.
[I-D.ietf-tls-oob-pubkey]
Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
T. Kivinen, "Out-of-Band Public Key Validation for
Transport Layer Security (TLS)", draft-ietf-tls-oob-
pubkey-09 (work in progress), July 2013.
[I-D.ietf-core-resource-directory]
Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource
Directory", draft-ietf-core-resource-directory-00 (work
in progress), June 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.
[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.
[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.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, February 2013.
[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security
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Architecture", RFC 3740, March 2004.
[RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, November 2008.
[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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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
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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