Internet DRAFT - draft-keoh-dice-multicast-security
draft-keoh-dice-multicast-security
DICE Working Group S. Keoh
Internet-Draft University of Glasgow Singapore
Intended status: Standards Track S. Kumar, Ed.
Expires: January 4, 2015 O. Garcia-Morchon
E. Dijk
Philips Research
A. Rahman
InterDigital
July 03, 2014
DTLS-based Multicast Security in Constrained Environments
draft-keoh-dice-multicast-security-08
Abstract
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.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 4, 2015.
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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
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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 . . . . . . . . . . . 8
3.1. IP Multicast . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Securing Multicast in Constrained Networks . . . . . . . 9
4. Multicast Data Security . . . . . . . . . . . . . . . . . . . 10
4.1. SecurityParameter derivation . . . . . . . . . . . . . . 11
4.2. Record layer adaptation . . . . . . . . . . . . . . . . . 11
4.3. Sending Secure Multicast Messages . . . . . . . . . . . . 13
4.4. Receiving Secure Multicast Messages . . . . . . . . . . . 14
4.5. Unicast Responses to Multicast Messages . . . . . . . . . 14
4.6. Proxy Operation . . . . . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
6.1. Group level security . . . . . . . . . . . . . . . . . . 16
6.2. Late joiners . . . . . . . . . . . . . . . . . . . . . . 16
6.3. Uniqueness of SenderIDs . . . . . . . . . . . . . . . . . 17
6.4. Reduced sequence number space . . . . . . . . . . . . . . 17
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
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).
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) [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 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 [RFC3830], [RFC4082], [RFC3740],
[RFC4046], and [RFC4535]. However, these approaches are not
necessarily suitable for constrained environments 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
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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 constrained networks.
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 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.
o Listener: The entity that receives multicast messages when
listening to a specific multicast IP address.
o Security Association (SA): A set of policy and cryptographic keys
that provide security services to network traffic that matches
that policy [RFC3740]. A Security Association usually contains
the following attributes:
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* selectors, such as source and destination identifiers.
* 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 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:
a. Lighting automation: enabling synchronous operation of a group of
6LoWPAN [RFC4944] [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
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time, hence providing a visual synchronicity of light effects to
the user.
b. Parameter update: configuration settings of a group of similar
devices are updated simultaneously and efficiently.
c. 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.
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.
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 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.
The security requirements which are out-of-scope of this draft and
assumed to be already fulfilled:
a. 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
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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].
b. 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 [RFC5116] ciphersuites. Other
ciphersuites that are defined for data record security in DTLS
should also be preferably supported.
c. 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.
d. 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.
The security requirements which need to be fulfilled by the solution
described in this draft:
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 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.
b. Multicast group size: The security solutions should support the
typical group sizes that "Group Communication for CoAP" draft
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[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.
c. 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.
d. Multicast data replay protection: It must not be possible to
replay a multicast message as this would disrupt the operation of
the group communication.
e. 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.
3. Overview of DTLS-based Secure Multicast
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 [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.
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3.1. IP Multicast
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.
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 network. 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 1: The roles of nodes in a 1-to-N multicast communication
topology
3.2. Securing Multicast in Constrained Networks
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
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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 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.
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.
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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:
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.
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+---------+---------+--------+--------+--------+------------+-------+
| 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
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:
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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
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 [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
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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.
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 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.
4.5. Unicast Responses to Multicast Messages
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 [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 [I-D.ietf-core-coap]. This
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requires that a unicast DTLS session is already established between
the multicast sender and the listener.
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
[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.
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.
For matching a CoAP response to its corresponding CoAP multicast
request, the matching rules for multicast CoAP in Section 8.2 of
[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 [I-D.ietf-core-coap] do not apply in the multicast
request case.
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
[I-D.ietf-core-coap].
4.6. 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
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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.
6. Security Considerations
Some of the security issues that should be taken into consideration
are discussed below.
6.1. Group level security
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.
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.
6.2. 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, 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
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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.
6.3. 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.
6.4. 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
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, 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.
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.
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[I-D.ietf-core-groupcomm]
Rahman, A. and E. Dijk, "Group Communication for CoAP",
draft-ietf-core-groupcomm-19 (work in progress), June
2014.
[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.
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-06
(work in progress), June 2014.
[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.
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[I-D.mcgrew-aero]
McGrew, D. and J. Foley, "Authenticated Encryption with
Replay prOtection (AERO)", draft-mcgrew-aero-01 (work in
progress), February 2014.
[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.
[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.
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[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).
o Ran IETF spell checker and ID-Nits tools and corrected various
issues throughout the document.
o Various editorial updates.
Changes from keoh-04 to keoh-05:
o 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.
o In Section 2.2, (b) reduced the group size to 100; (h) clarified
data source authenticity
o Added new Section 6.1 (Group level security) in security
considerations to make clear the risks of the single group key.
Changes from keoh-05 to keoh-06:
o Added description of protection of unicast responses to multicast
request in new Section 4.5 (Unicast Responses to Multicast
Messages).
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o Clarified that CoAP may be run over either LLNs or regular
networks. This also included changing the title of the I-D.
o Various editorial updates.
Changes from keoh-06 to keoh-07:
o Clarified that either the sender or receiver may initiate the
unicast DTLS handshake (for the protected unicast response) in
Section 4.5.
o Various editorial updates.
Changes from keoh-07 to keoh-08:
o Changed focus of usage of the DTLS-multicast solution from
"control applications" to a "Lighting automation" theme.
o Added request/response matching rules for DTLS-multicast.
o Various editorial updates for better clarity.
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
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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
Akbar Rahman
InterDigital
1000 Sherbrooke Street West
Montreal H3A 3G4
CA
Email: akbar.rahman@interdigital.com
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