Internet DRAFT - draft-ietf-dice-profile
draft-ietf-dice-profile
dice H. Tschofenig, Ed.
Internet-Draft ARM Ltd.
Intended status: Standards Track T. Fossati
Expires: April 21, 2016 Alcatel-Lucent
October 19, 2015
TLS/DTLS Profiles for the Internet of Things
draft-ietf-dice-profile-17.txt
Abstract
A common design pattern in Internet of Things (IoT) deployments is
the use of a constrained device that collects data via sensors or
controls actuators for use in home automation, industrial control
systems, smart cities and other IoT deployments.
This document defines a Transport Layer Security (TLS) and Datagram
TLS (DTLS) 1.2 profile that offers communications security for this
data exchange thereby preventing eavesdropping, tampering, and
message forgery. The lack of communication security is a common
vulnerability in Internet of Things products that can easily be
solved by using these well-researched and widely deployed Internet
security protocols.
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
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This Internet-Draft will expire on April 21, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. TLS and DTLS . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Communication Models . . . . . . . . . . . . . . . . . . 6
3.3. The Ciphersuite Concept . . . . . . . . . . . . . . . . . 18
4. Credential Types . . . . . . . . . . . . . . . . . . . . . . 20
4.1. Pre-Conditions . . . . . . . . . . . . . . . . . . . . . 20
4.2. Pre-Shared Secret . . . . . . . . . . . . . . . . . . . . 21
4.3. Raw Public Key . . . . . . . . . . . . . . . . . . . . . 24
4.4. Certificates . . . . . . . . . . . . . . . . . . . . . . 25
5. Signature Algorithm Extension . . . . . . . . . . . . . . . . 31
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 31
7. Session Resumption . . . . . . . . . . . . . . . . . . . . . 33
8. Compression . . . . . . . . . . . . . . . . . . . . . . . . . 34
9. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . . . 34
10. Keep-Alive . . . . . . . . . . . . . . . . . . . . . . . . . 35
11. Timeouts . . . . . . . . . . . . . . . . . . . . . . . . . . 37
12. Random Number Generation . . . . . . . . . . . . . . . . . . 38
13. Truncated MAC and Encrypt-then-MAC Extension . . . . . . . . 39
14. Server Name Indication (SNI) . . . . . . . . . . . . . . . . 39
15. Maximum Fragment Length Negotiation . . . . . . . . . . . . . 40
16. Session Hash . . . . . . . . . . . . . . . . . . . . . . . . 40
17. Re-Negotiation Attacks . . . . . . . . . . . . . . . . . . . 40
18. Downgrading Attacks . . . . . . . . . . . . . . . . . . . . . 41
19. Crypto Agility . . . . . . . . . . . . . . . . . . . . . . . 41
20. Key Length Recommendations . . . . . . . . . . . . . . . . . 43
21. False Start . . . . . . . . . . . . . . . . . . . . . . . . . 44
22. Privacy Considerations . . . . . . . . . . . . . . . . . . . 44
23. Security Considerations . . . . . . . . . . . . . . . . . . . 45
24. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
25. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 46
26. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
26.1. Normative References . . . . . . . . . . . . . . . . . . 46
26.2. Informative References . . . . . . . . . . . . . . . . . 48
Appendix A. Conveying DTLS over SMS . . . . . . . . . . . . . . 54
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A.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 54
A.2. Message Segmentation and Re-Assembly . . . . . . . . . . 55
A.3. Multiplexing Security Associations . . . . . . . . . . . 56
A.4. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 57
Appendix B. DTLS Record Layer Per-Packet Overhead . . . . . . . 57
Appendix C. DTLS Fragmentation . . . . . . . . . . . . . . . . . 58
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 59
1. Introduction
An engineer developing an Internet of Things (IoT) device needs to
investigate the security threats and decide about the security
services that can be used to mitigate these threats.
Enabling IoT devices to exchange data often requires authentication
of the two endpoints and the ability to provide integrity- and
confidentiality-protection of exchanged data. While these security
services can be provided at different layers in the protocol stack,
the use of Transport Layer Security (TLS)/Datagram TLS (DTLS) has
been very popular with many application protocols and it is likely to
be useful for IoT scenarios as well.
Fitting Internet protocols into constrained devices can be difficult
but thanks to the standardization efforts new profiles and protocols
are available, such as the Constrained Application Protocol (CoAP)
[RFC7252]. CoAP messages are mainly carried over UDP/DTLS, but other
transports can be utilized, such as SMS (as described in Appendix A)
or TCP (as currently being proposed with
[I-D.tschofenig-core-coap-tcp-tls]).
While the main goal for this document is to protect CoAP messages
using DTLS 1.2 [RFC6347], the information contained in the following
sections is not limited to CoAP nor to DTLS itself.
Instead, this document defines a profile of DTLS 1.2 [RFC6347] and
TLS 1.2 [RFC5246] that offers communication security services for IoT
applications and is reasonably implementable on many constrained
devices. Profile thereby means that available configuration options
and protocol extensions are utilized to best support the IoT
environment. This document does not alter TLS/DTLS specifications
and does not introduce any new TLS/DTLS extension.
The main target audience for this document is the embedded system
developer configuring and using a TLS/DTLS stack. This document may,
however, also help those developing or selecting a suitable TLS/DTLS
stack for an Internet of Things product. If you are familiar with
(D)TLS, then skip ahead to Section 4.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
This specification refers to TLS as well as DTLS and particularly to
version 1.2, which is the most recent version at the time of writing.
We refer to TLS/DTLS whenever the text is applicable to both versions
of the protocol and to TLS or DTLS when there are differences between
the two protocols. Note that TLS 1.3 is being developed but it is
not expected that this profile will "just work" due to the
significant changes being done to TLS for version 1.3.
Note that "Client" and "Server" in this document refer to TLS/DTLS
roles, where the client initiates the handshake. This does not
restrict the interaction pattern of the protocols on top of DTLS
since the record layer allows bi-directional communication. This
aspect is further described in Section 3.2.
RFC 7228 [RFC7228] introduces the notion of constrained-node
networks, which are made of small devices with severe constraints on
power, memory, and processing resources. The terms constrained
devices, and Internet of Things (IoT) devices are used
interchangeably.
The terms "Certification Authority" (CA) and "Distinguished Name"
(DN) are taken from [RFC5280]. The terms "trust anchor" and "trust
anchor store" are defined in [RFC6024] as
"A trust anchor represents an authoritative entity via a public
key and associated data. The public key is used to verify digital
signatures, and the associated data is used to constrain the types
of information for which the trust anchor is authoritative."
"A trust anchor store is a set of one or more trust anchors stored
in a device. A device may have more than one trust anchor store,
each of which may be used by one or more applications."
3. Overview
3.1. TLS and DTLS
The TLS protocol [RFC5246] provides authenticated, confidentiality-
and integrity-protected communication between two endpoints. The
protocol is composed of two layers: the Record Protocol and the
Handshaking Protocols. At the lowest level, layered on top of a
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reliable transport protocol (e.g., TCP), is the Record Protocol. It
provides connection security by using symmetric cryptography for
confidentiality, data origin authentication, and integrity
protection. The Record Protocol is used for encapsulation of various
higher-level protocols. The handshaking protocols consist of three
sub-protocols, namely the handshake protocol, the change cipher spec
protocol and the alert protocol. The handshake protocol allows the
server and client to authenticate each other and to negotiate an
encryption algorithm and cryptographic keys before the application
protocol transmits or receives data.
The design of DTLS [RFC6347] is intentionally very similar to TLS.
However, since DTLS operates on top of an unreliable datagram
transport, it must explicitly cope with the absence of reliable and
ordered delivery assumptions made by TLS. RFC 6347 explains these
differences in great detail. As a short summary, for those not
familiar with DTLS the differences are:
o An explicit sequence number and an epoch field is included in the
Record Protocol. Section 4.1 of RFC 6347 explains the processing
rules for these two new fields. The value used to compute the MAC
is the 64-bit value formed by concatenating the epoch and the
sequence number.
o Stream ciphers must not be used with DTLS. The only stream cipher
defined for TLS 1.2 is RC4 and due to cryptographic weaknesses it
is not recommended anymore even for use with TLS [RFC7465]. Note
that the term 'stream cipher' is a technical term in the TLS
specification. Section 4.7 of RFC 5246 defines stream ciphers in
TLS as follows: in stream cipher encryption, the plaintext is
exclusive-ORed with an identical amount of output generated from a
cryptographically secure keyed pseudorandom number generator.
o The TLS Handshake Protocol has been enhanced to include a
stateless cookie exchange for Denial of Service (DoS) resistance.
For this purpose a new handshake message, the HelloVerifyRequest,
was added to DTLS. This handshake message is sent by the server
and includes a stateless cookie, which is returned in a
ClientHello message back to the server. Although the exchange is
optional for the server to execute, a client implementation has to
be prepared to respond to it. Furthermore, the handshake message
format has been extended to deal with message loss, reordering,
and fragmentation.
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3.2. Communication Models
This document describes a profile of DTLS and, to be useful, it has
to make assumptions about the envisioned communication architecture.
Two communication architectures (and consequently two profiles) are
described in this document.
3.2.1. Constrained TLS/DTLS Clients
The communication architecture shown in Figure 1 assumes a unicast
communication interaction with an IoT device utilizing a constrained
TLS/DTLS client interacting with one or multiple TLS/DTLS servers.
Before a client can initiate the TLS/DTLS handshake it needs to know
the IP address of that server and what credentials to use.
Application layer protocols, such as CoAP, which is conveyed on top
of DTLS, may be configured with URIs of the endpoints to which CoAP
needs to register and publish data. This configuration information
(including non-confidential credentials, like certificates) may be
conveyed to clients as part of a firmware/software package or via a
configuration protocol. The following credential types are supported
by this profile:
o For PSK-based authentication (see Section 4.2), this includes the
paired "PSK identity" and shared secret to be used with each
server.
o For raw public key-based authentication (see Section 4.3), this
includes either the server's public key or the hash of the
server's public key.
o For certificate-based authentication (see Section 4.4), this
includes a pre-populated trust anchor store that allows the DTLS
stack to perform path validation for the certificate obtained
during the handshake with the server.
Figure 1 shows example configuration information stored at the
constrained client for use with respective servers.
This document focuses on the description of the DTLS client-side
functionality but, quite naturally, the equivalent server-side
support has to be available.
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+////////////////////////////////////+
| Configuration |
|////////////////////////////////////|
| Server A --> PSK Identity, PSK |
| |
| Server B --> Public Key (Server B),|
| Public/Private Key |
| (for Client) |
| |
| Server C --> Public/Private Key |
| (for Client) |
| Trust Anchor Store |
+------------------------------------+
oo
oooooo
o
+-----------+
|Constrained|
|TLS/DTLS |
|Client |-
+-----------+ \
\ ,-------.
,' `. +------+
/ IP-based \ |Server|
( Network ) | A |
\ / +------+
`. ,'
'---+---' +------+
| |Server|
| | B |
| +------+
|
| +------+
+----------------->|Server|
| C |
+------+
Figure 1: Constrained Client Profile.
3.2.1.1. Examples of Constrained Client Exchanges
3.2.1.1.1. Network Access Authentication Example
Re-use is a recurring theme when considering constrained environments
and is behind a lot of the directions taken in developments for
constrained environments. The corollary of re-use is to not add
functionality if it can be avoided. An example relevant to the use
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of TLS is network access authentication, which takes place when a
device connects to a network and needs to go through an
authentication and access control procedure before it is allowed to
communicate with other devices or connect to the Internet.
Figure 2 shows the network access architecture with the IoT device
initiating the communication to an access point in the network using
the procedures defined for a specific physical layer. Since
credentials may be managed and stored centrally, in the
Authentication, Authorization, and Accounting (AAA) server, the
security protocol exchange may need to be relayed via the
Authenticator, i.e., functionality running on the access point, to
the AAA server. The authentication and key exchange protocol itself
is encapsulated within a container, the Extensible Authentication
Protocol (EAP) [RFC3748], and messages are conveyed back and forth
between the EAP endpoints, namely the EAP peer located on the IoT
device and the EAP server located on the AAA server or the access
point. To route EAP messages from the access point, acting as a AAA
client, to the AAA server requires an adequate protocol mechanism,
namely RADIUS [RFC2865] or Diameter [RFC6733].
More details about the concepts and a description about the
terminology can be found in RFC 5247 [RFC5247].
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+--------------+
|Authentication|
|Authorization |
|Accounting |
|Server |
|(EAP Server) |
| |
+-^----------^-+
* EAP o RADIUS/
* o Diameter
--v----------v--
/// \\\
// \\
| Federation |
| Substrate |
\\ //
\\\ ///
--^----------^--
* EAP o RADIUS/
* o Diameter
+-------------+ +-v----------v--+
| | EAP/EAP Method | |
| Internet of |<***************************>| Access Point |
| Things | |(Authenticator)|
| Device | EAP Lower Layer and |(AAA Client) |
| (EAP Peer) | Secure Association Protocol | |
| |<--------------------------->| |
| | | |
| | Physical Layer | |
| |<===========================>| |
+-------------+ +---------------+
Legend:
<****>: Device-to-AAA Server Exchange
<---->: Device-to-Authenticator Exchange
<oooo>: AAA Client-to-AAA Server Exchange
<====>: Physical layer like IEEE 802.11/802.15.4
Figure 2: Network Access Architecture.
One standardized EAP method is EAP-TLS, defined in RFC 5216
[RFC5216], which re-uses the TLS-based protocol exchange and
encapsulates it inside the EAP payload. In terms of re-use this
allows many components of the TLS protocol to be shared between the
network access security functionality and the TLS functionality
needed for securing application layer traffic. In the EAP-TLS
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exchange shown in Figure 3 the IoT device as the EAP peer acts as a
TLS client.
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
TLS certificate_request,
TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
TLS certificate_verify,
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=EAP-TLS ->
<- EAP-Success
Figure 3: EAP-TLS Exchange.
The guidance in this document also applies to the use of EAP-TLS for
network access authentication. An IoT device using a network access
authentication solution based on TLS can re-use most parts of the
code for the use of DTLS/TLS at the application layer thereby saving
a significant amount of flash memory. Note, however, that the
credentials used for network access authentication and those used for
application layer security are very likely different.
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3.2.1.1.2. CoAP-based Data Exchange Example
When a constrained client uploads sensor data to a server
infrastructure it may use CoAP by pushing the data via a POST message
to a pre-configured endpoint on the server. In certain circumstances
this might be too limiting and additional functionality is needed, as
shown in Figure 4 and Figure 5, where the IoT device itself runs a
CoAP server hosting the resource that is made accessible to other
entities. Despite running a CoAP server on the IoT device it is
still the DTLS client on the IoT device that initiates the
interaction with the non-constrained resource server in our scenario.
Figure 4 shows a sensor starting a DTLS exchange with a resource
directory and uses CoAP to register available resources in Figure 5.
[I-D.ietf-core-resource-directory] defines the resource directory
(RD) as a web entity that stores information about web resources and
implements Representational State Transfer (REST) interfaces for
registration and lookup of those resources. Note that the described
exchange is borrowed from the OMA Lightweight Machine-to-Machine
(LWM2M) specification [LWM2M] that uses RD but adds proxy
functionality.
The initial DTLS interaction between the sensor, acting as a DTLS
client, and the resource directory, acting as a DTLS server, will be
a full DTLS handshake. Once this handshake is complete both parties
have established the DTLS record layer. Subsequently, the CoAP
client can securely register at the resource directory.
After some time (assuming that the client regularly refreshes its
registration) the resource directory receives a request from an
application to retrieve the temperature information from the sensor.
This request is relayed by the resource directory to the sensor using
a GET message exchange. The already established DTLS record layer
can be used to secure the message exchange.
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Resource
Sensor Directory
------ ---------
+---
|
| ClientHello -------->
| #client_certificate_type#
F| #server_certificate_type#
U|
L| <------- HelloVerifyRequest
L|
| ClientHello -------->
D| #client_certificate_type#
T| #server_certificate_type#
L|
S| ServerHello
| #client_certificate_type#
H| #server_certificate_type#
A| Certificate
N| ServerKeyExchange
D| CertificateRequest
S| <-------- ServerHelloDone
H|
A| Certificate
K| ClientKeyExchange
E| CertificateVerify
| [ChangeCipherSpec]
| Finished -------->
|
| [ChangeCipherSpec]
| <-------- Finished
+---
Note: Extensions marked with '#' were introduced with
RFC 7250.
Figure 4: DTLS/CoAP exchange using Resource Directory: Part 1 - DTLS
Handshake.
Figure 5 shows the DTLS-secured communication between the sensor and
the resource directory using CoAP.
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Resource
Sensor Directory
------ ---------
[[==============DTLS-secured Communication===================]]
+--- ///+
C| \ D
O| Req: POST coap://rd.example.com/rd?ep=node1 \ T
A| Payload: \ L
P| </temp>;ct=41; \ S
| rt="temperature-c";if="sensor", \
R| </light>;ct=41; \ R
D| rt="light-lux";if="sensor" \ E
| --------> \ C
R| \ O
E| \ R
G| Res: 2.01 Created \ D
.| <-------- Location: /rd/4521 \
| \ L
+--- \ A
\ Y
* \ E
* (time passes) \ R
* \
+--- \ P
C| \ R
O| Req: GET coaps://sensor.example.com/temp \ O
A| <-------- \ T
P| \ E
| Res: 2.05 Content \ C
G| Payload: \ T
E| 25.5 --------> \ E
T| \ D
+--- ///+
Figure 5: DTLS/CoAP exchange using Resource Directory: Part 2 - CoAP/
RD Exchange.
Note that the CoAP GET message transmitted from the Resource Server
is protected using the previously established DTLS Record Layer.
3.2.2. Constrained TLS/DTLS Servers
Section 3.2.1 illustrates a deployment model where the TLS/DTLS
client is constrained and efforts need to be taken to improve memory
utilization, bandwidth consumption, reduce performance impacts, etc.
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In this section, we assume a scenario where constrained devices run
TLS/DTLS servers to secure access to application layer services
running on top of CoAP, HTTP or other protocols. Figure 6
illustrates a possible deployment whereby a number of constrained
servers are waiting for regular clients to access their resources.
The entire process is likely, but not necessarily, controlled by a
third party, the authentication and authorization server. This
authentication and authorization server is responsible for holding
authorization policies that govern the access to resources and
distribution of keying material.
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+////////////////////////////////////+
| Configuration |
|////////////////////////////////////|
| Credentials |
| Client A -> Public Key |
| Server S1 -> Symmetric Key |
| Server S2 -> Certificate |
| Server S3 -> Public Key |
| Trust Anchor Store |
| Access Control Lists |
| Resource X: Client A / GET |
| Resource Y: Client A / PUT |
+------------------------------------+
oo
oooooo
o
+---------------+ +-----------+
|Authentication | +-------->|TLS/DTLS |
|& Authorization| | |Client A |
|Server | | +-----------+
+---------------+ ++
^ | +-----------+
\ | |Constrained|
\ ,-------. | Server S1 |
,' `. +-----------+
/ Local \
( Network )
\ / +-----------+
`. ,' |Constrained|
'---+---' | Server S2 |
| +-----------+
|
| +-----------+
+-----------------> |Constrained|
| Server S3 |
+-----------+
Figure 6: Constrained Server Profile.
A deployment with constrained servers has to overcome several
challenges. Below we explain how these challenges can be solved with
CoAP, as an example. Other protocols may offer similar capabilities.
While the requirements for the TLS/DTLS protocol profile change only
slightly when run on a constrained server (in comparison to running
it on a constrained client) several other eco-system factors will
impact deployment.
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There are several challenges that need to be addressed:
Discovery and Reachability:
A client must first and foremost discover the server before
initiating a connection to it. Once it has been discovered,
reachability to the device needs to be maintained.
In CoAP the discovery of resources offered by servers is
accomplished by sending a unicast or multicast CoAP GET to a well-
known URI. The CORE Link format specification [RFC6690] describes
the use case (see Section 1.2.1), and reserves the URI (see
Section 7.1). Section 7 of the CoAP specification [RFC7252]
describes the discovery procedure. [RFC7390] describes use case
for discovering CoAP servers using multicast (see Section 3.3),
and specifies the protocol processing rules for CoAP group
communications (see Section 2.7).
The use of Resource Directory (RD)
[I-D.ietf-core-resource-directory] is yet another possibility for
discovering registered servers and their resources. Since RD is
usually not a proxy, clients can discover links registered with
the RD and then access them directly.
Authentication:
The next challenge concerns the provisioning of authentication
credentials to the clients as well as servers. In Section 3.2.1
we assumed that credentials (and other configuration information)
are provisioned to the device and that those can be used with the
authorization servers. Of course, this leads to a very static
relationship between the clients and their server-side
infrastructure but poses fewer challenges from a deployment point
of view, as described in Section 2 of [RFC7452]. In any case,
engineers and product designers have to determine how the relevant
credentials are distributed to the respective parties. For
example, shared secrets may need to be provisioned to clients and
the constrained servers for subsequent use of TLS/DTLS PSK. In
other deployments, certificates, private keys, and trust anchors
for use with certificate-based authentication may need to be
utilized.
Practical solutions either use pairing (also called imprinting) or
a trusted third party. With pairing two devices execute a special
protocol exchange that is unauthenticated to establish a shared
key (for example using an unauthenticated Diffie-Hellman
exchange). To avoid man-in-the-middle attacks an out-of-band
channel is used to verify that nobody has tampered with the
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exchanged protocol messages. This out-of-band channel can come in
many forms, including:
* Human involvement by comparing hashed keys, entering passkeys,
scanning QR codes
* The use of alternative wireless communication channels (e.g.,
infra-red communication in addition to WiFi)
* Proximity-based information
More details about these different pairing/imprinting techniques
can be found in the smart object security workshop report
[RFC7397] and various position papers submitted on that topic,
such as [ImprintingSurvey]. The use of a trusted third party
follows a different approach and is subject to ongoing
standardization efforts in the 'Authentication and Authorization
for Constrained Environments (ACE)' working group [ACE-WG].
Authorization
The last challenge is the ability for the constrained server to
make an authorization decision when clients access protected
resources. Pre-provisioning access control information to
constrained servers may be one option but works only in a small
scale, less dynamic environment. For a finer-grained and more
dynamic access control solution the reader is referred to the
ongoing work in the IETF ACE working group.
Figure 7 shows an example interaction whereby a device, a thermostat
in our case, searches in the local network for discoverable resources
and accesses those. The thermostat starts the procedure using a
link-local discovery message using the "All CoAP Nodes" multicast
address by utilizing the RFC 6690 [RFC6690] link format. The IPv6
multicast address used for CoAP link-local discovery is FF02::FD. As
a result, a temperature sensor and a fan respond. These responses
allow the thermostat to subsequently read temperature information
from the temperature sensor with a CoAP GET request issued to the
previously learned endpoint. In this example we assume that
accessing the temperature sensor readings and controlling the fan
requires authentication and authorization of the thermostat and TLS
is used to authenticate both endpoints and to secure the
communication.
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Temperature
Thermostat Sensor Fan
---------- --------- ---
Discovery
-------------------->
GET coap://[FF02::FD]/.well-known/core
CoAP 2.05 Content
<-------------------------------
</3303/0/5700>;rt="temperature";
if="sensor"
CoAP 2.05 Content
<--------------------------------------------------
</fan>;rt="fan";if="actuation"
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
\ Protocol steps to obtain access token or keying /
\ material for access to the temperature sensor and fan. /
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
Read Sensor Data
(authenticated/authorized)
------------------------------->
GET /3303/0/5700
CoAP 2.05 Content
<-------------------------------
22.5 C
Configure Actuator
(authenticated/authorized)
------------------------------------------------->
PUT /fan?on-off=true
CoAP 2.04 Changed
<-------------------------------------------------
Figure 7: Local Discovery and Resource Access.
3.3. The Ciphersuite Concept
TLS (and consequently DTLS) has the concept of ciphersuites and an
IANA registry [IANA-TLS] was created to register the suites. A
ciphersuite (and the specification that defines it) contains the
following information:
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o Authentication and key exchange algorithm (e.g., PSK)
o Cipher and key length (e.g., Advanced Encryption Standard (AES)
with 128 bit keys [AES])
o Mode of operation (e.g., Counter with Cipher Block Chaining -
Message Authentication Code (CBC-MAC) Mode (CCM) for AES)
[RFC3610]
o Hash algorithm for integrity protection, such as the Secure Hash
Algorithm (SHA) in combination with Keyed-Hashing for Message
Authentication (HMAC) (see [RFC2104] and [RFC6234])
o Hash algorithm for use with pseudorandom functions (e.g., HMAC
with the SHA-256)
o Misc information (e.g., length of authentication tags)
o Information whether the ciphersuite is suitable for DTLS or only
for TLS
The TLS ciphersuite TLS_PSK_WITH_AES_128_CCM_8, for example, uses a
pre-shared authentication and key exchange algorithm. [RFC6655]
defines this ciphersuite. It uses the Advanced Encryption Standard
(AES) encryption algorithm, which is a block cipher. Since the AES
algorithm supports different key lengths (such as 128, 192 and 256
bits) this information has to be specified as well and the selected
ciphersuite supports 128 bit keys. A block cipher encrypts plaintext
in fixed-size blocks and AES operates on fixed block size of 128
bits. For messages exceeding 128 bits, the message is partitioned
into 128-bit blocks and the AES cipher is applied to these input
blocks with appropriate chaining, which is called mode of operation.
TLS 1.2 introduced Authenticated Encryption with Associated Data
(AEAD) ciphersuites (see [RFC5116] and [RFC6655]). AEAD is a class
of block cipher modes which encrypt (parts of) the message and
authenticate the message simultaneously. Examples of such modes
include the Counter with Cipher Block Chaining - Message
Authentication Code (CBC-MAC) Mode (CCM) mode, and the Galois/Counter
Mode (GCM) (see [RFC5288] and [RFC7251]).
Some AEAD ciphersuites have shorter authentication tags (i.e.,
message authentication codes) and are therefore more suitable for
networks with low bandwidth where small message size matters. The
TLS_PSK_WITH_AES_128_CCM_8 ciphersuite that ends in "_8" has an
8-octet authentication tag, while the regular CCM ciphersuites have,
at the time of writing, 16-octet authentication tags. The design of
CCM and the security properties are described in [CCM].
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TLS 1.2 also replaced the combination of MD5/SHA-1 hash functions in
the TLS pseudo random function (PRF) used in earlier versions of TLS
with cipher-suite-specified PRFs. For this reason authors of more
recent TLS 1.2 ciphersuite specifications explicitly indicate the MAC
algorithm and the hash functions used with the TLS PRF.
4. Credential Types
The mandatory-to-implement functionality will depend on the
credential type used with IoT devices. The sub-sections below
describe the implications of three different credential types, namely
pre-shared secrets, raw public keys, and certificates.
4.1. Pre-Conditions
All exchanges described in the subsequent sections assume that some
information has been distributed before the TLS/DTLS interaction can
start. The credentials are used to authenticate the client to the
server and vice versa. What information items have to be distributed
depends on the chosen credential types. In all cases the IoT device
needs to know what algorithms to prefer, particularly if there are
multiple algorithms choices available as part of the implemented
ciphersuites, as well as information about the other communication
endpoint (for example in form of a URI) a particular credential has
to be used with.
Pre-Shared Secrets: In this case a shared secret together with an
identifier needs to be made available to the device as well as to
the other communication party.
Raw Public Keys: A public key together with a private key are stored
on the device and typically associated with some identifier. To
authenticate the other communication party the appropriate
credential has to be known. If the other end uses raw public keys
as well then their public key needs to be provisioned (out-of-
band) to the device.
Certificates The use of certificates requires the device to store
the public key (as part of the certificate) as well as the private
key. The certificate will contain the identifier of the device as
well as various other attributes. Both communication parties are
assumed to be in possession of a trust anchor store that contains
CA certificates and, in case of certificate pinning, end-entity
certificates. Similarly to the other credentials the IoT device
needs information about which entity to use which certificate
with. Without a trust anchor store on the IoT device it will not
be possible to perform certificate validation.
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We call the above-listed information device credentials and these
device credentials may be provisioned to the device already during
the manufacturing time or later in the process, depending on the
envisioned business and deployment model. These initial credentials
are often called 'root of trust'. Whatever process for generating
these initial device credential is chosen it MUST be ensured that a
different key pair is provisioned for each device and installed in
as-secure a manner as possible. For example, it is preferable to
generate public / private keys on the IoT device itself rather than
generating them outside the device. Since an IoT device is likely to
interact with various other parties the initial device credential may
only be used with some dedicated entities and configuring further
configuration and credentials to the device is left to a separate
interaction. An example of a dedicated protocol used to distribute
credentials, access control lists and configuration information is
the Lightweight Machine-to-Machine (LWM2M) protocol [LWM2M].
For all the credentials listed above there is a chance that those may
need to be replaced or deleted. While separate protocols have been
developed to check the status of these credentials and to manage
these credentials, such as the Trust Anchor Management Protocol
(TAMP) [RFC5934], their usage is, however, not envisioned in the IoT
context so far. IoT devices are assumed to have a software update
mechanism built-in and it will allow updates of low-level device
information, including credentials and configuration parameters.
This document does, however, not mandate a specific software /
firmware update protocol.
With all credentials used as input to TLS/DTLS authentication it is
important that these credentials have been generated with care. When
using a pre-shared secret, a critical consideration is use sufficient
entropy during the key generation, as discussed in [RFC4086].
Deriving a shared secret from a password, some device identifiers, or
other low-entropy source is not secure. A low-entropy secret, or
password, is subject to dictionary attacks. Attention also has to be
paid when generating public / private key pairs since the lack of
randomness can result in the same key pair being used in many
devices. This topic is also discussed in Section 12 since keys are
generated during the TLS/DTLS exchange itself as well and the same
considerations apply.
4.2. Pre-Shared Secret
The use of pre-shared secrets is one of the most basic techniques for
TLS/DTLS since it is both computationally efficient and bandwidth
conserving. Pre-shared secret based authentication was introduced to
TLS with RFC 4279 [RFC4279].
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The exchange shown in Figure 8 illustrates the DTLS exchange
including the cookie exchange. While the server is not required to
initiate a cookie exchange with every handshake, the client is
required to implement and to react on it when challenged, as defined
in RFC 6347 [RFC6347]. The cookie exchange allows the server to
react to flooding attacks.
Client Server
------ ------
ClientHello -------->
<-------- HelloVerifyRequest
(contains cookie)
ClientHello -------->
(with cookie)
ServerHello
*ServerKeyExchange
<-------- ServerHelloDone
ClientKeyExchange
ChangeCipherSpec
Finished -------->
ChangeCipherSpec
<-------- Finished
Application Data <-------> Application Data
Legend:
* indicates an optional message payload
Figure 8: DTLS PSK Authentication including the Cookie Exchange.
Note that [RFC4279] used the term PSK identity to refer to the
identifier used to refer to the appropriate secret. While
'identifier' would be more appropriate in this context we re-use the
terminology defined in RFC 4279 to avoid confusion. RFC 4279 does
not mandate the use of any particular type of PSK identity and the
client and server have to agree on the identities and keys to be
used. The UTF-8 encoding of identities described in Section 5.1 of
RFC 4279 aims to improve interoperability for those cases where the
identity is configured by a human using some management interface
provided by a Web browser. However, many IoT devices do not have a
user interface and most of their credentials are bound to the device
rather than to the user. Furthermore, credentials are often
provisioned into hardware modules or provisioned alongside with
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firmware. As such, the encoding considerations are not applicable to
this usage environment. For use with this profile the PSK identities
SHOULD NOT assume a structured format (such as domain names,
Distinguished Names, or IP addresses) and a byte-by-byte comparison
operation MUST be used by the server for any operation related to the
PSK identity. RFC 6943 [RFC6943] calls these types of identifiers
"absolute".
Protocol-wise the client indicates which key it uses by including a
"PSK identity" in the ClientKeyExchange message. As described in
Section 3.2 clients may have multiple pre-shared keys with a single
server, for example in a hosting context. The TLS Server Name
Indication (SNI) extension allows the client to convey the name of
the server it is contacting. A server implementation needs to guide
the selection based on a received SNI value from the client.
RFC 4279 requires TLS implementations supporting PSK ciphersuites to
support arbitrary PSK identities up to 128 octets in length, and
arbitrary PSKs up to 64 octets in length. This is a useful
assumption for TLS stacks used in the desktop and mobile environments
where management interfaces are used to provision identities and
keys. Implementations in compliance with this profile MAY use PSK
identities up to 128 octets in length, and arbitrary PSKs up to 64
octets in length. The use of shorter PSK identities is RECOMMENDED.
Constrained Application Protocol (CoAP) [RFC7252] currently specifies
TLS_PSK_WITH_AES_128_CCM_8 as the mandatory to implement ciphersuite
for use with shared secrets. This ciphersuite uses the AES algorithm
with 128 bit keys and CCM as the mode of operation. The label "_8"
indicates that an 8-octet authentication tag is used. Note that the
shorted authentication tag increases the chance that an adversary
with no knowledge of the secret key can present a message with a MAC
that will pass the verification procedure. The likelihoods of
accepting forged data is explained in Section 5.3.5 of
[SP800-107-rev1] and depends on the lengths of the authentication tag
and allowed numbers of MAC verifications using a given key.
This ciphersuite makes use of the default TLS 1.2 Pseudorandom
Function (PRF), which uses an HMAC with the SHA-256 hash function.
Note: Starting with TLS 1.2 (and consequently DTLS 1.2) ciphersuites
have to specify the pseudorandom function. RFC 5246 states that 'New
cipher suites MUST explicitly specify a PRF and, in general, SHOULD
use the TLS PRF with SHA-256 or a stronger standard hash function.'.
The ciphersuites recommended in this document use the SHA-256
construct defined in Section 5 of RFC 5246.
A device compliant with the profile in this section MUST implement
TLS_PSK_WITH_AES_128_CCM_8 and follow the guidance from this section.
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4.3. Raw Public Key
The use of raw public keys with TLS/DTLS, as defined in [RFC7250], is
the first entry point into public key cryptography without having to
pay the price of certificates and a public key infrastructure (PKI).
The specification re-uses the existing Certificate message to convey
the raw public key encoded in the SubjectPublicKeyInfo structure. To
indicate support two new extensions had been defined, as shown in
Figure 9, namely the server_certificate_type and the
client_certificate_type.
Client Server
------ ------
ClientHello -------->
#client_certificate_type#
#server_certificate_type#
ServerHello
#client_certificate_type#
#server_certificate_type#
Certificate
ServerKeyExchange
CertificateRequest
<-------- ServerHelloDone
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Note: Extensions marked with '#' were introduced with
RFC 7250.
Figure 9: DTLS Raw Public Key Exchange.
The CoAP recommended ciphersuite for use with this credential type is
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251]. This elliptic curve
cryptography (ECC) based AES-CCM TLS ciphersuite uses the Ephemeral
Elliptic Curve Diffie-Hellman (ECDHE) as the key establishment
mechanism and an Elliptic Curve Digital Signature Algorithm (ECDSA)
for authentication. The named Diffie-Hellman groups
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[I-D.ietf-tls-negotiated-dl-dhe] are not applicable to this profile
since it relies on the ECC-based counterparts. This ciphersuite
makes use of the AEAD capability in DTLS 1.2 and utilizes an eight-
octet authentication tag. The use of a Diffie-Hellman key exchange
provides perfect forward secrecy (PFS). More details about PFS can
be found in Section 9.
[RFC6090] provides valuable information for implementing Elliptic
Curve Cryptography algorithms, particularly for choosing methods that
have been available in the literature for a long time (i.e., 20 years
and more).
A device compliant with the profile in this section MUST implement
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 and follow the guidance from this
section.
4.4. Certificates
The use of mutual certificate-based authentication is shown in
Figure 10, which makes use of the cached info extension
[I-D.ietf-tls-cached-info]. Support of the cached info extension is
REQUIRED. Caching certificate chains allows the client to reduce the
communication overhead significantly since otherwise the server would
provide the end entity certificate, and the certificate chain with
every full DTLS handshake.
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Client Server
------ ------
ClientHello -------->
*cached_info*
ServerHello
*cached_info*
Certificate
ServerKeyExchange
CertificateRequest
<-------- ServerHelloDone
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Note: Extensions marked with '*' were introduced with
[I-D.ietf-tls-cached-info].
Figure 10: DTLS Mutual Certificate-based Authentication.
TLS/DTLS offers a lot of choices when selecting ECC-based
ciphersuites. This document restricts the use to named curves
defined in RFC 4492 [RFC4492]. At the time of writing the
recommended curve is secp256r1 and the use of uncompressed points to
follow the recommendation in CoAP. Note that standardization for
Curve25519 (for ECDHE) is ongoing (see [I-D.irtf-cfrg-curves]) and
support for this curve will likely be required in the future.
A device compliant with the profile in this section MUST implement
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 and follow the guidance from this
section.
4.4.1. Certificates used by Servers
The algorithm for verifying the service identity, as described in RFC
6125 [RFC6125], is essential for ensuring proper security when
certificates are used. As a summary, the algorithm contains the
following steps:
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1. The client constructs a list of acceptable reference identifiers
based on the source domain and, optionally, the type of service
to which the client is connecting.
2. The server provides its identifiers in the form of a PKIX
certificate.
3. The client checks each of its reference identifiers against the
presented identifiers for the purpose of finding a match.
4. When checking a reference identifier against a presented
identifier, the client matches the source domain of the
identifiers and, optionally, their application service type.
For various terms used in the algorithm shown above consult RFC 6125.
It is important to highlight that comparing the reference identifier
against the presented identifier obtained from the certificate is
required to ensure the client is communicating with the intended
server.
It is worth noting that the algorithm description and the text in RFC
6125 assumes that fully qualified DNS domain names are used. If a
server node is provisioned with a fully qualified DNS domain then the
server certificate MUST contain the fully qualified DNS domain name
or "FQDN" as dNSName [RFC5280]. For CoAP, the coaps URI scheme is
described in Section 6.2 of [RFC7252]. This FQDN is stored in the
SubjectAltName or in the leftmost CN component of subject name, as
explained in Section 9.1.3.3 of [RFC7252], and used by the client to
match it against the FQDN used during the look-up process, as
described in [RFC6125]. For other protocols, the appropriate URI
scheme specification has to be consulted.
The following recommendation is provided:
1. Certificates MUST NOT use DNS domain names in the Common Name of
certificates and instead use the subjectAltName attribute, as
described in the previous paragraph.
2. Certificates MUST NOT contain domain names with wildcard
characters.
3. Certificates MUST NOT contain multiple names (e.g., more than one
dNSName field).
Note that there will be servers that are not provisioned for use with
DNS domain names, for example, IoT devices that offer resources to
nearby devices in a local area network, as shown in Figure 7. When
such constrained servers are used then the use of certificates as
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described in Section 4.4.2 is applicable. Note that the Service Name
Indication (SNI) extension cannot be used in this case since SNI does
not offer the ability to convey EUI-64 [EUI64] identifiers. Note
that this document does not recommend to use IP addresses in
certificates nor does it discuss the implications of placing IP
addresses in certificates.
4.4.2. Certificates used by Clients
For client certificates the identifier used in the SubjectAltName or
in the leftmost CN component of subject name MUST be an EUI-64.
4.4.3. Certificate Revocation
For certificate revocation neither the Online Certificate Status
Protocol (OCSP) nor Certificate Revocation Lists (CRLs) are used.
Instead, this profile relies on a software update mechanism to
provision information about revoked certificates. While multiple
OCSP stapling [RFC6961] has recently been introduced as a mechanism
to piggyback OCSP request/responses inside the DTLS/TLS handshake (to
avoid the cost of a separate protocol handshake), further
investigations are needed to determine its suitability for the IoT
environment.
As stated earlier in this section, modifications to the trust anchor
store depends on a software update mechanism as well. There are
limitations to the use of a software update mechanism because of the
potential inability to change certain types of keys, such as those
provisioned during manufacturing. For this reason, manufacturer
provisioned credentials are typically employed only to obtain further
certificates (for example via a key distribution server) for use with
servers the IoT device is finally communicating with.
4.4.4. Certificate Content
All certificate elements listed in Table 1 MUST be implemented by
clients and servers claiming support for certificate-based
authentication. No other certificate elements are used by this
specification.
When using certificates, IoT devices MUST provide support for a
server certificate chain of at least 3 not including the trust anchor
and MAY reject connections from servers offering chains longer than
3. IoT devices MAY have client certificate chains of any length.
Obviously, longer chains require more digital signature verification
operations to perform and lead to larger certificate messages in the
TLS handshake.
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Table 1 provides a summary of the elements in a certificate for use
with this profile.
+----------------------+--------------------------------------------+
| Element | Notes |
+----------------------+--------------------------------------------+
| version | This profile uses X.509 v3 certificates |
| | [RFC5280]. |
| | |
| serialNumber | Positive integer unique per certificate. |
| | |
| signature | This field contains the signature |
| | algorithm and this profile uses ecdsa- |
| | with-SHA256 or stronger [RFC5758]. |
| | |
| issuer | Contains the DN of the issuing CA. |
| | |
| validity | Values expressed as UTC time in notBefore |
| | and notAfter fields. No validity period |
| | mandated. |
| | |
| subject | See rules outlined in this section. |
| | |
| subjectPublicKeyInfo | The SubjectPublicKeyInfo structure |
| | indicates the algorithm and any associated |
| | parameters for the ECC public key.This |
| | profile uses the id-ecPublicKey algorithm |
| | identifier for ECDSA signature keys, as |
| | defined in specified in [RFC5480]. |
| | |
| signatureAlgorithm | The ECDSA signature algorithm with ecdsa- |
| | with-SHA256 or stronger. |
| | |
| signatureValue | Bit string containing the digital |
| | signature. |
| | |
| Extension: | See rules outlined in this section. |
| subjectAltName | |
| | |
| Extension: | Indicates whether the subject of the |
| BasicConstraints | certificate is a CA and the maximum depth |
| | of valid certification paths that include |
| | this certificate. This extension is used |
| | for CA certs only and then the value of |
| | the 'cA' field is set to TRUE. The default |
| | is FALSE. |
| | |
| Extension: Key Usage | The KeyUsage field MAY have the following |
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| | values in the context of this profile: |
| | digitalSignature or keyAgreement, |
| | keyCertSign for verifying signatures on |
| | public key certificates. |
| | |
| Extension: Extended | The ExtKeyUsageSyntax field MAY have the |
| Key Usage | following values in context of this |
| | profile: id-kp-serverAuth for server |
| | authentication, id-kp-clientAuth for |
| | client authentication, id-kp-codeSigning |
| | for code signing (for software update |
| | mechanism), id-kp-OCSPSigning for future |
| | OCSP usage in TLS. |
+----------------------+--------------------------------------------+
Table 1: Certificate Content.
There are various cryptographic algorithms available to sign digital
certificates; those algorithms include RSA, the Digital Signature
Algorithm (DSA), and the Elliptic Curve Digital Signature Algorithm
(ECDSA). As Table 1 shows, in this profile certificates are signed
using ECDSA. This is not only true for the end-entity certificates
but also for all other certificates in the chain, including CA
certificates. This profiling reduces the amount of flash memory
needed on IoT device to store the code of several algorithm
implementations due to the smaller number of options.
Further details about X.509 certificates can be found in
Section 9.1.3.3 of [RFC7252].
4.4.5. Client Certificate URLs
RFC 6066 [RFC6066] allows to avoid sending client-side certificates
and uses URLs instead. This reduces the over-the-air transmission.
Note that the TLS cached info extension does not provide any help
with caching client certificates.
TLS/DTLS clients MUST implement support for client certificate URLs
for those environments where client-side certificates are used and
the server-side is not constrained. For constrained servers this
functionality is NOT RECOMMENDED since it forces the server to
execute an additional protocol exchange, potentially using a protocol
it does not even support. The use of this extension also increases
the risk of a denial of service attack against the constrained server
due to the additional workload.
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4.4.6. Trusted CA Indication
RFC 6066 [RFC6066] allows clients to indicate what trust anchor they
support. With certificate-based authentication, a DTLS server
conveys its end entity certificate to the client during the DTLS
handshake. Since the server does not necessarily know what trust
anchors the client has stored, to facilitate certification path
construction and validation, it includes intermediate CA certs in the
certificate payload.
Today, in most IoT deployments there is a fairly static relationship
between the IoT device (and the software running on them) and the
server-side infrastructure. For these deployments where IoT devices
interact with a fixed, pre-configured set of servers this extension
is NOT RECOMMENDED.
In cases where client interact with dynamically discovered TLS/DTLS
servers, for example in the use cases described in Section 3.2.2, the
use of this extension is RECOMMENDED.
5. Signature Algorithm Extension
The "signature_algorithms" extension, defined in Section 7.4.1.4.1 of
RFC 5246 [RFC5246], allows the client to indicate to the server which
signature/hash algorithm pairs may be used in digital signatures.
The client MUST send this extension to select the use of SHA-256
since otherwise absent this extension RFC 5246 defaults to SHA-1 /
ECDSA for the ECDH_ECDSA and the ECDHE_ECDSA key exchange algorithms.
The "signature_algorithms" extension is not applicable to the PSK-
based ciphersuite described in Section 4.2.
6. Error Handling
TLS/DTLS uses the Alert protocol to convey errors and specifies a
long list of error types. However, not all error messages defined in
the TLS/DTLS specification are applicable to this profile. In
general, there are two categories of errors (as defined in
Section 7.2 of RFC 5246), namely fatal errors and warnings. Alert
messages with a level of fatal result in the immediate termination of
the connection. If possible, developers should try to develop
strategies to react to those fatal errors, such as re-starting the
handshake or informing the user using the (often limited) user
interface. Warnings may be ignored by the application since many IoT
devices will either have limited ways to log errors or no ability at
all. In any case, implementers have to carefully evaluate the impact
of errors and ways to remedy the situation since a commonly used
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approach for delegating decision making to users is difficult (or
impossible) to accomplish in a timely fashion.
All error messages marked as RESERVED are only supported for
backwards compatibility with SSL and MUST NOT be used with this
profile. Those include decryption_failed_RESERVED,
no_certificate_RESERVED, and export_restriction_RESERVED.
A number of the error messages MUST only be used for certificate-
based ciphersuites. Hence, the following error messages MUST NOT be
used with with PSK and raw public key authentication:
o bad_certificate,
o unsupported_certificate,
o certificate_revoked,
o certificate_expired,
o certificate_unknown,
o unknown_ca, and
o access_denied.
Since this profile does not make use of compression at the TLS layer
the decompression_failure error message MUST NOT be used either.
RFC 4279 introduced a new alert message unknown_psk_identity for PSK
ciphersuites. As stated in Section 2 of RFC 4279 the
decryption_error error message may also be used instead. For this
profile the TLS server MUST return the decryption_error error message
instead of the unknown_psk_identity since the two mechanisms exist
and provide the same functionality.
Furthermore, the following errors should not occur with devices and
servers supporting this specification but implementations MUST be
prepared to process these errors to deal with servers that are not
compliant to the profiles in this document:
protocol_version: While this document focuses only on one version of
the TLS/DTLS protocol, namely version 1.2, ongoing work on TLS/
DTLS 1.3 is in progress at the time of writing.
insufficient_security: This error message indicates that the server
requires ciphers to be more secure. This document specifies only
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one ciphersuite per profile but it is likely that additional
ciphersuites get added over time.
user_canceled: Many IoT devices are unattended and hence this error
message is unlikely to occur.
7. Session Resumption
Session resumption is a feature of the core TLS/DTLS specifications
that allows a client to continue with an earlier established session
state. The resulting exchange is shown in Figure 11. In addition,
the server may choose not to do a cookie exchange when a session is
resumed. Still, clients have to be prepared to do a cookie exchange
with every handshake. The cookie exchange is not shown in the
figure.
Client Server
------ ------
ClientHello -------->
ServerHello
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Figure 11: DTLS Session Resumption.
Constrained clients MUST implement session resumption to improve the
performance of the handshake. This will lead to a reduced number of
message exchanges, lower computational overhead (since only symmetric
cryptography is used during a session resumption exchange), and
session resumption requires less bandwidth.
For cases where the server is constrained (but not the client) the
client MUST implement RFC 5077 [RFC5077]. Note that the constrained
server refers to a device that has limitations in terms of RAM and
flash memory, which place restrictions on the amount of TLS/DTLS
security state information that can be stored on such a device. RFC
5077 specifies a version of TLS/DTLS session resumption that does not
require per-session state information to be maintained by the
constrained server. This is accomplished by using a ticket-based
approach.
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If both the client and the server are constrained devices both
devices SHOULD implement RFC 5077 and MUST implement basic session
resumption. Clients that do not want to use session resumption are
always able to send a ClientHello message with an empty session_id to
revert to a full handshake.
8. Compression
Section 3.3 of [RFC7525] recommends to disable TLS/DTLS-level
compression due to attacks, such as CRIME [CRIME]. For IoT
applications compression at the TLS/DTLS layer is not needed since
application layer protocols are highly optimized and the compression
algorithms at the DTLS layer increases code size and complexity.
TLS/DTLS layer compression is NOT RECOMMENDED by this TLS/DTLS
profile.
9. Perfect Forward Secrecy
Perfect forward secrecy (PFS) is a property that preserves the
confidentiality of past protocol interactions even in situations
where the long-term secret is compromised.
The PSK ciphersuite recommended in Section 4.2 does not offer this
property since it does not utilize a Diffie-Hellman (DH) exchange.
New ciphersuites that support PFS for PSK-based authentication, such
as proposed in [I-D.schmertmann-dice-ccm-psk-pfs], might become
available as standardized ciphersuite in the (near) future. The
recommended PSK-based ciphersuite offers excellent performance, a
very small memory footprint, and has the lowest on the wire overhead
at the expense of not using any public cryptography. For deployments
where public key cryptography is acceptable, the use of raw public
keys might offer a middle ground between the PSK ciphersuite in terms
of out-of-band validation and the functionality offered by asymmetric
cryptography.
Physical attacks create additional opportunities to gain access to
the crypto material stored on IoT devices. A PFS ciphersuite
prevents an attacker from obtaining the communication content
exchanged prior to a successful long-term key compromise; however, an
implementation that (for performance or energy efficiency reasons)
has been re-using the same ephemeral DH keys over multiple different
sessions partially defeats PFS, thus increasing the damage extent.
For this reason, implementations SHOULD NOT reuse ephemeral DH keys
over multiple protocol exchanges.
The impact of the disclosure of past communication interactions and
the desire to increase the cost for pervasive monitoring (as demanded
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by [RFC7258]) has to be taken into account when selecting a
ciphersuite that does not support the PFS property.
Client implementations claiming support of this profile MUST
implement the ciphersuites listed in Section 4 according to the
selected credential type.
10. Keep-Alive
Application layer communication may create state at the endpoints and
this state my expire at some time. For this reason, applications
define ways to refresh state, if necessary. While the application
layer exchanges are largely outside the scope of the underlying TLS/
DTLS exchange similar state considerations also play a role at the
level of TLS/DTLS. While TLS/DTLS also creates state in form of a
security context (see the security parameter described in Appendix A6
in RFC 5246) at the client and the server this state information does
not expire. However, network intermediaries may also allocate state
and require this state to be kept alive. Failure to keep state alive
at a stateful packet filtering firewall or at a NAT may result in the
inability for one node to reach the other since packets will get
blocked by these middleboxes. Periodic keep-alive messages exchanged
between the TLS/DTLS client and server keep state at these
middleboxes alive. According to measurements described in
[HomeGateway] there is some variance in state management practices
used in residential gateways but the timeouts are heavily impacted by
the choice of the transport layer protocol: timeouts for UDP are
typically much shorter than those for TCP.
RFC 6520 [RFC6520] defines a heartbeat mechanism to test whether the
other peer is still alive. As an additional feature, the same
mechanism can also be used to perform Path Maximum Transmission Unit
(MTU) Discovery.
A recommendation about the use of RFC 6520 depends on the type of
message exchange an IoT device performs and the number of messages
the application needs to exchange as part of their application
functionality. There are three types of exchanges that need to be
analyzed:
Client-Initiated, One-Shot Messages
This is a common communication pattern where IoT devices upload
data to a server on the Internet on an irregular basis. The
communication may be triggered by specific events, such as opening
a door.
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The DTLS handshake may need to be re-started (ideally using
session resumption, if possible) in case of an IP address change.
In this case there is no use for a keep-alive extension for this
scenario.
Client-Initiated, Regular Data Uploads
This is a variation of the previous case whereby data gets
uploaded on a regular basis, for example, based on frequent
temperature readings. If neither NAT bindings nor IP address
changes occurred then the record layer will not notice any
changes. For the case where the IP address and port number
changes, it is necessary to re-create the record layer using
session resumption.
In this scenario there is no use for a keep-alive extension. It
is also very likely that the device will enter a sleep cycle in
between data transmissions to keep power consumption low.
Server-Initiated Messages
In the two previous scenarios the client initiated the protocol
interaction and maintains it. Since messages to the client may
get blocked by middleboxes the initial connection setup is
triggered by the client and then kept alive by the server.
For this message exchange pattern the use of DTLS heartbeat
messages is quite useful but may have to be coordinated with
application exchanges (for example when the CoAP resource
directory is used) to avoid redundant keep-alive message
exchanges. The MTU discovery mechanism, which is also part of
[RFC6520], is less likely to be relevant since for many IoT
deployments the most constrained link is the wireless interface
between the IoT device and the network itself (rather than some
links along the end-to-end path). Only in more complex network
topologies, such as multi-hop mesh networks, path MTU discovery
might be appropriate. It also has to be noted that DTLS itself
already provides a basic path discovery mechanism (see
Section 4.1.1.1 of RFC 6347 by using the fragmentation capability
of the handshake protocol).
For server-initiated messages the heartbeat extension is RECOMMENDED.
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11. Timeouts
A variety of wired and wireless technologies are available to connect
devices to the Internet. Many of the low power radio technologies,
such as IEEE 802.15.4 or Bluetooth Smart, only support small frame
sizes (e.g., 127 bytes in case of IEEE 802.15.4 as explained in
[RFC4919]). Other radio technologies, such as the Global System for
Mobile Communications (GSM) using the short messaging service (SMS)
have similar constraints in terms of payload sizes, such as 140 bytes
without the optional segmentation and reassembly scheme known as
Concatenated SMS, but show higher latency.
The DTLS handshake protocol adds a fragmentation and reassembly
mechanism to the TLS handshake protocol since each DTLS record must
fit within a single transport layer datagram, as described in
Section 4.2.3 of [RFC6347]. Since handshake messages are potentially
bigger than the maximum record size, the mechanism fragments a
handshake message over a number of DTLS records, each of which can be
transmitted separately.
To deal with the unreliable message delivery provided by UDP, DTLS
adds timeouts and "per-flight" re-transmissions, as described in
Section 4.2.4 of [RFC6347]. Although the timeout values are
implementation specific, recommendations are provided in
Section 4.2.4.1 of [RFC6347], with an initial timer value of 1 second
and doubled with at each retransmission up to no less than 60
seconds.
TLS protocol steps can take longer due to higher processing time on
the constrained side. On the other hand, the way DTLS handles
retransmission, which is per-flight instead of per-segment, tends to
interact poorly with low bandwidth networks.
For these reasons, it's essential that the probability of a spurious
retransmit is minimized and, on timeout, the sending endpoint does
not react too aggressively. The latter is particularly relevant when
the WSN is temporarily congested: if lost packets are re-injected too
quickly, congestion worsens.
An initial timer value of 9 seconds with exponential back off up to
no less then 60 seconds is therefore RECOMMENDED.
This value is chosen big enough to absorb large latency variance due
to either slow computation on constrained endpoints or to intrinsic
network characteristics (e.g., GSM-SMS), as well as to produce a low
number of retransmission events and relax the pacing between them.
Its worst case wait time is the same as using 1s timeout (i.e. 63s),
while triggering less then half retransmissions (2 instead of 5).
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In order to minimise the wake time during DTLS handshake, sleepy
nodes might decide to select a lower threshold, and consequently a
smaller initial timeout value. If this is the case, the
implementation MUST keep into account the considerations about
network stability described in this section.
12. Random Number Generation
The TLS/DTLS protocol requires random numbers to be available during
the protocol run. For example, during the ClientHello and the
ServerHello exchange the client and the server exchange random
numbers. Also, the use of the Diffie-Hellman exchange requires
random numbers during the key pair generation.
It is important to note that sources contributing to the randomness
pool on laptops, or desktop PCs are not available on many IoT device,
such as mouse movement, timing of keystrokes, air turbulence on the
movement of hard drive heads, etc. Other sources have to be found or
dedicated hardware has to be added.
Lacking sources of randomness in an embedded system may lead to the
same keys generated again and again.
The ClientHello and the ServerHello messages contains the 'Random'
structure, which has two components: gmt_unix_time and a sequence of
28 random bytes. gmt_unix_time holds the current time and date in
standard UNIX 32-bit format (seconds since the midnight starting Jan
1, 1970, GMT). Since many IoT devices do not have access to an
accurate clock, it is RECOMMENDED that the receiver of a ClientHello
or ServerHello does not assume that the value in
'Random.gmt_unix_time' is an accurate representation of the current
time, and instead treats it as an opaque random string.
When TLS is used with certificate-based authentication the
availability of time information is needed to check the validity of a
certificate. Higher-layer protocols may provide secure time
information. The gmt_unix_time component of the ServerHello is not
used for this purpose.
IoT devices using TLS/DTLS must offer ways to generate quality random
numbers. There are various implementation choices for integrating a
hardware-based random number generator into a product: an
implementation inside the microcontroller itself is one option but
also dedicated crypto-chips are reasonable choices. The best choice
will depend on various factors outside the scope of this document.
Guidelines and requirements for random number generation can be found
in RFC 4086 [RFC4086] and in the NIST Special Publication 800-90a
[SP800-90A].
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Chip manufacturers are highly encouraged to provide sufficient
documentation of their design for random number generators so that
customers can have confidence about the quality of the generated
random numbers. The confidence can be increased by providing
information about the procedures that have been used to verify the
randomness of numbers generated by the hardware modules. For
example, NIST Special Publication 800-22b [SP800-22b] describes
statistical tests that can be used to verify random random number
generators.
13. Truncated MAC and Encrypt-then-MAC Extension
The truncated MAC extension was introduced with RFC 6066 [RFC6066]
with the goal to reduce the size of the MAC used at the Record Layer.
This extension was developed for TLS ciphersuites that used older
modes of operation where the MAC and the encryption operation was
performed independently.
The recommended ciphersuites in this document use the newer
Authenticated Encryption with Associated Data (AEAD) construct,
namely the CBC-MAC mode (CCM) with eight-octet authentication tags,
and are therefore not applicable to the truncated MAC extension.
RFC 7366 [RFC7366] introduced the encrypt-then-MAC extension (instead
of the previously used MAC-then-encrypt) since the MAC-then-encrypt
mechanism has been the subject of a number of security
vulnerabilities. RFC 7366 is, however, also not applicable to the
AEAD ciphers recommended in this document.
Implementations conformant to this specification MUST use AEAD
ciphers. RFC 7366 ("Encrypt-then-MAC") and RFC 6066 ("Truncated MAC
extension") are not applicable to this specification and MUST NOT be
used.
14. Server Name Indication (SNI)
The Server Name Indication extension defined in [RFC6066] defines a
mechanism for a client to tell a TLS/DTLS server the name of the
server it wants to contact. This is a useful extension for many
hosting environments where multiple virtual servers are run on single
IP address.
Implementing the Server Name Indication extension is REQUIRED unless
it is known that a TLS/DTLS client does not interact with a server in
a hosting environment.
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15. Maximum Fragment Length Negotiation
This RFC 6066 extension lowers the maximum fragment length support
needed for the Record Layer from 2^14 bytes to 2^9 bytes.
This is a very useful extension that allows the client to indicate to
the server how much maximum memory buffers it uses for incoming
messages. Ultimately, the main benefit of this extension is to allow
client implementations to lower their RAM requirements since the
client does not need to accept packets of large size (such as 16k
packets as required by plain TLS/DTLS).
Client implementations MUST support this extension.
16. Session Hash
In order to begin connection protection, the Record Protocol requires
specification of a suite of algorithms, a master secret, and the
client and server random values. The algorithm for computing the
master secret is defined in Section 8.1 of RFC 5246 but only includes
a small number of parameters exchanged during the handshake and does
not include parameters like the client and server identities. This
can be utilized by an attacker to mount a man-in-the-middle attack
since the master secret is not guaranteed to be unique across
sessions, as discovered in the 'Triple Handshake' attack [Triple-HS].
[I-D.ietf-tls-session-hash] defines a TLS extension that binds the
master secret to a log of the full handshake that computes it, thus
preventing such attacks.
Client implementations SHOULD implement this extension even though
the ciphersuites recommended by this profile are not vulnerable to
this attack. For Diffie-Hellman-based ciphersuites the keying
material is contributed by both parties and in case of the pre-shared
secret key ciphersuite, both parties need to be in possession of the
shared secret to ensure that the handshake completes successfully.
It is, however, possible that some application layer protocols will
tunnel other authentication protocols on top of DTLS making this
attack relevant again.
17. Re-Negotiation Attacks
TLS/DTLS allows a client and a server who already have a TLS/DTLS
connection to negotiate new parameters, generate new keys, etc by
using the re-negotiation feature. Renegotiation happens in the
existing connection, with the new handshake packets being encrypted
along with application data. Upon completion of the re-negotiation
procedure the new channel replaces the old channel.
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As described in RFC 5746 [RFC5746] there is no cryptographic binding
between the two handshakes, although the new handshake is carried out
using the cryptographic parameters established by the original
handshake.
To prevent the re-negotiation attack [RFC5746] this specification
REQUIRES to disable the TLS renegotiation feature. Clients MUST
respond to server-initiated re-negotiation attempts with an alert
message (no_renegotiation) and clients MUST NOT initiate them.
18. Downgrading Attacks
When a client sends a ClientHello with a version higher than the
highest version known to the server, the server is supposed to reply
with ServerHello.version equal to the highest version known to the
server and the handshake can proceed. This behavior is known as
version tolerance. Version-intolerance is when the server (or a
middlebox) breaks the handshake when it sees a ClientHello.version
higher than what it knows about. This is the behavior that leads
some clients to re-run the handshake with lower version. As a
result, a potential security vulnerability is introduced when a
system is running an old TLS/SSL version (e.g., because of the need
to integrate with legacy systems). In the worst case, this allows an
attacker to downgrade the protocol handshake to SSL 3.0. SSL 3.0 is
so broken that there is no secure cipher available for it (see
[RFC7568]).
The above-described downgrade vulnerability is solved by the TLS
Fallback Signaling Cipher Suite Value (SCSV) [RFC7507] extension.
However, the solution is not applicable to implementations conforming
to this profile since the version negotiation MUST use TLS/DTLS
version 1.2 (or higher). More specifically, this implies:
o Clients MUST NOT send a TLS/DTLS version lower than version 1.2 in
the ClientHello.
o Clients MUST NOT retry a failed negotiation offering a TLS/DTLS
version lower than 1.2.
o Servers MUST fail the handshake by sending a protocol_version
fatal alert if a TLS/DTLS version >= 1.2 cannot be negotiated.
Note that the aborted connection is non-resumable.
19. Crypto Agility
This document recommends software and chip manufacturers to implement
AES and the CCM mode of operation. This document references the CoAP
recommended ciphersuite choices, which have been selected based on
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implementation and deployment experience from the IoT community.
Over time the preference for algorithms will, however, change. Not
all components of a ciphersuite are likely to change at the same
speed. Changes are more likely expected for ciphers, the mode of
operation, and the hash algorithms. The recommended key lengths have
to be adjusted over time as well. Some deployment environments will
also be impacted by local regulation, which might dictate a certain
algorithm and key size combination. Ongoing discussions regarding
the choice of specific ECC curves will also likely impact
implementations. Note that this document does not recommend or
mandate a specific ECC curve.
The following recommendations can be made to chip manufacturers:
o Make any AES hardware-based crypto implementation accessible to
developers working on security implementations at higher layers in
the protocol stack. Sometimes hardware implementations are added
to microcontrollers to offer support for functionality needed at
the link layer and are only available to the on-chip link layer
protocol implementation. Such a setup does not allow application
developers to re-use the hardware-based AES implementation.
o Provide flexibility for the use of the crypto function with future
extensibility in mind. For example, making an AES-CCM
implementation available to developers is a first step but such an
implementation may not be usable due to parameter differences
between an AES-CCM implementations. AES-CCM in IEEE 802.15.4 and
Bluetooth Smart uses a nonce length of 13-octets while DTLS uses a
nonce length of 12-octets. Hardware implementations of AES-CCM
for IEEE 802.15.4 and Bluetooth Smart are therefore not re-usable
by a DTLS stack.
o Offer access to building blocks in addition (or as an alternative)
to the complete functionality. For example, a chip manufacturer
who gives developers access to the AES crypto function can use it
to build an efficient AES-GCM implementations. Another example is
to make a special instruction available that increases the speed
of speed-up carryless multiplications.
As a recommendation for developers and product architects we suggest
that sufficient headroom is provided to allow an upgrade to a newer
cryptographic algorithms over the lifetime of the product. As an
example, while AES-CCM is recommended throughout this specification
future products might use the ChaCha20 cipher in combination with the
Poly1305 authenticator [RFC7539]. The assumption is made that a
robust software update mechanism is offered.
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20. Key Length Recommendations
RFC 4492 [RFC4492] gives approximate comparable key sizes for
symmetric- and asymmetric-key cryptosystems based on the best-known
algorithms for attacking them. While other publications suggest
slightly different numbers, such as [Keylength], the approximate
relationship still holds true. Figure 12 illustrates the comparable
key sizes in bits.
Symmetric | ECC | DH/DSA/RSA
------------+---------+-------------
80 | 163 | 1024
112 | 233 | 2048
128 | 283 | 3072
192 | 409 | 7680
256 | 571 | 15360
Figure 12: Comparable Key Sizes (in bits) based on RFC 4492.
At the time of writing the key size recommendations for use with TLS-
based ciphers found in [RFC7525] recommend DH key lengths of at least
2048 bit, which corresponds to a 112-bit symmetric key and a 233 bit
ECC key. These recommendations are roughly inline with those from
other organizations, such as the National Institute of Standards and
Technology (NIST) or the European Network and Information Security
Agency (ENISA). The authors of [ENISA-Report2013] add that a 80-bit
symmetric key is sufficient for legacy applications for the coming
years, but a 128-bit symmetric key is the minimum requirement for new
systems being deployed. The authors further note that one needs to
also take into account the length of time data needs to be kept
secure for. The use of 80-bit symmetric keys for transactional data
may be acceptable for the near future while one has to insist on
128-bit symmetric keys for long lived data.
Note that the recommendations for 112-bit symmetric keys are chosen
conservatively under the assumption that IoT devices have a long
expected lifetime (such as 10+ years) and that this key length
recommendation refers to the long-term keys used for device
authentication. Keys, which are provisioned dynamically, for the
protection of transactional data (such as ephemeral Diffie-Hellman
keys used in various TLS/DTLS ciphersuites) may be shorter
considering the sensitivity of the exchanged data.
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21. False Start
A full TLS handshake as specified in [RFC5246] requires two full
protocol rounds (four flights) before the handshake is complete and
the protocol parties may begin to send application data.
An abbreviated handshake (resuming an earlier TLS session) is
complete after three flights, thus adding just one round-trip time if
the client sends application data first.
If the conditions outlined in [I-D.ietf-tls-falsestart] are met,
application data can be transmitted when the sender has sent its own
"ChangeCipherSpec" and "Finished" messages. This achieves an
improvement of one round-trip time for full handshakes if the client
sends application data first, and for abbreviated handshakes if the
server sends application data first.
The conditions for using the TLS False Start mechanism are met by the
public-key-based ciphersuites in this document. In summary, the
conditions are
o Modern symmetric ciphers with an effective key length of 128 bits,
such as AES-128-CCM
o Client certificate types, such as ecdsa_sign
o Key exchange methods, such as ECDHE_ECDSA
Based on the improvement over a full round-trip for the full TLS/DTLS
exchange this specification RECOMMENDS the use of the False Start
mechanism when clients send application data first.
22. Privacy Considerations
The DTLS handshake exchange conveys various identifiers, which can be
observed by an on-path eavesdropper. For example, the DTLS PSK
exchange reveals the PSK identity, the supported extensions, the
session id, algorithm parameters, etc. When session resumption is
used then individual TLS sessions can be correlated by an on-path
adversary. With many IoT deployments it is likely that keying
material and their identifiers are persistent over a longer period of
time due to the cost of updating software on these devices.
User participation poses a challenge in many IoT deployments since
many of the IoT devices operate unattended, even though they are
initially provisioned by a human. The ability to control data
sharing and to configure preferences will have to be provided at a
system level rather than at the level of the DTLS exchange itself,
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which is the scope of this document. Quite naturally, the use of
DTLS with mutual authentication will allow a TLS server to collect
authentication information about the IoT device (likely over a long
period of time). While this strong form of authentication will
prevent mis-attribution, it also allows strong identification.
Device-related data collection (e.g., sensor recordings) associated
with other data type will prove to be truly useful but this extra
data might include personal information about the owner of the device
or data about the environment it senses. Consequently, the data
stored on the server-side will be vulnerable to stored data
compromise. For the communication between the client and the server
this specification prevents eavesdroppers to gain access to the
communication content. While the PSK-based ciphersuite does not
provide PFS the asymmetric versions do. This prevents an adversary
from obtaining past communication content when access to a long-term
secret has been gained. Note that no extra effort to make traffic
analysis more difficult is provided by the recommendations made in
this document.
Note that the absence or presence of communication itself might
reveal information to an adversary. For example, a presence sensor
may initiate messaging when a person enters a building. While TLS/
DTLS would offer confidentiality protection of the transmitted
information it does not help to conceal all communication patterns.
Furthermore, the IP header, which is not protected by TLS/DTLS,
additionally reveals information about the other communication
endpoint. For applications where such privacy concerns exist
additional safeguards are required, such as injecting dummy traffic
and onion routing. A detailed treatment of such solutions is outside
the scope of this document and requires a system-level view.
23. Security Considerations
This entire document is about security.
We would also like to point out that designing a software update
mechanism into an IoT system is crucial to ensure that both
functionality can be enhanced and that potential vulnerabilities can
be fixed. This software update mechanism is important for changing
configuration information, for example, trust anchors and other
keying related information. Such a suitable software update
mechanism is available with the Lightweight Machine-to-Machine
(LWM2M) protocol published by the Open Mobile Alliance (OMA) [LWM2M].
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24. IANA Considerations
This document includes no request to IANA.
25. Acknowledgments
Thanks to Derek Atkins, Olaf Bergmann, Paul Bakker, Carsten Bormann,
Ben Campbell, Brian Carpenter, Robert Cragie, Spencer Dawkins, Russ
Housley, Rene Hummen, Jayaraghavendran K, Matthias Kovatsch, Sandeep
Kumar, Barry Leiba, Sye Loong Keoh, Simon Lemay, Alexey Melnikov,
Gabriel Montenegro, Manuel Pegourie-Gonnard, Akbar Rahman, Eric
Rescorla, Michael Richardson, Ludwig Seitz, Zach Shelby, Michael
StJohns, Rene Struik, Sean Turner, and Tina Tsou for their helpful
comments and discussions that have shaped the document.
Big thanks also to Klaus Hartke, who wrote the initial version of
this document.
Finally, we would like to thank our area director (Stephen Farrell)
and our working group chairs (Zach Shelby and Dorothy Gellert) for
their support.
26. References
26.1. Normative References
[EUI64] "GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER (EUI-64)
REGISTRATION AUTHORITY", URL:
https://standards.ieee.org/regauth/oui/tutorials/
EUI64.html, April 2010.
[GSM-SMS] ETSI, "3GPP TS 23.040 V7.0.1 (2007-03): 3rd Generation
Partnership Project; Technical Specification Group Core
Network and Terminals; Technical realization of the Short
Message Service (SMS) (Release 7)", March 2007.
[I-D.ietf-tls-cached-info]
Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", draft-ietf-tls-
cached-info-19 (work in progress), March 2015.
[I-D.ietf-tls-session-hash]
Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley,
A., and M. Ray, "Transport Layer Security (TLS) Session
Hash and Extended Master Secret Extension", draft-ietf-
tls-session-hash-06 (work in progress), July 2015.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)", RFC
4279, DOI 10.17487/RFC4279, December 2005,
<http://www.rfc-editor.org/info/rfc4279>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<http://www.rfc-editor.org/info/rfc5746>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, DOI
10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC6520] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) Heartbeat Extension", RFC 6520, DOI 10.17487/
RFC6520, February 2012,
<http://www.rfc-editor.org/info/rfc6520>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>.
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[RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
<http://www.rfc-editor.org/info/rfc7251>.
[WAP-WDP] Wireless Application Protocol Forum, "Wireless Datagram
Protocol", June 2001.
26.2. Informative References
[ACE-WG] IETF, "Authentication and Authorization for Constrained
Environments (ace) Working Group", URL:
https://datatracker.ietf.org/wg/ace/charter/, Jan 2015.
[AES] National Institute of Standards and Technology, "FIPS PUB
197, Advanced Encryption Standard (AES)", URL:
http://csrc.nist.gov/publications/fips/fips197/
fips-197.pdf, November 2001.
[CCM] National Institute of Standards and Technology, "Special
Publication 800-38C, Recommendation for Block Cipher Modes
of Operation: The CCM Mode for Authentication and
Confidentiality", http://csrc.nist.gov/publications/
nistpubs/800-38C/SP800-38C_updated-July20_2007.pdf, May
2004.
[CRIME] Wikipedia, "CRIME Security Exploit", HTML
https://en.wikipedia.org/wiki/CRIME, June 2001.
[ENISA-Report2013]
ENISA, "Algorithms, Key Sizes and Parameters Report -
2013", URL: https://www.enisa.europa.eu/activities/
identity-and-trust/library/deliverables/algorithms-key-
sizes-and-parameters-report, October 2013.
[HomeGateway]
Eggert, L., "An experimental study of home gateway
characteristics, In Proceedings of the '10th annual
conference on Internet measurement'", PDF
https://eggert.org/papers/2010-imc-hgw-study.pdf, 2010.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-04
(work in progress), July 2015.
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[I-D.ietf-tls-falsestart]
Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", draft-ietf-tls-
falsestart-00 (work in progress), May 2015.
[I-D.ietf-tls-negotiated-dl-dhe]
Gillmor, D., "Negotiated Discrete Log Diffie-Hellman
Ephemeral Parameters for TLS", draft-ietf-tls-negotiated-
dl-dhe-00 (work in progress), July 2014.
[I-D.irtf-cfrg-curves]
Langley, A. and M. Hamburg, "Elliptic Curves for
Security", draft-irtf-cfrg-curves-11 (work in progress),
October 2015.
[I-D.schmertmann-dice-ccm-psk-pfs]
Schmertmann, L. and C. Bormann, "ECDHE-PSK AES-CCM Cipher
Suites with Forward Secrecy for Transport Layer Security
(TLS)", draft-schmertmann-dice-ccm-psk-pfs-01 (work in
progress), August 2014.
[I-D.tschofenig-core-coap-tcp-tls]
Bormann, C., Lemay, S., Technologies, Z., and H.
Tschofenig, "A TCP and TLS Transport for the Constrained
Application Protocol (CoAP)", draft-tschofenig-core-coap-
tcp-tls-04 (work in progress), June 2015.
[IANA-TLS]
IANA, "TLS Cipher Suite Registry", URL:
https://www.iana.org/assignments/tls-parameters/tls-
parameters.xhtml#tls-parameters-4, 2014.
[ImprintingSurvey]
Chilton, E., "A Brief Survey of Imprinting Options for
Constrained Devices", URL: http://www.lix.polytechnique.fr
/hipercom/SmartObjectSecurity/papers/EricRescorla.pdf,
March 2012.
[Keylength]
Giry, D., "Cryptographic Key Length Recommendations", URL:
http://www.keylength.com, November 2014.
[LWM2M] Open Mobile Alliance, "Lightweight Machine-to-Machine,
Technical Specification, Candidate Version 1.0", HTML
http://openmobilealliance.org/about-oma/work-program/
m2m-enablers/, December 2013.
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[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
1996, <http://www.rfc-editor.org/info/rfc1981>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, DOI
10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2865, DOI 10.17487/RFC2865, June 2000,
<http://www.rfc-editor.org/info/rfc2865>.
[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
2003, <http://www.rfc-editor.org/info/rfc3610>.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<http://www.rfc-editor.org/info/rfc3748>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, DOI
10.17487/RFC4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals", RFC
4919, DOI 10.17487/RFC4919, August 2007,
<http://www.rfc-editor.org/info/rfc4919>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <http://www.rfc-editor.org/info/rfc5077>.
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[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>.
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
March 2008, <http://www.rfc-editor.org/info/rfc5216>.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, DOI 10.17487/RFC5247, August 2008,
<http://www.rfc-editor.org/info/rfc5247>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, DOI
10.17487/RFC5288, August 2008,
<http://www.rfc-editor.org/info/rfc5288>.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
<http://www.rfc-editor.org/info/rfc5480>.
[RFC5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
Polk, "Internet X.509 Public Key Infrastructure:
Additional Algorithms and Identifiers for DSA and ECDSA",
RFC 5758, DOI 10.17487/RFC5758, January 2010,
<http://www.rfc-editor.org/info/rfc5758>.
[RFC5934] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
Management Protocol (TAMP)", RFC 5934, DOI 10.17487/
RFC5934, August 2010,
<http://www.rfc-editor.org/info/rfc5934>.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, <http://www.rfc-editor.org/info/rfc6024>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, DOI 10.17487/
RFC6090, February 2011,
<http://www.rfc-editor.org/info/rfc6090>.
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[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI
10.17487/RFC6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655, DOI 10.17487/
RFC6655, July 2012,
<http://www.rfc-editor.org/info/rfc6655>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://www.rfc-editor.org/info/rfc6690>.
[RFC6733] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
Ed., "Diameter Base Protocol", RFC 6733, DOI 10.17487/
RFC6733, October 2012,
<http://www.rfc-editor.org/info/rfc6733>.
[RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
2013, <http://www.rfc-editor.org/info/rfc6943>.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
DOI 10.17487/RFC6961, June 2013,
<http://www.rfc-editor.org/info/rfc6961>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, DOI 10.17487/
RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, DOI 10.17487/
RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
[RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
<http://www.rfc-editor.org/info/rfc7366>.
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[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014,
<http://www.rfc-editor.org/info/rfc7390>.
[RFC7397] Gilger, J. and H. Tschofenig, "Report from the Smart
Object Security Workshop", RFC 7397, DOI 10.17487/RFC7397,
December 2014, <http://www.rfc-editor.org/info/rfc7397>.
[RFC7400] Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
IPv6 over Low-Power Wireless Personal Area Networks
(6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
2014, <http://www.rfc-editor.org/info/rfc7400>.
[RFC7452] Tschofenig, H., Arkko, J., Thaler, D., and D. McPherson,
"Architectural Considerations in Smart Object Networking",
RFC 7452, DOI 10.17487/RFC7452, March 2015,
<http://www.rfc-editor.org/info/rfc7452>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, DOI
10.17487/RFC7465, February 2015,
<http://www.rfc-editor.org/info/rfc7465>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<http://www.rfc-editor.org/info/rfc7507>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
<http://www.rfc-editor.org/info/rfc7539>.
[RFC7568] Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
DOI 10.17487/RFC7568, June 2015,
<http://www.rfc-editor.org/info/rfc7568>.
[SP800-107-rev1]
NIST, "NIST Special Publication 800-107, Revision 1,
Recommendation for Applications Using Approved Hash
Algorithms", URL: http://csrc.nist.gov/publications/
nistpubs/800-107-rev1/sp800-107-rev1.pdf, August 2012.
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[SP800-22b]
National Institute of Standards and Technology, "Special
Publication 800-22, Revision 1a, A Statistical Test Suite
for Random and Pseudorandom Number Generators for
Cryptographic Applications", URL:
http://csrc.nist.gov/publications/nistpubs/800-22-rev1a/
SP800-22rev1a.pdf, April 2010.
[SP800-90A]
NIST, "DRAFT Special Publication 800-90a, Revision 1,
Recommendation for Random Number Generation Using
Deterministic Random Bit Generators", URL:
http://csrc.nist.gov/publications/drafts/800-90/
sp800-90a_r1_draft_november2014_ver.pdf, November 2014.
[Triple-HS]
Bhargavan, K., Delignat-Lavaud, C., Pironti, A., and P.
Strub, "Triple Handshakes and Cookie Cutters: Breaking and
Fixing Authentication over TLS", IEEE Symposium on
Security and Privacy, pages 98-113, 2014.
Appendix A. Conveying DTLS over SMS
This section is normative for the use of DTLS over SMS. Timer
recommendations are already outlined in Section 11 and also
applicable to the transport of DTLS over SMS.
This section requires readers to be familiar with the terminology and
concepts described in [GSM-SMS], and [WAP-WDP].
The remainder of this section assumes Mobile Stations are capable of
producing and consuming 8-bit binary data encoded Transport Protocol
Data Units (TPDU).
A.1. Overview
DTLS adds an additional round-trip to the TLS [RFC5246] handshake to
serve as a return-routability test for protection against certain
types of DoS attacks. Thus a full blown DTLS handshake comprises up
to 6 "flights" (i.e., logical message exchanges), each of which is
then mapped on to one or more DTLS records using the segmentation and
reassembly (SaR) scheme described in Section 4.2.3 of [RFC6347]. The
overhead for said scheme is 6 bytes per Handshake message which,
given a realistic 10+ messages handshake, would amount around 60
bytes across the whole handshake sequence.
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Note that the DTLS SaR scheme is defined for handshake messages only.
In fact, DTLS records are never fragmented and MUST fit within a
single transport layer datagram.
SMS provides an optional segmentation and reassembly scheme as well,
known as Concatenated short messages (see Section 9.2.3.24.1 of
[GSM-SMS]). However, since the SaR scheme in DTLS cannot be
circumvented, the Concatenated short messages mechanism SHOULD NOT be
used during handshake to avoid redundant overhead. Before starting
the handshake phase (either actively or passively), the DTLS
implementation MUST be explicitly configured with the PMTU of the SMS
transport in order to correctly instrument its SaR function. The
PMTU SHALL be 133 bytes if WDP-based multiplexing is used (see
Appendix A.3), 140 bytes otherwise.
It is RECOMMENDED to use the established security context over the
longest possible period (possibly until a Closure Alert message is
received, or after a very long inactivity timeout) to avoid the
expensive re-establishment of the security association.
A.2. Message Segmentation and Re-Assembly
The content of an SMS message is carried in the TP-UserData field,
and its size may be up to 140 bytes. As already mentioned in
Appendix A.1, longer (i.e., up to 34170 bytes) messages can be sent
using Concatenated SMS.
This scheme consumes 6-7 bytes (depending on whether the short or
long segmentation format is used) of the TP-UserData field, thus
reducing the space available for the actual content of the SMS
message to 133-134 bytes per TPDU.
Though in principle a PMTU value higher than 140 bytes could be used,
which may look like an appealing option given its more efficient use
of the transport, there are disadvantages to consider. First, there
is an additional overhead of 7 bytes per TPDU to be paid to the SaR
function (which is in addition to the overhead introduced by the DTLS
SaR mechanism. Second, some networks only partially support the
Concatenated SMS function and others do not support it at all.
For these reasons, the Concatenated short messages mechanism SHOULD
NOT be used, and it is RECOMMENDED to leave the same PMTU settings
used during the handshake phase, i.e., 133 bytes if WDP- based
multiplexing is enabled, 140 bytes otherwise.
Note that, after DTLS handshake has completed, any fragmentation and
reassembly logic that pertains the application layer (e.g.,
segmenting CoAP messages into DTLS records and reassembling them
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after the crypto operations have been successfully performed) needs
to be handled by the application that uses the established DTLS
tunnel.
A.3. Multiplexing Security Associations
Unlike IPsec ESP/AH, DTLS records do not contain any association
identifiers. Applications must arrange to multiplex between
associations on the same endpoint which, when using UDP/IP, is
usually done with the host/port number.
If the DTLS server allows more than one client to be active at any
given time, then the WAP User Datagram Protocol [WAP-WDP] can be used
to achieve multiplexing of the different security associations. (The
use of WDP provides the additional benefit that upper layer protocols
can operate independently of the underlying wireless network, hence
achieving application-agnostic transport handover.)
The total overhead cost for encoding the WDP source and destination
ports is either 5 or 7 bytes out of the total available for the SMS
content depending on if 1-byte or 2-byte port identifiers are used,
as shown in Figure 13 and Figure 14.
0 1 2 3 4
+--------+--------+--------+--------+--------+
| ... | 0x04 | 2 | ... | ... |
+--------+--------+--------+--------+--------+
UDH IEI IE Dest Source
Length Length Port Port
Figure 13: Application Port Addressing Scheme (8 bit address).
0 1 2 3 4 5 6
+--------+--------+--------+--------+--------+--------+--------+
| ... | 0x05 | 4 | ... | ... |
+--------+--------+--------+--------+--------+--------+--------+
UDH IEI IE Dest Source
Length Length Port Port
Figure 14: Application Port Addressing Scheme (16 bit address).
The receiving side of the communication gets the source address from
the originator address (TP-OA) field of the SMS-DELIVER TPDU. This
way an unique 4-tuple identifying the security association can be
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reconstructed at both ends. (When replying to its DTLS peer, the
sender will swaps the TP-OA and TP-DA parameters and the source and
destination ports in the WDP.)
A.4. Timeout
If SMS-STATUS-REPORT messages are enabled, their receipt is not to be
interpreted as the signal that the specific handshake message has
been acted upon by the receiving party. Therefore, it MUST NOT be
taken into account by the DTLS timeout and retransmission function.
Handshake messages MUST carry a validity period (TP-VP parameter in a
SMS-SUBMIT TPDU) that is not less than the current value of the
retransmission timeout. In order to avoid persisting messages in the
network that will be discarded by the receiving party, handshake
messages SHOULD carry a validity period that is the same as, or just
slightly higher than, the current value of the retransmission
timeout.
Appendix B. DTLS Record Layer Per-Packet Overhead
Figure 15 shows the overhead for the DTLS record layer for protecting
data traffic when AES-128-CCM with an 8-octet Integrity Check Value
(ICV) is used.
DTLS Record Layer Header................13 bytes
Nonce (Explicit).........................8 bytes
ICV..................................... 8 bytes
------------------------------------------------
Overhead................................29 bytes
------------------------------------------------
Figure 15: AES-128-CCM-8 DTLS Record Layer Per-Packet Overhead.
The DTLS record layer header has 13 octets and consists of
o 1 octet content type field,
o 2 octet version field,
o 2 octet epoch field,
o 6 octet sequence number,
o 2 octet length field.
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The "nonce" input to the AEAD algorithm is exactly that of [RFC5288],
i.e., 12 bytes long. It consists of two values, namely a 4 octet
salt and an 8 octet nonce_explicit:
The salt is the "implicit" part and is not sent in the packet.
Instead, the salt is generated as part of the handshake process.
The nonce_explicit value is 8 octet long and it is chosen by the
sender and carried in each TLS record. RFC 6655 [RFC6655] allows
the nonce_explicit to be a sequence number or something else.
This document makes this use more restrictive for use with DTLS:
the 64-bit none_explicit value MUST be the 16-bit epoch
concatenated with the 48-bit seq_num. The sequence number
component of the nonce_explicit field at the AES-CCM layer is an
exact copy of the sequence number in the record layer header
field. This leads to a duplication of 8-bytes per record.
To avoid this 8-byte duplication RFC 7400 [RFC7400] provides help
with the use of the generic header compression technique for IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs). Note
that this header compression technique is not available when DTLS
is exchanged over transports that do not use IPv6 or 6LoWPAN, such
as the SMS transport described in Appendix A.
Appendix C. DTLS Fragmentation
Section 4.2.3 of [RFC6347] advises DTLS implementations to not
produce overlapping fragments. However, it requires receivers to be
able to cope with them. The need for the latter requisite is
explained in Section 4.1.1.1 of [RFC6347]: accurate path MTU (PMTU)
estimation may be traded for shorter handshake completion time.
In many cases, the cost of handling fragment overlaps has proved to
be unaffordable for constrained implementations, particularly because
of the increased complexity in buffer management.
In order to reduce the likelihood of producing different fragment
sizes and consequent overlaps within the same handshake, this
document RECOMMENDs:
o clients (handshake initiators) to use reliable PMTU information
for the intended destination;
o servers to mirror the fragment size selected by their clients.
The PMTU information comes either from a "fresh enough" discovery
performed by the client ([RFC1981], [RFC4821]), or from some other
reliable out-of-band channel.
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Authors' Addresses
Hannes Tschofenig (editor)
ARM Ltd.
110 Fulbourn Rd
Cambridge CB1 9NJ
Great Britain
Email: Hannes.tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Thomas Fossati
Alcatel-Lucent
3 Ely Road
Milton, Cambridge CB24 6DD
UK
Email: thomas.fossati@alcatel-lucent.com
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