A Datagram Transport Layer Security (DTLS) 1.2 Profile for the Internet of Things
draft-ietf-dice-profile-02
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| Document | Type | Active Internet-Draft (dice WG) | |
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| Author | Hannes Tschofenig | ||
| Last updated | 2014-07-04 | ||
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draft-ietf-dice-profile-02
dice H. Tschofenig, Ed.
Internet-Draft ARM Ltd.
Intended status: Standards Track July 4, 2014
Expires: January 5, 2015
A Datagram Transport Layer Security (DTLS) 1.2 Profile for the Internet
of Things
draft-ietf-dice-profile-02.txt
Abstract
This document defines a DTLS profile that is suitable for Internet of
Things applications and is reasonably implementable on many
constrained devices.
A common design pattern in IoT deployments is the use of a
constrained device (typically providing sensor data) that interacts
with the web infrastructure. This document focuses on this
particular pattern.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 5, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. The Communication Model . . . . . . . . . . . . . . . . . . . 5
4. The Ciphersuite Concept . . . . . . . . . . . . . . . . . . . 6
5. Pre-Shared Secret Authentication with DTLS . . . . . . . . . 8
6. Raw Public Key Use with DTLS . . . . . . . . . . . . . . . . 9
7. Certificate Use with DTLS . . . . . . . . . . . . . . . . . . 11
8. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 12
9. Session Resumption . . . . . . . . . . . . . . . . . . . . . 14
10. TLS Compression . . . . . . . . . . . . . . . . . . . . . . . 14
11. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . . . 14
12. Keep-Alive . . . . . . . . . . . . . . . . . . . . . . . . . 15
13. Random Number Generation . . . . . . . . . . . . . . . . . . 16
14. Client Certificate URLs . . . . . . . . . . . . . . . . . . . 17
15. Trusted CA Indication . . . . . . . . . . . . . . . . . . . . 17
16. Truncated MAC Extension . . . . . . . . . . . . . . . . . . . 18
17. Server Name Indication (SNI) . . . . . . . . . . . . . . . . 18
18. Maximum Fragment Length Negotiation . . . . . . . . . . . . . 18
19. Negotiation and Downgrading Attacks . . . . . . . . . . . . . 18
20. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19
21. Security Considerations . . . . . . . . . . . . . . . . . . . 19
22. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
23. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
24. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
24.1. Normative References . . . . . . . . . . . . . . . . . . 20
24.2. Informative References . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
This document defines a DTLS 1.2 [RFC6347] profile that offers
communication security for Internet of Things (IoT) applications and
is reasonably implementable on many constrained devices. It aims to
meet the following goals:
o Serves as a one-stop shop for implementers to know which pieces of
the specification jungle contain relevant details.
o Does not alter the DTLS 1.2 specification.
o Does not introduce any new extensions.
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o Aligns with the DTLS security modes of the Constrained Application
Protocol (CoAP) [RFC7252].
DTLS is used to secure a number of applications run over an
unreliable datagram transport. CoAP [RFC7252] is one such protocol
and has been designed specifically for use in IoT environments. CoAP
can be secured a number of different ways, also called security
modes. These security modes are as follows, see Section 5,
Section 6, Section 7 for additional details:
No Security Protection at the Transport Layer: No DTLS is used but
instead application layer security functionality is assumed.
Shared Secret-based DTLS Authentication: DTLS supports the use of
shared secrets [RFC4279]. This mode is useful if the number of
communication relationships between the IoT device and servers is
small and for very constrained devices. Shared secret-based
authentication mechanisms offer good performance and require a
minimum of data to be exchanged.
DTLS Authentication using Asymmetric Cryptography: TLS supports
client and server authentication using asymmetric cryptography.
Two approaches for validating these public keys are available.
First, [RFC7250] allows raw public keys to be used in TLS without
the overhead of certificates. This approach requires out-of-band
validation of the public key. Second, the use of X.509
certificates [RFC5280] with TLS is common on the Web today (at
least for server-side authentication) and certain IoT environments
may also re-use those capabilities. Certificates bind an
identifier to the public key signed by a certification authority
(CA). A trust anchor store has to be provisioned on the device to
indicate what CAs are trusted. Furthermore, the certificate may
contain a wealth of other information used to make authorization
decisions.
As described in [I-D.ietf-lwig-tls-minimal], an application designer
developing an IoT device needs to consider the security threats and
the security services that can be used to mitigate the threats.
Enabling devices to upload data and retrieve configuration
information, inevitably requires that Internet-connected devices be
able to authenticate themselves to servers and vice versa as well as
to ensure that the data and information exchanged is integrity and
confidentiality protected. While these security services can be
provided at different layers in the protocol stack the use of
communication security, as offered by DTLS, has been very popular on
the Internet and it is likely to be useful for IoT scenarios as well.
In case the communication security features offered by DTLS meet the
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security requirements of your application the remainder of the
document might offer useful guidance.
Not every IoT deployment will use CoAP but the discussion regarding
choice of credentials and cryptographic algorithms will be very
similar. As such, the discussions in this document are applicable
beyond the use of the CoAP protocol.
The design of DTLS is intentionally very similar to TLS. Since DTLS
operates on top of an unreliable datagram transport a few
enhancements to the TLS structure are, however necessary. 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
TLS Record Layer. 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.
o The TLS Handshake Protocol has been enhanced to include a
stateless cookie exchange for Denial of Service (DoS) resistance.
Furthermore, the header has been extended to deal with message
loss, reordering, and fragmentation. Retransmission timers have
been included to deal with message loss. For DoS protection 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. This type of DoS protection mechanism has also
been incorporated into the design of IKEv2. Although the exchange
is optional for the server to execute, a client implementation has
to be prepared to respond to it.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Note that "Client" and "Server" in this document refer to TLS roles,
where the Client initiates the TLS handshake. This does not restrict
the interaction pattern of the protocols carried inside TLS as the
record layer allows bi-directional communication. In the case of
CoAP the "Client" can act as a CoAP Server or Client.
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3. The Communication Model
This document describes a profile of DTLS 1.2 and, to be useful, it
has to make assumptions about the envisioned communication
architecture.
The communication architecture shown in Figure 1 assumes a uni-cast
communication interaction with an IoT device utilizing a DTLS client
and that client interacts with one or multiple DTLS servers.
Clients are preconfigured with the address or addresses of servers
(e.g., as part of the firmware) they will communicate with as well as
authentication information:
o For PSK-based authentication (see Section 5), this includes the
paired "PSK identity" and shared secret to be used with each
server.
o For raw public key-based authentication (see Section 6), this
includes either the server's public key or the hash of the
server's public key.
o For certificate-based authentication (see Section 7), this may
include a pre-populated trust anchor store that allows the client
to perform path validation for the certificate obtained during the
handshake with the server.
This document only focuses on the description of the DTLS client-side
functionality.
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+////////////////////////////////////+
| Configuration |
|////////////////////////////////////|
| Server A --> PSK Identity, PSK |
| Server B --> Public Key (Server B),|
| Public Key (Client) |
| Server C --> Public Key (Client), |
| Trust Anchor Store |
+------------------------------------+
oo
oooooo
o
+------+
|Client|---
+------+ \
\ ,-------.
,' `. +------+
/ IP-based \ |Server|
( Network ) | A |
\ / +------+
`. ,'
'---+---' +------+
| |Server|
| | B |
| +------+
|
| +------+
+----------------->|Server|
| C |
+------+
Figure 1: Constrained DTLS Client Profile.
4. 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:
o Authentication and Key Exchange Algorithm (e.g., PSK)
o Cipher and Key Length (e.g., AES with 128 bit keys)
o Mode of operation (e.g., CBC)
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o Hash Algorithm for Integrity Protection (e.g., SHA in combination
with HMAC)
o Hash Algorithm for use with the Pseudorandom Function (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. RFC 6655
[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 [RFC5116]. 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 CBC-
MAC (CCM) mode, and the Galois/Counter Mode (GCM).
Some AEAD ciphersuites have shorter authentication tags 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 16-octet authentication tags.
TLS 1.2 also replaced the combination of MD5/SHA-1 hash functions in
the TLS pseudo random function (PRF) 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.
This document references the CoAP recommended ciphersuite choices,
which have been selected based on implementation and deployment
experience from the IoT community. Over time the preference for
certain algorithms will, however, change. Not all components of a
ciphersuite change at the same speed. Changes are more likely to
expect for ciphers, the mode of operation, and the hash algorithms.
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Some deployment environments will also be impacted by local
regulation, which might dictate a certain and less likely for public
key algorithms (such as RSA vs. ECC).
5. Pre-Shared Secret Authentication with DTLS
The use of pre-shared secret credentials is one of the most basic
techniques for DTLS since it is both computational efficient and
bandwidth conserving. Pre-shared secret based authentication was
introduced to TLS with RFC 4279 [RFC4279]. The exchange shown in
Figure 2 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.
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 2: DTLS PSK Authentication including the Cookie Exchange.
[RFC4279] does not mandate the use of any particular type of
identity. Hence, the TLS client and server clearly have to agree on
the identities and keys to be used. The mandated encoding of
identities in Section 5.1 of RFC 4279 aims to improve
interoperability for those cases where the identity is configured by
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a person using some management interface. Many IoT devices do,
however, not have a user interface and most of their credentials are
bound to the device rather than the user. Furthermore, credentials
are provisioned into trusted hardware modules or in the firmware by
the developers. 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 (as domain
names, Distinguished Names, or IP addresses have) and a bit-by-bit
comparison operation can then be used by the server-side
infrastructure.
As described in Section 3 clients may have pre-shared keys with
several different servers. The client indicates which key it uses by
including a "PSK identity" in the ClientKeyExchange message. To help
the client in selecting which PSK identity / PSK pair to use, the
server can provide a "PSK identity hint" in the ServerKeyExchange
message. For IoT environments a simplifying assumption is made that
the hint for PSK key selection is based on the domain name of the
server. Hence, servers SHOULD NOT send the "PSK identity hint" in
the ServerKeyExchange message and client MUST ignore the message.
This approach is inline with RFC 4279 [RFC4279].
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 environment
where management interfaces are used to provision identities and
keys. For the IoT environment, however, many devices are not
equipped with displays and input devices (e.g., keyboards). Hence,
keys are distributed as part of hardware modules or are embedded into
the firmware. As such, these restrictions are not applicable to this
profile.
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. This
ciphersuite makes use of the default TLS 1.2 Pseudorandom Function
(PRF), which uses HMAC with the SHA-256 hash function.
6. Raw Public Key Use with DTLS
The use of raw public keys with DTLS, as defined in [RFC7250], is the
first entry point into public key cryptography without having to pay
the price of certificates and a 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 TLS
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extensions had been defined, as shown in Figure 3, namely the
server_certificate_type and the client_certificate_type. To operate
this mechanism securely it is necessary to authenticate and authorize
the public keys out-of-band. This document therefore assumes that a
client implementation comes with one or multiple raw public keys of
servers, it has to communicate with, pre-provisioned. Additionally,
a device will have its own raw public key. To replace, delete, or
add raw public key to this list requires a software update, for
example using a firmware update mechanism.
Client Server
------ ------
ClientHello -------->
client_certificate_type
server_certificate_type
<------- HelloVerifyRequest
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
Figure 3: DTLS Raw Public Key Exchange including the Cookie 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 Elliptic
Curve Diffie Hellman (ECDHE) as the key establishment mechanism and
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an Elliptic Curve Digital Signature Algorithm (ECDSA) for
authentication. This ciphersuite make use of the AEAD capability in
DTLS 1.2 and utilizes an eight-octet authentication tag. Based on
the Diffie-Hellman it provides perfect forward secrecy (PFS). More
details about the PFS can be found in Section 11.
RFC 6090 [RFC6090] provides valuable information for implementing
Elliptic Curve Cryptography algorithms.
Since many IoT devices will either have limited ways to log error or
no ability at all, any error will lead to implementations attempting
to re-try the exchange.
7. Certificate Use with DTLS
The use of mutual certificate-based authentication is shown in
Figure 4, 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.
Because certificate validation requires that root keys be distributed
independently, the self-signed certificate that specifies the root
certificate authority is omitted from the chain. Client
implementations MUST be provisioned with a trust anchor store that
contains the root certificates. The use of the Trust Anchor
Management Protocol (TAMP) [RFC5934] is, however, not envisioned.
Instead IoT devices using this profile MUST rely a software update
mechanism to provision these trust anchors.
When DTLS is used to secure CoAP messages then the server provided
certificates MUST contain the fully qualified DNS domain name or
"FQDN". The coaps URI scheme is described in Section 6.2 of
[RFC7252]. This FQDN is stored in the SubjectAltName or in the CN,
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 RFC 6125 [RFC6125]. For the profile in this
specification does not assume dynamic discovery of local servers.
For client certificates the identifier used in the SubjectAltName or
in the CN MUST be an EUI-64 [EUI64], as mandated in Section 9.1.3.3
of [RFC7252].
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. While
multiple OCSP stapling [RFC6961] has recently been introduced as a
mechanism to piggyback OCSP request/responses inside the DTLS/TLS
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handshake to avoid the cost of a separate protocol handshake further
investigations are needed to determine its suitability for the IoT
environment.
Client Server
------ ------
ClientHello -------->
cached_information
<------- HelloVerifyRequest
ClientHello -------->
cached_information
ServerHello
cached_information
Certificate
ServerKeyExchange
CertificateRequest
<-------- ServerHelloDone
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Figure 4: DTLS Mutual Certificate-based Authentication.
Regarding the ciphersuite choice the discussion in Section 6 applies.
Further details about X.509 certificates can be found in
Section 9.1.3.3 of [RFC7252].
QUESTION: What restrictions regarding the depth of the certificate
chain should be made? Is one level enough?
8. Error Handling
DTLS uses the Alert protocol to convey error messages and specifies a
longer list of errors. However, not all error messages defined in
the TLS specification are applicable to this profile. All error
messages marked as RESERVED are only supported for backwards
compatibility with SSL and are therefore not applicable to this
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profile. Those include decryption_failed_RESERVED,
no_certificate_RESERVE, and export_restriction_RESERVED.
A number of the error messages are applicable only for certificate-
based authentication ciphersuites. Hence, for PSK and raw public key
use the following error messages are not applicable:
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 is not applicable 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.
Furthermore, the following errors should not occur based on the
description in this specification:
protocol_version: This document only focuses on one version of the
DTLS protocol.
insufficient_security: This error message indicates that the server
requires ciphers to be more secure. This document does, however,
specify the only acceptable ciphersuites and client
implementations must support them.
user_canceled: The IoT devices in focus of this specification are
assumed to be unattended.
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9. Session Resumption
Session resumption is a feature of DTLS that allows a client to
continue with an earlier established session state. The resulting
exchange is shown in Figure 5. 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.
Client Server
------ ------
ClientHello -------->
ServerHello
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Figure 5: DTLS Session Resumption.
Clients MUST implement session resumption to improve the performance
of the handshake (in terms of reduced number of message exchanges,
lower computational overhead, and less bandwidth conserved).
Since the communication model described in Section 3 does not assume
that the server is constrained. RFC 5077 [RFC5077] describing TLS
session resumption without server-side state is not utilized by this
profile.
10. TLS Compression
[I-D.ietf-uta-tls-bcp] recommends to always disable DTLS-level
compression due to attacks. For IoT applications compression at the
DTLS is not needed since application layer protocols are highly
optimized and the compression algorithms at the DTLS layer increase
code size and complexity.
This DTLS client profile does not include DTLS layer compression.
11. Perfect Forward Secrecy
Perfect forward secrecy (PFS) is designed to prevent the compromise
of a long-term secret key from affecting the confidentiality of past
conversations. The PSK ciphersuite recommended in the CoAP
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specification [RFC7252] does not offer this property since it does
not utilize a Diffie-Hellman exchange. [I-D.ietf-uta-tls-bcp] on the
other hand recommends using ciphersuites offering this security
property and so do the public key-based ciphersuites recommended by
the CoAP specification.
The use of PFS is certainly a tradeoff decision since on one hand the
compromise of long-term secrets of embedded devices is more likely
than with many other Internet hosts but on the other hand a Diffie-
Hellman exchange requires emphemeral key pairs to be generated, which
can be demanding from a performance point of view. Finally, the
impact of the disclosure of past conversations and the desire to
increase the cost for pervasive monitoring (see [RFC7258]) has to be
taken into account.
Our recommendation is to stick with the ciphersuite suggested in the
CoAP specification. New ciphersuites support PFS for pre-shared
secret-based authentication, such as
[I-D.schmertmann-dice-ccm-psk-pfs], and might be available as a
standardized ciphersuite in the future.
12. Keep-Alive
RFC 6520 [RFC6520] defines a heartbeat mechanism to test whether the
other peer is still alive. 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. There are three types of
exchanges that need to be analysed:
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
communcation may be triggered by specific events, such as opening
a door.
Since the upload happens on an irregular and unpredicable basis
and due to renumbering and Network Address Translation (NAT) a new
DTLS session or DTLS session resumption can be used.
In this case there is no use for a keep-alive extension for this
scenario.
Client-Initiated, Regular Data Uploads
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This is a variation of the previous case whereby data gets
uploaded on a regular basis, for example, based on frequent
temperature readings. With such regular exchange it can be
assumed that the DTLS context is still in kept at the IoT device.
If neither NAT bindings nor IP address changes occurred then the
DTLS record layer will not notice any changes. For the case where
IP and port changes happened it is necessary to re-create the DTLS
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. In this case, we consider server-initiated messages.
Since messages to the client may get blocked by intermediaries,
such as NATs and stateful packet filtering firewalls, the initial
connection setup is triggered by the client and then kept alive.
Since state expires fairly quickly at middleboxes regular
heartbeats are necessary whereby these keep-alive messages may be
exchanged at the application layer or within DTLS itself.
For this message exchange pattern the use of DTLS heartbeat
messages is quite useful. The MTU discovery mechanism, on the
other hand, is less likely to be relevant since for many IoT
deployments the must constrained link is the wireless interface at
the IoT device itself rather than somewhere in the network. Only
in more complex network topologies the situation might be
different.
For server-initiated messages the heartbeat extension can be
recommended.
13. Random Number Generation
The 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. Special care has to
be paid when generating random numbers in embedded systems as many
entropy sources available on desktop operating systems or mobile
devices might be missing, as described in [Heninger]. Consequently,
if not enough time is given during system start time to fill the
entropy pool then the output might be predictable and repeatable, for
example leading to the same keys generated again and again.
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Recommendation: IoT devices using DTLS MUST offer ways to generate
quality random numbers. Guidelines and requirements for random
number generation can be found in RFC 4086 [RFC4086].
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.
14. Client Certificate URLs
This RFC 6066 [RFC6066] extension allows to avoid sending client-side
certificates and URLs instead. This reduces the over-the-air
transmission.
This is certainly a useful extension when a certificate-based mode
for DTLS is used since the TLS cached info extension does not provide
any help with caching information on the server side.
Recommendation: Add support for client certificate URLs for those
environments where client-side certificates are used.
15. Trusted CA Indication
This RFC 6066 extension 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
exchange provides. Since the server does not necessarily know what
trust anchors the client has stored it includes intermediate CA certs
in the certificate payload as well to facilitate with certification
path construction and path validation.
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 and no such dynamic indication of trust
anchors is needed.
Recommendation: For IoT deployments where clients talk to a fixed,
pre-configured set of servers and where a software update mechanism
is available this extension is not recommended. Environments where
the client needs to interact with dynamically discovered DTLS servers
this extension may be useful to reduce the communication overhead.
Note, however, in that case the TLS cached info extension may help to
reduce the communication overhead for everything but the first
protocol interaction.
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16. Truncated MAC Extension
This RFC 6066 extension was introduced to reduces 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.
For CoAP, however, the recommended ciphersuites use the newer
Authenticated Encryption with Associated Data (AEAD) construct,
namely the CBC-MAC mode (CCM) with eight-octet authentication tags.
Recommendation: Since this profile only supports AEAD ciphersuites
this extension is not applicable.
17. Server Name Indication (SNI)
This RFC 6066 extension defines a mechanism for a client to tell a
TLS 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.
Recommendation: Unless it is known that a DTLS client does not
interact with a server in a hosting environment that requires such an
extension we advice to offer support for the SNI extension in this
profile.
18. 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 it to
allows 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).
Recommendation: Client implementations must support this extension.
19. Negotiation and Downgrading Attacks
CoAP demands version 1.2 of DTLS to be used and the earlier version
of DTLS is not supported. As such, there is no risk of downgrading
to an older version of DTLS. The work described in
[I-D.bmoeller-tls-downgrade-scsv] is therefore also not applicable to
this environment since there is no legacy server infrastructure to
worry about.
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QUESTION: Should we say something for non-CoAP use of DTLS?
To prevent the TLS renegotiation attack [RFC5746] clients MUST
respond to server-initiated renegotiation attempts with an Alert
message (no_renegotiation) and clients MUST NOT initiate them. TLS
and DTLS allows a client and a server who already have a TLS
connection to negotiate new parameters, generate new keys, etc by
initiating a TLS handshake using a ClientHello message.
Renegotiation happens in the existing TLS connection, with the new
handshake packets being encrypted along with application data.
20. 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 with many IoT deployments poses a challenge since
many of the IoT devices operate unattended, even though they will
initially be enabled by a human. The ability to control data sharing
and to configure preference will have to be provided at a system
level rather than at the level of a DTLS profile, 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 (potentially over a long period of
time). While this strong form of authentication will prevent mis-
attribution it also allows strong identification. This device-
related data collection (e.g., sensor recordings) will be associated
with other data to be truly useful and this extra data might include
personal data 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
version does. No explicit techniques, such as extra padding, have
been provided to make traffic analysis more difficult.
21. Security Considerations
This entire document is about security.
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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 also useful for changing
configuration information, for example, trust anchors and other
keying related information.
22. IANA Considerations
This document includes no request to IANA.
23. Acknowledgements
Thanks to Rene Hummen, Sye Loong Keoh, Sandeep Kumar, Eric Rescorla,
Russ Housley, Michael Richardson, Zach Shelby, and Sean Turner 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.
24. References
24.1. Normative References
[EUI64] "GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER (EUI-64)
REGISTRATION AUTHORITY", April 2010,
<http://standards.ieee.org/regauth/oui/tutorials/
EUI64.html>.
[I-D.ietf-tls-cached-info]
Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", draft-ietf-tls-
cached-info-16 (work in progress), February 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279, December
2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010.
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[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[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, March 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6520] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) Heartbeat Extension", RFC 6520, February 2012.
[RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
T. Kivinen, "Using Raw Public Keys in Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7250, June 2014.
[RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
TLS", RFC 7251, June 2014.
24.2. Informative References
[Heninger]
Heninger, N., Durumeric, Z., Wustrow, E., and A.
Halderman, "Mining Your Ps and Qs: Detection of Widespread
Weak Keys in Network Devices", 21st USENIX Security
Symposium,
https://www.usenix.org/conference/usenixsecurity12/
technical-sessions/presentation/heninger, 2012.
[I-D.bmoeller-tls-downgrade-scsv]
Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", draft-bmoeller-tls-downgrade-scsv-02 (work in
progress), May 2014.
[I-D.cooper-ietf-privacy-requirements]
Cooper, A., Farrell, S., and S. Turner, "Privacy
Requirements for IETF Protocols", draft-cooper-ietf-
privacy-requirements-01 (work in progress), October 2013.
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[I-D.ietf-lwig-tls-minimal]
Kumar, S., Keoh, S., and H. Tschofenig, "A Hitchhiker's
Guide to the (Datagram) Transport Layer Security Protocol
for Smart Objects and Constrained Node Networks", draft-
ietf-lwig-tls-minimal-01 (work in progress), March 2014.
[I-D.ietf-uta-tls-bcp]
Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of TLS and DTLS", draft-
ietf-uta-tls-bcp-01 (work in progress), June 2014.
[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-00 (work in
progress), February 2014.
[IANA-TLS]
IANA, "TLS Cipher Suite Registry",
http://www.iana.org/assignments/tls-parameters/
tls-parameters.xhtml#tls-parameters-4, 2014.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, July
2003.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[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, May 2006.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[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, May 2008.
[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
August 2008.
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[RFC5934] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
Management Protocol (TAMP)", RFC 5934, August 2010.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655, July 2012.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
June 2013.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, July
2013.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, June 2014.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014.
Author's Address
Hannes Tschofenig (editor)
ARM Ltd.
110 Fulbourn Rd
Cambridge CB1 9NJ
Great Britain
Email: Hannes.tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
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