dice H. Tschofenig, Ed.
Internet-Draft ARM Ltd.
Intended status: Standards Track October 26, 2014
Expires: April 29, 2015

A Datagram Transport Layer Security (DTLS) 1.2 Profile for the Internet of Things


This document defines a DTLS 1.2 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/.

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This Internet-Draft will expire on April 29, 2015.

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Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

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. The DTLS 1.2 protocol is based on Transport Layer Security (TLS) 1.2 [RFC5246] and provides equivalent security guarantees. This document aims to meet the following goals:

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 5.1, Section 5.2, Section 5.3 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 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:

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.

RFC 7228 [RFC7228] introduces the notion of constrained-node networks, which are small devices with severe constraints on power, memory, and processing resources. The terms constrained devices, and Internet of Things (IoT) devices are used interchangeably.

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 unicast communication interaction with an IoT device utilizing a DTLS client and that client interacts with one or multiple DTLS servers.

Clients are provisioned with information about the servers they need to initiate their DTLS exchange with and with credentials. This information 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:

This document focuses on the description of the DTLS client-side functionality but, quite naturally, the equivalent server-side support has to be available.

           |          Configuration             |
           | Server A --> PSK Identity, PSK     |
           | Server B --> Public Key (Server B),|
           |              Public Key (Client)   |
           | Server C --> Public Key (Client),  |
           |              Trust Anchor Store    |
   +------+   \
               \  ,-------.
                ,'         `.            +------+
               /  IP-based   \           |Server|
              (    Network    )          |  A   |
               \             /           +------+
                `.         ,'
                  '---+---'                  +------+
                      |                      |Server|
                      |                      |  B   |
                      |                      +------+
                      |                  +------+
                                         |  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:

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. 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. Credential Types

5.1. Pre-Shared Secret

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. The cookie exchange allows the server to react to flooding attacks.

      Client                                               Server
      ------                                               ------
      ClientHello                 -------->

                                  <--------    HelloVerifyRequest
                                                (contains cookie)

      ClientHello                  -------->
      (with cookie)
                                   <--------      ServerHelloDone
      Finished                     -------->
                                   <--------             Finished
      Application Data             <------->     Application Data


* 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 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.

The client indicates which key it uses by including a "PSK identity" in the ClientKeyExchange message. As described in Section 3 clients may have multiple pre-shared keys with a single server and 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. If the hint for PSK key selection is based on the domain name of the server then servers SHOULD NOT send the "PSK identity hint" in the ServerKeyExchange message. 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]. Note: The TLS Server Name Indication (SNI) extension allows the client to tell a server the name of the server it is contacting, which is relevant for hosting environments. A server using the identity hint 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 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 an HMAC with the SHA-256 hash function. (Note that all IoT implementations will need a SHA-256 implementation due to the construction of the pseudo-random number function in TLS 1.2.)

A device compliant with this protocol MUST implement TLS_PSK_WITH_AES_128_CCM_8 and follow the guidance from this section.

5.2. Raw Public Key

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 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             -------->

                         <-------    HelloVerifyRequest

 ClientHello             -------->

                         <--------      ServerHelloDone

 Finished                -------->

                         <--------             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 Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) as the key establishment mechanism and an Elliptic Curve Digital Signature Algorithm (ECDSA) for authentication. Due to the use of Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) the recently introduced named Diffie-Hellman groups [I-D.ietf-tls-negotiated-dl-dhe] are not applicable to this profile. This ciphersuite make use of the AEAD capability in DTLS 1.2 and utilizes an eight-octet authentication tag. The use of a Diffie-Hellman key exchange adds perfect forward secrecy (PFS). More details about PFS can be found in Section 9.

RFC 6090 [RFC6090] provides valuable information for implementing Elliptic Curve Cryptography algorithms, particularly for choosing methods that have been published more than 20 years ago.

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. Implementers have to carefully evaluate the impact of errors and ways to remedy the situation since a commonly used approach for delegating decision making to users is difficult in a timely fashion (or impossible).

A device compliant with this protocol MUST implement TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 and follow the guidance from this section.

5.3. Certificates

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 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 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 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             -------->

                         <-------    HelloVerifyRequest

 ClientHello             -------->
                         <--------      ServerHelloDone

 Finished                -------->

                         <--------             Finished


Figure 4: DTLS Mutual Certificate-based Authentication.

Regarding the ciphersuite choice the discussion in Section 5.2 applies. Further details about X.509 certificates can be found in Section of [RFC7252]. The TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 ciphersuite description in Section 5.2 is also applicable to this section.

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 resources to process, transmit or receive.

A device compliant with this protocol MUST implement TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 and follow the guidance from this section.

6. 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 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:

  • bad_certificate,
  • unsupported_certificate,
  • certificate_revoked,
  • certificate_expired,
  • certificate_unknown,
  • unknown_ca, and
  • 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:

This document only focuses on one version of the DTLS protocol.
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.
The IoT devices in focus of this specification are assumed to be unattended.

7. 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                   -------->
                                    <--------             Finished
      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.

8. 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.

9. 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 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 trade-off 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 ephemeral 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.

10. 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 communication may be triggered by specific events, such as opening a door.

Since the upload happens on an irregular and unpredictable 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

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.

11. 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.

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.

The ClientHello and the ServerHello message contains the 'Random' structure, which has two components: gmt_unix_time and a random 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). [I-D.mathewson-no-gmtunixtime] argues that the entire value the ClientHello.Random and ServerHello.Random fields, including gmt_unix_time, should be set to a cryptographically random sequence because of privacy concerns (fingerprinting). Since many IoT devices do not have access to a real-time clock this recommendation is even more relevant in the embedded systems environment.

12. 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.

13. 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.

14. Truncated MAC Extension

The truncated MAC extension was introduced with RFC 6066 with the goal 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. Furthermore, the extension [RFC7366] introducing the encrypt-then-MAC security mechanism (instead of the MAC-then-encrypt) is also not applicable for this profile.

Recommendation: Since this profile only supports AEAD ciphersuites this extension is not applicable.

15. 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.

16. 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.

17. TLS Session Hash

The TLS master secret is not cryptographically bound to important session parameters such as 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.

[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.

Recommendation: Client implementations SHOULD implement this extension support this extension even though the ciphersuites recommended by this profile are not vulnerable 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.

18. 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.

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.

19. 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 provisioned 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 the DTLS exchange itself, 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) 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 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.

20. 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 also useful for changing configuration information, for example, trust anchors and other keying related information.

21. IANA Considerations

This document includes no request to IANA.

22. Acknowledgements

Thanks to Robert Cragie, Russ Housley, Rene Hummen, Sandeep Kumar, Sye Loong Keoh, Eric Rescorla, Michael Richardson, Zach Shelby, Michael StJohns, 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.

23. References

23.1. Normative References

, "
[I-D.ietf-tls-cached-info] Santesson, S. and H. Tschofenig, "Transport Layer Security (TLS) Cached Information Extension", Internet-Draft draft-ietf-tls-cached-info-14, March 2013.
[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", Internet-Draft draft-ietf-tls-session-hash-01, August 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.
[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.

23.2. 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", Internet-Draft draft-bmoeller-tls-downgrade-scsv-01, November 2013.
[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", Internet-Draft draft-ietf-lwig-tls-minimal-00, September 2013.
[I-D.ietf-tls-negotiated-dl-dhe] Gillmor, D., "Negotiated Discrete Log Diffie-Hellman Ephemeral Parameters for TLS", Internet-Draft draft-ietf-tls-negotiated-dl-dhe-00, July 2014.
[I-D.ietf-uta-tls-bcp] Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations for Secure Use of TLS and DTLS", Internet-Draft draft-ietf-uta-tls-bcp-01, June 2014.
[I-D.mathewson-no-gmtunixtime] Mathewson, N. and B. Laurie, "Deprecating gmt_unix_time in TLS", Internet-Draft draft-mathewson-no-gmtunixtime-00, December 2013.
[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)", Internet-Draft draft-schmertmann-dice-ccm-psk-pfs-00, February 2014.
[IANA-TLS] IANA, "TLS Cipher Suite Registry", http://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-4, 2014.
[RFC4086] Eastlake, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005.
[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.
[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.
[RFC7228] Bormann, C., Ersue, M. and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, May 2014.
[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.
[RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7366, September 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