dice H. Tschofenig, Ed.
Internet-Draft ARM Ltd.
Intended status: Standards Track T. Fossati
Expires: December 13, 2015 Alcatel-Lucent
June 11, 2015

A TLS/DTLS Profile for the Internet of Things


A common design pattern in Internet of Things (IoT) deployments is the use of a constrained device that collects data via sensor 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.

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This Internet-Draft will expire on December 13, 2015.

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Table of Contents

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]. UDP is mainly used to carry CoAP messages but other transports can be utilized, such as SMS or even TCP.

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.

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].

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 "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 4.

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

3. TLS/DTLS Protocol Overview

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 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 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:

4. 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.

4.1. 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 credentials) 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:

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.

           |          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    |
|Client     |-
+-----------+ \
               \  ,-------.
                ,'         `.            +------+
               /  IP-based   \           |Server|
              (    Network    )          |  A   |
               \             /           +------+
                `.         ,'
                  '---+---'                  +------+
                      |                      |Server|
                      |                      |  B   |
                      |                      +------+
                      |                  +------+
                                         |  C   |


Figure 1: Constrained Client Profile.

4.1.1. Examples of Constrained Client Exchanges 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 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].

                                             |Authorization |
                                             |Accounting    |
                                             |Server        |
                                             |(EAP Server)  |
                                             |              |
                                               * EAP      o RADIUS/
                                               *          o Diameter
                                          ///                \\\
                                        //                      \\
                                       |        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         |               |
 |             |<===========================>|               |
 +-------------+                             +---------------+

    <****>: 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 exchange shown in Figure 3 the IoT device as the EAP peer acts as a TLS client.

   Authenticating Peer     Authenticator
   -------------------     -------------
                           <- EAP-Request/
   Identity (MyID) ->
                           <- EAP-Request/
                           (TLS Start)
   (TLS client_hello)->
                           <- EAP-Request/
                           (TLS server_hello,
                             TLS certificate,
                    [TLS server_key_exchange,]
                     TLS certificate_request,
                        TLS server_hello_done)
   (TLS certificate,
    TLS client_key_exchange,
    TLS certificate_verify,
    TLS change_cipher_spec,
    TLS finished) ->
                           <- EAP-Request/
                           (TLS change_cipher_spec,
                            TLS finished)
   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. 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, 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 to register available resources. [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.

    Sensor                                       Directory
    ------                                       ---------

  | ClientHello             -------->
  | #client_certificate_type#
 F| #server_certificate_type#
 L|                         <-------    HelloVerifyRequest
  | ClientHello             -------->
 D| #client_certificate_type#
 T| #server_certificate_type#
 S|                                            ServerHello
  |                               #client_certificate_type#
 H|                               #server_certificate_type#
 A|                                            Certificate
 N|                                      ServerKeyExchange
 D|                                     CertificateRequest
 S|                         <--------      ServerHelloDone
 A| Certificate
 K| ClientKeyExchange
 E| CertificateVerify
  | [ChangeCipherSpec]
  | Finished                -------->
  |                                     [ChangeCipherSpec]
  |                         <--------             Finished

  +---                                                  ///+
 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
  +---                                                  ///+

   Note: Extensions marked with '#' were introduced with
         RFC 7250.


Figure 4: DTLS/CoAP exchange using Resource Directory.

4.2. Constrained TLS/DTLS Servers

Section 4.1 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. 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 5 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.

         |          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      |
+---------------+                +-----------+
|Authentication |      +-------->|TLS/DTLS   |
|& Authorization|      |         |Client A   |
|Server         |      |         +-----------+
+---------------+     ++
             ^        |                  +-----------+
              \       |                  |Constrained|
               \  ,-------.              | Server S1 |
                ,'         `.            +-----------+
               /    Local    \
              (    Network    )
               \             /        +-----------+
                `.         ,'         |Constrained|
                  '---+---'           | Server S2 |
                      |               +-----------+
                      |                   +-----------+
                      +-----------------> |Constrained|
                                          | Server S3 |


Figure 5: Constrained Server Profile.

A deployment with constrained servers has to overcome several challenges. Below we explain how these challenges have 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 factor will impact deployment.

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.

The next challenge concerns the provisioning of authentication credentials to the clients as well as servers. In Section 4.1 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] these different communication patterns. 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 an shared key (for example using an unauthenticated Diffie-Hellman exchange) key. To avoid man-in-the-middle attacks an out-of-band channel is used to verify that nobody has tampered with the 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 to 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].

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 more fine-grained and dynamic access control the reader is referred to the ongoing work in the ACE working group.

Figure 6 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 site-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 endpoint and to secure the communication.

  Thermostat                     Sensor              Fan
  ----------                   ---------             ---

    GET coap://[FF02::FD]/.well-known/core

                  CoAP 2.05 Content

                                     CoAP 2.05 Content

\                                                        /
\ Protocol steps to obtain access token or keying        /
\ material for access to the temperature sensor and fan. /
\                                                        /

   Read Sensor Data 
   GET /3303/0/5700

                 CoAP 2.05 Content
                            22.5 C

  Configure Actuator 
  PUT /fan?on-off=true

                                   CoAP 2.04 Changed


Figure 6: Local Discovery and Resource Access.

5. 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:

  • Authentication and key exchange algorithm (e.g., PSK)
  • Cipher and key length (e.g., Advanced Encryption Standard (AES) with 128 bit keys [AES])
  • Mode of operation (e.g., Counter with Cipher Block Chaining - Message Authentication Code (CBC-MAC) Mode (CCM) for AES) [RFC3610]
  • 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])
  • Hash algorithm for use with pseudorandom functions (e.g., HMAC with the SHA-256)
  • Misc information (e.g., length of authentication tags)
  • 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].

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.

6. 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.

6.1. Pre-Shared Secret

The use of pre-shared secrets is one of the most basic techniques for TLS/DTLS since it is both computational efficient and bandwidth conserving. Pre-shared secret based authentication was introduced to TLS with RFC 4279 [RFC4279]. When using a pre-shared secret, a critical consideration is how to assure the randomness of these secrets. The best practice is to ensure that any pre-shared key contains as much randomness as possible. Deriving a shared secret from a password, name, or other low-entropy source is not secure. A low-entropy secret, or password, is subject to dictionary attacks.

The exchange shown in Figure 7 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 7: 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 into the firmware by 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 (such as domain names, Distinguished Names, or IP addresses) and a bit-by-bit comparison operation MUST be used by the server for any operation related to the PSK identity.

Protocol-wise the client indicates which key it uses by including a "PSK identity" in the ClientKeyExchange message. As described in Section 4 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. 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.

6.2. 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 8, 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 key distribution step may, for example, be provided by a dedicated protocol, such as the OMA LWM2M [LWM2M]. 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. To replace, delete, or add raw public keys to this list requires a software update, for example using a firmware update mechanism. Additionally, a device will have its own raw public key and the corresponding private key. This key pair may, for example, be configured during the manufacturing process of the device.

 Client                                          Server
 ------                                          ------

 ClientHello             -------->

                         <--------      ServerHelloDone

 Finished                -------->

                         <--------             Finished

Note: Extensions marked with '#' were introduced with 
      RFC 7250.


Figure 8: 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. 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 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 11.

[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.

6.3. Certificates

The use of mutual certificate-based authentication is shown in Figure 9, 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. Because certificate validation requires that root keys be distributed independently, the self-signed certificate that specifies the root certification authority is omitted from the chain. Client implementations MUST be provisioned with a trust anchor store that contains these root certificates. The use of the Trust Anchor Management Protocol (TAMP) [RFC5934] is, however, not envisioned. Instead IoT devices using this profile MUST use a software update mechanism to populate the trust anchor store.

 Client                                          Server
 ------                                          ------

 ClientHello             -------->

                         <--------      ServerHelloDone

 Finished                -------->

                         <--------             Finished

Note: Extensions marked with '*' were introduced with 


Figure 9: DTLS Mutual Certificate-based Authentication.

TLS/DTLS offers a lot of freedom for the use with ECC. This document restricts the use of ECC ciphersuites 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. To offer elliptic-curve signatures using the Edwards-curve Digital Signature Algorithm (EdDSA) standardization work on Ed25519 is also ongoing (see [I-D.josefsson-eddsa-ed25519]).

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.

6.3.1. 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:

  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 certificate usage without comparing the reference identifier against the presented identifier obtained from the certificate breaks security.

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 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 contains 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 6. When such constrained servers are used then the use of certificates as described in Section 6.3.2 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.

6.3.2. 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, as mandated in Section of [RFC7252].

6.3.3. 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.

6.3.4. Certificate Content

All certificate elements listed in Table 1 are mandatory-to-implement for client 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.

Table 1 provides a summary of the elements in a certificate for use with this profile.

Certificate Content.
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: subjectAltName See rules outlined in this section.
Extension: BasicConstraints Indicates whether the subject of the 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 values in the context of this profile: digitalSignature or keyAgreement, keyCertSign for verifying signatures on public key certificates.
Extension: Extended Key Usage The ExtKeyUsageSyntax field MAY have the 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.

There are various algorithms used to sign digital certificates, such as RSA, the Digital Signature Algorithm (DSA), and the Elliptic Curve Digital Signature Algorithm (ECDSA). As Table 1 indicates certificate 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.

Further details about X.509 certificates can be found in Section of [RFC7252].

6.3.5. 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.

6.3.6. 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 exchange provides. Since the server does not necessarily know what trust anchors the client has stored and to facilitate certification path construction as well as path 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 4.2, the use of this extension is RECOMMENDED.

7. Signature Algorithm Extension

The "signature_algorithms" extension, defined in Section 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 6.1.

8. 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 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 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:

  • 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 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:

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.
This error message indicates that the server requires ciphers to be more secure. This document specifies only one ciphersuite per profile but it is likely that additional ciphersuites get added over time.
Many IoT devices are unattended and hence this error message is unlikely to occur.

9. 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 10. 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                   -------->
                                    <--------             Finished
      Finished                      -------->
      Application Data              <------->     Application Data


Figure 10: 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 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.

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.

10. Compression

Section 3.3 of [RFC7525] recommends to disable TLS/DTLS-level compression due to attacks, such as 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.

This TLS/DTLS profile MUST NOT implement TLS/DTLS layer compression.

11. Perfect Forward Secrecy

Perfect forward secrecy (PFS) is a property that preserves the confidentiality of past conversations even in situations where the long-term secret is compromised.

The PSK ciphersuite recommended in Section 6.1 does not offer this property since it does not utilize a Diffie-Hellman 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 raw public might offer an acceptable middle ground between the PSK ciphersuite in terms of out-of-band validation and the functionality offered by asymmetric cryptography.

The use of PFS is 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 is demanding from a performance point of view. For obvious performance improvement, some implementations re-use key pairs over multiple exchanges (rather than generating new keys for each exchange). However, note that such key re-use over long periods voids the benefits of forward secrecy when an attack gains access to this DH key pair.

The impact of the disclosure of past conversations and the desire to increase the cost for pervasive monitoring (as demanded by [RFC7258]) has to be taken into account when making a deployment decision.

Client implementations claiming support of this profile MUST implement the ciphersuites listed in Section 6 according to the selected credential type.

12. 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.

Since the upload happens on an irregular and unpredictable basis and due to renumbering and Network Address Translation (NAT) the DTLS handshake may need to be re-started (ideally using session resumption, if possible).

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 of RFC 6347 by using the fragmentation capability of the handshake protocol).

For server-initiated messages the heartbeat extension is RECOMMENDED.

13. Timeouts

To connect to the Internet a variety of wired and wireless technologies are available. 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 RFC 4919 [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 re-transmissions, as described in Section 4.2.4 of [RFC6347]. Although the timeout values are implementation specific, recommendations are provided in Section of [RFC6347], with an initial timer value of 1 second and doubled with at each retransmission up to no less than 60 seconds. Due to the nature of some radio technologies, these values are too aggressive and lead to spurious failures when messages in flight need longer.

Note: If a round-trip time estimator (such as proposed in [I-D.bormann-core-cocoa]) is available in the protocol stack of the device, it could be used to dynamically update the setting of the retransmit timeout.

Choosing appropriate timeout values is difficult with changing network conditions, and large variance in latency. This specification therefore RECOMMENDS an initial timer value of 10 seconds with exponential back off up to no less then 60 seconds. Appendix A provides additional normative text for carrying DTLS over SMS.

14. 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. Special care has to be taken 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.

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 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). [I-D.mathewson-no-gmtunixtime] argues that the entire ClientHello.Random value (including gmt_unix_time) should be a sequence of random bits because of device fingerprinting privacy concerns. Since many IoT devices do not have access to an accurate clock, it is RECOMMENDED to follow the guidance outlined in [I-D.mathewson-no-gmtunixtime] regarding the content of the ClientHello.Random field. However, for the ServerHello.Random structure it is RECOMMENDED to maintain the existing structure with gmt_unix_time followed by a sequence of 28 random bytes since the client can use the received time information to securely obtain time information. For constrained servers it cannot be assumed that they maintain accurate time information; these devices MUST include time information in the Server.Random structure when they actually obtain accurate time information that can be utilized by clients. Clients MUST only use time information obtained from servers they trust and the use of this approach has to be agreed out-of-band.

IoT devices using TLS/DTLS MUST offer ways to generate quality random numbers using hardware-based random number generators. Note that these hardware-based random number generators do not necessarily need to be implemented inside the microcontroller itself but could be made available in dedicated crypto-chips as well. Guidelines and requirements for random number generation can be found in RFC 4086 [RFC4086] and in the NIST Special Publication 800-90a [SP800-90A].

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.

15. 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. Hence, RFC 7366 and RFC 6066 are not applicable to this specification and MUST NOT be implemented.

16. 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.

This specification RECOMMENDs the implementation of the Server Name Indication extension unless it is known that a TLS/DTLS client does not interact with a server in a hosting environment.

17. 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.

18. 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.

19. 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.

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 RECOMMENDS 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.

20. 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 [I-D.ietf-tls-sslv3-diediedie]).

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:

  • Clients MUST NOT send a TLS/DTLS version lower than version 1.2 in the ClientHello.
  • Clients MUST NOT retry a failed negotiation offering a TLS/DTLS version lower than 1.2.
  • 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.

If at some time in the future this profile reaches the quality of SSL 3.0 a software update is needed since constrained devices are unlikely to run multiple TLS/DTLS versions due to memory size restrictions.

21. 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 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. Some deployment environments will also be impacted by local regulation, which might dictate a certain cipher and key size. 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:

  • Make any AES hardware-based crypto implementation accessible to developers working on security implementations at higher layers. 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.
  • 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.
  • 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 recommend 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.

22. 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 11 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 11: 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.

23. 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.bmoeller-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

  • Modern symmetric ciphers with an effective key length of 128 bits, such as AES-128-CCM
  • Client certificate types, such as ecdsa_sign
  • 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.

24. 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) 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.

25. 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].

26. IANA Considerations

This document includes no request to IANA.

27. Acknowledgments

Thanks to Derek Atkins, Olaf Bergmann, Paul Bakker, Robert Cragie, Russ Housley, Rene Hummen, Jayaraghavendran K, Matthias Kovatsch, Sandeep Kumar, Sye Loong Keoh, Simon Lemay, Alexey Melnikov, Manuel Pegourie-Gonnard, Akbar Rahman, Eric Rescorla, Michael Richardson, Ludwig Seitz, Zach Shelby, Michael StJohns, Rene Struik, 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.

Finally, we would like to thank our area director (Stephen Farrell) and our working group chairs (Zach Shelby and Dorothy Gellert) for their support.

28. References

28.1. Normative References

, "
[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", Internet-Draft draft-ietf-tls-cached-info-19, 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", Internet-Draft draft-ietf-tls-session-hash-05, April 2015.
[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.
[WAP-WDP] Wireless Application Protocol Forum, "Wireless Datagram Protocol", June 2001.

28.2. 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)", http://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-4, 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.
[ENISA-Report2013] ENISA, "Algorithms, Key Sizes and Parameters Report - 2013", http://www.enisa.europa.eu/activities/identity-and-trust/library/deliverables/algorithms-key-sizes-and-parameters-report, October 2013.
[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.
[HomeGateway] Eggert, L., "An experimental study of home gateway characteristics, In Proceedings of the '10th annual conference on Internet measurement'", 2010.
[I-D.bmoeller-tls-falsestart] Langley, A., Modadugu, N. and B. Moeller, "Transport Layer Security (TLS) False Start", Internet-Draft draft-bmoeller-tls-falsestart-01, November 2014.
[I-D.bormann-core-cocoa] Bormann, C., Betzler, A., Gomez, C. and I. Demirkol, "CoAP Simple Congestion Control/Advanced", Internet-Draft draft-bormann-core-cocoa-02, July 2014.
[I-D.ietf-core-resource-directory] Shelby, Z. and C. Bormann, "CoRE Resource Directory", Internet-Draft draft-ietf-core-resource-directory-02, November 2014.
[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-tls-sslv3-diediedie] Barnes, R., Thomson, M., Pironti, A. and A. Langley, "Deprecating Secure Sockets Layer Version 3.0", Internet-Draft draft-ietf-tls-sslv3-diediedie-03, April 2015.
[I-D.irtf-cfrg-curves] Langley, A. and R. Salz, "Elliptic Curves for Security", Internet-Draft draft-irtf-cfrg-curves-02, March 2015.
[I-D.josefsson-eddsa-ed25519] Josefsson, S. and N. Moller, "EdDSA and Ed25519", Internet-Draft draft-josefsson-eddsa-ed25519-03, May 2015.
[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-01, August 2014.
[IANA-TLS] IANA, "TLS Cipher Suite Registry", http://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", http://www.keylength.com, November 2014.
[LWM2M] Open Mobile Alliance, "Lightweight Machine-to-Machine, Technical Specification, Candidate Version 1.0", December 2013.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990.
[RFC1981] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000.
[RFC3610] Whiting, D., Housley, R. and N. Ferguson, "Counter with CBC-MAC (CCM)", RFC 3610, September 2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H. Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004.
[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.
[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, August 2007.
[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.
[RFC5216] Simon, D., Aboba, B. and R. Hurst, "The EAP-TLS Authentication Protocol", RFC 5216, March 2008.
[RFC5247] Aboba, B., Simon, D. and P. Eronen, "Extensible Authentication Protocol (EAP) Key Management Framework", RFC 5247, August 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.
[RFC5288] Salowey, J., Choudhury, A. and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, August 2008.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R. and T. Polk, "Elliptic Curve Cryptography Subject Public Key Information", RFC 5480, March 2009.
[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, January 2010.
[RFC5934] Housley, R., Ashmore, S. and C. Wallace, "Trust Anchor Management Protocol (TAMP)", RFC 5934, August 2010.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management Requirements", RFC 6024, October 2010.
[RFC6090] McGrew, D., Igoe, K. and M. Salter, "Fundamental Elliptic Curve Cryptography Algorithms", RFC 6090, February 2011.
[RFC6234] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for Transport Layer Security (TLS)", RFC 6655, July 2012.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, August 2012.
[RFC6733] Fajardo, V., Arkko, J., Loughney, J. and G. Zorn, "Diameter Base Protocol", RFC 6733, October 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.
[RFC7390] Rahman, A. and E. Dijk, "Group Communication for the Constrained Application Protocol (CoAP)", RFC 7390, October 2014.
[RFC7397] Gilger, J. and H. Tschofenig, "Report from the Smart Object Security Workshop", RFC 7397, December 2014.
[RFC7400] Bormann, C., "6LoWPAN-GHC: Generic Header Compression for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC 7400, November 2014.
[RFC7452] Tschofenig, H., Arkko, J., Thaler, D. and D. McPherson, "Architectural Considerations in Smart Object Networking", RFC 7452, March 2015.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, February 2015.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks", RFC 7507, April 2015.
[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, May 2015.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF Protocols", RFC 7539, May 2015.
[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", 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", 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 13 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.

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 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 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 12 and Figure 13.

0        1        2        3        4
| ...    | 0x04   | 2      | ...    | ...    |
  UDH      IEI      IE       Dest     Source
  Length            Length   Port     Port


Figure 12: 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 13: 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 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 14 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 14: AES-128-CCM-8 DTLS Record Layer Per-Packet Overhead.

The DTLS record layer header has 13 octets and consists of

  • 1 octet content type field,
  • 2 octet version field,
  • 2 octet epoch field,
  • 6 octet sequence number,
  • 2 octet length field.

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

[Editor's Note: Proposed text that requires discussion. ]

Section 4.2.3 of [RFC6347] advises DTLS implementations to not produce overlapping fragments, but requires receivers to be able to cope with them. The need for the latter requisite is explained in Section of [RFC6347]: accurate path MTU (PMTU) estimation may be traded for shorter handshake completion time. This approach may be beneficial in unconstrained networks where a PMTU of 1280 bytes can be pretty much universally assumed. However, an handshake that is carried over a narrow-band radio technology, such as IEEE 802.15.4, Bluetooth Smart or GSM-SMS, and the client is lacking reliable PMTU data to inform fragmentation (e.g., using [RFC1981] or [RFC1191]) can place a cost on the constrained implementation in terms of memory (due to re-buffering) and latency (due to re-transmission) much higher than the benefit that it would get from a shorter handshake.

In order to reduce the likelihood of producing different fragment sizes (and consequent overlaps) within the same handshake, this document RECOMMENDs:

  • for clients (handshake initiators), to perform PMTU discovery towards the server before handshake starts, and not rely on any guesses (unless the network path characteristics are reliably known from another source);
  • for servers, to mirror the fragment size selected by their clients.

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