TLS/DTLS 1.3 Profiles for the Internet of Things

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Table of Contents

1. Introduction

This document defines a profile of DTLS 1.3 [DTLS13] and TLS 1.3 [RFC8446] 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.¶

For IoT profiles using TLS/DTLS 1.2 please consult [RFC7925]. This document re-uses the communication pattern defined in [RFC7925] and makes IoT-domain specific recommendations for version 1.3 (where necessary).¶

TLS 1.3 has been re-designed and several previously defined extensions are not applicable to the new version of TLS/DTLS anymore. This clean-up also simplifies this document. Furthermore, many outdated ciphersuites have been omitted from the TLS/DTLS 1.3 specification.¶

2. Credential Types

In accordance with the recommendations in [RFC7925], a compliant implementation MUST implement TLS_AES_128_CCM_8_SHA256. It SHOULD implement TLS_CHACHA20_POLY1305_SHA256.¶

Pre-shared key based authentication is integrated into the main TLS/DTLS 1.3 specification and has been harmonized with session resumption.¶

A compliant implementation supporting authentication based on certificates and raw public keys MUST support digital signatures with ecdsa_secp256r1_sha256. A compliant implementation MUST support the key exchange with secp256r1 (NIST P-256) and SHOULD support key exchange with X25519.¶

A plain PSK-based TLS/DTLS client or server MUST implement the following extensions:¶

• Supported Versions,¶
• Cookie,¶
• Server Name Indication (SNI),¶
• Pre-Shared Key,¶
• PSK Key Exchange Modes, and¶
• Application-Layer Protocol Negotiation (ALPN).¶

For use of external pre-shared keys [RFC9258] makes the following recommendation:¶

• Applications SHOULD provision separate PSKs for (D)TLS 1.3 and prior versions.¶

Where possible, the importer interface defined in [RFC9258] MUST be used for external PSKs. This ensures that external PSKs used in (D)TLS 1.3 are bound to a specific key derivation function (KDF) and hash function.¶

The SNI extension is discussed in this document and the justification for implementing and using the ALPN extension can be found in [RFC9325].¶

For TLS/DTLS clients and servers implementing raw public keys and/or certificates the guidance for mandatory-to-implement extensions described in Section 9.2 of [RFC8446] MUST be followed.¶

3. Error Handling

TLS 1.3 simplified the Alert protocol but the underlying challenge in an embedded context remains unchanged, namely what should an IoT device do when it encounters an error situation. The classical approach used in a desktop environment where the user is prompted is often not applicable with unattended devices. Hence, it is more important for a developer to find out from which error cases a device can recover from.¶

4. Session Resumption

TLS 1.3 has built-in support for session resumption by utilizing PSK-based credentials established in an earlier exchange.¶

5. Compression

TLS 1.3 does not have support for compression of application data traffic, as offered by previous versions of TLS. Applications are therefore responsible for transmitting payloads that are either compressed or use a more efficient encoding otherwise.¶

With regards to the handshake itself, various strategies have been applied to reduce the size of the exchanged payloads. TLS and DTLS 1.3 use less overhead, depending on the type of key confirmations, when compared to previous versions of the protocol. Additionally, the work on Compact TLS (cTLS) [I-D.ietf-tls-ctls] has taken compression of the handshake a step further by utilizing out-of-band knowledge between the communication parties to reduce the amount of data to be transmitted at each individual handshake, among applying other techniques.¶

6. Perfect Forward Secrecy

TLS 1.3 allows the use of PFS with all ciphersuites since the support for it is negotiated independently.¶

7. Keep-Alive

The discussion in Section 10 of [RFC7925] is applicable.¶

8. Timeouts

The recommendation in Section 11 of [RFC7925] is applicable. In particular this document RECOMMENDED to use an initial timer value of 9 seconds with exponential back off up to no less then 60 seconds.¶

9. Random Number Generation

The discussion in Section 12 of [RFC7925] is applicable with one exception: the ClientHello and the ServerHello messages in TLS 1.3 do not contain gmt_unix_time component anymore.¶

10. Server Name Indication

This specification mandates the implementation of the Server Name Indication (SNI) extension. Where privacy requirements require it, the Encrypted Client Hello extension [I-D.ietf-tls-esni] prevents an on-path attacker to determine the domain name the client is trying to connect to.¶

Note: To avoid leaking DNS lookups from network inspection altogether further protocols are needed, including DoH [RFC8484] and DPRIVE [RFC7858] [RFC8094]. Since the Encrypted Client Hello extension requires use of Hybrid Public Key Encryption (HPKE) [I-D.irtf-cfrg-hpke] and additional protocols require further protocol exchanges and cryptographic operations, there is a certain overhead associated with this privacy feature.¶

11. Maximum Fragment Length Negotiation

The Maximum Fragment Length Negotiation (MFL) extension has been superseded by the Record Size Limit (RSL) extension [RFC8449]. Implementations in compliance with this specification MUST implement the RSL extension and SHOULD use it to indicate their RAM limitations.¶

12. Crypto Agility

The recommendations in Section 19 of [RFC7925] are applicable.¶

13. Key Length Recommendations

The recommendations in Section 20 of [RFC7925] are applicable.¶

14. 0-RTT Data

Appendix E.5 of [TLS13] establishes that:¶

• Application protocols MUST NOT use 0-RTT data without a profile that defines its use. That profile needs to identify which messages or interactions are safe to use with 0-RTT and how to handle the situation when the server rejects 0-RTT and falls back to 1-RTT.¶

At the time of writing, no such profile has been defined for CoAP [CoAP]. Therefore, 0-RTT MUST NOT be used by CoAP applications.¶

15. Certificate Profile

This section contains updates and clarifications to the certificate profile defined in [RFC7925]. The content of Table 1 of [RFC7925] has been split by certificate "type" in order to clarify exactly what requirements and recommendations apply to which entity in the PKI hierarchy.¶

16. Certificate Revocation Checks

The considerations in Section 4.4.3 of [RFC7925] hold.¶

Since the publication of RFC 7925 the need for firmware update mechanisms has been reinforced and the work on standardizing a secure and interoperable firmware update mechanism has made substantial progress, see [RFC9019]. RFC 7925 recommends to use a software / firmware update mechanism to provision devices with new trust anchors.¶

The use of device management protocols for IoT devices, which often include an onboarding or bootstrapping mechanism, has also seen considerable uptake in deployed devices and these protocols, some of which are standardized, allow provision of certificates on a regular basis. This enables a deployment model where IoT device utilize end-entity certificates with shorter lifetime making certificate revocation protocols, like OCSP and CRLs, less relevant.¶

Hence, instead of performing certificate revocation checks on the IoT device itself this specification recommends to delegate this task to the IoT device operator and to take the necessary action to allow IoT devices to remain operational.¶

17. Certificate Overhead

In a public key-based key exchange, certificates and public keys are a major contributor to the size of the overall handshake. For example, in a regular TLS 1.3 handshake with minimal ECC certificates and no subordinate CA utilizing the secp256r1 curve with mutual authentication, around 40% of the entire handshake payload is consumed by the two exchanged certificates.¶

Hence, it is not surprising that there is a strong desire to reduce the size of certificates and certificate chains. This has lead to various standardization efforts. Here is a brief summary of what options an implementer has to reduce the bandwidth requirements of a public key-based key exchange:¶

• Use elliptic curve cryptography (ECC) instead of RSA-based certificate due to the smaller certificate size.¶
• Avoid deep and complex CA hierarchies to reduce the number of subordinate CA certificates that need to be transmitted.¶
• Pay attention to the amount of information conveyed inside certificates.¶
• Use session resumption to reduce the number of times a full handshake is needed. Use Connection IDs [DTLS-CID], when possible, to enable long-lasting connections.¶
• Use the TLS cached info [RFC7924] extension to avoid sending certificates with every full handshake.¶
• Use client certificate URLs [RFC6066] instead of full certificates for clients.¶
• Use certificate compression as defined in [I-D.ietf-tls-certificate-compression].¶
• Use alternative certificate formats, where possible, such as raw public keys [RFC7250] or CBOR-encoded certificates [I-D.ietf-cose-cbor-encoded-cert].¶

The use of certificate handles, as introduced in cTLS [I-D.ietf-tls-ctls], is a form of caching or compressing certificates as well.¶

Whether to utilize any of the above extensions or a combination of them depends on the anticipated deployment environment, the availability of code, and the constraints imposed by already deployed infrastructure (e.g., CA infrastructure, tool support).¶

18. Ciphersuites

Section 4.5.3 of [DTLS13] flags AES-CCM with 8-octet authentication tags (CCM_8) as unsuitable for general use with DTLS. In fact, due to its low integrity limits (i.e., a high sensitivity to forgeries), endpoints that negotiate ciphersuites based on such AEAD are susceptible to a trivial DoS. (See also Section 5.3 and 5.4 of [I-D.irtf-cfrg-aead-limits] for further discussion on this topic, as well as references to the analysis supporting these conclusions.)¶

Specifically, [DTLS13] warns that:¶

• "TLS_AES_128_CCM_8_SHA256 MUST NOT be used in DTLS without additional safeguards against forgery. Implementations MUST set usage limits for AEAD_AES_128_CCM_8 based on an understanding of any additional forgery protections that are used."¶

Since all the ciphersuites mandated by [RFC7925] and [CoAP] are based on CCM_8, there is no stand-by ciphersuite to use for applications that want to avoid the security and availability risks associated with CCM_8 while retaining interoperability with the rest of the ecosystem.¶

In order to ameliorate the situation, this document RECOMMENDS that implementations support the following two ciphersuites:¶

• TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256¶
• TLS_ECDHE_ECDSA_WITH_AES_128_CCM¶

and offer them as their first choice. These ciphersuites provide confidentiality and integrity limits that are considered acceptable in the most general settings. For the details on the exact bounds of both ciphersuites see Section 4.5.3 of [DTLS13]. Note that the GCM-based ciphersuite offers superior interoperability with cloud services at the cost of a slight increase in the wire and peak RAM footprints.¶

When the GCM-based ciphersuite is used with TLS 1.2, the recommendations in Section 6.2.1 of [RFC9325] related to deterministic nonce generation apply. In addition, the integrity limits on key usage detailed in Section 4.4 of [RFC9325] also apply.¶

19. Fault Attacks on Deterministic Signature Schemes

A number of passive side-channel attacks as well as active fault-injection attacks (e.g., [Ambrose2017]) have been demonstrated that allow a malicious third party to gain information about the signing key if a fully deterministic signature scheme (e.g., [RFC6979] ECDSA or EdDSA [RFC8032]) is used.¶

Most of these attacks assume physical access to the device and are therefore especially relevant to smart cards as well as IoT deployments with poor or non-existent physical security.¶

In this security model, it is recommended to combine both randomness and determinism, for example, as described in [I-D.mattsson-cfrg-det-sigs-with-noise].¶

20. Open Issues

A list of open issues can be found at https://github.com/thomas-fossati/draft-tls13-iot/issues¶

21. Security Considerations

This entire document is about security.¶

22. IANA Considerations

This document makes no requests to IANA.¶

Acknowledgments

We would like to thank Ben Kaduk and John Mattsson.¶

Authors' Addresses