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 [I-D.ietf-tls-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 Furthermore, many outdated ciphersuites have been omitted from the TLS/DTLS 1.3 specification.¶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¶
• pre_shared_key¶
• psk_key_exchange_modes¶

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.¶

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.¶

Question: DTLS 1.3 now offers per-record retransmission and therefore introduces much less congestion risk associated with spurious retransmissions. Hence, should we relax the 9s initial timeout?¶

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 (SNI)

This specification mandates the implementation of the SNI extension. Where privacy requirements require it, the encrypted SNI extension [I-D.ietf-tls-esni] prevents an on-path attacker to determine the domain name the client is trying to connect to. Note, however, that the extension is still at an experimental state.¶

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

When clients and servers share a PSK, TLS/DTLS 1.3 allows clients to send data on the first flight ("early data"). This features reduces communication setup latency but requires application layer protocols to define its use with the 0-RTT data functionality.¶

For HTTP this functionality is described in [RFC8470]. This document specifies the application profile for CoAP, which follows the design of [RFC8470].¶

For a given request, the level of tolerance to replay risk is specific to the resource it operates upon (and therefore only known to the origin server). In general, if processing a request does not have state-changing side effects, the consequences of replay are not significant. The server can choose whether it will process early data before the TLS handshake completes.¶

It is RECOMMENDED that origin servers allow resources to explicitly configure whether early data is appropriate in requests.¶

This specification specifies the Early-Data option, which indicates that the request has been conveyed in early data and that a client understands the 4.25 (Too Early) status code. The semantic follows [RFC8470].¶

+-----+---+---+---+---+-------------+--------+--------+---------+---+
| No. | C | U | N | R | Name        | Format | Length | Default | E |
+-----+---+---+---+---+-------------+--------+--------+---------+---+
| TBD | x |   |   |   | Early-Data  | empty  | 0      | (none)  | x |
+-----+---+---+---+---+-------------+--------+--------+---------+---+

        C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable,
        E=Encrypt and Integrity Protect (when using OSCORE)

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 [I-D.ietf-suit-architecture]. 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. Security Considerations

This entire document is about security.¶

18. Acknowledgements

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

19. IANA Considerations

IANA is asked to add the Option defined in Figure 2 to the CoAP Option Numbers registry.¶

+--------+------------+-----------+
| Number | Name       | Reference |
+--------+------------+-----------+
| TBD    | Early-Data | RFCThis   |
+--------+------------+-----------+

IANA is asked to add the Response Code defined in Figure 3 to the CoAP Response Code registry.¶

+--------+-------------+-----------+
| Code   | Description | Reference |
+--------+-------------+-----------+
| 4.25   | Too Early   | RFCThis   |
+--------+-------------+-----------+

Authors' Addresses