TLS 1.3 Extended Key Schedule

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Table of Contents

1. Introduction

Introducing additional key material into the TLS handshake is a non-trivial process because both parties need to agree on the injection content and context. If the two parties do not agree then an attacker may exploit the mismatch in so-called channel synchronization attacks, such as those described by [SLOTH].¶

Injecting key material into the TLS handshake allows other protocols to be bound to the handshake. For example, it may provide additional protections to the ClientHello message, which in the standard TLS handshake only receives protections after the server's Finished message has been received. It may also permit the use of combined shared secrets, possibly from multiple key exchange algorithms, to be included in the key schedule. This pattern is common for Post Quantum key exchange algorithms, as discussed in [I-D.ietf-tls-hybrid-design].¶

The goal of this document is to provide a standardised way for binding extra context into TLS 1.3 handshakes in a way that is easy to analyse from a security perspective, reducing the need for security analysis of extensions that affect the key schedule. It separates the concerns of whether an extension achieves its goals from the concerns of whether an extension reduces the security of a TLS handshake, either directly or through some unforseen interaction with another extension.¶

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Key Schedule Extension

This section describes two places in which additional secrets can be injected into the TLS 1.3 key schedule.¶

                                 |
                                 v
                           Derive-Secret(., "derived", "")
                                 |
                                 v
  KeyScheduleInput || (EC)DHE -> HKDF-Extract = Handshake Secret
                                 |
                                 v

                           |
                           v
                     Derive-Secret(., "derived", "")
                           |
                           v
  KeyScheduleInput || 0 -> HKDF-Extract = Main Secret
                           |
                           v

4. Key Schedule Injection Negotiation

Applications which make use of additional key schedule inputs MUST define a mechanism for negotiating the content and type of that input. This input MUST be framed in a KeyScheduleSecret struct, as defined in Section 5. Applications must take care that any negotiation that takes place unambiguously agrees a secret. It must be impossible, even under adversarial conditions, that a client and server agree on the transcript of the negotiation, but disagree on the secret that was negotiated.¶

5. Key Schedule Extension Structure

In some cases, protocols may require more than one secret to be injected at a particular stage in the key schedule. Thus, we require a generic and extensible way of doing so. To accomplish this, we use a structure - KeyScheduleInput - that encodes well-ordered sequences of secret material to inject into the key schedule. KeyScheduleInput is defined as follows:¶

struct {
    KeyScheduleSecretType type;
    opaque secret_data<0..2^16-1>;
} KeyScheduleSecret;

enum {
    (65535)
} KeyScheduleSecretType;

struct {
    KeyScheduleSecret secrets<0..2^16-1>;
} KeyScheduleInput;

Each secret included in a KeyScheduleInput structure has a type and corresponding secret data. Each secret MUST have a unique KeyScheduleSecretType. When encoding KeyScheduleInput as the key schedule Input value, the KeyScheduleSecret values MUST be in ascending sorted order. This ensures that endpoints always encode the same KeyScheduleInput value when using the same secret keying material.¶

6. Security Considerations

[BINDEL] provides a proof that the concatenation approach in Section 3 is secure as long as either the concatenated secret is secure or the existing KDF input is secure.¶

[[OPEN ISSUE: Is this guarantee sufficient? Do we also need to guarantee that a malicious prefix can't weaken the resulting PRF output?]]¶

7. IANA Considerations

This document requests the creation of a new IANA registry: TLS KeyScheduleInput Types. This registry should be under the existing Transport Layer Security (TLS) Parameters heading. It should be administered under a Specification Required policy [RFC8126].¶

[[OPEN ISSUE: specify initial registry values]]¶

Acknowledgments

We thank Karthik Bhargavan for his comments.¶

Authors' Addresses