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.¶

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.stebila-tls-hybrid-design].¶

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

                 |
                 v
           Derive-Secret(., "derived early", "")
                 |
                 v
        Input -> HKDF-Extract
                 |
                 v
           Derive-Secret(., "derived", "")
                 |
                 v
   Input || 0 -> HKDF-Extract = Master Secret
                 |
                 v

4. 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.¶

5. Security Considerations

[[OPEN ISSUE: This draft has not seen any security analysis.]]¶

6. IANA Considerations

[[TODO: define secret registry structure]]¶

Acknowledgments

We thank Karthik Bhargavan for his comments.¶

Authors' Addresses