SSLKEYLOGFILE Extension for Encrypted Client Hello (ECH)

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Table of Contents

1. Introduction

Debugging protocols with TLS can be difficult due to encrypted communications. Analyzing these messages in diagnostic and debug tools requires inspecting the encrypted content. Various TLS implementations have informally adopted a file format to log the secret values generated by the TLS key schedule, aiding in this analysis.¶

In many implementations, the file that the secrets are logged to is specified in an environment variable named "SSLKEYLOGFILE". [I-D.ietf-tls-keylogfile] standardizes this format. With the introduction of [I-D.ietf-tls-esni] additional secrets are derived during the handshake to encrypt the ClientHello message using Hybrid Public Key Encryption (HPKE) [RFC9180]. This document extends the SSLKEYLOGFILE format to also offer support for the ECH extension to enable debugging of ECH-enabled connections. The proposed extension can also be used with all protocols that support ECH, including TLS 1.3 [RFC8446], DTLS 1.3 [RFC9147] and QUIC [RFC9000][RFC9001].¶

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. SSLKEYLOGFILE Labels for ECH

This document defines two new labels for SSLKEYLOGFILE format: ECH_SECRET and ECH_CONFIG. The client SHOULD log the labels if it offered ECH regardless of server acceptance. The server MAY log the labels only if it successfully decrypted and accepted ECH offered by the client. The 32-byte random value from the Outer ClientHello message is used as the client_random value for these log records. The client MUST NOT log the labels for connections that use the GREASE ECH extension (see Section 6.2 of [I-D.ietf-tls-esni]).¶

4. Client_random for other TLS 1.3 SSLKEYLOGFILE Entries

The SSLKEYLOGFILE uses the random value from the ClientHello message as a "connection identifier". This creates ambiguity since the TLS handshake with ECH contains two different random values, one in the Outer ClientHello structure and the second one in the Inner ClientHello.¶

The SSLKEYLOGFILE entries corresponding to TLS 1.3 secrets for connections that successfully negotiated ECH MUST use the random from the Inner ClientHello structure. In all other cases the random value from the Outer ClientHello structure MUST be used.¶

5. Security Considerations

The applicability statement of [I-D.ietf-tls-keylogfile] also applies to this document: if unauthorized entities gain access to the logged secrets then the core guarantees that TLS provides are completely undermined.¶

This specification extends the SSLKEYLOGFILE specification [I-D.ietf-tls-keylogfile] and therefore introduces the following threats:¶

• Access to the ECH_SECRET record in the SSLKEYLOGFILE allows the attacker to decrypt the ECH extension and thereby reveal the content of the ClientHello message, including the payload of the Server Name Indication (SNI) extension.¶

• Access to the HPKE-established shared secret introduces a potential attack surface against the HPKE library since access to this keying material is not ncessarily available otherwise.¶

Implementers MUST take measures to prevent unauthorized access to the SSLKEYLOGFILE text file.¶

According to SSLKEYLOGFILE specification [I-D.ietf-tls-keylogfile], this extension is intended for use in systems where TLS only protects test data. While the access this information provides to TLS connections can be useful for diagnosing problems during development, this mechanism MUST NOT be used in a production environment.¶

6. IANA Considerations

This document has no IANA actions.¶

Acknowledgments

We would like to thank Stephen Farrell, Martin Thomson and Peter Wu for their review comments.¶

Authors' Addresses