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SSLKEYLOGFILE Extension for Encrypted Client Hello (ECH)
draft-ietf-tls-ech-keylogfile-00

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Yaroslav Rosomakho , Hannes Tschofenig
Last updated 2024-10-08 (Latest revision 2024-10-03)
Replaces draft-rosomakho-tls-ech-keylogfile
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draft-ietf-tls-ech-keylogfile-00
Transport Layer Security                                    Y. Rosomakho
Internet-Draft                                                   Zscaler
Intended status: Informational                             H. Tschofenig
Expires: 6 April 2025     University of Applied Sciences Bonn-Rhein-Sieg
                                                          3 October 2024

        SSLKEYLOGFILE Extension for Encrypted Client Hello (ECH)
                    draft-ietf-tls-ech-keylogfile-00

Abstract

   This document specifies an extension to the SSLKEYLOGFILE format to
   support the logging of information about Encrypted Client Hello (ECH)
   related secrets.  Two new labels are introduced, namely ECH_SECRET
   and ECH_CONFIG, which log the Hybrid Public Key Encryption (HPKE)-
   derived shared secret and the ECHConfig used for the ECH,
   respectively.

   This extension aims to facilitate debugging of TLS connections
   employing ECH.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://tlswg.github.io/draft-ietf-tls-ech-keylogfile/draft-ietf-tls-
   ech-keylogfile.html.  Status information for this document may be
   found at https://datatracker.ietf.org/doc/draft-ietf-tls-ech-
   keylogfile/.

   Discussion of this document takes place on the Transport Layer
   Security Working Group mailing list (mailto:tls@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/tls/.  Subscribe
   at https://www.ietf.org/mailman/listinfo/tls/.

   Source for this draft and an issue tracker can be found at
   https://github.com/tlswg/draft-ietf-tls-ech-keylogfile.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 6 April 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  SSLKEYLOGFILE Labels for ECH  . . . . . . . . . . . . . . . .   3
     3.1.  ECH_SECRET  . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  ECH_CONFIG  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Client_random for other TLS 1.3 SSLKEYLOGFILE Entries . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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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]).

3.1.  ECH_SECRET

   This label corresponds to the KEM shared secret derived during the
   HPKE key schedule process.  Length of the secret is defined by the
   KEM negotiated for use with ECH.

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3.2.  ECH_CONFIG

   This label is used to log the ECHConfig used for construction of the
   ECH extension.  Note that the value is logged in hexadecimal
   representation, similarly to other entries in the SSLKEYLOGFILE.

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.

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6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  Normative References

   [I-D.ietf-tls-esni]
              Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-22, 15 September 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              esni-22>.

   [I-D.ietf-tls-keylogfile]
              Thomson, M., "The SSLKEYLOGFILE Format for TLS", Work in
              Progress, Internet-Draft, draft-ietf-tls-keylogfile-02, 29
              April 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-tls-keylogfile-02>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

7.2.  Informative References

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [RFC9001]  Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9001>.

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   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/rfc/rfc9147>.

   [RFC9180]  Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.

Acknowledgments

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

Authors' Addresses

   Yaroslav Rosomakho
   Zscaler
   Email: yrosomakho@zscaler.com

   Hannes Tschofenig
   University of Applied Sciences Bonn-Rhein-Sieg
   Email: Hannes.Tschofenig@gmx.net

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