PEM file format for ECH
draft-farrell-tls-pemesni-11

Internet Engineering Task Force (IETF)                        S. Farrell
Internet-Draft                                    Trinity College Dublin
Intended status: Standards Track                         4 December 2025
Expires: 7 June 2026

                        PEM file format for ECH
                      draft-farrell-tls-pemesni-11

Abstract

   Encrypted ClientHello (ECH) key pairs need to be configured into TLS
   servers, that can be built using different TLS libraries, so there is
   a benefit and little cost in documenting a file format to use for
   these key pairs, similar to how RFC7468 defines other PEM file
   formats.

Status of This Memo

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   This Internet-Draft will expire on 7 June 2026.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  ECHConfig file  . . . . . . . . . . . . . . . . . . . . . . .   2
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   3
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   3
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Appendix A.  Changes  . . . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   Encrypted ClientHello (ECH) [I-D.ietf-tls-esni] for TLS1.3 [RFC8446]
   defines a confidentiality mechanism for server names and other
   ClientHello content in TLS.  That requires publication of an
   ECHConfigList data structure in an HTTPS or SVCB RR [RFC9460] in the
   DNS.  An ECHConfigList can contain one or more ECHConfig values.  An
   ECHConfig structure contains the public component of a key pair that
   will typically be periodically (re-)generated by some key manager for
   a TLS server.  TLS servers then need to be configured to use these
   key pairs, and given that various TLS servers can be built with
   different TLS libraries, there is a benefit in having a standard
   format for ECH key pairs and configs, just as was done with
   [RFC7468].

2.  Terminology

   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.  ECHConfig file

   A PEM ECH file contains zero or one private key and one encoded
   ECHConfigList.

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   The public and private keys MUST both be PEM encoded.  The file
   contains the concatenation of the PEM encoding of the private key (if
   present) followed by the PEM encoding of the public key as an
   ECHConfigList containing an ECHConfig that matches the private key.
   The private key MUST be encoded as a PKCS#8 PrivateKey.  The public
   key MUST be the base64 encoded form of an ECHConfigList value that
   can also be published in the DNS.  The string "ECHCONFIG" MUST be
   used in the PEM file delimiter for the public key.

   There MUST only be one key pair in each file even if a server
   publishes multiple public keys in the DNS in one ECHConfigList
   structure.

   Any content after the PEM encoded ECHConfigList SHOULD be ignored.

   Figure 1 shows an example ECHConfig PEM File

   -----BEGIN PRIVATE KEY-----
   MC4CAQAwBQYDK2VuBCIEICjd4yGRdsoP9gU7YT7My8DHx1Tjme8GYDXrOMCi8v1V
   -----END PRIVATE KEY-----
   -----BEGIN ECHCONFIG-----
   AD7+DQA65wAgACA8wVN2BtscOl3vQheUzHeIkVmKIiydUhDCliA4iyQRCwAEAAEA
   AQALZXhhbXBsZS5jb20AAA==
   -----END ECHCONFIG-----

                    Figure 1: Example ECHConfig PEM file

4.  Security Considerations

   Storing cryptographic keys in files leaves them vulnerable should
   anyone get read access to the filesystem on which they are stored.
   The same protection mechanims that would be used for a server's PEM
   encoded HTTPS certificate private key should be used for the PEM ECH
   configuration.

5.  Acknowledgements

   Thanks to Daniel McCarney for comments.

6.  IANA Considerations

   This document contains no IANA considerations.

7.  References

7.1.  Normative References

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   [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/info/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/info/rfc8174>.

   [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/info/rfc8446>.

   [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-25, 14 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              esni-25>.

7.2.  Informative References

   [RFC7468]  Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
              PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
              April 2015, <https://www.rfc-editor.org/info/rfc7468>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/info/rfc9460>.

Appendix A.  Changes

   From -10 to -11:

   *  Change to standards track as agreed with shepherd/AD.

   From -09 to -10:

   *  Tweaks to fit being AD sponsored.

   From -08 to -09:

   *  Minor clarificatrion of encoding based on current OpenSSL ECH
      feature branch code.

   From -07 to -08:

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   *  Processed some github comments

   From -06 to -07:

   *  Refresh due to expiry.

   From -05 to -06:

   *  Refresh due to expiry.

   From -04 to -05:

   *  Refresh due to expiry.

   From -03 to -04:

   *  Refresh due to expiry.

   From -02 to -03:

   *  Refresh due to expiry and not possible ISE destination

   From -01 to -02:

   *  ECHO -> ECH

   From -00 to -01:

   *  ESNI -> ECHO

Author's Address

   Stephen Farrell
   Trinity College Dublin
   Dublin
   2
   Ireland
   Phone: +353-1-896-2354
   Email: stephen.farrell@cs.tcd.ie

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