



Internet Engineering Task Force (IETF)                        S. Farrell
Request for Comments: 9934                        Trinity College Dublin
Category: Standards Track                                     March 2026
ISSN: 2070-1721


Privacy-Enhanced Mail (PEM) File Format for Encrypted ClientHello (ECH)

Abstract

   Encrypted ClientHello (ECH) key pairs need to be configured into TLS
   servers, which can be built using different TLS libraries.  This
   document specifies a standard file format for this purpose, similar
   to how RFC 7468 defines other Privacy-Enhanced Mail (PEM) file
   formats.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9934.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  ECHConfig File
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgements
   Author's Address

1.  Introduction

   Encrypted ClientHello (ECH) [RFC9849] 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.

   The public and private keys MUST both be PEM encoded [RFC7468].  The
   file contains the concatenation of the PEM encoding of the private
   key (if present) followed by the PEM encoding of the public key(s) as
   an ECHConfigList.  When a private key is present, the ECHConfigList
   MUST contain an ECHConfig that matches the private key.  The private
   key MUST be encoded as a PKCS #8 PrivateKey [RFC7468].  The public
   key(s) MUST be the base64-encoded form (see Section 4 of [RFC4648])
   of an ECHConfigList value that can be published in the DNS using an
   HTTPS RR as described in [RFC9848].  The string "ECHCONFIG" MUST be
   used in the PEM file delimiter for the public key.

   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

   If the above ECHConfigList were published in the DNS for
   foo.example.com, then one could access that as shown in Figure 2.

   $ dig +short HTTPS foo.example.com
   1 . ech=AD7+DQA65wAgACA8wVN2BtscOl3vQheUzHeIkVmKIiydUhDCliA4iyQR
   wAEAAEAAQALZXhhbXBsZS5jb20AAA==

           Figure 2: Use of Dig to Get the ECHConfigList from DNS

   TLS servers using this file format might configure multiple file
   names as part of their overall configuration, if, for example, only
   the ECHConfigList values from a subset of those files are to be used
   as the value for retry_configs in the ECH fallback scenario.

   The ECHConfigList in a PEM file might contain more than one ECHConfig
   if, for example, those ECHConfig values contain different extensions
   or different public_name values.  Consistent with [RFC9849], the
   ECHConfig values within an ECHConfigList appear in decreasing order
   of preference.  If the ECHConfigList value is to be used as the
   retry_configs value, then that might contain different public keys.
   (Nonetheless, when a private key is present, that MUST match the
   public key from one of the ECHConfig values.)

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 mechanisms that would be used for a server's PEM-
   encoded HTTPS certificate private key should be used for the PEM ECH
   configuration.

   The security considerations of [RFC9848] apply when retrieving an
   ECHConfigList from the DNS.

   For clarity, only the ECHConfigList is to be published in the DNS -
   the private key from an ECH PEM file MUST NOT be published in the
   DNS.

5.  IANA Considerations

   This document has no IANA actions.

6.  References

6.1.  Normative References

   [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>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [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>.

   [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>.

   [RFC9849]  Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", RFC 9849, DOI 10.17487/RFC9849,
              March 2026, <https://www.rfc-editor.org/info/rfc9849>.

6.2.  Informative References

   [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>.

   [RFC9848]  Schwartz, B., Bishop, M., and E. Nygren, "Bootstrapping
              TLS Encrypted ClientHello with DNS Service Bindings",
              RFC 9848, March 2026,
              <https://www.rfc-editor.org/info/rfc9848>.

Acknowledgements

   Thanks to Daniel McCarney, Jim Reid, and Peter Yee for comments.

Author's Address

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