Trinity College Dublin

Dublin 2 Ireland +353-1-896-2354 stephen.farrell@cs.tcd.ie

Security Area TLS ESNI 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 .

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Table of Contents

• .  Introduction
• .  Terminology
• .  ECHConfig File
• .  Security Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Author's Address

Introduction Encrypted ClientHello (ECH) for TLS1.3 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 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 .

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 when, and only when, they appear in all capitals, as shown here.

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 . 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 . The public key(s) MUST be the base64-encoded form (see ) of an ECHConfigList value that can be published in the DNS using an HTTPS RR as described in . 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. shows an example ECHConfig PEM File

Example ECHConfig PEM File -----BEGIN PRIVATE KEY----- MC4CAQAwBQYDK2VuBCIEICjd4yGRdsoP9gU7YT7My8DHx1Tjme8GYDXrOMCi8v1V -----END PRIVATE KEY----- -----BEGIN ECHCONFIG----- AD7+DQA65wAgACA8wVN2BtscOl3vQheUzHeIkVmKIiydUhDCliA4iyQRCwAEAAEA AQALZXhhbXBsZS5jb20AAA== -----END ECHCONFIG-----

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

Use of Dig to Get the ECHConfigList from DNS $ dig +short HTTPS foo.example.com 1 . ech=AD7+DQA65wAgACA8wVN2BtscOl3vQheUzHeIkVmKIiydUhDCliA4iyQR wAEAAEAAQALZXhhbXBsZS5jb20AAA==

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 , 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.)

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 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.

IANA Considerations This document has no IANA actions.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] This document describes and discusses the textual encodings of the Public-Key Infrastructure X.509 (PKIX), Public-Key Cryptography Standards (PKCS), and Cryptographic Message Syntax (CMS). The textual encodings are well-known, are implemented by several applications and libraries, and are widely deployed. This document articulates the de facto rules by which existing implementations operate and defines them so that future implementations can interoperate. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. Independent Fastly Cryptography Consulting LLC Cloudflare Informative References This document specifies the "SVCB" ("Service Binding") and "HTTPS" DNS resource record (RR) types to facilitate the lookup of information needed to make connections to network services, such as for HTTP origins. SVCB records allow a service to be provided from multiple alternative endpoints, each with associated parameters (such as transport protocol configuration), and are extensible to support future uses (such as keys for encrypting the TLS ClientHello). They also enable aliasing of apex domains, which is not possible with CNAME. The HTTPS RR is a variation of SVCB for use with HTTP (see RFC 9110, "HTTP Semantics"). By providing more information to the client before it attempts to establish a connection, these records offer potential benefits to both performance and privacy. Meta Platforms, Inc. Akamai Technologies Akamai Technologies

Acknowledgements Thanks to , , and for comments.

Author's Address Trinity College Dublin

Dublin 2 Ireland +353-1-896-2354 stephen.farrell@cs.tcd.ie