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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.

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

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

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 Independent Fastly Cryptography Consulting LLC Cloudflare Informative References Meta Platforms, Inc. Akamai Technologies Akamai Technologies

Acknowledgements Thanks to , , and for comments.