| Internet-Draft | PEM file format for ECH | November 2024 |
| Farrell | Expires 3 June 2025 | [Page] |
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.¶
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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 [I-D.ietf-dnsop-svcb-https] 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].¶
[[At present, based on TLS WG list discussion, it seems most likely that this draft will be sent to the Independent stream once ECH is done and dusted (but not before). The source for this is in https://github.com/sftcd/pemesni/ PRs are welcome there too.]]¶
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.¶
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 as an ECHConfigList containing one 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.¶
Figure 1 shows an example ECHConfig PEM File¶
-----BEGIN PRIVATE KEY----- MC4CAQAwBQYDK2VuBCIEICjd4yGRdsoP9gU7YT7My8DHx1Tjme8GYDXrOMCi8v1V -----END PRIVATE KEY----- -----BEGIN ECHCONFIG----- AD7+DQA65wAgACA8wVN2BtscOl3vQheUzHeIkVmKIiydUhDCliA4iyQRCwAEAAEA AQALZXhhbXBsZS5jb20AAA== -----END ECHCONFIG-----
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.¶
Thanks to Daniel McCarney for comments.¶
There are none so this section can be deleted later.¶
From -07 to -08:¶
Processed some github comments¶
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Refresh due to expiry.¶
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ECHO -> ECH¶
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ESNI -> ECHO¶