curl/docs/ECH.md

TLS: ECH support in curl and libcurl

Summary

ECH means Encrypted Client Hello, a TLS 1.3 extension which is currently the subject of an IETF Draft. (ECH was formerly known as ESNI).

This file is intended to show the latest current state of ECH support in curl and libcurl.

At end of August 2019, an experimental fork of curl, built using an experimental fork of OpenSSL, which in turn provided an implementation of ECH, was demonstrated interoperating with a server belonging to the DEfO Project.

Further sections here describe

• resources needed for building and demonstrating curl support for ECH,

• progress to date,

• TODO items, and

• additional details of specific stages of the progress.

Resources needed

To build and demonstrate ECH support in curl and/or libcurl, you will need

• a TLS library, supported by libcurl, which implements ECH;

• an edition of curl and/or libcurl which supports the ECH implementation of the chosen TLS library;

• an environment for building and running curl, and at least building OpenSSL;

• a server, supporting ECH, against which to run a demonstration and perhaps a specific target URL;

• some instructions.

The following set of resources is currently known to be available.

Set	Component	Location	Remarks
DEfO	TLS library	sftcd/openssl	Tag esni-2019-08-30 avoids bleeding edge
	curl fork	niallor/curl	Tag esni-2019-08-30 likewise
	instructions	ESNI-README

Progress

PR 4011 (Jun 2019) expected in curl release 7.67.0 (Oct 2019)

• Details below;

• New configuration option: --enable-ech;

• Build-time check for availability of resources needed for ECH support;

• Pre-processor symbol USE_ECH for conditional compilation of ECH support code, subject to configuration option and availability of needed resources.

TODO

• (next PR) Add libcurl options to set ECH parameters.

• (next PR) Add curl tool command line options to set ECH parameters.

• (WIP) Extend DoH functions so that published ECH parameters can be retrieved from DNS instead of being required as options.

• (WIP) Work with OpenSSL community to finalize ECH API.

• Track OpenSSL ECH API in libcurl

• Identify and implement any changes needed for CMake.

• Optimize build-time checking of available resources.

• Encourage ECH support work on other TLS/SSL backends.

Additional detail

PR 4011

TLS: Provide ECH support framework for curl and libcurl

The proposed change provides a framework to facilitate work to implement ECH support in curl and libcurl. It is not intended either to provide ECH functionality or to favour any particular TLS-providing backend. Specifically, the change reserves a feature bit for ECH support (symbol CURL_VERSION_ECH), implements setting and reporting of this bit, includes dummy book-keeping for the symbol, adds a build-time configuration option (--enable-ech), provides an extensible check for resources available to provide ECH support, and defines a compiler pre-processor symbol (USE_ECH) accordingly.

Proposed-by: @niallor (Niall O'Reilly)
Encouraged-by: @sftcd (Stephen Farrell)
See-also: this message

Limitations:

• Book-keeping (symbols-in-versions) needs real release number, not 'DUMMY'.

• Framework is incomplete, as it covers autoconf, but not CMake.

• Check for available resources, although extensible, refers only to specific work in progress (described here) to implement ECH for OpenSSL, as this is the immediate motivation for the proposed change.

References

Cloudflare blog: Encrypting SNI: Fixing One of the Core Internet Bugs

Cloudflare blog: Encrypt it or lose it: how encrypted SNI works

IETF Draft: Encrypted Server Name Indication for TLS 1.3

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