Encrypted Client Hello (ECH) PR

Hi all,

I've proposed a PR [1] for including ECH support in apache2.
Be very happy to discuss that, on this list, or via github.
I've included the text describing the PR below in case some
people prefer that via mail rather than github.

Cheers,
Stephen.

PS: This is my first PR for this project, apologies in
advance for the things I've inevitably gotten wrong;-)

[1] https://github.com/apache/httpd/pull/551

Text describing the PR:

This PR adds Encrypted Client Hello (ECH) functionality to apache2, when 
using OpenSSL for TLS.

Notes:

- ECH is not yet part of an OpenSSL release. We'd hope ECH will be part 
of OpenSSL 4.0 in April 2026. However, we have been working with OpenSSL 
maintainers on the so-called "ECH feature branch" and that branch 
(subject to the same OpenSSL maintainer approval process as the OpenSSL 
master branch) now includes sufficient ECH code for web servers like 
apache2. So there's plenty of time for this PR to be discussed, but 
starting now may be timely.
- This PR includes documentation in a markdown document in the repo's 
top directory, which is certainly the wrong place, but may be useful 
short-term. That describes how to do the build, configuration and 
logging changes, and the code changes for ECH. (So should be a good 
place for reviewers to start.)
- While OpenSSL releases do not yet include ECH support, some other TLS 
libraries do, in particular boringssl. If useful, we could extend this 
PR to also support boringssl, or that could be a follow-up. (It'd be 
good if the server configuration were the same regardless of the TLS 
library.)
- ECH support using these OpenSSL ECH APIs was included in the ligthttpd 
web server (in January 2025) so some code and patterns are common with 
that. We also plan to submit similar PRs to apache2 and haproxy, and 
ideally all would share some commonality.
- We have proposed a very similar PR for the NGINX project (OpenSSL ECH 
integration nginx/nginx#840).
- All that said, we're not fixated at all on things being done this way, 
and would be happy to make whatever changes are desired for apache2 and 
there are some notes on potential changes in the documentation.

Lastly, for open-ness, our work on this has been funded by the Open 
Technology Fund (OTF) in the DEfO project.

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