Goal

Add opt-in Encrypted Client Hello (ECH, draft-ietf-tls-esni-22) support to proxybroker's outbound TLS connections (judges, providers, proxy testing) so that on-path observers can't see the SNI hostnames being queried.

Why

When proxybroker connects to a TLS server (https://httpbin.org etc.), the server name appears in cleartext in the TLS handshake's SNI extension. Network operators (ISPs, employers, captive portals) routinely log or filter based on SNI — a 2024 measurement study found ~90% of HTTPS traffic has plaintext-SNI-based filtering deployed somewhere along its path.

For a tool whose sole purpose is privacy-related (proxies, anonymity), leaking the destination hostname in cleartext defeats the point. Users who deploy proxybroker on hostile networks (corporate wifi, repressive states, monitored mobile carriers) need the encrypted-DNS + encrypted-SNI combo to keep their proxy-testing activity off network logs.

State of ECH in 2026

• Cloudflare rolled out ECH at the edge in 2023 (limited rollout); reactivated 2024.
• Firefox 119+ ships ECH enabled by default (2024).
• Chrome 117+ ships ECH behind a flag.
• Python 3.12+ has tentative ECH support in the ssl module via ECH_OUTER_SNI and friends (subject to platform OpenSSL ≥3.2).
• OpenSSL 3.2+ ships ECH support; 3.5+ has stable API.
• aiohttp doesn't expose ECH knobs directly but accepts a custom SSLContext.

Scope

1. Resolver(ech=True) and Judge(ech=True) opt-in flags. Default False to preserve current behavior on systems without OpenSSL ECH support.
2. Build the SSLContext with ECH enabled when the flag is set:

   ctx = ssl.create_default_context()
   if ech:
       try:
           ctx.set_ech_outer_sni("cloudflare-ech.com")  # or per-judge
       except (AttributeError, ssl.SSLError):
           log.warning("ECH unavailable on this platform; falling back to plaintext SNI")

3. Per-judge ECH config (some judges may not support ECH; let users opt out per host).
4. Detection helper: proxybroker.utils.has_ech_support() -> bool — returns True if Python+OpenSSL versions support ECH.
5. Tests: mock SSLContext interaction; verify ech=False is bit-identical to current behavior; verify ech=True raises useful error on platforms without support.

Acceptance criteria

1. Resolver(ech=True) constructs without error on supported platforms.
2. Resolver(ech=True) falls back gracefully (with warning) on unsupported platforms; doesn't crash startup.
3. has_ech_support() accurately reflects Python + OpenSSL capabilities at runtime.
4. Documentation: a "Privacy mode" section in README explaining when to enable ECH and what guarantees it provides (and doesn't — it doesn't hide the destination IP, only the SNI).
5. Existing tests pass; no behavior change with default ech=False.

Out of scope

• ECH server-side support in serve mode — proxybroker's serve mode is HTTP CONNECT, doesn't terminate TLS.
• DoH/DoT for Resolver itself — separate ticket (see SOTA polish: connection-layer Happy Eyeballs + DNSSEC + IDN (post-#212) #213 for DNS privacy items).
• ECHConfigList retrieval automation — usually advertised via DNS HTTPS records (RFC 9460); aioquic/aiodns may need support.

References

• draft-ietf-tls-esni
• Python 3.12+ ssl module ECH API
• Cloudflare ECH announcement (2024)
• Mozilla ECH deployment notes

Estimated effort

~4-6 hours. Most of the work is figuring out the cross-platform SSL context setup and writing the docs that explain what ECH does and doesn't protect against.