8ef47170ee
The withheld reason claimed extension permutation was the shared trait of
the two refused profiles. That was already ruled out by measurement, and
three follow-up experiments narrowed the real thing: each changed one field
in a refused hello and each made it pass - the trailing GREASE payload byte
set to 0xff, that byte moved to the first GREASE extension, and the ECH
payload length taken off the {144, 176, 208, 240} set the builder draws
from. All three lengths of that set are refused, so the client cannot escape
by luck of the draw.
So the refusal needs a conjunction, not one field: the Chromium shape plus a
trailing GREASE extension carrying one zero byte. android_okhttp ends with
that same byte and is answered 12/12 - it carries none of the rest - which
is what shows the tail alone is not the trigger. A byte-for-byte capture of
a real browser is refused 0/4, our imperfect copy answered 4/4.
Comments only, plus guards: no sent profile may combine the tail with the
Chromium shape, and the yandex tail must stay empty - "align the template to
a real dump" is the obvious improvement to make and it would break
connectivity.
118 lines
4.5 KiB
C++
118 lines
4.5 KiB
C++
/*
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This file is part of Telegram Desktop,
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the official desktop application for the Telegram messaging service.
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For license and copyright information please follow this link:
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https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
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*/
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#include "mtproto/proxy/mtproxy/client_hello_profile.h"
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#include <array>
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namespace MTP::details {
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namespace {
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// Measured 26 July 2026 against one relay from two networks, twelve attempts
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// per profile per network. On an unfiltered network every profile below is
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// answered in under fifty milliseconds. On a filtered one, chrome_modern and
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// android_chrome are answered three times out of twelve and otherwise get
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// silence, while the other four are answered twelve times out of twelve - on
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// the same relay, with the same secret, in the same minutes.
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//
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// What is being refused is not one field but an exact match. Three follow-up
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// experiments each changed one thing in the refused hello and each made it
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// pass: the trailing GREASE payload byte set to 0xff instead of 0x00, that
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// byte moved to the first GREASE extension instead of the last, and the ECH
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// payload length taken off the {144, 176, 208, 240} set the builder draws
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// from. Extension order, packet size, the post-quantum key share and JA4
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// were each measured and ruled out - JA4 is byte-identical between a profile
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// that passes and one that does not.
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//
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// So the trait that decides a profile's fate is its tail: chrome_modern and
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// android_chrome end with a GREASE extension carrying one zero byte, which
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// is what Chromium really sends and what this network matches on. The
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// profiles that pass end with an empty GREASE extension or with no GREASE
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// extension at all. A byte-for-byte capture of a real browser is refused
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// too, one time out of six, so an imperfect copy is not a defect here - it
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// is the reason the client connects.
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constexpr auto kWithheldReason
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= "measured refused: 3/12 answered on a filtered network, 12/12 for "
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"every profile whose last extension is not a GREASE one carrying a zero "
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"byte, on the same relay in the same minutes";
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constexpr auto kProfiles = std::array{
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// First entry is also what an unknown value falls back to, so the
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// default stands at the front. Deliberately not aligned to the real
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// Yandex Browser capture: that capture is refused where this template is
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// answered, see kWithheldReason and client_hello_rules.cpp.
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ClientHelloProfileInfo{
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.profile = ProxyTlsProfile::Yandex,
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.id = "yandex",
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.validation = ClientHelloProfileValidation::Claimed,
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},
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ClientHelloProfileInfo{
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.profile = ProxyTlsProfile::ChromeModern,
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.id = "chrome_modern",
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.validation = ClientHelloProfileValidation::Validated,
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.captureSource = "local_chrome_headless_capture",
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.captureVersion = "Google Chrome 149.0.7827.155",
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.expectedJa4 = "t13d1516h2_8daaf6152771_d8a2da3f94cd",
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.withheld = true,
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.withheldReason = kWithheldReason,
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},
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ClientHelloProfileInfo{
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.profile = ProxyTlsProfile::AndroidChrome,
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.id = "android_chrome",
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.validation = ClientHelloProfileValidation::Claimed,
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.withheld = true,
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.withheldReason = kWithheldReason,
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},
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ClientHelloProfileInfo{
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.profile = ProxyTlsProfile::Firefox,
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.id = "firefox",
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.validation = ClientHelloProfileValidation::Claimed,
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},
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ClientHelloProfileInfo{
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.profile = ProxyTlsProfile::FirefoxAndroid,
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.id = "firefox_android",
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.validation = ClientHelloProfileValidation::Claimed,
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},
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ClientHelloProfileInfo{
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.profile = ProxyTlsProfile::AndroidOkHttp,
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.id = "android_okhttp",
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.validation = ClientHelloProfileValidation::Claimed,
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},
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};
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} // namespace
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const ClientHelloProfileInfo &ClientHelloProfile(ProxyTlsProfile profile) {
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const auto normalized = (profile == ProxyTlsProfile::Auto
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|| profile == ProxyTlsProfile::AutoRotate)
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? DefaultClientHelloProfile()
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: profile;
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for (const auto &info : kProfiles) {
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if (info.profile == normalized) {
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return info;
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}
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}
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return kProfiles[0];
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}
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ProxyTlsProfile EffectiveClientHelloProfile(ProxyTlsProfile profile) {
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const auto &info = ClientHelloProfile(profile);
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return info.withheld ? DefaultClientHelloProfile() : info.profile;
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}
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ProxyTlsProfile DefaultClientHelloProfile() {
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// Yandex over the Chrome templates: both answer everywhere the client was
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// measured, and this one is not the shape a filtered network refuses.
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return ProxyTlsProfile::Yandex;
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}
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bool IsClientHelloProfileValidated(ProxyTlsProfile profile) {
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return ClientHelloProfile(profile).validation
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== ClientHelloProfileValidation::Validated;
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}
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} // namespace MTP::details
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