Improve FakeTLS server-flight fidelity and macOS portability

2026-04-17 18:44:10 +03:00 · 2026-04-15 19:35:09 +05:00
parent 9303c7854a
commit f5b5ea3bbf
7 changed files with 653 additions and 47 deletions
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2780,7 +2780,7 @@ checksum = "7b2093cf4c8eb1e67749a6762251bc9cd836b6fc171623bd0a9d324d37af2417"

 [[package]]
 name = "telemt"
-version = "3.3.39"
+version = "3.3.40"
 dependencies = [
 "aes",
 "anyhow",
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "telemt"
-version = "3.3.39"
+version = "3.3.40"
 edition = "2024"

 [features]
@@ -98,4 +98,3 @@ harness = false
 [profile.release]
 lto = "fat"
 codegen-units = 1
-
--- a/docs/Architecture/Fronting-splitting/TLS_FRONT_PROFILE_FIDELITY.en.md
+++ b/docs/Architecture/Fronting-splitting/TLS_FRONT_PROFILE_FIDELITY.en.md
@@ -0,0 +1,225 @@
+# TLS Front Profile Fidelity
+
+## Overview
+
+This document describes how Telemt reuses captured TLS behavior in the FakeTLS server flight and how to validate the result on a real deployment.
+
+When TLS front emulation is enabled, Telemt can capture useful server-side TLS behavior from the selected origin and reuse that behavior in the emulated success path. The goal is not to reproduce the origin byte-for-byte, but to reduce stable synthetic traits and make the emitted server flight structurally closer to the captured profile.
+
+## Why this change exists
+
+The project already captures useful server-side TLS behavior in the TLS front fetch path:
+
+- `change_cipher_spec_count`
+- `app_data_record_sizes`
+- `ticket_record_sizes`
+
+Before this change, the emulator used only part of that information. This left a gap between captured origin behavior and emitted FakeTLS server flight.
+
+## What is implemented
+
+- The emulator now replays the observed `ChangeCipherSpec` count from the fetched behavior profile.
+- The emulator now replays observed ticket-like tail ApplicationData record sizes when raw or merged TLS profile data is available.
+- The emulator now preserves more of the profiled encrypted-flight structure instead of collapsing it into a smaller synthetic shape.
+- The emulator still falls back to the previous synthetic behavior when the cached profile does not contain raw TLS behavior information.
+- Operator-configured `tls_new_session_tickets` still works as an additive fallback when the profile does not provide enough tail records.
+
+## Practical benefit
+
+- Reduced distinguishability between profiled origin TLS behavior and emulated TLS behavior.
+- Lower chance of stable server-flight fingerprints caused by fixed CCS count or synthetic-only tail record sizes.
+- Better reuse of already captured TLS profile data without changing MTProto logic, KDF routing, or transport architecture.
+
+## Limitations
+
+This mechanism does not aim to make Telemt byte-identical to the origin server.
+
+It also does not change:
+
+- MTProto business logic;
+- KDF routing behavior;
+- the overall transport architecture.
+
+The practical goal is narrower:
+
+- reuse more captured profile data;
+- reduce fixed synthetic behavior in the server flight;
+- preserve a valid FakeTLS success path while changing the emitted shape on the wire.
+
+## Validation targets
+
+- Correct count of emulated `ChangeCipherSpec` records.
+- Correct replay of observed ticket-tail record sizes.
+- No regression in existing ALPN and payload-placement behavior.
+
+## How to validate the result
+
+Recommended validation consists of two layers:
+
+- focused unit and security tests for CCS-count replay and ticket-tail replay;
+- real packet-capture comparison for a selected origin and a successful FakeTLS session.
+
+When testing on the network, the expected result is:
+
+- a valid FakeTLS and MTProto success path is preserved;
+- the early encrypted server flight changes shape when richer profile data is available;
+- the change is visible on the wire without changing MTProto logic or transport architecture.
+
+This validation is intended to show better reuse of captured TLS profile data.
+It is not intended to prove byte-level equivalence with the real origin server.
+
+## How to test on a real deployment
+
+The strongest practical validation is a side-by-side trace comparison between:
+
+- a real TLS origin server used as `mask_host`;
+- a Telemt FakeTLS success-path connection for the same SNI;
+- optional captures from different Telemt builds or configurations.
+
+The purpose of the comparison is to inspect the shape of the server flight:
+
+- record order;
+- count of `ChangeCipherSpec` records;
+- count and grouping of early encrypted `ApplicationData` records;
+- lengths of tail or continuation `ApplicationData` records.
+
+## Recommended environment
+
+Use a Linux host or Docker container for the cleanest reproduction.
+
+Recommended setup:
+
+1. One Telemt instance.
+2. One real HTTPS origin as `mask_host`.
+3. One Telegram client configured with an `ee` proxy link for the Telemt instance.
+4. `tcpdump` or Wireshark available for capture analysis.
+
+## Step-by-step test procedure
+
+### 1. Prepare the origin
+
+1. Choose a real HTTPS origin.
+2. Set both `censorship.tls_domain` and `censorship.mask_host` to that hostname.
+3. Confirm that a direct TLS request works:
+
+```bash
+openssl s_client -connect ORIGIN_IP:443 -servername YOUR_DOMAIN </dev/null
+```
+
+### 2. Configure Telemt
+
+Use a configuration that enables:
+
+- `censorship.mask = true`
+- `censorship.tls_emulation = true`
+- `censorship.mask_host`
+- `censorship.mask_port`
+
+Recommended for cleaner testing:
+
+- keep `censorship.tls_new_session_tickets = 0`, so the result depends primarily on fetched profile data rather than operator-forced synthetic tail records;
+- keep `censorship.tls_fetch.strict_route = true`, if cleaner provenance for captured profile data is important.
+
+### 3. Refresh TLS profile data
+
+1. Start Telemt.
+2. Let it fetch TLS front profile data for the configured domain.
+3. If `tls_front_dir` is persisted, confirm that the TLS front cache is populated.
+
+Persisted cache artifacts are useful, but they are not required if packet captures already demonstrate the runtime result.
+
+### 4. Capture a direct-origin trace
+
+From a separate client host, connect directly to the origin:
+
+```bash
+openssl s_client -connect ORIGIN_IP:443 -servername YOUR_DOMAIN </dev/null
+```
+
+Capture with:
+
+```bash
+sudo tcpdump -i any -w origin-direct.pcap host ORIGIN_IP and port 443
+```
+
+### 5. Capture a Telemt FakeTLS success-path trace
+
+Now connect to Telemt with a real Telegram client through an `ee` proxy link that targets the Telemt instance.
+
+`openssl s_client` is useful for direct-origin capture and fallback sanity checks, but it does not exercise the successful FakeTLS and MTProto path.
+
+Capture with:
+
+```bash
+sudo tcpdump -i any -w telemt-emulated.pcap host TELEMT_IP and port 443
+```
+
+### 6. Decode TLS record structure
+
+Use `tshark` to print record-level structure:
+
+```bash
+tshark -r origin-direct.pcap -Y "tls.record" -T fields \
+  -e frame.number \
+  -e ip.src \
+  -e ip.dst \
+  -e tls.record.content_type \
+  -e tls.record.length
+```
+
+```bash
+tshark -r telemt-emulated.pcap -Y "tls.record" -T fields \
+  -e frame.number \
+  -e ip.src \
+  -e ip.dst \
+  -e tls.record.content_type \
+  -e tls.record.length
+```
+
+Focus on the server flight after ClientHello:
+
+- `22` = Handshake
+- `20` = ChangeCipherSpec
+- `23` = ApplicationData
+
+### 7. Build a comparison table
+
+A compact table like the following is usually enough:
+
+| Path | CCS count | AppData count in first encrypted flight | Tail AppData lengths |
+| --- | --- | --- | --- |
+| Origin | `N` | `M` | `[a, b, ...]` |
+| Telemt build A | `...` | `...` | `...` |
+| Telemt build B | `...` | `...` | `...` |
+
+The comparison should make it easy to see that:
+
+- the FakeTLS success path remains valid;
+- the early encrypted server flight changes when richer profile data is reused;
+- the result is backed by packet evidence.
+
+## Example capture set
+
+One practical example of this workflow uses:
+
+- `origin-direct-nginx.pcap`
+- `telemt-ee-before-nginx.pcap`
+- `telemt-ee-after-nginx.pcap`
+
+Practical notes:
+
+- `origin` was captured as a direct TLS 1.2 connection to `nginx.org`;
+- `before` and `after` were captured on the Telemt FakeTLS success path with a real Telegram client;
+- the first server-side FakeTLS response remains valid in both cases;
+- the early encrypted server-flight segmentation differs between `before` and `after`, which is consistent with better reuse of captured profile data;
+- this kind of result shows a wire-visible effect without breaking the success path, but it does not claim full indistinguishability from the origin.
+
+## Stronger validation
+
+For broader confidence, repeat the same comparison on:
+
+1. one CDN-backed origin;
+2. one regular nginx origin;
+3. one origin with a multi-record encrypted flight and visible ticket-like tails.
+
+If the same directional improvement appears across all three, confidence in the result will be much higher than for a single-origin example.
--- a/docs/Architecture/Fronting-splitting/TLS_FRONT_PROFILE_FIDELITY.ru.md
+++ b/docs/Architecture/Fronting-splitting/TLS_FRONT_PROFILE_FIDELITY.ru.md
@@ -0,0 +1,225 @@
+# Fidelity TLS Front Profile
+
+## Обзор
+
+Этот документ описывает, как Telemt переиспользует захваченное TLS-поведение в FakeTLS server flight и как проверять результат на реальной инсталляции.
+
+Когда включена TLS front emulation, Telemt может собирать полезное серверное TLS-поведение выбранного origin и использовать его в emulated success path. Цель здесь не в побайтном копировании origin, а в уменьшении устойчивых synthetic признаков и в том, чтобы emitted server flight был структурно ближе к захваченному profile.
+
+## Зачем нужно это изменение
+
+Проект уже умеет собирать полезное серверное TLS-поведение в пути TLS front fetch:
+
+- `change_cipher_spec_count`
+- `app_data_record_sizes`
+- `ticket_record_sizes`
+
+До этого изменения эмулятор использовал только часть этой информации. Из-за этого оставался разрыв между захваченным поведением origin и тем FakeTLS server flight, который реально уходил на провод.
+
+## Что реализовано
+
+- Эмулятор теперь воспроизводит наблюдаемое значение `ChangeCipherSpec` из полученного `behavior_profile`.
+- Эмулятор теперь воспроизводит наблюдаемые размеры ticket-like tail ApplicationData records, когда доступны raw или merged TLS profile data.
+- Эмулятор теперь сохраняет больше структуры профилированного encrypted flight, а не схлопывает его в более маленькую synthetic форму.
+- Для профилей без raw TLS behavior по-прежнему сохраняется прежний synthetic fallback.
+- Операторский `tls_new_session_tickets` по-прежнему работает как дополнительный fallback, если профиль не даёт достаточного количества tail records.
+
+## Практическая польза
+
+- Снижается различимость между профилированным origin TLS-поведением и эмулируемым TLS-поведением.
+- Уменьшается шанс устойчивых server-flight fingerprint, вызванных фиксированным CCS count или полностью synthetic tail record sizes.
+- Уже собранные TLS profile data используются лучше, без изменения MTProto logic, KDF routing или transport architecture.
+
+## Ограничения
+
+Этот механизм не ставит целью сделать Telemt побайтно идентичным origin server.
+
+Он также не меняет:
+
+- MTProto business logic;
+- поведение KDF routing;
+- общую transport architecture.
+
+Практическая цель уже:
+
+- использовать больше уже собранных profile data;
+- уменьшить fixed synthetic behavior в server flight;
+- сохранить валидный FakeTLS success path, одновременно меняя форму emitted traffic на проводе.
+
+## Цели валидации
+
+- Корректное количество эмулируемых `ChangeCipherSpec` records.
+- Корректное воспроизведение наблюдаемых ticket-tail record sizes.
+- Отсутствие регрессии в существующем ALPN и payload-placement behavior.
+
+## Как проверять результат
+
+Рекомендуемая валидация состоит из двух слоёв:
+
+- focused unit и security tests для CCS-count replay и ticket-tail replay;
+- сравнение реальных packet capture для выбранного origin и успешной FakeTLS session.
+
+При проверке на сети ожидаемый результат такой:
+
+- валидный FakeTLS и MTProto success path сохраняется;
+- форма раннего encrypted server flight меняется, когда доступно более богатое profile data;
+- изменение видно на проводе без изменения MTProto logic и transport architecture.
+
+Такая проверка нужна для подтверждения того, что уже собранные TLS profile data используются лучше.
+Она не предназначена для доказательства побайтной эквивалентности с реальным origin server.
+
+## Как проверить на реальной инсталляции
+
+Самая сильная практическая проверка — side-by-side trace comparison между:
+
+- реальным TLS origin server, используемым как `mask_host`;
+- Telemt FakeTLS success-path connection для того же SNI;
+- при необходимости capture от разных Telemt builds или configurations.
+
+Смысл сравнения состоит в том, чтобы посмотреть на форму server flight:
+
+- порядок records;
+- количество `ChangeCipherSpec` records;
+- количество и группировку ранних encrypted `ApplicationData` records;
+- размеры tail или continuation `ApplicationData` records.
+
+## Рекомендуемое окружение
+
+Для самой чистой проверки лучше использовать Linux host или Docker container.
+
+Рекомендуемый setup:
+
+1. Один экземпляр Telemt.
+2. Один реальный HTTPS origin как `mask_host`.
+3. Один Telegram client, настроенный на `ee` proxy link для Telemt instance.
+4. `tcpdump` или Wireshark для анализа capture.
+
+## Пошаговая процедура проверки
+
+### 1. Подготовить origin
+
+1. Выберите реальный HTTPS origin.
+2. Установите и `censorship.tls_domain`, и `censorship.mask_host` в hostname этого origin.
+3. Убедитесь, что прямой TLS request работает:
+
+```bash
+openssl s_client -connect ORIGIN_IP:443 -servername YOUR_DOMAIN </dev/null
+```
+
+### 2. Настроить Telemt
+
+Используйте config, где включены:
+
+- `censorship.mask = true`
+- `censorship.tls_emulation = true`
+- `censorship.mask_host`
+- `censorship.mask_port`
+
+Для более чистой проверки рекомендуется:
+
+- держать `censorship.tls_new_session_tickets = 0`, чтобы результат в первую очередь зависел от fetched profile data, а не от операторских synthetic tail records;
+- держать `censorship.tls_fetch.strict_route = true`, если важна более чистая provenance для captured profile data.
+
+### 3. Обновить TLS profile data
+
+1. Запустите Telemt.
+2. Дайте ему получить TLS front profile data для выбранного домена.
+3. Если `tls_front_dir` хранится persistently, убедитесь, что TLS front cache заполнен.
+
+Persisted cache artifacts полезны, но не обязательны, если packet capture уже показывают runtime result.
+
+### 4. Снять direct-origin trace
+
+С отдельной клиентской машины подключитесь напрямую к origin:
+
+```bash
+openssl s_client -connect ORIGIN_IP:443 -servername YOUR_DOMAIN </dev/null
+```
+
+Capture:
+
+```bash
+sudo tcpdump -i any -w origin-direct.pcap host ORIGIN_IP and port 443
+```
+
+### 5. Снять Telemt FakeTLS success-path trace
+
+Теперь подключитесь к Telemt через реальный Telegram client с `ee` proxy link, который указывает на Telemt instance.
+
+`openssl s_client` полезен для direct-origin capture и для fallback sanity checks, но он не проходит успешный FakeTLS и MTProto path.
+
+Capture:
+
+```bash
+sudo tcpdump -i any -w telemt-emulated.pcap host TELEMT_IP and port 443
+```
+
+### 6. Декодировать структуру TLS records
+
+Используйте `tshark`, чтобы вывести record-level structure:
+
+```bash
+tshark -r origin-direct.pcap -Y "tls.record" -T fields \
+  -e frame.number \
+  -e ip.src \
+  -e ip.dst \
+  -e tls.record.content_type \
+  -e tls.record.length
+```
+
+```bash
+tshark -r telemt-emulated.pcap -Y "tls.record" -T fields \
+  -e frame.number \
+  -e ip.src \
+  -e ip.dst \
+  -e tls.record.content_type \
+  -e tls.record.length
+```
+
+Смотрите на server flight после ClientHello:
+
+- `22` = Handshake
+- `20` = ChangeCipherSpec
+- `23` = ApplicationData
+
+### 7. Собрать сравнительную таблицу
+
+Обычно достаточно короткой таблицы такого вида:
+
+| Path | CCS count | AppData count in first encrypted flight | Tail AppData lengths |
+| --- | --- | --- | --- |
+| Origin | `N` | `M` | `[a, b, ...]` |
+| Telemt build A | `...` | `...` | `...` |
+| Telemt build B | `...` | `...` | `...` |
+
+По такой таблице должно быть легко увидеть, что:
+
+- FakeTLS success path остаётся валидным;
+- ранний encrypted server flight меняется, когда переиспользуется более богатое profile data;
+- результат подтверждён packet evidence.
+
+## Пример набора capture
+
+Один практический пример такой проверки использует:
+
+- `origin-direct-nginx.pcap`
+- `telemt-ee-before-nginx.pcap`
+- `telemt-ee-after-nginx.pcap`
+
+Практические замечания:
+
+- `origin` снимался как прямое TLS 1.2 connection к `nginx.org`;
+- `before` и `after` снимались на Telemt FakeTLS success path с реальным Telegram client;
+- первый server-side FakeTLS response остаётся валидным в обоих случаях;
+- сегментация раннего encrypted server flight отличается между `before` и `after`, что согласуется с лучшим использованием captured profile data;
+- такой результат показывает заметный эффект на проводе без поломки success path, но не заявляет полной неотличимости от origin.
+
+## Более сильная валидация
+
+Для более широкой проверки повторите ту же процедуру ещё на:
+
+1. одном CDN-backed origin;
+2. одном regular nginx origin;
+3. одном origin с multi-record encrypted flight и заметными ticket-like tails.
+
+Если одно и то же направление улучшения повторится на всех трёх, уверенность в результате будет значительно выше, чем для одного origin example.
--- a/src/daemon/mod.rs
+++ b/src/daemon/mod.rs
@@ -9,6 +9,7 @@ use std::os::unix::fs::OpenOptionsExt;
 use std::path::{Path, PathBuf};

 use nix::fcntl::{Flock, FlockArg};
+use nix::errno::Errno;
 use nix::unistd::{self, ForkResult, Gid, Pid, Uid, chdir, close, fork, getpid, setsid};
 use tracing::{debug, info, warn};

@@ -337,6 +338,31 @@ fn is_process_running(pid: i32) -> bool {
    nix::sys::signal::kill(Pid::from_raw(pid), None).is_ok()
 }

+// macOS gates nix::unistd::setgroups differently in the current dependency set,
+// so call libc directly there while preserving the original nix path elsewhere.
+fn set_supplementary_groups(gid: Gid) -> Result<(), nix::Error> {
+    #[cfg(target_os = "macos")]
+    {
+        let groups = [gid.as_raw()];
+        let rc = unsafe {
+            libc::setgroups(
+                i32::try_from(groups.len()).expect("single supplementary group must fit in c_int"),
+                groups.as_ptr(),
+            )
+        };
+        if rc == 0 {
+            Ok(())
+        } else {
+            Err(Errno::last())
+        }
+    }
+
+    #[cfg(not(target_os = "macos"))]
+    {
+        unistd::setgroups(&[gid])
+    }
+}
+
 /// Drops privileges to the specified user and group.
 ///
 /// This should be called after binding privileged ports but before entering
@@ -368,7 +394,7 @@ pub fn drop_privileges(

    if let Some(gid) = target_gid {
        unistd::setgid(gid).map_err(DaemonError::PrivilegeDrop)?;
-        unistd::setgroups(&[gid]).map_err(DaemonError::PrivilegeDrop)?;
+        set_supplementary_groups(gid).map_err(DaemonError::PrivilegeDrop)?;
        info!(gid = gid.as_raw(), "Dropped group privileges");
    }

--- a/src/tls_front/emulator.rs
+++ b/src/tls_front/emulator.rs
@@ -11,6 +11,7 @@ use crc32fast::Hasher;

 const MIN_APP_DATA: usize = 64;
 const MAX_APP_DATA: usize = MAX_TLS_CIPHERTEXT_SIZE;
+const MAX_TICKET_RECORDS: usize = 4;

 fn jitter_and_clamp_sizes(sizes: &[usize], rng: &SecureRandom) -> Vec<usize> {
    sizes
@@ -62,6 +63,51 @@ fn ensure_payload_capacity(mut sizes: Vec<usize>, payload_len: usize) -> Vec<usi
    sizes
 }

+fn emulated_app_data_sizes(cached: &CachedTlsData) -> Vec<usize> {
+    match cached.behavior_profile.source {
+        TlsProfileSource::Raw | TlsProfileSource::Merged => {
+            if !cached.behavior_profile.app_data_record_sizes.is_empty() {
+                return cached.behavior_profile.app_data_record_sizes.clone();
+            }
+        }
+        TlsProfileSource::Default | TlsProfileSource::Rustls => {}
+    }
+
+    let mut sizes = cached.app_data_records_sizes.clone();
+    if sizes.is_empty() {
+        sizes.push(cached.total_app_data_len.max(1024));
+    }
+    sizes
+}
+
+fn emulated_change_cipher_spec_count(cached: &CachedTlsData) -> usize {
+    usize::from(cached.behavior_profile.change_cipher_spec_count.max(1))
+}
+
+fn emulated_ticket_record_sizes(
+    cached: &CachedTlsData,
+    new_session_tickets: u8,
+    rng: &SecureRandom,
+) -> Vec<usize> {
+    let mut sizes = match cached.behavior_profile.source {
+        TlsProfileSource::Raw | TlsProfileSource::Merged => {
+            cached.behavior_profile.ticket_record_sizes.clone()
+        }
+        TlsProfileSource::Default | TlsProfileSource::Rustls => Vec::new(),
+    };
+
+    let target_count = sizes
+        .len()
+        .max(usize::from(new_session_tickets.min(MAX_TICKET_RECORDS as u8)))
+        .min(MAX_TICKET_RECORDS);
+
+    while sizes.len() < target_count {
+        sizes.push(rng.range(48) + 48);
+    }
+
+    sizes
+}
+
 fn build_compact_cert_info_payload(cert_info: &ParsedCertificateInfo) -> Option<Vec<u8>> {
    let mut fields = Vec::new();

@@ -180,39 +226,21 @@ pub fn build_emulated_server_hello(
    server_hello.extend_from_slice(&message);

    // --- ChangeCipherSpec ---
-    let change_cipher_spec = [
-        TLS_RECORD_CHANGE_CIPHER,
-        TLS_VERSION[0],
-        TLS_VERSION[1],
-        0x00,
-        0x01,
-        0x01,
-    ];
+    let change_cipher_spec_count = emulated_change_cipher_spec_count(cached);
+    let mut change_cipher_spec = Vec::with_capacity(change_cipher_spec_count * 6);
+    for _ in 0..change_cipher_spec_count {
+        change_cipher_spec.extend_from_slice(&[
+            TLS_RECORD_CHANGE_CIPHER,
+            TLS_VERSION[0],
+            TLS_VERSION[1],
+            0x00,
+            0x01,
+            0x01,
+        ]);
+    }

    // --- ApplicationData (fake encrypted records) ---
-    let sizes = match cached.behavior_profile.source {
-        TlsProfileSource::Raw | TlsProfileSource::Merged => cached
-            .app_data_records_sizes
-            .first()
-            .copied()
-            .or_else(|| {
-                cached
-                    .behavior_profile
-                    .app_data_record_sizes
-                    .first()
-                    .copied()
-            })
-            .map(|size| vec![size])
-            .unwrap_or_else(|| vec![cached.total_app_data_len.max(1024)]),
-        _ => {
-            let mut sizes = cached.app_data_records_sizes.clone();
-            if sizes.is_empty() {
-                sizes.push(cached.total_app_data_len.max(1024));
-            }
-            sizes
-        }
-    };
-    let mut sizes = jitter_and_clamp_sizes(&sizes, rng);
+    let mut sizes = jitter_and_clamp_sizes(&emulated_app_data_sizes(cached), rng);
    let compact_payload = cached
        .cert_info
        .as_ref()
@@ -299,17 +327,13 @@ pub fn build_emulated_server_hello(
    // --- Combine ---
    // Optional NewSessionTicket mimic records (opaque ApplicationData for fingerprint).
    let mut tickets = Vec::new();
-    let ticket_count = new_session_tickets.min(4);
-    if ticket_count > 0 {
-        for _ in 0..ticket_count {
-            let ticket_len: usize = rng.range(48) + 48;
-            let mut rec = Vec::with_capacity(5 + ticket_len);
+    for ticket_len in emulated_ticket_record_sizes(cached, new_session_tickets, rng) {
+        let mut rec = Vec::with_capacity(5 + ticket_len);
            rec.push(TLS_RECORD_APPLICATION);
            rec.extend_from_slice(&TLS_VERSION);
            rec.extend_from_slice(&(ticket_len as u16).to_be_bytes());
            rec.extend_from_slice(&rng.bytes(ticket_len));
            tickets.extend_from_slice(&rec);
-        }
    }

    let mut response = Vec::with_capacity(
@@ -334,6 +358,10 @@ pub fn build_emulated_server_hello(
 #[path = "tests/emulator_security_tests.rs"]
 mod security_tests;

+#[cfg(test)]
+#[path = "tests/emulator_profile_fidelity_security_tests.rs"]
+mod emulator_profile_fidelity_security_tests;
+
 #[cfg(test)]
 mod tests {
    use std::time::SystemTime;
@@ -478,7 +506,7 @@ mod tests {
    }

    #[test]
-    fn test_build_emulated_server_hello_ignores_tail_records_for_raw_profile() {
+    fn test_build_emulated_server_hello_replays_tail_records_for_profiled_tls() {
        let mut cached = make_cached(None);
        cached.app_data_records_sizes = vec![27, 3905, 537, 69];
        cached.total_app_data_len = 4538;
@@ -500,11 +528,19 @@ mod tests {

        let hello_len = u16::from_be_bytes([response[3], response[4]]) as usize;
        let ccs_start = 5 + hello_len;
-        let app_start = ccs_start + 6;
-        let app_len =
-            u16::from_be_bytes([response[app_start + 3], response[app_start + 4]]) as usize;
+        let mut pos = ccs_start + 6;
+        let mut app_lengths = Vec::new();
+        while pos + 5 <= response.len() {
+            assert_eq!(response[pos], TLS_RECORD_APPLICATION);
+            let record_len = u16::from_be_bytes([response[pos + 3], response[pos + 4]]) as usize;
+            app_lengths.push(record_len);
+            pos += 5 + record_len;
+        }

-        assert_eq!(response[app_start], TLS_RECORD_APPLICATION);
-        assert_eq!(app_start + 5 + app_len, response.len());
+        assert_eq!(app_lengths.len(), 4);
+        assert_eq!(app_lengths[0], 64);
+        assert_eq!(app_lengths[3], 69);
+        assert!(app_lengths[1] >= 64);
+        assert!(app_lengths[2] >= 64);
    }
 }
--- a/src/tls_front/tests/emulator_profile_fidelity_security_tests.rs
+++ b/src/tls_front/tests/emulator_profile_fidelity_security_tests.rs
@@ -0,0 +1,95 @@
+use std::time::SystemTime;
+
+use crate::crypto::SecureRandom;
+use crate::protocol::constants::{
+    TLS_RECORD_APPLICATION, TLS_RECORD_CHANGE_CIPHER, TLS_RECORD_HANDSHAKE,
+};
+use crate::tls_front::emulator::build_emulated_server_hello;
+use crate::tls_front::types::{
+    CachedTlsData, ParsedServerHello, TlsBehaviorProfile, TlsProfileSource,
+};
+
+fn make_cached() -> CachedTlsData {
+    CachedTlsData {
+        server_hello_template: ParsedServerHello {
+            version: [0x03, 0x03],
+            random: [0u8; 32],
+            session_id: Vec::new(),
+            cipher_suite: [0x13, 0x01],
+            compression: 0,
+            extensions: Vec::new(),
+        },
+        cert_info: None,
+        cert_payload: None,
+        app_data_records_sizes: vec![1200, 900, 220, 180],
+        total_app_data_len: 2500,
+        behavior_profile: TlsBehaviorProfile {
+            change_cipher_spec_count: 2,
+            app_data_record_sizes: vec![1200, 900],
+            ticket_record_sizes: vec![220, 180],
+            source: TlsProfileSource::Merged,
+        },
+        fetched_at: SystemTime::now(),
+        domain: "example.com".to_string(),
+    }
+}
+
+fn record_lengths_by_type(response: &[u8], wanted_type: u8) -> Vec<usize> {
+    let mut out = Vec::new();
+    let mut pos = 0usize;
+    while pos + 5 <= response.len() {
+        let record_type = response[pos];
+        let record_len = u16::from_be_bytes([response[pos + 3], response[pos + 4]]) as usize;
+        if pos + 5 + record_len > response.len() {
+            break;
+        }
+        if record_type == wanted_type {
+            out.push(record_len);
+        }
+        pos += 5 + record_len;
+    }
+    out
+}
+
+#[test]
+fn emulated_server_hello_replays_profile_change_cipher_spec_count() {
+    let cached = make_cached();
+    let rng = SecureRandom::new();
+
+    let response = build_emulated_server_hello(
+        b"secret",
+        &[0x71; 32],
+        &[0x72; 16],
+        &cached,
+        false,
+        &rng,
+        None,
+        0,
+    );
+
+    assert_eq!(response[0], TLS_RECORD_HANDSHAKE);
+    let ccs_records = record_lengths_by_type(&response, TLS_RECORD_CHANGE_CIPHER);
+    assert_eq!(ccs_records.len(), 2);
+    assert!(ccs_records.iter().all(|len| *len == 1));
+}
+
+#[test]
+fn emulated_server_hello_replays_profile_ticket_tail_lengths() {
+    let cached = make_cached();
+    let rng = SecureRandom::new();
+
+    let response = build_emulated_server_hello(
+        b"secret",
+        &[0x81; 32],
+        &[0x82; 16],
+        &cached,
+        false,
+        &rng,
+        None,
+        0,
+    );
+
+    let app_records = record_lengths_by_type(&response, TLS_RECORD_APPLICATION);
+    assert!(app_records.len() >= 4);
+    assert_eq!(&app_records[app_records.len() - 2..], &[220, 180]);
+}