Hands-On Lab: Build a Test RCS E2EE Environment and Validate Interoperability

Hook: Why you should build a local Rich Communication Services (RCS) E2EE lab now

If you are responsible for mobile messaging security, you face two simultaneous pressures in 2026: the rapid roll‑out of Rich Communication Services (RCS) with end‑to‑end encryption (E2EE), and the need to validate interoperability across Android and Apple devices before production traffic hits your services. Carriers, Google and Apple began rolling E2EE for RCS in late 2024–2025 under GSMA Universal Profile updates, and by early 2026 many operator stacks and consumer clients advertise MLS‑based E2EE. That means your test matrix must include cross‑platform validation, logging and forensic evidence that messages are actually encrypted in transit — not just that UIs claim ‘secure’.

Lab objective and outcomes

Objective: Deploy a repeatable test environment to send RCS messages between Android and iPhone clients, capture logs and network traces, and validate E2EE properties and interoperability scenarios.

After this lab you will be able to:

• Provision an Android test client and an iPhone test client that speak RCS E2EE
• Capture and filter client logs that show MLS handshakes and key events
• Collect network captures demonstrating ciphertext in transit
• Run interoperability scenarios (1:1, group, attachments, downgrades) and automate checks
• Produce evidence artifacts (logs, pcaps, signed test report) suitable for audit or vendor escalation

What changed in 2025–2026 — quick context for engineers

Recent industry momentum matters for this lab. The GSMA Universal Profile and vendor implementations converged on MLS (Message Layer Security) as the E2EE primitive for RCS group and 1:1 encryption. By late 2025 several major carriers in Europe and APAC enabled E2EE capability flags, and both Google Messages and newer iOS builds (Apple began shipping RCS capability flags in 2024–2025 betas) shipped user‑agent support for encrypted RCS. For engineering teams this means:

• Client logs now contain MLS handshake events. Look for explicit MLS session creation and epoch updates.
• Servers still mediate presence and delivery receipts. You must verify that servers cannot read message plaintext.
• Interop edge cases proliferated. Apple/Android differences in group semantics, attachment chunking and fallback behavior require targeted tests.

Prerequisites — hardware, software and accounts

Assemble the following before starting the step‑by‑step sections.

• Two test devices: one Android device with a recent Google Messages / RCS client (beta if needed), and one iPhone running an RCS‑capable iOS build (developer or public beta where RCS flags are present). Prefer physical devices over simulators for carrier/IMS features.
• Wi‑Fi network and a macOS or Linux workstation for captures, logging and automation.
• USB cables and adb (Android SDK tools) installed. Xcode + macOS Console or libimobiledevice for iPhone logs.
• Optional: control of an RCS hub (operator or testbed). If you cannot control a server, you can still validate client behavior and produce network evidence.
• Installable tooling: Wireshark/tshark, tcpdump, OpenMLS (for MLS test harnesses), Appium/XCTest (for automation), and a secure artifact store for your logs.

High‑level lab topology

We will validate three layers:

1. Client UI/UX — confirm clients mark conversations as encrypted and display key continuity info.
2. Client logs — collect MLS handshake logs, key IDs and session IDs.
3. Network level — capture traffic to confirm ciphertext and evaluate downgrade attempts.

Step 1 — Prepare devices and enable RCS E2EE

1. On Android:

• Install or update Google Messages to the latest beta channel where E2EE is supported.
• Enable Chat features (RCS) and Opt into developer logging if the client offers it. For Messages beta there's often a Developer settings toggle: Settings → Chat features → Advanced → Developer settings.
• Note the device phone number and ensure it’s RCS registered (green status in the client).

2. On iPhone:

• Install the iOS beta that contains RCS capability flags (as of early 2026 this is widely available in Apple’s public beta program for many regions — check your carrier list).
• Enable the carrier/Settings flag that allows RCS (Settings → Messages → Carrier RCS or similar). If your device does not show a toggle, work with a test carrier profile or use a carrier that advertises RCS E2EE support.
• Confirm the iPhone reports RCS chat capability with the Android test device (UI indicator or contact card).

Step 2 — Capture client logs

Client logs are the most actionable artifact for E2EE validation. They show MLS handshake steps, key IDs, and error conditions.

Android: adb logcat

Connect the Android device and run adb. Use a focused filter to reduce noise.

adb devices
adb logcat -v time | grep -iE "(Rcs|rcs|mls|ims|E2EE|SecureConversation)"

Look for log lines that indicate:

• MLS handshake initiation: MLS group create or initiating MLS handshake.
• Key IDs and fingerprints: abbreviated hashes shown in logs for session verification.
• Epoch updates and message counters: indicate forward secrecy and group membership events.

iPhone: macOS Console or libimobiledevice

Attach the iPhone to your Mac and open Console.app. Filter logs by “RCS”, “mls”, “IMCore”, or the Messages process name.

# alternative using libimobiledevice on Linux/macOS
idevicesyslog | grep -iE "(RCS|mls|ims|E2EE|SecureConversation)"

On Apple builds expect similar handshake messages; note that Apple log formats differ. Collect full logs for the time window around message exchanges.

Step 3 — Network captures and cryptographic verification

Next, capture traffic to and from each device. Because RCS uses TLS to connect to carrier/hub servers, you will see TLS records; E2EE means even a server with TLS access should not be able to read message plaintext.

Capture method

• Put devices on a test Wi‑Fi controlled by your workstation or a small lab router.
• On the workstation, run tcpdump/tshark capturing the device IP addresses.

sudo tcpdump -i any host 192.168.1.13 -w android_dev.pcap
sudo tcpdump -i any host 192.168.1.14 -w iphone_dev.pcap

Open captures in Wireshark and inspect TLS application data packets. You should observe:

• TLS connections established to the operator/hub endpoints (SNI and IPs visible).
• Application data blobs that cannot be parsed as cleartext — ciphertext only.

If you control an RCS hub (test server), inspect server logs to verify received payloads are opaque ciphertext and that no plaintext body is recorded in server logs.

Step 4 — Interoperability test cases and expected results

Run this test matrix. For each test record client logs, server logs (if available), and network capture.

1. 1:1 text message (small payload)
   • Expected: Clients show “secure/locked” indicator; logs show MLS session established; network shows ciphertext.
2. Attachment transfer (image > 200 KB)
   • Expected: Attachment is uploaded to carrier blob storage only as encrypted content or uploaded in a way that the client performs out‑of‑band encryption. Confirm ciphertext on network and successful download on recipient.
3. Group chat (3+ members, mixed Android/iPhone)
   • Expected: MLS group handshake with epoch and membership info; logs show group keys and epoch updates; message delivered to group members encrypted under group epoch keys.
4. Linked device / multi‑device scenario
   • Expected: New device join triggers key synchronization; validate that message history handling follows client policy (e.g., recent history synced or not).
5. Downgrade simulation (force SMS fallback)
   • Expected: Clients detect inability to establish RCS and fallback to SMS or MMS; this is a plaintext fallback — log and capture a downgrade event for risk assessment.

Step 5 — Validate E2EE properties in logs (what to look for)

Logs are where you’ll prove E2EE. Focus on these artifacts:

• Handshake records: MLS proposals, commit messages, and key package identifiers.
• Key identifiers / fingerprints: Short hex or base64 identifiers that both clients log — they must match for the same conversation.
• Epoch transitions: When group membership changes, logs should show epoch updates and new group secrets derived.
• No plaintext bodies in client or server logs: Search for send/recv log lines — ensure bodies are not printed or are replaced by placeholders.

Advanced validation: MLS test harness with OpenMLS

If you control the service or want deep MLS verification, use OpenMLS (an open MLS implementation) to build a test harness that mirrors MLS handshakes and verifies transcript signatures and epochs.

Quick start (Rust + cargo):

git clone https://github.com/openmls/openmls.git
cd openmls
cargo build --release
# run example MLS client/server to reproduce handshake events
cargo run --example simple_group

Use the example’s debug output to compare key package IDs and commits with the mobile client logs. This proves both sides derive compatible key material and that the client’s MLS implementation is functioning as expected.

Automating tests and CI integration

Automate the scenarios above to run on commit and report regressions.

• Use Appium or Espresso for Android and XCTest for iOS to drive UI actions (send message, attach file, join group).
• On each run, collect adb logs and idevicesyslog output, and upload pcaps to a secure artifacts bucket.
• Parse logs with a small script (Python) to extract MLS key IDs, epoch numbers and match them across clients.

Example log extraction pseudocode (Python):

import re
with open('android_log.txt') as f:
    logs = f.read()
match = re.search(r"KeyPackageID: ([A-Za-z0-9+/=]+)", logs)
print(match.group(1))

Simulating attacks and failure modes

To validate robustness, simulate common attack or failure modes:

• Active MITM on TLS: Interpose a TLS proxy (e.g., mitmproxy) at the network level. With E2EE, you will see TLS content decrypted by the proxy but the application payloads remain ciphertext — confirm messages cannot be read.
• Replay attempts: Replay captured ciphertext to server and observe server/client detection of replays (ML S uses sequence numbers / epoch-specific info).
• Downgrade attempts: Remove or block RCS control messages (server/feature flags) to force fallback; detect and log fallback events as a policy failure.

Collecting evidence for audits or vendor escalation

When you need to prove E2EE in an audit or to a carrier/vendor, package the following:

1. Client logs (timestamped) from both endpoints covering the test window.
2. Network captures (pcap) with device IPs and timestamps.
3. If available, server logs showing reception of encrypted payloads (no plaintext bodies).
4. A short reproducible script or automation artifact and the exact device/build versions used.

Tip: Time synchronization across devices (NTP) is critical. Use NTP or manual time alignment to make log correlation reliable.

Common pitfalls and troubleshooting

• RCS capability mismatch — one client not registered: verify Chat features registration and carrier provisioning first.
• Beta OS differences — build mismatch on iOS/Android: confirm both clients are running supported builds for E2EE.
• Missing logs — enable verbose/developer logging on clients where available before starting tests.
• Attachment encryption — some implementations upload encrypted blobs differently; correlate logs for upload/download steps.

2026 trends and predictions engineers should track

As of 2026, keep an eye on these trends that will affect your testing strategy:

• Wider MLS maturity: MLS libraries and reference implementations will stabilize; expect conformity tests from GSMA and IETF working groups.
• Operator E2EE adoption: More carriers will flip E2EE flags in 2026; run regional test matrices to capture differences in behavior and attachment handling.
• Multi‑device key synchronization: Solutions for history sync under E2EE are evolving—test linked‑device sync flows carefully.
• Third‑party SDKs: RCS Business Messaging SDKs will add hooks for E2EE metadata — validate they don’t leak plaintext or key info.

Checklist — run before production launch

• Reproduce all interoperability scenarios across captive carrier/test operator networks.
• Collect and store logs and pcaps for each test run.
• Automate nightly regression suite to catch client/OS updates that break E2EE behavior.
• Produce a short technical report summarizing findings and, if problems found, include repro steps and artifacts for vendor escalation.

Appendix: Useful commands and filters

Quick reference commands for your lab.

• adb logcat filter (Android):

  adb logcat -v time | grep -iE "(Rcs|mls|E2EE|SecureConversation|Ims)"

• iPhone syslog (macOS):

  idevicesyslog | grep -iE "(RCS|mls|E2EE|IMCore)"

• tcpdump capture by IP:

  sudo tcpdump -i any host 192.168.1.13 -w device.pcap

• Wireshark filter to show TLS application data:

  tls.record.version && tls.app_data

• OpenMLS example runner (Rust):

  git clone https://github.com/openmls/openmls.git
  cd openmls
  cargo run --example simple_group

Final takeaways — what matters most

Practical E2EE validation requires three coordinated artifacts: client logs showing MLS handshake and key IDs, network captures showing ciphertext in transit, and, when possible, server logs proving no plaintext is available server‑side. In 2026, interoperability between Android and iPhone for RCS E2EE is real but fragile — test early, automate often, and keep vendor escalation packages ready.

Call to action

Ready to harden your messaging stack? Start this lab with the devices and tooling checklist, run the test matrix above, and produce an audit bundle. If you want a turnkey lab kit — including an OpenMLS harness and log‑parsing scripts tailored for your fleet — contact our team at defensive.cloud for a walkthrough and customized automation templates.

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