Custom WebRTC for Rosetta Android (Audio E2EE Timestamp)

This setup builds a custom libwebrtc.aar for Android and patches audio E2EE so FrameEncryptor/FrameDecryptor receive non-empty additional_data with RTP timestamp bytes.

Why

Stock io.github.webrtc-sdk:android:125.6422.07 can call audio frame encryptor with empty additional_data (ad=0), so nonce derivation based on timestamp is unavailable.

Desktop uses frame timestamp for nonce. This patch aligns Android with that approach by passing an 8-byte big-endian timestamp payload in additional_data (absolute RTP timestamp, including sender start offset):

bytes 0..3 = 0
bytes 4..7 = RTP timestamp (big-endian)

Files

build_custom_webrtc.sh — reproducible build script
patches/0001-audio-e2ee-pass-rtp-timestamp-as-additional-data.patch — WebRTC patch
patches/0002-android-build-on-mac-host.patch — allows Android target build on macOS host
patches/0003-macos-host-java-ijar.patch — enables host tools (ijar/jdk) on macOS
patches/0004-macos-linker-missing-L-dirs.patch — skips invalid host -L... paths for lld
patches/0005-macos-server-utils-socket.patch — handles macOS socket errno in Android Java compile helper

Build

Recommended on Linux (macOS is supported via additional patches in this folder).

Bootstrap depot_tools first:

cd /path/to/rosetta-android/tools/webrtc-custom
./bootstrap_depot_tools.sh

Then run:

cd /path/to/rosetta-android/tools/webrtc-custom
./build_custom_webrtc.sh

Optional env vars:

WEBRTC_ROOT — checkout root (default: $HOME/webrtc_android)
WEBRTC_SRC — direct path to src/
WEBRTC_BRANCH — default branch-heads/6422
WEBRTC_TAG — use a specific tag/commit instead of branch
OUT_AAR — output AAR path (default: app/libs/libwebrtc-custom.aar)
SYNC_JOBS — gclient sync jobs (default: 1, safer for googlesource limits)
SYNC_RETRIES — sync retry attempts (default: 8)
SYNC_RETRY_BASE_SEC — base retry delay in seconds (default: 20)
MAC_ANDROID_NDK_ROOT — local Android NDK path on macOS (default: ~/Library/Android/sdk/ndk/27.1.12297006)

Troubleshooting (HTTP 429 / RESOURCE_EXHAUSTED)

If build fails with:

The requested URL returned error: 429
RESOURCE_EXHAUSTED
Short term server-time rate limit exceeded

run with conservative sync settings:

SYNC_JOBS=1 SYNC_RETRIES=12 SYNC_RETRY_BASE_SEC=30 ./build_custom_webrtc.sh

The script now retries fetch, git fetch, and gclient sync with backoff.

Integration in app

app/build.gradle.kts already prefers local app/libs/libwebrtc-custom.aar if present. If file exists, Maven WebRTC dependency is not used.

Maintenance policy

Keep patch small and isolated to audio/channel_send.cc + audio/channel_receive.cc.
Pin WebRTC branch/tag for releases.
Rebuild AAR on version bumps and verify e2ee_diag.txt shows ad=8 (or non-zero).

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