# Custom WebRTC for Rosetta iOS (Audio E2EE Timestamp)

This setup builds a custom `WebRTC.xcframework` for iOS and patches audio E2EE so
`FrameEncryptor/FrameDecryptor` receive non-empty `additional_data` with RTP timestamp bytes.

## Why

Stock `stasel/WebRTC` M146 calls 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 iOS 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)

Same patch as Android: `mobile-android/tools/webrtc-custom/patches/0001-...`

## Files

- `build_custom_webrtc_ios.sh` — reproducible build script
- `patches/0001-audio-e2ee-pass-rtp-timestamp-as-additional-data.patch` — WebRTC patch (identical to Android)

## Build

Requirements: macOS, Xcode, `depot_tools` in PATH.

Bootstrap depot_tools first (if not installed):

```bash
git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git ~/depot_tools
export PATH=$PATH:$HOME/depot_tools
```

Then build:

```bash
cd /path/to/Rosetta/tools/webrtc-custom-ios
./build_custom_webrtc_ios.sh
```

Optional env vars:

- `WEBRTC_ROOT` — checkout root (default: `$HOME/webrtc_ios`)
- `WEBRTC_BRANCH` — default `branch-heads/6422` (M125, same as Android)
- `WEBRTC_TAG` — use a specific tag/commit instead of branch
- `OUT_FRAMEWORK` — output xcframework path (default: `Frameworks/WebRTC.xcframework`)
- `SYNC_JOBS` — `gclient sync` jobs (default: `1`)
- `SYNC_RETRIES` — sync retry attempts (default: `8`)

## Integration in Xcode

1. Remove `stasel/WebRTC` SPM dependency from Xcode project
2. Add `Frameworks/WebRTC.xcframework` as Embedded Framework
3. Build and verify: diagnostic logs should show `ad=8` instead of `ad=0`

## Verification

After building and integrating:

```
ENC frame#0 sz=53 ad=8 mode=raw-abs nonce=[00000000XXXXXXXX]
DEC frame#0 sz=48 ad=8 mode=raw-abs ok=1 nonce=[00000000XXXXXXXX]
```

Where `XXXXXXXX` is the RTP timestamp in hex. This matches Desktop and Android behavior.

## Maintenance

- Keep patch small: only `audio/channel_send.cc` + `audio/channel_receive.cc`
- Pin WebRTC branch/tag for releases (M125 = `branch-heads/6422`)
- Rebuild xcframework on version bumps
- Verify `ad=8` in diagnostic logs after each rebuild