Add Android Secure Capture & Timestamp – Technical Blueprint
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## 1. Product vision
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A minimal **Android app (Kotlin)** that can, on user demand, **capture video + audio in the background**, stream it live through **WebRTC** to a backend, and locally buffer a loss‑tolerant copy. On the server side we **hash** each media chunk and immediately obtain a **qualified timestamp (QTSP)** or blockchain attestation, preserving an end‑to‑end, court‑ready chain of custody.
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---
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## 2. Functional requirements
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| ID | Requirement | Notes |
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| ----- | ----------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------- |
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| FR‑1 | User can start/stop a secure recording session from a single UI toggle. | Must show foreground notification due to Android 13+ background recording rules. |
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| FR‑2 | App records **front or rear camera** and **microphone** simultaneously. | 1080 p @ 30 fps baseline. |
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| FR‑3 | App survives screen‑off / app switch (uses **Foreground Service**). | "MediaProjection + CameraX" or pure CameraX route. |
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| FR‑4 | Each encoded frame is sent via **WebRTC data/media channel** to server. | SRTP for privacy. |
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| FR‑5 | If network is lost, chunks are stored encrypted in app sandbox and retried with **WorkManager**. | |
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| FR‑6 | Server receives RTP, hashes every GOP with SHA‑256, requests timestamp, stores tuple *(hash, RFC 3161 TST, chunk path)* in DB + S3‑like object store. | |
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| FR‑7 | App exposes a "Verify" screen: download TST receipt & proof for any session, recompute local hash, and show green / red indicator. | |
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---
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## 3. High‑level architecture
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```
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┌───────────────┐ SRTP / ICE ┌──────────────────┐
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│ Android App │ ─────── peer connection ─► │ Signalling + │
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│ Kotlin + │ ◄────── (TURN fallback) ◄─ │ Media SFU/Router│
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│ Foreground │ │ (Janus / Pion) │
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│ Service │ REST/WebSocket │ │
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└──────▲────────┘ │ └─────────▲────────┘
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│ local AES‑256 store │ SHA‑256(chunk) │ gRPC
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│ (Room + SQLCipher) ▼ ▼
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┌──────┴────────┐ ┌───────────────┐ ┌───────────────┐
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│ WorkManager │ │ Hasher + │ TSP │ Qualified │
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│ retry queue │ │ TimeStamper │◄────►│ TS Authority │
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└───────────────┘ └───────────────┘ └───────────────┘
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```
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* **Media path**: CameraX → MediaCodec (H.264) → `SurfaceTexture` → WebRTC **VideoTrack**.
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* **Audio path**: `AudioRecord` (OPUS) → WebRTC **AudioTrack**.
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---
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## 4. Android implementation details
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### 4.1 Permissions & privacy
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* `CAMERA`, `RECORD_AUDIO`, `FOREGROUND_SERVICE_CAMERA`, `POST_NOTIFICATIONS` (Tiramisu+).
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* Runtime request flow wrapped in coroutine `suspend fun requestPermissions()`.
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### 4.2 Foreground RecordingService
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```kotlin
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@ForegroundService
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class RecordingService : Service() {
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private val scope = CoroutineScope(Dispatchers.Default + SupervisorJob())
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override fun onStartCommand(i: Intent?, flags: Int, startId: Int): Int {
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startForeground(NOTIF_ID, buildNotif())
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scope.launch { startCaptureSession() }
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return START_STICKY
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}
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private suspend fun startCaptureSession() {
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val camera = CameraXInitializer(cameraSelector).open()
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val rtcClient = WebRtcClient(iceServers)
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val videoSrc = CameraXVideoSource(camera)
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rtcClient.addTrack(videoSrc.track)
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rtcClient.addTrack(MicAudioSource().track)
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rtcClient.connect(signalServer)
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}
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override fun onDestroy() {
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scope.cancel();
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super.onDestroy()
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}
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}
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```
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*Notification must be **user‑dismissible** and display recording timer — meets Android policy.*
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### 4.3 Local buffering & WorkManager
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```kotlin
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class ChunkWriter(private val ctx: Context){
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suspend fun saveChunk(bytes: ByteArray){
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val encrypted = crypto.encrypt(bytes)
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room.chunkDao().insert(ChunkEntity(ts=now(), blob=encrypted))
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}
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}
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```
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`PeriodicWorkRequest` checks DB → upload to server via REST → on success delete row.
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---
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## 5. Server‑side pipeline (Golang example)
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```go
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// Media handler (Pion SFU hook)
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func onRTPPacket(pkt *rtp.Packet){
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chunkBuf.Write(pkt.Payload)
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if gopFinished(pkt){
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h := sha256.Sum256(chunkBuf.Bytes())
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tst, _ := tspClient.Timestamp(h[:])
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store.Save(chunkBuf.Bytes(), h[:], tst)
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chunkBuf.Reset()
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}
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}
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```
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*Timestamp sources*:
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1. **QTSP (eIDAS)** – Camerfirma TSA, FNMT, Poste Italiane, etc.
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2. **Blockchain fallback** – embed hash in Bitcoin OP\_RETURN; use public API for proof.
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---
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## 6. Performance & energy
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| Component | Trade‑off | Recommendation |
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| ------------------------- | --------------------------------------------------------- | ------------------------------------------------------- |
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| CameraX vs. MediaRecorder | CameraX gives **Surface** for WebRTC; slightly higher CPU | Acceptable on devices ≥ Snapdragon 730 |
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| Video codec | H.264 Baseline profile | Widely HW‑accelerated, 1 Mbps @1080p 30fps ≈ 2 h ≈ 1 GB |
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| Audio | OPUS 48 kHz 16 kbps | Low bandwidth, good quality |
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| Encryption | SRTP inline; local AES‑256 | negligible on ARMv8 AES‑NI |
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---
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## 7. Security considerations
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* **Zero‑Config Keys**: generate on first run, stored in Android Keystore.
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* **Pinning**: TLS cert pin to signalling + REST host.
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* **Tamper‑evident log**: every state change appended to SQLCipher DB with incremental hash chain.
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---
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## 8. Roadmap
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1. **P0** – Local capture + save MP4.
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2. **P1** – Add WebRTC streaming via public TURN.
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3. **P2** – Server hash + QTSP integration.
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4. **P3** – Verification UI + export proof bundle (ZIP: video + JSON hash + TST).
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5. **P4** – Battery optimisation, Doze‑mode job scheduling.
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---
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## 9. References / Tooling
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* *CameraX* Jetpack lib.
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* *WebRTC‑KMP* for multiplatform client.
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* **Pion SFU** (Go) or **Janus** (C) backend.
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* **RFC 3161** Time‑Stamp Protocol.
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* **ETS I EN 319 421** QTSP TSA policy.
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* **SQLCipher** for on‑device encrypted DB.
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* **gRPC** for low‑latency hash receipt.
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---
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### Appendix A – Minimal Client ↔ Server handshake (pseudo‑sequence)
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```
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User taps Record
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│
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├─► RecordingService.startForeground()
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│ ├─ CameraX open & startPreview
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│ ├─ WebRtcClient.createOffer()
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│ └─ Signal via WebSocket
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│
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Signalling Server ↔ offers/answers ICE
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│
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RTP/RTCP flow
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│
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Hasher splits GOP → SHA‑256 → TSA → OK
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│
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Server sends `hashReceipt` JSON over data‑channel ➜ App shows ✅
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```
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*End of document*
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