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# Stream Chat
# Client (Mobile/Web App)
*   **UI:** React Native / Flutter
*   **Chat encryption:** libsignal / WebCrypto
*   **File encryption:** AES-256-GCM (client-side)
*   **Progressive streaming:** MediaSource API (Web) + native player (Mobile)
*   **WebRTC:** P2P calls/stream sync

# Backend (Supabase / Node.js + Postgres)

*   **Auth service:** JWT + refresh tokens
*   **Chatroom metadata store:** rooms, members, keys
*   **File metadata store:** filename, encrypted key, expiry, storage path
*   Pre-signed short TTL links for downloads
*   **Admin dashboard:** reports, takedowns

# Storage Layer

*   S3-compatible storage (Wasabi/Backblaze/S3)
*   File chunks encrypted client-side before upload

# Streaming Infra

*   WebRTC P2P mesh (small groups ≤4 users)
*   **TURN server:** Coturn for NAT traversal
*   **Optional SFU:** mediasoup/LiveKit for larger groups
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2. Data Flow

(a) Chat Message Flow

1. User types message → encrypts with Signal protocol.
2. Message sent to Supabase Realtime.
3. Recipient client pulls message → decrypts using session key.

(b) File Upload & Share Flow

1. User selects file → client generates AES key.
2. Encrypt file chunks locally.
3. Upload encrypted chunks → S3 storage.
4. AES key encrypted with recipient’s public key → stored in DB.
5. Recipient fetches file → decrypts key with private key → decrypts chunks → plays.

(c) Streaming Flow

1. Uploader sends encrypted chunks to storage while upload continues.
2. Recipient fetches progressive chunks (pre-signed URL).
3. Decrypt + buffer → playback.
4. For co-watch → WebRTC data channel syncs playback position.

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3. Database Schema (Postgres / Supabase)

users

sql id UUID (PK)
email TEXT UNIQUE
public_key TEXT
created_at TIMESTAMP

chatrooms

sql id UUID (PK)
name TEXT
owner_id UUID (FK → users.id)
created_at TIMESTAMP

chatroom_members

sql id UUID (PK)
chatroom_id UUID (FK → chatrooms.id)
user_id UUID (FK → users.id)
role TEXT (admin/member)
joined_at TIMESTAMP

messages

sql id UUID (PK)
chatroom_id UUID (FK → chatrooms.id)
sender_id UUID (FK → users.id)
cipher_text TEXT
created_at TIMESTAMP

files

sql id UUID (PK)
uploader_id UUID (FK → users.id)
chatroom_id UUID (FK → chatrooms.id)
file_name TEXT
storage_path TEXT
encrypted_key TEXT
expiry TIMESTAMP
created_at TIMESTAMP

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4. API Endpoints (REST/GraphQL)

Auth

• POST /auth/register → create user
• POST /auth/login → login & JWT
• GET /auth/me → current user

Chatrooms

• POST /chatrooms → create chatroom
• POST /chatrooms/:id/join → join room
• GET /chatrooms/:id/messages → fetch messages
• POST /chatrooms/:id/message → send encrypted message

Files

• POST /files/upload → request pre-signed URL + upload metadata
• GET /files/:id → fetch file metadata + pre-signed download link
• DELETE /files/:id → delete file

Streaming (WebRTC)

• POST /webrtc/offer → exchange SDP offer
• POST /webrtc/answer → exchange SDP answer
• POST /webrtc/candidate → ICE candidate exchange

Admin / Reports

• POST /report → report file/message
• GET /admin/reports → view reports
• DELETE /admin/files/:id → force delete file

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5. Encryption Details

• Chat: Signal Protocol (X3DH + Double Ratchet).
• Files: AES-256-GCM (per-file Data Encryption Key).
• Key Wrapping: DEK encrypted with recipient’s X25519 public key.
• Transport: HTTPS + TLS 1.3.
• Streaming (WebRTC): DTLS-SRTP + Insertable Streams for E2E.

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6. Tech Stack

• Frontend: React Native (Expo) / Flutter
• Backend: Node.js (NestJS/Express) OR Supabase (faster MVP)
• DB: Postgres (Supabase)
• Storage: S3-compatible (Wasabi/Backblaze)
• Streaming: WebRTC (mediasoup / LiveKit for SFU)
• TURN/STUN: Coturn server
• Encryption: libsignal, WebCrypto, Insertable Streams

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7. DevOps & Deployment

• Infra: Docker + Kubernetes (later scale)
• CI/CD: GitHub Actions → Deploy to AWS / GCP / Fly.io
• Monitoring: Prometheus + Grafana
• Error tracking: Sentry
• Logs: Encrypted logs for debugging without exposing user data

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8. Developer To-Do (MVP)

• Auth system (JWT + refresh)
• Signal protocol integration for chat
• File encryption + pre-signed upload/download
• Progressive decryption + playback
• WebRTC P2P chatroom sync (small groups)
• Basic admin dashboard

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