Design camera-footage upload with custody chain

## System design prompt You’re building a backend service for uploading **recorded body-camera / dashcam footage** from client devices (often on unreliable networks). Uploaded videos may later be used as **legal evidence**, so the system must support large files, security scanning, and a **tamper-evident chain of custody**. ### Core requirements 1. **Large blob uploads** - Video files can be multiple GBs. - Upload must be **resumable** and tolerate flaky connectivity. - Clients may upload in **chunks**. - Assume you **cannot fully trust the cloud provider’s built-in multipart “finalization”** (i.e., you must be able to prove server-side what bytes were received and assembled). 2. **Malicious content detection** - Newly uploaded blobs must be treated as untrusted. - Run security checks (e.g., malware scan, file-type validation) before making content available to users or downstream systems. 3. **Chain of custody** - You must produce an auditable, tamper-evident history of all actions on a piece of footage (upload, scan results, moves, access/download/export, retention/legal hold changes, deletion). - The system should help prove that footage was not altered. ### What to cover - APIs and data model (upload session, chunk tracking, metadata) - Storage layout (quarantine vs public/evidence) - Background processing pipeline (assembly, scanning, promotion) - Integrity verification approach (checksums/hashes) - Chain-of-custody design (immutable audit log) - Access control and security considerations - Key tradeoffs and failure/edge cases