GitHub - unprovable/ShadowCat: Single file optical file transfer using a browser

2 min read Original article ↗

A fully offline, single-file HTML page for moving data between two devices via QR codes — intended for old phones whose radios (BLE, NFC, etc.) arevdead but whose cameras and browsers still work.

Tabs

  • Generate — encode text into a single QR code.
  • Scan — decode a single QR via the camera.
  • Send file — pick a file, choose chunk size / FPS / ECC, hit Start. Cycles through [header, chunk1…chunkN] forever at the chosen FPS. Pause / Resume / Stop.
  • Start from — begin the loop at a chosen frame index; it then continues forward and wraps back to the header normally.
  • Show frame + Show / − / + — display exactly one frame static, for resending a specific missing chunk. The number matches the chunk index shown in the receiver's missing-chunks grid (0 = header).
  • Receive file — start the camera and point at the sender. Header autodetects, progress bar fills in, missing-chunks grid shows which ones haven't arrived yet. When complete, the file's CRC is verified and a Download button appears.

Protocol

  • Header: QRX1|H|<total>|<filename>|<sizeBytes>|<crc32hex>
  • Data: QRX1|D|<idx>|<base64chunk> (1-indexed)
  • Base64 alphabet has no |, so parsing is just split('|').
  • Receiver tracks chunks by index, ignores duplicates, dedupes header by CRC.

Practical notes for old phones

  • Camera needs HTTPS or localhost — file:// won't grant getUserMedia permission. Serve with python3 -m http.server 8000 and visit http://<your-laptop-ip>:8000/qrcode.html over the local network. iOS Safari additionally requires HTTPS for cross-device access — for a LAN setup, caddy or a self-signed cert helps.
  • If render fails on a frame ("code length overflow"), drop chunk size or drop ECC level.
  • 500 chars × 3 fps ≈ 1.1 KB/s base64 ≈ 0.83 KB/s raw. A 100 KB file is roughly 2 minutes per loop; receiver typically needs 1-2 loops.
  • If old devices struggle to decode: lower FPS, raise ECC to Q, shrink chunk to ~300 chars — produces smaller, less dense QRs.