Show and tell: LX-D02 thermal printer on Windows (React Native/Hermes app reversed)

[bigtower17]

Sharing a small success story that started with REA.

Goal: use an LX-D02 Bluetooth thermal mini printer, which normally only works with the Funny Print Android and iOS app, as a regular Windows printer.

Result: bigtower17/picoprint. The printer now shows up in Windows and prints from any app with Ctrl+P.

How it went:

• REA (rea-agents 6.5.0) was the entry point. My agent used doctor to check readiness and list-adb-devices, and REA framed the overall Android investigation.
• An existing open-source LX-D02 driver (rusq/thermoprint) connected fine, but nothing came out of the printer. The printer rejected a hard-coded 5a 0a/5a 0b handshake.
• The app turned out to be React Native with a Hermes bundle (bytecode v98). The logic was in JS, not in the dex or native code. I decompiled it with hermes-dec. Function names survived (sendHandshake, checkCrc, sendAPPCheck…), so the challenge–response came out quickly: crc16xmodem(random byte || printer MAC), with the low bytes going to the printer and the high bytes going back to the app. It matched the captured traffic byte for byte. Write-up: doc/lx-d02-app-check.md. Upstream PR: lx-d02: compute app check handshake; ippsrv: Windows-compatible printer attributes rusq/thermoprint#15.

Feedback / idea: the step REA didn't cover for this target was Hermes bytecode. REA has identify-flutter-build / inspect-dart-aot for Flutter, and something similar for React Native would have made this a fully in-REA workflow:

• detect a Hermes index.android.bundle and its bytecode version;
• list function names and strings;
• decompile one function.

Many cheap IoT and BLE companion apps are React Native.

Thanks for the tool!