LittleCPM3

Little Code on Proxmark3

LittleCPM3 is an Electron desktop application focused on storing and retrieving text/code payloads inside MIFARE Classic 1K NFC cards using a Proxmark3 device.

Project Intent

This project is designed to:

• Store source code snippets, notes, or plain text on NFC cards.
• Read that content back in a structured and reliable way.
• Support content larger than one card by splitting data across multiple cards.
• Provide a practical GUI on top of Proxmark3 operations.

This is useful for offline demos, physical payload transfer, lab experiments, and NFC workflow prototyping.

How the App Works (High Level)

The app has three layers:

• Renderer (UI): editor, hex preview, card info, chunk controls, logs.
• Electron main/preload bridge: safe IPC channel between UI and backend.
• PM3 backend utility: runs Proxmark3 CLI commands, parses results, reads/writes blocks.

Operational flow:

1. User triggers Read/Write/Wipe from UI.
2. Renderer calls window.pm3.* APIs from preload.
3. Main process invokes backend PM3 operations.
4. Backend executes Proxmark3 commands (hf mf autopwn, dump/read/write flows).
5. Parsed results are returned to the UI.
6. UI updates editor, metadata, storage bar, and logs.

Data Format on Card

LittleCPM3 writes a custom metadata header and payload blocks.

Block 0   : Manufacturer block (read-only, ignored)
Block 1   : PM3C metadata (16 bytes)
                            [0-3]   magic "PM3C"
                            [4-7]   payload size (uint32, big-endian)
                            [8]     chunk index
                            [9]     total chunks
                            [10-11] CRC-16
                            [12-15] reserved
Blocks 2+ : payload bytes (sector trailers are skipped)

For MIFARE Classic 1K, usable payload is:

• 45 data blocks × 16 bytes = 720 bytes per card

Prerequisites

Assuming you already have a functional Proxmark3 setup, you still need:

• Node.js 21 or newer
• npm (bundled with Node.js)
• Proxmark3 CLI available in your PATH (for example proxmark3 in /usr/local/bin)
• A connected Proxmark3 device with card read/write access

Setup and Run

cd LittleCPM3
npm install
npm start

Optional development command:

npm run dev

First Use Checklist (with Proxmark3 Base Already Ready)

1. Connect Proxmark3 and place a single MIFARE Classic card on the antenna.
2. Start the app with npm start.
3. Click Read Card to load existing payload (or detect blank state).
4. Edit content in the main editor.
5. Click Write Card to persist the payload and metadata.
6. Read again to verify round-trip consistency.

Main Features

• Read Card

  • Reads PM3C metadata and payload.
  • Detects blank/non-initialized PM3C cards.
  • Updates UID, port, chunk info, storage usage.

• Write Card

  • Writes current editor content to the card.
  • Validates payload size against 720-byte single-card limit.
  • Stores metadata for chunked reconstruction.

• Wipe Card

  • Removes PM3C metadata markers used by this app.
  • Resets app-side state and editor for a clean workflow.

• Split Editor Content

  • Splits payloads larger than 720 bytes into card-sized chunks.
  • Loads first chunk automatically.
  • Uses chunk index/total metadata to coordinate multi-card storage.

• Hex View

  • Displays current editor content as hex + ASCII representation.

• Line Counter in Editor

  • Shows dynamic line numbers similar to code editors.
  • Stays synchronized while typing and scrolling.

• Runtime/Log Feedback

  • Real-time operation logs.
  • Toast notifications for success/errors.
  • Overlay and button lock while operations are running.

Authentication and Keys

The app uses an autopwn-first strategy:

• Runs hf mf autopwn to discover usable keys.
• Reuses discovered keys for read/write operations when possible.
• Cleans generated artifacts (.json, .bin) after operations.

This improves compatibility with cards that do not use default keys.

Keyboard Shortcuts

Shortcut	Action
Ctrl+D	Read Card
Ctrl+R	Read Card
Ctrl+S	Write Card

Typical Multi-Card Workflow

If your content is larger than 720 bytes:

1. Paste full text/code into the editor.
2. Click Split editor content.
3. Write chunk 1 to card 1.
4. Set next chunk index and write card 2.
5. Repeat until all chunks are written.

When reading, metadata indicates chunk order and total chunk count.

Troubleshooting Notes

• Keep only one card in the RF field to avoid tag collision.
• If authentication fails, retry with the card stable on antenna so autopwn can finish.
• Ensure your OS user has serial permissions for Proxmark3 (/dev/ttyACM* on Linux).
• If no payload is found, card may be blank for PM3C format.

Scope and Limitations

• Focused on MIFARE Classic 1K workflows.
• Designed for text/code payload storage, not for general card cloning use cases.
• Very large payloads require manual multi-card handling.

License

© 2026 Lucas Gabriel (lucmsilva). Licensed under BSD-3-Clause.