In this guide, we'll explore how to pair your Coldcard wallet with a mobile device using virtual disk mode and integrate it with Nunchuck Wallet. This setup allows you to manage your Bitcoin with added security and mobility.

Prerequisites

Before beginning, ensure you have:

• A Coldcard wallet set up with a seed phrase and wallet allocated.
• Familiarity with basic Bitcoin transactions and wallet use.
• A mobile device (Android or iPhone).
• A compatible cable to connect your Coldcard to your mobile device (e.g., USB-C to USB-C for newer devices, USB-C to Lightning for older iPhones).

Section 1: Exporting Wallet from Coldcard

1. Power On and Log In: Connect your Coldcard to your mobile device using the appropriate cable. The Coldcard will draw power from your phone. Enter your pin to access the main menu.
2. Enable Virtual Disk Mode: Navigate to Settings > Hardware on/off. Ensure that both the USB Port and Virtual Disk Mode are turned on.
3. Export Wallet File: Go to Advanced Tools > Export Wallet > Generic JSON and choose to save the file to the virtual disk. This file will include your wallet's xPub (extended public key).

Section 2: Setting Up Nunchuck Wallet on Mobile

1. Download and Open Nunchuck: Install the Nunchuck Wallet app on your mobile device and open it.
2. Importing Wallet File:
   • Tap the + icon to add an air-gapped key.
   • Opt to import via file and navigate to the Coldcard drive in your file explorer.
   • Select the recently exported JSON file and import it into Nunchuck.
3. Create a Wallet:
   • On the home screen, tap the + beside Wallet.
   • Choose Create Wallet, and name your wallet (e.g., CC Single for Coldcard Single Sig).
   • Confirm the key associated with the wallet and the number of signatures required (for single sig, this will be one).
4. Finalize Setup: Review the summary of the wallet setup and confirm. The wallet now appears on the main screen.

Section 3: Receiving Funds

1. Receive Address: In Nunchuck Wallet, tap on your wallet and select Receive to display a QR code and address. Share this with the sender or use another wallet to send funds to this address.
2. Confirm Transaction: Once the transaction is sent, it will appear as pending in Nunchuck Wallet. After confirmation on the Bitcoin blockchain, the funds will be available in your wallet.

Section 4: Sending Funds with Coldcard

1. Construct Transaction: In Nunchuck Wallet, tap Send. Enter the amount and recipient's address (either by pasting or scanning a QR code).
2. Customize Transaction: Opt to customize the transaction to adjust the fee or select a manual fee rate.
3. Export Unsigned Transaction: Confirm and create the transaction, then export the unsigned transaction file to the virtual disk on the Coldcard.
4. Sign Transaction on Coldcard: With the Coldcard still plugged into your phone, navigate to Ready to Sign on the Coldcard to sign the transaction. Confirm the details and approve the transaction.
5. Import Signature: Back in Nunchuck Wallet, import the signed transaction file from the virtual disk. Confirm the signature and broadcast the transaction.
6. Completion: The transaction is now sent, and your balance in Nunchuck Wallet will update accordingly.

Final Thoughts

Using Coldcard with virtual disk mode and Nunchuck Wallet on your mobile device provides a secure and convenient way to manage Bitcoin transactions. This method is efficient, user-friendly, and preserves the security benefits of a hardware wallet.