How to perform CoinJoin transactions with Coldcard Mk4?

Coldcard Mk4 supports CoinJoin for enhanced Bitcoin privacy; use Sparrow Wallet to set up, initiate, sign, and broadcast transactions securely.

Apr 03, 2025 at 06:15 pm

Introduction to CoinJoin and Coldcard Mk4

CoinJoin is a privacy-enhancing technique used in the cryptocurrency world, particularly for Bitcoin, to obscure the trail of transactions and enhance user anonymity. By mixing multiple transactions from different users into a single transaction, it becomes difficult to trace the flow of funds. Coldcard Mk4, a highly secure hardware wallet, supports this technique, allowing users to maintain privacy without compromising the security of their funds. This article will guide you through the process of performing CoinJoin transactions using the Coldcard Mk4.

Preparing for a CoinJoin Transaction

Before you can perform a CoinJoin transaction with your Coldcard Mk4, you need to ensure that your device is set up correctly and that you have the necessary software and funds.

• Ensure your Coldcard Mk4 is updated to the latest firmware. You can check and update the firmware using the Coldcard's built-in update tool.
• Install and set up a compatible wallet software that supports CoinJoin transactions. Popular options include Sparrow Wallet and Wasabi Wallet. For this guide, we will use Sparrow Wallet.
• Transfer the amount you wish to mix into your Coldcard Mk4. Make sure you have enough Bitcoin to cover the transaction fees as well.

Setting Up Sparrow Wallet with Coldcard Mk4

To perform a CoinJoin transaction, you need to connect your Coldcard Mk4 to a compatible wallet software. Here’s how to set up Sparrow Wallet with your Coldcard Mk4:

• Download and install Sparrow Wallet from the official website.
• Launch Sparrow Wallet and click on "Add Wallet".
• Select "Hardware Wallet" and then choose "Coldcard".
• Connect your Coldcard Mk4 to your computer using a USB cable.
• Enter your PIN on the Coldcard Mk4 to unlock it.
• Select the derivation path you want to use. The default is usually fine, but you can choose a custom path if needed.
• Click "Next" and then "Finish" to complete the setup.

Initiating a CoinJoin Transaction

Once your Coldcard Mk4 is connected to Sparrow Wallet, you can initiate a CoinJoin transaction. Follow these steps:

• Navigate to the "Send" tab in Sparrow Wallet.
• Click on "CoinJoin" to start the process.
• Select the amount you want to mix. Ensure you have enough funds to cover the transaction fees.
• Choose the number of rounds for the CoinJoin. More rounds increase privacy but also increase fees.
• Review the transaction details and confirm the amount and fees.
• Click "Create Transaction" to generate the CoinJoin transaction.

Signing the CoinJoin Transaction with Coldcard Mk4

After creating the CoinJoin transaction in Sparrow Wallet, you need to sign it using your Coldcard Mk4. Here’s how to do it:

• The transaction details will be displayed on your Coldcard Mk4. Review them carefully.
• Enter your PIN to unlock the Coldcard Mk4 if prompted.
• Use the navigation buttons to scroll through the transaction details.
• Press the "OK" button to sign the transaction.
• The signed transaction will be sent back to Sparrow Wallet.

Broadcasting the CoinJoin Transaction

Once the transaction is signed, you need to broadcast it to the Bitcoin network. Here’s how to do it in Sparrow Wallet:

• Return to Sparrow Wallet and you will see the signed transaction ready to be broadcast.
• Click "Broadcast" to send the transaction to the Bitcoin network.
• Wait for the transaction to be confirmed. This may take some time depending on the network congestion and the number of rounds you chose for the CoinJoin.

Verifying the CoinJoin Transaction

After the transaction is confirmed, you should verify that the CoinJoin was successful. Here’s how to do it:

• Check the transaction on a blockchain explorer like Blockchain.com or Mempool.space.
• Look for the transaction ID provided by Sparrow Wallet after broadcasting.
• Verify that the transaction has multiple inputs and outputs, which is a sign of a successful CoinJoin.

Frequently Asked Questions

Q: Can I use other hardware wallets for CoinJoin transactions?

A: Yes, other hardware wallets like Trezor and Ledger also support CoinJoin transactions, but the process may differ slightly depending on the wallet software you use.

Q: How much does a CoinJoin transaction cost?

A: The cost of a CoinJoin transaction depends on the number of rounds you choose and the current Bitcoin network fees. Generally, more rounds increase the privacy but also increase the fees.

Q: Is it safe to perform CoinJoin transactions with Coldcard Mk4?

A: Yes, Coldcard Mk4 is designed with high security in mind, and performing CoinJoin transactions through a reputable wallet like Sparrow Wallet is considered safe. However, always ensure you are using the latest firmware and software versions.

Q: How long does a CoinJoin transaction take to confirm?

A: The confirmation time for a CoinJoin transaction can vary based on the number of rounds and the current state of the Bitcoin network. Typically, it can take anywhere from a few minutes to several hours.