Signing a transaction with Coldcard

After adding your Coldcard to your Casa vault, you can use your Coldcard to sign a transaction from your BTC vault in Casa.

If you haven’t added your Coldcard to Casa yet, please see our article Adding Coldcard to your Casa vault.

If you want to learn how to perform a Health Check, see Performing a Health Check with Coldcard.

When signing a transaction, you will need:

Coldcard device
A laptop or desktop computer
Micro SD card
Micro SD card adapter (if your computer does not have a micro SD card slot)

First, initiate a transaction from your vault within your Casa app.

Note: Coldcard has a default setting where it will not sign transactions if the fee is set as more than 10% of the transaction. This is to prevent accidentally sending a transaction with a larger fee than intended, but can be an issue if you're attempting to sign smaller outgoing transactions. You can disable this default setting if needed by following Coldcard's instructions here (see section on "Max Network Fee").

Next, check your email inbox for an email from Casa with a link. Clicking that link will initiate the signing process.

Physically insert your micro SD card into your computer (you may need an adapter for this step).

Coldcard 1

When you click the link, you’ll be prompted to download a .psbt file onto your computer. Once the file is downloaded, drag and drop the file onto the Coldcard’s micro SD card.

Coldcard 2

Next, take the micro SD card out of your computer and insert it into your Coldcard.

Coldcard 3

When your Coldcard is powered up and unlocked with your PIN, click the checkmark button to select "Ready To Sign."

If there is only one .psbt file on the Coldcard, it will immediately begin the transaction verification and signing process. If there are multiple files then you will need to select which one to sign.

Once you finish verifying the transaction details, the Coldcard will save the signed psbt to the SD card.

Now that the transaction’s signature is saved to the SD card, you’re ready to transfer the signature back to your computer to complete the signing process.

Remove the SD card from the Coldcard and insert it into your computer.

Coldcard 4

Back in your web browser, click "Next," then drag and drop the file ending in "-part.psbt" into the window when prompted.

Coldcard 5

We recommend deleting the -part.psbt file from the micro SD card once finished uploading.

That’s it!

If you’re signing a transaction, you’ll need to apply as many other signatures as are required to send the transaction, if necessary, and complete the transaction within your Casa app.