Coldcard, a popular hardware device manufactured by Coinkite, recommend for advanced Casa multisig users, has the unique benefit of offering offline signing via PSBTs (Partially Signed Bitcoin Transactions).
NOTE: Due to device memory constraints in earlier Coldcard iterations, only Coldcard versions MK3 and higher are supported. If you have a "MK1" or "MK2" Coldcard, we strongly recommend upgrading!
Before Adding Coldcard to the Casa App...
Because our integration with Coldcard is done via airgapped transfers of PSBTs (Partially Signed Bitcoin Transactions), which involve use of file transfers between the Coldcard device and your computer via an SD card, we recommend use of Coldcard with the Casa App for advanced users only!
Coldcard may be less convenient to set up and use than Ledger or Trezor, but you gain some additional security benefits in exchange.
If you’re new to Bitcoin, or just starting out with the Casa App , we recommend using a Trezor or a Ledger device for the simplest possible Casa App experience.
Prior to adding the Coldcard to the Casa App , you will need:
- Coldcard device (only MK3 or higher is supported!)
- A laptop or desktop computer
- Micro SD card
- Micro SD card adapter (if your computer does not have a micro SD card slot)
- Micro USB to USB cable (the same used by Trezor and Ledger)
Optionally, you can use a battery pack for power (to avoid plugging into a computer).
Note that buying the right power bank can be tricky; the Coldcard uses so little power that many power banks won’t detect it and will automatically turn off.
- AUKEY PB-N54 (Press the power button twice to enable low current mode)
- Anker batteries with a trickle charging mode:
- PowerCore 15000 Redux
- PowerCore 10000 Redux
- PowerCore Essential 2000
Before adding your Coldcard to the Casa App it will be necessary to first initialize your Coldcard using the instructions found on Coldcard's website.
We recommend that the Coldcard hardware device you use with the Casa App should be used exclusively with the Casa App; there is no need to add funds to your hardware device directly. The hardware device is used just as one of the keys needed to sign and send transactions from the Casa App.
If you already have a Coldcard device that you've set up and added funds to, it will still work to sign transactions with the Casa App (despite this not being recommended).
During the initialization process, you must write down the seed phrase for your Coldcard device in order to verify it. As long as you're using your Coldcard exclusively with the Casa App, we recommend not keeping the seed phrase backup for your Coldcard device beyond the initial requirement for verification.
This means that once your Coldcard has been set up, you should destroy the seed phrase. That being said, it is vitally important that you do not forget the PIN code to your Coldcard device! For simplicity, we recommend using the same PIN as for your other hardware devices and storing it in a safe place, such as an encrypted password manager.
Once your Coldcard device has been initialized, make sure you're running the latest firmware version (3.1.x), available ONLY on the Mk2 or Mk3. You can find instructions for upgrading the firmware on Coldcard's website.
To find the latest version of your Coldcard, you would boot up your Coldcard, unlock it with your PIN code, and navigate to:
Advanced > Upgrade > Show Version
IMPORTANT: in order to "future-proof" your Coldcard you should navigate to Settings > Multisig Wallets > Trust PSBT? and select "Trust PSBT." Otherwise, your Coldcard may stop.
Adding Coldcard to the Casa App
Coldcard can be added to the Casa App like any other key.
If you have not yet set up the Casa App, you would tap your hardware device icon and follow the steps to add the key to your keyset.
If you have previously set up the Casa App, you will need to choose an existing key to rotate out of your keyset, so that it will be replaced with the Coldcard, and no longer used.
To replace a key that you’ve previously set up, you first need to mark it as compromised. After it’s marked compromised, you can choose to replace the key. Make sure you keep that key you've marked compromised unchanged until you've completed the transfer to the new keyset.
Once you’ve chosen to replace the key, we will send you an email with further instructions on how to complete the replacement process.
For Gold users who are using the standard setup of Mobile Key + Hardware key + Casa Recovery Key, you can start adding a Coldcard device as a second hardware key in their keyset, which replaces the mobile key, by tapping the Casa Icon and then tapping “Switch Key Setup.”
Whether you’re setting up the Casa App for the first time, or rotating out an existing key you’ve already added, you will get an email from Casa when setting up your Coldcard device.
When you get the email to connect your Coldcard device, click the link (from a laptop or desktop computer), then choose “Coldcard.”
You’ll see a prompt to add a file to verify your Coldcard. Before doing this, you’ll need to find the file on your Coldcard and download it to the SD card.
Go ahead and insert the SD card into the SD card slot on your Coldcard.
Next, plug the Coldcard into a power source (doesn’t have to be plugged into your computer).
Unlock your device using your PIN code, then navigate to:
Advanced > MicroSD > Dump Summary
Once in “Dump Summary,” click “Ok” to export the Dump Summary to the SD card.
Once that file has been exported onto the SD card, remove your SD card from the Coldcard and insert it into your computer.
Once you locate the public.txt file on your MicroSD, you can simply drag and drop into the window you’ve opened from our email.
We recommend deleting the public.txt file from the MicroSD once finished uploading.
If successful, the new added key should automatically reflect in the Casa App.
That’s it! You’ve successfully added a Coldcard device.
If you have funds on your previous keyset, you’ll need to send funds to the new keyset using the minimum required keys in your multisig setup.
For more instructions on replacing a hardware device in the Casa App, see our detailed tutorial.
Next, you'll want to complete a health check for your Coldcard. Click here for instructions on how to perform a Health Check and send funds from the Casa App using your Coldcard.