Coldcard, a popular hardware device manufactured by Coinkite, is recommend for advanced Casa users, and 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 Casa
Because our integration with Coldcard is done via airgapped transfers of PSBTs (Partially Signed Bitcoin Transactions), which involve the use of file transfers between the Coldcard device and your computer via a micro 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, Ledger, Keystone, or Passport device for the simplest possible Casa experience. You can check out our full list of supported devices here.
Prior to adding the Coldcard to your Casa app, you will need:
- Coldcard device (only MK3 or higher are 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 the Coldcard 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.
These devices have been tested and confirmed to work with Coldcard:
- 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 your Casa app, it will be necessary first to initialize your Coldcard using the instructions found on Coldcard's website.
We recommend that the Coldcard hardware device you use with Casa be used exclusively with Casa; there is no need to add funds to your hardware device's single key wallet directly. The hardware device is only used as one of the keys for signing and sending 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 Casa (despite this not being recommended).
During the initialization process, you will be prompted to write down the seed phrase for your Coldcard device in order to verify it. As long as you're using your Coldcard exclusively with Casa, it's not strictly necessary to write down your seed phrase. For more information about the advantages and disadvantages of retaining the seed phrase, see our article on Mostly Seedless multisig.
It is vitally important that you do not forget the PIN to your Coldcard device! For simplicity, we recommend using the same PIN as for your other hardware device(s) 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 (4.1.x), available ONLY on the Mk3 or higher. You can find instructions for upgrading the firmware on Coldcard's website.
To find the current 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 Casa like any other key.
Casa will send you an email with a link you can use to add the Coldcard device to your Casa vault. The email will be titled "Connect your device" and will come from no-reply@mail.keys.casa.
Click the link in the email, select "Coldcard," and follow the on-screen instructions to add the device to Casa.
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:
Mk3: Advanced > MicroSD Card > Dump Summary
Mk4 v.5.0.3 or less: Advanced > File Management > Dump Summary
Mk4 v.5.0.4 or greater: Advanced/Tools > Export Wallet > Dump Summary
Once in "Dump Summary," click "OK" to export the Dump Summary to the micro SD card.
Once that file has been exported onto the micro SD card, remove your micro SD card from the Coldcard and insert it into your computer.
Once you locate the public.txt file on your micro SD card, you can simply drag and drop into the window you’ve opened from our email.
We recommend deleting the public.txt file from the micro SD card once finished uploading.
If successful, the newly added key should reflect in your Casa app automatically.
That’s it! You’ve successfully added a Coldcard device.
Also check out our instructions on Performing a health check with Coldcard and Signing a transaction with Coldcard.