Hardware wallets already keep keys off your computer. Air-gapped wallets go a step further and avoid any direct connection. This advanced guide is part of our guide to hardware wallets explained. If you are new, start with do I need a hardware wallet?.
What air-gapped means
An air gap means that there is no electrical or wireless link. The device has no USB data connection, Bluetooth or Wi-Fi. Data crosses between your computer and the wallet by a visible, one-step method.
How signing works
- Prepare. Your computer's wallet app creates an unsigned transaction, often in a format called a PSBT (partially signed Bitcoin transaction).
- Transfer. It is shown as a QR code, or saved to a microSD card.
- Check. The device reads it and shows the amount and address on its screen.
- Sign. You approve it, and the device signs with its private key.
- Return. The signed transaction goes back by QR or card.
- Broadcast. Your computer sends it to the network.
Your keys never leave the device.
QR vs microSD
| Question | QR codes | microSD |
|---|---|---|
| Speed | Fast | Slower |
| Needs a camera | Yes | No |
| Reusable media risk | None | Card holds data |
| Large transactions | Can need several codes | Easy |
| Visible data transfer | Yes | No |
What it protects against
- Malware that attacks the USB connection.
- Faults in device drivers and connection software.
- Accidental exposure through a connected computer.
What it does not protect against
- Approving a bad transaction. Malware can change what your computer shows, so check details on the device.
- A pre-generated phrase.
- Weak backups, or exposed words.
- Physical threats.
- Flaws in the way that the device generates randomness. Advanced users may add dice rolls.
The trade-offs
Air-gapped setups are slower and easier to get wrong. For most people, the extra steps add little compared with checking the screen and protecting the backup. They are a good fit for large, long-term holdings, particularly for those using multisig.
Keeping an air-gapped wallet updated
Firmware usually arrives on a card, and it should be verified by the device. See firmware updates. Avoid reusing cards with other devices.