Air Gap

An air gap is the deliberate physical separation of a device from all networks, so that the machine holding your private keys never connects to the internet, Bluetooth, or any other channel an attacker could reach through. In cryptocurrency, air-gapped setups are used to sign transactions on an offline device while a separate online device handles broadcasting them to the network.

A typical workflow: an unsigned transaction is created on an online computer and passed to the offline device as a QR code or on a microSD card; the offline device, which holds the keys, signs it and returns only the signed transaction the same way; the online computer then broadcasts it. Some hardware wallets are designed around this model, communicating exclusively through QR codes so no cable ever links the keys to a networked machine. For example, a long-term holder might keep keys on a permanently offline device in a safe and sign a withdrawal twice a year via QR codes.

The security logic is that malware cannot steal what it cannot reach: even a fully compromised online computer sees only unsigned and signed transactions, never the keys. The remaining risks are physical theft, compromised devices before setup, and user error, and the data crossing the gap must still be verified, since a malicious online computer could present a doctored transaction to sign. A common misconception is that any hardware wallet is air-gapped; USB-connected devices are close in practice, but strict air-gapping means no direct connection at all.