An air-gapped wallet is a signing device that has never connected and will never connect to the internet or any networked device via cable. It communicates transaction data only through physically isolated channels, typically QR codes displayed on screen or microSD cards, eliminating any digital attack surface between the wallet and the outside world.
What Is an Air-Gapped Wallet?
3 min read
The short version
An air-gapped wallet takes the idea of cold storage to its logical extreme. Imagine signing a document in a sealed room with no phone lines, no wifi, and no windows, you pass the document under the door on paper, get it stamped inside, and the stamped copy comes back under the door. No electronic connection ever exists between the signing keys and the internet.
How It Works
Air-gapped wallets (Coldcard with microSD, Keystone with QR codes, Passport by Foundation) generate and store keys on a device that has no USB data connection, no WiFi, no Bluetooth, and no cellular radio. Transaction signing workflow: (1) You create an unsigned transaction (PSBT for Bitcoin) on your internet-connected computer. (2) Transfer it to the air-gapped device via QR code (scan with device camera) or microSD card. (3) The device displays transaction details on its own trusted screen. (4) You approve and sign on the device. (5) The signed transaction is transferred back via QR code or SD card. (6) Your computer broadcasts the signed transaction to the network. At no point does the signing device have a data connection to any other electronic device.
Signing a Bitcoin transaction with Coldcard via microSD
You want to send 0.5 BTC from cold storage. In Sparrow Wallet (on your PC), you create the transaction and export an unsigned PSBT file to a microSD card. You physically carry the SD card to your Coldcard (which is battery-powered, never plugged into USB for data). The Coldcard reads the PSBT, shows: "Send 0.5 BTC to bc1q... Fee: 3,200 sats." You press the checkmark. The Coldcard writes the signed PSBT back to the SD card. You carry the SD card back to your PC, import the signed transaction into Sparrow, and broadcast it. Your private key existed only inside the Coldcard, the SD card carried only transaction data, never key material.
What People Get Wrong
All hardware wallets are air-gapped
Most hardware wallets (Ledger, Trezor) connect via USB or Bluetooth for signing, they are not air-gapped. Their secure elements still protect keys well, but the device has a direct electronic connection to your computer during use.
Air-gapping is overkill for normal users
For most users, a standard hardware wallet provides excellent security. Air-gapping adds protection against: sophisticated USB-based attacks, evil-maid scenarios, and supply-chain firmware compromises. It is most relevant for very large holdings or high-threat environments.
QR codes can contain malware
QR codes are just data encoding. The air-gapped device firmware only accepts properly formatted transaction data, it cannot execute arbitrary code received via QR. The QR is a transport mechanism, not an attack vector.
Keep Reading
Sources & Further Reading
- Coldcard Documentation
Official docs for the Coldcard air-gapped Bitcoin signing device
- SeedSigner Project
Open-source DIY air-gapped signing device built on Raspberry Pi Zero
Questions People Also Ask
- Which wallets are truly air-gapped?
- Coldcard (microSD-only mode), Keystone (QR-only), Foundation Passport (QR + microSD), SeedSigner (DIY, QR-only), and Jade (in QR-only mode). Each avoids USB data connections during normal signing operations.
- Can air-gapped wallets work with Ethereum?
- Yes. Keystone supports Ethereum, EVM chains, and many other networks. The QR-code workflow works the same, unsigned transactions are shown as QR codes by MetaMask (via the Keystone integration) and scanned by the device.
- How do air-gapped wallets get firmware updates?
- Via microSD card (Coldcard, Passport) or by temporarily connecting via USB for the update only (verifying the update signature first). The device verifies update authenticity cryptographically before applying.