An Air-gapped Wallet signs payments on a device separated from ordinary network communication with the online computer or phone. Private keys are meant to stay in that separate environment; payment proposals and signatures travel by a controlled route, such as a QR code or memory card.
A wallet is an application or device for receiving and sending bitcoin. Anna browses the internet on her laptop. She does not want it to hold her private keys as well: the secret information used to create signatures authorizing bitcoin spending. She divides the work between devices. Software on the laptop follows payments and prepares new ones; this is the coordinator. A separate device checks the proposal and signs after her approval; this is the signer. A Hardware Wallet can work this way. Air gap mainly describes the separation and data transfer, rather than an automatic property of every hardware wallet. Temporarily switching off Wi-Fi on an ordinary laptop is not enough: its other connections, software and later reconnection with secrets still inside also matter. [3] [5]
Anna enters the recipient and amount in the coordinator. It prepares a PSBT: a package containing a proposed transaction and the information needed to sign it. This includes information about UTXOs, the unspent outputs of earlier payments. These are separate amounts referenced by the inputs of the new transaction. She transfers the package to the signer, perhaps by scanning a QR code. The device processes supported data and adds a signature after checking and approval. The result returns to the laptop. Only once the required signing conditions are met can the completed transaction be assembled and sent to the network. That does not yet confirm it in a block. A block groups transactions that are added together to Bitcoin’s public transaction history. The proposal and signature make this trip, not the secret key or wallet backup. [1] [6]
Imagine an input of 100000 sat. A sat is bitcoin's smallest unit. Anna wants to send 60000 sat, return 39000 sat to herself and leave 1000 sat as the fee. The returning amount has its own output, called a Change Output. The signer must verify it against the correct keys and wallet configuration; a “change” label supplied by the laptop is not enough. Anna also compares the entire recipient address, amount and fee on the signer's trusted display with her intended payment. An infected laptop can propose an attacker's address while getting the arithmetic exactly right. If Anna approves that proposal, keeping the devices separate will not protect her money. [1] [4]
A QR code represents data as an image; it does not certify that the data are safe. A larger package can travel as a sequence of images using BBQr or UR, and both devices must understand that format. Software that reads the image and PSBT can have bugs like any other program. The crucial distinction is between a transaction QR and SeedQR. SeedQR contains a backup of secret information, not a payment proposal. It must therefore stay away from an online laptop's or phone's camera. Whether a code may leave the isolated environment depends on its contents and purpose, not on its resemblance to other little squares. [2] [4]
Another method uses MicroSD: Anna saves the PSBT to a card, inserts it in the signer and brings the result back. No network cable is needed, but the device still reads an external file. Security depends on the software handling it and on the controller and filesystem, which handle reading and organizing data. A dedicated card helps keep files orderly; it does not guarantee harmless contents. Power deserves the same attention. USB can carry both electricity and data. If it is intended only for power, there must actually be no active data path. A “charging” label alone does not establish that property. [2] [3]
COLDCARD also lists NFC among its air-gap methods. NFC is short-range radio, whereas QR carries an image and MicroSD carries a file on physical media. Treating these channels as identical would be misleading. If your requirement is to exclude radio communication, enabled NFC does not meet it. With a particular device, finding out which channel is active, when it turns on and what can pass through it is more useful than relying on the word air-gapped. A persistent USB data connection has a different boundary again from manually carrying a file. [2]
The coordinator can be watch-only: it follows the wallet without being able to sign a payment itself. It can use an xpub, an extended public key for deriving further public keys, or a public Descriptor, which describes the keys and rules used to produce addresses. These data alone do not authorize spending, but can link addresses and payments to one wallet. A PSBT may also contain public keys, their derivation paths and UTXO information. With Multisig, where several keys authorize spending according to a specified rule, the correct configuration of all participants matters for recovery and checking change. Isolating the signer therefore does not make the laptop anonymous or its network queries private. [1] [4]
Anna still needs a safely stored Seed Phrase, a backup in words, and any other information her wallet requires. SeedSigner loads secrets temporarily; after power-off, its memory cannot be relied on as a backup. An air gap does not replace trustworthy firmware, the program inside the device, or careful payment approval. A Secure Element addresses a different part of protection by making certain attacks on the chip harder. Cold Storage describes keeping keys away from an online environment; air gap describes working with a separate signing device. Before relying on the arrangement for substantial savings, rehearse transfer, verification and recovery with separate test keys. The aim is to understand every step, not simply to put more devices on the table. [3] [4]
For the clearest picture, read this entry together with Hardware Wallet, PSBT, Cold Storage, Seed Phrase, Output Descriptor, Multisig. The reverse links also lead from Cold Storage, Secure Element, SeedQR, BBQr.