Watch-only address

Watching is a read operation. You give a wallet an address, an extended public key, or a descriptor, and it scans the chain for matching outputs and adds them up. The keys stay wherever you left them, normally a hardware device in a drawer, and the machine doing the watching can be compromised without a single coin moving.

How it works

A watch-only wallet is an ordinary wallet with the secret half removed. Bitcoin Core builds one through importdescriptors, fed a descriptor containing an extended public key and no private material. Descriptor wallets, standardized in BIP-380, arrived in version 0.21.0 and became the default for newly created wallets in version 23.0. The older importaddress call did the same job one address at a time and demanded a rescan of the chain afterward.

Consumer software takes the same input in a friendlier form. Sparrow, Electrum and BlueWallet accept an extended public key pasted or scanned from a hardware wallet, derive addresses forward from it, and ask an indexing server about each one. That forward derivation is where the gap limit of 20 applies: a watch-only wallet that gives up after twenty consecutive empty addresses will under-report a balance sitting further along the branch.

Spending from a watch-only setup is possible in two moves. The watch-only side builds an unsigned transaction as a partially signed bitcoin transaction under BIP-174, the offline signer adds the signatures, and the watch-only side broadcasts the finished result. Nothing secret ever touches the connected machine, which is the whole point of the arrangement.

Where you see it

Watch-only wallets are how people check on cold coins without waking anything up.

The standard setup is a hardware wallet initialized and then stored, with its extended public key loaded into a phone or laptop app. You can confirm that an exchange withdrawal landed, watch confirmations accumulate, and prepare a transaction, all without connecting the device. Accountants and tax software take the same input, since a complete dated history is exactly what a capital gains calculation needs.

Businesses use the pattern at scale. A self-hosted payment processor watches a merchant's extended public key, derives a new invoice address for every order, and has no ability to spend, which is the difference between a processor and a custodian.

The risk here is disclosure rather than theft. Whoever holds your extended public key can enumerate every address on that branch and every satoshi that has ever passed through it, including balances you would never volunteer, and they can keep doing so indefinitely. Cloud-synced apps and browser extensions that request an xpub are requesting your complete financial history, and there is no way to revoke it afterward short of moving everything to a new seed.

Watch-only address vs cold storage

A watch-only address holds no keys; cold storage holds keys that are kept offline. The two get confused because they normally appear together, but a watch-only wallet running on an internet-connected phone is not cold and does not make the arrangement any colder. It is the online window onto a cold wallet. Security comes entirely from where the private keys live and how the device holding them was obtained and verified, not from the watching software, which an attacker could replace tomorrow without touching your bitcoin.