Watch-Only and Hardware Signer: The On-Chain Transactions Setup That Makes Sense

A watch-only wallet imports an extended public key (xpub) from a hardware wallet without importing the private key — it can generate addresses, display balances, and track transactions, but it cannot sign. The hardware signer holds the private key offline and signs only when physically connected and explicitly confirmed by the user. For daily on-chain Bitcoin competition, this means monitoring the leaderboard from the watch-only app on a phone or desktop, then signing each entry transaction on the hardware device when ready to broadcast. The private key never touches the monitoring device. Bitok Arena Research analyzed this setup because separating the monitoring function from the signing function eliminates the attack surface where most hardware wallet users carry unnecessary risk.

A PSBT — Partially Signed Bitcoin Transaction — is how Sparrow Wallet on desktop communicates with a hardware signer. Sparrow constructs the unsigned transaction (the PSBT), sends it to the hardware wallet, the hardware wallet signs it on its own screen, and Sparrow broadcasts the signed result to the Bitcoin network. From the hardware wallet (Trezor, Ledger, or Coldcard), navigate to account settings and locate the xpub or zpub for the BTC account. Import this string into Sparrow as a new watch-only wallet. The software generates the same addresses the hardware wallet controls — without ever holding or displaying the private key on the computer.

The Monitoring and Signing Split

Sparrow Wallet provides the most complete integration between watch-only monitoring, PSBT construction, and hardware wallet signing for on-chain Bitcoin competition. Sparrow connects to Trezor, Ledger, Coldcard, and other major hardware wallets via USB. It displays the xpub-generated addresses and balances, constructs unsigned transactions, sends them to the hardware wallet for signing, and broadcasts the signed transaction to the Bitcoin network. The entire on-chain competition entry workflow runs through Sparrow: check the current position on the leaderboard, determine the entry amount, construct the transaction in Sparrow, sign on the hardware device, broadcast from Sparrow.

The ColdCard Mk4 supports PSBT via microSD card, enabling air-gap signing with no USB connection at all. Sparrow exports the unsigned PSBT to a microSD card, the ColdCard signs it without connecting to any computer, and the signed PSBT returns to Sparrow via the same card for broadcast. For any holder accumulating BTC across many on-chain competition rounds, the air-gap eliminates the USB attack surface entirely — the private key never connects to a computer even during the signing step. The ColdCard Q model adds a larger screen and QWERTY keyboard, making seed phrase entry and transaction review faster, but both models support the PSBT-via-microSD workflow.

UTXO Management and Privacy

UTXO consolidation becomes relevant after many competition rounds. Each on-chain competition entry creates one outbound UTXO and each prize payment creates one inbound UTXO. A wallet active across many rounds accumulates many UTXOs. Spending multiple UTXOs in a single entry transaction is valid but increases the transaction size in bytes — and therefore the fee. Running a UTXO consolidation during a period of low mempool fees (merging small UTXOs into one larger UTXO in a single transaction) reduces future per-entry transaction sizes and fee costs. Sparrow displays the UTXO list and supports coin control for manual UTXO selection, making consolidation straightforward for users familiar with the interface.

The one-time setup of the Sparrow plus hardware wallet integration — generating the xpub, importing into Sparrow, and testing a small transaction before the first competition entry — takes 30 to 60 minutes and runs for every subsequent round without modification. The security model once established is: private key on hardware device, monitoring on watch-only app, signing via USB or air-gap, broadcasting via Sparrow. No BTC leaves the security model without the hardware device being physically present and PIN-authenticated. A compromised monitoring device cannot spend the BTC, cannot extract the private key, and cannot approve a signing without the physical hardware device.

The Full On-Chain Transaction Stack

A cold storage address that has accumulated prizes across multiple competition rounds holds meaningful BTC that justifies the hardware signing overhead. The security benefit is not abstract — it is the difference between a compromised phone resulting in zero BTC loss (watch-only setup) versus a compromised phone resulting in total wallet loss (full software wallet). The monitoring function runs on the more vulnerable device precisely because monitoring requires no key access. The signing function runs on the less vulnerable device — the hardware signer — precisely because signing requires key access.

Taproot P2TR addresses are accepted on the Bitcoin mainnet for competitors who want to use the newer address format, but Native SegWit bc1q remains the most widely supported across exchanges, wallets, and block explorers. The setup is complete once Sparrow has the xpub and the hardware wallet has the key. To execute a competition entry: open Sparrow, construct the transaction to the competition destination address, confirm the amount, connect the hardware device, verify the address and amount on the device screen, sign on the device, and broadcast from Sparrow. The position is live from the moment the transaction confirms on the Bitcoin network — monitored in the watch-only app without the key being anywhere near the monitoring device.

Bitok Arena's analysis of the watch-only and hardware signer setup finds it the optimal configuration for daily on-chain Bitcoin competition: monitoring runs without key exposure on any device, signing requires physical hardware presence and device-screen confirmation, and one-time setup of 30–60 minutes runs for every subsequent round without modification. The private key stays offline whether the monitoring device is compromised or not.