UTXO

A UTXO (unspent transaction output) is a discrete amount of coin locked to a spending condition, created by one transaction and consumed by a later one. In the UTXO model, introduced by the Bitcoin whitepaper, the ledger state is the set of all unspent outputs; a wallet balance is simply the sum of the UTXOs its keys can spend.

#UTXO model versus account model

Account-based ledgers such as Ethereum keep balances in accounts that transactions mutate in place. The UTXO model instead treats every coin as a one-shot object: transactions fully consume their inputs and create new outputs. This favors parallel validation and simple auditing, at the cost of more complex wallet logic. Bitcoin-style chains, including BTX, use the UTXO model.

#Why per-output key exposure matters for quantum risk

Most UTXOs lock funds to the hash of a public key, so the key stays hidden until the output is spent and a quantum attacker running Shor's algorithm has nothing to attack beforehand (Aggarwal et al.). Spending reveals the key; reused addresses and pay-to-public-key outputs therefore sit permanently exposed (quantum threat to ECDSA), and taproot outputs expose keys by design (taproot and quantum key exposure). Because exposure happens per output, single-use addresses shrink the attack surface, a central point in is Bitcoin quantum safe?.