Dust Expiry: Clean the UTXO set from spam

In addressing the challenges associated with maintaining a commitment over the Unspent Transaction Output (UTXO) set, which can significantly undermine the efficiency gains by escalating the costs involved, an alternative approach is proposed that sidesteps the need for a UTXO-set commitment. This solution hinges on leveraging the existing blockchain infrastructure to serve as a de facto Transaction Output (TXO) commitment framework. By doing so, it obviates the necessity for elaborate and resource-intensive commitments to the UTXO set, which have been identified as a substantial bottleneck in optimizing blockchain operations.

The crux of the proposed method involves the spender furnishing a Simplified Payment Verification (SPV) proof, which is essentially a Merkle inclusion proof demonstrating that a particular transaction is included within its corresponding block. This proof is then verified against the blockchain's header chain, thereby utilizing the blockchain itself as an implicit commitment mechanism to the state of transaction outputs. This leverages the inherent properties of the blockchain, with its sequential and immutable record of transactions, to efficiently ascertain the validity of transactions without necessitating a separate, extensive commitment structure for the UTXO set.

To address the issue of unspent but expired UTXOs—a scenario that typically requires tracking to prevent fraudulent re-spending or to ensure the integrity of the ledger—a novel yet lightweight data structure is introduced. This structure comprises a list of 8-byte "locations," uniquely identifying the positions of these unspent, expired UTXOs within the blockchain. Each "location" entry is delineated by a combination of the block height, the transaction index within the block, and the output index within the transaction. This compact representation provides a streamlined approach to tracking and managing expired UTXOs without imposing significant storage or processing burdens, thus enhancing the overall efficiency and scalability of the system.