Recipient-Agnostic Hash Locks for Cross-Chain Auctions with Bitcoin Bidding

Cross-chain auction is a promising paradigm for enabling on-chain price discovery across blockchain systems. While existing bridge solutions can functionally support cross-chain auctions, their deployment across heterogeneous blockchains remains challenging. In addition, conventional hash time-lock contracts (HTLCs) inherently predetermine the receiver, making them incompatible with the flexible recipient assignment required in auction settings. In this paper, we propose recipient-agnostic hash locks (RAHL), a new primitive that extends the HTLC framework to support dynamic recipient selection. By leveraging adaptor signatures and the UTXO model, we construct mutually exclusive pre-payments that allow multiple potential receivers to participate, while ensuring that only one is ultimately finalized. Furthermore, we embed adaptor signatures with polynomial commitments such that the secret revealed during settlement encodes recipient information, thereby enabling recipient-agnostic functionality. We formally model RAHL under a rational cross-chain communication (CCC) setting, and prove that it satisfies effectiveness and atomicity, while providing resilience against front-running and temporary censorship attacks. Experimental results demonstrate that RAHL primitive supports auctions between Bitcoin and Ethereum, reducing cross-chain settlement overhead by 33% compared to representative light-client approaches.