Paper 2026/879

Accountable Fair Exchange Protocols on Blockchain

Ryosuke Abe, Keio University

Shigeya Suzuki, Keio University

Abstract

Fair exchange protocols on blockchain enable atomic exchange of digital goods and cryptocurrency between untrusted parties. Two prominent protocols, Zero-Knowledge Contingent Payment (ZKCP) and FairSwap, guarantee fairness: either both parties receive the expected items (digital goods and cryptocurrency) or neither does. However, both protocols lack accountability: when an exchange terminates abnormally, the protocol cannot identify which party caused the failure. This lack of accountability undermines the applicability of fair exchange protocols in decentralized settings. To fill this gap, we identify the common accountability issue in both protocols and revise them by adding signature exchange and chaining. We formally define the revised protocols in the universal composability framework and provide a proof sketch, and measure the computational overhead of the revised protocols in our implementation and experiment.

@misc{cryptoeprint:2026/879,
      author = {Riku Mochizuki and Ryosuke Abe and Shigeya Suzuki},
      title = {Accountable Fair Exchange Protocols on Blockchain},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/879},
      year = {2026},
      url = {https://eprint.iacr.org/2026/879}
}