| Electronic contract signing is an essential cryptographic service in the digital office.Traditional electronic contract signing protocols introduce a centralized third party to ensure the fairness of the contract signing process.However,in practice,the security of data transmission with the third party cannot be guaranteed,which threatens the signatory’s privacy.If a dishonest third party conspires with a signatory,the contract signing process will become unfair,causing financial damages.Therefore,the third-party service provider must be trustworthy to ensure the fairness and privacy of the contract signing process.Blockchain technology,the foundation of digital currency,is deployed as a trusted third party decentralized to avoid the traditional centralized third-party problem.However,the publicly accessible character of blockchain introduces new challenges to the privacy of signatories.In addition,most of the existing blockchain-based fair contract signing protocols serve two parties,so it is imperative to design a multi-party privacy-protecting fair contract signing protocol on the blockchain.This paper investigates the multi-party privacy-preserving fair contract signing protocol based on the blockchain and verifiably encrypted signatures.The main contributions are as follows:(1)An efficient bilinear pair-based certificateless aggregate verifiably encrypted signature(CLA-VES)scheme is proposed based on Hashimoto and Ogata’s certificateless aggregated signatures scheme.First,we propose the definition and security model of the CLA-VES scheme.Besides,the correctness,confidentiality,unforgeability,non-repudiation,opaqueness,and verifiability of our scheme are reduced to solve the Computational Diffie-Hellman problem in the random oracles model.Finally,the feasibility and efficiency of the scheme are verified by comparison with the existing schemes and experiments.The CLA-VES scheme combines the benefits of certificateless signatures and aggregated signatures,aggregating the generated verifiable encrypted signatures into a fixed-size aggregated signature and solving key recovery and certificate management problems.Moreover,the scheme reduces signature communication and computation and improves verification efficiency compared with similar schemes.(2)A multi-party privacy-preserving fair contract signing protocol(MP~2FCSP)on the blockchain is designed based on the CLA-VES scheme.The protocol satisfies the requirements of fairness,privacy,immutability,timeliness and non-repudiation.Ethereum smart contracts are used to actualize as the offline trusted third party,which can achieve the goal of decentralization and fairness for the signature protocol.Moreover,the signatories jointly negotiate the blind factor to encrypt signature and contract content,which can solve the privacy-preserving problem in the contract signing process.Finally,we implement the protocol in Ethereum and verify the feasibility of the protocol by the Ethers cost of experiments. |
