基于区块链的多方隐私保护公平合同签署协议

A Multi-Party Privacy Preserving Fair Contract Signing Protocol based on Blockchains

传统的公平合同签署协议通过引入中心化的可信第三方来保证协议的公平性。当第三方不诚实且和签署一方进行合谋,就会对另一方产生不公平。同时,第三方可能会泄露参与方的敏感信息,这将极大地威胁参与方的隐私。故合同签署的公平性和隐私性均依赖于第三方的可靠性。基于区块链的公平合同签署协议可去中心化从而避免依赖第三方来解决公平性,但区块链可被公开访问和验证,这为参与方的隐私问题又带来新的挑战。已有的基于公开区块链的隐私保护公平合同签署协议利用参与方共享的秘密因子对合同信息及公钥进行加密从而隐藏了参与方数字证书中的真实身份信息;但在协议的承诺阶段,由于区块链的假名性,执行两笔保证金交易时可能会泄露正在签署合同的参与方信息。为最大限度保护参与方的身份隐私,本文基于混币技术,通过引入半诚实可信第三方来提供参与者身份的混淆服务,并结合盲的可验证加密签名方案,设计出新的隐私保护公平合同签署协议。该协议可支持多个合同签署人通过区块链完成公平的合同签署,不仅可以保护与合同相关的隐私内容,还可以保护参与方的身份隐私。

Traditional fair contract signing protocols need a centralized trusted third party （TTP） to ensure the fairness of the protocols. When the third party becomes dishonest and colludes with one participant, it is unfair for the other partici- pant. Meanwhile, the third party may reveal sensitive information from the participants, which is a great threat to the pri- vacy of the participants. Therefore, the fairness and privacy are dependent on the reliability of the third party. Using blockchain technology, fair contract signing protocols can be decentralized to achieve fairness. It poses new challenges to the privacy issues since anyone can access and verify a public blockchain. In the existing privacy preserving fair contract signing protocol based on public blockchain, the secret information shared by the participants is used to encrypt the con- tract information and the public keys so as to conceal the true identity information in the participants＇ digital certificates. However, in the commit phase of the protocol, two deposits would probably reveal the identity information of the partici- pants because of the pseudonymity in the blockchain. For maximum protection of participants＇ privacy, this paper uses the coin mixing technique, which introduces a semi-honest third party to provide the mixing service, and designs a new fair contract signing protocol based on blind verifiably encryption signature scheme. The new protocol allows multi-party contract signers to fulfill the task of fair contract signing on blockchain, and protects not only the privacy contents related to the contract but also the orivacv of identities of contract signers.