Solution Probing Attack Against Coin Mixing Based Privacy-Preserving Crowdsourcing Platforms

Conventional crowdsourcing platforms primarily rely on a central server as the broker for information exchange. Although many efforts have been made, centralized platforms are still vulnerable to underlying security issues, such as an untrusted central server and single-point failure. Fortunately, blockchain has emerged as an alternative infrastructure for building crowdsourcing platforms. Many excellent designs of blockchain-based decentralized crowdsourcing (BDCS) solutions have been proposed. Benefiting from blockchain, BDCS can provide fascinating features, like tampering resistance and anonymity. However, a new attack surface appears in BDCS. Recently, a new attack against BDCS named solution probing attack has been identified. The solution-probing adversary can take advantage of the anonymity of BDCS to probe valid solutions using a generative model. Due to the transparency of blockchain transactions, the probing attack is effective even if solutions are encrypted. Nevertheless, we find transaction-mixing techniques effective in defending against probing attacks. In this paper, we introduce the solution probing attack and an improved variant, which can attack coin mixing-based BDCS. We evaluate probing attacks on large-scale crowdsourcing tasks. Experimental results show that the adversary is capable of deceiving BDCS with a limited number of probing, even if the BDCS is protected by solution encryption and coin mixing techniques.