On the Security of a Proxy-free Privacy-preserving Task Matching with Efficient Revocation

Task matching is an important part to realize task assignment in crowdsourcing computing. However, privacy of tasks and workers is usually ignored in most of exiting task matching schemes. To solve this issue, recently, Shu et al. proposed a privacy-preserving task matching with efficient revocation in Crowdsourcing (IEEE Transactions on Dependable and Secure Computing DOI 10.1109/TDSC.2018.2875682) to ensure privacy protection of tasks and workers and achieve the worker revocation. Their scheme had claimed to be selective IND-CKA secure, and realized efficient revocation of the worker. Unfortunately, in this work, by analyzing the security of Shu et al. scheme, we show that their scheme is insecure. It cannot really provide IND-CKA security and realize the revocation of the worker. This is to say, their scheme does not satisfy the confidentiality of keyword since an adversary can distinguish the ciphertexts of arbitrary keywords without trapdoor information. Finally, after the corresponding attacks are given, we analyze the reason to produce such attacks.