Defending Privacy Against More Knowledgeable Membership Inference Attackers

Membership Inference Attack (MIA) in deep learning is a common form of privacy attack which aims to infer whether a data sample is in a target classifier's training dataset or not. Previous studies of MIA typically tackle either a black-box or a white-box adversary model, assuming an attacker not knowing (or knowing) the structure and parameters of the target classifier while having access to the confidence vector of the query output. With the popularity of privacy protection methods such as differential privacy, it is increasingly easier for an attacker to obtain the defense method adopted by the target classifier, which poses extra challenge to privacy protection. In this paper, we name such attacker a crystal-box adversary. We present definitions for utility and privacy of target classifier, and formulate the design goal of the defense method as an optimization problem. We also conduct theoretical analysis on the respective forms of the optimization for three adversary models, namely black-box, white-box, and crystal-box, and prove that the optimization problem is NP-hard. Thereby we solve a surrogate problem and propose three defense methods, which, if used together, can make trade-off between utility and privacy. A notable advantage of our approach is that it can be used to resist attacks from three adversary models, namely black-box, white-box, and crystal-box, simultaneously. Evaluation results show effectiveness of our proposed approach for defending privacy against MIA and better performance compared to previous defense methods.