You can hide, but your periodic schedule can't

The enterprise Wi-Fi networks enable the collection of large-scale users' trajectory datasets, which are highly desired for both research and commercial purposes. Meanwhile, releasing these mobility data also raises serious privacy concerns. A large body of work tries to achieve k-anonymity as the first step to solve the privacy problem and it has been qualitatively recognized that k-anonymity is still risky when the diversity of sensitive information in the k-anonymity set is low. However, there lacks a study that provides a quantitative understanding for trajectory data. In this work, we investigate the schedule-leakage risk for the first time, by presenting a large-scale measurement based analysis of the high schedule-leakage risk over sixteen weeks of trajectory data collected from Tsinghua University, a campus with 2,670 access points deployed in 111 buildings. Using this dataset, we recognize the high risk of the schedule-leakage, i.e., even when 4-anonymity is satisfied, 28% of individuals' schedules are totally disclosed, and 56% are partly disclosed.