The increasing use of RFID tags in many applications have brought forth valid concerns of privacy and anonymity among users. One of the primary concerns with RFID tags is their ability to track an individually tagged entity. While this capability is currently thought to be necessary for supporting some features of RFID systems, such practice can lead to potential privacy violations. In this paper, we propose a privacy-preserving scheme that enables anonymous estimation of the cardinality of a dynamic set of RFID tags, while allowing the set membership to vary in both the spatial and temporal domains. In addition, the proposed scheme can identify the dynamics of the changes in the tag set population. The main idea of the scheme is to avoid explicit identification of tags. We demonstrate that the proposed scheme is highly adaptive and can accurately estimate tag populations across many orders of magnitude, ranging from a few tens to millions of tags. The associated probing latency is also substantially lower (<= 10 %) than that of the schemes which require explicit tag identification. We also show that our proposed scheme performs well even in highly dynamic environments, where the tag set keeps changing rapidly.
