Location tracking accessories (or ``tracking tags'') such as those sold by Apple, Samsung, and Tile, allow owners to track the location of their property via offline finding networks. The tracking protocols were designed to ensure that no entity (including the vendor) can use a tag's broadcasts to surveil its owner. These privacy guarantees, however, seem to be at odds with the phenomenon of {\em tracker-based stalking}, where attackers use these very tags to monitor a target's movements. Numerous such criminal incidents have been reported, and in response, manufacturers have chosen to substantially weaken privacy guarantees in order to allow users to detect stalker tags. This compromise has been adopted in a recent IETF draft jointly proposed by Apple and Google.
We put forth the notion of {\em abuse-resistant offline finding protocols} that aim to achieve a better balance between user privacy and stalker detection. We present an efficient protocol that achieves stalker detection under realistic conditions without sacrificing honest user privacy. At the heart of our result, and of independent interest, is a new notion of {\em multi-dealer secret sharing} which strengthens standard secret sharing with novel privacy and correctness guarantees. We show that this primitive can be instantiated efficiently on edge devices using variants of Interleaved Reed-Solomon codes combined with new lattice-based decoding algorithms.
Gabrielle Beck is an independent researcher and consultant who defended her PhD at Johns Hopkins last semester. Before Hopkins, she completed her bachelor's degree at the University of Michigan where she studied computer science. Her research interests mainly involve developing new practical cryptographic constructions to solve the problems of everyday people with a special focus in metadata-hiding and other unconventional problem areas. Separately, she has recently become very interested in some select topics within coding theory.