  Wireless Contact tracing has emerged as an important tool for managing the
COVID19 pandemic and relies on continuous broadcasting of a person's presence
using Bluetooth Low Energy beacons. The limitation of current contact tracing
systems in that a reception of a single beacon is sufficient to reveal the user
identity, potentially exposing users to malicious trackers installed along the
roads, passageways, and other infrastructure. In this paper, we propose a
method based on Shamir secret sharing algorithm, which lets mobile nodes reveal
their identity only after a certain predefined contact duration, remaining
invisible to trackers with short or fleeting encounters. Through data-driven
evaluation, using a dataset containing 18 million BLE sightings, we show that
the method drastically reduces the privacy exposure of users. Finally, we
implemented the approach on Android phones to demonstrate its feasibility and
measure performance for various network densities.
