  Research has shown that counting WiFi packets called probe requests (PRs)
implicitly provides a proxy for the number of people in an area. In this paper,
we discuss a crowd counting system involving WiFi sensors detecting PRs over
the air, then extracting and anonymizing their media access control (MAC)
addresses using a hash-based approach. This paper discusses an anonymization
procedure and shows time-synchronization inaccuracies among sensors and hashing
collision rates to be low enough to prevent anonymization from interfering with
counting algorithms. In particular, we derive an approximation of the collision
rate of uniformly distributed identifiers, with analytical error bounds.
