  In this paper, we show that by using inertial sensor data generated by a
smart ring, worn on the finger, the user can be authenticated when making
mobile payments or when knocking on a door (for access control). The proposed
system can be deployed purely in software and does not require updates to
existing payment terminals or infrastructure. We also demonstrate that smart
ring data can authenticate smartwatch gestures, and vice versa, allowing either
device to act as an implicit second factor for the other. To validate the
system, we conduct a user study (n=21) to collect inertial sensor data from
users as they perform gestures, and we evaluate the system against an active
impersonation attacker. Based on this data, we develop payment and access
control authentication models for which we achieve EERs of 0.04 and 0.02,
respectively.
