  Biometric authentication prospered because of its convenient use and
security. Early generations of biometric mechanisms suffer from spoofing
attacks. Recently, unobservable physiological signals (e.g.,
Electroencephalogram, Photoplethysmogram, Electrocardiogram) as biometrics
offer a potential remedy to this problem. In particular, Photoplethysmogram
(PPG) measures the change in blood flow of the human body by an optical method.
Clinically, researchers commonly use PPG signals to obtain patients' blood
oxygen saturation, heart rate, and other information to assist in diagnosing
heart-related diseases. Since PPG signals contain a wealth of individual
cardiac information, researchers have begun to explore their potential in cyber
security applications. The unique advantages (simple acquisition, difficult to
steal, and live detection) of the PPG signal allow it to improve the security
and usability of the authentication in various aspects. However, the research
on PPG-based authentication is still in its infancy. The lack of
systematization hinders new research in this field. We conduct a comprehensive
study of PPG-based authentication and discuss these applications' limitations
before pointing out future research directions.
