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.

翻译：生物特征认证因其便捷性和安全性而蓬勃发展。早期的生物特征认证机制易受伪造攻击。近年来，不可观测的生理信号（如脑电图、光电容积描记图、心电图）作为生物特征为此问题提供了潜在的解决方案。特别是光电容积描记图（PPG），通过光学方法测量人体血流变化。临床上，研究人员通常利用PPG信号获取患者的血氧饱和度、心率等信息，辅助诊断心脏相关疾病。由于PPG信号蕴含丰富的个体心电信息，研究人员开始探索其在网络安全应用中的潜力。PPG信号独特的优势（采集简单、难以窃取、活体检测）使其能够在多方面提升认证的安全性与可用性。然而，基于PPG的身份认证研究仍处于起步阶段，缺乏系统性阻碍了这一领域的新探索。我们对基于PPG的身份认证进行了全面研究，探讨了这些应用的局限性，并指出了未来的研究方向。