# Biometrics for Internet-of-Things Security: A Review
## 1 Introduction
Second, users' private data or sensitive information stored in IoT devices should only be accessed by genuine users. A number of research articles integrate biometrics into IoT systems to secure IoT devices and applications. [PERSON] et al. [9] surveyed the technologies and challenges of applying biometrics to the IoT. Moreover, the authors provided their visions on using biometrics in areas such as big data and mobile cloud computing. [PERSON] [10] presented the  
Figure 1: Application domains of the IoT (adapted from [7]). advantages and disadvantages brought to the IoT by biometrics. Distinctive features of biometrics in relation to IoT security are summarized in [10]. [PERSON] et al. [11] conducted a survey on biometrics for wearable IoT devices. The authors discussed the differences between biometrics for wearable devices (called wearable biometrics in [11]) and traditional biometrics. In addition, the computing cost, system structure and experiment setup of different schemes are compared and analyzed in [11]. [PERSON] et al. [12] discussed and compared popular biometric traits applied to the IoT in terms of accuracy, ease of use and cost. The authors also covered the biometric feature extraction techniques (e.g., the Fourier transform and Gabor filtering) and gave an example of a biometric-based eHealth system. While the above-mentioned research articles provided insights into incorporating biometrics in IoT applications, to the best of our knowledge, little comprehensive survey over biometrics for IoT security has been conducted thus far, and in particular, the topics such as biometric data protection and biometric-cryptography in the IoT are not covered. To fill this gap, this paper presents an in-depth review of current research in biometrics for IoT security, especially focusing on authentication and encryption. The main contributions of this paper are highlighted as follows. 1. To the best of our knowledge, there has been little survey which adequately considers biometric authentication and encryption simultaneously for the IoT environment prior to this review paper. This paper gives a comprehensive review of the contemporary biometric-based systems that provide authentication and encryption for the IoT;
2. Regarding authentication, we classify and analyze IoT-related biometric authentication systems based on different biometric traits and the number of biometric traits used in the biometric systems. Following this, we investigate biometric-cryptographic systems that integrate biometrics with cryptography for data encryption. The study of these systems sheds light on the latest development in handling IoT-related security vulnerabilities or possible attacks targeting the IoT;
3. Challenges brought by the deployment of biometric systems in the IoT are identified and potential solutions are discussed and highlighted;
4. Several insights into future research directions concerning biometrics for IoT security are provided in this paper. The database search strategy using keywords is adopted in this review paper such that a more complete list of articles can be obtained. In search of research papers, a number of databases (e.g., IEEE Xplore, Web of Science, Science Direct and Scopus) are included, followed by applying the inclusion and exclusion criteria based on titles and abstracts to filter out irrelevant articles. Then, the full text of each article is examined to determine whether the article should be included in this review paper, according to certain guidelines (e.g., [13]). The remainder of the paper is organized as follows. Section 2 presents challenges and vulnerabilities identified in the IoT, while Section 3 describes common biometric traits that can be applied to the IoT. Section 4 discusses and classifies existing biometric-based systems that are being used purposely for IoT authentication. In Section 5, biometric systems that are applied to IoT data encryption are analyzed and discussed. Challenges brought by applying biometrics to IoT are highlighted in Section 6.