# Secure D2D Group Authentication Employing Smartphone Sensor Behavior Analysis
## 1 Introduction
Additionally, with multiple advanced features and functionalities, smartphones have played a significant role in our daily lives [12]. Generally, the modern smartphone is equipped with various built-in sensors including an accelerometer and a gyroscope, which are capable of unobtrusively collecting abundant personal biometric data whenever the user conducts daily activities such as sitting, sleeping, running, and walking [13]. Particularly, the gathered personal activity data reflects certain users' unique characteristics, which can be adopted for continuous authentication. We assume a particular D2D scenario intended for public safety or field trip applications for extreme environments, where the smartphone acts as the UEs for every terminal user [14]. In most of the isolated nature landscapes including the mountainous regions, desert areas, or tropical rainforests, infrastructures for a cellular network are not always available, especially in the depopulated zones. That is, most of the UEs are not within the cellular coverage. In this assumption, the D2D communication between UEs could provide real time communication for individual users within this area [15; 16]. The BS is able to interact with certain UEs and provide cellular connection for all the participating UEs within the D2D network. As for individual users with UE, vital biometric data including accelerometer and gyroscope signals are acquired by the smartphone sensors, revealing personal behavioral features. Subsequently, the user's behavioral patterns can be adopted for continuous authentication. That is, after the initial authentication operation, the specific verification is conducted periodically for the purpose of guaranteeing secure data transmission throughout the entire process. Therefore, through analysis of a user's behavioral patterns, continuous authentication can spot vulnerabilities at any point in a session [17]. Note that the above assumption is suitable for both field trip assistance and emergency rescue. The detected anomaly feature demonstrates certain distinctness from existing personal record. That is, the user may encounter unexpected physical danger that causes severe deterioration of health, which is of great significance for real time healthcare monitoring and subsequent emergency aid in complex circumstances [6]. In this paper, a secure certificateless authentication scheme for D2D communication is presented. The nontrivial contributions of this paper can be briefly summarized:  
* _Secure certificateless authentication scheme:_ According to our design, a certificateless cryptography mechanism is applied so as to provide improved security assurance. BS and UE itself generate the partial private key respectively so as to prevent the key escrow problem of identity-based encryption. Moreover, conditional privacy-preserving authentication (CPPA) is deployed. That is, the user anonymity is provided through the entire authentication session, preventing illegal tracing towards particular UEs, while the valid identity-related information is recorded in BS side in the preliminary registration phase. Hence the tracking and revocation towards malicious UEs can be conducted by trustworthy authority. Additionally, bilinear pairing is adopted in order for advanced security properties. * _Efficient Group key distribution with updating mechanism:_ During the authentication process, the allocated group key computed by BS will be delivered to all legitimate UEs through one broadcasting operation, which drastically alleviates the communication cost compared with conventional one-to-one key distribution. Note that only the authentic UEs have the capability of deriving the valid group key. Therefore, the designed key updating mechanism only require small modification in the BS side, while the decrypting information in the UEs side remains constant as soon as the UEs are validated. Similarly, fast UE revocation process can be operated by BS without extensive computation.