# Secure D2D Group Authentication Employing Smartphone Sensor Behavior Analysis
## 1 Introduction  
Device-to-Device (D2D) communication is defined as a promising short-distance communicating strategy, which is capable of directly conducting effective data exchange among the proximate entities without involvement of the cellular core network. As for conventional cellular networking services, connectivity between the mobile devices is subjected to the coverage of access point or base stations, where direct communication among mobile devices is not offered [1]. Instead, all the involved traffic floods should be processed through the core cellular network, which severely restrains the large-scale implementation of massive and reliable data exchange between mobile devices. In this case, D2D communication is capable of complementing the traditional cellular network strategy by leveraging the physical proximity of participating devices, which is essential in sparse environments [2]. D2D communication performs as the vital component in the upcoming 5G mobile networks and wireless systems [3, 4], which is anticipated to support the deluge of data traffic. Accordingly, the network performance regarding effective coverage, spectrum efficiency, real-time network delay and transmission fairness can be significantly improved. Various emerging D2D applications scenarios regarding social networking and local information aggregating have been widely implemented. Particularly, its combination with Internet of Things (IoT) enables extensive services including vehicle-to-vehicle (V2V) communication, smart grid and early warning systems for natural disasters like hurricanes and earthquakes [5]. Currently, various D2D communicating infrastructures have been designed and investigated by the Third Generation Partnership Project (3 GPP), which can be classified into standalone D2D and network-assisted D2D [6]. Standalone D2D communication fully depends on the local hardware capabilities, where the D2D devices organize the interaction themselves. In contrast, the network-assisted D2D network operates with the assistance of certain infrastructure, such as a base station or access point for cellular connection. In typical D2D infrastructure, the user equipment (UE) is considered one of the essential components, which performs as mobile devices operated by terminal users themselves. It can be a portable cellphone or a laptop computer with a wireless broadband adapter. Due to the open wireless connecting features [7, 8, 9], D2D data exchange suffers from various security risks and privacy threats, especially in the D2D group communication involving large numbers of participating devices. It is worth emphasizing that the security and privacy requirements vary for different D2D management and application scenarios. In this case, advanced security strategies and privacy preservation techniques are vital for general D2D environments [10]. Effective and efficient authentication mechanism between user equipments (UEs) and the regarding base station (BS) could provide preliminary protection for D2D data exchange, which is particularly essential for group communication. Accordingly, various charted and uncharted secure threats including eavesdropping, impersonation and replaying can be prevented, which is indispensable for the upcoming 5G practical implementation. Currently, emphasizing the D2D secure authentication issue, lots of research achievements have been made, adopting diverse cryptographic design and verification methods [2]. Note that in some, the key information for individual UEs are fully organized by the key generation center (KGC), resulting in a potential key escrow problem [1, 11]. For this consideration, it is of great importance for the UE to independently generate its own partial key information and keep it secret from all other entities including the KGC. For this consideration, the certificateless encryption outperforms other methods by generating the partial secret key from both the KGC and the UE itself. Note that both the KGC and the UE have no access to the partial secret generated by the other party.