# Two-Level-Composite-Hashing Facilitating Highly Efficient Anonymous IoT and D2D Authentication
## 2 Related Work  
Several publications like [2; 6] have examined the vulnerabilities of IoT systems and have surveyed the existent authentication mechanisms. Regarding the cryptographic algorithms of IoT authentication, they could be either asymmetric-key-cryptographies or symmetric-key-cryptographies. In general, a symmetric-key-based scheme demands fewer computations than an asymmetric-key-based scheme. As many IoT devices are resource-limited, and intensive computations and frequent connections would consume large resources and quickly deplete the battery, it is desirable to have very lightweight solutions for authenticating the devices and protecting their identities. IoT systems range from small-scale ones to very-large-scale systems. For those widely deployed IoT systems, it is cost-effective to choose public mobile systems like 4G/5G as the IoT communication backbone to leverage the advantages of the ubiquitous coverage, high reliability, and very competitive cost [5; 6]. The Third Generation Partnership Project (3 GPP) has standardized the D2D communication as one critical technology and application domain for the 5G systems. Potential applications of D2D communications include social network services, gaming, information sharing, advertising, conferencing services, etc. Since then, several works like [7; 8; 9; 10; 11; 12] aim at proposing IoT authentication schemes and the works like [13; 14; 15; 16; 17; 18; 19; 20; 21; 22] aim at securing D2D authentications in the 5G architecture. Regarding mobile-system-based IoT authentications, the aggregated communication overhead and the long latency would seriously degrade the performance; therefore, reducing the two metrics are important. In these scenarios, both cloud-based architectures or edge-service-based architecture are popular. [PERSON] et al. [23] designed a lightweight edge computation for wide-range IoT applications. [PERSON] et al. [24] proposed to adopt edge computation to improve the latencies and the response time of cloud-based IoT services. In [PERSON]'s works [9; 25], the systems distribute some tokens to the visited domains for authentication delegation and delicately apply hash functions to greatly improve the computational efficiency; however, the systems do not provide device anonymity and do not provide D2D communications. [PERSON] et al. [12] consider an interesting mobile-system-based IoT architecture, where there are two levels of IoT devices; the level-1 IoT devices are registered at the servers while the level-2 devices are registered at the level-1 IoT devices, which act as a relay for the level-2 devices to access the core networks; the level-2 devices use the same pseudonyms in the communications, which facilitates attackers link the transmissions from the same device. [PERSON] et al. [21] considered the D2D authentication via the help of servers; however, they do not consider device identity privacy. [PERSON] et al. [22] consider both the one-to-one D2D authentication and the one-to-many D2D authentication; however, the schemes use the identity-based prefix encryption (IDPE) [26], which involves several costly pairing operations to protect a device's identity privacy. As the potential applications of D2D communications cover a wide range, the corresponding security requirements also vary a lot. Even so, most of secure D2D communications highlight the importance of device anonymity protection and efficiency, as a person might carry the devices or the devices might be deployed at some sensitive locations. Toachieve anonymity, many of the existent authentication protocols adopt pseudonyms, and many of them link pseudonyms to some kinds of public-key certificates (PKC) [16] or certifiateless public keys (CLPK) [12; 13; 14; 20].