## 2 Related Work  
In a WSN, security must be enabled before the initialization phase, at the time of network deployment, because node cloning attacks can be executed only during this phase, as stated by [PERSON] [28]. Moreover, clustering-based secure routing mechanisms are infeasible for WSNs because of the energy constraints of wireless sensor nodes [29]. Elliptic curve cryptography (ECC) can be implemented on miniaturized wireless sensor nodes, thereby making public key cryptography (PKC) a realistic possibility for these sensors [30; 31]. However, the implementation of PKC-based security at higher layers on top of the efficient primitives is not a simple process because of the unique requirements and characteristics of wireless sensor nodes. Consequently, studies of the performance of PKC-based security schemes have been limited in scope. A secure energy-efficient routing protocol (SERP) for densely deployed WSNs was proposed by [PERSON] and [PERSON] [32]. This tree-based routing method depends on the distance and the remaining energy parameters of nodes for routing decisions. The authors use one-way hash chains and shared secret keys that are pre-stored in the wireless sensor nodes before deployment. During initialization, all nodes in a WSN are in active mode for the construction of the tree and the one-way hash chain. In SERP, the encryption key changes occasionally, providing a security advantage. However, a major disadvantage of SERP is that a high traffic overhead is generated when the shared key is changed throughout the entire network. Furthermore, the mechanism presented by [PERSON] and [PERSON] [32] involves a key refreshment system that is called when all nodes on a certain path are offline until the destination is reached, which entails the re-initialization of part of or even the entire network. Because of the involvement of a time parameter, feedback and other processes, such as shared key updating, long processing times are required, which cause delays and consume large amounts of energy. In addition, because the shared secret key--which is not generated by the system in real time and is non-dynamic--is preloaded in the base station and the sensor nodes, the mechanism is unrealistic for application in highly dynamic WSN scenarios. In SERP [32], the source node ID is open and can be easily read by searching packet headers, which opens the system to brute-force search and eavesdropping attacks. The authors conducted a detailed comparison of their proposed SERP with LEACH and EAD in an ns-2-based simulation. LEACH energy consumption is high because it uses one-hop transmission to reach the cluster head or the sink node, and therefore, EAD performs much better than LEACH because of its multi-hop transmission capability. However, by virtue of its adaptive transmission range, SERP consumes less energy than does either LEACH or EAD. [PERSON] and [PERSON] [26] proposed a secure real-time load distribution (SRTLD) routing protocol to ensure secure packet transfer by means of packet header authentication. SRTLD was developed as an enhancement of the real-time load distribution (RTLD) routing protocol [26] through the incorporation of a security module. The security module contains a mathematical function that is based on a unique random number generator function [26]. A hash function is further utilized to encrypt the source address (_S_ID_), destination address (_D_ID_), and packet ID (_Pkt_ID_) fields in the packet header. When a node receives a packet, it attempts to decrypt the encrypted fields, and thus, the data authenticity in the network is verified by SRTLD. The authors showed that SRTLD is capable of securing WSNs from wormholes, HELLO flood attacks, Sybil attacks, selective forwarding, and sinkholes [26]. The main shortcoming of SRTLD is that each node is programmed with a predefined location and with routing optimization values depending on the target location or territory.