# AEF: Adaptive En-Route Filtering to Extend Network Lifetime in Wireless Sensor Networks
## 2 Related Work  
Dynamic en-route filtering (DEF) [3], uses the hill-climbing approach to distribute keys and effectively counters attacks to save scarce energy. Each node transmits its key to the forwarding node. Sending nodes reveal their keys after sending reports, enabling the forwarding node (s) to validate their reports. It utilizes the broadcasting nature of wireless networks to defend against denial of service (DoS) attacks and multipath routing to counter topology changes. DEF can counter attacks requiring lower memory as compared to other schemes. Statistical en-route filtering (SEF) [5] was the first work to detect fabricated reports. A global key pool is split into nonoverlapping segments. A small number of keys is preloaded on each node before deployment. Each event detecting node generates a message authentication code (MAC) using one of its stored keys. Event detection nodes elect a center of stimulus (CoS) node to collect and summarize all the received detection results to produce a report including the respective MACs. A report with an inadequate number of MACs will not be forwarded. False reports reached at the base station (BS) will be finally detected since BS has all the keys. SEF has only a limited filtering capacity and fails to counter impersonating attacks. Commutative Cipher-based En-route Filtering (CCEF) [4] saves energy by early detection. It has the following limitations: fix path routing, not considering residual energy, and fix detection capacity. It does not respond according to the change of attack frequency. Underlying fixed path routing is not energy aware so it exhibits adverse effects on network lifetime. Secure ticket-based en-route filtering (STEF) [6] uses tickets issued by a sink for report verification. It is similar to SEF and DEF. The interleave hop-by-hop authentication (IIA)[7] scheme, which is also based on fixed path routing, suffers similar limitations as in CCEF. It needs interleaved upper and lower associations between the sensor nodes to share sensor secrets. Due to the constantly changing and unreliable nature of WSNs, it is implausible to have determined routes. Moreover, these associations demand global knowledge which is complex and costly task for energy-scar WSNs. Each node in the bandwidth-efficient cooperative authentication (BECAN) [8] scheme requires a fixed number of neighbors for authentication. It disseminate the authentication keys of en-routing to all sensor nodes in the path. Bit compression is used to save bandwidth. BECAN cannot counter false routing information and selective dropping attacks. Moreover, this novel scheme cannot counter selective dropping and false routing information along with some limitations exhibited by the above schemes. Greedy perimeter-based stateless routing (GPSR) [9] was generally used in en-route filtering schemes which relied on its simple and robust approach. However, this routing scheme was designed for ad hoc networks and did not performed well for WSNs. It selected a forwarding node among its candidate nodes which is closest to the BS. In case it could not find a path around the perimeter, a right hand rule is employed. The authors of Reference [43] present multipath routing to mitigate improvements in network security including eavesdropping or modification attacks. This work proposed that multipath routing is effective against this type of attack. Furthermore, loading balancing work has been proposed using opportunistic routing [44]. In order to measure energy consumption, the first-order radio model was used [11]. Moreover, the parameters used in the performance measurement of the first-order radio model are adapted from it. An acceptable energy-to-noise ratio is presented in Reference [12]. A lot of works, for instance, Reference [13], has been done to increase network lifetime by improving underlying routing schemes. There is a need to improve network lifetime in security schemes which often ignore routing to prolong network lifetime.