## 4 Security Benefits of Directional Antennas in WSNs
### Sybil Attack  
Usually, in a wireless sensor network each node has its own identity (ID), a one-to-one relationship between nodes and their unique IDs being a prerequisite for many network mechanisms [59]. In a Sybil attack [60], a malicious node forges the identities of authenticated network nodes and, as a consequence, can spread its aggressive activities to other nodes or even throughout the entire network. An example of such an attack is presented in Figure 4, where the Sybil node, shown in dotted line, uses the identity of three network nodes (A, B and C) to maliciously alter the nodes' normal behavior. [PERSON] _et al._[48] identified the ways Sybil attacks can be used to disrupt WSN operations implying distributed storage, routing algorithms, data aggregation mechanisms, voting algorithms, fair resource allocation and misbehavior detection. They provide a list of possible defenses which include node validation and authentication, resource testing (computation, storage, or communication resource testing), random key predistribution, identity registration and position verification. From this comprehensive list, one of the most efficient methods to discover the Sybil nodes is undeniably the position verification technique. Accordingly, the Sybil nodes can be identified by  
Figure 4: Sybil attack in WSN. comparing their exact position with the previously known locations of network nodes from which the Sybil nodes stole the identities. This type of methods usually employs two elements: (a) radio signal characteristics (signal strength and/or direction); and (b) trusted nodes cooperation for node identification and authentication. Directional antennas are inherently offering the direction of captured signals. If two messages coming from two nodes having the same IDs are concurrently gathered from two different directions, then we can come to a logical decision: one of the two network nodes is undoubtedly malicious. Such a methodology can be derived from the one described in [49], which uses nodes equipped with GPS devices and directional antennas. Thus, the precise location of all WSNs components are known, while the position of Sybil nodes may be calculated using triangulation [61] based on information captured by directional antennas and by employing the cooperation of at least one trusted node. A simplified Sybil attack named evil-twin, in which the malicious node is using only one stolen identity, is addressed by [PERSON] _et al._[50] using four-sector directional antennas. If two messages with the same sender ID come from two different angles a logical conclusion is drawn: one of them is bogus. Subsequently an algorithm named Hyperbolic Position Bounding (HPB) [62] is employed to obtain the location of the two twin nodes (the real node and the malicious node). Approaches for coping with Sybil attacks in wireless sensor networks based on directional antennas can also be derived from methods proposed for other types of wireless networks. For example, some methods developed for mobile _ad hoc_ networks (MANETs) or one of their subcategories (vehicle _ad hoc_ networks--VANETs) can be simply particularized to address the Sybil attack in WSNs. Two such methods attracted our interest. [PERSON] and [PERSON] [51] proposed an integrated key (a type of cryptographic key that encloses a symmetric node's ID, geographic location of the node and round trip response time)-based Strict Friendliness Verification (SFV) of neighboring nodes. As a result, a set of verifier nodes discover the Sybil nodes by dynamically changing the symmetric node's ID every time a new wireless connection is established, and by encrypting/decrypting each packet by different integrated keys. A cross-layer scheme to detect Sybil attacks in VANETs is proposed in [52]. A trial packet is sent to the mobile node's claimed location employing a directional antenna. If the mobile node is in the claimed position, it can receive the packet and reply with a response packet.