# Cryptography in Wireless Multimedia Sensor Networks: A Survey and Research Directions
## 2 Security Issues in Multimedia Sensor Networks
### Security Threats  
Wireless sensor networks are vulnerable to several types of attacks, which may compromise one or more of the security requirements of the applications. Actually, the maintenance of security requirements depends on how threats are understood and how they can be avoided or minimized. For typical WMSN applications, the wireless channel may be \"easily\" accessed by unauthorized people, mostly due to the nature of such applications. Actually, hundreds or thousands of sensor nodes may be deployed on large areas, turning control of whom is accessing the covered area into a very hard task. In such a way, insertion of malicious nodes or even DoS attacks may not be easily avoided. Besides physical access to sensor nodes, the transmission flow may also be subject to attacks, especially due to the inherent characteristics of wireless ad hoc communications. The resulting scenario is prone to many security attacks that may compromise the effectiveness of sensors-based monitoring applications. In short, any vulnerability is a weakness that can be exploited by attackers, and so, they should be properly known and prevented. When there is a vulnerability, attackers may exploit it, creating a threat. In general, attacks may be centered on exploiting vulnerabilities in some communication layer, eavesdropping on transmitted data, altering confidential data or prejudicing the network operation with artificial malicious information. The work in [15] classifies security threats for wireless sensor networks as internal or external, depending on the origin of possible attacks. An external attack comes from outside the network, while internal attacks will be executed by legitimate nodes that will behave in unintended ways. That work also defines that an attack may be passive, with no modification of the network, or active, where data streams are modified or created. Security attacks in wireless sensor networks may also be of four different types: interruption, interception, modification and fabrication [15; 16]. Although there may be differences according to the application characteristics, interruption attacks compromise availability; interception compromises confidentiality; modification prejudices integrity; and fabrication impairs authentication. Therefore, their impacts will depend on how prejudicial the impairment of some security requirement may be for the applications. Interruption attacks may be designed to congest communication channels or to disconnect sensor nodes (e.g., due to inflicted energy wasting). Among the possibilities, \"denial of service\" attacks inject malicious or useless packets into the network in order to reduce applications' quality [5]. As more packets will need to be handled, DoS attacks may rapidly deplete the processing, memory and energy resources of nodes [17]. Moreover, DoS attacks in the physical layer (also referred to as jamming) may also be extremely prejudicial to wireless multimedia sensor networks, since they may interfere in the operation of MAC layer protocols. Interception and modification attacks can generally happen when sensor nodes are \"captured\". Besides data stealing, faked information may be artificially fabricated, compromising the monitoring quality of applications. Nodes deployed on outdoor environments are especially vulnerable to tampering attacks, which is the physical access to nodes for the modification of their configurations. Security threats may be presented in different conceptual communication layers in WSN. Routing and transport-layer protocols may be attacked to create loops, to redirect packets to malicious nodes or to compromise reliability [7]. Attacks that produce excessive packet retransmissions are also extremely prejudicial, since they may increase congestion, transmission latency and energy consumption. The monitoring functions of applications may have inherent threats that may be hard to avoid.