# Cryptography in Wireless Multimedia Sensor Networks: A Survey and Research Directions
## 2 Security Issues in Multimedia Sensor Networks
### Security Defenses  
The numerous security threats in wireless sensor networks have demanded the use of defense mechanisms. Actually, there are different approaches that may be employed to try to preserve security requirements in sensing applications, each one with advantages and drawbacks. In general, the particularities and limitations of wireless sensor networks will dictate what are the most appropriate approaches for security protection.  
Security defenses may be focused on the network or on the data. When protecting the network, secure protocols may be used to avoid attacks, as denial of service, man-in-the-middle and general packet redirection [19]. Authentication mechanisms may also be used to control the access of new nodes to the network. On the other hand, when protecting data, cryptography is the most effective approach [8; 9].  
A wireless sensor network should continue its function under DoS attacks [5; 7]. As may also happen in Internet-based networks, implementing a DoS-resilient sensor network is very challenging, mainly due to the resource constrained nature of such networks, which place DoS attacks as one of the biggest threats to WSN. Additionally, this is due to the fact that DoS attacks can be performed in many different ways and against any of the different communication layers. In general, a reasonable security defense against these attacks is the monitoring of the network operation. Some sensors may be used to monitor the network, identifying the abnormal operation of nodes [19; 20]. Physical-layer signals may also be monitored to identify jamming attacks [21]. Such monitoring approaches can then be used to disconnect the sources of the attacks or to trigger alarms [19]. Frequency hopping [15] and admission control mechanisms may also be effective defense mechanisms in these cases [22].  
Other security defenses may also apply for threats to the network operation. The work in [23] discusses security issues of routing protocols in wireless sensor networks. A secure transport protocol for WSN is proposed in [24], which employs an authentication mechanism to avoid the processing of fake control messages. A security defense mechanism in the MAC-layer is proposed in [25]. Overall, there are many works that have been proposed addressing security defenses in wireless sensor networks [26] and many of them may influence security protection in wireless multimedia sensor networks.  
The basic defense mechanism in wireless sensor networks is cryptography, which directly protects the data. In short, cryptography is the set of techniques for transforming original information into a set of unreadable data, allowing it to be read only by the correct recipient [27; 28]. Due to all constraints that are inherent to wireless sensor networks, especially when processing and transmitting multimedia data, traditional cryptography with high computing and communication overhead may not be feasible for WSN, depending on the chosen hardware. Nevertheless, although the initial age of sensor networks was based on highly constrained sensor nodes, more recent technology has allowed the use of affordable sensor nodes with reasonable processing power and memory, benefiting multimedia processing and powerful cryptography alike.  
In short, cryptography may be employed to provide authenticity, confidentiality and integrity for wireless multimedia sensor networks. The use of cryptography keys allows the authentication of source nodes, since they must have the proper keys. Additionally, as such keys would be required to recover the original data, confidentiality is also assured. At last, if the original information cannot be accessed, it cannot be adulterated, also providing integrity. Securing data with cryptography can then be valuable when preserving security requirements, which has fostered much research in this area. Cryptography algorithms and keys management [29; 30] are some of the bases of security protection in wireless multimedia sensor networks.