# Cryptography in Wireless Multimedia Sensor Networks: A Survey and Research Directions
## 2 Security Issues in Multimedia Sensor Networks  
Wireless multimedia sensor networks are subject to many threats that can compromise applications in different ways. In recent years, the increasing use of sensing technologies in emerging networks has brought attention to sensors' weak security. However, the resource-constrained nature of sensor nodes may discourage the adoption of security mechanisms, which may leave open vulnerabilities to be exploited. The proper understanding of threats and countermeasures is then of paramount importance.  
Actually, one of the fundamentals of wireless sensor networks is resource constraints. These networks are typically comprised of a great number of tiny low-cost electronic devices with limited sensing and computing capabilities, allowing sensor nodes to be inexpensive and (theoretically) disposable. Constrained sensors are useful for massive deployment and for energy efficiency, which are central concepts of WSN. As a result, security measures have an additional challenge when employed in such networks, when compared to Internet-based systems. The literature on this subject has been devoted to create or adapt security solutions for this constrained environment.  
Monitoring and control applications may be concerned with different threats, which will demand some specialized defense measures. In general, some security requirements can be defined, and it is expected that most applications will be concerned with at least one of them. Such requirements are defined as follows.  
* Authenticity: Sensor nodes and transmitted packets should be authenticated. Malicious sensor nodes could enter the network to steal information or inject packets. The verification of the packets' origins is desired in many cases.
* Availability: Some applications may require that a minimum level of availability is assured during the network operation, but attacks may reduce the attainable availability in different ways (compromising communication, processing or sensing coverage).
* Confidentiality: Sensed data and control information may be confidential, since their content must not be accessible by intruders or external elements.  
* Freshness: Attackers should not be able to exploit old messages, which require the adoption of efficient mechanisms to control time scopes.
* Integrity: While confidentiality avoids that attackers can steal data, integrity will be concerned with data changing and manipulation.
* Localization: Secure localization is required to assure only accurate information is considered.  
The authors in [7] define different scopes for the security requirements, which may be node-centric, data-centric, network-centric or user-centric, according to the perspective for the requirement guarantees. Node-centric security is related to all aspects directly related to the device, including both its software and its hardware, while data-centric security focuses on confidentiality for recorded data. Moreover, network-centric security guarantees are only valid during transmission. At last, user-centric security relates to the user perspectives, who have to be able to check if and how their personal data are protected.  
The understanding of the security requirements is important when designing wireless multimedia sensor networks, since they will guide the adoption of security mechanisms for the set of expected security threats. For this, the next subsections discuss security threats and countermeasures in wireless sensor networks.