# Cryptography in Wireless Multimedia Sensor Networks: A Survey and Research Directions
## 5 Research Directions  
Although security is highly required for many applications, wireless multimedia sensor networks may have severe constraints in processing, memory, transmission and energy supply, which bring some important challenges to be considered. Actually, multimedia processing will demand more resources than scalar data handling, putting security in a critical position. In this context, the surveyed works present promising solutions for this complex scenario, but much more research effort is still required, guiding future research directions. Selective encryption will be central in wireless multimedia sensor networks. Although heterogeneous sensor networks should become the most common option for monitoring functions, with the employment of resource-rich multimedia nodes, processing, memory and transmission restrictions should still guide the way cryptography will be employed in some cases. When applying selective encryption, which is centered on combining encryption and encoding/compression algorithms, processing burden can be softened. In such a way, a promising research trend could be focused on the application of selective encryption, but compression particularities of image, video and audio streams have to be properly considered. For example, selective encryption of still images may exploit the characteristics of different algorithms/transforms, such as DCT, DWT, quadtree, EZW, SPIHT, EBCOT, SPECK, among others [81; 82]. For video streams, it seems to be reasonable that most works will exploit the characteristics of predictive encoding, with H.264/AVC being the most used codec. At last, for selective encryption for audio streams, the use of transform-based algorithms also seems to be a highly reasonable approach when producing compressed data with different significance for the reconstruction process. Following this trend, the combination of selective encryption with QoS (Quality of Service) parameters can bring significant results. Roughly speaking, the QoS is an indication of the expected quality of communications, which may be associated with some characteristics, such as throughput, latency, jitter and packet error rates [11]. In such a way, when employing cryptography mechanisms, QoS-based solutions could adapt error recovery, routing, congestion control, security and the energy consumption pattern (just to cite some of the most relevant aspects) when encryption multimedia data, achieving optimized solutions. Actually, possible approaches may be based on QoS with different scopes, which may have a local or global significance [11]. For QoS at the local level, encrypted packets containing most relevant data may have, for example, higher traffic priority over other packets or even stronger protection against packet errors during transmissions. On the other hand, QoS can be applied so that some source nodes may have a global significance for all deployed nodes, where source nodes with different priorities may apply different encryption strategies for higher performance [18]. Besides these two relevance scopes, QoS parameters may be associated with a broader perception of relevance, which comprises all concurrent systems in an environment. Some of these approaches are discussed in [11], as priorities may be computed according to many different kinds of information with different scopes. Actually, such prioritization mechanisms could be wealthy when defining the most suitable cryptography approach for multimedia data, in an adaptive way. Cryptanalysis of different encryption algorithms, both symmetric and asymmetric, are extremely relevant for the use of cryptography in wireless sensor networks [8]. Different algorithms exist, and new ones should be created in future; and some of them may be suitable for the particularities of wireless multimedia sensor networks, for example when exploiting chaos theory [83; 84; 85].