# Cryptography in Wireless Multimedia Sensor Networks: A Survey and Research Directions
## 3 Multimedia Data Encryption
### Image Cryptography  
In general, we can say that images are snapshots retrieved by camera-enabled sensors, according to the Field of View (FoV) of the camera and the coding characteristics of the employed hardware and \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Media** & **Size** & **Coding** & **Some Relevant Issues** \\ \hline \multirow{2}{*}{image} & \multirow{2}{*}{medium} & DCT(JPEG); DWT(JPG2000); & Selective encryption; \\  & & SPIHT; EZW [42] & watermarking \\ \hline \multirow{2}{*}{video} & \multirow{2}{*}{high} & H.264/AVC; DVC; compressive & Multipath routing; \\  & & Sensing [43; 44] & real-time delivery \\ \hline \multirow{2}{*}{audio} & \multirow{2}{*}{low/medium} & PCM/ADPCM; CVSD; MDCT & Aggregation \\  & & & \\ \hline \multirow{2}{*}{scalar} & \multirow{2}{*}{low} & Raw data or compressed data & Redundancy \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Multimedia data encryption. DVC, Distributed Video Coding; MDCT, Modified Discrete Cosine Transform. software. The resolution, color pattern and compression quality depend on such characteristics and also the application requirements: grayscale low-resolution images may be suitable for some types of monitoring, while colored high-resolution images are expected for high-definition applications. The encryption of all retrieved images at source nodes may be very costly in time and computing power, which may render its adoption unfeasible for some sensor networks. Therefore, optimization approaches may be adopted to lower the burden of image cryptography in wireless multimedia sensor networks. Among those approaches, many works have exploited the principle of selective encryption to achieve that goal [8]. Partial or selective encryption is an optimized method that exploits the characteristics of media coding algorithms to provide secrecy while reducing computational complexity [47; 48]. In practical means, only part of the original data is protected, but authenticity, confidentiality and integrity are still assured. Some coding algorithms are naturally suitable for selective encryption. Among them, most works have concentrated efforts in quadtree and wavelet-based coding algorithms. Quadtree coding is based on a computational rooted tree, which decomposes the original image into different sub-quadrants, and the relevance of the quadrants for the reconstruction process is related to their position in the tree [49; 50]. On the other hand, DWT-based (Discrete Wavelet Transform) algorithms apply a wavelet transform on the original data, generating a hierarchy of frequency bands [51; 52]. Therefore, in wavelet-based coding, the band of highest compression level contains the most important visual information for the reconstruction process. In both cases, as there may be resulting parts with higher importance for the reconstruction of the original image (at the destination), cryptography might be applied only over those parts. As a result, the overall cryptography burden could be reduced, once image coding is already required in most cases for compression purposes. Figure 1 presents a general schema of selective encryption of an image compressed using DWT. Only the most relevant sub-band (roughly 25% of the original image) is encrypted in this example, but the entire image is protected, since it cannot be reconstructed without the most relevant DWT sub-band. Cryptography may also be applied for watermarking of sensed images, which is focused on authentication. A digital watermark is a special marker that is embedded into scalar, audio, image or video data, aiming at providing a mechanism to identify ownership and copyright [53; 54]. The watermarking process will hide authentication information into original data, which may be visible or not. Actually, as it is relatively simple to implement, watermarking is a technique that has been used for a long time, being popular in Internet-based networks. In wireless sensor networks, this lightweight technique can be valuable when providing authentication [55].