# Cryptography in Wireless Multimedia Sensor Networks: A Survey and Research Directions
## 3 Multimedia Data Encryption  
Cryptography in wireless multimedia sensor networks may be required in many monitoring scenarios, since it can assure acceptable levels of security at relatively low cost. However, differently from scalar sensors that retrieve small numerical data, multimedia sensors can sense and transmit much more data in the form of image, video and audio. Therefore, as more data have to be encrypted and decrypted (at the destination), cryptography naturally becomes more complex and resource-demanding in WMSN. Moreover, as image, video and audio have different particularities \begin{table}
\begin{tabular}{l l l} \hline \hline
**Algorithm** & **Type** & **Description** \\ \hline AES (Advanced Encryption & \\ Standard) [31; 32] & Symmetric & It is a block cipher, taking blocks of a fixed size. \\ \hline RC4 (Rivest Cipher 4) [33] & Symmetric & It is a popular stream cipher algorithm. \\ \hline RC5 (Rivest Cipher 5) [34] & Symmetric & It is also a block cipher algorithm, but with blocks with variable sizes. \\ \hline RSA (Ron Shamir and Adleman) [35; 36] & Asymmetric & It exploits factoring of large prime numbers. RSA may be computationally intensive. \\ \hline ECC (Elliptic Curve & & It is based on the elliptic curve discrete logarithm \\ Cryptography) [37; 38] & Asymmetric & problem. ECC employs small keys compared to other algorithms. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Some popular cryptography algorithms for WSNs. among them, cryptography is usually different depending on the transmitted data, requiring proper strategies. In general, secure data transmissions can be achieved through symmetric or asymmetric cryptography, which may be performed through different algorithms. Actually, both of them present advantages and drawbacks that should be properly evaluated for each type of application. In short, while the symmetric cryptography paradigm defines a single shared key for both encryption and decryption functions, asymmetric encryption employs a pair of keys to perform data encryption [8]. This particularity guides the implementation of the cryptography algorithms, and it has direct impact on performance and computational costs. When performing symmetric cryptography, which is simpler to implement due to the fact that a single key is used for both encryption and decryption, the biggest challenge is how to securely distribute that key. However, once it is accomplished, it is often argued that symmetric cryptography is more suitable for wireless sensor networks due to lower computational costs [5]. On the other hand, asymmetric cryptography utilizes a public key (that is known by all nodes) to perform data encryption, while a private one is used for decryption, resulting in higher memory occupation and processing time. Nevertheless, the adoption of multimedia sensing has also brought sensor nodes with more computational resources to this scene, which encourages the use of more robust cryptography for wireless multimedia sensor networks [5]. As cryptography keys are central when protecting data, so are management key approaches [29; 30]. Such approaches should support the addition and revocation of nodes in the network, with special concern to nodes' mobility and sensor networks based on constant redeployment. As cryptography becomes more complex in WMSN scenarios, key management acquires an important role in data protection. Overall, cryptography in wireless multimedia sensor networks may be classified according to the type of the considered media. As there are significant differences between the type of sensed and transmitted data, cryptography may be optimized to achieve higher performance according to the media type. In fact, WMSNs may sense and transmit images, video, audio and scalar data, following a query-based, a time-based or an event-based paradigm [41]. Moreover, real-time transmissions may be required, also adding complexity to multimedia cryptography. Table 2 summarizes the main characteristics of cryptography in WMSNs according to the media type.