## 3 Image Cryptography
### Key Management  
Key management is central for most modern cryptography algorithms [40]. The main goal of key management is to establish a key exchange between sensor nodes and between nodes and base stations, safely and reliably [6; 40]. Such schemes should support the addition and revocation of nodes in the network, and they must be extremely light due to the restrictions of memory, processing and energy resources in wireless sensor networks. Most WSN key management protocols are based on symmetric encryption, and there are two possible classifications for establishing keys between nodes: by network structure or by probability.  
Classification of key management in relation to the network structure can be centralized or distributed. In centralized schemes, there is an entity whose function is to generate, re-generate, distribute and revoke keys. A protocol based on this scheme is the logical key hierarchy for wireless sensor networks (LKHW) [41]. Its drawback is that if the central unit fails, the schema as a whole becomes unavailable. In addition, centralized key management has some problems, such as scalability [12], _i.e._, the scheme becomes inefficient for use in large-scale WSN. On the other hand, distributed schemes are those where many different controllers are used, allowing better scalability and increased fault tolerance. According to [6], most key management approaches in WSN are distributed by nature.  
Besides classification by the network structure, key management can also be classified by their operation: deterministic or probabilistic. In the deterministic scheme, protocols and algorithms require that keys are pre-distributed among nodes. As an example protocol, we can mention the Localized Encryption and Authentication Protocol (LEAP) proposed in [42]. On the other hand, the probabilistic scheme is based on the probability of communications to happen due to the proximity of nodes. In other words, each sensor node finds out its neighbors inside the wireless communication range with which it shares keys. Additionally, it may share keys either through broadcast or using a challenge-response technique in order to hide key-sharing patterns [43]. According to [6; 43], most key management schemes for WSN are distributed and probabilistic. An example of such a protocol based on probability is the random key scheme proposed in [44].  
Figure 1 summarizes key management approaches, presenting schemes and some examples of protocols for wireless sensor networks.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Algorithm** & \multicolumn{2}{c}{**Signature**} & \multicolumn{2}{c}{**Key exchange**} \\ \hline - & Sign & Verify & Client & Server \\ RSA-1024 & 304 & 11.9 & 15.4 & 304 \\ ECC-160 & 22.82 & 45.09 & 22.3 & 22.3 \\ RSA-2048 & 2302.7 & 53.7 & 57.2 & 2302.7 \\ ECC-224 & 61.54 & 121.98 & 60.4 & 60.4 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Analysis of energy cost for signature and key exchange of the algorithms RSAand elliptic curve cryptography (ECC) (mJ) [39].  
In general, encryption mechanisms for wireless scalar sensor networks may also be employed for wireless image sensor networks, once the particularities of visual data sensing, processing, storage and transmissions are properly considered.