## 2 Related Work
### Security in WMSNs  
Several surveys [19; 20; 21] have been conducted for security protocols in WSN in general. They discussed the existing security mechanisms and reviewed the proposed security protocols in authentication and encryption, secure routing, key management and distribution, and intrusion detection systems. Most of the Preexisting secure protocols -as shown in this subsection- have very strong requirements, like having different shared keys between each of the nodes and the base station or hardware modification, which are arguably too strong requirements for those proposals to be feasible.The following works discussed security implementations in WMSNs:  
A security scheme is proposed in [22] for collaborative image transmission from image sensors to cluster heads where heterogeneous cluster-based wireless image sensor network architecture is assumed. The proposed scheme does not depend on a key-based security management, but it uses an approach for secret image sharing on multiple node-disjoint paths for image delivery. In this approach, the image is separated into overlapped and non-overlapped regions where the later is directly transmitted without encryption after compression, while the former is conveyed via secret sharing and transmitted with appropriate distribution ratio via multiple paths. The proposed scheme also exploits the inter-image correlation among sensors sharing overlapped regions in order to achieve high-security image transmission and to be more energy efficient. In order to assure authentication and data integrity for multimedia data delivered over WMSN, an energy-aware adaptive wavelet-based watermarking technique for real time image delivery is presented in [23]. This technique embeds additional data called a watermark into some location in an image object so it can be detected later to make an affirmation about the object. The watermarking locations or positions are adaptively chosen by using two thresholds to insert the watermark according to network conditions so that the energy efficiency and security can be achieved. In order to degrade the effect of the distortion of watermarked image, the proposed scheme embeds the watermark into few positions as possible to make it invisible and allocates extra network resources to protect this embedded watermark from high distortion so it can be detectable. In addition, it also embeds watermark coding redundancies into the original image so the watermark becomes more robust to packet loss. The frequency-based (middle band) discrete wavelet transform (DWT) has been selected because it is more robust, easy to recognizable and authenticated at the receiver side, and it reduces computation complexity and process delay by exploiting the correlation of the inter-frames. In [24], a privacy paradigm called HoLiSTiC is proposed that secures routing and topology information for WMSNs against outsider attacks. The paper assumes a clustered network with some nodes equipped with free-space optical (FSO) capabilities. Also it assumes that the BS and CHs have bidirectional communication links and the camera and transport nodes have unidirectional links. The proposed protocol requires that each node to have an individual key shared with the BS and pairwise keys shared between adjacent visual nodes in a cluster. In addition, every network entity has two pre-deployed network-wide keys. All keys are employed for symmetric cryptography to provide a variety of security services. A secure data converter architecture is proposed in [25] for WMSN that employs fingerprinting and encryption capabilities for simultaneously digitize and authenticate sensor readings. The proposed architecture suggested hardware modifications to the data converter and aims to reduce the computational complexity of the security algorithms at the aggregation points in the systems that need in-network processing.