## 5 Watermarking  
Cryptography is an effective way to provide confidentiality, integrity and authenticity in different types of data networks. However, processing, memory and energy constraints in wireless sensor  
Figure 6: Discrete wavelet transform (DWT) coding generating one and two levels of resolution [57]. networks may be too stringent for some cryptography algorithms. This context fosters the adoption of other security mechanisms for WISN, such as watermarking. Many image monitoring applications will require authentication mechanisms. The major challenge for data verification and authentication is to exactly know which sensor nodes are malicious. False visual data may be inserted into the network, and source nodes may be subject to tampering attacks. In such a way, some mechanism should be employed to authenticate received data at the sink side. The watermarking technique comes then as a way to provide a lightweight mechanism for authentication in wireless sensor networks. In general, any image transmission over wireless sensor networks may be protected using watermarks. Actually, this technique has been used in many visual applications on the Internet, but it may also bring significant results for WISN. In short, 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. The watermarking process will hide authentication information in original data, and the marks may be visible or not. In a secure context, watermarks must not be detected or removed by attackers, and there are different ways to provide such protection [62]. The watermarking process is composed of three general stages: generation, embedding and detection [62]. The generation process depends on the nature of the considered sensed data, and it may require complex computation, where a watermarking key is generally employed. The embedding process will insert the computed watermark into the desired data. Such a process may be performed exploiting characteristics of transforms, as in discrete cosine transform (DCT), DWT, discrete Fourier transform (DFT), among others. As popular image codecs are based on such transforms, such as JPEG (DCT) and JPEG2000 (DWT), image watermarking may be thought of as an effective authentication resource for wireless sensor networks. At last, the detection process is responsible for detecting and extracting watermarks. Figure 7 presents a general scheme for image watermarking. For wireless image sensor networks, watermarks may be valuable as authentication information, which may be desired in many monitoring environments. Moreover, sensitive data may be embedded as watermarks in unprotected data, as for example when scalar data are transmitted as watermarks \"inside\" multimedia data [62]. Nevertheless, the resource-constrained nature of typical sensor nodes demands efficient mechanisms when employing watermarking for image transmissions, which should be optimized for that context. Many works have investigated watermarking in wireless sensor networks [63]. Depending on the nature of data being protected, the watermarking technique may have different characteristics. For scalar  
Figure 7: Watermarking in wireless image sensor networks (WISN). data, watermarks may impact transmitted data, as additional information may be added to the network. In [64], the authors propose a reversible watermarking technique for scalar-sensed data. Their idea is to remove the watermarks after successful verification at the sink side, allowing processing of original data without undesired additional information. The same concern is presented in [65], where watermarking is performed considering control information within data packets. Doing so, original data are preserved after authentication. For visual data, watermarks may be \"invisible\" for people, not affecting processing. In [66], the authors investigate data watermarking in wireless multimedia sensor networks, raising the particularities that should be considered.