# Image Watermarking Scheme Using LSB and Image Gradient
###### Abstract  
In the modern age, watermarking techniques are mandatory to secure digital communication over the internet. For an optimal technique, a high signal-to-noise ratio and normalized correctional is required. In this paper, a digital watermarking technique is proposed on the basis of the least significant bit through an image gradient and chaotic map. The image is segmented into noncorrelated blocks, and the gradient of each block is calculated. The gradient of the image expresses the rapid changes in an image. A chaotic substitution box (S-Box) is used to scramble the watermark according to a piecewise linear chaotic map (PWLCM). PWLCM has a positive Lyapunov exponent and better balance property as compared to other chaotic maps. This S-Box technique is capable of producing a disperse sequence with high nonlinearity in the generated sequence. Least significant bit is a simple technique for embedding but it has a high payload capacity and direct pixel manipulation. The embedding payload introduces a tradeoff between robustness and imperceptibility; hence, the image gradient is a technique to identify the best-suited place to embed a watermark and avoid image degradation. By modifying the least significant bits of the original image, the watermark signal is embedded according to the image gradient. In the image gradient, the direction and magnitude decide how much embeding can be done. In comparison with other methods, the experimental results show satisfactory progress in robustness against several image processing and geometrical attacks while maintaining the imperceptibility of the watermark signal. substitution box; chaotic map; piecewise linear chaotic map; least significant bit; image gradient +
Footnote †: journal: Medical Image  
watermarking related to the type of data being watermarked. Image watermarking is further categorized into spatial-domain watermarking and transform-domain watermarking. Watermarks can be embedded in the spatial domain by manipulating pixel strength of the cover image, while watermarks in the transform domain are embedded by manipulating the transform coefficient [1]. Watermarking in the spatial domain is easy to implement and has high perceptual quality. The simplest method in the spatial domain is least significant bit substitution. Transform-domain techniques are more robust at the cost of computational overhead. The common methods in the transform domain are discrete wavelet transform (DWT) and discrete cosine transform (DCT), but the choice of transform depends on the complexity, cost of implementation, and speed of operation [2]. The imperceptibility and robustness of the scheme depend on the embedded watermark capacity. In common signal operations such as filtering, JPEG compression, geometric attacks, and addition of noise, watermarking techniques have shown satisfactory progress [3]. To manipulate information is the main purpose of attack. The goal of the watermarking technique is robustness, high capacity, and imperceptibility [4,5]. Depending on the embedding purpose, the watermarking technique can be divided into robust and fragile watermarking. A fragile watermark is destroyed after a slight modification and used for authentication and tamper detection. A robust watermark is strong and has the ability to resist slight modification, which can be used for copyright protection and owner confirmation. Base on the type of watermark insertion, techniques can be classified into visible and invisible watermarking. In visible watermarking, human eyes perceive the watermark without extraction, while, in invisible watermarking, it cannot be perceived without extraction. According to watermark detection, watermarking practices are grouped into blind, nonblind, and semi-blind. In blind watermarking, watermark is only visible by using secret key [6]; however, in semi-blind and nonblind watermarking, extra information is needed for extraction [7].