## 1 Introduction  
There is a large number of areas where the security of multimedia content is crucial for ensuring privacy and citizens' rights in general. Digital watermarking is an efficient and versatile technical means for embedding secret information into multimedia objects, such as still images, videos, and audio files. An example of such secret, sensitive information can be medical data related to patients. Watermarking technology can assure protection of the digital content against unauthorized access, tampering, sensitive information disclosure, or copyright infringement. Methods based on watermarking may be also used for such applications as steganography and pseudonymization of private data. A graphic or audio file marked in this way can help locate websites or FTP servers where these files are unlawfully shared. As a result, a digital watermark now has high hopes for an effective fight against fraud. The efficient watermark should be characterized by the following features: Imperceptibility--the watermark should be imperceptible to the human eye, and the inserted information should not deteriorate the visual quality of an original image. Robustness--the watermark is detectable even after the original image transformation and is difficult to be removed. Consideration of local image properties--the watermark is inserted with varying intensity in different areas, depending on the characteristics of the area (e.g., brightness) Watermark decoding method--the watermark can be read based on the watermarked image only, without the need to verify against the original image. Image watermarking can be performed in the spatial or transform domain. Spatial domain methods usually result in direct modifications of image data, such as color bands,and brightness. The common method for embedding a watermark in the spatial domain is the Least Significant Bit (LSB) method where the secret information is inserted into the original image by modifying or replacing the least significant bits of pixels. On the other hand, transform-based techniques rely on changing spectral factors in the domain of a specific transform. To retrieve the image with an embedded watermark one needs to perform the corresponding inverse transform operation. Watermarks embedded in the transform domains are typically more reliable in comparison with the watermarks inserted in the spatial domain [1; 2]. The most widely used transforms used in digital watermarking include discrete cosine transform (DCT) [3; 4; 5; 6], discrete wavelet transforms (DWT) [7; 8] and discrete Fourier transform (DFT) [9; 10]. Combination of different transform methods can be implemented, (e.g., DCT and DWT transform) [11; 12; 13]. Additionally, transform-based techniques can be used jointly with other methods, such as, (e.g., singular value decomposition (SVD) [14] or discrete fractional random transform (DFRNT)) [2]. There are also new approaches that apply novel types of transforms that are orthogonal and can be parameterized [15]. In [16], [PERSON] et al. presented a data hiding scheme based on LSB modification in the Piecewise-Linear Haar transform for audio signals. [PERSON] et al. in [17] proposed a reversible data hiding method dedicated to images using symmetrical histogram expansion also in the domain of this transform. However, Periodic Haar Piecewise-Linear (PHL) transform is only mentioned in the literature with regard to image compression tasks [18]. For obvious reasons medical images are private to the patient and authorized medical personnel and should be protected from unauthorized viewers. One method to protect such images is using cryptography including traditional symmetric cryptosystems and biometrics [19; 20; 21]. Digital content, in particular this related to medical images, is more and more often protected by a combination of tools, such as encryption and watermarking. As defined in [22] encryption algorithms can be considered as an \"a priori\" protection mechanism since once data is decrypted, it is no longer protected.