The Digital Image Watermarking

Published Date: 02 Nov 2017

The transmission of information takes place in different forms and is used in many applications. In a large number of these applications, it is desired that the communication to be done in secret. Such secret communication includes transfer of medical data, bank transfers, corporate communications, purchases using bank cards, a large percentage of everyday emails and etc.

N.Provos and P.Honeyman [26] says that steganography is different from cryptography and watermarking, although they all have overlapping usages in the information hiding process. Steganography security hides the knowledge that there is information in the cover medium, where cryptography reveals this knowledge but encodes the data as cipher-text. Similar to steganography, watermarking is about hiding information in other image, but difference is that watermark must be somewhat resilience against attempts to remove it. This technique of information hiding can be extended to copyright protection of multimedia content. Digital watermarking and steganography techniques are used to address digital right management, protect information, and conceal secrets. Information hiding techniques provide an interesting challenge for digital forensic investigations [23].

DIGITAL STEGANOGRAPHY

The word steganography comes from the Greek word Steganos, which means covered or protected, and – the word graphy, which means writing or drawing. Therefore, steganography means that literally, covered writing. Steganography is the technique of hiding information such that its presence cannot be detected and a communication is happening [27]. The advantage of steganography over cryptography is that messages do not attract attention to themselves. Therefore, whereas cryptography protects the content of a message, steganography can be said to protect both messages and communicating parties [23].

2.1.1 Properties of Steganography

All the steganographic algorithms need to fulfill the following basic requirements.

Invisibility- The invisibility of steganographic algorithm is the first and foremost requirement, since the steganography lies in its ability to be unnoticed by the human eye.

Payload Capacity- Steganography aims at hidden communication and therefore requires sufficient embedding capacity.

Robustness against Stastical Attacks- Statistical steganalysis is the practice of detecting hidden information through applying statistical tests on image data.

Independent of file format-The most powerful steganographic algorithms lies in the ability to embed information in any type of file.

2.1.2 Applications of Steganography

To have secure secret communication, where strong cryptography is not possible. In military applications, where even the knowledge that two parties communicate can be of large importance.

2.2 DIGITAL IMAGE WATERMARKING

Digital image watermarking is a kind of technology that embeds copyright information into multimedia content. An effective image watermarking mainly includes watermark generation, watermark embedding, watermark detection, and watermark attack [5], [1]. Digital image watermarking provides copyright protection to image by hiding appropriate information in original image to declare rightful ownership [12]. There are four essential factors those are commonly used to determine quality of watermarking scheme. They are robustness, imperceptibility, capacity, and blindness. Robustness is a measure of immunity of watermark against attempts to image modification and manipulation like compression, filtering, rotation, scaling, noise attacks, resizing, cropping etc. Imperceptibility is the quality that the cover image should not be destroyed by the presence of watermark. Capacity includes techniques that make it possible to embed majority of information. Extraction of watermark from watermarked image without the need of original image is referred to as blind watermarking. The non-blind watermarking technique requires that the original image to exist for detection and extraction. The semi-blind watermarking scheme requires the secrete key and watermark bit sequence for extraction. Another categorization of watermarks based on the embedded data is visible or invisible [6], [25]. Based on the robustness of the watermarks, watermarks are classified as robust watermarks, fragile watermarks and semi-fragile watermarks. Robust watermarks can resist malicious distortions, whereas fragile watermarks can easily destroyed by all image distortions and semi-fragile watermarks can be destroyed by certain type of distortions while resisting other minor changes.

The main applications of digital image watermarking include Digital Rights Management (DRM)/ Owner Identification, copyright protection and authentication. DRM can be defined as "the description, identification, trading, protecting, monitoring and tracking of all forms of usages over tangible and intangible assets [19]. It concerns the management of digital rights and enforcement of rights digitally. Copyright enables the identification of the copyright holder and thus protects rights in content distribution. It is used to prevent third party from copying or claiming the ownership of the multimedia content. Authentication in image watermarking refers to the integrity assurance of the image.

From application point of view, robust watermarks are suitable for copyright protection, because they can resist common image processing operations. On the other hand, fragile watermarks can be used to detect tampering and authenticate an image, because it is sensitive to changes. Semi-fragile watermarks are in some special cases of authentication and tamper detection.

According to the domain of watermark insertion, the watermarking techniques fall into two categories: spatial domain methods and transform domain methods. Many techniques have been proposed in the spatial domain such as LSB (Least Significant Bit) insertion method, the patch work method and the texture block coding method [6]. These techniques process the location and luminance of the image pixel directly. The LSB method has a major disadvantage that the least significant bits may be easily destroyed by lossy compression. Transform domain method based on special transformations, and process the coefficients in frequency domain to hide the data. Transform domain methods include Fast Fourier Transform (FFT),Discrete Cosine Transform(DCT), Discrete Wavelet Transform(DWT),Curvelet Transform(CT), Counterlet Transform(CLT) etc. In these methods the watermark is hidden in the high and middle frequency coefficients of the cover image. The low frequency coefficients are suppressed by filtering as noise, hence watermark is not inserted in low frequency coefficients [6]. The transform domain method is more robust than the spatial domain method against compression, filtering, rotation, cropping and noise attacks.

2.2.1 Properties of Digital Image Watermarking

The efficiency of a digital image watermarking process is evaluated according to the properties of perceptual transparency, robustness, computational cost, bit rate of data embedding process, false positive rate, recovery of data with or without access to the original image, the speed of imbedding and retrieval process, the ability of embedding and retrieval module to integrate into standard encoding and decoding process etc. [27-29].

To understand watermarking methods and determine their applications, one needs to know the properties of digital image watermarking.

Robustness- of a watermark refers to its ability to withstand non-malicious distortions. The watermarking scheme should be robust to any possible signal processing operations, as long as the quality of the watermarked data preserved.

Data Payload- is the encoded message size of a watermark in an image. On the other hand, multi-bit watermarks can carry textual or pictorial information [27].

Capacity- is the amount of information in an image. If multiple watermarks are embedded into an image, then the watermarking capacity of the image is the sum of all individual watermarks data payload [27].

While the robustness of the watermarking method increases, the capacity also increases where the imperceptibility decreases. There is a tradeoff between these requirements and this tradeoff should be taken into while the watermarking method is being proposed [19].

Imperceptibility – is the characteristic of hiding of the watermark so that it does not degrade the visual quality of an image. The imperceptibility of the watermark is tested by peak signal to noise ratio.

Fidelity- is the visual similarity between the watermarked image and its cover image.

Security- of the watermark is the ability of the watermark to resist malicious attacks. These attacks include intentional operations of another watermark insertion, modification, removal and estimation which aim at defeating the purpose of the watermarks.

Computational cost-is the measure of computing resources required to perform watermark embedding or detection processes. It can be measured using the processing time for a given computer configuration.

There are several ways of classifying watermarking methods. One of the widely adopted classifications is based on watermark robustness. Under this classification, watermark can be grouped into 3 types:

Robust watermarks are watermarks that can resist malicious distortions.

Fragile watermarks are easily destroyed by all image distortions.

Semi-fragile watermarks can be destroyed by certain types of distortions while resisting other minor changes.

Besides watermark robustness, watermarks can also be categorized into visible and invisible types. Visible watermarks are perceptible to a viewer. On the other hand, watermarks are imperceptible and do not change the visual appearance of the images.

Depending upon the application, the properties, which are used mainly in the evaluation process, vary. For example, in the video indexing application, evaluating the robustness of a watermarking scheme to any signal processing is meaningless, since there is no case that the video passes through some signal processing operation. In the covert communication application, it is better to use a watermarking scheme that does not need the original data during the watermark detection process, if real television broadcasting is used as the communication channel, while most of the watermarking schemes in other applications need the original data during the detection process. If the application is the copyright protection, the other owner of the original data may wait for several days to insert or detect watermark, if the data is valuable for the owner. On the other hand, in a broadcast monitoring application, the speed of the watermark detection algorithm should be as fast as the speed of real time broadcasting. As a result, each watermarking application has its own requirements and the efficiency of the watermarking scheme should be evaluated according to these requirements [30].

The owner of the original data wants to prove his ownership in case of original data is copied, edited and used without permission of the owner. In the watermarking research world, this problem has been analyzed in a more detailed manner [19].

2.2.2 Applications of Digital Image Watermarks

Digital image watermarking techniques have been proposed to be implemented in many applications. Some major groups of its applications are:

Digital Rights Management(DRM)/Owner identification

DRM can be defined as the description, identification, trading, protecting, monitoring and tacking of all forms of usages over tangible and intangible assets. It concerns the management of digital rights and enforcement of rights digitally.

Copyright protection

It enables the identification of the copyright holder and thus protects the rights in content distribution. It is used to prevent third parties from copying or claiming the ownership of the digital media. Robust watermarks are embedded into an image to protect the rights of the owners. It should be possible to detect the watermark despite common image processing, geometrical distortions, image compression, and many other image manipulations. The successful detection of the watermark can positively identify the owner.

Authentication

Authentication in image watermarking refers to the integrity assurance of the image. The applications related to image authentication are the validation of cultural heritage paintings, medical records and digital artworks.

Other Applications:

There are many other applications where digital image watermarking methods have been proposed as a technology enabling tool. Some of them are:

Broadcast monitoring- watermarks embedded into advertisement sections of broadcast. It is used to track the broadcast of a particular file over a channel.

Device control- watermarks embedded into radio and television signals can be used to control features of a receiver.

Medical Applications- used in X-ray film references where they are marked with a unique ID of the patient.

Fingerprinting- to convey information about the recipient of the digital media.

Copy control- watermarks detected in a video content are used to control the functionality of a watermark complaint recorder.

Application wise robust watermarks are suitable for copyright protection, because they can resist common image processing operations. On the other hand, fragile watermarks can be used to detect tampering and authenticate an image, because it is sensitive to changes. Semi-fragile watermarks are usually applied in some special cases of authentication and tamper detection. These cases may consider lossy image compression as legitimate changes while highlighting geometrical distortions as intentional attacks.

2.2.3 Key differences between watermarking and Steganography

Digital Image Watermarking

Inserts information related to either to host signal or its owner.

Main goals are copyright protection and information authentication.

It is either visible or imperceptible.

It is for communications point-to-multiple points.

Capacity is not an important issue

Robustness is an important issue

Digital Steganography

Must not only be imperceptible but also statistically undectable.

Is for point-to-point communications.

Main goal is covert communication.

Inserts any kind of information.

Capacity is an important issue.

May or may not be robust.