| In the digital world, watermarking has gained popularity not only as a research topic but also for its critical applications. As the purpose of digital watermarks is to provide copyright protection, its association with steganography is also an interesting factor. Our goal in presenting this article is to provide the readers in understanding the design principles and application of this technology.
Until recently, information hiding techniques received very much less attention from the research community and from industry than cryptography. This situation is, however, changing rapidly with the applications of this technology in public domain. The main driving force is concern over protecting copyright; as audio, video and other works become available in digital form, the ease with which perfect copies can be made may lead to large-scale unauthorized copying, and this is of great concern to the music, film, book and software publishing industries. Billions of dollars allegedly lost to piracy of multimedia content have recently triggered the industry to rethink the way music and films are distributed on the Internet.
At the same time, moves by various governments to restrict the availability of encryption services have motivated people to study methods by which private messages can be embedded in seemingly innocuous cover messages. As encryption is vulnerable to digital or analog re-recording, currently almost all copyright protection mechanisms rely to a certain extent on watermarking. A watermark is an imperceptive secret hidden into a host signal. This article surveys the basics of technical research in the fields of watermarking.
Digital Watermark
Also referred to as simply watermarking, a pattern of bits inserted into a digital image, audio or video file that identifies the file's copyright information (author, rights, etc.). The name comes from the faintly visible watermarks imprinted on stationery that identify the manufacturer of the stationery. The purpose of digital watermarks is to provide copyright protection for intellectual property that's in digital format.
Unlike printed watermarks, which are intended to be somewhat visible, digital watermarks are designed to be completely invisible, or in the case of audio clips, inaudible. Moreover, the actual bits representing the watermark must be scattered throughout the file in such a way that they cannot be identified and manipulated. And finally, the digital watermark must be robust enough so that it can withstand normal changes to the file, such as reductions from lossy compression algorithms.
Satisfying all these requirements is no easy feat, but there are a number of companies offering competing technologies. All of them work by making the watermark appear as noise—that is, random data that exists in most digital files anyway. To view a watermark, you need a special program that knows how to extract the watermark data.
Watermarking is also called data embedding and information hiding.
Figure - 1 : An image with visible digital watermarking. The text "Brian Kell 2006" can be seen across the center of the image.
The Term
In technical terms, digital watermarking is the process of possibly irreversibly embedding information into a digital signal. The signal may be audio, pictures or video, for example. If the signal is copied, then the information is also carried in the copy. In visible watermarking, the information is visible in the picture or video. Typically, the information is text or a logo which identifies the owner of the media. The image on the right has a visible watermark. When a television broadcaster adds its logo to the corner of transmitted video, this is also a visible watermark.
In invisible watermarking, information is added as digital data to audio, picture or video, but it cannot be perceived as such (although it is possible to detect the hidden information). An important application of invisible watermarking is to copyright protection systems, which are intended to prevent or deter unauthorized copying of digital media. Steganography is an application of digital watermarking, where two parties communicate a secret message embedded in the digital signal. Annotation of digital photographs with descriptive information is another application of invisible watermarking. While some file formats for digital media can contain additional information called metadata, digital watermarking is distinct in that the data is carried in the signal itself.
The use of the word of watermarking is derived from the much older notion of placing a visible watermark on paper.
Applications
Digital Watermarking can be used for a wide range of applications such as:
1. Copyright protection: A secret copyright notice or watermark can be embedded inside an image to identify it as intellectual property. This is the watermarking scenario where the message is the watermark. The “watermark” can be a relatively complicated structure. In addition, when an image is sold or distributed an identification of the recipient and time stamp can be embedded to identify potential pirates. A watermark can also serve to detect whether the image has been subsequently modified. Detection of an embedded watermark is performed by a statistical, correlation, or similarity test, or by measuring other quantity characteristic to the watermark in a stego-image. The insertion and analysis of watermarks to protect copyrighted material is responsible for the recent surge of interest in digital steganography and data embedding.
2. Fingerprinting: In this the different recipients get differently watermarked content.
3. Broadcast Monitoring: Some of the television channels often contains watermarked video while being broadcasted.
4. Feature Tagging: Captions, annotations, time stamps, and other descriptive elements can be embedded inside an image, such as the names of individuals in a photo or locations in a map. Copying the stego-image also copies all of the embedded features and only parties who possess the decoding stego-key will be able to extract and view the features. In an image database, keywords can be embedded to facilitate search engines. If the image is a frame of a video sequence, timing markers can be embedded in the image for synchronization with audio. The number of times an image has been viewed can be embedded for “pay-perview” applications.
5. Tamper Resistance: Beyond robustness to destruction, tamper-resistance refers to the difficulty for an attacker to alter or forge a message once it has been embedded in a stego-image, such as a pirate replacing a copyright mark with one claiming legal ownership. Applications that demand high robustness usually also demand a strong degree of tamper resistance. In a copyright protection application, achieving good tamper resistance can be difficult because a copyright is effective for many years and a watermark must remain resistant to tampering even when a pirate attempts to modify it using computing technology decades in the future.
6. Covert Communication: The best example of this application is steganography. In many situations, transmitting a cryptographic message draws unwanted attention. The use of cryptographic technology may be restricted or forbidden by law. However, the use steganography does not advertise covert communication and therefore avoids scrutiny of the sender, message, and recipient. A trade secret, blueprint, or other sensitive information can be transmitted without alerting potential attackers or eavesdroppers.
Steganography
Since steganography is one of the main components of digital watermarking, let us understand the concept in brief. Steganography is the practice of encoding secret information in a manner such that the very existence of the information is concealed. Throughout history, many steganographic techniques have been documented, including the use of cleverly-chosen words, invisible ink written between lines, modulation of line or word spacing, and microdots [1,2,3]. Usually the secret information is concealed by the use of an innocuous cover as to not arouse suspicion if hostile agents discover the cover. As an example, the cover text:
I’m feeling really stuffy. Emily’s medicine wasn’t strong enough without another febrifuge.
hides the sentence “Meet me at nine” if the reader retains the second letter of each word in sequence.
Life-Cycle Phases of Watermarking
The information to be embedded is called a digital watermark, although in some contexts the phrase digital watermark means the difference between the watermarked signal and the cover signal. The signal where the watermark is to be embedded is called the host signal. A watermarking system is usually divided into three distinct steps, embedding, attack and detection. In embedding, an algorithm accepts the host and the data to be embedded and produces a watermarked signal.
The watermarked signal is then transmitted or stored, usually transmitted to another person. If this person makes a modification, this is called an attack. While the modification may not be malicious, the term attack arises from copyright protection application, where pirates attempt to remove the digital watermark through modification. There are many possible modifications, for example, lossy compression of the data, cropping an image or video, or intentionally adding noise.
Detection (often called extraction) is an algorithm which is applied to the attacked signal to attempt to extract the watermark from it. If the signal was unmodified during transmission, then the watermark is still present and it can be extracted. In robust watermarking applications, the extraction algorithm should be able to correctly produce the watermark, even if the modifications were strong. In fragile watermarking, the extraction algorithm should fail if any change is made to the signal.
Figure - 2: General watermark life-cycle phases with embedding, attacking and detection/retrieval functions.
Classification
A digital watermark is called robust with respect to transformations if the embedded information can reliably be detected from the marked signal even if degraded by any number of transformations. Typical image degradations are JPEG (Joint Photographic Experts Group) compression, rotation, cropping, additive noise and quantization. For video content temporal modifications and MPEG (Moving Picture Experts Group) compression are often added to this list. A watermark is called imperceptible if the cover signal and marked signal are indistinguishable with respect to an appropriate perceptual metric. In general it is easy to create robust watermarks or imperceptible watermarks, but the creation of robust and imperceptible watermarks has proven to be quite challenging. Robust imperceptible watermarks have been proposed as tool for the protection of digital content, for example as an embedded 'no-copy-allowed' flag in professional video content.
Digital watermarking techniques can be classified in several ways. The most important of them are
1. Robustness: A watermark is called fragile if it fails to be detected after the slightest modification. Fragile watermarks are commonly used for tamper detection (integrity proof). Modifications to an original work that are clearly noticeable are commonly not referred to as watermarks, but as generalized barcodes.
A watermark is called semi-fragile if it resists benign transformations but fails detection after malignant transformations. Semi-fragile watermarks are commonly used to detect malignant transformations. A watermark is called robust if it resists a designated class of transformations. Robust watermarks may be used in copy protection applications to carry copy and access control information.
Here, robustness refers to the ability of embedded data to remain intact if the stego-image undergoes transformations, such as linear and non-linear filtering, addition of random noise, sharpening or blurring, scaling and rotations, cropping or decimation, lossy compression, and conversion from digital to analog form and then reconversion back to digital form (such as in the case when a hard copy of a stego-image is printed and then a digital image is formed by subsequently scanning the hardcopy.) Robustness is critical in copyright protection watermarks because pirates will attempt to filter and destroy any watermarks embedded in images. Anti-watermarking software is already available on the Internet and have been shown effective in removing some watermarks. These techniques can also be used to destroy the message in a stego-image.
2. Perceptibility / Perceptual Transparency: A watermark is called imperceptible if the original cover signal and the marked signal are (close to) perceptually indistinguishable. A watermark is called perceptible if its presence in the marked signal is noticeable, but non-intrusive.
The act of hiding the message in the cover necessitates some noise modulation or distortion of the cover image. It is important that the embedding occur without significant degradation or loss of perceptual quality of the cover. In a secret communications application, if an attacker notices some distortion that arouses suspicion of the presence of hidden data in a stegoimage, the steganographic encoding has failed even if the attacker is unable to extract the message. Preserving perceptual transparency in an embedded watermark for copyright protection is also of paramount importance because the integrity of the original work must be maintained.
For applications where the perceptual transparency of embedded data is not critical, allowing more distortion in the stego-image can increase hiding capacity, robustness, or both.
3. Hiding Capacity: Hiding capacity is the size of information that can be hidden relative to the size of the cover. A larger hiding capacity allows the use of a smaller cover for a message of fixed size, and thus decreases the bandwidth required to transmit the stego-image.
The length of the embedded message determines two different main classes of watermarking schemes:
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The message is conceptually zero-bit long and the system is designed in order to detect the presence or the absence of the watermark in the marked object. This kind of watermarking schemes is usually referred to as Italic zero-bit or Italic presence watermarking schemes. Sometimes, this type of watermarking scheme is called 1-bit watermark, because a 1 denotes the presence (and a 0 the absence) of a watermark.
4. Embedding method: A watermarking method is referred to as spread-spectrum if the marked signal is obtained by an additive modification. Spread-spectrum watermarks are known to be modestly robust, but also to have a low information capacity due to host interference.
A watermarking method is said to be of quantization type if the marked signal is obtained by quantization. Quantization watermarks suffer from low robustness, but have a high information capacity due to rejection of host interference.
A watermarking method is referred to as amplitude modulation if the marked signal is embedded by additive modification which is similar to spread spectrum method but is particularly embedded in spatial domain.
Evaluation / Benchmarking
The evaluation of digital watermarking schemes can provide detailed information for watermark designer or end users. Therefore, different evaluation strategies exists. Often used from watermark designer is the evaluation of single properties to show, for example, an improvement. End-users, are mostly not interested in detailed information. They want to know if a given digital watermarking algorithm can be used for their application scenario, and if yes, which parameter sets seems to be the best.
Conclusion
An overview of digital watermarking was presented along with applications that can benefit from the technology. Features of water marking systems were also discussed, followed by general descriptions of how current systems work. As technology advances, immense research in steganography continues to expand the perceptual transparency, robustness and capacity of information hiding systems.
Since ancient times, man has found a desire in the ability to communicate covertly. The recent explosion of research in watermarking to protect intellectual property is evidence that steganography or image hiding is not just limited to military or espionage applications. Digital watermarking, like cryptography, will play an increasing role in the future of secure communication in the “digital world.”
—By: R. Manoj. The author is an Assistant Editor at Fanatic Media, Bangalore. He is also an Independent Researcher, specializing in Software Security. He has an active interest in designing security algorithms for securing softwares. He can reached at infosecurity@fanaticmedia.com |
