Digital watermarking is the process of embedding information into a digital signal which may be used to verify its authenticity or the identity of its owners, in the same manner as paper bearing a
watermarkA watermark is a recognizable image or pattern in paper that appears as various shades of lightness/darkness when viewed by transmitted light , caused by thickness or density variations in the paper...
for visible identification. In digital watermarking, the signal may be audio, pictures, or video. If the signal is copied, then the information also is carried in the copy. A signal may carry several different watermarks at the same time.
In
visible digital 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 also is a visible watermark.
In
invisible digital watermarking, information is added as digital data to audio, picture, or video, but it cannot be perceived as such (although it may be possible to
detect that some amount of information is hidden in the signal). The watermark may be intended for widespread use and thus, is made easy to retrieve or, it may be a form of
steganographySteganography is the art and science of writing hidden messages in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message, a form of security through obscurity...
, where a party communicates a secret message embedded in the digital signal. In either case, as in visible watermarking, the objective is to attach ownership or other descriptive information to the signal in a way that is difficult to remove. It also is possible to use hidden embedded information as a means of covert communication between individuals.
One application of watermarking is in
copyrightCopyright is a legal concept, enacted by most governments, giving the creator of an original work exclusive rights to it, usually for a limited time...
protection systems, which are intended to prevent or deter unauthorized copying of digital media. In this use, a copy device retrieves the watermark from the signal before making a copy; the device makes a decision whether to copy or not, depending on the contents of the watermark. Another application is in
source tracing. A watermark is embedded into a digital signal at each point of distribution. If a copy of the work is found later, then the watermark may be retrieved from the copy and the source of the distribution is known. This technique reportedly has been used to detect the source of illegally copied movies.
Annotation of digital photographs with descriptive information is another application of invisible watermarking.
While some file formats for digital media may contain additional information called
metadataThe term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
, digital watermarking is distinctive in that the data is carried right in the signal.
Applications
Digital watermarking may be used for a wide range of applications, such as:
- Copyright protection
Copy protection, also known as content protection, copy obstruction, copy prevention and copy restriction, refer to techniques used for preventing the reproduction of software, films, music, and other media, usually for copyright reasons.- Terminology :Media corporations have always used the term...
- Source tracking (different recipients get differently watermarked content)
- Broadcast monitoring (television news often contains watermarked video from international agencies)
- Covert communication
Digital watermarking life-cycle phases
The information to be embedded in a signal 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.
Then the watermarked digital signal is 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 (in which resolution is diminished), 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 still is present and it may be extracted. In
robust digital watermarking applications, the extraction algorithm should be able to produce the watermark correctly, even if the modifications were strong. In
fragile digital watermarking, the extraction algorithm should fail if any change is made to the signal.
Classification
A digital watermark is called
robust with respect to transformations if the embedded information may be detected reliably from the marked signal, even if degraded by any number of transformations. Typical image degradations are JPEG compression, rotation, cropping, additive noise, and
quantizationQuantization, in mathematics and digital signal processing, is the process of mapping a large set of input values to a smaller set – such as rounding values to some unit of precision. A device or algorithmic function that performs quantization is called a quantizer. The error introduced by...
. For video content, temporal modifications and MPEG compression often are added to this list. A digital watermark is called
imperceptible if the watermarked content is perceptually equivalent to the original, unwatermarked content. 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 may be classified in several ways.
Robustness
A digital watermark is called
fragile if it fails to be detectable after the slightest modification. Fragile watermarks are commonly used for tamper detection (integrity proof). Modifications to an original work that clearly are noticeable, commonly are not referred to as watermarks, but as generalized
barcodeA barcode is an optical machine-readable representation of data, which shows data about the object to which it attaches. Originally barcodes represented data by varying the widths and spacings of parallel lines, and may be referred to as linear or 1 dimensional . Later they evolved into rectangles,...
s.
A digital watermark is called
semi-fragile if it resists benign transformations, but fails detection after malignant transformations. Semi-fragile watermarks commonly are used to detect malignant transformations.
A digital 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 no access control information.
Perceptibility
A digital watermark is called
imperceptible if the original cover signal and the marked signal are (close to) perceptually indistinguishable.
A digital watermark is called
perceptible if its presence in the marked signal is noticeable, but non-intrusive.
Capacity
The length of the embedded message determines two different main classes of digital watermarking schemes:
- 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 scheme is usually referred to as zero-bit or 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.
- The message is a n-bit-long stream (
, with 
) or 
and is modulated in the watermark. These kinds of schemes usually are referred to as multiple-bit watermarking or non-zero-bit watermarking schemes.
Embedding method
A digital watermarking method is referred to as
spread-spectrumSpread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
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
interferenceIn communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
.
A digital 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 digital watermarking method is referred to as
amplitude modulationAmplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
if the marked signal is embedded by additive modification which is similar to spread spectrum method, but is particularly embedded in the spatial domain.
Evaluation and benchmarking
The evaluation of digital watermarking schemes may provide detailed information for a watermark designer or for end-users, therefore, different evaluation strategies exist. Often used by a watermark designer is the evaluation of single properties to show, for example, an improvement. Mostly, end-users are not interested in detailed information. They want to know if a given digital watermarking algorithm may be used for their application scenario, and if so, which parameter sets seems to be the best.
Cameras
Epson and Kodak have produced cameras with security features such as the Epson PhotoPC 3000Z and the Kodak DC-290. Both cameras added irremovable features to the pictures which distorted the original image, making them unacceptable for some applications such as forensic evidence in court. According to Blythe and Fridrich, "[n]either camera can provide an undisputable proof of the image origin or its author".
A secure digital camera (SDC) was proposed by Mohanty, et al. in 2003 and published in January 2004. This was not the first time this was proposed. Blythe and Fridrich also have worked on SDC in 2004 for a
digital cameraA digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...
that would use lossless watermarking to embed a biometric identifier together with a cryptographic hash.
Reversible data hiding
Reversible data hiding is a technique which enables images to be authenticated and then restored to their original form by removing the digital watermark and replacing the image data that had been overwritten. This would make the images acceptable for legal purposes. The U.S. Army also is interested in this technique for authentication of
reconnaissanceReconnaissance is the military term for exploring beyond the area occupied by friendly forces to gain information about enemy forces or features of the environment....
images.
Watermarking for relational databases
Digital watermarking for relational databases emerged as a candidate solution to provide copyright protection, tamper detection, traitor tracing, maintaining integrity of relational data. Many watermarking techniques have been proposed in the literature to address these purposes. A survey of the current state-of-the-art and a classification of the different techniques according to their intent, the way they express the watermark, the cover type, the granularity level, and their verifiability, can be found in
jucs.org.
See also
- Audio watermark detection
Watermarking is the process of embedding information into a signal in a way that is difficult to remove. If the signal is copied, then the information is also carried in the copy. A signal may carry several different watermarks at the same time...
- Coded Anti-Piracy
Coded Anti-Piracy is an anti-copyright infringement technology which marks each film print of a motion picture with a distinguishing patterns of dots, used as a forensic identifier to identify the source of illegal copies....
- Copy attack
The copy attack is an attack on certain digital watermarking systems proposed by M. Kutter, S. Voloshynovskiy, and A. Herrige in a paper presented in January, 2000 at the Photonics West SPIE convention....
- EURion constellation
The EURion constellation is a pattern of symbols found on a number of banknote designs worldwide since about 1996. It is added to help software detect the presence of a banknote in a digital image. Such software can then block the user from reproducing banknotes to prevent counterfeiting using...
- Pattern Recognition (novel)
Pattern Recognition is a novel by science fiction writer William Gibson published in 2003. Set in August and September 2002, the story follows Cayce Pollard, a 32-year-old marketing consultant who has a psychological sensitivity to corporate symbols...
- Steganography
Steganography is the art and science of writing hidden messages in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message, a form of security through obscurity...
- Watermark (data file)
A watermark stored in a data file refers to a method for ensuring data integrity which combines aspects of data hashing and digital watermarking. Both are useful for tamper detection, though each has its own advantages and disadvantages.- Data hashing :...
- Watermark detection
Further reading
External links