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Defense & Security

Digital watermarking: perfecting the art of security

From OE Reports Number 191 - November 1999
31 November 1999, SPIE Newsroom. DOI: 10.1117/2.6199911.0001

Figure 1. Original image (left), image with Digimark watermark (center), exaggerated view of imperceptible digital message (right).

The World Wide Web is, in many respects, a world without barriers. Its openness is highly appealing, if not downright noble. Unfortunately, unscrupulous people can take advantage of this electronic latitude to unlawfully take what is not theirs.

On the web, hard-won intellectual property (IP) becomes a pirate's booty for hackers and less-than-ethical businesses. Efforts aimed at protecting IP are limited to pop-up windows and gateways that show lengthy legal documents establishing ownership of digital files. However, while displaying this text establishes ownership, it does not provide enforcement or any real method of tracking stolen IP in the digital world.

The lack of online security for IP has many of the world's consumer electronics, computer, and entertainment companies nervous. In response, companies such as Sony, Hitachi, Pioneer, and Phillips are investigating ways to imperceptibly mark and track their IP should it be stolen or misused.

Watermarks without paper

Traditional watermarking techniques used in currency involve changing the density of the paper to create a mark that cannot be duplicated or erased without destroying the note itself. Digital watermarking takes a similar track by changing the image, movie, or music in the spatial or frequency domains in a way that only machines can detect. More than that, the coded information remains with the picture regardless of whether the image is printed, packaged, and transmitted across the Internet or, in some cases, compressed.

Digimarc Corp. (Lake Oswego, OR) is applying digital watermarking to applications, which not only protects digital files but also provides management mechanisms while enhancing a company's marketing abilities.

Watermark information can include just about anything. Digimarc President Bruce Davis said a watermark payload varies from several to100 bits of data. The block size is established by the need to repeat the watermark information throughout the image, movie, or musical recording. "The more times you repeat the data, the stronger the signal and the more robust the watermark is to typical image operations," Perry said.

When the image is viewed by a Digimarc-enabled software program, such as Corel PhotoPaint or Adobe Photoshop, the payload information automatically appears in a dialogue box. It can be as simple as a copyright year or a link to a website with information about who owns the picture.

In addition to tagging the image, the watermark can act as a beacon for sophisticated search programs, such as Digimarc's MarcSpider. MarcSpider constantly searches the web for watermarked images, noting their location and use. The information is stored and supplied to clients so that they know when their image has been posted or used, enabling them to contact the website owner about licensing arrangements or potential copyright infringement. "It doesn't disable their computer, though," Davis said. "People have actually asked me about that."

Figure 2. When opened by a watermark-enabled software program, information about the proper legal uses for the image and hyperlinks to ownership information automatically open in a special diaglogue window. In addition to establishing ownership, this can also be used by marketing departments to draw customers directly to a company's web site for special information or offers.
Embedding the watermark

Perry said digital watermarking uses one of two methods to embed a payload. One way involves special algorithms that can change the intensity or luminance of individual pixels of a digital picture to encode a predetermined number of bits of embedded, imperceptible data. The other method involves changing the image in the frequency domain. By slightly altering the discrete cosine transform (DCT) coefficients at the heart of the compression algorithms used in MPEG and JPEG, Digimarc encodes the watermark data into the picture without perceptibly changing the picture.

Compression algorithms represent the greatest hurdle and danger to a watermark. While a watermark will remain in a picture even if printed into hardcopy, lossy compression techniques invariably throw away some of the image data, and therefore, part of the watermark. For MPEG, one of the most popular video stream compression techniques, Digimarc countered the possibility of data loss through compression by embedding the payload in the lower frequencies (MPEG and JPEG tend to quantize or 'cut away' high frequency data that is less perceptible to the human eye. According to Perry, by encoding the watermark in the lower frequencies, it ensures that the watermark will remain with the video until compression ratios reach a point that significantly degrades the video itself.).

Figure 3. Digimarc's batch encoder allows digital artists and archivists to watermark multiple images easily and quickly while fine tuning the strength of the watermark and compressing the file for transmission.

Like the numerous compression techniques themselves, digital watermarking is not perfect or absolute. Successful compression of an image, video, or audio stream is a tradeoff between compression ratios and quality of the compressed image. The more you reduce a digital file, the less likely it is to contain all the original information when decoded, Davis said.

Watermarking suffers from some of the same trade-offs. The stronger the embedded signal, the easier and quicker it is to read, although the more likely it is to impact the aesthetic quality of the image. For this reason, production software for digital images, video, and audio produced by Digimarc or packaged through license agreements with major software developers such as Corel and Adobe, give the operator the ability to change the digital watermark intensity.

Finding the fine line that represents the most robust watermark that does not degrade the original work is an art in itself. To make the process easier, Digimarc's software can automate the process based on the work's final destination, such as the Internet, a print publication, or strictly remaining a digital file (Internet pictures are likely to be lower resolution and, therefore, can contain a higher intensity watermark, while a picture that is set for publication or destined for a studio-quality photo CD might require a lower intensity watermark).

According to Perry, other factors also influence the strength of the watermark. A busy picture, such as a field of flowers, can hold more watermark data and a higher watermark intensity than a picture of a blue sky with a few puffy clouds because the data is more easily hidden from the human eye. Generally, after an operator has inserted his/her watermark into the image and determined the correct intensity level, the Digimarc software checks the image to make sure the watermark will be readable at its destination.

"Everyone has a different aesthetic threshold [for what they can visually detect]," Perry said, "but in general not even the 'golden eyes' of Hollywood can detect the watermark in a movie unless they do a freeze-frame analysis side-by-side with the original print. It's very difficult to see."

Birthing pains

As with the Internet itself, digital watermarking is a new technology that is still finding its legs. The sheer number of computer platforms, transmission and storage methods, compression, software, and countless other hardware factors combined with a lack of knowledge on behalf of photographers and distributors has slowed the acceptance of digital watermarking as a copyright enforcement tool.

Fotunately, for software designers and artists around the world, several factors are coming together to change that. Digimarc, in conjunction with Phillips and analog-format copyright protector MacroVision, are one of two teams bidding to supply digital watermarking services to the Digital Versatile Disk (DVD) industry.

According to Davis, the Copyright Protection Technical Working Group, an international association of executives in the computer, consumer electronics, and motion picture industries, have been weighing the pros and cons of different watermarking technologies for two years. Unlike still photography, which uses software codecs packaged in Adobe Systems Inc., Corel Corp., Micrografx, and Live Picture programs, a DVD will have a hardwired solution. According to Davis, the working group has spent considerable effort and time determining the proper payload sizes in regards to robustness, the need for real-time decoding, and the additional cost to a consumer system based on the need for additional microchips.

The question is not simple, Davis said. "When people say, 'how robust is it,' the answers have to be given in the context of a specific application. It depends on what the customer wants to be encoded. For instance, video copyright protection wants a solution with minimum quality impact and processing requirements with high reliability, so a short payload is good for that. Whereas with commercial photography, we're working with PCs that have high [processing speeds] and so can accommodate a larger payload than video space without any difficulty. If someone just wants to stop copying of a DVD, they don't need a lot of payload."

A second piece of industry regulation, this time outside the industry association, could also give digital watermarking the sort of public approval that turns a fledgling industry into a standard practice. The U.S. government has proposed the Digital Millenium Copyright Act to establish copyright protection and legal recourse for copyright infringement across international boundaries. Currently, the act is in the public comment stage. This phase can take one or more years depending on the political questions that develop; however, once completed digital watermarks would stand out as one of the more mature technologies for tracking digital intellectual property.

Davis said the majority of major consumer electronic companies are researching or developing some form of digital watermark technology. During the writing of this article, Hewlett-Packard announced that it had invested in Digimarc.

Watermark expansion

As Digimarc continues to market its existing digital watermarking technology, Perry's group is expanding development into new areas. According to company officials, Digimarc is developing two new applications.

The first makes use of digital watermarks in print media. Digimarc is developing a system that would use a camera mounted on a standard PC and run with proprietary software that could check for watermarks in printed material. By holding a magazine or printed picture up to the camera, the system would automatically check for a digital watermark. If one is present, the software would read the embedded message, determine who owns the picture, and automatically launch the PC's web browser, taking the operator directly to the web page of the image's owner based on information in the Digimarc locator database.

A second application challenges the smart card industry. Called self-authenticating documents, watermarks could be used as security checks for legal documents such as drivers' licenses and passports (Figure 4).

Figure 4. Passports, drivers' licenses, and work IDs are all examples of legal documents that could use watermarking as an additional security feature by encoding personal data about the legal user within visual elements of the ID.

"The concept is to use the watermark to help authenticate data in a document," Perry said. "One example is a passport. If you look at one today, there's no way to tell if the picture goes with the document or not. Our concept is to produce a document where the picture contains a watermark with some of the text data from the passport, or photo ID. Then, an automated reader could authenticate the document by comparing the printed data on the card with the data in the picture."

Despite its potential, digital watermarking is still caught in a market that is sometimes blasé to the dangers of copyright infringement. However, Davis expects that will change as the world's analog infrastructure changes to a digital world. "We're just in the early embryonic states of the digital economy. Most of the analog control systems are being pretty firmly obsoleted by digital processing and so attention needs to be paid to protection. Otherwise, we'll have a severe economic loss among rights holders. Leaders in several media industries, including commercial photography, movies, music, financial instruments, and identity documents are becoming increasingly convinced that digital watermarking is an integral component deterring unauthorized use of their valuable properties. This bodes well for digital watermarking."

Company info:

Digimarc Corp
One Centerpointe Dr.
Ste. 500
Lake Oswego, OR 97035
Phone: (1) 503/968-2908
Fax: (1) 503/968-0219
Web: www.digimarc.com

R. Winn Hardin
R. Winn Hardin is a journalist based in Jacksonville, FL.