Content protection
Mar 1, 2008 12:00 PM, ERIC DIEHL
In this digital age, protecting your assets is essential.
Forensic marking
The third barrier complements the previous one. Ultimately, any digital content has to be rendered in the analog world. In this context, the protection provided by encryption does not work anymore. The objective of forensic marking is to append information about the source rendering the content. Watermarking is the technology used. A watermark is a message embedded into digital content that can be detected or extracted later. The watermark may be visible, for example, adding an ownership notice or the name of the recipient. Visible watermarks are not extremely robust but may prove a deterrent.
Figure 2. Watermarking content, where M is the message, Co is the initial image, Cw is the modified image and Cwn is the transmitted image with noise.
Click image to enlarge.
Alternatively, the watermark may be invisible, meaning that the information cannot be seen by viewers. Figure 2 shows an example of watermarking content. Invisible watermarking hides a message (M) by slightly altering the initial image (Co). This is the role of the watermark embedder. The watermark key enables the message to be hidden in a deterministic way.
The result is a modified image (Cw), which carries the message. To be invisible, modification of the image takes into account the physiological characteristics of human vision. Transmission of the modified image possibly introduced some noise (Cwn). If noise is acceptable, the person will perceive an image that is identical to the original one. Nevertheless, an informed watermark detector with the right watermark key will be able to extract the hidden message. The same image may contain several hidden messages.
Typical watermark information includes copyright details, or the identifier of the expected user. In case of leakage, detectors can extract this information and thus trace back the source of leak. Forensic marking is useful in numerous contexts, including spotting leakage in the post-production environment, protecting screeners used for awards selection or reviewing, and detecting the source of illegal rebroadcasting of content by spotting the infringing set-top box.
Thwarting illegal distribution
Unfortunately, content will always leak. So, the fourth barrier attempts to limit the losses that are incurred. The first step is to detect illegal content. The most efficient technology to do this is fingerprinting. A reference database includes fingerprints that contain unique characteristics of the content. These features may be based on visual hashes, color, time characteristics or point of interests. Fingerprinting may be audio-based, video-based or both. Then the system spots suspect content, extracts the relevant fingerprints and compares them with those in the reference database.
Once illegal content is identified, the corrective action taken depends on context. Currently, some user-generated content (UGC) sites filter the content files submitted by Internet users. In the case of peer-to-peer (P2P), a take-down notification may be sent to the sharers. In this context, fingerprinting is superior to identification using cryptographic hash values because it is robust to geometrical modifications, mash-ups or camcording.
The second step is to slow down dissemination. The first objective is to deter the downloaders by providing the wrong piece of content instead of the expected one. Typical techniques spread decoys, fake content or even encrypted content that requires a payment to be made. The second objective is to inhibit access either by slowing down the bandwidth or by routing the requester to controlled peers.
Scoping the solutions
Is all this complexity really required? Is the first barrier (controlling physical access) insufficient? It has proven to be so for many years. Unfortunately, dematerialization of processes and content has opened the door to many new threats, including the growing threat posed by insiders. A recent, well-known study called “Analysis of security vulnerabilities in the movie production and distribution process” by Simon Byers and colleagues highlighted the importance of leakages caused by insiders. Limiting protection to the first barrier — physical access controls — is clearly risky in the digital era.
Encryption provides the best way to securely receive master content files from copyright owners, to avoid any risk of leaks taking place during subtitling at external facilities, or to secure storage. It can also be employed to enable journalists or other authorized third parties to preview content prior to airing.
Content can be encrypted using the state-of-the-art AES algorithm. One way to perform key management is through USB smart cards offering an extremely secure implementation of this key asset. In addition, content protection can embed visible and invisible watermarks when content is decrypted and can thus trace back to where and when the content was decrypted.
Traceability of all or some of audiovisual content is also made possible by the use of a forensic marking when content is transmitted from content owners or post houses to a broadcaster's facilities. An invisible watermark identifies the emitting location and operator.
For traceability of content in the broadcast world, a monitoring system can automatically alert a broadcaster if and when images from its broadcast feed are aired by other channels across monitored territories. Invisible watermarks included in the content identify the broadcasting channel and time of airing.
Web monitoring, through video fingerprints, can identify copyrighted contents on UGC sites or P2P exchanges. In conjunction with forensic marking, it allows rights owners to trace where leaks are coming from. It is then possible to seal the gaps.
Help is at hand
The emergence of the digital era has led to a dramatic increase in the number and range of threats to content in the broadcast environment. Controlling physical access to an asset is no longer sufficient to guarantee its protection. It has become an increasingly urgent imperative for content owners to find new ways of protecting their assets throughout the broadcast process. Fortunately, in recent years, a range of complementary techniques have emerged to tackle this key issue.
Eric Diehl is security domain director for Thomson R&D, France.
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