Electronic watermarking is a new research area, combining aspects of digital signal processing, cryptography, statistical communication theory and human perception. It aims at embedding additional data into clear content (images, audio etc) in a way that is difficult to remove. Principal applications of electronic watermarks are in copyright enforcement, automatic metering of asset usage in multi-media applications, piracy tracing, and in providing additional information, such as image captions.
New multi-media networks and services facilitate the distribution of content, but at the same time make copying and copyright piracy simple. Here we see a clear need to embed copyright data, such as the ownership or the identity of the authorized user in an indelable way. In case of a legal dispute over copy rights, this provides a proof of the origin of the material, and it can provide a tool to formally establish which user received and illegally re-disseminated the material. This need particularly exists for audio, video (frames) and still images in broadcast multimedia networks. Similar requirements also exist for executable computer programs or information distribution through packaged (storage) media. One application can be to to protect and enforce film copyright infringements, for instance with Digital Video Disc.
Some abiguity exists over terms used. In particular, we distinguish between watermarking, fingerprinting, posing a signature or label and transmitting over a subliminal channel
By watermarking we mean the process of electronically attaching the identity (secret or clear, analogue or digital) of the owner of the copyright in a way that is difficult to erase. Watermarking is comparable to placing an electronic stamp on the document. In some cases, the word watermark is used exclusively for a hidden identity code.
Watermarking differs from authentication or digital signature that proves to a receiver that the message could only have come from one particular transmitter. Mostly, authentication messages in the form of conventional hash functions can easily be deleted by a pirate who wishes to use copyrighted material for illegal purposes. The goal is to give the copyright owner of a digital image (or other piece of information) the possibility to attest technically the origin of the image. Watermarking does not address authentication explicitly, although certain forms of watermarks can be used for authentication as well.
The requirements for fingerprinting are contradictory. On one hand the broadcaster or copyright holder may want to easily recognize the fingerprint, preferably visually. This allows easy tracing of a decoder that is used for illegal purposes. This approach is very similar to the commonly used watermarking by means of the logo of a TV station that is continuously visible in one of the corners of the screen. On the other hand, the fingerprint should be hidden, in order not to disturb paying viewers with program-unrelated messages on their screen, or to avoid that any pirate can detect and erase the fingerprint electronically. In the latter case, it may involve specific equipment to detect and decode a fingerprint.
However, fingerprinting is also used to describe the process of extracting identifiers from content, without modifying that content.
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Tutorial |
How the struggle to protect Intellectual Property Rights shapes the information society. Technical aspects of copy protection for DVD, audio, etc. |
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Quiz |
Given a image of N pixels, watermarked with a secret pattern of N watermark values (+1 or -1). An attacker has a black-box watermark detector, with a binary output, saying whether or not it sees a watermark. How many experiments must a smart hacker do to learn about the watermark secret?
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Read more about the sensitivity attack (file: ps.gz) |
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A do-it-yourself watermark It doesn't take a lot programming experience to implement a basic watermarking system. Take a watermarking method that decreases the luminance of half of the pixels by one step and increases the luminance of the other half, i.e., W in {-1,1}N, with N the number of pixels in the image. The embedder and the detector share this secret sequence W. The embedder simply adds the sequence to the pixel luminance values of the image. In the detector, the correlation operation now reduces to calculating the sum S+ of all pixel values where W = 1, and subtracting the sum S- of all pixel luminaries where it has been decreased (W = -1). If the image is watermarked, the expected value of the result S+ - S- equals the number of pixels in the image. If the image is not watermarked, the expected outcome is zero. Let the detector decide that a watermark is present if the result is larger than, say N/2. It is interesting to see how this works properly for most images. It appears robust against several processing operations, including some lossy compression (JPEG), noise addition or quantization, but a detector of this basic form will not recover the watermark from a scaled image. To detect watermarks after filtering, JPEG compression and similar operations, usually a threshold setting lower than N/2 gives best performance. |
link to watermarks.TV mirror web