JPL's Wireless Communication Reference Website

Chapter: Capita Selecta

Fingerprinting and Watermarks

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

Some vocabulary

  • 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.
  • By fingerprinting we mean the process of attaching the identity of (crypto-) decoder to a signal in a way that is difficult to erase. This allows the copyright owner to trace pirates (or at least their decoder) if the clear signal is re-disseminated illegally.

    The main difference between watermarks and fingerprints is whether the identity of the transmitter or that of the recipient is embedded in the signal. In many technical solutions, this also determines where (thus, in which device) the identifier is being added to the clear signal.

    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.

    Watermarking Images

    PC Windows tool to add a pseudo-random noise sequence to an image and to detect this watermark by correlation.

    Watermarking as a mechanism for copy control

    Watermarking is regarded as a useful tool for copy control, e.g. for (DVD or DVB) video or audio A watermark preferably is undeletable by a hacker, perceptually and statistically invisible and resistant to addition of random noise. With statistically invisible is meant that the watermark cannot be deleted by statistical analysis (like Kalman filtering) and lossy compression algorithms (like JPEG and MPEG). Another name for watermarking is "Tattooing". Watermarks can represent registration numbers of the copyright holders, terminal numbers of the machine which was used to duplicate the image, registration number of legal users.

    Embedding and extracting watermarks requires additional hardware, for example, a watermark based on masking will require image edges extraction, a watermark hidden in the frequency domain will require many fast Fourier transforms. Another problem is the prove of the legal validity of the watermark in case of court pursuits. The energy required to prove the legitimacy of the watermark is directly related to the watermark quality.

    For example, a watermark can be embedded in a picture by using the following method. Bits, which represent registration numbers etc., can be hidden in the picture by modificating the luminance values inside blocks of nxn pixels. Each block can contain for example one bit of the additional information. Other methods modify the DCT coefficients generated by JPEG or MPEG coders.


    A watermark can also be hidden in audio signals.


    link to MP2 files
    Audio Engineering Society AES workshop on copy management, May 1998
    Audio clips by
    • Paul Jessop, Technical Director of International Federation of Phonographic Industries,
    • Marcus Erne, ETH Zurich, on watermarking and
    • Don Jerrard on legal issues.

    Data hiding in audio signals provides a special challenge because the human auditory system is extremely sensitive. The pertibations in a sound file can be detected as low as one part in ten million (-80dB). Although the limit of perceptible noise increases as the noise contents of the host audio signal increases, the typical allowable noise level is very low. The human auditory system has very low sensitivity to the phase of the sound. Unfortunately, this hole has been exploited by numerous audio compression algorithms. According to experiments, 8 bits of hidden data per second can be added to host sounds with quiet backgrounds. Twice as much data can be added to host sounds with noisy background by using phase coding.

    Marking Documents

    Data can also be hidden in digital document images, for example by using word and line shifting. Each document recipient (i.e. a subscriber) receives a document containing a unique set of fingerprint marks. Each mark corresponds to an imperceptible horizontal displacement of a textual object. Since the information is not observable upon casual inspection of a document, a recipient may not be aware of its presence. A recovered, unauthorized document copy can be traced to the original, authorized recipient.

    Some References

    JPL's Wireless Communication Reference Website 1997.