Papers in PDF format

Watermarking for the HyperWave Hypermedia System
General
Chris Anderson,
Chris Burns,
Michael Klemme
Hypermedia Unit, Department of Computer Science
University of Auckland
Private Bag 92019, Auckland, New Zealand
chris-a, chris-b, michael-k@hmu.auckland.ac.nz
Digital information can be copied without loss of quality, so it is possible to alter documents and incorporate them into
new products without leaving a trace. Documents can be transferred over electronic networks with ease and at high
speeds. Owners of documents that are of value, which will be made available electronically, need to take precautions to
secure their assets. Two basic approaches to prevent misuse of copyrighted material can be identified:
Access control and logging:
Access to the information is restricted to certain users or groups of users. Logging all requests for a document
can help in detecting excessive use by a single customer. Unfortunately, this method still does not prevent
registered users from passing on documents to others.
Watermarking
Including some additional information in the document allows the provider of the information to prove their
ownership and trace the source of ``stolen'' information. Watermarking is also known as labelling or
fingerprinting.
With the current exponential growth of the Internet and World Wide Web, much of the information provided (images,
text, videos, etc.) may appear on other web servers, without giving credit or payment to the original author. It is obvious
that any serious publishing effort on the Internet involving copyrighted material must include some kind of
watermarking scheme.
Watermarking Techniques
Visible watermarks change the visible information content of the document and can be seen by the viewer. For images
the watermark normally consists of some kind of overlayed image. The pixels of the overlay are computationally
merged with those of the original. This process alters the image information and should not be reversible. Removing the
watermark from the image will damage it substantially. Watermarks can decrease the clarity and accuracy of an image,
but they might be seen as an advertisement of the institution offering the document.
The application of invisible watermarks does not visibly alter the document. One of the base technologies used is
steganography, which hides additional information in the background noise or randomness of digitized information.
This information can not be detected without further information. To be of any use the watermark must still be present,
even if the document is modified (i.e. scaled, rotated, cropped, etc.). This can be insured by adding redundant
information in the digital signature.
Currently, research groups worldwide are working on sustems using the algorithms described above.
Providing watermarked Images: Our Approach
The ``Watermarking'' project was born from the desire for the general public to have access to a large digital image
database, whilst safeguarding the images from unauthorised use and duplication. The image database at the time
consisted of approximately 1100 full screen images (with associated text data) and thumbnail images. The requirements
of the system dictated a scheme whereby certain categories of user could view the unmodified original image, while
others view only the watermarked version.
HyperWave provides a mechanism for identification of users and enforces access rights based on this identification on a