A watermark stored in a data file refers to a method for ensuring
data integrity which combines aspects of
data hashing and
digital watermarking. Both are useful for
tamper detection, though each has its own advantages and disadvantages.
Data hashing
A typical data hash will process an input file to produce an
alphanumeric
Alphanumericals or alphanumeric characters are a combination of alphabetical and numerical characters. More specifically, they are the collection of Latin letters and Arabic digits. An alphanumeric code is an identifier made of alphanumeric c ...
string unique to the data file. Should the file be modified, such as if one or more bit changes occur within this original file, the same hash process on the modified file will produce a different alphanumeric. Through this method, a
trusted source can calculate the hash of an original data file and subscribers can verify the integrity of the data. The subscriber simply compares a hash of the received data file with the known hash from the trusted source. This can lead to two situations: the hash being the same or the hash being different.
If the hash results are the same, the systems involved can have an appropriate degree of confidence to the integrity of the received data. On the other hand, if the hash results are different, they can conclude that the received data file has been altered.
This process is common in
P2P networks, for example the
BitTorrent protocol. Once a part of the file is downloaded, the data is then checked against the hash key (known as a hash check). Upon this result, the data is kept or discarded.
Digital watermarking
Digital watermarking is distinctly different from
data hashing. It is the process of altering the original data file, allowing for the subsequent recovery of embedded auxiliary data referred to as a
watermark.
A subscriber, with knowledge of the watermark and how it is recovered, can determine (to a certain extent) whether significant changes have occurred within the data file. Depending on the specific method used, recovery of the embedded auxiliary data can be robust to post-processing (such as
lossy compression
In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
).
If the data file to be retrieved is an
image
An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
, the provider can embed a watermark for protection purposes. The process allows tolerance to some change, while still maintaining an association with the original image file. Researchers have also developed techniques that embed components of the image within the image. This can help identify portions of the image that may contain unauthorized changes and even help in recovering some of the lost data.
A disadvantage of digital watermarking is that a subscriber cannot significantly alter some files without sacrificing the quality or utility of the data. This can be true of various files including
image data,
audio data
Digital audio is a representation of sound recorded in, or converted into, digital form. In digital audio, the sound wave of the audio signal is typically encoded as numerical samples in a continuous sequence. For example, in CD audio, samp ...
, and
computer code
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These progra ...
.
See also
*
Steganography
External links
{{no footnotes, date=September 2009
The Digital Watermarking Alliance - Furthering the Adoption of Digital WatermarkingOpen Platform for testing digital watermarking systems
Authentication methods
Watermarking
cs:Vodoznak (watermark)