sniff out secrets
Technology Research News
As digitized pictures, audio and text proliferate,
people are exploring ways to exploit these media by hiding messages within
the information, which leads others to try to detect these hidden messages.
Although steganography -- the practice of hiding a secret message in written
or audio information -- is hardly new, computers and the Web have added
a new twist simply because the volume of information that makes up digitized
media is so large. This provides for historically large haystacks that
easily obscure needles.
A Dartmouth College researcher has found a method that makes it easier
to detect hidden messages in digital images, which can contain a megabyte
-- a string of one million ones and zeros -- of information, or more.
Digital images are made up of pixels, or dots of color. Especially with
high-resolution digital images that have one million or more different
shades of color, it's easy to hide a message by slightly altering these
colors in ways that are imperceptible to the human eye.
In an image that has not been tampered with, however, the information
that makes up the image is not simply random. The key to the Dartmouth
detection method is creating a statistical profile of the compressed data
files that make up natural, or undisturbed images, then checking a given
image against the profile, according to Hany Farid, an assistant professor
of computer science at Dartmouth. "In order to detect hidden messages
in an image we need to start by characterizing the statistics of natural
images. The hope, then, is that when a message is hidden in an image,
these statistics are disturbed," he said.
When images are compressed so they can be stored as smaller files, the
digital information that indicates the color of each pixel is changed
into wavelet information. Wavelet mathematics includes functions like
spatial position, orientation and scale. Wavelets allow for compression
because all the information that makes up a wavelet can be reconstructed
from only a portion of that information. An image is compressed by storing
only the portion that is needed to reconstruct the whole.
Farid collected two types of wavelet statistics: variations like mean,
variance, skewness and kurtosis in the coefficients, or numbers that make
up the wavelets, and information about the rate of errors that occur when
reconstructing full wavelets from compressed information.
Variance shows how spread out the data is from the mean, or average; skewness
shows how evenly distributed the data is on either side of the mean, and
kurtosis shows how peaked the distribution of data is around the mean,
He then combined the variation and error rate statistics into a vector
-- a mathematical construction that is like a virtual sculpture with 70
to 100 dimensions rather than the usual three.
By comparing the statistical vector information with the same information
in an individual image, Farid was able to tell if the image had been disturbed
with a hidden message, he said.
The practice of information hiding, or steganography is related to, but
different from cryptography.
In cryptography a message is encrypted and then transmitted. If you saw
the transmission you wouldn't be able to decipher the message, but you
would know the sender and receiver might be trading secrets. The goal
of information hiding is to go a step further by camouflaging the transmission
entirely, said Farid.
The statistical vector method only detects hidden messages, and cannot
read or remove them, but may eventually be adapted to do so, said Farid.
"This work cannot obviously be adapted to remove or decipher the hidden
message. I do believe, however, that it is possible to do so," he said.
The technique is an extension of previous steganography detection schemes,
said Neil F. Johnson, associate director of the Center for Secure Information
Systems. "It is potentially useful if the techniques for detection are
repeatable," he said.
In addition to determining if there's information embedded in a message,
it is also useful for a detection method to identify the steganography
technique used to hide the information, said Johnson. Another goal is
to be able to extract the embedded information, he said. Steganography
tools exist that can do this in at least some cases, he added.
Steganography has many applications, both good and bad, said Farid. "It
can be used to protect copyrights in digital media, for unobtrusive military
and intelligence communication, covert criminal communication, trafficking
of illegal pornography, and for the protection of civilian speech against
An unfortunate side effect of research that reveals hidden messages, is
"repressive governments could use this research to limit civilian speech,"
Farid said. Because of the possible unpleasant applications, "some will
be very critical of this research, possibly with good reason," said Farid.
"Nevertheless, I believe that the development of these techniques are
inevitable and... will lead to better techniques for hiding information,
which in turn will lead to better detection schemes and so on. My larger
research vision is in authenticating digital media so that [neither] the
'good-guys' [nor] the 'bad-guys' will... be able to manipulate digital
sound, image or video to suit their needs," he said.
The method can also eventually be applied to analyzing works of art to
detect forgeries or to determine if more than one artist painted a single
painting, Farid said.
The method could be used practically in less than two years, said Farid.
Farid's research was funded by the National Science Foundation (NSF) and
the Department of Justice (DOJ).
Timeline: < 2 years
TRN Categories: Cryptography and Security; Pattern Recognition
Story Type: News
Related Elements: Technical paper, "Detecting Steganographic
Messages in Digital Images," posted at http://www.cs.dartmouth.edu/farid/publications/tr01.html
Statistics sniff out
Quantum bit withstands
Image search sorts by
Study finds Web quality
Powerless memory gains
Research News Roundup
Research Watch blog
View from the High Ground Q&A
How It Works
News | Blog
Buy an ad link