Steganography, Compsci 100, Spring 2010
You should snarf assignment stego from
http://www.cs.duke.edu/courses/spring10/cps100/snarf or browse the
code directory for code files provided. See the Stenagraphy howto file for help and more
instructions.
The code provided in this assignment uses the Picture class
provided as part of An Introduction to
Programming with Java (an Interdisciplinary Approach) by Kevin Wayne
and Robert Sedgewick.
A steganography assignment appears in
the Nifty
Assignments Archive from 2009, that idea gave birth to this
assignment, though this one is substantially different. The nifty
assignment was originally developed by Tom Murtaugh and
Brent Heeringa from
Williams College.
Genesis of Assignment
The images to the left are of the 1908 Picasso painting
Three
Women which is held at the Hermitage Museum in St Petersburg,
Russia. One of the images (perhaps both) has a hidden watermark so that
the use of the image can be tracked by the Hermitage. Watermarks are a
form of steganography where the hidden information is related to the
object in which the information is hidden. The other image has been
altered by a program equivalent to the one you'll write as part of this
assignment to store another, hidden image. Clicking on
this link will reveal the hidden
image extracted from the
Three Women image. The extracted
hidden image is a lower-quality version of the
original image that was hidden,
because it's not possible to hide the complete, full quality image
without essentially erasing the
Three Women image. (In this
case the hidden image isn't of a very high quality to begin with.)
Steganography (see the Wikipedia entry
for details) deals with hiding information, typically so that even the
existence of the hidden information is hidden. An encrypted message might
attract attention --- even if it's not possible to decrypt it, some
people will try. But a message whose existence is hidden is hard to
decipher because ... well hopefully you get the picture.
You'll also write a program to hide text in an image.
This version of Three Women has
the complete text of Melville's
Bartleby, the Scrivener (a story
of Wall-street) hidden in it. Your programs will be
able to hide and extract text in an image.
There are several websites that hide text in an image --- exactly what
one of the programs you write will do. Of course these sites will
extract the hidden text as well, just as your suite of programs
will. Site for hiding text in an image include imagecipher, mozaiq/encrypt, and utilitymill.com
are the sites.
What You Will Do
You will write five classes for this assignment. Most of them consist of
simple changes to the class HideImage
which you'll write first and for which you're given code with which
you'll start. Each of the classes you write will also have a
main method to run it. Hiding and extracting images is
required, extra credit can be earned by hiding and extracting text.
credit.
-
HideImage prompts the user for two images and a number
of bits and hides one image (the source) in the other (the target) using
the specified number of bits. A starter version of this class HideImage.java is provided that prompts
the user for two image files and the number of bits. For this class you
only complete part of the method hide that alters one pixel.
More details are in
the howto.
-
ExtractImage extracts an image from an image when
the user specifies the number of bits to use in the
extraction. A starter version of this class ExtractImage.java is provided that
prompts the user for the image file and the number of bits. More details
are in the howto.
The text processing classes and benchmarking code are A credit/extra
credit.
-
HideText is similar to HideImage but
hides text in an image using either one or two bits. The user specifies
whether one or two bits will be used as well as both the image file and
the file of text to be hidden. More details
are in the howto.
-
ExtractText extracts text hidden in an image specified
by the user. The user also specifies whether one or two bits will be
used in extracting text --- see the howto for details.
-
StegoBenchmark processes every image file in a
directory chosen by the user and determines which of the image files
contains hidden text as created by the HideText
program. You'll need to use method(s) from ExtractText that
you write and try both one- and two-bit encodings. You'll need to write
a method to determine if a string represents text, ideas can be found in
the howto. A starter file StegoBenchmark.java is provided that
processes all files in a directory chosen by the user.
Grading
You'll have algorithm/correctness and engineering points for this
assignment. Analysis points will be given for the benchmark code and
explanation, though that's only done if you're doing the extra part of
this assignment.
Each program you write should be robust in the sense that if
the inputs don't work, your program should exit gracefully. For example,
when hiding a source image in a target, the images must be the same size
--- you're given two directories of images that are the same size. This
makes things simpler for you as the programmer, so if the images are not
the same size, your program should exit gracefully with a message to the
user, not crash. When hiding text, you may have more text than can fit
in an image. If this is the case, your program should certainly not
crash. Ideally you'd inform the user that not all the text was hidden
when there's not enough space.
Your analysis file should list your testing results; your README should
provide all the people with whom you collaborated, and the TAs/UTAs you
consulted with. You should include an estimate of how long you spent on
the program and what your thoughts are about the assignment.
Submit your analysis, README, and all of your source code using Eclipse
with assignment name stego.
More Extra Credit
For even more extra credit you should write HideText and
ExtracText to allow the user the choice of hiding the
text in row-major order as described in the howto or in column-major order. Essentially
your code will process the pixels in the target image either
row-by-row starting with the top row of pixels (this is row-major)
or column-by-column starting with the left most column (this is
column major). You should give the user the choice of which order to
use by appropriate use of the
JOptionPane.showOptionDialog method, e.g., as shown below.
<![CDATA[
int opt = JOptionPane.showOptionDialog(null,
"Choose Row or Column",
"Hiding Text Options",
JOptionPane.YES_NO_CANCEL_OPTION,
JOptionPane.QUESTION_MESSAGE,
null,
new String[]{"row", "column"},
"row");
]]>
You should ask the user for which order to use when extracting and when
hiding. You should also try both row- and column-major orders when
writing your StegoBenchmark program. Be sure to document in
your analysis that you tried the extra credit and how much of it you
did.
