Steganography is the art and science of hiding information. Most people
have probably used it as a child - writing on a blank sheet of paper
using lemon juice. The lemon juice appears "invisible" until
the paper is heated - and because it burns before the paper does the
invisible writing magically appears. More modern applications have become
much more sophisticated, and in the last 10 years there has been more
and more people using computers to hide information.
the idea is to replace some redundant information in a cover object
with the hidden message. With the lemon juice example the paper is the
"cover". In this project, images are used as the cover to
hide the information. There are programs available that hide in other
digital mediums such as email content, word documents, music/sound files,
unused diskspace and many others. Regardless of what type of cover is
used, the resulting stego-object (the cover containing the hidden information)
should have nothing noticeably wrong with it.
images it's possible to change the colours a little bit so that the
message can be hidden. It's possible to completely overwrite the 4 least
significant bits of each colour value (there are 8 possible bits - meaning
half of the picture is overwritten) without the picture suffering too
much degradation. However, there are techniques known as "steganalysis"
which can look at the image and provide a probability of it containing
hidden information. If you are using least significant steganography
the steganalysis can very often get it right. There is a notion of security
here - steganography is secure if even knowing how the message was embedded
does not necessarily mean you can retrieve it - or even tell that there
is something hidden.
is often confused with watermarking. Watermarking often uses similar
techniques - but there are two big differences. Firstly, watermarking
is highly robust. If a watermarked picture is rotated, resized, flipped,
recoloured - it should still be possible to retrieve the watermark.
If the same is done to a picture that has had steganography applied
it is highly unlikely you will be able to get any of the message out
at all. Secondly, watermarking isn't necessarily hidden. It may be possible
to look at a watermarked item and see the watermark. With steganography
we are hiding information - so if it's hidden well, we shouldn't be
able to see anything.
this project, least significant bit steganography is being used. The
reason that this method was chosen over others is as follows:
- It allows a huge amount
of information to be hidden compared to other techniques.
- Techniques that hide
in the discrete cosine transform coefficients of jpeg's leave a
very predictable marking on the file format - because we are changing
the colours and not the file format this can be avoided.
- If the right algorithm
is used, it can be extremely secure.
- There are no free user-friendly
applications that I've found that implement this technique using
a graphical interface and a selection of techniques.
Ink Toolkit allows you to pick from several different hiding techniques
and hide a file of any type (message) inside an image. It can currently
handle reading 24-bit colour JPG, PNG and BMP formats, and allows you
to save the stego-image as either a 24-bit PNG or BMP. As long as you
know what algorithm was used to hide the message (and the password used)
it is possible to retrieve the hidden file intact. The toolkit also
provides some benchmarking and steganalysis information on any given
pictures. There is more information on the documentation
page about how this is done.