Steganography:
Steganography is the art of hiding data. When it comes
to cryptography, steganography is a way to transfer "sercet" data without
anyone knowing the data is present. Conventional encryption simply
aims to stops someone without the key from obtaining the orignal data.
More than likely, if the encrypted data was intercepted durring transfer
or found at all, the ones who found it know it is encrypted.
The idia behind steganography is to store and transfer data
without anyone being aware the data is present. This is done by "hidding"
the privliged data in inside some other "harmless" data.
There are many ways this can be acomplished. Some are
more detectable than others. Some are nearly imposibal to detect.
Encode to BMP is a program designed to hide data inside an
image. More specificly, a "real-world" image saved in 24-bit BMP format.
Encode to BMP is able to store data inside the BMP image by making tinny
changes to the image-- changes too suttle to be seen in the result.
In fact, the changes made to the image are so small, it is nealy imposibal
to deduce the modified image wasn't the orignal image. More on that
latter.
Encode to BMP works best on "real-world" images. That
is, a BMP image not computer generated such as a scanned photograph or digital
camera image. 24-bit images have so much color detail that the color
resolution exceeds that of the hummen eye. One can change much of
the fineite data and visualy leave the image unchanged. Encode to
BMP uses the fineite data to store the "hidden" information.
The way this works reduces the color resolution from 16.8 million
colors down to around 1 million. 4-bits of fineite data is taken from
of each pixel. In this manner, it will take two pixels for ever one
byte of data to be hidden in the bitmap.
Undetectable:
In a real world image, the finite vareations from pixel to
pixel apear random. That is, one can not predict how the finite
vareations will change from pixel to the next. Now, the output of
a good encryption algorithm is also random. If one were to take
an image and replace the ture finite vareations with random ones, there
would be no way to tell which image was the orignal (with out knowing
prehand of corse).
Encode to BMP encrypts all data stored in the BMP with a
strong stream cipher. Reguardless the input, the data stored in
the BMP apears random-- just like the orignal. If the an orignal
image was being transfered with hidden data, there is no way to know if
that image contains the data or not. Even if one suspects the image
of holding secret data and has Encode to BMP, without the key, they can
not prove it.
Now, I asked the people of "sci.crypt" if the above
statment was true. I mostly got a lot of off the subject replys
(as usual), but I did get a bit of useful information. Basicly,
it might be posibal to predict how LSBs should move near edges in an image.
By anilysing the sharp edges of an image, one may be able to find that
there might be iregularities caused by hidden data. Unforchently,
I wasn't able to find good information about how this process is done,
how much change is requiered for detection or how sharp the edges need
to be before detection is posibal. There is a system for detecting
hidden data in JPEG images. However, I wasn't able to deduce from
the write up if the detection was based on anylise of the image or if
detection was done by checking iregulearitys in the commen findings of
the JPEG format itself. That is, this system of detection may only
aply to images using JPEG compression.
In any case, there isn't an off the shelf method for detecting
data hidden in .BMP files. If someone has one, they don't talk about
it.
Weaknesses:
This steganography system has some ovious weaknesses.
The data is likely safe, but an atacker could stop the data from getting
to a recepiant.
The simplest and easyest method would be to fill the
finite data of the image with random data. It will not effect the
visual quality of the image, but it will distroy any "hidden" data by
making it imposibal to extract.
The second weakness is lossy compression. This system
only works on 24-bit BMP files. The files could be saved in other
formats, transfered and converted back into BMP with the hidden data intact
only if no or lossless compression is used. Lossy compression,
such as JPEG, take advantage of the fact 24-bit color is too good and
use this as part of the compression. Lossy compression changes the
image to make it easyer to compress. An atacker could simple use
JPEG compression on all BMP images and distroy all hidden data.
A note from Punkroy:
Steganography with encryption is probly one of my favroit
feilds of encryption. It's hits a would be atacker with two problems.
One, cracking encryption. But harder, trying to find the encrypted
data to crack. Steganography is defently sneaky. It alows
secret data to be stored in plain sight, amonst harmless data. The
data can exist without anyone knowing it's there. And the best way
to stop an atacker is not to let them have anything to atack.
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