Have you ever wanted to hide something from:

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Have you ever wanted to hide something from

• Your friends?

• Your family?

• The Government?

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If the answer is yes, then you need to learn about
Steganography
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Thousands of years ago, the Greeks used
steganography to hide information from their
enemies.
One hiding method was to engrave a message in a
block of wood, then cover it with wax, so it
looked like a blank wax tablet. When they wanted
to retrieve the message, they would simply melt
off the wax.
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You can try steganography at home by writing on a
piece of paper with lemon juice.
If you heat the paper with a hair dryer the juice
will burn and reveal the hidden message.
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We can also perform steganography using a
computer.
Everything in a computer is stored as 1s and 0s
(a bit). Bits are grouped in sets of eight, one
set is called a byte.
One byte can be used to represent each letter of
the alphabet. This is what is used in text files.
01000001 A 01000010 B 01000011 C
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Pictures are made up of lots of little dots
called pixels. Each pixel is represented as 3
bytes one for red, one for green and one for
blue.
11111000 11001001 00000011
248 201 3
Each byte is interpreted as a number, which is
how much of that colour is used to make the final
colour of the pixel.
248 201 3 Orange Colour
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The difference between two colours that differ by
one in either one red, green or blue value is
impossible to see with the human eye.
248 201 3 Original Colour
If we change the least significant (last) bit in
a byte, we either add or subtract one from the
value it represents.
This means we can overwrite the last bit in a
byte without affecting the colour it appears to
be.
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We can use images to hide things if we replace
the last bit of every colours byte with a bit
from the message.
Now we hide our message in the image
11111000 11001001 00000010
11111000 11001000 00000010
11111000 11001001 00000011
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Even if we do this across a big image and with a
really large message, it is still hard to tell
that anything is wrong.
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Normally when we hide a message in an image we
just start at the top left pixel and keep writing
across the image until we are done.
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Example
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This may appear to work quite well, but if we
zoom right in and look at the pixels in a block
of plain colour then we can see that some pixels
arent all the same.
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If we change an edge it is harder to notice
because two pixels next to each other will
already have very different colours.
So what we want to do is hide in the edges of a
picture because then we can avoid hiding in
blocks of colour.
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This method of hiding is called FilterFirst,
because we filter the image to find the edges
before we start picking the order of pixels to
hide in.
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Example
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Imagine our example (above) has a pink square in
the middle.
The algorithm will write to the edges of the
square before moving out and writing around the
rest of the image.
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And if you dont believe its any of this, you
can try it out yourself! Theres a free program
you can download called Digital Invisible Ink
Toolkit, which can hide a computer file inside a
colour image.
http//diit.sourceforge.net
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Can you pick the picture with the hidden message?
http//diit.sourceforge.net
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Can you pick the picture with the hidden message?
If you guessed this one, you were right.
http//diit.sourceforge.net
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A proud University of Waikato, School of
Computing and Mathematical Sciences Honours
Project, 2005
http//diit.sourceforge.net