Steganography
Steganography is what this program is about. It allows to hide
plaintext or encrypted text into an image.
We need to type (or paste) our text first into "Open text"
window.
We click on "Open text" tab and we type our message.
I have actually pasted poem: "The Twelve days of Christmas"
from clipboard (CTRL+C)
When we are done with our message then we click
"Steganography" tab.
Next thing we need is the cover image. We need to click the
blue file icon in the right top corner.
Dialog will appear and will allow us to select an image.
I have selected this nice photo of a squirrel, which I have
resized earlier from much larger JPG
using very nice program called Irfan View.
Picture is in PNG format, and that is what we want. We don't
want to load in JPG files directly.
I will explain why later.
To insert our open text into this image we need to select
"Method" tab:
We should select Random X method, which allows for password.
We won't use password this time, so we leave "Use password"
checkbox non-checked.
We will be using open text as our message so we leave "Use
encrypted text" checkbox
also non-checked.
We press "Insert" button and a message box appears on the
screen:
Our short poem occupies only 4.4% of an image. We could fit
much longer text.
When we look back at the open image then it looks identical to
the naked eye,
yet the secret text is already there. We can save the file
back to disk by pressing
"Save image As" button on the "Open image" page. Saved file
contains secret message.
We can prove it by closing the program down, re-opening it.
It should no longer contain open text in "Open text" tab
window.
To extract secret message we need to:
- select "Steganography" page, then
- load open image by clicking the blue file icon in the right top corner and then selecting relevant file.
Message will be extracted automatically and "Open text" page
containing extracted text will open.
Voila! Magic.
Now the details. Secret messages are hidden in seventh (least
significant) bit of
pseudo-randomly selected pixels of the image.
This is how that invisible 7th bit plane looks like when it is
exposed.
Need to click "Covert image" to see that:
Looks random to the naked eye, and yet there are some
patterns which are natural
and are part of the picture. The image above already contains
the message.
Now another image:
This one does not contain secret text. There are some
differences between these two
covert images, but they are really hard to spot. It would be
possible to write a program
to find pixels that are different, but they won't constitute
full message - some pixels will
have exactly the same colour as in the source picture, on
average 12.5% of all pixels
will remain unidentified by differential analysis. What we are
discussing here is a very
comfortable situation for the attacker - being able to perform
differential analysis
means that whoever has prepared steganographic image has left
both the original and
steganographic images somewhere where attacker had an access
to. Big mistake.
OK, the user has made big mistake. Now let's see what attacker
has got in such situation:
bunch or random pixels, but not even all of them.
On average, the attacker has only 87.5% of all the pixels.
That is not enough to reconstruct
the message. But even these pixels that are in possession of
the attacker are in random order.
In a message that contains 1000 characters there will be about
3000 pixels.
Now the attacker has to arrange these pixels in the right
order. Good luck with that.
There is about 3000! possibilities to consider. I am saying
about because the attacker
doesn't know exactly how long exactly the message is.
For those unfamiliar with math I will explain here that !
symbol stands for factorial,
which is calculated as follows: n! = 1*2*3*...*(n-1)*n.
So 10! will be 1*2*3*4*5*6*7*8*9*10=3628800. And this is just
for 10.
For 3000! it will be a very, very very long number.
I have used online calculator to give you an idea how big this
number is.
n! = 3000!
= 4.149359603 E+9130=
414935960343785408555686709308661217095111919493180991768946765
769755856512353195008600076521780034200751846353836171184957508
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331577388153284389297376130211063129303744014853787254460796102
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249600000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000
Pretty big number. But hey, that is not all. Even if the attacker by some miracle
had the right sequence of pixels (which is practically impossible),
all these pixels are XOR-ed with bytes from yet another pseudo random sequence,
regardless whether encryption was used or not while constructing an image.
XOR-ing a message with a pseudo random sequence can be considered light encryption,
which is very difficult to decrypt if the attacker doesn't know the initial position
of the random number generator which emitted the sequence.
To sum things up. There are three pseudo random generators in RandomX algorithm
involved in scrambling the message, each generator is initialized to a different value
which appears completely random to the attacker
(but each of these three values are precisely determined by the password,
so extraction of the message is possible only by the person who knows the password)
There are no eyecatchers and no backdoor left in a message generated by RandomX
algorithm. Without the right password it is impossible to extract a message even for me,
and I am the designer of the RandomX algorithm and creator of the Squirrel program.
Even in case when user makes big mistake and gives an attacker both pictures
(pure carrier and carrier with the message), the attacker won't be able to extract the message.
On average 12.5% of pixels in the message perfectly blend with the input picture
and therefore are impossible for the attacker to identify by means of differential analysis.
As a result the attacker is unable to precisely determine even the length of the message.
In practice the attacker would need to try way more than 3000! possibilities,
and I have doubts whether such an attack is even possible. Most probably it is not.
However when an attacker has both pictures originating from the user, he will suspect
that one of pictures may contain a message. He won't know which one,
and he won't even know whether these pixel differences are result of a message
being inserted, or maybe a random noise being inserted as a decoy. At this point suspicions arise,
but these are only suspicions, nothing is for certain.
However, at this point you may be tortured in some countries ( North Korea, others may follow ),
(yes, because of few subtle pixel differences, I know how ridiculous it may sound today)
so please don't keep both original carrier image and image copy with a message on your hard drive.
My advice is:
Please destroy or overwrite carrier image as soon as it has been used.
Use one carrier photo only once. Destroy the original picture.
I will now explain what happens if the user doesn't use password while using RandomX method.
The user will allow message extraction for anybody who is in possession of a copy of Squirrel program.
It may be desirable in some cases. A message becomes a secret hidden in a plain sight
accessible to everybody who knows where to look and has the right tool.
Maybe good for games one can play with their friends, with no real enemies involved.
Naive algorithm can be used exactly for the same purpose, except in this case message
has been inserted deliberately in such a naive way, that makes it likely to be discovered
by steganography detection tools. Still invisible to he naked eye though.
Good for playing games with your friends.
If there is enough interest generated by this program I will implement more cool features
such as insertion of image into image, insertion of files,
and other more serious "James Bond style" features,
but I don't want to reveal them at this point yet.
Now let's discuss other potential mistakes the user can do.
I have selected 12days.jpg.
Choosing JPG directly as a cover image is possible but it is far from ideal choice.
It will not provide plausible deniability. It is possible to detect existence of secret message
by using algorithmic methods and even by visual inspection if you know what to look for.
Don't get me wrong. It will NOT be possible to extract secret message if a password was used
for message insertion, but there could be a suspicion that something is hidden inside.
I am deliberately making a mistake here to demonstrate it.
I have opened 12days.jpg.
Here is how the 7th bit looks like in a plain file:
You can see black rectangles and rectangles with uniform stripes.
This image is not so bad, I have seen much worse JPGs with big white
or black areas in the 7th bit.
These features (rectangles) are typical for high compression JPG.
Ok, now I inserted message into 12days.jpg.
This is how the 7th bit now looks like. There are pseudo-random multi-coloured dots
inside black rectangles and inside striped rectangles if you look closely enough.
These could not be the result of JPG compression, so what are they?
Something else must be going on. Somebody tampered with an image,
inserting secret message perhaps?
Open image still looks good to the naked eye,
but it becomes suspicious because of those unusual dots inside JPG artifacts.
Now the naive method for comparison:
Well, it is obvious where the message is: on the top :-)
Use this method to tell the world that you are using steganography.
Open image still looks perfect to the naked eye, just like the original.
