Hello guys and welcome!
We start a series of lectures on a steganography.
Maybe you wonder what Steganography is; is it in fact stenography
and I made a mistake?
No, actually there is no mistake here!
While stenography is the skill or work of recording spoken works by writing them down in shorthand and
later transcribing them using a machine such as a typewriter Steganography is something completely different.
It's the art of hiding secret information in plain text or in clear sight. Steganography tools can
easily embed secret files into images, movies, audio files or other file formats. Let's
take a look at this picture that I've taken using my mobile phone.
Do you notice something strange?
I suppose not!
You are just seeing a lake and some trees,
a common park in the city.
But inside this nice image I've hidden the tragedy of Julius Caesar by William Shakespeare.
Except for me and the people I choose to tell this,
like you, no one could ever imagine that I can post the image somewhere on the Internet and later access
the historical play of William Shakespeare easily and in privacy.
The word steganography comes from the Greek word “steganos” which means “hidden”
and “graph” or “graphia” which means writing. Let me tell you a nice story!
One of the first recorded use of steganography
is when Herodotus mentions how a Greek ruler sent a message to his vessel by shaving the head of his
most trusted servant marking the message onto his scalp then sending him on his way
once his hair had regrown! Nowadays though we make use of the modern techniques and we insert secret
files into other files. Maybe you wonder why someone would use steganography when there are so many strong
encryption algorithms available, so strong that virtually no one can ever track them.
The answer is very simple!
Even though the encryption is so secure it doesn't actually hide the existence of the message. Anyone
can see that an encrypted message has been sent
and even though no one can read it merely communicating a secret can trigger alarms and make others
suspicious.
Once a third party determines that you are communicating in secret they can force you or the person
you are communicating with to tell them what you are hiding or they can simply intercept the communication
and destroy the encrypted message.
This is where steganography comes in.
Unlike cryptography the purpose of Steganography is to hype even the mere existence of the message that
is being sent. Let's see step by step how it really works!
First the secret file is encrypted.
This is not mandatory but this highly recommended and mainy steganography tool do that automatically
for you.
The file that is hidden is called “embedded file”.
Then the information (encrypted or in clear text) is embedded into a cover file according to an algorithm
in most cases.
This is an image an audio or video file.
The steganography algorithm decides how to hide data and how to randomize the placement of the data
in such a way that the cover file remains visually the same; if someone is looking simultaneously at
the original cover file and at the file that contains the embedded message he would see no difference
at all.
The cover file that contains the script the message or the embedded file is called “stego file”. After
the secret file was embedded the “stego file” is sent normally to the destination.
If the “stego file” file is anemic the user can post this and other images on a website form or simply send
them in clear text per email.
If someone such as an attacker captures the traffic he won't be suspicious because it's just an image.
Eventually the user sends more images is a distraction.
The steganography algorithm should be strong enough so that no one could tell the difference between
the initial image and the one that contains the secret message.
And even if somehow the attacker is strong enough to extract the embedded message this will be encrypted
anyway.
Take a look at these two images.
The origin alone and the one that contains the historical play of William Shakespeare.
I'm sure you won't see any visual difference, no matter how close you look and how much you zoom in.
They look the same.
At the destination
the one that knows that there is a hidden message there applies the same steganography algorithm and
extracts the message. Most of the time
he needs a secret key that was previously shared with him to extract and then decrypt the message.
I'll show you in a short while
a practical example.
Now that we've seen what Steganography is and how it can be used maybe you wonder how someone could
modify an image or an audio file without anyone else to find out that there is a secret hidden.
If you use an audio file as the cover file you should know that there are a lot of frequencies too high
or too low that the human ear cannot detect anyway. So modifying those frequencies doesn't change
the way it normally sounds.
Or if you take an image you should know that there are a lot of pixels that cannot to be seen by the
human eye and if those pixels are modified the picture will still look the same. At the end
of this lecture
let's see what are use cases of steganography.
It's useful in situations where sending encrypted messages might raise suspicion such as in countries
where free speech is suppressed.
It's also frequently used as a digital watermark when the owner or the copyright holder embeds a secret
message into a media to find when images or audio files are stolen.
You can also use steganography to embed a private key like the one used by ssh or by crypto currencies
or any other secret files like the one that contains passwords or other sensitive information in an
image and then post that image somewhere on the Internet where it's always available.
It's like a backup but a very very secure one. No matter where you are
you will be able to access the secret information. Or imagine that you want to take with you a secret
document. You don't want to use a USB stick that can be easily discovered or sent the document by e-mail.
You can simply embedded in an image and do whatever you like with that image.
Note that if you post a photo on a website like Facebook or Instagram there is the risk that the image
will be automatically scaled and that means changed! The embedded message will be lost.
The stego file has to remain unchanged.
At the end of this video let me demonstrate to you that the tragedy of Julius Caesar was indeed hidden
in that image.
This is the image img.jpg
It's the imagine
you have already seen! I use a steganography tool called steghide like this: steghide extract
-sf from stego file and the image
and I'm entering the secret passphrase
And the secret has been extracted! It wrote extracted data to julius_caesar_ws.txt
This is the file! It's of the entire historical play of William Shakespeare!
In the next lectures will dive deeper into steganography and I'll also show your many live examples.