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Home > Key Stage 3 (Ages 11 - 14) > Secret messages using steganography
Secret messages using steganography
Have you ever heard of Steganography? If you havent then you will think that this is really cool... The online definition from Wikipedia describes it as “the art and science of writing hidden messages in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message…” The word comes from the Greek language meaning "concealed writing" and also the Greek word 'steganos' which translates to "covered or protected".
How would this relate to my classroom curriculum?
This is an excellent opportunity to look at data representation and Digital Literacy (responible usage of copyrighted materials). If you have found these resources useful then why not click here to use the resources on teaching data encryption or click here to view the resources for teaching data compression.
So what is this? 
There has been some really imaginative uses of Steganography, including:
  • In Ancient Greece, hidden messages were placed within wax tablets.
  • During World War II, the French Resistance sent some messages written on the backs of couriers using invisible ink.
  • Messages written in Morse code on knitting yarn and then knitted into a piece of clothing worn by a courier.
  • Hiding files in "plain sight" (e.g., what better place to "hide" a file than with an important sounding name in the c:\winnt\system32 directory?) See the example video below that highlights the number of files in this folder:

Part 1a: Basic steganography
With the advance of technology, steganography has become more attainable for the every day person. In a digital age there are many different techniques. As a simple example, a sender might start with an innocuous image file and adjust the color of every 1st pixel in the bottom left corner to correspond to a letter in the alphabet, a change so subtle that someone not specifically looking for it is unlikely to notice it. Please feel free to download the original and update images below to look for your self... Please see the video below for details:
  DSH Original   DSHupdated.png
Original Encrypted message

Part 1b: Least Significant Bit Insertion
Developing the method above further by looking at an image at it's most basic level i.e. binary, we see how the simplest approach to hiding data within an image file is through a technique called 'least significant bit insertion' where we take the binary representation e.g. a character converted into binary, and use it to overwrite the least significant bit within a image. Imagine that we have three adjacent pixels of nine bytes (each byte made up of 8 bits), with the following Red Green Blue (RGB) encoding:


Now imagine that we wanted to hide the following 9 bits of data (101101101) which represents a character . If we overlay these 9 bits over the least significant bit for each of the 9 bytes above, you will get the following result. 


Note that we have successfully hidden 9 bits but at a cost of only changing 4 of the bits which is roughly 50% of the least significant bits, these bits have also been spread out over between adjacent pixels, as a result, this amount of change will be minimal and not noticable to the human eye. This description given is only meant to work as a high-level overview. This same method can be applied to 8-bit colour the changes, as you can imagine, are more dramatic due to the reduced colour pallette. For more details on understanding why, you need to read about how colour depth is calculated.

Part 2: Digital Water marking
You can also use a 'digital watermark' is another kind of indicator or marker that can be placed in the image data. It is typically used to identify owbership when copyright has been infringed or for banknote authtication. The water mark is usually used to identify the ownership of a image i.e. copyright. The process of 'digitally water marking' an image i.e. embedding hidden information, is undertaken by a computer.  Like traditional watermarks, digital watermarks are only perceptible under certain conditions, i.e. after using some algorithm, and imperceptible anytime else. You could teach this technique to your students to prevent others from stealing and wrongly submitting homework/classwork as their own.

This second  video on watermarking is from Digimarc, the person in the video describes how a watermark in an image can be tracked...!  Click here for more details on how to track images.


Part 3: Chaffing and winnowing
With the use of technology, steganography is significantly more sophisticated than the examples above might suggest. There is the potential for someone to hide large amounts of information within both image and audio files. Often steganography is used in conjunction with cryptography i.e. first it is encrypted, then this encrypted data is doubly protected by being hidden in a file. Think about the use of email, by using the 'chaffing and winnowing' techique (desribed below), the sender of a message could send out their messages and cover thier tracks at the same time. 
One method for using these two techniques is to put a message into art i.e. hiding a message within an image. You can send messages containing an image or place an image on a website for anyone to see without anyone else (but the intended recipient) being the wiser. As teachers we could use this techique to put together games and puzzles for our students, website visitors or blog readers by hiding messages in specific images allowing people to hunt for the hideen messages. 

The process is simple:  type a message, upload an image, enter a password, click a button and download the image with the message encrypted in it. Click here to link to the service. For example, I have uploaded an image of the Digital Schoolhouse main website image. 

A screen shots of the process:

 step1.gif  step2.gif  step3.gif
Step 1: Encrypt image Step 2: Download image Step 3: Decrypt image

 A short video of the process:  

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