All About Steganography
An innocent looking picture contains a lot of secret information. Is
it possible? Steganography makes it possible. Find out more about this
technique of hidden communication
Simply put, Steganography is the technique of hidden communication. Using steganography a secret message is embedded in a medium, such as an image or a sound clip and sent. The existence of the hidden message is not known except by the sender and receiver. The word is derived from the Greek words stegos meaning covered and graphia meaning writing.
How is it different from cryptography
While cryptography and steganography are related, there is a difference between the two. Cryptography is used to scramble messages so that they cannot be understood. It does not hide the fact that the message exists. Steganography, on the other hand, conceals the fact that the message exists by hiding the actual message in another.
The concept of steganography has been in use for centuries. In ancient times, text was written on wax poured on stone tablets. To hide the message the wax was scraped off, the message was inscribed directly onto the tablet and then the tablet was covered with a fresh layer of wax. To anyone inspecting the tablet, it was a blank tablet. On the receiving end the wax was scraped off to reveal the message.
In the 20th century, invisible inks were widely used. Fluids like vinegar, milk and fruit juices were used to write messages that could not be seen unless subjected to special treatment. When heated, these fluids turn dark and the message can be read.
The current form of this technique is also quite similar, the idea being to prevent the detection of a message by hiding it rather than distorting it… Figuratively speaking usually!
How it works
The "cover" is the medium which is used to hide the secret information. The information to be hidden can be a plain text message, a cipher text, another image, or anything that can be represented in binary.
Cover media can be a lot of things: text, images, audio and video. As images are the most commonly used medium, let's look at that closer.
Images can be altered in the noisy areas with a lot of color variations so that the alterations are less obvious. The message can also be scattered randomly throughout the image.
Common methods of concealing data in digital images include:
- Least significant bit insertion: This is a very popular method because of its simplicity. In this method, the LSB of each byte in the image is used to store the secret data. The resulting changes are too small to be recognized by the human eye. The disadvantage of this technique is that since it uses each pixel in an image, a lossless compression format like bmp or gif has to be used for the image. If lossy compression is used, some of the hidden information might be lost.
- Masking and filtering: These methods hide information in a manner similar to paper watermarks. This can be done, for example, by modifying the luminance of parts of the image. It does change the visible properties of an image, but if done with care the distortion is barely discernable. This method is much more robust than LSB modification with respect to compression since the information is hidden in the visible parts of the image.
- Transformations: This is a more complex way of hiding information in an image. Various algorithms and transformations are applied on the image to make hide information in it. DCT (Direct cosine transformation) is one such method. DCT is used by the JPEG compression algorithm to transform successive 8 x 8 pixel blocks of the image, into 64 DCT coefficients each. Steganography tools can use the LSB of the quantized DCT coefficient can be used to hide information. In addition to DCT, images can be processed with fast Fourier transformation and wavelet transformation. Other image properties such as luminance can also be manipulated.
S-Tools and EzStego are tools that use LSB method for hiding information. These tools, in addition to hiding the information in the LSBs, also do some additional processing to make the hiding less detectable. For example, the EzStego tool arranges the palette to reduce the occurrence of adjacent index colors that contrast too much before it inserts the message. This ensures that there is not to much change in the color of the pixel once the LSB is modified. White Noise Storm is a tool that uses the transformation technique. It applies spread spectrum technology and frequency hopping, which scatters the message throughout the image.
Steganography is a powerful tool that enables people to communicate without eavesdroppers even being aware that the communication is taking place. It can even be combined with cryptography so that the message is not just hidden but also scrambled so that even on being discovered it cannot be read. In the future, the most important use of steganography techniques will probably be in the field of digital watermarking to track the copyright and ownership of electronic media. There has also been growing concern in law enforcement agencies about the use of steganography for exchange of illegal material via web page images, audio and other files. But then as with all such concealment techniques, as the latest conspiracy theorist Dan Brown in his fiction tome Digital Fortress would have us believe, there always is a way to reveal the message. Steganalysis is one such branch of digital crime detection and prevention.
Lookout for an article on Steganalysis in the next issue…