Steganography is the art of covered, or hidden, writing. The purpose of steganography is covert communication to hide a message from a third party. This differs from cryptography, the art of secret writing, which is intended to make a message unreadable by a third party but does not hide the very existence of the secret communication. Although the term steganography was only coined at the very end of the fifteenth century, the use of stego dates back several millennia. Steganography hides the covert message but not the fact that two parties are communicating with each other.
The stego process generally involves placing a hidden message within some transport medium, called the carrier. The secret message is embedded within the carrier to form the stego medium. The use of a stego key may be employed for encryption of the hidden message and/or for randomization within the stego scheme. As an increasing amount of data is stored on computers and transmitted over networks, it is no surprise that steganography has entered the digital age.
On computers and networks, stego applications allow for someone to hide any type of binary file into any other binary file, although image and audio files are today's most common carriers. Steganography provides some very useful and commercially important functions in the digital world, most notably digital watermarking. In this application, an author can embed a hidden message in a file so that he/she can later assert their ownership of intellectual property and/or copyright. An artist, for example, could post some original artwork on a Web site. If someone else should "steal" the file and claims the work as their own, the artist can later prove ownership because only he/she can recover the watermark.
While conceptually similar to stego, digital watermarking usually has different technical goals; generally only a small amount of repetitive information is inserted into the carrier, it is not necessary to hide the watermarking information, and it is useful for the watermark to be able to be removed while maintaining the integrity of the carrier. Steganography has been a field of interest from ancient times and has flourished a lot in recent times due to copyright issues and transmission of secret data. Although cryptography provides a secure way of encrypting data, however the very existence of the data is known; in this case Steganography scores higher than cryptography. This does not mean that Steganography can replace cryptography; it’s just an alternative.
There are two types of Steganography: Fragile and Robust.
Fragile steganography embeds information into a file. If the file is modified then the information is destroyed automatically. It is mainly advantageous when it is required to prove the modification of the file.
Robust Steganography embeds information into a file as is the case with fragile Steganography but here information is not easily destroyed. The information should be hidden in that part of the file where its removal can be easily perceived. Fragile steganography techniques are easier to implement than robust methods.
The application of Steganography lies in the field of digital watermarking where unauthorized and illegal copying of material can be detected. Steganography also finds its applications in secret data transmission where a user can transmit secret data through multimedia mediums without the knowledge of the existence of the data to others. Steganography can be divided in two domains, depending on the component which is used for hiding data i.e. spatial or frequency.
Example of Steganography Hiding Image inside another Image: