This post is cut and (partially pasted)
from teachengineering.org website.
Steganography, which is the art and science of hiding information in plain sight.
Your inkjet printer generates subliminal messages. Steganography is barely visible tiny yellow dots on each page contain encoded printer serial numbers as well as date and time stamps.
Steganography is a method for concealing information within seemingly innocent media. The true craft comes from sending the information in a manner so that only the sender and the intended recipient realize its existence. Steganography may be as simple as altering the language of a message. It can also involve using a “container,” such as a jpg image, to carry a “cargo” of data, such as a text file. However the message is concealed, the art is in hiding the data in plain sight so as to hide its true intention. With steganography, seemingly unaltered images, video files, sound files and even blank disc space can all inconspicuously carry extra data. Software engineers are on the front lines in detecting steganographic methods employed by modern terrorists to hide details of their plans. Due to the prolific multimedia content on the Internet that is easily shared using social media, terrorists are able use this information to convey their plans. Modern software engineers or cyber terrorism specialists scan Internet traffic in the attempt to detect these hidden messages.
History of Steganography
The first recorded use of steganography dates back to the early Greeks and the ruler, Demaratus. As king of Sparta, Demaratus ruled from 515 to 491 BC. At this time, it was common to write on waxed tablets. To communicate a warning of an impending attack on Greece by the Persian ruler Xerxes, Demaratus scraped the wax off of a wooden tablet, inscribed a message, and then covered the tablet in wax once again. In this way, the tablets carried secret information to the intended recipient without arousing suspicion. Other times of conflict are rich with examples of steganography. The Underground Railroad used quilt patterns to direct slaves on their journey northward, cryptographic writing was used numerous times in both World Wars I and II, and it is suspected the 9/11 terrorists communicated through messages hidden in jpg images. Even gang graffiti can covertly pass information.