Steganography is 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, a form of security through obscurity. The word steganography is of Greek origin and means "concealed writing" from the Greek words steganos meaning "covered or protected", and graphein meaning "to write". The first recorded use of the term was in 1499 by Johannes Trithemius in his Steganographia, a treatise on cryptography and steganography disguised as a book on magic. Generally, messages will appear to be something else: images, articles, shopping lists, or some other covertext and, classically, the hidden message may be in invisible ink between the visible lines of a private letter.
 
The advantage of steganography, over cryptography alone, is that messages do not attract attention to themselves. Plainly visible encrypted messages—no matter how unbreakable—will arouse suspicion, and may in themselves be incriminating in countries where encryption is illegal. Therefore, whereas cryptography protects the contents of a message, steganography can be said to protect both messages and communicating parties.
 
Steganography includes the concealment of information within computer files. In digital steganography, electronic communications may include steganographic coding inside of a transport layer, such as a document file, image file, program or protocol. Media files are ideal for steganographic transmission because of their large size. As a simple example, a sender might start with an innocuous image file and adjust the color of every 100th pixel to correspond to a letter in the alphabet, a change so subtle that someone not specifically looking for it is unlikely to notice it.
 
The first recorded uses of steganography can be traced back to 440 BC when Herodotus mentions two examples of steganography in The Histories of Herodotus. Demaratus sent a warning about a forthcoming attack to Greece by writing it directly on the wooden backing of a wax tablet before applying its beeswax surface. Wax tablets were in common use then as reusable writing surfaces, sometimes used for shorthand. Another ancient example is that of Histiaeus, who shaved the head of his most trusted slave and tattooed a message on it. After his hair had grown the message was hidden. The purpose was to instigate a revolt against the Persians.
 
Steganography has been widely used, including in recent historical times and the present day. Possible permutations are endless and known examples include:
* Hidden messages within wax tablets: in ancient Greece, people wrote messages on the wood, then covered it with wax upon which an innocent covering message was written.
* Hidden messages on messenger's body: also used in ancient Greece. Herodotus tells the story of a message tattooed on a slave's shaved head, hidden by the growth of his hair, and exposed by shaving his head again. The message allegedly carried a warning to Greece about Persian invasion plans. This method has obvious drawbacks, such as delayed transmission while waiting for the slave's hair to grow, and the restrictions on the number and size of messages that can be encoded on one person's scalp.
* In WWII, the French Resistance sent some messages written on the backs of couriers using invisible ink.
* Hidden messages on paper written in secret inks, under other messages or on the blank parts of other messages.
* Messages written in Morse code on knitting yarn and then knitted into a piece of clothing worn by a courier.
* Messages written on the back of postage stamps.
* During and after World War II, espionage agents used photographically produced microdots to send information back and forth. Microdots were typically minute, approximately less than the size of the period produced by a typewriter. WWII microdots needed to be embedded in the paper and covered with an adhesive (such as collodion). This was reflective and thus detectable by viewing against glancing light. Alternative techniques included inserting microdots into slits cut into the edge of post cards.
* During World War II, a spy for Japan in New York City, Velvalee Dickinson, sent information to accommodation addresses in neutral South America. She was a dealer in dolls, and her letters discussed how many of this or that doll to ship. The stegotext was the doll orders, while the concealed "plaintext" was itself encoded and gave information about ship movements, etc. Her case became somewhat famous and she became known as the Doll Woman.
* Cold War counter-propaganda. In 1968, crew members of the USS Pueblo (AGER-2) intelligence ship held as prisoners by North Korea, communicated in sign language during staged photo opportunities, informing the United States they were not defectors but rather were being held captive by the North Koreans. In other photos presented to the US, crew members gave "the finger" to the unsuspecting North Koreans, in an attempt to discredit photos that showed them smiling and comfortable.
 
--
http://e...content-available-to-author-only...a.org/wiki/Steganography
 

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