Introduction
In this paper, we will introduce what steganography is and what kindof applications can be expected. Steganography is an art and scienceof hidding information within other information. The word itself comesfrom Greek and means hidden writting. First complex book coveringsteganography was written by Johannes Trithemius in 1499. Thebook Steganographia itself was published later in 1606 and immediatelyplaced on the Index Librorum Prohibitorum.
In recent years cryptography become very popular science. As steganographyhas very close to cryptography and its applications, we can withadvantage highlight the main differences. Cryptography is about concealingthe content of the message. At the same time encrypted data packageis itself evidence of the existence of valuable information. Steganographygoes a step further and makes the ciphertext invisible to unauthorizedusers. Hereby we can dene steganography as cryptography withthe additional property that its output looks unobtrusively.
One can ask what is it good for. Well, image the common situationwhen you encrypt your important business data. Suddenly robbers captureand torture you into revealing cryptographics keys. As well policepower may be abused. They ask you to give them the private keys or youare highly suspicious of committing crime. Next, what if the police isbribed. Would not it be better, if you can plausibly deny the existence ofimportant data?
A famous example of steganography is Simmons' Prisoners' problem, see [1]. Bob and Alice are in a jail and wish to escape. Their cellsare far apart from each other and the only allowed communication issending messages via prison ofcer. If warden detects any sign of conspiracy,he will secure their cells even more. Bob and Alice are wellaware of these facts.
Happily, before they got arrested, they have agreed a stegosystem.Stegosystem describes the way the secret message is embedded into a covertext(seemingly innocent message). According to the standard terminologyof information hidding a covertext with hidden information iscalled stegotext.
Examples of historical stegosystems can be secret inks, wax tabletsor microdots used during WWII. In modern era these methods can invokesmile on face, but image their power, when they were not widely known.Nowadays methods still hold the same simplicity, just exploit advantagesof digital media and communications.
We can distinguish between stegosystems with passive warder andactive warder. Passive warder just monitors the communication channel.He can pass the covertexts throught several statistical tests, but do notmodify them. It is the same situation as when the network packets gothrought Intrusion Detect System. Applications from this eld are oftenrefered to as trafc security.
On the other side, active warden manipulates covertexts in order topreclude the possibility of hidden communication. Bob and Alice haveto use very sophisticated embedding algorithm. Hidden information mustwithstand various recoding of covering medium, the use of error correctioncodes is recommended. Typical real-life application is watermarkingand ngerprinting.
Watermark is a small piece of embedded information which can proofcopyrighted material. Fingerprint is very similar, but is intended to trackthe concrete copy of copyrighted data.
