Definition
Steganography includes a vast array of techniques for hiding messages in a variety of media. Among these methods are invisible inks, microdots, digital signatures, covert channels and spread-spectrum communications. Today, thanks to modern technology, steganography is used on text, images, sound, signals, and more. The advantage of steganography is that it can be used to secretly transmit messages without the fact of the transmission being discovered. Often, using encryption might identify the sender or receiver as somebody with something to hide. For example, that picture of your cat could conceal the plans for your company's latest technical innovation.

However, steganography has a number of disadvantages as well. Unlike encryption, it generally requires a lot of overhead to hide a relatively few bits of information. However, there are ways around this. Also, once a steganographic system is discovered, it is rendered useless. This problem, too, can be overcome if the hidden data depends on some sort of key for its insertion and extraction.

In fact, it is common practice to encrypt the hidden message before placing it in the cover message. However, it should be noted that the hidden message does not need to be encrypted to qualify as steganography. The message itself can be in plain English and still be a hidden message. However, most steganographers like the extra layer of protection that encryption provides. If your hidden message is found, and then at least make it as protected as possible.

Methods for hiding data in three varied media (text, image, and audio) will be described, and some guidelines for users of steganography will be provided where necessary. In addition, we will take a brief look at steganalysis, the science of detecting steganography, and destroying it. In the field of steganography, some terminology has developed.
The adjectives cover, embedded and stego were defined at the Information Hiding Workshop held in Cambridge, England. The term ``cover'' is used to describe the original, innocent message, data, audio, still, video and so on. When referring to audio signal steganography, the cover signal is sometimes called the ``host'' signal.
The information to be hidden in the cover data is known as the ``embedded'' data. The ``stego'' data is the data containing both the cover signal and the ``embedded'' information. Logically, the processing of putting the hidden or embedded data, into the cover data, is sometimes known as embedding. Occasionally, especially when referring to image steganography, the cover image is known as the container.



Steganography under Various Media
Often, although it is not necessary, the hidden messages will be encrypted. This meets a requirement posed by the ``Kerckhoff principle'' in cryptography. This principle states that the security of the system has to be based on the assumption that the enemy has full knowledge of the design and implementation details of the steganographic system. The only missing information for the enemy is a short, easily exchangeable random number sequence, the secret key. Without this secret key, the enemy should not have the chance to even suspect that on an observed communication channel, hidden communication is taking place. Most of the software that we will discuss later meets this principle.