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Introduction to Steganography and Data Hiding Methods
Steganography is a technique for hiding information. The word steganography comes from Greek roots, literally meaning "covered writing". The goal of steganography is to conceal the existence of a hidden message or a piece of information. Steganography and cryptography, although closely related, are nevertheless different in a fundamental way. While the goal of cryptography is to conceal the contents of a message, steganography tries to hide the very existence of such. However, these two techniques can be integrated very efficiently by first encrypting the secret message and then hiding it. A variety of steganographic methods have been used throughout history. Ancient Greeks, for example, used to shave the hair of a slave messenger, tatoo the message on his bare head, and let the hair grow again. The recepient would shave off the hair and recover the message. Steganographic techniques had also been used extensively during World Wars I and II, among them invisible inks (chemicals that react only with other specific chemicals and reveal the invisible writing), microdot (photographs the size of a printed period) and null-cipher messages (unencrypted innocent-sounding messages that camouflage the real ones). Today, with most of communication occuring electronically, digital multimedia signals are widely used as cover signals. There are many possible applications that utilize various data hiding schemes:
We will concentrate on applications using digital images as the cover signals. Descriptions of other applications (audio, text) can be found in [5]. Data hiding schemes can be characterized using the following parameters [6]:
From information theory perspective, the steganographic channel can be viewed as one having a noise of large power, which is the cover image, and the signal itself is the embedded message. Therefore, a low signal-to-noise ratio is desired in order to satisfy the imperceptibility requirement (steganographic SNR). It is not possible to simultaneously achieve high robustness and capacity while maintaining low SNR. Therefore, trade-offs must be made between these parameters, as required by specific application. For example, in an information-hiding application invisibility and capacity are more important than resistance to removal; watermarking application, on the other hand, will prefer high resistance to removal, but not high capacity and even invisibility. Additionally, data hiding schemes can be classified as follows:
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