Editing Steganography (section)

===Example from modern practice===
The larger the cover message (in binary data, the number of [[bit]]s) relative to the hidden message, the easier it is to hide the hidden message (as an analogy, the larger the "haystack", the easier it is to hide a "needle"). So [[digital image|digital picture]]s, which contain much data, are sometimes used to hide messages on the [[Internet]] and on other digital communication media. It is not clear how common this practice actually is.

For example, a 24-bit [[bitmap]] uses 8 bits to represent each of the three color values (red, green, and blue) of each [[pixel]]. The blue alone has 2<sup>8</sup> different levels of blue intensity. The difference between 11111111<sub>2</sub> and 11111110<sub>2</sub> in the value for blue intensity is likely to be undetectable by the human eye. Therefore, the [[least significant bit]] can be used more or less undetectably for something else other than color information. If that is repeated for the green and the red elements of each pixel as well, it is possible to encode one letter of [[ASCII]] text for every three [[pixel]]s.

Stated somewhat more formally, the objective for making steganographic encoding difficult to detect is to ensure that the changes to the carrier (the original signal) because of the injection of the payload (the signal to covertly embed) are visually (and ideally, statistically) negligible. The changes are indistinguishable from the [[noise floor]] of the carrier. All media can be a carrier, but media with a large amount of redundant or compressible information is better suited.

From an [[information theory|information theoretical]] point of view, that means that the [[Communication channel|channel]] must have more [[channel capacity|capacity]] than the "surface" [[signal (information theory)|signal]] requires. There must be [[redundancy (information theory)|redundancy]]. For a digital image, it may be [[noise]] from the imaging element; for [[digital audio]], it may be noise from recording techniques or [[amplifier|amplification]] equipment. In general, electronics that digitize an [[analog signal]] suffer from several noise sources, such as [[Johnson-Nyquist noise|thermal noise]], [[flicker noise]], and [[shot noise]]. The noise provides enough variation in the captured digital information that it can be exploited as a noise cover for hidden data. In addition, [[lossy compression]] schemes (such as [[JPEG]]) always introduce some error to the decompressed data, and it is possible to exploit that for steganographic use, as well.

Although steganography and digital watermarking seem similar, they are not. In steganography, the hidden message should remain intact until it reaches its destination. Steganography can be used for [[digital watermark]]ing in which a message (being simply an identifier) is hidden in an image so that its source can be tracked or verified (for example, [[Coded Anti-Piracy]]) or even just to identify an image (as in the [[EURion constellation]]). In such a case, the technique of hiding the message (here, the watermark) must be robust to prevent tampering. However, digital watermarking sometimes requires a brittle watermark, which can be modified easily, to check whether the image has been tampered with. That is the key difference between steganography and digital watermarking.