STEGANOGRAPHY

STEGANOGRAPHY is the practice of concealing a file, message, image, or video within another file, message, image, or video. The word steganography combines the Greek words steganos (στεγανός), meaning "covered, concealed, or protected", and graphein (γράφειν) meaning "writing".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, the hidden messages appear to be (or be part of) something else: images, articles, shopping lists, or some other cover text. For example, 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 the intended secret message does not attract attention to itself as an object of scrutiny. Plainly visible encrypted messages—no matter how unbreakable—arouse interest, and may in themselves be incriminating in countries where encryption is illegal.The advantage of steganography over cryptography alone is that the intended secret message does not attract attention to itself as an object of scrutiny. Plainly visible encrypted messages—no matter how unbreakable—arouse interest, and may in themselves be incriminating in countries where encryption is illegal.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. For 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.

Techniques

• Physical
• Digital messages
• Digital Text
• Social Steganography
• Network
• Printed
• Using Puzzles

Additional terminology

Discussions of steganography generally use terminology analogous to (and consistent with) conventional radio and communications technology. However, some terms show up in software specifically, and are easily confused. These are most relevant to digital steganographic systems.

The payload is the data covertly communicated. The carrier is the signal, stream, or data file that hides the payload—which differs from the channel (which typically means the type of input, such as a JPEG image). The resulting signal, stream, or data file with the encoded payload is sometimes called the package, stego file, or covert message. The percentage of bytes, samples, or other signal elements modified to encode the payload is called the encoding density, and is typically expressed as a number between 0 and 1.

In a set of files, those files considered likely to contain a payload are suspects. A suspect identified through some type of statistical analysis might be referred to as a candidate.

Countermeasures and detection

In computing, steganographically encoded package detection is called steganalysis. The simplest method to detect modified files, however, is to compare them to known originals. For example, to detect information being moved through the graphics on a website, an analyst can maintain known-clean copies of these materials and compare them against the current contents of the site. The differences, assuming the carrier is the same, comprise the payload. In general, using extremely high compression rate makes steganography difficult, but not impossible. Compression errors provide a hiding place for data—but high compression reduces the amount of data available to hold the payload, raising the encoding density, which facilitates easier detection (in extreme cases, even by casual observation).

Applications

The larger the cover message (in binary data, the number of bits) relative to the hidden message, the easier it is to hide the latter. For this reason, digital pictures (which contain large amounts of data) are used to hide messages on the Internet and on other communication media. It is not clear how commonly this actually is. For example: a 24-bit bitmapuses 8 bits to represent each of the three color values (red, green, and blue) at each pixel. The blue alone has 2⁸ different levels of blue intensity. The difference between 11111111 and 11111110 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 this 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 pixels.

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) due to the injection of the payload (the signal to covertly embed) are visually (and ideally, statistically) negligible; that is to say, the changes are indistinguishable from the noise floor of the carrier. Any medium can be a carrier, but media with a large amount of redundant or compressible information are better suited.

- Swagat Gaire

source: wikipedia.org