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Steganography is the art and science of writing hidden messages in such a way that no one apart from the intended recipient knows of the existence of the message; this is in contrast to cryptography, where the existence of the message itself is not disguised, but the content is obscured. Quite often, steganography is hidden in pictures.
The word "Steganography" is of Greek origin and means "covered, or hidden writing". Its ancient origins can be traced back to 440 BC. Herodotus mentions two examples of Steganography in The Histories of Herodotus . Demeratus sent a warning about a forthcoming attack to Greece by writing it on a wooden panel and covering it in wax. Wax tablets were in common use then as re-usable writing surface, sometimes used for shorthand. Another ancient example is that of Histiaeus, who shaved the head of his most trusted slave and tattooed a message on it. After his hair had grown the message was hidden. The purpose was to instigate a revolt against the Persians. Later, Johannes Trithemius's book Steganographia is a treatise on cryptography and steganography disguised as a book on black magic.
Generally, a steganographic message will appear to be something else: a picture, an article, a shopping list, or some other message. This apparent message is the covertext. For instance, a message may be hidden by using invisible ink between the visible lines of innocuous documents.
The advantage of steganography over cryptography alone is that messages do not attract attention to themselves, to messengers, or to recipients. An unhidden coded message, no matter how unbreakable it is, will arouse suspicion and may in itself be incriminating, as in some countries encryption is illegal .
Steganography used in electronic communication include steganographic coding inside of a transport layer, such as an MP3 file, or a protocol, such as UDP.
A steganographic message (the plaintext) is often first encrypted by some traditional means, and then a covertext is modified in some way to contain the encrypted message (ciphertext), resulting in stegotext. For example, the letter size, spacing, typeface, or other characteristics of a covertext can be manipulated to carry the hidden message; only the recipient (who must know the technique used) can recover the message and then decrypt it. Francis Bacon is known to have suggested such a technique to hide messages (see Bacon's cipher).
Steganography has been widely used in historical times, especially before cryptographic systems were developed. Examples of historical usage include:
In general, terminology analogous to (and consistent with) more conventional radio and communications technology is used; however, a brief description of some terms which show up in software specifically, and are easily confused, is appropriate. These are most relevant to digital steganographic systems.
The payload is the data it is desirable to transport (and, therefore, to hide). The carrier is the signal, stream, or data file into which the payload is hidden; contrast "channel" (typically used to refer to the type of input, such as "a JPEG image"). The resulting signal, stream, or data file which has the payload encoded into it is sometimes referred to as the package. The percentage of bytes, samples, or other signal elements which are modified to encode the payload is referred to as the encoding density and is typically expressed as a floating-point number between 0 and 1.
In a set of files, those files considered likely to contain a payload are called suspects. If the suspect was identified through some type of statistical analysis, it may be referred to as a candidate.
The detection of steganographically encoded packages is called steganalysis. The simplest method to detect modified files, however, is to compare them to the originals. To detect information being moved through the graphics on a website, for example, 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) will compose the payload.
In general, using an extremely high compression rate makes steganography difficult, but not impossible; while compression errors provide a good place to hide data, high compression reduces the amount of data available to hide the payload in, raising the encoding density and facilitating easier detection (in the extreme case, even by casual observation).
Steganography is used by some modern printers, including HP and Xerox brand color laser printers. Tiny yellow dots are added to each page. The dots are barely visible and contain encoded printer serial numbers, as well as date and time stamps.
The larger the cover message is (in data content terms — 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 is actually done. For example: a 24-bit bitmap will have 8 bits representing each of the three color values (red, green, and blue) at each pixel. If we consider just the blue there will be 28 different values of blue. 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 we do it with the green and the red as well we can get 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.
From an information theoretical point of view, this means that the channel must have more capacity than the 'surface' signal requires, that is, there must be redundancy. For a digital image, this may be noise from the imaging element; for digital audio, it may be noise from recording techniques or amplification equipment. In general, electronics that digitize an analog signal suffer from several noise sources such as thermal noise, flicker noise, and shot noise. This 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 into the decompressed data; it is possible to exploit this for steganographic use as well.
Steganography can be used for digital watermarking, where a message (being simply an identifier) is hidden in an image so that its source can be tracked or verified.
In the era of Digital video recorder and devices like TiVo, TV commercials authors have figured out how to make use of such devices as well - by putting a hidden message which becomes visible when played at frame-by-frame speed (see KFC Unveils 'TiVo-proof' Ad).
The rumors about terrorists using steganography started first in the daily newspaper USA Today on February 5, 2001.
The articles are still available online, and were titled "Terrorist instructions hidden online", and the same day, "Terror groups hide behind Web encryption". In July of the same year, the information looked even more precise: "Militants wire Web with links to jihad".
A citation from the USA Today article: "Lately, al-Qaeda operatives have been sending hundreds of encrypted messages that have been hidden in files on digital photographs on the auction site eBay.com". These rumors were cited many times—without ever showing any actual proof—by other media worldwide, especially after the terrorist attack of 9/11.
For example, the Italian newspaper Corriere della Sera reported that an Al Qaeda cell which had been captured at the Via Quaranta mosque in Milan had had pornographic images on their computers, and that these images had been used to hide secret messages (although no other Italian paper ever covered the story).
The USA Today articles were written by veteran foreign correspondent Jack Kelley, who in 2004 was fired after allegations emerged that he had fabricated stories and invented sources.
In October 2001, the New York Times published an article claiming that al-Qaeda had used steganographic techniques to encode messages into images, and then transported these via email and possibly via USENET to prepare and execute the September 11, 2001 Terrorist Attack.
Despite being dismissed by security experts , the story has been widely repeated and resurfaces frequently. It was noted that the story apparently originated with a press release from "iomart" , a vendor of steganalysis software. No corroborating evidence has been produced by any other source.
Moreover, a captured al-Qaeda training manual makes no mention of this method of steganography. The chapter on communications in the al-Qaeda manual acknowledges the technical superiority of US security services, and generally advocates low-technology forms of covert communication.
The chapter on "codes and ciphers" places considerable emphasis on using invisible inks in traditional paper letters, plus simple ciphers such as simple substitution with nulls; computerized image steganography is not mentioned.
Nevertheless public efforts were mounted to detect the presence of steganographic information in images on the web (especially on eBay, which had been mentioned in the New York Times article).
To date these scans have examined millions of images without detecting any steganographic content (see "Detecting Steganographic Content on the Internet" under external links), other than test images used to test the system, and instructional images on web sites about steganography.