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This article is about steganography (hidden writing), not to be confused with stenography (shorthand).

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 meaning is obscured.

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 Xerxes by writing it on a wooden panel and covering it in wax, which was itself inscribed with a message. Wax tablets were in common use then as re-usable writing surface, sometimes used for shorthand. The second ancient example is Histaeus who shaved the head of his most trusted slave and tattooed a message on his head. 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 - the covertext. Classically, it may be hidden by using invisible ink between the visible lines of innocuous documents, or even written onto clothing. In WW2 a message was once written in morse code along two-colored knitting yarn. Another method is invisible ink underlining, or simply pin pricking of individual letters in a newspaper article, thus forming a message. It may even be a few words written under a postage stamp, the stamp then being the covertext.

The advantage of steganography over cryptography alone, is that messages do not attract attention to themselves, messengers or to recipients. An unhidden coded message, no matter how unbreakable it is, will arouse suspicion and may in itself be incriminating. In some countries encryption is illegal.

Steganography uses in electronic communication include steganographic coding inside of a transport layer such an MP3 file or a protocol such as UDP.

Steganographic messages are often first encrypted by some traditional means, and then a covertext is modified in some way to contain the encrypted message, 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.

An example from modern practice

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 2 different values of blue. The difference between say 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, i.e., there is 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. Any system with an analog (signal) amplification stage will also introduce so-called thermal or "1/f" noise, which can be exploited as a noise cover. 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.

Steganographic techniques

Steganography has been widely used in historical times, especially before cryptographic systems were developed. Examples of historical usage include:

• Hidden messages in wax tablets: in ancient Greece, people wrote messages on the wood, then covered it with wax so that it looked like an ordinary, unused, tablet.
• Hidden messages on messenger's body: also in ancient Greece. Herodotus tells the story of a message tattooed on a slave's shaved head, hidden by the growth of his hair, and exposed by shaving his head again. The message, if the story is true, carried a warning to Greece about Persian invasion plans.
• Hidden messages on paper written in secret inks under other messages or on the blank parts of other messages.
• During and after World War II, espionage agents used microdots to send information back and forth. Since the dots were typically extremely small -- the size of a period produced by a typewriter (perhaps in a font with 10 or 12 characters per inch) or even smaller -- the stegotext was whatever the dot was hidden within. If a letter or an address, it was some alphabetic characters. If under a postage stamp, it was the presence of the stamp.
• More obscurely, during World War II, a Japanese spy in New York City, Velvalee Dickinson, sent information to accommodation addresses in neutral South America. She was a dealer in dolls, and her letters discussed how many of this or that doll to ship. The stegotext in this case was the doll orders; the 'plaintext' being concealed was itself a codetext giving information about ship movements, etc. Her case became somewhat famous and she became known as the Doll Woman.
• The one-time pad is a theoretically unbreakable cipher that produces ciphertexts indistinguishable from random texts: only those who have the private key can distinguish these ciphertexts from any other perfectly random texts. Thus, any perfectly random data can be used as a covertext for a theoretically unbreakable steganography.
• In the manga Lone Wolf and Cub, a main plot device is a Yagyū letter which has a message written in mulberry extract. They see the message by placing it in a bowl of silkworms and seeing where they eat.

Some more recent steganography techniques:

• Chaffing and winnowing
• Invisible ink
• Null ciphers
• Concealed messages in tampered executable files, exploiting redundancy in the i386 instruction set .

Additional terminology

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.

Rumored usage in terrorism

The rumors about terrorists using steganography started first in the daily newspaper USA Today on February 5th 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: "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.

The USA Today articles were written by veteran foreign correspondent Jack Kelley, who in 2004 was fired in a big scandal, because it appeared that he fabricated a lot of stories and invented sources that didn't exist.

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.

Effective detection of steganographically encoded materials in communications intercepts between suspected terrorists is therefore extremely important, but very complicated, as we will see below.

Countermeasures

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)..

See also

• Covert channel
• DeCSS
• Security engineering
• Canary traps
• Polybius square

External links

Steganography articles

• Steganography Articles, Links, and Whitepapers at Forensics.nl
• Examples showing images hidden in other images
• FBI Article: An Overview of Steganography for the Computer Forensics Examiner

Steganalysis

• Cyber warfare: steganography vs. steganalysis For every clever method and tool being developed to hide information in multimedia data, an equal number of clever methods and tools are being developed to detect and reveal its secrets.
• "Detecting Steganographic Content on the Internet", PDF file, 813 KB.
• Some sample pages of Gaspar Schott's Schola steganographica
• Research Group An example of ongoing research on Steganography.

Implementations

• Phonebook FS protects your disks with Deniable Encryption, aka data hidden in another encrypted data.
• spammimic.com will take a sentence that you provide and turn it into text that looks to all the world like spam.
• Software for steganography
• StegoMagic 1.0 A Cutting Edge Free Steganographic Software
• StegaNote Hiding text and files in images
• stego and winstego Steganography by justified plain text.
• Digital Invisible Ink Toolkit An open-source cross-platform image steganography suite that includes both steganography and steganalysis implementations.
• Steghide Free .jpeg and .wav encryption for Linux and other operating systems.
• Hermetic Stego Windows steganography software for hiding a file of any size in one or more BMP image files.
• TrueCrypt Free, open-source Windows/Linux disk encryption software that supports hiding one encrypted volume inside another, without leaving any evidence that the second encrypted volume exists.

• Steganography
• Espionage