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Steganography ( ) is the practice of representing information within another message or physical object, in such a manner that the presence of the information is not evident to human inspection. In computing/electronic contexts, a
computer file A computer file is a computer resource for recording data in a computer storage device, primarily identified by its file name. Just as words can be written to paper, so can data be written to a computer file. Files can be shared with and transfe ...
, message, image, or video is concealed within another file, message, image, or video. The word ''steganography'' comes from
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
''steganographia'', which combines the words ''steganós'' (), meaning "covered or concealed", and ''-graphia'' () meaning "writing". The first recorded use of the term was in 1499 by
Johannes Trithemius Johannes Trithemius (; 1 February 1462 – 13 December 1516), born Johann Heidenberg, was a German Benedictine abbot and a polymath who was active in the German Renaissance as a lexicographer, chronicler, cryptographer, and occultist. He is consi ...
in his ''
Steganographia ''Steganographia'' is a book on steganography, written in c. 1499 by the German Benedictine abbot and polymath Johannes Trithemius. General Trithemius' most famous work, ''Steganographia'' (written c. 1499; published Frankfurt, 1606), was p ...
'', a treatise on
cryptography Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adver ...
and steganography, disguised as a book on magic. Generally, the hidden messages appear to be (or to be part of) something else: images, articles, shopping lists, or some other cover text. For example, the hidden message may be in
invisible ink Invisible ink, also known as security ink or sympathetic ink, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by some means, such as heat or ultraviolet light. Invisible ...
between the visible lines of a private letter. Some implementations of steganography that lack a
shared secret In cryptography, a shared secret is a piece of data, known only to the parties involved, in a secure communication. This usually refers to the key of a symmetric cryptosystem. The shared secret can be a password, a passphrase, a big number, or a ...
are forms of
security through obscurity Security through obscurity (or security by obscurity) is the reliance in security engineering on design or implementation secrecy as the main method of providing security to a system or component. History An early opponent of security through ob ...
, and key-dependent steganographic schemes adhere to
Kerckhoffs's principle Kerckhoffs's principle (also called Kerckhoffs's desideratum, assumption, axiom, doctrine or law) of cryptography was stated by Dutch-born cryptographer Auguste Kerckhoffs in the 19th century. The principle holds that a cryptosystem should be s ...
. The advantage of steganography over
cryptography Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adver ...
alone is that the intended secret message does not attract attention to itself as an object of scrutiny. Plainly visible
encrypted In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decip ...
messages, no matter how unbreakable they are, arouse interest and may in themselves be incriminating in countries in which
encryption In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decip ...
is illegal. Whereas cryptography is the practice of protecting the contents of a message alone, steganography is concerned with concealing the fact that a secret message is being sent and its contents. 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 hundredth
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the smal ...
to correspond to a letter in the alphabet. The change is so subtle that someone who is not specifically looking for it is unlikely to notice the change.


History

The first recorded uses of steganography can be traced back to 440 BC in
Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders with ...
, when
Herodotus Herodotus ( ; grc, , }; BC) was an ancient Greek historian and geographer from the Greek city of Halicarnassus, part of the Persian Empire (now Bodrum, Turkey) and a later citizen of Thurii in modern Calabria ( Italy). He is known f ...
mentions two examples in his '' Histories''.
Histiaeus Histiaeus (, died 493 BC), the son of Lysagoras, was a Greek ruler of Miletus in the late 6th century BC. Histiaeus was tyrant of Miletus under Darius I, king of Achaemenid Empire, Persia, who had subjugated Miletus and the other Ionian ...
sent a message to his vassal,
Aristagoras Aristagoras ( grc-gre, Ἀρισταγόρας ὁ Μιλήσιος), d. 497/496 BC, was the leader of the Ionian city of Miletus in the late 6th century BC and early 5th century BC and a key player during the early years of the Ionian Revolt a ...
, by shaving the head of his most trusted servant, "marking" the message onto his scalp, then sending him on his way once his hair had regrown, with the instruction, "When thou art come to Miletus, bid Aristagoras shave thy head, and look thereon." Additionally,
Demaratus Demaratus ( el, Δημάρατος ; Doric: ) was a king of Sparta from around 515 BC to 491 BC. The 15th of the Eurypontid line, he was the first son born to his father, King Ariston. As king, Demaratus is known chiefly for his opposition t ...
sent a warning about a forthcoming attack to Greece by writing it directly on the wooden backing of a
wax tablet A wax tablet is a tablet made of wood and covered with a layer of wax, often linked loosely to a cover tablet, as a "double-leaved" diptych. It was used as a reusable and portable writing surface in Antiquity and throughout the Middle Ages. C ...
before applying its beeswax surface. Wax tablets were in common use then as reusable writing surfaces, sometimes used for
shorthand Shorthand is an abbreviated symbolic writing method that increases speed and brevity of writing as compared to longhand, a more common method of writing a language. The process of writing in shorthand is called stenography, from the Greek ''ste ...
. In his work ''Polygraphiae,''
Johannes Trithemius Johannes Trithemius (; 1 February 1462 – 13 December 1516), born Johann Heidenberg, was a German Benedictine abbot and a polymath who was active in the German Renaissance as a lexicographer, chronicler, cryptographer, and occultist. He is consi ...
developed his so-called " Ave-Maria-Cipher" that can hide information in a Latin praise of God. "''Auctor Sapientissimus Conseruans Angelica Deferat Nobis Charitas Potentissimi Creatoris''" for example contains the concealed word ''VICIPEDIA''.


Techniques


Physical

Steganography has been widely used for centuries. Some examples include: * Hidden messages on a paper written in secret inks. *Hidden messages distributed, according to a certain rule or key, as smaller parts (e.g. words or letters) among other words of a less suspicious cover text. This particular form of steganography is called a
null cipher A null cipher, also known as concealment cipher, is an ancient form of encryption where the plaintext is mixed with a large amount of non-cipher material. Today it is regarded as a simple form of steganography, which can be used to hide ciphertext ...
. * Messages written in
Morse code Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...
on
yarn Yarn is a long continuous length of interlocked fibres, used in sewing, crocheting, knitting, weaving, embroidery, ropemaking, and the production of textiles. Thread is a type of yarn intended for sewing by hand or machine. Modern manufact ...
and then knitted into a piece of clothing worn by a courier. * Messages written on envelopes in the area covered by
postage stamp A postage stamp is a small piece of paper issued by a post office, postal administration, or other authorized vendors to customers who pay postage (the cost involved in moving, insuring, or registering mail), who then affix the stamp to the fa ...
s. * In the early days of the printing press, it was common to mix different typefaces on a printed page because the printer did not have enough copies of some letters in one typeface. Thus, a message could be hidden by using two or more different typefaces, such as normal or italic. *During and after World War II,
espionage Espionage, spying, or intelligence gathering is the act of obtaining secret or confidential information (intelligence) from non-disclosed sources or divulging of the same without the permission of the holder of the information for a tangibl ...
agents used photographically-produced
microdot A microdot is text or an image substantially reduced in size to prevent detection by unintended recipients. Microdots are normally circular and around in diameter but can be made into different shapes and sizes and made from various materials su ...
s to send information back and forth. Microdots were typically minute (less than the size of the
period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...
produced by a
typewriter A typewriter is a mechanical or electromechanical machine for typing characters. Typically, a typewriter has an array of keys, and each one causes a different single character to be produced on paper by striking an inked ribbon selectivel ...
). World War II microdots were embedded in the paper and covered with an adhesive, such as
collodion Collodion is a flammable, syrupy solution of nitrocellulose in ether and alcohol. There are two basic types: flexible and non-flexible. The flexible type is often used as a surgical dressing or to hold dressings in place. When painted on the skin, ...
that was reflective and so was detectable by viewing against glancing light. Alternative techniques included inserting microdots into slits cut into the edge of postcards. * During World War II, Velvalee Dickinson, a spy for
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
in
New York City New York, often called New York City or NYC, is the List of United States cities by population, most populous city in the United States. With a 2020 population of 8,804,190 distributed over , New York City is also the L ...
, sent information to accommodation addresses in neutral
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
. She was a dealer in
doll A doll is a physical model, model typically of a human or humanoid character, often used as a toy for children. Dolls have also been used in traditional religious rituals throughout the world. Traditional dolls made of materials such as clay and ...
s, and her letters discussed the quantity and type of doll to ship. The stegotext was the doll orders, and the concealed "plaintext" was itself encoded and gave information about ship movements, etc. Her case became somewhat famous and she became known as the Doll Woman. * During World War II,
photosensitive glass Photosensitive glass, also known as photostructurable glass (PSG) or photomachinable glass, is a crystal-clear glass that belongs to the lithium-silicate family of glasses, in which an image of a mask can be captured by microscopic metallic par ...
was declared secret, and used for transmitting information to Allied armies. *
Jeremiah Denton Jeremiah Andrew Denton Jr. (July 15, 1924 – March 28, 2014) was an American politician and military officer who served as a U.S. Senator representing Alabama from 1981 to 1987. He was the first Republican to be popularly elected to a Sena ...
repeatedly blinked his eyes in
Morse code Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...
during the 1966 televised press conference that he was forced into as an American
prisoner-of-war A prisoner of war (POW) is a person who is held captive by a belligerent power during or immediately after an armed conflict. The earliest recorded usage of the phrase "prisoner of war" dates back to 1610. Belligerents hold prisoners of w ...
by his North Vietnamese captors, spelling out "T-O-R-T-U-R-E". That confirmed for the first time to the
US Naval Intelligence The Office of Naval Intelligence (ONI) is the military intelligence agency of the United States Navy. Established in 1882 primarily to advance the Navy's modernization efforts, it is the oldest member of the U.S. Intelligence Community and serve ...
and other Americans that the North Vietnamese were torturing American prisoners-of-war. *In 1968, crew members of the USS ''Pueblo'' intelligence ship, held as prisoners by
North Korea North Korea, officially the Democratic People's Republic of Korea (DPRK), is a country in East Asia. It constitutes the northern half of the Korea, Korean Peninsula and shares borders with China and Russia to the north, at the Yalu River, Y ...
, communicated in sign language during staged photo opportunities, to inform the United States that they were not defectors but captives of the North Koreans. In other photos presented to the US, crew members gave "
the finger In Western culture, "the finger", or the middle finger (as in giving someone the (middle) finger, the bird or flipping someone off) is an obscene hand gesture. The gesture communicates moderate to extreme contempt, and is roughly equivalent i ...
" to the unsuspecting North Koreans, in an attempt to discredit photos that showed them smiling and comfortable. *In 1985, a
klezmer Klezmer ( yi, קלעזמער or ) is an instrumental musical tradition of the Ashkenazi Jews of Central and Eastern Europe. The essential elements of the tradition include dance tunes, ritual melodies, and virtuosic improvisations played for l ...
saxophonist The saxophone (often referred to colloquially as the sax) is a type of single-reed woodwind instrument with a conical body, usually made of brass. As with all single-reed instruments, sound is produced when a reed on a mouthpiece vibrates to pr ...
smuggled secrets into and out of the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen national ...
by coding them as pitches of
musical notes In music, a note is the representation of a musical sound. Notes can represent the pitch and duration of a sound in musical notation. A note can also represent a pitch class. Notes are the building blocks of much written music: discretization ...
in
sheet music Sheet music is a handwritten or printed form of musical notation that uses List of musical symbols, musical symbols to indicate the pitches, rhythms, or chord (music), chords of a song or instrumental Musical composition, musical piece. Like ...
.


Digital messages

* Concealing messages within the lowest bits of noisy images or sound files. A survey and evaluation of relevant literature/techniques on the topic of digital image steganography can be found here. * Concealing data within encrypted data or within random data. The message to conceal is encrypted, then used to overwrite part of a much larger block of encrypted data or a block of random data (an unbreakable cipher like the
one-time pad In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is not smaller than the message being sent. In this technique, a plaintext is paired with a ran ...
generates ciphertexts that look perfectly random without the private key). *
Chaffing and winnowing Chaffing and winnowing is a cryptographic technique to achieve confidentiality without using encryption when sending data over an insecure channel. The name is derived from agriculture: after grain has been harvested and threshed, it remains mix ...
. *
Mimic function A mimic function changes a file A so it assumes the statistical properties of another file B. That is, if p(t,A) is the probability of some substring t occurring in A, then a mimic function f, recodes A so that p(t,f(A)) approximates p(t,B) for al ...
s convert one file to have the statistical profile of another. This can thwart statistical methods that help brute-force attacks identify the right solution in a
ciphertext-only attack In cryptography, a ciphertext-only attack (COA) or known ciphertext attack is an attack model for cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts. While the attacker has no channel providing access to the pl ...
. * Concealed messages in tampered executable files, exploiting redundancy in the targeted
instruction set In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ' ...
. * Pictures embedded in video material (optionally played at a slower or faster speed). * Injecting imperceptible delays to packets sent over the network from the keyboard. Delays in keypresses in some applications (
telnet Telnet is an application protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. User data is interspersed in-band with Telnet control i ...
or
remote desktop software In computing, the term remote desktop refers to a software- or operating system feature that allows a personal computer's desktop environment to be run remotely off of one system (usually a PC, but the concept applies equally to a server or a ...
) can mean a delay in packets, and the delays in the packets can be used to encode data. * Changing the order of elements in a set. * Content-Aware Steganography hides information in the semantics a human user assigns to a
datagram A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The del ...
. These systems offer security against a nonhuman adversary/warden. *
Blog A blog (a truncation of "weblog") is a discussion or informational website published on the World Wide Web consisting of discrete, often informal diary-style text entries (posts). Posts are typically displayed in reverse chronological order ...
-Steganography. Messages are fractionalized and the (encrypted) pieces are added as comments of orphaned web-logs (or pin boards on social network platforms). In this case, the selection of blogs is the symmetric key that sender and recipient are using; the carrier of the hidden message is the whole
blogosphere The blogosphere is made up of all blogs and their interconnections. The term implies that blogs exist together as a connected community (or as a collection of connected communities) or as a social networking service in which everyday authors can p ...
. * Modifying the echo of a sound file (Echo Steganography). * Steganography for audio signals. * Image bit-plane complexity segmentation steganography * Including data in ignored sections of a file, such as after the logical end of the carrier file. * Adaptive steganography: Skin tone based steganography using a secret embedding angle. * Embedding data within the
control-flow diagram A control-flow diagram (CFD) is a diagram to describe the control flow of a business process, process or review. Control-flow diagrams were developed in the 1950s, and are widely used in multiple engineering disciplines. They are one of the cla ...
of a program subjected to
control flow analysis In computer science, control-flow analysis (CFA) is a static-code-analysis technique for determining the control flow of a program. The control flow is expressed as a control-flow graph (CFG). For both functional programming languages and object- ...


Digital text

* Using non-printing Unicode characters
Zero-Width Joiner The zero-width joiner (ZWJ, ) is a non-printing character used in the computerized typesetting of writing systems in which the shape or positioning of a grapheme depends on its relation to other graphemes ( complex scripts), such as the Arabic s ...
(ZWJ) and
Zero-Width Non-Joiner The zero-width non-joiner (ZWNJ) is a non-printing character used in the computerization of writing systems that make use of ligatures. When placed between two characters that would otherwise be connected into a ligature, a ZWNJ causes them to ...
(ZWNJ). These characters are used for joining and disjoining letters in Arabic and Persian, but can be used in Roman alphabets for hiding information because they have no meaning in Roman alphabets: because they are "zero-width" they are not displayed. ZWJ and ZWNJ can represent "1" and "0". This may also be done with
en space An en (from English '' en quadrat'') is a typographic unit, half of the width of an em. By definition, it is equivalent to half of the body height of the typeface (e.g., in 16- point type it is 8 points). As its name suggests, it is also tradi ...
,
figure space A figure space or numeric space is a typographic unit equal to the size of a single numerical digit. Its size can fluctuate somewhat depending on which font is being used. This is the preferred space to use in numbers. It has the same width as a d ...
and
whitespace characters In computer programming, whitespace is any character or series of characters that represent horizontal or vertical space in typography. When rendered, a whitespace character does not correspond to a visible mark, but typically does occupy an are ...
. * Embedding a secret message in the pattern of deliberate errors and marked corrections in a word processing document, using the word processor's change tracking feature. * In 2020, Zhongliang Yang et al discovered that for text generative steganography, when the quality of the generated steganographic text is optimized to a certain extent, it may make the overall statistical distribution characteristics of the generated steganographic text more different from the normal text, making it easier to be recognized. They named this phenomenon Perceptual-Statistical Imperceptibility Conflict Effect (Psic Effect).


Hiding an image within a soundfile

An image or a text can be converted into a soundfile, which is then analysed with a
spectrogram A spectrogram is a visual representation of the spectrum of frequencies of a signal as it varies with time. When applied to an audio signal, spectrograms are sometimes called sonographs, voiceprints, or voicegrams. When the data are represen ...
to reveal the image. Various artists have used this method to conceal hidden pictures in their songs, such as
Aphex Twin Richard David James (born 18 August 1971), best known as Aphex Twin, is an Irish-born British musician, composer and DJ. He is known for his idiosyncratic work in electronic music, electronic styles such as techno, ambient music, ambient, and jun ...
in "
Windowlicker "Windowlicker" is a song by British electronic musician Aphex Twin, released on 22 March 1999 as a single by Warp Records. The artwork for the single was created by Chris Cunningham, with additional work by The Designers Republic. Cunningham als ...
" or Nine Inch Nails in their album ''
Year Zero A year zero does not exist in the Anno Domini (AD) calendar year system commonly used to number years in the Gregorian calendar (nor in its predecessor, the Julian calendar); in this system, the year is followed directly by year . However, the ...
''.


Social steganography

In communities with social or government taboos or censorship, people use cultural steganography—hiding messages in idiom, pop culture references, and other messages they share publicly and assume are monitored. This relies on social context to make the underlying messages visible only to certain readers. Examples include: * Hiding a message in the title and context of a shared video or image. * Misspelling names or words that are popular in the media in a given week, to suggest an alternate meaning. * Hiding a picture that can be traced by using Paint or any other drawing tool.


Steganography in streaming media

Since the era of evolving network applications, steganography research has shifted from image steganography to steganography in streaming media such as
Voice over Internet Protocol Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
(VoIP). In 2003, Giannoula et al. developed a data hiding technique leading to compressed forms of source video signals on a frame-by-frame basis. In 2005, Dittmann et al. studied steganography and watermarking of multimedia contents such as VoIP. In 2008, Yongfeng Huang and Shanyu Tang presented a novel approach to information hiding in low bit-rate VoIP speech stream, and their published work on steganography is the first-ever effort to improve the codebook partition by using Graph theory along with Quantization Index Modulation in low bit-rate streaming media. In 2011 and 2012, Yongfeng Huang and Shanyu Tang devised new steganographic algorithms that use codec parameters as cover object to realise real-time covert VoIP steganography. Their findings were published in ''IEEE Transactions on Information Forensics and Security''.


Cyber-physical systems/Internet of Things

Academic work since 2012 demonstrated the feasibility of steganography for
cyber-physical system A cyber-physical system (CPS) or intelligent system is a computer system in which a mechanism is controlled or monitored by computer-based algorithms. In cyber-physical systems, physical and software components are deeply intertwined, able to oper ...
s (CPS)/the
Internet of Things The Internet of things (IoT) describes physical objects (or groups of such objects) with sensors, processing ability, software and other technologies that connect and exchange data with other devices and systems over the Internet or other comm ...
(IoT). Some techniques of CPS/IoT steganography overlap with network steganography, i.e. hiding data in communication protocols used in CPS/the IoT. However, specific techniques hide data in CPS components. For instance, data can be stored in unused registers of IoT/CPS components and in the states of IoT/CPS actuators.


Printed

Digital steganography output may be in the form of printed documents. A message, the ''
plaintext In cryptography, plaintext usually means unencrypted information pending input into cryptographic algorithms, usually encryption algorithms. This usually refers to data that is transmitted or stored unencrypted. Overview With the advent of comp ...
'', may be first encrypted by traditional means, producing a ''
ciphertext In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext ...
''. Then, an innocuous ''cover text'' is modified in some way so as to contain the ciphertext, resulting in the ''stegotext''. For example, the letter size, spacing,
typeface A typeface (or font family) is the design of lettering that can include variations in size, weight (e.g. bold), slope (e.g. italic), width (e.g. condensed), and so on. Each of these variations of the typeface is a font. There are list of type ...
, or other characteristics of a cover text can be manipulated to carry the hidden message. Only a recipient who knows the technique used can recover the message and then decrypt it.
Francis Bacon Francis Bacon, 1st Viscount St Alban (; 22 January 1561 – 9 April 1626), also known as Lord Verulam, was an English philosopher and statesman who served as Attorney General and Lord Chancellor of England. Bacon led the advancement of both ...
developed
Bacon's cipher Bacon's cipher or the Baconian cipher is a method of steganographic message encoding devised by Francis Bacon in 1605. A message is concealed in the presentation of text, rather than its content. Cipher details To encode a message, each letter of ...
as such a technique. The ciphertext produced by most digital steganography methods, however, is not printable. Traditional digital methods rely on perturbing noise in the channel file to hide the message, and as such, the channel file must be transmitted to the recipient with no additional noise from the transmission. Printing introduces much noise in the ciphertext, generally rendering the message unrecoverable. There are techniques that address this limitation, one notable example being ASCII Art Steganography. Although not classic steganography, some types of modern color laser printers integrate the model, serial number, and timestamps on each printout for traceability reasons using a dot-matrix code made of small, yellow dots not recognizable to the naked eye — see
printer steganography A Machine Identification Code (MIC), also known as printer steganography, yellow dots, tracking dots or secret dots, is a digital watermark which certain color laser printers and copiers leave on every printed page, allowing identification of th ...
for details.


Using puzzles

The art of concealing data in a puzzle can take advantage of the degrees of freedom in stating the puzzle, using the starting information to encode a key within the puzzle/puzzle image. For instance, steganography using
sudoku Sudoku (; ja, 数独, sūdoku, digit-single; originally called Number Place) is a logic-based, combinatorial number-placement puzzle. In classic Sudoku, the objective is to fill a 9 × 9 grid with digits so that each column, each row ...
puzzles has as many keys as there are possible solutions of a Sudoku puzzle, which is .


Network

In 1977, Kent concisely described the potential for covert channel signaling in general network communication protocols, even if the traffic is encrypted (in a footnote) in "Encryption-Based Protection for Interactive User/Computer Communication," Proceedings of the Fifth Data Communications Symposium, September 1977. In 1987, Girling first studied covert channels on a local area network (LAN), identified and realised three obvious covert channels (two storage channels and one timing channel), and his research paper entitled “Covert channels in LAN’s” published in ''IEEE Transactions on Software Engineering'', vol. SE-13 of 2, in February 1987. In 1989, Wolf implemented covert channels in LAN protocols, e.g. using the reserved fields, pad fields, and undefined fields in the TCP/IP protocol. In 1997, Rowland used the IP identification field, the TCP initial sequence number and acknowledge sequence number fields in TCP/IP headers to build covert channels. In 2002, Kamran Ahsan made an excellent summary of research on network steganography. In 2005, Steven J. Murdoch and Stephen Lewis contributed a chapter entitled "Embedding Covert Channels into TCP/IP" in the "''Information Hiding''" book published by Springer. All information hiding techniques that may be used to exchange steganograms in telecommunication networks can be classified under the general term of network steganography. This nomenclature was originally introduced by Krzysztof Szczypiorski in 2003. Contrary to typical steganographic methods that use digital media (images, audio and video files) to hide data, network steganography uses communication protocols' control elements and their intrinsic functionality. As a result, such methods can be harder to detect and eliminate. Typical network steganography methods involve modification of the properties of a single network protocol. Such modification can be applied to the
protocol data unit In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data. In the layered architectures of c ...
(PDU), to the time relations between the exchanged PDUs, or both (hybrid methods). Moreover, it is feasible to utilize the relation between two or more different network protocols to enable secret communication. These applications fall under the term inter-protocol steganography. Alternatively, multiple network protocols can be used simultaneously to transfer hidden information and so-called control protocols can be embedded into steganographic communications to extend their capabilities, e.g. to allow dynamic overlay routing or the switching of utilized hiding methods and network protocols. Network steganography covers a broad spectrum of techniques, which include, among others: * Steganophony – the concealment of messages in
Voice-over-IP Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
conversations, e.g. the employment of delayed or corrupted packets that would normally be ignored by the receiver (this method is called LACK – Lost Audio Packets Steganography), or, alternatively, hiding information in unused header fields. * WLAN Steganography – transmission of steganograms in Wireless Local Area Networks. A practical example of WLAN Steganography is the HICCUPS system (Hidden Communication System for Corrupted Networks)


Terminology and Taxonomy

In 2015, a taxonomy of 109 network hiding methods was presented by Steffen Wendzel, Sebastian Zander et al. that summarized core concepts used in network steganography research. The taxonomy was developed further in recent years by several publications and authors and adjusted to new domains, such as CPS steganography.


Additional terminology

Discussions of steganography generally use terminology analogous to and consistent with conventional radio and communications technology. However, some terms appear specifically in software and are easily confused. These are the most relevant ones 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 proportion 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, the files that are considered likely to contain a payload are ''suspects''. A ''suspect'' identified through some type of statistical analysis can be referred to as a ''candidate''.


Countermeasures and detection

Detecting physical steganography requires a careful physical examination, including the use of magnification, developer chemicals, and
ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
. It is a time-consuming process with obvious resource implications, even in countries that employ many people to spy on their fellow nationals. However, it is feasible to screen mail of certain suspected individuals or institutions, such as prisons or prisoner-of-war (POW) camps. During
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, prisoner of war camps gave prisoners specially-treated
paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distributed ...
that would reveal
invisible ink Invisible ink, also known as security ink or sympathetic ink, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by some means, such as heat or ultraviolet light. Invisible ...
. An article in the 24 June 1948 issue of ''Paper Trade Journal'' by the Technical Director of the
United States Government Printing Office The United States Government Publishing Office (USGPO or GPO; formerly the United States Government Printing Office) is an agency of the legislative branch of the United States Federal government. The office produces and distributes information ...
had Morris S. Kantrowitz describe in general terms the development of this paper. Three prototype papers (''Sensicoat'', ''Anilith'', and ''Coatalith'') were used to manufacture postcards and stationery provided to German prisoners of war in the US and Canada. If POWs tried to write a hidden message, the special paper rendered it visible. The US granted at least two
patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...
s related to the technology, one to Kantrowitz, , "Water-Detecting paper and Water-Detecting Coating Composition Therefor," patented 18 July 1950, and an earlier one, "Moisture-Sensitive Paper and the Manufacture Thereof," , patented 20 July 1948. A similar strategy issues prisoners with writing paper ruled with a water-soluble ink that runs in contact with water-based invisible ink. In computing, steganographically encoded package detection is called
steganalysis Steganalysis is the study of detecting messages hidden using steganography; this is analogous to cryptanalysis applied to cryptography. Overview The goal of steganalysis is to identify suspected packages, determine whether or not they have a payl ...
. 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 the materials and then compare them against the current contents of the site. The differences, if the carrier is the same, comprise the payload. In general, using extremely high compression rates 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). There are a variety of basic tests that can be done to identify whether or not a secret message exists. This process is not concerned with the extraction of the message, which is a different process and a separate step. The most basic approaches of
steganalysis Steganalysis is the study of detecting messages hidden using steganography; this is analogous to cryptanalysis applied to cryptography. Overview The goal of steganalysis is to identify suspected packages, determine whether or not they have a payl ...
are visual or aural attacks, structural attacks, and statistical attacks. These approaches attempt to detect the steganographic algorithms that were used.Wayner, Peter (2009). ''Disappearing Cryptography: Information Hiding: Steganography & Watermarking'', Morgan Kaufmann Publishers, Amsterdam; Boston These algorithms range from unsophisticated to very sophisticated, with early algorithms being much easier to detect due to statistical anomalies that were present. The size of the message that is being hidden is a factor in how difficult it is to detect. The overall size of the cover object also plays a factor as well. If the cover object is small and the message is large, this can distort the statistics and make it easier to detect. A larger cover object with a small message decreases the statistics and gives it a better chance of going unnoticed. Steganalysis that targets a particular algorithm has much better success as it is able to key in on the anomalies that are left behind. This is because the analysis can perform a targeted search to discover known tendencies since it is aware of the behaviors that it commonly exhibits. When analyzing an image the least significant bits of many images are actually not random. The camera sensor, especially lower-end sensors are not the best quality and can introduce some random bits. This can also be affected by the file compression done on the image. Secret messages can be introduced into the least significant bits in an image and then hidden. A steganography tool can be used to camouflage the secret message in the least significant bits but it can introduce a random area that is too perfect. This area of perfect randomization stands out and can be detected by comparing the least significant bits to the next-to-least significant bits on an image that hasn't been compressed. Generally, though, there are many techniques known to be able to hide messages in data using steganographic techniques. None are, by definition, obvious when users employ standard applications, but some can be detected by specialist tools. Others, however, are resistant to detection—or rather it is not possible to reliably distinguish data containing a hidden message from data containing just noise—even when the most sophisticated analysis is performed. Steganography is being used to conceal and deliver more effective cyber attacks, referred to as ''Stegware''. The term Stegware was first introduced in 2017 to describe any malicious operation involving steganography as a vehicle to conceal an attack. Detection of steganography is challenging, and because of that, not an adequate defence. Therefore, the only way of defeating the threat is to transform data in a way that destroys any hidden messages, a process called Content Threat Removal.


Applications


Use in modern printers

Some modern computer printers use steganography, including
Hewlett-Packard The Hewlett-Packard Company, commonly shortened to Hewlett-Packard ( ) or HP, was an American multinational information technology company headquartered in Palo Alto, California. HP developed and provided a wide variety of hardware components ...
and
Xerox Xerox Holdings Corporation (; also known simply as Xerox) is an American corporation that sells print and electronic document, digital document products and services in more than 160 countries. Xerox is headquartered in Norwalk, Connecticut (ha ...
brand color laser printers. The printers add tiny yellow dots to each page. The barely-visible dots contain encoded printer serial numbers and date and time stamps.


Example from modern practice

The larger the cover message (in binary data, the number of
bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
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 pictures, which contain much data, are sometimes used to hide messages on the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...
and on other digital communication media. It is not clear how common this practice actually is. For example, a 24-bit
bitmap In computing, a bitmap is a mapping from some domain (for example, a range of integers) to bits. It is also called a bit array A bit array (also known as bitmask, bit map, bit set, bit string, or bit vector) is an array data structure that c ...
uses 8 bits to represent each of the three color values (red, green, and blue) of each
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the smal ...
. 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 In computing, bit numbering is the convention used to identify the bit positions in a binary number. Bit significance and indexing In computing, the least significant bit (LSB) is the bit position in a binary integer representing the binary 1 ...
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 ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
text for every three
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the smal ...
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 In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where noise is defined as any signal other than the one being monitored. In radio com ...
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 theoretical point of view, that means that the
channel Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Austral ...
must have more capacity than the "surface"
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
requires. There must be redundancy. For a digital image, it may be
noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
from the imaging element; for
digital audio Digital audio is a representation of sound recorded in, or converted into, digital form. In digital audio, the sound wave of the audio signal is typically encoded as numerical samples in a continuous sequence. For example, in CD audio, sa ...
, it may be noise from recording techniques or amplification equipment. In general, electronics that digitize an
analog signal An analog signal or analogue signal (see spelling differences) is any continuous signal representing some other quantity, i.e., ''analogous'' to another quantity. For example, in an analog audio signal, the instantaneous signal voltage varies c ...
suffer from several noise sources, such as
thermal noise A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
,
flicker noise Flicker noise is a type of electronic noise with a 1/''f'' power spectral density. It is therefore often referred to as 1/''f'' noise or pink noise, though these terms have wider definitions. It occurs in almost all electronic devices and can show ...
, and
shot noise Shot noise or Poisson noise is a type of noise which can be modeled by a Poisson process. In electronics shot noise originates from the discrete nature of electric charge. Shot noise also occurs in photon counting in optical devices, where shot ...
. 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 In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
schemes (such as
JPEG JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...
) 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 A digital watermark is a kind of marker covertly embedded in a noise-tolerant signal such as audio, video or image data. It is typically used to identify ownership of the copyright of such signal. "Watermarking" is the process of hiding digital inf ...
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 Coded anti-piracy (CAP) is an anti-copyright infringement technology which marks each film print of a motion picture with a distinguishing pattern of dots, used as a forensic identifier to identify the source of illegal copies. They are not to b ...
) or even just to identify an image (as in the
EURion constellation The EURion constellation (also known as Omron rings or doughnuts) is a pattern of symbols incorporated into a number of secure documents such as banknotes and ownership title certificates designs worldwide since about 1996. It is added to help ...
). 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.


Alleged use by intelligence services

In 2010, the
Federal Bureau of Investigation The Federal Bureau of Investigation (FBI) is the domestic intelligence and security service of the United States and its principal federal law enforcement agency. Operating under the jurisdiction of the United States Department of Justice, ...
alleged that the Russian foreign intelligence service uses customized steganography software for embedding encrypted text messages inside image files for certain communications with "illegal agents" (agents without diplomatic cover) stationed abroad. On April 23, 2019 the U.S. Department of Justice unsealed an indictment charging Xiaoqing Zheng, a Chinese businessman and former Principal Engineer at General Electric, with 14 counts of conspiring to steal intellectual property and trade secrets from General Electric. Zheng had allegedly used steganography to exfiltrate 20,000 documents from General Electric to Tianyi Aviation Technology Co. in Nanjing, China, a company the FBI accused him of starting with backing from the Chinese government.


Distributed steganography

There are distributed steganography methods, including methodologies that distribute the payload through multiple carrier files in diverse locations to make detection more difficult. For example, by cryptographer William Easttom (
Chuck Easttom William "Chuck" Easttom II (born October 5, 1968) is an American computer scientist specializing in cyber security, cryptography, quantum computing, and systems engineering. Education Chuck Easttom holds a B.A. from Southeastern Oklahoma State ...
).


Online challenge

The puzzles that are presented by
Cicada 3301 Cicada 3301 is a nickname given to three sets of puzzles posted under the name "3301" online between 2012 and 2014. The first puzzle started on January 4, 2012, on 4chan and ran for nearly a month. A second round began one year later on January 4, ...
incorporate steganography with cryptography and other solving techniques since 2012. Puzzles involving steganography have also been featured in other
alternate reality game An alternate reality game (ARG) is an interactive networked narrative that uses the real world as a platform and employs transmedia storytelling to deliver a story that may be altered by players' ideas or actions. The form is defined by inten ...
s. The communications of The May Day mystery incorporate steganography and other solving techniques since 1981.


Stegoanalysis


Stegoanalytical algorithms

Stegoanalytical algorithms can be cataloged in different ways, highlighting: according to the available information and according to the purpose sought.


According to the information available

There is the possibility of cataloging these algorithms based on the information held by the stegoanalyst in terms of clear and encrypted messages. It is a technique similar to cryptography, however, they have several differences: * Chosen stego attack: the stegoanalyst perceives the final target stego and the stenographic algorithm used. * Known cover attack: the stegoanalyst comprises the initial conductive target and the final target stego. * Known stego attack: the stegoanalyst knows the initial carrier target and the final target stego, in addition to the algorithm used. * Stego only attack: the stegoanalyst perceives exclusively the stego target. * Chosen message attack: the stegoanalyst, following a message selected by him, originates a stego target. * Known message attack: the stegoanalyst owns the stego target and the hidden message, which is known to him.


According to the purpose sought

The principal purpose of steganography is to transfer information unnoticed, however, it is possible for an attacker to have two different pretensions: * Passive stegoanalysis: does not alter the target stego, therefore, it examines the target stego in order to establish whether it carries hidden information and recovers the hidden message, the key used or both. * Active stegoanalysis: changes the initial stego target, therefore, it seeks to suppress the transfer of information, if it exists.


See also

* * * * * * * * * * * * * * * * * *


References


Sources

* * * * *


External links


An overview of digital steganography, particularly within images, for the computationally curious
by Chris League, Long Island University, 2015 *


Information Hiding: Steganography & Digital Watermarking.
Papers and information about steganography and steganalysis research from 1995 to the present. Includes Steganography Software Wiki list. Dr. Neil F. Johnson.
Detecting Steganographic Content on the Internet.
2002 paper by
Niels Provos Niels Provos is a German-American researcher in security engineering, malware, and cryptography. He received a PhD in computer science from the University of Michigan. From 2003 to 2018, he worked at Google as a Distinguished Engineer on security ...
and Peter Honeyman published in ''Proceedings of the Network and Distributed System Security Symposium'' (San Diego, CA, February 6–8, 2002). NDSS 2002. Internet Society, Washington, D.C.
Covert Channels in the TCP/IP Suite
996 paper by Craig Rowland detailing the hiding of data in TCP/IP packets.

How-to articles on the subject of network steganography (Wireless LANs, VoIP – Steganophony, TCP/IP protocols and mechanisms, Steganographic Router, Inter-protocol steganography). By Krzysztof Szczypiorski and Wojciech Mazurczyk from Network Security Group.
Invitation to BPCS-Steganography.

Steganography by Michael T. Raggo
DefCon 12 (1 August 2004)
File Format Extension Through Steganography
by Blake W. Ford and Khosrow Kaikhah
Computer steganography. Theory and practice with Mathcad (Rus)
2006 paper by Konakhovich G. F., Puzyrenko A. Yu. published in ''MK-Press'' Kyiv, Ukraine
stegano
a Free and Open Source steganography web service. {{Authority control Espionage techniques