Steganography (, ) is the practice of representing information within another message or physical object, in such a manner that the presence of the concealed information would not be evident to an unsuspecting person's examination. In computing and electronic contexts, a computer file, message, image, or video is concealed within another file, message, image, or video. 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 between the visible lines of a private letter. Some implementations of steganography that lack a formal shared secret are forms of security through obscurity, while key-dependent steganographic schemes try to adhere to Kerckhoffs's principle.
The word steganography comes from Greek 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 in his Steganographia, a treatise on cryptography and steganography, disguised as a book on magic.
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 they are, arouse interest and may in themselves be incriminating in countries in which encryption is illegal. Whereas cryptography is the practice of protecting the contents of a message alone, steganography is concerned with concealing both 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 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 to correspond to a letter in the alphabet. The change is so subtle that someone who is not 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, when Herodotus mentions two examples in his Histories. Histiaeus sent a message to his vassal, Aristagoras, 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 sent a warning about a forthcoming attack to Greece by writing it directly on the wooden backing of a wax tablet before applying its beeswax surface. Wax tablets were in common use then as reusable writing surfaces, sometimes used for shorthand.
Greek writer Aeneas Tacticus also devised a method of steganography now known as the pinhole cipher.Aeneas Tacticus Siege Defense XXXIDavid Kahn (December 1996). The Codebreakers. Simon and Schuster. . He tells of a method where you are to write a letter and mark specific characters inside the letter with a dot-shaped hole. The recipient of said letter will simply note down all letters below the holes and string them together to form a message. He also suggested writing down a message and whitening it, making it invisible, so that when soaked in water the message would be revealed. He suggests many other hiding methods, such as hiding a message in leaves, in the foot of a messenger's sandal, and even in animal bladders.
In his Compendium of Mechanics, Philo of Byzantium describes a recipe for invisible ink. He says to write down a message with crushed gallnuts dissolved in water. Then it could be revealed by being soaked by a sponge of ferrous sulfate. This recipe was inspired by the recipe for gall ink. Ferrous sulfate and crushed gallnuts dissolved in water both react to make ink so when they combine the message is revealed.
In his Ars Amatoria, Roman poet Ovid noted of a primitive form of invisible ink using milk to be used in writing love letters. Naturalist Pliny the Elder also wrote of the tithymalus plant in his Natural History, and how its milk could be used for invisible ink.
The first recorded use of the term steganography was by Johannnes Trithemius with his Steganographia, a book seemingly about summoning angels to hide messages. In reality however, the book was about steganography, the angel summoning being a cover text. Every angel corresponded to a different method of steganography.
He had written a letter to Arnold Bostius about his book, how it contained many methods of secret writing, how it could be used to communicate without speaking, signing or even nodding and much more. Bostius had died before he could receive the letter, however his colleagues had read and published it, accusing him of conversing with demons. Trithemius had also invited Charles de Bovelles to read his book before de Bovelles threw it away, claiming Trithemius was a magician. This situation led Trithemius to quit the book, not however, his interest in steganography.
In his work Polygraphiae, Trithemius developed his Ave Maria cipher, a polyalphabetic substitution cipher and form of steganography that can hide information in a Latin praise of God.Hermes, J. (2022). Polygraphia III: The Cipher that Pretends to be an Artificial Language. In VOY. It consists of around 384 alphabets of 24 letters corresponding to 24 words to praise God. To encode a message, take the first letter of your message and replace that letter with the corresponding word in the first alphabet. Do that for the second, the third, and so on. "", for example, contains the concealed word SECRET.
In the year 1606, Frankfurt published Book I and II of the Steganographia, along with an incomplete Book III, and the Clavis Steganographie. The Clavis explained how to extract the secret of the Steganographia from Book I and II. Simply select an incantation to summon an "angel" and extract every other letter of every other word to find the secret steganographic method associated with that angel. With this publication came a slew of books vindicating Trithemius by explaining the steganography, acquitting him of magic. However, Book III was still widely regarded as a work of magic, with no clear way to extract its secrets. It would take until the late 1990s when Jim Reeds and Thomas Ernst independently discovered that it too, was secretly a work of steganography.Ernst, Thomas (1996). "Schwarzweiße Magie: Der Schlüssel zum dritten Buch der Stenographia des Trithemius". Daphnis: Zeitschrift für Mittlere Deutsche Literatur. 25 (1): 1–205.
Techniques
Numerous techniques throughout history have been developed to embed a message within another medium.
Physical Placing the message in a physical item has been widely used for centuries. Some notable examples include invisible ink on paper, writing a message in Morse code on yarn worn by a courier, microdots, or using a music cipher to hide messages as musical notes in sheet music.
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.Social Steganography: how teens smuggle meaning past the authority figures in their lives, Boing Boing, 22 May 2013. Retrieved 7 June 2014.Social Steganography , Scenario Magazine, 2013. 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 alternative meaning. Hiding a picture that can be traced by using Paint or any other drawing tool.
Digital messages
Since the dawn of computers, techniques have been developed to embed messages in digital cover mediums. The message to conceal is often 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 generates ciphertexts that look perfectly random without the private key).
Examples of this include changing pixels in image or sound files, properties of digital text such as spacing and font choice, chaffing and winnowing, mimic functions, modifying the echo of a sound file (echo steganography), and including data in ignored sections of a file.
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 (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.B. Xiao, Y. Huang, and S. Tang, "An Approach to Information Hiding in Low Bit-Rate Speech Stream", in IEEE GLOBECOM 2008, IEEE, pp. 371–375, 2008. .
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.
In 2024, Cheddad & Cheddad proposed a new framework for reconstructing lost or corrupted audio signals using a combination of machine learning techniques and latent information. The main idea of their paper is to enhance audio signal reconstruction by fusing steganography, halftoning (dithering), and state-of-the-art shallow and deep learning methods (e.g., RF, LSTM). This combination of steganography, halftoning, and machine learning for audio signal reconstruction may inspire further research in optimizing this approach or applying it to other domains, such as image reconstruction (i.e., inpainting).
Adaptive steganography Adaptive steganography is a technique for concealing information within digital media by tailoring the embedding process to the specific features of the cover medium. An example of this approach is demonstrated in the work. Their method develops a skin tone detection algorithm, capable of identifying facial features, which is then applied to adaptive steganography. By incorporating face rotation into their approach, the technique aims to enhance its adaptivity to conceal information in a manner that is both less detectable and more robust across various facial orientations within images. This strategy can potentially improve the efficacy of information hiding in both static images and video content.
Cyber-physical systems/Internet of Things
Academic work since 2012 demonstrated the feasibility of steganography for cyber-physical systems (CPS)/the Internet of Things (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.
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