View source for Steganography

{{short description|Hiding messages in other messages}}
{{Use dmy dates|date=November 2023}}
{{for-multi|the process of writing in shorthand|Stenography|the prefix "Stego-" as used in taxonomy|List of commonly used taxonomic affixes}}
[[File:Steganography.png|upright=1.4|thumb|The same image viewed by white, blue, green, and red lights reveals different hidden numbers.]]

'''Steganography''' ({{IPAc-en|audio=en-us-steganography.ogg|ˌ|s|t|ɛ|ɡ|ə|ˈ|n|ɒ|ɡ|r|ə|f|i}} {{respell|STEG|ə|NOG|rə-fee}}) 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/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]].<ref name=stegokey>{{cite journal|last=Fridrich|first=Jessica|author2=M. Goljan|author3=D. Soukal|title=Searching for the Stego Key|journal=Proc. SPIE, Electronic Imaging, Security, Steganography, and Watermarking of Multimedia Contents VI|year=2004|volume=5306|pages=70–82|url=http://www.ws.binghamton.edu/fridrich/Research/Keysearch_SPIE.pdf|access-date=23 January 2014|doi=10.1117/12.521353|series=Security, Steganography, and Watermarking of Multimedia Contents VI|bibcode=2004SPIE.5306...70F|s2cid=6773772|editor1-last=Delp Iii|editor1-first=Edward J|editor2-last=Wong|editor2-first=Ping W}}</ref>

The word ''steganography'' comes from [[Greek language|Greek]] ''steganographia'', which combines the words ''steganós'' ({{Wikt-lang|el|στεγανός}}), meaning "covered or concealed", and ''-graphia'' ({{Wikt-lang|el|γραφή}}) meaning "writing".<ref>{{Cite web|url=https://www.merriam-webster.com/dictionary/steganography|title=Definition of STEGANOGRAPHY|website=Merriam-webster.com|access-date=14 December 2021}}</ref> 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.<ref>{{cite web|url=http://www.alternet.org/story/11986/|title=Confounding Carnivore: How to Protect Your Online Privacy|publisher=[[AlterNet]]|archive-url=https://web.archive.org/web/20070716093719/http://www.alternet.org/story/11986/|archive-date=16 July 2007|access-date=2 September 2008|last=Pahati|first=OJ|date=29 November 2001}}</ref> 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==
[[File:Chart in the hand of Dr John Dee. Steganographiae.png|thumb|A chart from [[Johannes Trithemius]]'s ''[[Steganographia]]'' copied by [[Dr John Dee]] in 1591]]

The first recorded uses of steganography can be traced back to 440 BC in [[Greece]], when [[Herodotus]] mentions two examples in his ''[[The Histories of Herodotus|Histories]]''.<ref>{{cite journal |url=http://www.cl.cam.ac.uk/~fapp2/publications/ieee99-infohiding.pdf |title=Information Hiding: A survey |journal=Proceedings of the IEEE |last= Petitcolas |first=FAP |author2=Anderson RJ |author3=Kuhn MG  |volume=87 |issue=7 |pages=1062–78 |access-date=2 September 2008 |doi=10.1109/5.771065 |year=1999|citeseerx=10.1.1.333.9397 }}</ref> [[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]].

In his work ''[[Polygraphia (book)|Polygraphiae]],'' [[Johannes Trithemius]] developed his [[Ave Maria cipher|''Ave Maria'' cipher]] that can hide information in a Latin praise of God.<ref>{{cite web |url=http://daten.digitale-sammlungen.de/~db/0002/bsb00026190/images/index.html?seite=71|title=Polygraphiae (cf. p. 71f) |publisher = Digitale Sammlungen |language = de |access-date=27 May 2015}}</ref>{{better source|reason=This citation is for the book itself rather than to a reliable source that explains it.|date=March 2025}} "{{lang|la|Auctor sapientissimus conseruans angelica deferat nobis charitas potentissimi creatoris}}", for example, contains the concealed word ''VICIPEDIA''.{{cn|date=March 2025}}

==Techniques==
{{main|List of steganography techniques}}

[[File:Steganography in the hand of John Dee.png|thumb|Deciphering the code. ''Steganographia'']]

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.<ref name=knit>{{Cite web|url=https://getpocket.com/explore/item/the-wartime-spies-who-used-knitting-as-an-espionage-tool|title=The Wartime Spies Who Used Knitting as an Espionage Tool – Atlas Obscura|website=Pocket|access-date=4 March 2020}}</ref> Some notable examples include [[invisible ink]] on paper, writing a message in [[Morse code]] on [[yarn]] worn by a courier,<ref name=knit />  [[microdot]]s, or using a [[music cipher#Musical Steganography|music cipher]] to hide messages as [[musical note]]s in [[sheet music]].<ref>{{Cite magazine |last=Newman |first=Lily Hay |title=How a Saxophonist Tricked the KGB by Encrypting Secrets in Music |language=en-US |magazine=Wired |url=https://www.wired.com/story/merryl-goldberg-music-encryption-ussr-phantom-orchestra/ |access-date=9 June 2022 |archive-url=https://web.archive.org/web/20220608222058/https://www.wired.com/story/merryl-goldberg-music-encryption-ussr-phantom-orchestra/ |archive-date=8 June 2022 |issn=1059-1028}}</ref>

==== 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.<ref>[http://boingboing.net/2013/05/22/social-steganography-how-teen.html Social Steganography: how teens smuggle meaning past the authority figures in their lives], Boing Boing, 22 May 2013.  Retrieved 7 June 2014.</ref><ref>[http://www.scenariomagazine.com/social-steganography/ Social Steganography] {{Webarchive|url=https://web.archive.org/web/20140714143900/http://www.scenariomagazine.com/social-steganography/ |date=14 July 2014 }}, Scenario Magazine, 2013.</ref> 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.{{citation needed|date=December 2016}}

=== Digital messages ===
[[Image:Steganography original.png|frame|Image of a tree with a steganographically hidden image. The hidden image is revealed by removing all but the two least significant [[bit]]s of each [[color component]] and a subsequent [[Normalization (image processing)|normalization]]. The hidden image is shown below.]] 
[[Image:Steganography recovered.png|frame|Image of a cat extracted from the tree image above.]]

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,<ref>{{cite journal|doi=10.1016/j.sigpro.2009.08.010|title=Digital image steganography: Survey and analysis of current methods|journal=Signal Processing|volume=90|issue=3|pages=727–752|year=2010|last1=Cheddad|first1=Abbas|last2=Condell|first2=Joan|last3=Curran|first3=Kevin|last4=Mc Kevitt|first4=Paul|bibcode=2010SigPr..90..727C }}</ref> properties of digital text such as spacing and font choice, [[chaffing and winnowing]], [[mimic function]]s, modifying the echo of a sound file (echo steganography).{{citation needed|date=April 2024}}<!-- Original citation irretrievably lost -->, and including data in ignored sections of a file.<ref>{{cite journal | last1= Bender | first1= W. | last2= Gruhl | first2= D. | last3= Morimoto | first3= N. | last4= Lu | first4= A. | title= Techniques for data hiding | journal= IBM Systems Journal | publisher= IBM Corp.| date=1996 | volume= 35 | issn= 0018-8670 | url= https://pdfs.semanticscholar.org/8c82/c93dfc7d3672e58efd982a23791a8a419053.pdf | archive-url= https://web.archive.org/web/20200611050549/https://pdfs.semanticscholar.org/8c82/c93dfc7d3672e58efd982a23791a8a419053.pdf | url-status= dead | archive-date= 11 June 2020 |issue = 3.4| pages=313–336| doi= 10.1147/sj.353.0313 | s2cid= 16672162 }}</ref>

{{Multiple image
|align=center
|direction=horizontal
|header=Images hidden in sound files
|image1=Drawing of the word Wikipedia in Coagula.jpg
|caption1=1. The word "Wikipedia" is drawn using computer software
|alt1=The word "Wikipedia" written in green and red on black background
|width1=250
|image2=Drawing of the word Wikipedia transformed into a wav file (Coagula software).wav
|caption2=2. The image is converted into an audio file
|alt2=
|image3=Wikipedia wavefile by Coagula - logarihtmic spectrogram.svg
|caption3=3. Finally, the audio is analysed through a spectrogram, revealing the initial image
|alt3=The word "Wikipedia" in yellow over a dark blue/black background
|width3=250
|image4=Spectrogram - Nine Inch Nails - My Violent Heart.png
|caption4=[[Spectrogram]] of a [[Easter egg (media)|hidden image]] encoded as sound in the song "My Violent Heart" by [[Nine Inch Nails]] from the ''[[Year Zero (album)|Year Zero]]'' album (2007)
|alt4=image of a blue arm and hand over white, pixelated dots
|width4=200
}}

=== 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.<ref>{{Cite book|last1=Giannoula|first1=A.|last2=Hatzinakos|first2=D.|title=Proceedings 2003 International Conference on Image Processing (Cat. No.03CH37429) |chapter=Compressive data hiding for video signals |volume=1|pages=I–529–32|publisher=IEEE|doi=10.1109/icip.2003.1247015|isbn=0780377508|year=2003|s2cid=361883}}</ref>

In 2005, Dittmann et al. studied steganography and watermarking of multimedia contents such as VoIP.<ref>{{Cite journal|last1=Dittmann|first1=Jana|last2=Hesse|first2=Danny|last3=Hillert|first3=Reyk|date=21 March 2005|title=Steganography and steganalysis in voice-over IP scenarios: operational aspects and first experiences with a new steganalysis tool set|journal=Security, Steganography, and Watermarking of Multimedia Contents VII|volume=5681|pages=607|publisher=SPIE|doi=10.1117/12.586579|bibcode=2005SPIE.5681..607D|s2cid=206413447|editor1-last=Delp Iii|editor1-first=Edward J|editor2-last=Wong|editor2-first=Ping W}}</ref>

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.<ref>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. {{ISBN|978-1-4244-2324-8}}.</ref>

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''.<ref>{{Cite journal|last1=Huang|first1=Yong Feng|last2=Tang|first2=Shanyu|last3=Yuan|first3=Jian|date=June 2011|title=Steganography in Inactive Frames of VoIP Streams Encoded by Source Codec|journal=IEEE Transactions on Information Forensics and Security|volume=6|issue=2|pages=296–306|doi=10.1109/tifs.2011.2108649|s2cid=15096702|issn=1556-6013|url=https://repository.uwl.ac.uk/id/eprint/3935/1/Steganography%20in%20inactive%20frames%20of%20VoIP%20streams%20encoded%20by%20source%20codec.pdf}}</ref><ref>{{Cite journal|last1=Huang|first1=Yongfeng|last2=Liu|first2=Chenghao|last3=Tang|first3=Shanyu|last4=Bai|first4=Sen|date=December 2012|title=Steganography Integration Into a Low-Bit Rate Speech Codec|journal=IEEE Transactions on Information Forensics and Security|volume=7|issue=6|pages=1865–1875|doi=10.1109/tifs.2012.2218599|s2cid=16539562|issn=1556-6013|url=https://repository.uwl.ac.uk/id/eprint/3932/1/Steganography%20Integration%20into%20a%20low-bit%20rate%20speech%20codec.pdf}}</ref><ref>{{Cite journal|title=Application of Lah transform for security and privacy of data through information hiding in telecommunication|journal=Transactions on Emerging Telecommunications Technologies|year=2020|doi=10.1002/ett.3984|last1=Ghosal|first1=Sudipta Kr|last2=Mukhopadhyay|first2=Souradeep|last3=Hossain|first3=Sabbir|last4=Sarkar|first4=Ram|volume=32|issue=2|s2cid=225866797}}</ref>

In 2024, Cheddad & Cheddad proposed a new framework <ref>{{Cite book|last1=Cheddad|first1= Zohra Adila|last2=Cheddad|first2=Abbas|title=Proceedings 2023 Intelligent Systems Conference (IntelliSys'23)|chapter= Active Restoration of Lost Audio Signals Using Machine Learning and Latent Information |series= Lecture Notes in Networks and Systems|volume= 822|publisher=LNCS,Springer|doi=10.1007/978-3-031-47721-8_1|year=2024 |pages= 1–16|isbn= 978-3-031-47720-1}}</ref> 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.<ref>{{cite journal|title=A skin tone detection algorithm for an adaptive approach to steganography|journal=Signal Processing|volume=89|issue=12|pages=2465–2478|year=2009|doi=10.1016/j.sigpro.2009.04.022|last1=Cheddad|first1=Abbas|last2=Condell|first2=Joan|last3=Curran|first3=Kevin|last4=Mc Kevitt|first4=Paul|bibcode=2009SigPr..89.2465C }}</ref> 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 system]]s (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.<ref>{{cite journal|last1=Wendzel|first1=Steffen|last2=Mazurczyk|first2=Wojciech|last3=Haas|first3=Georg|title=Don't You Touch My Nuts: Information Hiding In Cyber Physical Systems Using Smart Buildings|journal=Proceedings of the 2017 IEEE Security & Privacy Workshops|language=en|publisher=IEEE}}</ref><ref>{{cite journal|last1=Tuptuk|first1=Nilufer|last2=Hailes|first2=Stephen|title=Covert channel attacks in pervasive computing|journal=Proceedings 2015 IEEE International Conference on Pervasive Computing and Communications (PerCom)}}</ref>

===Printed===
Digital steganography output may be in the form of printed documents. A message, the ''[[plaintext]]'', may be first encrypted by traditional means, producing a ''[[ciphertext]]''. 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]], 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]] developed [[Bacon's cipher]] 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.<ref>{{cite web|url=https://pictureworthsthousandwords.appspot.com/|title=ASCII Art Steganography|author=Vincent Chu|website=Pictureworthsthousandwords.appspot.com}}</ref>

[[File:HP Color Laserjet 3700 schutz g.jpg|thumb|Yellow dots from a laser printer]]
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]] for details.

=== Network ===
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.<ref>{{cite journal |last1=Wendzel |first1=Steffen |last2=Zander |first2=Sebastian |last3=Fechner |first3=Bernhard |last4=Herdin |first4=Christian |title=Pattern-Based Survey and Categorization of Network Covert Channel Techniques |journal=ACM Computing Surveys |date=16 April 2015 |volume=47 |issue=3 |pages=1–26 |doi=10.1145/2684195 |arxiv=1406.2901 |s2cid=14654993 |url=https://www.researchgate.net/publication/263048788}}</ref> The taxonomy was developed further in recent years by several publications and authors and adjusted to new domains, such as CPS steganography.<ref>{{cite book |last1=Mazurczyk |first1=Wojciech |last2=Wendzel |first2=Steffen |last3=Cabaj |first3=Krzysztof |title=Proceedings of the 13th International Conference on Availability, Reliability and Security |chapter=Towards Deriving Insights into Data Hiding Methods Using Pattern-based Approach |date=27 August 2018 |pages=1–10 |doi=10.1145/3230833.3233261|isbn=9781450364485 |s2cid=51976841 }}</ref><ref>{{cite journal |last1=Hildebrandt |first1=Mario |last2=Altschaffel |first2=Robert |last3=Lamshöft |first3=Kevin |last4=Lange |first4=Matthias |last5=Szemkus |first5=Martin |last6=Neubert |first6=Tom |last7=Vielhauer |first7=Claus |last8=Ding |first8=Yongjian |last9=Dittmann |first9=Jana |title=Threat Analysis of Steganographic and Covert Communication in Nuclear I&C Systems |journal=International Conference on Nuclear Security: Sustaining and Strengthening Efforts |date=2020}}</ref><ref>{{cite journal |last1=Mileva |first1=Aleksandra |last2=Velinov |first2=Aleksandar |last3=Hartmann |first3=Laura |last4=Wendzel |first4=Steffen |last5=Mazurczyk |first5=Wojciech |title=Comprehensive analysis of MQTT 5.0 susceptibility to network covert channels |journal=Computers & Security |date=May 2021 |volume=104 |pages=102207 |doi=10.1016/j.cose.2021.102207|s2cid=232342523 |doi-access=free }}</ref>

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.<ref>{{Cite journal|last=Girling|first=C.G.|date=February 1987|title=Covert Channels in LAN's|journal=IEEE Transactions on Software Engineering|volume=SE-13|issue=2|pages=292–296|doi=10.1109/tse.1987.233153|s2cid=3042941|issn=0098-5589}}</ref>

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.<ref>M. Wolf, “Covert channels in LAN protocols,” in Proceedings of the Workshop on Local Area Network Security (LANSEC’89) (T.A. Berson and T. Beth, eds.), pp. 91–102, 1989.</ref>

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.<ref>{{Cite journal|last=Rowland|first=Craig H.|date=5 May 1997|title=Covert channels in the TCP/IP protocol suite|journal=First Monday|volume=2|issue=5|doi=10.5210/fm.v2i5.528|issn=1396-0466 |doi-access=free }}</ref>

In 2002, Kamran Ahsan made an excellent summary of research on network steganography.<ref>Kamran Ahsan, “Covert Channel Analysis and Data Hiding in TCP/IP,” MSc Thesis, University of Toronto, 2002.</ref>

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.<ref>{{Citation|last1=Murdoch|first1=Steven J.|title=Embedding Covert Channels into TCP/IP|date=2005|work=Information Hiding|pages=247–261|publisher=Springer Berlin Heidelberg|isbn=9783540290391|last2=Lewis|first2=Stephen|doi=10.1007/11558859_19}}</ref>

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.<ref>{{cite web |url=http://www.tele.pw.edu.pl/~krzysiek/pdf/steg-seminar-2003.pdf |title=Steganography in TCP/IP Networks. State of the Art and a Proposal of a New System – HICCUPS |author=Krzysztof Szczypiorski |access-date=17 June 2010 |work=Institute of Telecommunications Seminar |date=4 November 2003}}</ref> 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.<ref>{{cite web |url=http://irevolution.wordpress.com/2009/06/05/steganography-2-0-digital-resistance-against-repressive-regimes/ |title=Steganography 2.0: Digital Resistance against Repressive Regimes |author=Patrick Philippe Meier |access-date=17 June 2010 |work=irevolution.wordpress.com |date=5 June 2009}}</ref>

Typical network steganography methods involve modification of the properties of a single network protocol. Such modification can be applied to the [[protocol data unit]] (PDU),<ref>{{cite web |url=http://firstmonday.org/htbin/cgiwrap/bin/ojs/index.php/fm/issue/view/80 |title=Covert Channels in the TCP/IP Suite |author=Craig Rowland |access-date=16 June 2010 |work=First Monday Journal |date=May 1997 |archive-date=26 January 2013 |archive-url=https://web.archive.org/web/20130126135920/http://firstmonday.org/htbin/cgiwrap/bin/ojs/index.php/fm/issue/view/80 |url-status=dead }}</ref><ref>{{cite web |url=http://www.cl.cam.ac.uk/~sjm217/papers/ih05coverttcp.pdf |title=Embedding Covert Channels into TCP/IP |author1=Steven J. Murdoch  |author2=Stephen Lewis  |name-list-style=amp |access-date=16 June 2010 |work=Information Hiding Workshop |year=2005}}</ref><ref>{{cite web |url=http://wwwiti.cs.uni-magdeburg.de/iti_amsl/acm/acm02/ahsan_kundur.pdf |title=Practical Data Hiding in TCP/IP |author1=Kamran Ahsan |author2=Deepa Kundur |name-list-style=amp |access-date=16 June 2010 |work=ACM Wksp. Multimedia Security |date=December 2002 |archive-date=29 October 2012 |archive-url=https://web.archive.org/web/20121029155725/http://wwwiti.cs.uni-magdeburg.de/iti_amsl/acm/acm02/ahsan_kundur.pdf |url-status=dead }}</ref> to the time relations between the exchanged PDUs,<ref>{{cite web |url=http://www.ece.tamu.edu/~deepa/pub/KunAhsTXSecWrkshp03.pdf |title=Practical Internet Steganography: Data Hiding in IP |author1=Kundur D. |author2=Ahsan K. |name-list-style=amp |access-date=16 June 2010 |work=Texas Wksp. Security of Information Systems |date=April 2003 |archive-date=29 October 2012 |archive-url=https://web.archive.org/web/20121029155725/http://www.ece.tamu.edu/~deepa/pub/KunAhsTXSecWrkshp03.pdf |url-status=dead }}</ref> or both (hybrid methods).<ref>{{cite book |chapter=Steganography of VoIP Streams |author1=Wojciech Mazurczyk  |author2=Krzysztof Szczypiorski  |title=On the Move to Meaningful Internet Systems: OTM 2008 |name-list-style=amp |doi=10.1007/978-3-540-88873-4_6 |series=Lecture Notes in Computer Science |date=November 2008|volume=5332 |pages=1001–1018 |arxiv=0805.2938 |isbn=978-3-540-88872-7 |s2cid=14336157 }}</ref>

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.<ref>{{cite arXiv |eprint=1005.1925 |title=Information Hiding Using Improper Frame Padding |author1=Bartosz Jankowski |author2=Wojciech Mazurczyk |author3=Krzysztof Szczypiorski  |name-list-style=amp |date=11 May 2010 |class=cs.CR}}</ref> 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.<ref>{{cite book|last1=Wendzel|first1=Steffen|last2=Keller|first2=Joerg|title=Communications and Multimedia Security |chapter=Low-Attention Forwarding for Mobile Network Covert Channels |volume=7025|date=20 October 2011|pages=122–133|doi=10.1007/978-3-642-24712-5_10|url=https://www.researchgate.net/publication/215661202|access-date=4 September 2016|series=Lecture Notes in Computer Science|isbn=978-3-642-24711-8}}</ref><ref name="Wiley-IEEE">{{cite book|url=http://eu.wiley.com/WileyCDA/WileyTitle/productCd-1118861698.html|title=Information Hiding in Communication Networks: Fundamentals, Mechanisms, and Applications|last1=Mazurczyk|first1=Wojciech|last2=Wendzel|first2=Steffen|last3=Zander|first3=Sebastian|last4=Houmansadr|first4=Amir|last5=Szczypiorski|first5=Krzysztof|date=2016|publisher=Wiley-IEEE|isbn=978-1-118-86169-1|edition=1}}</ref>

Network steganography covers a broad spectrum of techniques, which include, among others:
* Steganophony – the concealment of messages in [[Voice-over-IP]] 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.<ref>{{cite web |url=https://spectrum.ieee.org/vice-over-ip-the-voip-steganography-threat |title=Vice Over IP: The VoIP Steganography Threat |author1=Józef Lubacz |author2=Wojciech Mazurczyk |author3=Krzysztof Szczypiorski |access-date=11 February 2010 |work=IEEE Spectrum |date=February 2010}}</ref>
* 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)<ref>{{cite web |url=http://krzysiek.tele.pw.edu.pl/pdf/acs2003-hiccups.pdf |title=HICCUPS: Hidden Communication System for Corrupted Networks |author= Krzysztof Szczypiorski |access-date=11 February 2010 |work=In Proc. of: The Tenth International Multi-Conference on Advanced Computer Systems ACS'2003, pp. 31–40  |date=October 2003}}</ref>

==Additional terminology==
{{further|Security through obscurity}}
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|ultraviolet light]]. It is a time-consuming process with obvious resource implications, even in countries that employ many people to spy on other citizens. 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]], prisoner of war camps gave prisoners specially-treated [[paper]] that would reveal [[invisible ink]]. An article in the 24 June 1948 issue of ''Paper Trade Journal'' by the Technical Director of the [[United States Government Printing Office]] 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]]s related to the technology, one to Kantrowitz, {{US Patent|2515232}}, "Water-Detecting paper and Water-Detecting Coating Composition Therefor," patented 18 July 1950, and an earlier one, "Moisture-Sensitive Paper and the Manufacture Thereof," {{US Patent|2445586}}, 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]]. 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]] are visual or aural attacks, structural attacks, and statistical attacks. These approaches attempt to detect the steganographic algorithms that were used.<ref name="Wanyer2009">Wayner, Peter (2009). ''Disappearing Cryptography: Information Hiding: Steganography & Watermarking'',  Morgan Kaufmann Publishers, Amsterdam; Boston {{ISBN?}}</ref> 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.<ref name=Wanyer2009 />

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<ref>{{cite web|url=https://www.mcafee.com/blogs/enterprise/seeing-through-stegware/|title=What's Hidden in That Picture Online? Seeing Through "Stegware"|publisher=[[McAfee]]|last=Lancioni|first=German|date=16 October 2017}}</ref> 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,<ref>{{cite report |url=https://www.researchgate.net/publication/319943090 |title=Defenders Guide to Steganography |year=2017 |doi=10.13140/RG.2.2.21608.98561 |last1=Wiseman |first1=Simon}}</ref> a process called [[Content Threat Removal]].

==Applications==
===Use in modern printers===
{{Main|Printer steganography}}
Some modern computer printers use steganography, including [[Hewlett-Packard]] and [[Xerox]] 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.<ref>{{cite web |url=https://www.eff.org/press/archives/2005/10/16 |title=Secret Code in Color Printers Lets Government Track You; Tiny Dots Show Where and When You Made Your Print |publisher=[[Electronic Frontier Foundation]] |date=16 October 2005}}</ref>

===Example from modern practice===
The larger the cover message (in binary data, the number of [[bit]]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 image|digital picture]]s, which contain much data, are sometimes used to hide messages on the [[Internet]] and on other digital communication media. It is not clear how common this practice actually is.

For example, a 24-bit [[bitmap]] uses 8 bits to represent each of the three color values (red, green, and blue) of each [[pixel]]. The blue alone has 2<sup>8</sup> different levels of blue intensity. The difference between 11111111<sub>2</sub> and 11111110<sub>2</sub> 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 that is repeated for the green and the red elements of each pixel as well, it is possible to encode one letter of [[ASCII]] text for every three [[pixel]]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]] 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 theory|information theoretical]] point of view, that means that the [[Communication channel|channel]] must have more [[channel capacity|capacity]] than the "surface" [[signal (information theory)|signal]] requires. There must be [[redundancy (information theory)|redundancy]]. For a digital image, it may be [[noise]] from the imaging element; for [[digital audio]], it may be noise from recording techniques or [[amplifier|amplification]] equipment. In general, electronics that digitize an [[analog signal]] suffer from several noise sources, such as [[Johnson-Nyquist noise|thermal noise]], [[flicker noise]], and [[shot noise]]. 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]] schemes (such as [[JPEG]]) 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]]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]]) or even just to identify an image (as in the [[EURion constellation]]). 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]] alleged that the [[Foreign Intelligence Service (Russia)|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.<ref>{{cite web|title=Criminal complaint by Special Agent Ricci against alleged Russian agents|url=https://www.justice.gov/opa/documents/062810complaint2.pdf|publisher=United States Department of Justice}}</ref>

On 23 April 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.<ref>{{cite web|title=GE Engineer Charged in Elaborate Theft of Trade Secrets|url=https://blog.twinstate.com/news/ge-trade-secrets-theft|publisher=Twinstate Technologies}}</ref>

===Distributed steganography===
There are distributed steganography methods,<ref>{{cite book |chapter=Distributed Steganography |publisher=[[IEEE]] |year= 2011|doi=10.1109/IIHMSP.2011.20 |isbn=978-1-4577-1397-2 |title=2011 Seventh International Conference on Intelligent Information Hiding and Multimedia Signal Processing |last1=Liao |first1=Xin |last2=Wen |first2=Qiao-yan |last3=Shi |first3=Sha |pages=153–156 |s2cid=17769131 }}</ref> including methodologies that distribute the payload through multiple carrier files in diverse locations to make detection more difficult. For example, {{US Patent|8527779}} by cryptographer William Easttom ([[Chuck Easttom]]).

===Online challenge===
The puzzles that are presented by [[Cicada 3301]] incorporate steganography with cryptography and other solving techniques since 2012.<ref>{{cite news|title=Cicada 3301: The darknet treasure trail reopens|author=Jane Wakefield |url=https://www.bbc.co.uk/news/technology-25667292|work=BBC News|access-date=11 January 2014 |date=9 January 2014}}</ref> Puzzles involving steganography have also been featured in other [[alternative reality game]]s.

The communications<ref>{{Cite web|url=http://www.maydaymystery.org/mayday/texts/index.html|title=The texts|website=Maydaymystery.org|access-date=23 November 2017}}</ref><ref>{{Cite web|url=http://www.maydaymystery.org/mayday/recent.html|title=Recent things|website=Maydaymystery.org|access-date=23 November 2017}}</ref> of [[Arizona Daily Wildcat#The May Day mystery|The May Day mystery]] incorporate steganography and other solving techniques since 1981.<ref>{{Cite web|url=http://www.maydaymystery.org/mayday/mystery.html|title=The Mystery|website=Maydaymystery.org|access-date=23 November 2017}}</ref>

=== Computer malware ===
{{Main|Stegomalware}}
It is possible to steganographically hide computer malware into digital images, videos, audio and various other files in order to evade detection by [[antivirus software]]. This type of malware is called stegomalware. It can be activated by external code, which can be malicious or even non-malicious if some vulnerability in the software reading the file is exploited.<ref name="Chaganti-2021">{{Cite report |last1=Chaganti |first1=Raj |last2=R |first2=Vinayakumar |last3=Alazab |first3=Mamoun |last4=Pham |first4=Tuan |date=12 October 2021 |title=Stegomalware: A Systematic Survey of Malware Hiding and Detection in Images, Machine Learning Models and Research Challenges |url=https://www.techrxiv.org/articles/preprint/Stegomalware_A_Systematic_Survey_of_Malware_Hiding_and_Detection_in_Images_Machine_Learning_Models_and_Research_Challenges/16755457/1 |language=en |doi=10.36227/techrxiv.16755457.v1|arxiv=2110.02504 }} (pre-print, not peer reviewed)</ref>

Stegomalware can be removed from certain files without knowing whether they contain stegomalware or not. This is done through [[Content Disarm & Reconstruction|content disarm and reconstruction]] (CDR) software, and it involves reprocessing the entire file or removing parts from it.<ref>{{Cite web |last=Votiro |date=30 November 2021 |title=Finding a Content Disarm & Reconstruction (CDR) Vendor |url=https://votiro.com/blog/what-to-look-for-in-cdr-file-sanitization-vendors/ |access-date=11 January 2023 |website=Votiro |language=en-US}}</ref><ref>{{Cite web |title=Content Disarm and Reconstruct – SecureIQLab |date=12 April 2022 |url=https://secureiqlab.com/content-disarm-and-reconstruct/ |access-date=11 January 2023 |language=en-US}}</ref> Actually detecting stegomalware in a file can be difficult and may involve testing the file behaviour in [[Virtualization|virtual]] environments or [[deep learning]] analysis of the file.<ref name="Chaganti-2021" />

== Steganalysis ==
{{main|Steganalysis}}

=== 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 steganographic 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 them.

==== 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 steganalysis: 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 steganalysis: changes the initial stego target, therefore, it seeks to suppress the transfer of information, if it exists.

==See also==
<!-- Please respect alphabetical order -->
{{div col}}
* {{annotated link|40-track mode}}
* {{annotated link|Acrostic}}
* {{annotated link|Bible code}}
* {{annotated link|BPCS-Steganography}}
* {{annotated link|Hacktivismo#Camera.2FShy|Camera/Shy}}
* {{annotated link|Canary trap}}
* {{annotated link|Warrant canary}}
* {{annotated link|Covert channel}}
* {{annotated link|Cryptography}}
* {{annotated link|Deniable encryption}}
* {{annotated link|Digital watermarking}}
* {{annotated link|Invisible ink}}
* [[Music cipher#Musical Steganagraphy|Musical Steganography]] – Hiding a message using musical notation
* {{annotated link|Polybius square}}
* {{annotated link|Security engineering}}
* {{annotated link|Semiotics}}
* {{annotated link|Steganographic file system}}
* {{annotated link|Steganography tools}}
* {{annotated link|Audio watermark}}
* {{annotated link|Visual cryptography}}
* {{annotated link|Security printing}}

{{div col end}}

==References==
{{Reflist}}

==Sources==
{{Refbegin}}
* {{cite book |author=Wayner, Peter |title=Disappearing cryptography: information hiding: steganography & watermarking |publisher=MK/Morgan Kaufmann Publishers |location=Amsterdam |year=2002 |isbn=978-1-558-60769-9 |url=http://www.wayner.org/node/6 |access-date=4 November 2010 |archive-date=24 December 2010 |archive-url=https://web.archive.org/web/20101224093251/http://www.wayner.org/node/6 |url-status=dead }}
* {{cite book |author=Wayner, Peter |title=Disappearing cryptography 3rd Edition: information hiding: steganography & watermarking |publisher=MK/Morgan Kaufmann Publishers |location=Amsterdam |year=2009 |isbn=978-0-123-74479-1 |url=http://www.wayner.org/node/13 |access-date=4 November 2010 |archive-date=24 December 2010 |archive-url=https://web.archive.org/web/20101224093238/http://www.wayner.org/node/13 |url-status=dead }}
* {{cite book |author1=Petitcolas, Fabien A.P. |author2=Katzenbeisser, Stefan |title=Information Hiding Techniques for Steganography and Digital Watermarking |publisher=Artech House Publishers |year=2000 |isbn=978-1-580-53035-4 |url=http://petitcolas.net/fabien/publications/book99-ih/ }}
* {{cite book |author1=Johnson, Neil |author2=Duric, Zoran |author3=Jajodia, Sushil |title=Information hiding: steganography and watermarking: attacks and countermeasures |publisher=Springer |year=2001 |isbn=978-0-792-37204-2}}
* {{cite book |author1=Petitcolas, Fabien A.P. |author2=Katzenbeisser, Stefan |title=Information Hiding |publisher=Artech House Publishers |year=2016 |isbn=978-1608079285 |url=http://petitcolas.net/book15-ih/ }}
{{Refend}}

==External links==
{{External links|section|date=May 2024}}
{{Commons category}}
* [https://www.youtube.com/watch?v=-7FBPgQDX5o An overview of digital steganography, particularly within images, for the computationally curious] by Chris League, Long Island University, 2015
* [http://petitcolas.net/fabien/steganography/image_downgrading/index.html Examples showing images hidden in other images]
* [http://www.jjtc.com/Steganography 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.
* [http://niels.xtdnet.nl/papers/detecting.pdf Detecting Steganographic Content on the Internet.]   2002 paper by [[Niels Provos]] and Peter Honeyman published in ''Proceedings of the Network and Distributed System Security Symposium'' (San Diego, CA, 6–8 February 2002). NDSS 2002. Internet Society, Washington, D.C.
* [http://firstmonday.org/htbin/cgiwrap/bin/ojs/index.php/fm/article/view/528/449 Covert Channels in the TCP/IP Suite] {{Webarchive|url=https://web.archive.org/web/20121023122054/http://firstmonday.org/htbin/cgiwrap/bin/ojs/index.php/fm/article/view/528/449 |date=23 October 2012 }}{{snd}}1996 paper by Craig Rowland detailing the hiding of data in TCP/IP packets.
* [http://stegano.net/tutorials.html Network Steganography Centre Tutorials] {{Webarchive|url=https://web.archive.org/web/20171216153846/http://stegano.net/tutorials.html |date=16 December 2017 }}.  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.
* [http://datahide.org/BPCSe/ Invitation to BPCS-Steganography.]
* [http://www.spy-hunter.com/Steganography_V7.0_DefCon_V3_S.pdf Steganography by Michael T. Raggo], DefCon 12 (1 August 2004)
* [http://ecommons.txstate.edu/cscitad/7 File Format Extension Through Steganography] by Blake W. Ford and Khosrow Kaikhah
* [https://web.archive.org/web/20140221205846/http://er.nau.edu.ua/bitstream/NAU/8049/1/CompSteganoRU.pdf Computer steganography. Theory and practice with Mathcad (Rus)] 2006 paper by Konakhovich G. F., Puzyrenko A. Yu. published in ''MK-Press'' Kyiv, Ukraine
* [https://bztsrc.gitlab.io/stegano stegano] a Free and Open Source steganography web service.

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