Editing Steganography (section)

==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>