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Proceedings of the IEEE, special issue on protection of multimedia content, 87(7):1062–1078, July 1999.

Information Hiding—A Survey

Fabien A. P. Petitcolas, Ross J. Anderson and Markus G. Kuhn

Abstract— Information hiding techniques have recently be- come important in a number of application areas. Digital audio, video, and pictures are increasingly furnished with distinguishing but imperceptible marks, which may contain a hidden copyright notice or serial number or even help to prevent unauthorised copying directly. Military communi- cations systems make increasing use of traffic security tech- niques which, rather than merely concealing the content of a message using encryption, seek to conceal its sender, its receiver or its very existence. Similar techniques are used in some mobile phone systems and schemes proposed for digital elections. Criminals try to use whatever traffic se- curity properties are provided intentionally or otherwise in the available communications systems, and police forces try to restrict their use. However, many of the techniques pro- posed in this young and rapidly evolving field can trace their history back to antiquity; and many of them are surpris- ingly easy to circumvent. In this article, we try to give an overview of the field; of what we know, what works, what does not, and what are the interesting topics for research.

Keywords— Information hiding, steganography, copyright marking.

I. Introduction

It is often thought that communications may be secured by encrypting the traffic, but this has rarely been adequate in practice. Æneas the Tactician, and other classical writ- ers, concentrated on methods for hiding messages rather than for enciphering them [1]; and although modern cryp- tographic techniques started to develop during the Renais- sance, we find in 1641 that John Wilkins still preferred hiding over ciphering [2, IX pp. 67] because it arouses less suspicion. This preference persists in many operational contexts to this day. For example, an encrypted email mes- sage between a known drug dealer and somebody not yet under suspicion, or between an employee of a defence con- tractor and the embassy of a hostile power, has obvious implications.

So the study of communications security includes not just encryption but also traffic security, whose essence lies in hiding information. This discipline includes such tech- nologies as spread spectrum radio, which is widely used in tactical military systems to prevent transmitters being located; temporary mobile subscriber identifiers, used in digital phones to provide users with some measure of loca- tion privacy; and anonymous remailers, which conceal the identity of the sender of an email message [3].

An important subdiscipline of information hiding is ste- ganography. While cryptography is about protecting the content of messages, steganography is about concealing their very existence. It comes from Greek roots (στεγανo´- ς, γραφ-ειν), literally means ‘covered writing’ [4], and is

The authors are with the University of Cambridge Computer Labo- ratory, Security Group, Pembroke Street, Cambridge CB2 3QG, UK. E-mail {fapp2, rja14, mgk25}@cl.cam.ac.uk.

Part of this work was supported by the Intel Corporation under the grant ‘Robustness of Information Hiding Systems’.

usually interpreted to mean hiding information in other in- formation. Examples include sending a message to a spy by marking certain letters in a newspaper using invisible ink, and adding sub-perceptible echo at certain places in an audio recording.

Until recently, information hiding techniques received much less attention from the research community and from industry than cryptography, but this is changing rapidly (table I), and the first academic conference on the sub- ject was organised in 1996 [5]. The main driving force is concern over copyright; as audio, video and other works be- come available in digital form, the ease with which perfect copies can be made may lead to large-scale unauthorised copying, and this is of great concern to the music, film, book, and software publishing industries. There has been significant recent research into digital ‘watermarks’ (hid- den copyright messages) and ‘fingerprints’ (hidden serial numbers); the idea is that the latter can help to identify copyright violators, and the former to prosecute them.

In another development, the DVD consortium has called for proposals for a copyright marking scheme to enforce serial copy management. The idea is that DVD players available to consumers would allow unlimited copying of home videos and time-shifted viewing of TV programmes, but could not easily be abused for commercial piracy. The proposal is that home videos would be unmarked, TV broadcasts marked ‘copy once only’, and commercial videos marked ‘never copy’; compliant consumer equipment would act on these marks in the obvious way [7], [8].

There are a number of other applications driving interest in the subject of information hiding (figure 1).

  • Military and intelligence agencies require unobtrusive

communications. Even if the content is encrypted, the de- tection of a signal on a modern battlefield may lead rapidly to an attack on the signaller. For this reason, military com- munications use techniques such as spread spectrum mod- ulation or meteor scatter transmission to make signals hard for the enemy to detect or jam.

  • Criminals also place great value on unobtrusive com-

munications. Their preferred technologies include prepaid mobile phones, mobile phones which have been modified to change their identity frequently, and hacked corporate switchboards through which calls can be rerouted.

  • Law enforcement and counter intelligence agencies are

interested in understanding these technologies and their weaknesses, so as to detect and trace hidden messages.

  • Recent attempts by some governments to limit online free

speech and the civilian use of cryptography have spurred people concerned about liberties to develop techniques for anonymous communications on the net, including anony- mous remailers and Web proxies.

  • Schemes for digital elections and digital cash make use

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