TABLE I Number of publications on digital watermarking during the past few years according to INSPEC, January 99. Courtesy of J.-L. Dugelay .
of anonymous communication techniques.
Marketeers use email forgery techniques to send out huge
numbers of unsolicited messages while avoiding responses from angry users.
We will mention some more applications later. For the time being, we should note that while the ethical positions of the players in the cryptographic game are often thought to be clear cut (the ‘good’ guys wish to keep their com- munications private while the ‘bad’ eavesdropper wants to listen in), the situation is much less clear when it comes to hiding information. Legitimate users of the net may need anonymous communications to contact abuse helplines or vote privately in online elections ; but one may not want to provide general anonymous communication mechanisms that facilitate attacks by people who maliciously overload the communication facilities. Industry may need tools to hide copyright marks invisibly in media objects, yet these tools can be abused by spies to pass on secrets hidden in in- conspicuous data over public networks. Finally, there are a number of non-competitive uses of the technology, such as marking audio tracks with purchasing information so that someone listening to a piece of music on his car radio could simply press a button to order the CD.
The rest of this paper is organised as follows. Firstly, we will clarify the terminology used for information hiding, including steganography, digital watermarking and finger- printing. Secondly we will describe a wide range of tech- niques that have been used in a number of applications, both ancient and modern, which we will try to juxtapose in such a way that the common features become evident. Then, we will describe a number of attacks against these techniques; and finally, we will try to formulate general def- initions and principles. Moving through the subject from practice to theory may be the reverse of the usual order of presentation, but appears appropriate to a discipline in which rapid strides are being made constantly, and where general theories are still very tentative.
As we have noted previously, there has been a growing interest, by different research communities, in the fields of steganography, digital watermarking and fingerprinting. This led to some confusion in the terminology. We shall now briefly introduce the terminology which will be used in the rest of the paper and which was agreed at the first international workshop on the subject ,  (figure 1).
The general model of hiding data in other data can be described as follows. The embedded data is the message that one wishes to send secretly. It is usually hidden in
A classification of information hiding techniques based on
. Many of the ancient systems presented in Sections III-A and III-B are a form of ‘technical steganography’ (in the sense that messages are hidden physically) and most of the recent ex- amples given in this paper address ‘linguistic steganography’ and ‘copyright marking’.
an innocuous message referred to as a cover-text, or cover- image or cover-audio as appropriate, producing the stego- text or other stego-object. A stego-key is used to control the hiding process so as to restrict detection and/or recovery of the embedded data to parties who know it (or who know some derived key value).
As the purpose of steganography is having a covert com- munication between two parties whose existence is un- known to a possible attacker, a successful attack consists in detecting the existence of this communication. Copy- right marking, as opposed to steganography, has the addi- tional requirement of robustness against possible attacks. In this context, the term ‘robustness’ is still not very clear; it mainly depends on the application. Copyright marks do not always need to be hidden, as some systems use visi- ble digital watermarks , but most of the literature has focussed on invisible (or transparent) digital watermarks which have wider applications. Visible digital watermarks are strongly linked to the original paper watermarks which appeared at the end of the XIII century to differentiate paper makers of that time  (figure 6). Modern visible watermarks may be visual patterns (e.g., a company logo or copyright sign) overlaid on digital images.
In the literature on digital marking, the stego-object is usually referred to as the marked object rather than stego- object. We may also qualify marks depending on the ap- plication. Fragile watermarks1 are destroyed as soon as the object is modified too much. This can be used to prove that an object has not been ‘doctored’ and might be useful if digital images are used as evidence in court. Robust marks
1Fragile watermarks have also wrongly been referred to as ‘signa- ture’, leading to confusion with digital signatures used in cryptogra- phy.