Main Subdisciplines of Information Hiding
Covert channels have been defined by Lampson , in the context of multilevel secure systems (e.g., military computer systems), as communication paths that were neither designed nor intended to transfer information at all. These channels are typically used by untrustworthy programs to leak information to their owner while performing a service for another program. These communication channels have been studied at length in the past to find ways to confine such programs . We will not extend much more on this topic except as an example of covert communication n Ethernet networks (see Section 2.7.2) and in the context of image downgrading (see Section 3.2.3).
Anonymity is finding ways to hide the metacontent of messages, that is, the sender and the recipients of a message. Early examples include anonymous remailers as described by Chaum  and onion routing, proposed by Goldschlag, Reed, and Syverson . The idea is that one can obscure the trail of a message by using a set of remailers or routers as long as the intermediaries do not collude; so trust remains the cornerstone of these tools. Note that there are different variants depending on who is “anonymized”; sender, receiver, or both. Web applications have focused on receiver anonymity while email users are concerned with sender anonymity.
An important subdiscipline of information hiding is steganography. While cryptography is about protecting the content of messages, steganography is about concealing their very existence. This modern adaptation of steganographia (Trithemius, 1462–1516), assumed from Greek , literally means “covered writing” , and is usually interpreted to mean hiding information in other information. Examples include sending a message to a spy by marking certain letters in a newspaper using invisible ink, and adding subperceptible echo at certain places in an audio recording. The general model of hiding data in other data will be illustrated in Chapter 2 and the main steganographic techniques will be reviewed in Chapter 3.
Watermarking, as opposed to steganography, has the additional requirement of robustness against possible attacks. In this context, the term “robustness” is still not very clear; it mainly depends on the application, but a successful attack will simply try to make the mark undetectable. We will show ways to achieve this in Chapter 7. Robustness has strong implications in the overall design of a watermarking system and this is one of the reasons why we will treat steganography and digital watermarking separately in this book.
Watermarks do not always need to be hidden, as some systems use visible digital watermarks , but most of the literature has focussed on imperceptible (invisible, transparent, or inaudible, depending on the context) digital watermarks which have wider applications. Visible digital watermarks are strongly linked to the original paper watermarks which appeared at the end of the 13th century (see Section 5.2.1). Modern visible watermarks may be visual patterns (e.g., a company logo or copyright sign) overlaid on digital images and are widely used by many photographers who do not trust invisible watermarking techniques.
From this brief overview the reader may have already noticed another fundamental difference between steganography and watermarking. The information hidden by a watermarking system is always associated to the digital object to be protected or to its owner while steganographic systems just hide any information. The “robustness” criteria are also different, since steganography is mainly concerned with detection of the hidden message while watermarking concerns potential removal by a pirate. Finally, steganographic communications are usually point-to-point (between sender and receiver) while watermarking techniques are usually one-to-many. Precise terminology for these two subdisciplines of information hiding will be given in Chapters 2 and 5.<< Introduction to Information Hiding ||