tures of the Dutch passport in . Such products tend to combine overt marks that are expensive to reproduce (holo- grams, kinegrams, intaglios and optically variable inks) with tamper-evidence features (such as laminates and reac- tive inks) and secondary features whose presence may not be obvious (such as micro-printing, diffraction effects visi- ble only under special illumination, and alias band struc- tures – dithering effects that normal scanners cannot cap- ture), , . In a more recent application called sub- graving, variable information (such as a serial number) is printed on top of a uniform offset background. The printed area is then exposed to an excimer laser: this removes the offset ink everywhere but underneath the toner. Fraudu- lent removal of the toner by a solvent reveals the hidden ink .
Monograms figuring TGE RG (Thomas Goodrich Eliensis
Bishop of Ely, England – and Remy/Remigius Guedon, the
paper-maker). One of the oldest watermarks found in the Cam- bridge area (c.1550). At that time, watermarks were mainly used to identify the mill producing the paper; a means of guaranteeing quality. Courtesy of Dr E. Leedham-Green, Cambridge Univer- sity Archives. Reproduction technique: beta radiography.
Increasingly, features are incorporated that are designed to be verified by machines rather than humans. Marks can be embedded in the magnetic strips of bank cards, giving each card a unique serial number that is hard to repro- duce ; they are used in phone cards too in some coun- tries. Magnetic fibres can be embedded randomly in paper or cardboard, giving each copy of a document a unique fingerprint.
are encoded at locations separated by spaces of pseudo- random length. The cepstral transform  is used to ma- nipulate the echo signals.
E. Techniques specific to the environment
Echo hiding leads naturally to the broader topic of in- formation hiding techniques that exploit features of a par- ticular application environment. One technology that is emerging from the military world is meteor burst commu- nication, which uses the transient radio paths provided by ionised trails of meteors entering the atmosphere to send data packets between a mobile station and a base . The transient nature of these paths makes it hard for an enemy to locate mobiles using radio direction finding, and so me- teor burst is used in some military networks.
More familiar application-specific information hiding and marking technologies are found in the world of secu- rity printing. Watermarks in paper are a very old anti- counterfeiting technique (figure 6); more recent innovations include special ultra-violet fluorescent inks used in printing travellers’ cheques. As the lamps used in photocopiers have a high UV content, it can be arranged that photocopies come out overprinted with ‘void’ in large letters. Inks may also be reactive; one of the authors has experience of trav- ellers’ cheques coming out ‘void’ after exposure to perspi- ration in a money belt. Recent developments address the problem of counterfeiting with scanners and printers whose capabilities have improved dramatically over the last few years .
Many other techniques are used. For a survey of op- tically variable devices, such as diffraction products and thin film interference coatings, see ; the design of the US currency is described in , ; and the security fea-
The importance of these technologies is not limited to protecting currency and securities. Forgery of drugs, vehi- cle spares, computer software and other branded products is said to have cost over $24 billions in 1995, and to have directly caused over 100 deaths worldwide . Security printing techniques are a significant control measure, al- though many fielded sealing products could be much bet- ter designed given basic attention to simple issues such as choice of pressure-sensitive adhesives and nonstandard ma- terials . Fashion designers are also concerned that their product might be copied and wish to find techniques to enable easy detection of counterfeit clothes or bags. As a greater percentage of the gross world product comes in the form of digital objects, the digital marking techniques described here may acquire more economic significance.
Also important are covert channels: communication paths that were neither designed nor intended to trans- fer information at all. Common examples include timing variations and error messages in communication protocols and operating system call interfaces , . Covert chan- nels are of particular concern in the design and evaluation of mandatory access control security concepts, where the operating system attempts to restrict the flow of informa- tion between processes in order to protect the user from computer viruses and Trojan horse software that transmits secrets to third parties without authorization.
The electromagnetic radiation produced by computers forms another covert channel. It not only interferes with reception on nearby radio receivers, but can also convey in- formation. For instance, the video signal emitted by CRT or liquid-crystal displays can be reconstructed using a sim- ple modified TV set at several hundred meters distance . Many military organizations use especially shielded ‘Tem- pest’ certified equipment to process classified information, in order to eliminate the risk of losing secrets via compro-