noise shaper is used to reduce to minimise the effect of the embedded signal on the quality of the cover music.
For more details about the use of perceptual models in digital watermarking, the reader is referred to .
C. Hiding the location of the embedded information
Fig. 5. A typical use of masking and transform space for digital wa- termarking and fingerprinting. The signal can be an image or an audio signal. The perceptual analysis is based on the properties of the human visual or auditory systems respectively. ⊕ corre- sponds to the embedding algorithm and ⊗ to the weighting of the mark by the information provided by the perceptual model.
the effects on the surfaces of the fibres [32, pp. 523–525]. Nowadays, in the field of currency security, special inks or materials with particular structure (such as fluorescent dyes or DNA) are used to write a hidden message on bank notes or other secure documents. These materials provide a unique response to some particular excitation such as a reagent or laser light at a particular frequency .
By 1860 the basic problems of making tiny images had been solved . In 1857, Brewster suggested hiding secret messages ‘in spaces not larger than a full stop or small dot of ink’ . During the Franco-Prussian War of 1870–1871, while Paris was besieged, messages on microfilm were sent out by pigeon post , . During the Russo-Japanese war of 1905, microscopic images were hidden in ears, nos- trils, and under finger nails . By World War I messages to and from spies were reduced to microdots by several stages of photographic reduction and then stuck on top of printed periods or commas in innocuous cover material such as magazines , .
The digital equivalent of these camouflage techniques is the use of masking algorithms , , , , . Like most source-coding techniques (e.g., ), these rely on the properties of the human perceptual system. Au- dio masking, for instance, is a phenomenon in which one sound interferes with our perception of another sound . Frequency masking occurs when two tones which are close in frequency are played at the same time: the louder tone will mask the quieter one. Temporal masking occurs when a low-level signal is played immediately before or after a stronger one; after a loud sound stops, it takes a little while before we can hear a weak tone at a nearby frequency.
Because these effects are used in compression standards such as MPEG , many systems shape the embedded data to emphasise it in the perceptually most significant components of the data so it will survive compression ,  (figure 5). This idea is also applied in buried data channels where the regular channels of an audio CD contain other embedded sound channels ; here, an optimised
In a security protocol developed in ancient China, the sender and the receiver had copies of a paper mask with a number of holes cut at random locations. The sender would place his mask over a sheet of paper, write the secret mes- sage into the holes, remove the mask and then compose a cover message incorporating the code ideograms. The re- ceiver could read the secret message at once by placing his mask over the resulting letter. In the early 16th century Cardan (1501–1576), an Italian mathematician, reinvented this method which is now known as the Cardan grille. It appears to have been reinvented again in 1992 by a British bank, which recommended that its customers conceal the personal information number used with their cash machine card using a similar system. In this case, a poor implemen- tation made the system weak .
A variant on this theme is to mark an object by the pres- ence of errors or stylistic features at predetermined points in the cover material. An early example was a technique used by Francis Bacon (1561–1626) in his biliterarie alpha- bet [55, pp. 266], which seems to be linked to the con- troversy whether he wrote the works attributed to Shake- speare . In this method each letter is encoded in a five bit binary code and embedded in the cover-text by printing the letters in either normal or italic fonts. The variability of sixteenth century typography acted as camouflage.
Further examples come from the world of mathematical tables. Publishers of logarithm tables and astronomical ephemerides in the 17th and 18th century used to introduce errors deliberately in the least significant digits (e.g., ). To this day, database and mailing list vendors insert bogus entries in order to identify customers who try to resell their products.
In an electronic publishing pilot project copyright mes- sages and serial numbers have been hidden in the line spac- ing and other format features of documents (e.g., ). It was found that shifting text lines up or down by one-three- hundredth of an inch to encode zeros and ones was robust against multi-generation photocopying and could not be noticed by most people.
However, the main application area of current copyright marking proposals, lies in digital representations of ana- logue objects such as audio, still pictures, video and mul- timedia generally. Here there is considerable scope for em- bedding data by introducing various kinds of error. As we noted above, many writers have proposed embedding the data in the least significant bits , . An obvi- ously better technique, which has occurred independently to many writers, is to embed the data into the least signif- icant bits of pseudo-randomly chosen pixels or sound sam- ples , . In this way, the key for the pseudo-random sequence generator becomes the stego-key for the system and Kerckhoffs’ principle is observed.