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have the property that it is infeasible to remove them or make them useless without destroying the object at the same time. This usually means that the mark should be embedded in the most perceptually significant components of the object [14].

Mark (M)

Stego-image (I)

Secret/public key (K)

Marking algorithm


~ image (I)

Authors also make the distinction between various types of robust marks. Fingerprints (also called labels by some authors) are like hidden serial numbers which enable the in- tellectual property owner to identify which customer broke his license agreement by supplying the property to third parties. Watermarks tell us who is the owner of the ob- ject.

Figure 2 illustrates the generic embedding process. Given an image I, a mark M and a key K (usually the seed of a random number generator) the embedding process can

˜ be defined as a mapping of the form: I × K × M I and

Fig. 2. Generic digital watermark embedding scheme. The mark M can be either a fingerprint or a watermark.

Mark (M) and/or original image (I)


Detection algorithm

Mark or confidence measure

Secret/public key (K)

is common to all watermarking methods.

Fig. 3. Generic digital watermark recovery scheme.

The generic detection process is depicted in figure 3. Its output is either the recovered mark M or some kind of confidence measure indicating how likely it is for a given

III. Steganographic techniques

˜ m a r k a t t h e i n p u t t o b e p r e s e n t i n t h e i m a g e I u n d e r


There are several types of robust copyright marking sys- tems. They are defined by their inputs and outputs:

We will now look at some of the techniques used to hide information. Many of these go back to antiquity, but unfor- tunately many modern system designers fail to learn from the mistakes of their predecessors.

  • Private marking systems require at least the original im-

age. Type I systems, extract the mark M from the pos-

A. Security through obscurity

˜ s i b l y d i s t o r t e d i m a g e I a n d u s e t h e o r i g i n a l i m a g e a s a

˜ h i n t t o fi n d w h e r e t h e m a r k c o u l d b e i n I . T y p e I I s y s -

tems (e.g., [15], [16], [17]) also require a copy of the em- bedded mark for extraction and just yield a ‘yes’ or ‘no’

˜ a n s w e r t o t h e q u e s t i o n : d o e s I c o n t a i n t h e m a r k M ?

˜ ( I × I × K × M { 0 , 1 } ) . O n e m i g h t e x p e c t t h a t t h i s

kind of scheme will be more robust than the others since it conveys very little information and requires access to se- cret material [14]. Semi-private marking does not use the

˜ o r i g i n a l i m a g e f o r d e t e c t i o n ( I × K × M { 0 , 1 } ) b u t

answers the same question. The main uses of private and semi-private marking seem to be evidence in court to prove ownership and copy control in applications such as DVD where the reader needs to know whether it is allowed to play the content or not. Many of the currently proposed schemes fall in this category [18], [19], [20], [21], [22], [23], [24].

  • Public marking (also referred to as blind marking) re-

mains the most challenging problem since it requires nei- ther the secret original I nor the embedded mark M. In- deed such systems really extract n bits of information (the

˜ m a r k ) f r o m t h e m a r k e d i m a g e : I × K M [ 2 5 ] , [ 2 6 ] ,

[27], [28], [29]. Public marks have much more applications than the others and we will focus our benchmark on these systems. Indeed the embedding algorithms used in pub- lic systems can usually be used in private ones, improving robustness at the same time.

  • There is also asymmetric marking (or public key marking)

which should have the property that any user can read the mark, without being able to remove it.

In the rest of the paper, ‘watermark’ will refer to ‘digital watermark’ unless said otherwise.

By the 16–17th centuries, there had arisen a large lit- erature on steganography and many of the methods de- pended on novel means of encoding information. In his four hundred page book Schola Steganographica [30], Gas- par Schott (1608–1666) explains how to hide messages in music scores: each note corresponds to a letter (figure 4). Another method, based on the number of occurrences of notes and used by J. S. Bach, is mentioned in [11]. Schott also expands the ‘Ave Maria’ code proposed by Johannes Trithemius (1462–1516) in Steganographiæ, one of the first known books in the field. The expanded code uses forty tables, each of which contains 24 entries (one for each let- ter of the alphabet of that time) in four languages: Latin, German, Italian and French. Each letter of the plain-text is replaced by the word or phrase that appears in the corre- sponding table entry and the stego-text ends up looking like a prayer or a magic spell. It has been shown recently that these tables can be deciphered by reducing them modulo 25 and applying them to a reversed alphabet [31]. In [2], John Wilkins (1614–1672), Master of Trinity College, Cam- bridge, shows how ‘two Musicians may discourse with one another by playing upon their instruments of musick as well as by talking with their instruments of speech’ [2, XVIII, pp. 143–150]. He also explains how one can hide secretly a message into a geometric drawing using points, lines or triangles. ‘The point, the ends of the lines and the angles of the figures do each of them by their different situation express a several letter’ [2, XI, pp. 88–96].

A very widely used method is the acrostic. In his book, The Codebreakers [32], David Kahn explains how a monk wrote a book and put his lover’s name in the first letters of successive chapters. He also tells of prisoners of war who


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