What is watermark ?
Although different authors use different meanings for the word 'watermark', it is mostly agreed that the watermark is one, which is imperceptibly added to the cover-signal in order to convey the hidden data.

What is the difference between 'copy protection' 'copyright protection' ?
Copy protection attempts to find ways, which limits the access to copyrighted material and/or inhibit the copy process itself. Examples of copy protection include encrypted digital TV broadcast, access controls to copyrighted software through the use of license servers and technical copy protection mechanisms on the media. A recent example is the copy protection mechanism on DVDs. However, copy protection is very difficult to achieve in open systems, as recent incidents (like the DVD hack - DeCss) show.

Copyright protection inserts copyright information into the digital object without the loss of quality. Whenever the copyright of a digital object is in question, this information is extracted to identify the rightful owner. It is also possible to encode the identity of the original buyer along with the identity of the copyright holder, which allows tracing of any unauthorized copies. The most prominent way of embedding information in multimedia data is the use of digital watermarking.

Whereas copy protection seems to be difficult to implement, copyright protection protocols based on watermarking and strong cryptography are likely to be feasible.

What is a Original image ?
Consider the following scenario: Alice, the copyright holder, inserts her own watermark into some object, locks the original away and keeps selling the marked image. Bob can now try to insert his own watermark into the already marked object. In case of a dispute, both Alice and Bob are able to prove the presence of "their" watermark and claim ownership of the document. How can this situation be resolved?

The "traditional" answer is: look at the objects, Alice and Bob claim to be the original. Alice's original should not contain a watermark, whereas Bob's "original" must contain Alice's watermark (if we assume that Bob cannot remove marks). This situation would indicate that Bob inserted his watermark after Alice and so one may conclude that Alice is the rightful owner.

Unfortunately, sometimes the situation is not that simple. It has been shown that, in particular class of watermarking schemes, Bob can insert his watermark in a way that it also seems to be present in the copy Alice locked away (although he has no access to it). So Alice's original contains Bob's mark and Bob's "original" contains Alice's mark. This type of attack is called "inversion attack" or more "dead lock attack". There is no way to resolve copyright ownership in this case. This result indicates that watermarking "alone", that is without a carefully designed protocol around it, will not suffice to resolve the copyright situation.

Why not add the copyright information into the file format ?
One could define a new audio file format, in which the watermark is a part of the header block but is not removable without destroying the original signal, because part of the definition of the file format requires the watermark to be therein. In this case the signal would not really be literally 'destroyed' but any application using this file format would not touch it without a valid watermark. Some electronic copyright management system propose mechanisms like this. Such schemes are weak as anyone with a computer or a digital editing workstation would be able to convert the information to another format and remove the watermark at the same time. Finally this new audio format would be incompatible with the existing one. Thus the watermark should really be embedded in the audio signal.

This is very similar to S.C.M.S (Serial Code Management System). When Philips and Sony introduced the 'S/PDIF' (Sony/Phillips Digital Interchange Format), they included the S.C.M.S. which provides a way to regulate copies of digital music in the consumer market. This information is added to the stream of data that contains the music when one makes a digital copy (a 'clone'). This is in fact just a bit saying: digital copy prohibited or permitted. Some professional equipment are exempt for needing S.C.M.S.

With watermarking however, the copy control information is part of the audio-visual signal and aim at surviving file format conversion and other transformations.

What are 'public watermarking', 'blind watermarking', 'semi-blind watermarking', 'private watermarking', 'non-blind watermarking' and 'asymmetric watermarking' ?
There as been some confusion about the naming of various types of watermarking techniques and the main reason is that people involved in this field come from different backgrounds (in particular signal processing and computer security). On top of this some terminology has been imported from the related field of steganography.

Originally, public watermarking and blind watermarking mean the same, but the wording was confusing with public-key watermarking. 'Signal processing people' took over the field, so only the later tends to remain. In these schemes, the cover signal (the original signal) is not needed during the detection process to detect the mark. Solely the key, which is typically used to generate some random sequence used during the embedding process, is required. These types of schemes can be used easily in mass market electronic equipment or software.

In some cases you may need extra information to help your detector (in particular to synchronise its random sequence on the possibly distorted test signal). In particular some watermarking schemes require access to the 'published' watermarked signal, that is the original signal just after adding the watermark. People refer to these schemes as semi-blind watermarking schemes.

Private watermarking and non-blind-watermarking mean the same: the original cover signal is required during the detection process.

At last, by asymmetric watermarking or public-key watermarking, people refer to watermarking schemes with properties reminding asymmetric cryptosystem (or public key cryptosystem). No such system really exists yet although some possible suggestions have been made. In this case, the detection process (and in particular the detection key) is fully known to anyone as opposed to blind watermarking where a secret key is required. So here, only a 'public key' is needed for verification and a 'private key' (secret) is used for the embedding though. Knowledge of the public key does not help to compute the private key (at least in a reasonable time), it does not either allow removal of the mark nor it allows an attacker to forge a mark

What is the difference between (semi-) fragile and robust watermarks ?
The aims of such watermarks are completely different: A (semi-)fragile watermark is a mark which is (highly) sensitive to a modification of the stego-medium. A fragile watermarking scheme should be able to detect any change in the signal and identify where it has taken place and possibly what the signal was before modification. It serves at proving the authenticity of a document. On the opposite, a robust watermark should be stuck to the document it has been embedded in, in such a way that any signal transform of reasonable strength cannot remove the watermark. Hence a pirate willing to remove the watermark will not succeed unless they debase the document too much to be of commercial interest. The latter form is the very challenging and attracts most research.

What are the different attributes associated with watermarking ?
The characteristics of an watermarking algorithm is normally tied to the application is was designed for. The following merely explain the words used in the context of watermarking.

What are fingerprints ?
Fingerprints are characteristics of an object that tend to distinguish it from other similar objects. They enable the owner to trace authorized users distributing them illegally. In the case of encrypted satellite television broadcasting, for instance, users could be issued a set of keys to decrypt the video streams and the television station could insert fingerprint bits into each packet of the traffic to detect unauthorized uses. If a group of users give their subset of keys to unauthorized people (so that they can also decrypt the traffic) at least one of the key donors can be traced, when the unauthorized decoder is captured. In this respect, fingerprinting is usually discussed in the context of the traitor tracing problem.

What are people referring to when they say, they are working on watermarking. Do they all develop their own techniques?
[Jeffrey A Bloom] Yes. Most people who publish papers in this field are developing new algorithms. It may be helpful to think of watermarking "backwards", i.e. from the perspective of detection. Consider an image watermarking system.

Define some algorithm to "extract" a watermark (this could be taking the 1000 highest amplitude DCT coeffs, or averaging the 8x8 blocks of an image, or subtracting the original and projecting onto some subspace, or finding salient points, finding the Delaunay triangulation of those points and representing the result as a graph, etc.)

Modify the image so that the extracted watermark will be "similar" to some predefined watermark (or set of watermarks). This may be done by adding something to the image or by multiplying the image by some spatially variant map. We may modify some values relative to others, increase one subset and decrease another subset of pixels or coefficients, warp the image to obtain a particular arrangement of salient points, etc. The modification might be done under the control of a perceptual model to limit the fidelity impact. It may be done under the control of a distortion model to maximize the robustness.

Is there a secure/robust/removal resilient watermarking technique ?
The use of these terms on an application specific case might be true but not universally. So, a better question is ``Is this watermarking technique secure/robust for this application ?''. There is the same problem in cryptography: people think their system is secure because it uses RSA. This is an illusion: hackers focus their effort on protocols or on implementations but they never try to break RSA

[Jeffrey A Bloom] Try the early Digimarc patents. Geoffrey Rhoads does an excellent job in the disclosures describing "knots" and "rings" and "tapestries". That technique is robust to rotation, crop, and resize, it is a blind detection technique, it is an n-bit watermark, i.e. it has a payload rather than a 0-bit watermark which is simply present or absent, but carries 0-bits worth of information. I suspect that these patents are the foundation of the Mediabridge technology. That is clearly blind, multi-bit, and robust to the distortions you mention (as well as others).