2. Security: A watermark should be secret and must be undetectable by an unauthorized user in general. It should only be accessible by authorized parties. This requirement is regarded as security and the watermark is usually achieved by the use of cryptographic keys. Everyone may know the details of a digital watermarking algorithm, however, the owner of the intellectual property is the only one who holds the private secret keys.
   
   
3. Imperceptibility: One of the main requirements for watermarking is the perceptual transparency. The digital watermark should not be noticeable to the user. The data embedding process should not introduce any perceptible artifacts into the original signal and not degrade the perceived quality of the signal.
   
   
4. Robustness: The digital watermark should resist the attacks and must be detected by the watermark detector. Possible attacks include linear or non-linear filtering, enhancements, requantization, resizing, and compression.

2. GENERIC WATERMARKING SCHEME

A generic watermark embedding process is as shown in Fig 1. The extraction process of watermark from a watermarked signal is shown in Fig 2. There are a number of desirable characteristics that a watermark should exhibit. It at least should comply with the following two basic requirements of image watermark:

1. A digital watermark should not be noticeable to the user, nor should the digital watermark degrade the perceived quality of the signal. This property is called the transparency of digital watermark.
   
   
2. A digital watermark should still be present in the signal after distortion and it should be detected by the watermark detector. Ideally, the amount of signal distortion necessary to remove the watermark should degrade the desired signal quality to a point of becoming commercially valueless. It is called the robustness of digital watermark to signal distortion.

In fact, the two requirements contradict each other from the view of signal processing. The key to the watermarking technique is to compromise between the transparency and the robustness. Watermarking on a signal n(t) is given by the following equation

s(t) = P[ f (t)] + n(t)                  (1)

where f(t) is the watermark and P is some linear transformation. For the purpose of authentication of the signal n(t), f(t) must be a code that can be reconstructed accurately and robustly.

To satisfy this condition it is reasonable to consider f(t) to represent a bit stream, i.e. to consider a digitized version of f(t) – the vector f_i – to be composed of a set of elements with values 0 or 1. This binary code can be generated by using a key or a set of keys, which, when reconstructed, is compared to the key(s) for the purpose of authentication of the data. However, this requires the distribution of the keys (public and/or private).

The principal design challenge is to embed the watermark so that it reliably fulfils its intended task. For copy protection applications, the watermark must be recoverable even when the watermarked signal undergoes a reasonable level of distortion, and for tamper assessment applications, the watermark must effectively characterize the signal distortions. Security of a system comes from the uncertainty of the key. Without access to this information, the watermark cannot be extracted or be effectively removed or forged. It assumes presence of an established key management system that securely assigns required codes to the rightful watermark embedding and extraction parties.

Watermarking is in an evolution phase and currently researchers are developing general guidelines for effective watermarking algorithm design, improving reliability within the constraints of computational complexity and tailoring to the constantly changing needs of multimedia industries. Although there are commercially available digital watermarking systems, the area is still in its infancy.

3. TYPES OF WATERMARKS

Many watermarking approaches have been proposed till date [1,4]. A broad classification of watermarks is based on; perceptibility, robustness, inserted media, processing method and necessary data for extraction.

Perceptibility and robustness

For most applications two basic criteria used in evaluating a watermark scheme are perceptual quality and robustness to intentional/unintentional attacks that tend to remove the watermarks [5-7]. The watermarks inserted should be transparent i.e. they should not interfere with the media content.