Prepared by Antonius Cahya Prihandoko
Digital watermark is generally a code that is physically or non physically embedded inside host content and acts as a digital signature which provides information of ownership or authenticity of the content. In a typical content screening system, the client’s equipment searches a hidden information inside the content, in order to verify whether a valid license is present. The embedded watermark can also later be extracted or detected to make an assertion about the content for security purposes.
Digital watermarking can be done in two domains: spatial and frequency domain. In spatial domain, watermarking process is relatively easy: watermark signal is directly embedded into host content signal. However, weaknesses of almost all spatial techniques include altering the host content during embedding proses, having the lowest bit capacity and the lowest resistance to signal compression. Frequency domain watermarking is more robust and compatible to common signal compression standards. It is undertaken by first applying a transform, such as fast Fourier transform (FFT), discrete Fourier transform (DFT), discrete cosine transform (DCT), discrete wavelet transform (DWT), and others. Watermarking process is then carried out by altering the frequency value in the host content spectrum. Though having different robustness, the spatial and frequency domain watermarking result the same appearance.
Based on watermark verification mechanism, watermarking schemes can be classified into three categories. Firstly, non-blind scheme or private watermarking. This scheme require both the original host content and the secret key(s) for extracting or detecting the watermark. Secondly, semi-blind scheme or semi-private watermarking, which requires the secret key(s) and the watermark bit sequence. Finally, blind scheme or public watermarking remains the most challenging problem as it only needs the secret key in its watermark detecting stage.
In the electronic commerce applications, digital contents can be categorized into four basic types of media data: audio, video, text, and image. Each type requires a specific watermark according to watermark design patterns (WDPs) Watermark for audio data has to be inaudible and robust. Watermark for video data has to be invisible and robust. Watermark for text data has to be invisible and fragile. Watermark for image data can be either visible and fragile, or invisible and robust. These watermark specifications are provided to meet customers satisfaction and content security.
In the term of rights management, digital watermark is applied to accomplish copyright protection. Initially, watermark can be used to prove the ownership of a watermarked content. The capability of watermark is then extended for content encryption to provide a secure content delivery. The watermarking application enables content provider to protect their intellectual property by integrating essential information of content in a hidden mark. For this purpose, watermark needs to be secure and robust; hard to remove and resists non malicious distortions.