Image Authentication using Digital Watermarks
Many watermarks have been proposed so far. Below are examples of spatial and frequency domain watermarks. Digital watermarking assumes inserting of a digital watermark at the source (e.g., camera) and verifying the marks integrity at the detection side. Digital watermarks are mostly imperceptible; they are inseparable from the digital media they are embedded in, and they undergo the same transformations as the digital media itself. A major drawback of approaches based on watermarks is that the watermarks must be inserted either at the time of recording the image, or later by a person authorized to do so. This limitation requires specially equipped cameras or subsequent processing of the original image. Furthermore, some watermarks may degrade the image quality. It also requires hiding the watermark key from the device owner.
Digital watermarks are classified as visible or invisible. The visible group is perceptible to the human eye. In the case of the latter group, the existence can only be determined using a detection algorithm. In addition, watermarks can also be designed to be fragile or robust. Fragile watermarks become corrupted when any part of the image is modified. Thus, the most fundamental property of invisible fragile watermarks is the test of image authenticity and tamper detection. Robust watermarks are not affected by common image- manipulation procedures. Therefore, they are a proper way of ownership protection.