Reversible watermarking could thwart digital photo tricksters
In these days of PhotoShop and its brethren, it’s becoming almost impossible to tell whether or not an image has been digitally manipulated. While some ‘shopping is done simply for whimsical reasons (see picture above), the matter becomes a bit more serious when things such as military images are altered. Visible watermarks are sometimes overlaid on digital photos, but these permanently alter and obscure that copy of the picture. Recently, however, researchers in India came up with a system for verifying a photo’s authenticity, without altering it in any way.
Dakshinamurthi Sivakumar and Govindarajan Yamuna of Annamalai University, in Tamil Nadu, have developed a process that they call reversible watermarking. Using a relatively small amount of computer power, their system measures the parameters of every pixel in an image. That data is converted into a Hash Message Authentication Code (HMAC), wherein the original pixel values are stored as a key.
