A hybrid watermark for tamper detection in digital images

A new hybrid image authentication watermark obtained as a combination of a fragile and a robust watermark is presented. The fragile watermark has good localization and security properties but cannot be used to distinguish malicious changes, such as feature adding or removal, from innocent image processing operations. The hybrid watermark can be used to accurately pinpoint changes as well as distinguish malicious tamper from innocent operations. The authentication can be done without accessing any information about the original image. Examples of tamper detection on real imagery are given. Powerful publicly available image processing software packages such as Adobe PhotoShop or PaintShop Pro make digital forgeries a reality. Feathered cropping enables replacing or adding features without causing detectable edges. It is also possible to carefully cut out portions of several images and combine them together while leaving barely detectable traces. Thus, the classical forensic techniques are not readily applicable to digital imagery.