Watermarks are images or patterns printed onto paper that serve both to authenticate documents and to prevent the forging or reusing of those pictures or documents. They can be subtle or obvious, depending on the purpose of the watermark. What they haven’t been, up until now, is reversible. Govindarajan Yamuna and Dakshinamurthi Sivakumar of Annamalai University in India have solved that problem.
Briefly, the designers convert every pixel in a picture into a code they term a Hash Message Authentication Code (HMAC). This code can be used both to embed watermarks at specific pixels and to restore the original image. The two images (with and without watermarks) can be compared to ensure authenticity.
Why is this important? Some photographs, particularly those with military or legal relevance must be authenticated to ensure that the images haven’t been altered in any way. Digital signatures and watermarks are a way to do this. However, part of the digital data is usually destroyed by the placement of the watermark. Reversible watermarks can certify the authenticity of a photograph without corrupting the original data.