A new reversible watermarking system developed by researchers at Annamalai University in India could be used to authenticate military images. "Traditionally, source authentication and integrity verification of digital data have been carried out with digital signatures and encrypted watermarks," the researchers say. "Unfortunately, watermarking techniques modify original data as a modulation of the watermark information and unavoidably cause permanent distortion to the original data." The reversible watermarking scheme works by calculating the parameters of every pixel in an image and converting the information into a code. The Hash Message Authentication Code (HMAC) is where the distinct pixel values are selected for embedding watermark bits and the preferred pixel values are stored as a key. The generated key is used for both the watermark extraction and restoration of the original image. The authenticity of the received image can be determined by comparing the extracted HMAC to the HMAC of the restored image.

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kinda like EXIF? :D
interesting... but will the HMAC be retained in the picture when uploaded to various image host sites ?