Knowing whether a digital photo has been altered is sometimes of vital importance. Being able to confirm that the picture is authentic and that it, indeed, comes from the declared source is crucial in a military setting.
So far, visible watermarks and digital signatures are the only things available to those bent on proving the authenticity of an image, but it is only a partial solution because a visible watermark can destroy information contained in the image and can’t be removed without damaging it.
Science Daily reports that the answer to this problem comes from India. Yamuna Govindarajan and Sivakumar Dakshinamurthi of Annamalai University in the Indian state of Tamil Nadu, have devised a reversible system of watermarking digital documents.
The scheme is based on histogram modification and – according to the abstract of their paper published in the International Journal of Signal and Imaging Systems Engineering – consists of calculating the Hash Message Authentication Code (HMAC) of the image and embedding it into the image for authentication.
“Based on a criterion, the distinct pixel values with Non-Zero Frequencies of the histogram are selected for embedding watermark bits and the preferred pixel values are stored as a key. This key is used for both the watermark extraction and the original image restoration. The bits of extracted HMAC from the marked image and HMAC of the restored image are compared for authentication,” they claim.