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Reversible watermarking could thwart digital photo tricksters

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July 7, 2010

Reversible watermarking could keep digitally-altered images from going undetected

Reversible watermarking could keep digitally-altered images from going undetected

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.

After a visible watermark has been added and the image has been sent, the recipient simply uses the embedded HMAC to extract the watermark and restore the original image. Even if some unsavory character had intercepted the image and manipulated it in a way that didn’t alter the appearance of the watermark, the HMAC would still not match the pixel value of the current image, so the recipient would know something was up.

One would assume that the HMAC is unalterable after being initially created, although those hackers can be a pretty crafty bunch.

The research was recently published in the International Journal of Signal and Imaging Systems Engineering.

About the Author
Ben Coxworth An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away.   All articles by Ben Coxworth
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3 Comments

Absolute dribble!

There's hundreds of ways image integrity can be "ensured" during transmission. Ever heard of encryption?

Omer Qadir
8th July, 2010 @ 08:32 am PDT

Simply opening an image with an editor then resaving it with a higher compression to shrink the filesize would invalidate the HMAC for the purpose of detecting if the image's visual content was altered.

At best in such cases, it could be used to create an uncertainty value.

Facebook User
8th July, 2010 @ 03:10 pm PDT

I hate to nitpick, but it's just 'Photoshop' - not 'PhotoShop'.

Neil
8th July, 2010 @ 03:26 pm PDT
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