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Digital images that are tamper-proof

Digital pictures that have been tampered with can now be detected by a 'watermark' developed by scientists at Bristol University, England. Digital images such as CCTV footage are increasingly being used as evidence in court cases in the UK, but tampering with them has been difficult to detect.

Researchers have created a digital version of a watermark to tackle this problem and validate the image. Prof Dave Bull says that the watermark can be introduced on either still or video images to validate their authenticity and reveal any changes made to the original version. Prof Bull said: "With a high-tech problem, we needed a high-tech solution, but drew our inspiration from the humble paper watermarks first created in the 18th century."

Paper watermarks were patterns put into paper during its production while still wet. This gave the paper an identity mark that was used by manufacturers as a security measure to avoid forgery of important documents such as bank notes and passports.

The old watermarks were created by forming a seal around the image in its original state. The digital watermark is applied automatically at the point when the image is taken by incorporating appropriate software into the camera. Encoded information within the watermark will show if it is subsequently tampered with.

"The idea is that if the image file is passed through a special statistical detector it will be able to read the digital watermark and determine whether it has been altered. For example, the hands on a clock might have been changed to show a different time, or a face might have been obscured," said Prof Bull.

He hopes that the research will be adopted by digital film and camera manufacturers, and applied as part of an industry-wide security standard. The watermark uses patent-pending technology based on the use of mathematical algorithms.

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