Digital watermark to fight crime

Compact discs, videos and images used as evidence in courts could soon be made tamper-proof by a digital watermark.

Digital CCTV images are increasingly being used as evidence in courts, so a reliable method of protecting them from interference is needed.

The global music and video industries are also suffering to the tune of millions a year at the hands of pirates. Existing methods of encryption have largely failed to deter criminals, and distinguishing tampered CD and video images from original copies can be extremely difficult.

The digital watermark, developed by researchers at Bristol University, is made up of information embedded within the image, music track or film frame.

Every digital image is made up of a series of ones and zeros, and the pattern of these numbers is used to create an individual code – the watermark’s signature.

The system can be built into the hardware of all digital cameras, and applies the watermark to the image as it is taken, said Prof David Bull, head of electrical and electronic engineering at Bristol. ‘The information for the watermark is embedded within every frame of the video image. It is in fact a low-level distortion of the image. It is a signature that can be detected using a special algorithm.’ The algorithm is used as a mathematical key to verify the image’s code.

The watermark will allow experts to tell whether an image has been tampered with, and where within the image changes have been made, he said. ‘We can also say what kind of change has taken place.’

Because the technology can detect how an image has been changed, it is able to recreate the original details using the information embedded in the picture.

The research team is hoping the mathematical system will become an industry standard. Bull has been working on the project with the Metropolitan Police, which has provided expertise on digital information used in court, and helped with image verification in tests. As well as detecting possible criminal changes to images, the system can also be used by police to verify images that have been deliberately changed to protect individuals.

The project has received £250,000 from the EPSRC, as well as funding from Motorola.