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Holograms to detect photo fraud
June 25, 2005
By Sam Rae, Institute of Physics
and World Science Staff
A new technique for detecting forged photographs will help newspapers and magazines check pictures that might have been doctored, physicists say.
If it works, the new technology could also prevent hackers from tampering with sensitive legal images, including fingerprint records and medical scans used as evidence in court, and help intelligence officers verify the authenticity of photos.
Developed by Giuseppe Schirripa Spagnolo and colleagues at the University of Roma Tre in Rome, Italy, research
describing the technique was published on June 21 in the
Journal of Optics A, a research journal.
In the system, an image, such as a company logo, is added to a digital photograph as an invisible “watermark.” Any subsequent attempt to alter the content of the photograph
would also damage the watermark. Forgery could be detected using a computer to extract the watermark and check it for damage.
In a forged picture, a computer could even identify the object or section which has been tampered with, the researchers maintained.
To ensure that only an authorised recipient could extract the watermark, which could otherwise be put in a fraudulent image to pass it off as genuine, the watermark would be encrypted, or converted into a code, before
being added. Thus only someone who knows a private key could reconstruct it. The encryption also
would make it hard to detect whether an image had been watermarked at all.
Before its addition to the photograph, the encrypted mark is turned into a computer-generated hologram, the researchers explained. A normal hologram, an image that appears to be three-dimensional, is created with a process that uses lasers.
Making the computer-generated version involves simulating the pattern of light waves recorded when a real hologram is made.
As with all holograms, a small part of the hologram contains enough information to recreate the entire image. This means only a small part of the watermarked image is needed to extract the watermark.
When the hologram is added to the image it replaces the image “noise,” which has been filtered out beforehand. Noise is random information in a picture that
doesn’t contribute to the image that you see, and can be removed without damaging the picture.
The watermark information uses the same part of the light spectrum as the noise, making it invisible to the human eye in the photograph. The watermark is
then embedded in the digital image file, in a separate part of the spectrum to the picture information, making it easy for the recipient to isolate and extract it. If the watermark can’t be reconstructed using the private key it means that someone has destroyed the watermark by trying to modify the
image, the researchers explained.
Testing their technique, the team showed how a hologram watermark could be used to find out which part of an image
had been tampered with. They changed colours in certain areas of a watermarked image and divided it into 16 parts, extracting a watermark from each.
The parts where the colour
had been changed showed a significantly damaged watermark, the researchers said.
“We hope that this technique can be used to improve the reliability of photographs in the media,” said Lorenzo Cozella,
a co-author of the paper.
“Digital cameras could be developed so that an invisible watermark is added when a picture is taken. A newspaper buying a photo from a freelancer could then check for a watermark to confirm that it
hasn’t been tampered with to make it more newsworthy.”
The system could also protect databases of images that serve as evidence in court,
such as fingerprint records or medical scans, the researchers said. These could be used in cases of alleged
malpractice. The technique would be especially useful for electronically stored information exposed to external users via the
Internet, and thus to fraud, the researchers added.
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