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Mask secures personal displays

26 Apr 2004

A mask that decodes encrypted pixels ensures that sensitive information can be viewed securely.

When you view secure information on a screen, there is always the risk that someone is peering over your shoulder and taking notes. To remove this worry, scientists in Japan have been experimenting with a technique known as visual cryptography. The team believes this approach could ensure the security of information displayed on PDAs, computer screens or bank terminals. (Optics Express 12 1258)

In a visual cryptography system, the image containing the sensitive information is encrypted and appears as a random pattern. The only way to view the information is to place a decoding mask over the encrypted image.

Hirotsugu Yamamoto and colleagues from the University of Tokushima have developed a decoding mask that has two functions: to decrypt the display and limit the viewing zone of the decrypted image.

Key to the technique is an algorithm that generates the pixels in both the encrypted image and the mask. The team’s paper describes its algorithm to encrypt images containing 8 colours.

To test their algorithm, the researchers printed the mask out on an overhead transparency and placed it in front of an LCD showing an encrypted image. Secret images were only perceived when the viewer stood in front of the LCD. “The viewing zone covers about 10 cm to the right and 10 cm to the left of the center at the designated viewing distance of 1.5 m,” Yamamoto explained.

Improving both the security of the decoding mask and the image quality are now Yamamoto’s priorities. “In the next version, I’m making the mask pattern on a plastic plate,” he told OLE. “The mask pattern has some layers and prevents someone from copying it. The number of encrypted colours has also been increased to 216 and 343.” The team is also working on a decoding goggle similar to the glasses that cinema audiences have to wear to see movies in 3D.

Yamamoto is also optimistic that this technique will find commercial applications. “Display of secret information on PDA and computer screens are practical applications,” he explained. “Other business applications include: securing the screen of a terminal at a bank; an operator screen that shows personal information; and a touch panel screen of a safe.”

This work was funded by the Japanese Ministry of Education, Culture, Sports, Science and Technology; the Mazda Foundation; and the Secom Science and Technology Foundation.

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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