Making Light Work of Encryption

Researchers have developed a way to encode information transmitted by light. The advance, described today at the International Society for Optical Engineering's AeroSense 98 Conference in Orlando, Florida, could lead to encryption techniques that are several orders of magnitude harder to break than current methods and would require codebreakers to have specialized equipment.

To encrypt a message, computer programmers create keys--usually random strings of ones and zeros--that mathematically transform information into apparent gibberish. Deciphering the message without the keys takes massive amounts of computer power. Light can make the challenge even tougher, because its intensity can vary through many shades of gray rather than being confined to the black and white of binary. In addition, light also has phase--a shifting of waves that cannot be seen by the naked eye or by most imaging equipment, but that is used to create holograms.

Bahram Javidi of the University of Connecticut and colleagues have developed a way to encrypt 2-dimensional arrays of data, such as pictures, using keys that contain phase information. In their test setup, the 2-D message to be encrypted is shown on a 1-cm by 1-cm liquid crystal display (LCD). A laser beam is shot through the front side of the LCD and picks up the message. Since the beam is as wide as the display, each pixel can alter a different portion of the beam's cross section. Clear pixels don't affect the beam intensity, but dark pixels block or dampen it.

The message is encrypted when it passes through two keys--specialized LCDs that alter the beam's phase according to the orientation of the liquid crystals. The keys garble the message into what appear to be random dots. The beam can only be decoded if the receiver can reverse the process using the second key. Javidi's group has recently developed methods for turning the encrypted optical signal into binary code that can be transmitted over computer networks.

The new method is "technically important" and could have "broad practical impact," says physicist Eung Gi Paek of the National Institute of Standards and Technology in Gaithersburg, Maryland. But this technique is years away from the market, Paek says, because the resolution of commercially available LCDs will have to be much higher than they are today for the laser to contain a large and secure message.