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47: Code Breakers Stumped By Photon-Based System

By Elizabeth SvobodaJanuary 3, 2005 6:00 AM


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Code breakers have always found ways to tap into secret transmissions, but in June researchers at Harvard and Boston universities, BBN Technologies, and the Defense Advanced Research Projects Agency showed how to put eavesdroppers out of business for good. The scientists deployed a computer link between their institutions so secure that any attempts to listen in automatically interrupts the flow of data.

Most secure online communications are encoded using strings of numbers so long it could theoretically take hackers hundreds of years to guess what they are. Nevertheless, each advance in computer algorithms increases the likelihood that the process could be vulnerable. “There’s no mathematical guarantee that numbers are secure, so we had a strong desire to try something else,” says physicist John Myers of Harvard.

His team decided to use a sparse stream of photons—particles of light—instead of random numbers to encrypt information. A sender directs photons through a fiber-optic cable at specific intervals. The receiver then analyzes the timing of the photons. If it matches a portion of the pattern, which is conveyed separately by the sender, the recipient is permitted to store this “key” and use it to unlock data from the network. The beam is so dim it cannot be monitored without altering it, so a third party snooping on the line would unavoidably disrupt the photons. The receiver’s device would detect the intrusion, end the communication, and sound an alert.

This approach isn’t perfect. It relies on an extremely weak light signal, so it has a range of less than 60 miles. Also, the system requires its own freestanding fiber-optic network, making it unlikely that photon cryptography will be widespread anytime soon. “Let’s put it this way: If you want to go from Boston to New York and it’s 1905, you might not want to take an airplane,” Myers says. He predicts that banks and government agencies willing to pay a lot for security will be among the first adopters.

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