Chaotic lasers lock messages

By Eric Smalley, Technology Research News

People separated by distance can exchange a secure message by first encrypting it using a random mathematical key. The challenge is ensuring that the intended recipient -- and no one else -- can get a copy of the key required to decrypt the message.

Researchers from Advanced Telecommunications Research Institute International (ATR) in Japan have found a way for two people in different locations to work out a cryptographic key using the signal from the rapidly and randomly fluctuating light signal of a chaotic laser.

The method allows a sender and receiver to exchange and compare parts of a chaotic signal in order to identify portions that they can use to generate a cryptographic key.

The method could be used to transmit secure keys through space or over fiber-optic lines that are tens or hundreds of kilometers long. The method could also be used with chaotic radio signals, which could lead to cheap, general-purpose encryption systems, said Peter Davis, a senior researcher at Advanced Telecommunications Research Institute International.

To make a key, a sender and receiver independently record random sections of the chaotic signal, select marked portions that delineate segments that can be used to make a key, then compare marks over an unsecure channel to identify a string of segments they recorded in common. The security comes from the very long odds against an eavesdropper being able to record all the segments that the sender and receiver recorded in common.

A chaotic semiconductor laser signal generates a very large amount of information. The signal is analog, unlike the on-off pulses normally used to send digital information over fiber-optic lines. "State-of-the-art semiconductor lasers can generate chaotic signals equivalent to one gigabyte of random bits per second, and potentially at least 10 times more than this," said Davis.

A gigabyte, or one billion bytes, is equivalent to the information in a 10-meter high stack of books, or a little more than a fifth of a DVD's worth of data.

The researchers showed they could tune the system's variables to assure that the eavesdropper had less than a one in a trillion trillion trillion trillion trillion trillion trillion trillion -- that's a one followed by 100 zeros -- chance of acquiring a key composed of 100 segments shared between the sender and receiver.

The eavesdropper's chances improve with better recording equipment, but only so far. Put 100 lasers together, and it becomes even more difficult to make a complete record of the chaotic signal. "We could use an array of 100 lasers to generate 100 gigabytes per second, or 360 terabytes of data per hour," said Davis. In comparison, the printed material contained in the Library of Congress contains about 10 terabytes.

Three hundred sixty terabytes of data per hour is "not only... a huge volume, but the processing and communication bandwidth required to sample and record at 100 gigabytes is formidable," Davis said.

As time goes on, progress in laser chaos technologies should lead to even higher random bit rates. Rates could reach 100 gigabits per second for a single laser in the next 10 years, he said.

There has been a lot of research in recent years into using chaotic laser-generated waveforms for encryption, but most of the work has focused on hiding data within chaotic waveforms. The security comes from the long odds against an eavesdropper being able to separate the data from the chaotic noise of the signal. The drawback to this approach is that it assumes the sender and receiver already have the key necessary to extract the data, according to Davis.

Because the signal the researchers use to generate their keys is analog, its transmission is limited to tens or hundreds of kilometers. "Hence we envision its use mainly in local or special-purpose analog fiber lines," said Davis.

The signal could also be transmitted through space using narrow or wide light beams. Narrow beams carry more data and are more difficult to intercept. Wide beams are easier to receive while moving around, said Davis.

In principle the researchers' scheme could be used with any type of chaotic signal source. The researchers are looking into microwave and millimeterwave radio sources, said Davis.

Special purpose applications like secure networks for intelligent buildings could be practical in two to five years; less expensive general-purpose systems and schemes that require radio signal sources could become practical in 5 to 10 years, according to the researchers.

Davis' research colleagues were Atsushi Uchida of ATR and S. Itaya of Abel Systems Inc. in Japan. The work appeared in the October 13, 2003 issue of Applied Physics Letters. The research was funded by the Japanese Telecommunication Advancement Organization (TAO) and Abel Systems Inc.

Timeline:   5-10 years
Funding:   Corporate; Government
TRN Categories:  Cryptography and Security; Optical Computing, Optoelectronics and Photonics;Telecommunications
Story Type:   News
Related Elements:  Technical paper, "Generation of Information Theoretic Secure Keys using a Chaotic Semiconductor Laser," Applied Physics Letters, October 13, 2003


December 3/10, 2003

Page One

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