Encryption Using Chaos

Lasers that "hide" messages could mean more foolproof security.

You know that eBay purchase you made? The online credit card payment you sent? The bank statement you checked at your computer? These transactions contained sensitive information about you that, for the most part, is kept private thanks to encryption software that scrambles the message before it's sent (and unscramble it once it's received by the intended party).

But software is not the only way to protect digital information. Now researchers are looking at ways to exploit lasers with chaotically fluctuating signals, to add an extra layer of privacy to messages sent over fiber-optic lines. By slipping a message into such a laser beam, decrypting the message requires a nearly identical laser to receive it – a process that's not readily accessible to most people.

To demonstrate the feasibility of the technology, Claudio Mirasso of the Universitat de les Illes Balears in Palma de Mallorca, Spain and his team recently showed that chaotic lasers can send and receive a message over 120 kilometers of commercially laid fiber optics. Even more impressive: the transmission rate was one gigabyte of chaos-encrypted information per second -- comparable to that of most commercial data transmissions. It was a major step that, for the first time, put this exotic encryption technique into the real world.

In order to send a message within a chaotic beam of light, Mirasso explains, the message must first be converted into an optical signal. It is then fed into a laser that passes it along within the laser's beam. The researchers then heighten the naturally occurring chaos in the beam and feed the message into it. This message-plus-chaos is sent to a nearly identical laser that receives it within its lasing cavity -- the innards of a laser where photons are stimulated and emitted.

At this point, Mirasso says, a phenomenon called chaotic synchronization takes over. This process, admittedly not entirely understood by scientists, makes the receiving laser's output match the message-plus-chaos of the sending laser. Then, to decrypt the original message, the chaos – a known signal from the sending laser – is subtracted from the receiving laser's beam, revealing the hidden information.

Before chaotic message encryption hits the big time, however, it must be shown to be as robust as traditional optical signals. In the January 1 issue of IEEE Photonic Technology Letters, a group has announced that they've tested the mettle of a chaos-encrypted message by relaying it through an intermediary laser. This step is crucial, explains Alan Shore of the University of Wales in Bangor, because commercial systems use relay stations to boost the distance a message can travel, and chaos-encrypted messages need to be just as strong as other information sent through a network. Shore's research also shows that it is possible to "send out messages to more than one receiver and extract messages at an intermediary stage," a common occurrence in standard optical networks.

Mirasso's next project involves developing "compact devices for chaos-based optical communication." Still, he notes, some issues with the technology need to be addressed. For instance, researchers still need to quantify the level of security they can offer "compared to other [techniques] like software-based encryption or quantum cryptography," he says.

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Mirasso estimates that using lasers to keep information private is roughly five years away from commercial viability.