March 11, 2004

New cryptography method takes advantage of light

By Megan Fellman

Put aside images of World War II espionage and codebreaking. Today cryptography is vital to the security of a form of communication and commerce never imagined 60 years ago: the Internet.

Prem Kumar

Researchers at Northwestern have demonstrated a new high-speed quantum cryptography method that uses the properties of light to encrypt information into a form of code that can only be cracked by violating the physical laws of nature.

Once optimized, the Northwestern method could replace the mathematical cryptography currently used by businesses, financial institutions and the military for secure communication.

“New cryptographic methods are needed to continue ensuring that the privacy and safety of each person’s information is secure,” said Prem Kumar, professor of electrical and computer engineering at the McCormick School of Engineering and Applied Science and co-principal investigator on the project.

“Our research team has succeeded in encrypting real information, sending the message over a University fiber optics system at very high speeds, and decrypting the information. Other quantum cryptography methods are slow and impractical for long-distance or high-speed communication, whereas ours shows great potential for real-world applications.”

The researchers transmitted encrypted data at the rate of 250 megabits per second. Because it uses standard lasers, detectors and other existing optical technology to transmit large bundles of photons, the Northwestern protocol is more than 1,000 times faster than its main competitor, a technique based on single photons that is difficult and expensive to implement.

The Northwestern method, which is geared toward securing the public fiber optic infrastructure, uses a form of “secret key” cryptography. In this type of cryptography, the two people communicating, say Alice and Bob, have the same secret key. If Alice wants to send a secure message to Bob, she sends a message in a “locked box,” which Bob can open.

To encode her message Alice uses the key to manipulate the light, creating a pattern more complex than just “on” or “off.” The method takes advantage of the granularity of light, known as quantum noise, which is integrated with the secret key’s pattern. To someone without the key, say the eavesdropper Eve, the information is indecipherable — the stolen message contains too much “noise.” Bob, with the secret key, has the pattern and can receive the signal with much less noise, allowing him to read Alice’s encoded message.