Keeping information secure with noisy light

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. Researchers at Northwestern University now 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.

In the open and global communication world of the Internet, information security is a critical issue because conventional cryptographic technologies cannot be relied upon for long-term security. Once optimized, the Northwestern method could replace the mathematical cryptography currently used by businesses, financial institutions and the military for secure communication. The innovative protocol promises security even against information security’s greatest foe: the not-yet-invented but still-feared powerful quantum computer, which could break almost any conventional code.

“As computing power and data traffic grow and information speeds get faster, cryptography is having a hard time keeping up,” 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. “New cryptographic methods are needed to continue ensuring that the privacy and safety of each person’s information is secure.

“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, which is no small feat. 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.

“No one else is doing encryption at these high speeds,” said Kumar, who has a secondary appointment in the department of physics and astronomy at the Judd A. and Marjorie Weinberg College of Arts and Sciences.

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.

In the case of the Northwestern method, 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. (Random polarization is one way to change the light’s granularity.) 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.

Having demonstrated that their high-speed encryption protocol works on a real network with real data, the researchers now are working toward speeds of 2.5 gigabits per second, which is the rate at which regular information is currently transmitted over the Internet’s fiber optic network.

“Current cryptographic schemes are vulnerable because as computers get more powerful the cryptography gets slower due to longer and longer keys,” said Horace Yuen, professor of electrical and computer engineering with a secondary appointment in physics and astronomy. He is principal investigator and theorist on the cryptography project.

“What we offer is a quantum cryptography system that is unconditionally secure, fast, easy to manage and cost-efficient. Our technology promises a realistic security solution to increasing computing power. We expect to develop a practical application within five years.”

The Northwestern research team is working with two industrial partners, Telcordia Technologies of Red Bank, N.J., and BBN Technologies of Cambridge, Mass., to develop prototype systems for integration into the core optical networks of the Internet.

Northwestern has filed a number of patents based on the technology developed at the University.

The quantum cryptographic research project is supported by a five-year, $4.7 million grant from the Defense Advanced Research Projects Agency (DARPA). The communication protocol that is the backbone of today’s Internet came out of a computer networking system begun by DARPA in the 1960s.