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GCN Home > 03/20/06 issue

A quantum leap for cryptography

Blasting photons can create completely secure random keys

By William Jackson , GCN Staff

An international team of government, academic and commercial researchers has made significant strides in their quest to accelerate secure quantum cryptography. The group has devised a photon detector operating as much as 20 times faster than previously, making it possible to continuously generate and exchange cryptographic keys at up to 100 Mbps.

“This is a classic example of experiments from the physics lab being transitioned to a working technology,” said Jonathan Habif, a research scientist with BBN Technologies of Cambridge, Mass.

The equipment, built from off-the-shelf components, is running on the Defense Advanced Research Projects Agency’s Quantum Key Distribution test bed network in Cambridge and Boston.

No eavesdropping

Quantum cryptography relies on the phase state of single photons—elemental particles of light—to generate secure cryptographic keys. A random series of photons representing ones or zeroes, depending on their spin and polarization, are streamed to a detector where they are used to create a key. Because the act of “reading” a photon alters its state, the key cannot be detected by an eavesdropper. This makes feasible the rapid creation and distribution of one-time pads, considered the most secure type of cryptography.

A one-time pad is a list of random crypto keys shared by a sender and receiver. Because each key is random instead of being generated by a scheme known to both sender and receiver, it should be impossible to break a message encrypted using a one-time pad.

The trick is generating the keys quickly and distributing copies to the sender and receiver. But why go to the trouble of generating keys this way?

“Right now, the standard encryption schemes are good enough,” Habif said. “But they’re not based on a provable security model. Quantum cryptography is nice because it is provably secure.”

Computing power is advancing rapidly, which presents a double-edged sword because existing crypto schemes are based on computational complexity, said Robert Schwall, project leader in the quantum information and measurements project at the National Institute of Standards and Technology’s Boulder, Colo., lab.

“The first guy who gets a quantum computer can read everybody else’s mail,” Schwall said. “You need a technology that is not vulnerable to that kind of computing. That’s where quantum cryptography comes in,” because its security is based on the laws of quantum physics, not complexity.

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