Quantum cryptography could shed test for hackers

A proposed quantum encryption technique would ensure secure communication while removing the painstaking step of checking for potential eavesdroppers. The efficient approach could eventually form the basis of a secure quantum network for exchanging sensitive information.

Computer-generated encryption protects data such as credit card numbers, passwords and confidential communication from would-be snoopers. However, this encryption is breakable, and there’s no smoking-gun signal that someone has cracked the code. A hacker can steal information for a long time before anyone finds out.

In 1984, physicists Charles Bennett and Gilles Brassard proposed the first quantum cryptography protocol, a way to use quantum mechanics to create a guaranteed secure link. Their approach calls for a receiver to measure the delicate quantum properties of photons in laser pulses and compare notes with the sender to establish a secret key (SN: 11/20/10, p. 20). An eavesdropper tapping in to the communication can also measure the photons but would leave a detectable trail.

Masato Koashi, a quantum physicist at the University of Tokyo, was irked by the method’s price of detecting a snoop: the sender and receiver have to divulge part of their encryption key to each other. “You cannot be certain about the quality of the final product, so you need a quality assurance measurement,” Koashi says. Depending on the amount of data transferred and the integrity of the connection, establishing the key while ensuring nobody is snooping can become difficult or impossible.

So Koashi and his colleagues developed a new quantum encryption approach that seems to make the potential presence of snoops moot. The proposed scheme, detailed in the May 22 Nature, is similar to the 1984 version except that the receiver introduces another layer of protection by measuring two sets of laser pulses and arbitrarily deciding a time delay between measuring them. The randomness of both the photons’ quantum properties and the time delay chosen by the receiver makes it nearly impossible for a hacker to determine the secret key. As a result, the communicating parties have no need to even test for eavesdroppers. “You can believe in the product,” Koashi says. “You don’t need quality assurance.”

Horace Yuen, a quantum physicist at Northwestern University in Evanston, Ill., calls Koashi’s proposal “a good start” toward an improved form of quantum encryption. But he warns that physicists have come up with some clever methods for quantum eavesdropping, and he wants to see proof that the new scheme could ward off all such snooping.