Researchers develop a digital eavesdropping alarm

Scientists reported Wednesday that they had taken a step toward bringing improved security to computer networks, developing an encryption technique that will extend protection to a small group of computer users.

The researchers at Toshiba’s European research laboratory in Cambridge, England, in a paper published Wednesday in the journal Nature, wrote that they had figured out a way to allow a group of users to exchange encryption keys — long numbers that are used to mathematically encode digital messages — through an experimental technique known as quantum key distribution.

The technique is believed to be more practical and less expensive than existing technologies. It also extends the scale of the current quantum key systems to as many as 64 computer users from just two users.


The system does not prevent eavesdropping — it simply serves as a kind of burglar alarm, alerting computer users that an outsider is listening to a transmission on an optical network.

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Nevertheless, the advance comes at a time of growing concern about the relative ease of breaching computer security, prompted by recent disclosures based on the documents leaked from the National Security Agency and the British Government Communications Headquarters intelligence agencies by Edward J. Snowden. One worry is that the initial exchange of the key material in modern encryption systems has become vulnerable.

Today many digital encryption systems are based on the ability of two computer users to secretly exchange a “key,” which is then used to establish a secure communication channel to exchange messages over a computer network.

The encryption key is encoded in a special stream of photons or bits. The Toshiba work is based on the ability to make the infinitesimally short time measurements required to capture pulses of quantum light hidden in streams of photons transmitted over fiber optic links — and to do that in a network of dozens of users.

The key exchange is usually protected by the use of mathematical formulas based on the challenge of factoring large numbers. In recent years public key cryptographic systems have been improved by lengthening the factored numbers used in the formula. That, in principal, would require vastly more computing resources to break into the system.


Quantum cryptography relies instead on encoding the key in a stream of quantum information — photons that are specially polarized.

If a third party eavesdrops on the communication, the fact will be immediately obvious to the parties of the secret communication.

“One of the attractive things about quantum cryptography is that security comes in the form of the laws of nature,” said Andrew J. Shields, one of the authors and the assistant managing director for Toshiba Research Europe. “It should in principal be secure forever.”

Encryption systems that are now commercially available are used to secured the wires by which digital information is transferred, but they are costly and function only for limited distances.

Allowing multiple users to share a network connection while using a quantum encryption system could significantly lower costs, Shields said.