Quantum leap for secret codes

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Secure keys can help keep data secret

Thursday, 5 June, 2003

Within three years, companies could be using quantum cryptography to protect sensitive messages.

British researchers say they are close to producing an off-the-shelf system that exploits quantum physics to create a secure communications channel.

The system encodes bits of information on individual particles of light.

This week, the researchers demonstrated their system working over fibres 100 kilometres in length.

Key controls

Secure ways of communicating are hugely important to companies as more business is done via the net.

Contemporary encryption algorithms ensure sensitive data stays secret simply because it takes so long to work out which mathematical key was used to scramble the information.

But companies could soon be turning to quantum physics to help them swap the keys they use to turn meaningful messages into scrambled characters.

"It is a radically different approach to computer security," said Andrew Shields, leader of the Quantum Information Group at Toshiba's Cambridge lab where the encryption system is being developed.

"Quantum cryptography allows users to swap a shared secret key that can be used to encrypt messages or authenticate messages sent across the network," said Dr Shields.

"We can absolutely guarantee that each key sent is secret," he said.

Quantum cryptography works by using particular properties of individual particles of light, photons, to represent individual bits of information.

"The sender sends the photons in a particular state which determines whether it is bit 0 or bit 1," said Dr Shields.

The system being developed by the Toshiba researchers can convert up to 2 kilobits of data per second into photons.

Companies were unlikely to use the system to transmit entire messages, said Dr Shields, but it was perfect for sending the keys used to scramble and unscramble messages.

Bits and PCs

Dr Shields said photon physics meant eavesdroppers changed the properties of any photons they intercepted. As a result, it became obvious if a message had been read by anyone it was not meant for, he added.

"The critical part is that bits have to be encoded on single photons because it is impossible to copy them. It gives them a way of testing whether someone else has read their message."

Unfortunately, the delicate states of the photons used to represent bits also mean that quantum cryptography can only work along unbroken, and relatively short, fibre optic cables.

By contrast, data sent across nations and oceans travels via fibre optic cables regularly broken by repeaters that boost signals to ensure they reach their destination.

Dr Shields' team have demonstrated quantum cryptography working over distances of 100 km, which should be enough to cover large metropolitan areas such as London and Tokyo.

Dr Shields said he expected the first users to be financial firms keen to establish secure communication between different offices.

One weakness with the system was what happened to encryption keys once they started to be used in the leaky, insecure computers many of us had on our desks, said Dr Shields.

"There are other vulnerabilities in the system that's true," said Dr Shields. "The key has to be used in the right way to ensure the security of the entire system."

The Department of Trade and Industry has pledged cash to help the researchers refine their work and bring commercial quantum cryptography products to market.

Dr Shields said commercial products should be ready in three years' time.

The researchers will be making their work public at the Conference on Lasers and Electro Optics being held in Baltimore this week.

Bongme