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Last Updated: Thursday, 22 February 2007, 12:52 GMT
Decoys fix quantum key swapping
Laser light, Eyewire
The technique uses photons of light as decoys

British scientists have found a way to fix a vulnerability in quantum cryptographic systems.

If left unpatched, the flaw would make it possible to grab information about the keys used to scramble information without being detected.

The flaw emerges because of the way that laser diodes emit the photons of light used to carry quantum key data.

Using decoy photons, researchers can spot when attackers are eavesdropping on secure communication channels.

Decoy data

Quantum cryptography exploits the laws of physics to create a secure communications channel over which can be sent conventional keys to unscramble encrypted data.

The technology has found a role in protecting high value communications between financial institutions or the secret messages governments need to swap.

Typically, quantum cryptography uses single photons of light to carry the bits of data making up the encryption/decryption key.

Using the quantum state of a photon to carry data means that any attempt to read that information while the tiny packet of light is travelling changes it in an obvious way revealing when eavesdroppers are listening in.

However, quantum cryptography researchers from Toshiba Europe in Cambridge found that the laser diodes used to generate the photons of light have a flaw in that they sometimes emit two photons at once.

By grabbing the second photon malicious hackers could, theoretically, gradually grab information about quantum keys without revealing that they were listening in.

Researchers from the Cambridge lab have found a way to foil this attack by using decoy pulses of photons.

By monitoring how many decoy pulses make it across the wires, it becomes possible to work out if anyone is listening in.

One side effect of this advance is that it allows keys to be transmitted faster than before.

"Using these new methods for Quantum Key Distribution we can distribute many more secret keys per second, while at the same time guaranteeing the unconditional security of each," said Dr Andrew Shields, leader of the quantum research at Toshiba Europe.

The team is now working to create a communications network which secures encryption keys using quantum cryptography.

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