Japanese researchers have put another dent in Quantum cryptograpy's reputation as the final word in secure communications.

Transmissions sent using the technique are protected from eavesdroppers by the fundamental rules of quantum physics, at least in theory.

In practice, implementation weaknesses can leave a narrow door for attackers, and Japanese boffins have developed an idea for a quantum eavesdropping device that can exploit one such weakness.

Quantum cryptography is designed to allow users to exchange secret keys. The polarisation of individual light photons determines one bit of a key. The rules of quantum mechanics mean that any attempt to intercept this data irreversibly alters it.

Because of this effect, any attempt to eavesdrop a key would be detected as a unacceptably noisy communications path.

The loophole exploited by Japanese boffins is that it might be possible to make a partial copy of a quantum key without tripping an alert that a communications path has been compromised. This partial copy might be used in subsequent cryptoanalysis. The technique relies on constructing an optical cloning circuit and a measuring device, as explained in a paper by the researchers here (pdf).

Boffins led by Yuta Okubo at the University of Tskuba in Japan have not yet built a device that implements the approach. Nonetheless the research is a concern for banks and government agencies that bought quantum cryptography systems in the belief they were inherently secure.

The Japanese research follows an earlier study by boffins in Sweden examining another practical shortcoming with quantum cryptography systems. As previously reported, the weakness identified by the Swedish team involved shortcomings in how systems verify that the content of a message has not been altered in transit. ®

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