Uncrackable quantum crypto undermined by new attack

Bell test blinded by lasers

Hopes of building an uncrackable cryptographic system using quantum mechanics have been called into question, after scientists devised a way to cheat a test used to detect secret keys that have been intercepted.

By blinding detectors with laser beams, the scientists were able to defeat what's known as the Bell test. In theory, the test is supposed to show when a quantum encryption key shared between two parties has been intercepted by an eavesdropper. In principle, it's impossible for a third party to tamper with it without disturbing its entanglement, a property that's measured by the Bell test.

“There have been some strong statements about quantum cryptography being robust against any attack,” an article in Nature quotes Christian Kurtsiefer, an expert of quantum optics at the Center for Quantum Technologies of the National University of Singapore, as saying. “But it isn't that simple.”

The attack works by shining a laser beam at a detector used by one of the parties sharing the key and intercepting some of its photons. The laser temporarily blinds the detector, allowing the attacker to register faked correlations. In theory, the Bell test should be able to recognize the counterfeit readings, but the difficulty of getting the test to work in practical settings requires scientists to discount a certain number of mismatches in the test results.

The findings appear in Physical Review Letters. ®

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