Norwegian computer scientists have perfected a laser-based attack against quantum cryptography systems that allows them to eavesdrop on communications without revealing their presence.

One of the biggest commercial uses for quantum cryptography to date has been to securely exchange keys. Unlike traditional key distribution techniques, using quantum mechanics offers a seemingly foolproof upside: any attempt by an attacker to measure quantum data disturbs it -- per the Heisenberg uncertainty principle -- which a quantum cryptography system can detect, thus ensuring that communications remain secure.

Enter the laser. The team of researchers from the Norwegian University of Science and Technology (NTNU), the University of Erlangen-Nürnberg and the Max Planck Institute for the Science of Light in Erlangen developed a quantum eavesdropping technique that remotely controls the photon detector, which is a key component in most quantum cryptography systems.

According to Gerd Leuchs, who's part of the research team, "the security of quantum cryptography relies on quantum physics but … it must also be properly implemented." In other words, any system may have implementation loopholes. The attack works by blinding the photon detector with a bright light -- the laser. Over its continuous wave of light, the researchers then added even stronger laser pulses to transmit data, but in a way that the quantum cryptography security system can't detect, because it doesn't involve quantum mechanics.

The researchers reported their findings in an advance online version of Nature Photonics, released last week. "Based on these experimental results," they write, someone "can attack the systems with off-the-shelf components, obtaining a perfect copy of the raw key without leaving any trace of her presence." With the key in hand, communications between the two key sharers could then be intercepted and decrypted, again without either party knowing.

One of the authors of the study, Vadim Makarov, who's a researcher in the quantum hacking group at NTNU, said that "unlike previously published attempts, this attack is implementable with current, off-the-shelf components."

The researchers demonstrated the technique against two products -- MagiQ Technology's QPN 5505 and ID Quantique Clavis2 systems -- but the vulnerability potentially applies to any such product. "The security loophole we have exposed is intrinsic to a whole class of single-photon detectors, regardless of their manufacturer and model," said Makarov.

Before publishing their research, the team disclosed the vulnerabilities to both of the manufacturers, and also worked with ID Quantique to create a countermeasure. While that countermeasure hasn't been detailed, in their published article, the researchers suggest adding an optical power meter to quantum key-sharing systems, to watch for laser-wielding attackers.

MagiQ, meanwhile, has discontinued selling the affected product but not made any of its newer models available to the researchers for testing.