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Quantum crypto felled by 'Perfect Eavesdropper' exploit

All your photons are belong to us

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Researchers have devised a technique for eavesdropping on communications secured through quantum cryptography that allows an attacker to surreptitiously construct the secret key encrypting the secret content.

The so-called Perfect Eavesdropper uses off-the-shelf hardware to defeat a key benefit of the alternative crypto system, namely that the use of properties rooted in quantum physics offers a theoretically fool-proof way for parties to exchange the secret key securing their communications without being intercepted. QKD, or quantum key distribution, allows a trusted party to construct a key by transmitting light to the other trusted party one photon at a time and then measuring their properties.

In theory, anyone monitoring the transmissions passing between the two parties will automatically be detected because in the world of quantum mechanics the act of eavesdropping taints the key in ways that are clear to the trusted parties.

The researchers, from the the National University of Singapore, the Norwegian University of Science and Technology, and the University Graduate Center in Norway, were able to compromise the QKD by making the key exchange behave in a classical way. Using readily available equipment that fits inside a suitcase, they intercepted single photons traveling over a 290-meter fiber link network and then re-emitted the corresponding pulses of light.

The re-emitted pulses in effect blinded the photodiodes used by the trusted party receiving the transmission of photons. As a result, the photodiodes were no longer sensitive to single photons, making them behave like classical detectors that generate a current proportional to the intensity of the incoming light.

“Quantum key distribution has matured into a true competitor to classical key distribution,” Christian Kurtsiefer, a professor at the Center for Quantum Technologies at the National University of Singapore, said in a release. “This attack highlights where we need to pay attention to ensure the security of this technology.”

One of the biggest challenges faced by cryptographers throughout history is finding a secure way for trusted parties to share their secret key. Public key cryptography solved this problem by using a public key to encrypt communications and a separate private key that's unique to each recipient to decrypt the content. As a result, the key never has to be transmitted. Quantum cryptography takes a different approach by allowing one party to securely transmit the key to another party using principles at the heart of quantum mechanics.

The findings are similar to those published last year by researchers from the University of Toronto, who claimed to carry out the first successful attack against a commercial system based on theoretically uncrackable quantum cryptography. The researchers behind the more recent Perfect Eavesdropper said it's the first practical exploit that surreptitiously steals a key during a typical QKD setup.

The researchers have already identified the loopholes that allow the Perfect Eavesdropper to function and are working on countermeasures.

The findings were reported in the most recent issue of Nature Communications (abstract here). More from Physics World and Forbes is here and here. ®

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