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Keeping eavesdroppers from tapping the lightlines

Secure transmission in quantum cryptography

Munich, 08/24/2011

Quantum cryptography permits the secure transmission of messages by taking advantage of the quantum nature of photons, the smallest units of light. Although quantum cryptography – or more precisely, quantum key distribution (QKD) – can theoretically be shown to be completely secure, the devil is in the details of its physical implementation. As a result, weaknesses have been revealed in various technical schemes, and instances of successful eavesdropping have been reported. A team of researchers led by LMU physicist Professor Harald Weinfurter, in collaboration with thecompany qutools GmbH, Munich, has now pulled off a successful attack on a QKD system using an unprecedentedly simple approach – yet, they promptly devised an effective countermeasure. To listen in, all the investigators had to do was to inject extremely weak light impulses into the transmission line between the transmitter and the receiver. And unlike previous attack schemes,  there was no need to measure the photons exchanged between transmitter and receiver. It turned out to be perfectly sufficient for the eavesdropper to selectively blind the receiver’s detectors. Without being noticed, the researchers were able to ascertain 98% of the key exchanged between the two parties, and then to decode the message subsequently transmitted. “This once again demonstrates how important it is to carry out rigorous tests to detect and eliminate weak links in the system,” says Weinfurter. “It makes no difference whether one is dealing with QKD or classical methods of cryptography. Fortunately, in this case, there is a very simple way to fix the leak.” With more sophisticated electronics it is possible to monitor whether or not all detectors are ready to record the arrival of signal photons – as a prerequisite for the secure transmission of the key. In this way, not only this latest type of attack, but all previously reported attacks using more intense light pulses, can be deterred. “Given careful construction and control circuitry, quantum cryptography can provide complete security, as reliable as only the laws of nature can be,” says Weinfurter. Following successful demonstrations in networks (SECOQC/TOKIO), commercially available systems are sure to become more popular in the near future. (suwe/PH)

Quantum eavesdropping without interception: an attack exploiting the dead time of single-photon detectors
Henning Weier, Harald Krauss, Markus Rau, Martin Fürst, Sebastian Nauerth and Harald Weinfurter
New Journal of Physics online, July 2011

Henning Weier
qutools GmbH
Phone: +49 89 32164959-0
Email: or

Professor Harald Weinfurter
Faculty of Physics, LMU Munich
Phone: +49089 2180 2044
Fax: +49 89 2180 5032


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