In the technology of quantum cryptography found a weak spot

The reliability of the quantum cryptography method used is now questioned by a group of scientists from the National University of Singapore and the University of Trondheim headed by Christian Kurtsiefer. Two facts add solidity and thoroughness to the study of this issue: its authors are the notorious British scientists (among the authors are Russian Vadim Makarov ) and the essence of vulnerability lies in the imperfection of the hardware of the “cryptographic device” and not in the ability to influence the laws of physics in any mythical way.

It is known that the message transmitted using the method of quantum cryptography is encoded in a sequence of photons of different polarization, moving along a data transmission channel. Some attacker, willing to read the message, will be forced to intercept photons in order to measure their polarization moment. However, according to the uncertainty principle of Heisenberg quantum systems, it is impossible to measure any photon parameter without inevitably distorting another parameter. This circumstance guaranteed the reliability of the quantum cryptography method, because the recipient of the message will receive the modified message and the fact of the interference will be detected.

To explain the essence of the vulnerability, the following should be clarified: when sending an encoded message, the author chooses one or another photon polarization, and the choice of the polarization moment is random. The recipient of the message uses a certain device - “detector” - to consider the moments of polarization of photons, and due to the randomness of their choice by the author of the message, the reading results will be either true or erroneous.

After all the readings of the polarization have been completed, the recipient sends the information about the reading parameters through the open channel to the message author without informing the measurement results themselves. The author also responds through an open channel to the recipient, in which cases he was mistaken. Discarding the results of incorrect measurements, the recipient will receive data on the sequence of photons encoded by the author - this transmitted secret information is called the primary key. In order to detect the fact of interception of messages, the author and the recipient use the open channel to compare the results of reading the polarization of photons — if they are intercepted, these parameters will not match the author and the recipient.
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In all this complex system, the weak point is the device for reading polarization moments (“detector”), with which the recipient works with photons. It turns out that if you “saturate” it with photons to a certain value, then the detector will lose the ability to work with individual quanta of light and will work as a classic device. In other words, an attacker can do the following: intercepting a photon of a message, he sends a “laser beam” ahead of him, the polarization of photons in which coincides with the polarization of the intercepted. In this case, the “detector” of the recipient will not be able to determine whether the polarization of the “legal” photon has changed and, as a result, the fact of unauthorized interference will be impossible to detect when reconciling.

For the sake of fairness, it should be noted that the authors, having discovered the problem, began to work on its solution, having already proposed one of the possible solutions.

[Source - Physics World ]