Weak spot found in quantum cryptography technology

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

It is known that a 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, wanting to read the message, will be forced to intercept photons in order to measure their polarization moment. However, as the principle of uncertainty in Heisenberg quantum systems states , it is impossible to measure any parameter of a photon without necessarily distorting another parameter. This circumstance guaranteed the reliability of the quantum cryptography method, because the recipient of the message will receive a modified message and the fact of interference will be detected.

To explain the essence of the vulnerability, it is necessary to clarify the following - when sending an encoded message, its author selects one or another polarization of photons, and the choice of polarization moment is random. The recipient of the message uses a certain device - a “detector” - to count the moments of polarization of the photons, and due to the randomness of their choice by the author of the message, the reading results will be either correct or erroneous.

After all the acts of reading the polarization are completed, the receiver sends the information about the parameters of the reading to the message author without informing the measurement results themselves. The author also answers the recipient through an open channel 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, both 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 coincide between the author and the recipient.

In this complex system, the weak point is the device for reading polarization moments (“detector”), with which the receiver works with photons. It turns out that if you “saturate” it with photons to a certain value, the detector will lose the ability to work with individual light quanta and will work like 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 one. In this case, the “detector” of the recipient will not be able to determine the fact of a change in the polarization of the “legal” photon, and as a result, during reconciliation, the fact of unauthorized interference cannot be detected.

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

[Source - Physics World ]