Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Journal of Experimental and Theoretical Physics Letters

On a New Attack on Quantum Key Distribution: Joint Unambiguous Measurements of Probe States and the PNS Attack on Information States

S. N. Molotkov

Published Sep 1, 2020• 2 citations
Publisher

Abstract

A new attack on the quantum key distribution is proposed involving joint collective unambiguous measurements of reflected probe states from an intensity modulator and photon number splitting (PNS) measurements, i.e., nondestructive measurements of the number of information photons in the quantum communication channel. This attack does not change the relative statistics of photocounts of states with different numbers of photons, does not lead to errors on the receiver side, and, thereby, is not detected by any of known methods, including the modified decoy state method. The attack results only in additional losses in the communication channel, which are not “controlled” by the decoy state method. The dependence of introduced losses on the intensity of reflected probe states is estimated. The critical level of losses depends on a particular physical implementation of the quantum cryptography system, which determines the upper bound of the intensity of reflected probe states. The knowledge of this bound is fundamentally necessary to ensure the security of keys. The fact that the attack does not lead to errors on the receiver side and does not change the relative statistics of photocounts but only results in additional losses, which depend on the intensity according to the distinguishability of reflected probe states, does not mean that this attack transforms quantum cryptography systems from the type of cryptographic systems where the security of keys is guaranteed by fundamental quantum mechanical laws to the type of systems where this security is guaranteed by technical restrictions. Even in the presence of side channels of information leakage, the security of keys is still guaranteed by fundamental quantum mechanical restrictions on the distinguishability of states. A low level of distinguishability (“intensity”) of quantum states in side channels is naturally reached by technical tools.

DOI
10.1134/s0021364020180095

Related papers

Unambiguous measurements and Trojan-horse attack in quantum cryptography

S. N. Molotkov

Abstract Early security proofs of the keys in quantum cryptography systems were based on the assumption that the transmitting and receiving stations were completely isolated from the outside world—the eavesdropper. However, this condition cannot be implemented in practice, since quantum cryptography systems are open systems, in the sense that an eavesdropper can have indirect access, for example, through a fiber communication channel to critical equipment elements (phase modulators, intensity modulators, etc) using active sensing of the state of these elements—Trojan-horse attacks. A new Trojan-horse attack based on joint unambiguous measurements of reflected probing states from an intensity modulator and information states in a quantum communication channel is proposed. Estimates of attack parameters for a number of quantum key distribution protocols are given, when an attack leads to compromise of keys.

Laser Physics Letters•Dec 20, 2019•3 citations

Are There Enough Decoy States to Ensure Key Secrecy in Quantum Cryptography?

S. N. Molotkov, Konstantin Kravtsov, M. I. Ryzhkin

Abstract The decoy state method has been proposed to detect a photon number splitting (PNS) attack. Today, the decoy state method is considered to be an almost universal method for proving the secrecy of quantum cryptography protocols and calculating the length of a secret key. In this paper, it is shown that there exist attacks, for example, a beam splitting (BS) attack, to which the decoy state method turns out to be insensitive. The decoy state method is oriented to the detection of changes in the photocount statistics of information states and decoy states under a PNS attack. Under a BS attack, the photocount statistics is not changed. As a result, the decoy state method significantly overestimates the length of the key. Thus, the decoy state method is not a universal method that allows the detection of various attacks. Apparently, due to a large number of publications on the decoy state method, a widespread opinion has been formed that this method is universal. This fact has led to attempts to adopt this method as an international standard in quantum cryptography, which is clearly premature.

Journal of Experimental and Theoretical Physics•Apr 1, 2019•9 citations

Trojan Horse Attacks, Decoy State Method, and Side Channels of Information Leakage in Quantum Cryptography

S. N. Molotkov

Abstract Early proofs of key secrecy in quantum cryptography systems were based on the assumption that the transmitting and receiving stations are completely isolated from the outside world—the eavesdropper. However, this condition cannot be implemented in practice since quantum cryptography systems are open systems in the sense that the eavesdropper may have indirect access, for example, through a fiber communication channel, to the critical elements of the equipment (phase modulators, intensity modulators, etc.) using active probing of the state of these elements. The state of the elements carries information about the transmitted key. In addition, the eavesdropper can use passive detection of side radiation from the receiving and transmitting equipment. Signals in side channels of information leakage may have extremely low intensity and are actually quantum signals. The eavesdropper may use the joint measurement of quantum information states in the communication channel and of states in various side channels of information leakage. The paper considers both passive attacks with measurement of side radiation and active attacks involving the probing of the states of the phase modulator and the intensity modulator, as well as backscattering radiation of single-photon avalanche detectors, which occurs during detecting information states on the receiver side. Combined attacks are also considered. The decoy state method is generalized with regard to active probing attacks, and boundaries for state parameters in side communication channels are obtained that guarantee secret key distribution for a given length of the communication channel.

Journal of Experimental and Theoretical Physics•Jun 1, 2020•16 citations

Detecting the possibility of a type of photon number splitting attack in decoy-state quantum key distribution

Xiao-Ming Chen, Lei Chen, Yalong Yan

The existing decoy-state quantum key distribution (QKD) beating photon-number-splitting (PNS) attack provides a more accurate method to estimate the secure key rate, while it still considers that only single-photon pulses can generate secure keys in any case. However, multiphoton pulses can also generate secure keys if we can detect the possibility of PNS attack in the channel. The ultimate goal of this line of research is to confirm the absence of all types of PNS attacks. In particular, the PNS attack mentioned and detected in this paper is only the weaker version of PNS attack which significantly changes the observed values of the legitimate users. In this paper, under the null hypothesis of no weaker version of PNS attack, we first determine whether there is an attack or not by retrieving the missing information of the existing decoy-state protocols, extract a Cauchy distribution statistic, and further provide a detection method and the type I error probability. If the result is judged to be an attack, we can use the existing decoy-state method and the GLLP formula to estimate the secure key rate. Otherwise, the pulses with the same basis received including both single-photon pulses and multiphoton pulses, can be used to generate the keys and we give the secure key rate in this case. Finally, the associated experiments we performed (i.e., the significance level is 5%) show the correctness of our method.

Chinese Physics B•Oct 17, 2022•2 citations

On Eavesdropping in Quantum Cryptography through Side Channels of Information Leakage

S. N. Molotkov

In quantum cryptography, in addition to attacks on transmitted quantum states, states in side channels of information leakage can be detected. The security of keys in real quantum cryptography systems cannot be analyzed without the inclusion of information leakage through side channels. A quantum-mechanical method has been proposed in this work to take into account key information leakage through side channels. This method is based on the detection of side electromagnetic radiation, active probing of a phase modulator at the transmitter station, and re-emission of avalanche detectors at the receiver side. The method takes into account joint collective measurements of quantum states in all channels of information leakage and operates at any intensity and structure of states in side channels. The choice of special prolate spheroidal functions makes it possible to “match” the quantum and classical descriptions of signals in side channels. A relation has been found between information leakage and the fundamental Holevo quantity. A transparent and intuitively clear physical interpretation of the results has been given.

Journal of Experimental and Theoretical Physics Letters•Jun 1, 2020•5 citations

Side channels of information leakage in quantum cryptography based on geometrically uniform coherent states

S. N. Molotkov, Kirill A. Balygin

Abstract Quantum key distribution systems are open systems in the sense that in addition to attacking quantum states in the quantum communication channel, the eavesdropper can obtain additional information about the transmitted key using passive radiation detection of the transmitting and receiving stations, as well as active probing of the state of the equipment through a fiber communication line. In this paper, we consider the cryptographic robustness of a quantum cryptography system that uses a protocol with phase coding based on strongly weakened coherent geometrically uniform states. Non-strict single-photon information states, as well as information leakage through side channels, are taken into account by the generalized Decoy State method, taking into account joint collective measurements of information quantum states and quantum states in side channels. An estimate of the secret key length is obtained, which is expressed only in terms of the observed values at the receiving station and the parameters of quantum states in the side channels.

Laser Physics•May 4, 2020•3 citations
A new attack on the quantum key distribution is proposed involving joint collective unambiguous measurements of reflected probe states from an intensity modulator and photon number splitting (PNS) measurements, i.e., nondestructive measurements of the number of information photons in the quantum communication channel. This attack does not change the relative statistics of photocounts of states with different numbers of photons, does not lead to errors on the receiver side, and, thereby, is not detected by any of known methods, including the modified decoy state method. The attack results only in additional losses in the communication channel, which are not “controlled” by the decoy state method. The dependence of introduced losses on the intensity of reflected probe states is estimated. The critical level of losses depends on a particular physical implementation of the quantum cryptography system, which determines the upper bound of the intensity of reflected probe states. The knowledge of this bound is fundamentally necessary to ensure the security of keys. The fact that the attack does not lead to errors on the receiver side and does not change the relative statistics of photocounts but only results in additional losses, which depend on the intensity according to the distinguishability of reflected probe states, does not mean that this attack transforms quantum cryptography systems from the type of cryptographic systems where the security of keys is guaranteed by fundamental quantum mechanical laws to the type of systems where this security is guaranteed by technical restrictions. Even in the presence of side channels of information leakage, the security of keys is still guaranteed by fundamental quantum mechanical restrictions on the distinguishability of states. A low level of distinguishability (“intensity”) of quantum states in side channels is naturally reached by technical tools.
Abstract Early security proofs of the keys in quantum cryptography systems were based on the assumption that the transmitting and receiving stations were completely isolated from the outside world—the eavesdropper. However, this condition cannot be implemented in practice, since quantum cryptography systems are open systems, in the sense that an eavesdropper can have indirect access, for example, through a fiber communication channel to critical equipment elements (phase modulators, intensity modulators, etc) using active sensing of the state of these elements—Trojan-horse attacks. A new Trojan-horse attack based on joint unambiguous measurements of reflected probing states from an intensity modulator and information states in a quantum communication channel is proposed. Estimates of attack parameters for a number of quantum key distribution protocols are given, when an attack leads to compromise of keys.
Abstract The decoy state method has been proposed to detect a photon number splitting (PNS) attack. Today, the decoy state method is considered to be an almost universal method for proving the secrecy of quantum cryptography protocols and calculating the length of a secret key. In this paper, it is shown that there exist attacks, for example, a beam splitting (BS) attack, to which the decoy state method turns out to be insensitive. The decoy state method is oriented to the detection of changes in the photocount statistics of information states and decoy states under a PNS attack. Under a BS attack, the photocount statistics is not changed. As a result, the decoy state method significantly overestimates the length of the key. Thus, the decoy state method is not a universal method that allows the detection of various attacks. Apparently, due to a large number of publications on the decoy state method, a widespread opinion has been formed that this method is universal. This fact has led to attempts to adopt this method as an international standard in quantum cryptography, which is clearly premature.
Abstract Early proofs of key secrecy in quantum cryptography systems were based on the assumption that the transmitting and receiving stations are completely isolated from the outside world—the eavesdropper. However, this condition cannot be implemented in practice since quantum cryptography systems are open systems in the sense that the eavesdropper may have indirect access, for example, through a fiber communication channel, to the critical elements of the equipment (phase modulators, intensity modulators, etc.) using active probing of the state of these elements. The state of the elements carries information about the transmitted key. In addition, the eavesdropper can use passive detection of side radiation from the receiving and transmitting equipment. Signals in side channels of information leakage may have extremely low intensity and are actually quantum signals. The eavesdropper may use the joint measurement of quantum information states in the communication channel and of states in various side channels of information leakage. The paper considers both passive attacks with measurement of side radiation and active attacks involving the probing of the states of the phase modulator and the intensity modulator, as well as backscattering radiation of single-photon avalanche detectors, which occurs during detecting information states on the receiver side. Combined attacks are also considered. The decoy state method is generalized with regard to active probing attacks, and boundaries for state parameters in side communication channels are obtained that guarantee secret key distribution for a given length of the communication channel.
The existing decoy-state quantum key distribution (QKD) beating photon-number-splitting (PNS) attack provides a more accurate method to estimate the secure key rate, while it still considers that only single-photon pulses can generate secure keys in any case. However, multiphoton pulses can also generate secure keys if we can detect the possibility of PNS attack in the channel. The ultimate goal of this line of research is to confirm the absence of all types of PNS attacks. In particular, the PNS attack mentioned and detected in this paper is only the weaker version of PNS attack which significantly changes the observed values of the legitimate users. In this paper, under the null hypothesis of no weaker version of PNS attack, we first determine whether there is an attack or not by retrieving the missing information of the existing decoy-state protocols, extract a Cauchy distribution statistic, and further provide a detection method and the type I error probability. If the result is judged to be an attack, we can use the existing decoy-state method and the GLLP formula to estimate the secure key rate. Otherwise, the pulses with the same basis received including both single-photon pulses and multiphoton pulses, can be used to generate the keys and we give the secure key rate in this case. Finally, the associated experiments we performed (i.e., the significance level is 5%) show the correctness of our method.
In quantum cryptography, in addition to attacks on transmitted quantum states, states in side channels of information leakage can be detected. The security of keys in real quantum cryptography systems cannot be analyzed without the inclusion of information leakage through side channels. A quantum-mechanical method has been proposed in this work to take into account key information leakage through side channels. This method is based on the detection of side electromagnetic radiation, active probing of a phase modulator at the transmitter station, and re-emission of avalanche detectors at the receiver side. The method takes into account joint collective measurements of quantum states in all channels of information leakage and operates at any intensity and structure of states in side channels. The choice of special prolate spheroidal functions makes it possible to “match” the quantum and classical descriptions of signals in side channels. A relation has been found between information leakage and the fundamental Holevo quantity. A transparent and intuitively clear physical interpretation of the results has been given.
Abstract Quantum key distribution systems are open systems in the sense that in addition to attacking quantum states in the quantum communication channel, the eavesdropper can obtain additional information about the transmitted key using passive radiation detection of the transmitting and receiving stations, as well as active probing of the state of the equipment through a fiber communication line. In this paper, we consider the cryptographic robustness of a quantum cryptography system that uses a protocol with phase coding based on strongly weakened coherent geometrically uniform states. Non-strict single-photon information states, as well as information leakage through side channels, are taken into account by the generalized Decoy State method, taking into account joint collective measurements of information quantum states and quantum states in side channels. An estimate of the secret key length is obtained, which is expressed only in terms of the observed values at the receiving station and the parameters of quantum states in the side channels.