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

arXiv:2605.02951

Binary Classifier Wire-Resistance Attack on KLJN: Impact of Narrowing the Resistor Gap

MEHMET YILDIRIM, László B. Kish

Published May 1, 2026• 0 citations
arXivHTMLPDF

Abstract

It is shown that narrowing the difference between the high and low resistor values in the Kirchhoff Law-Johnson Noise (KLJN) key exchange strongly affects security against a recently introduced binary classifier-based wire resistance attack. Using time domain simulations of a non-ideal KLJN loop with finite cable resistance, we generate large ensembles of secure (HL/LH) bits and evaluate the mean-square noise voltages at Alice's and Bob's ends. For each bit, these mean-square values form a point in a two-dimensional classifier plane, where the separation between the HL and LH point clouds characterizes the information available to an eavesdropper (Eve). We quantify Eve's success probability p by a simple decision rule based on the sign of the difference between the measured mean-square voltages. For strongly asymmetric resistors (for example RL = 4 kOhm and RH = 10 kOhm) and realistic wire resistances, the HL and LH clouds are fully separable and Eve's p approaches 1, which confirms that the classifier attack can practically recover all secure bits. As the low resistor value approaches the high one (for example RL = 9 kOhm and RH = 10 kOhm) at the same cable resistance, the HL and LH clouds increasingly overlap, and the measured p drops close to 0.7, approaching the ideal limit p = 0.5 as RL approaches RH. A surprising phenomenon is that, in this classifier-based scenario, increasing the wire resistance can decrease the information leak. This counterintuitive effect is strikingly the opposite of the behavior in the classical Bergou-Scheuer-Yariv wire resistance attack, where the mean-square voltages at the two ends of the wire are simply compared.

DOI
10.48550/arxiv.2605.02951

Related papers

Binary Classifier Wire-Resistance Attack on KLJN: Impact of Narrowing the Resistor Gap

MEHMET YILDIRIM, László B. Kish

It is shown that narrowing the difference between the high and low resistor values in the Kirchhoff Law–Johnson Noise (KLJN) key exchange strongly affects security against a recently introduced binary classifier-based wire resistance attack. Using time domain simulations of a non-ideal KLJN loop with finite cable resistance, we generate large ensembles of secure (HL/LH) bits and evaluate the mean-square noise voltages at Alice’s and Bob’s ends. For each bit, these mean-square values form a point in a two-dimensional classifier plane, where the separation between the HL and LH point clouds characterizes the information available to an eavesdropper (Eve). We quantify Eve’s success probability p by a simple decision rule based on the sign of the difference between the measured mean-square voltages. For strongly asymmetric resistors (for example [Formula: see text] kOhm and [Formula: see text] kOhm) and realistic wire resistances, the HL and LH clouds are fully separable and Eve’s p approaches 1, which confirms that the classifier attack can practically recover all secure bits. As the low resistor value approaches the high one (for example [Formula: see text] kOhm and [Formula: see text] kOhm) at the same cable resistance, the HL and LH clouds increasingly overlap and the measured p drops close to 0.7, approaching the ideal limit [Formula: see text] as [Formula: see text] approaches [Formula: see text]. A surprising phenomenon is that, in this classifier-based scenario, increasing the wire resistance can decrease the information leak. This counterintuitive effect is strikingly the opposite of the behavior in the classical Bergou–Scheuer–Yariv wire resistance attack, where the mean-square voltages at the two ends of the wire are simply compared.

Fluctuation and Noise Letters•Jun 5, 2026•0 citations

A Binary Classifier-Based Wire Resistance Attack on the KLJN Secure Key Exchanger

MEHMET YILDIRIM, Fahrettin Ay, László B. Kish

The statistical fluctuations of the mean-square noise voltages measured at Alice's and Bob's ends in the KLJN scheme are used to implement a binary classifier for a new type of wire resistance-based attack. The data are plotted on a two-dimensional graph, where the x- and y- axes represent the mean-square voltages at Alice's and Bob's ends, respectively. When the wire resistance is nonzero, the data form distinct lines for the LH and HL cases, allowing Eve to extract the secure bits with nearly 100% success. Further analysis shows that swapping the x and y axes for the LH data reproduces the curve for the HL case, effectively reducing the number of independent measurements by half. These results suggest that machine learning tools could exploit this property for enhanced detection performance, although such methods are unnecessary here since the LH and HL cases are completely separable. The only effective defense against this attack remains the traditional approach: properly increase the noise temperature on the side with lower resistance, or equivalently, scale down the noise temperature on the higher-resistance side. All claims are confirmed through computer simulations.

arXiv:2603.16101•Mar 17, 2026•0 citations

Making the eavesdropper's life harder

Gergely Vadai, László B. Kish

The unconditionally (that is, information theoretically) secure Kirchhoff-law-Johnson-noise (KLJN) key exchanger operates with two pairs of resistors, one at Alice's side and another one at Bob's end.Whenever the cable resistance is not negligible, the original scheme leaks information to the eavesdropper (Eve).The typical way to reduce this leak is treating the exchanged key by privacy amplification.In the present work our goal is to reduce the leak earlier, during the bit exchange level.So far, the common belief has been that the resistance values in Alice's/Bob's pairs should be very different for an easy separation of the bit values to yield low bit error probability in the key exchange.However, such situation is helping Eve during all the currently known types of attacks owing to higher information leak.We explore the possibility of enhancing the security by narrowing the difference between the resistance values.The impact on security and the cost of reduced communication speed are demonstrated for the Bergou-Scheuer-Yariv attack.

International Conference on Noise and Fluctuations•Aug 21, 2019•0 citations

A Binary Classifier-Based Wire Resistance Attack on the KLJN Secure Key Exchanger

MEHMET YILDIRIM, Fahrettin Ay, László B. Kish

The statistical fluctuations of the mean-square noise voltages measured at Alice’s and Bob’s ends in the KLJN scheme are used to implement a binary classifier for a new type of wire-resistance-based attack. The data are plotted on a two-dimensional graph, where the x- and y-axes represent the mean-square voltages at Alice’s and Bob’s ends, respectively. When the wire resistance is non-zero, the data form distinct lines for the LH and HL cases, allowing Eve to extract the secure bits with nearly 100% success. Further analysis shows that swapping the x and y axes for the LH data reproduces the curve for the HL case, effectively reducing the number of independent measurements by half. These results suggest that machine learning tools could exploit this property for enhanced detection performance, although such methods are unnecessary here since the LH and HL cases are completely separable. The only effective defense against this attack remains the traditional approach: properly increase the noise temperature on the side with lower resistance, or equivalently, scale down the noise temperature on the higher-resistance side. All claims are confirmed through computer simulations.

Fluctuation and Noise Letters•May 12, 2026•1 citation

A Static-Loop-Current Attack against the KLJN Secure Key Exchange System

Mutaz Y. Melhem, László B. Kish

A new attack against the Kirchhoff-Law-Johnson-Noise (KLJN) key distribution system is introduced. The attack is based on 1) Utilizing the dc-voltage-source - which we put at Alice’s end in our mathematical modeling of the attack-that could exist due to errors, imbalances, Electromagnetic interference and etc.2) On studying the number of samples per bit in the security key that the measured Alice/Bob voltages exceeds or falls below a threshold voltage, respectively. The threshold voltage is the average between dc voltages across low and high resistors- generated by a dc-voltage source. We count the number of samples the voltage at Bob’s end (containing both the noise and dc components) exceeds the threshold voltage and how many times it falls below the threshold to judge whether the resistor can be guessed as low or high for every cycle. Also for a pre-specified key-length we count the number of high resistance estimations per bit –at Bob’s end-according to the previous criterion and the non-successful estimations per bit to judge the final guessed resistor value at Alice’s end and Bob’s end, where if we have more bits with most of its measured samples are above the threshold voltage then we will predict Bob’s resistance as high resistance, otherwise we predict Bob’s resistance to be low resistance. The Simulation was conducted and the attack proved that it is successful unless the temperature increased dramatically to ranges more than a threshold temperature ~ Kelvin that increases when the number of samples per bit increases.

arXiv:1806.05596•Jun 11, 2018•2 citations

Transient attacks against the Kirchhoff–Law–Johnson–Noise (KLJN) secure key exchanger

Shahriar Faysal Ferdous, László B. Kish

We demonstrate the security vulnerability of an ideal Kirchhoff–Law–Johnson–Noise key exchanger against transient attacks. Transients start when Alice and Bob connect a wire to their chosen resistor at the beginning of each clock cycle. A transient attack takes place during a short duration of time, before the transients reflected from the end of Alice and Bob mix together. The information leak arises from the fact that Eve (the eavesdropper) monitors the cable and analyzes the transients during this time period. We will demonstrate such a transient attack, and then, we introduce a defense protocol to protect against the attack. Computer simulations demonstrate that after applying the defense method the information leak becomes negligible.

Applied Physics Letters•Apr 3, 2023•9 citations
It is shown that narrowing the difference between the high and low resistor values in the Kirchhoff Law-Johnson Noise (KLJN) key exchange strongly affects security against a recently introduced binary classifier-based wire resistance attack. Using time domain simulations of a non-ideal KLJN loop with finite cable resistance, we generate large ensembles of secure (HL/LH) bits and evaluate the mean-square noise voltages at Alice's and Bob's ends. For each bit, these mean-square values form a point in a two-dimensional classifier plane, where the separation between the HL and LH point clouds characterizes the information available to an eavesdropper (Eve). We quantify Eve's success probability p by a simple decision rule based on the sign of the difference between the measured mean-square voltages. For strongly asymmetric resistors (for example RL = 4 kOhm and RH = 10 kOhm) and realistic wire resistances, the HL and LH clouds are fully separable and Eve's p approaches 1, which confirms that the classifier attack can practically recover all secure bits. As the low resistor value approaches the high one (for example RL = 9 kOhm and RH = 10 kOhm) at the same cable resistance, the HL and LH clouds increasingly overlap, and the measured p drops close to 0.7, approaching the ideal limit p = 0.5 as RL approaches RH. A surprising phenomenon is that, in this classifier-based scenario, increasing the wire resistance can decrease the information leak. This counterintuitive effect is strikingly the opposite of the behavior in the classical Bergou-Scheuer-Yariv wire resistance attack, where the mean-square voltages at the two ends of the wire are simply compared.
It is shown that narrowing the difference between the high and low resistor values in the Kirchhoff Law–Johnson Noise (KLJN) key exchange strongly affects security against a recently introduced binary classifier-based wire resistance attack. Using time domain simulations of a non-ideal KLJN loop with finite cable resistance, we generate large ensembles of secure (HL/LH) bits and evaluate the mean-square noise voltages at Alice’s and Bob’s ends. For each bit, these mean-square values form a point in a two-dimensional classifier plane, where the separation between the HL and LH point clouds characterizes the information available to an eavesdropper (Eve). We quantify Eve’s success probability p by a simple decision rule based on the sign of the difference between the measured mean-square voltages. For strongly asymmetric resistors (for example [Formula: see text] kOhm and [Formula: see text] kOhm) and realistic wire resistances, the HL and LH clouds are fully separable and Eve’s p approaches 1, which confirms that the classifier attack can practically recover all secure bits. As the low resistor value approaches the high one (for example [Formula: see text] kOhm and [Formula: see text] kOhm) at the same cable resistance, the HL and LH clouds increasingly overlap and the measured p drops close to 0.7, approaching the ideal limit [Formula: see text] as [Formula: see text] approaches [Formula: see text]. A surprising phenomenon is that, in this classifier-based scenario, increasing the wire resistance can decrease the information leak. This counterintuitive effect is strikingly the opposite of the behavior in the classical Bergou–Scheuer–Yariv wire resistance attack, where the mean-square voltages at the two ends of the wire are simply compared.
The statistical fluctuations of the mean-square noise voltages measured at Alice's and Bob's ends in the KLJN scheme are used to implement a binary classifier for a new type of wire resistance-based attack. The data are plotted on a two-dimensional graph, where the x- and y- axes represent the mean-square voltages at Alice's and Bob's ends, respectively. When the wire resistance is nonzero, the data form distinct lines for the LH and HL cases, allowing Eve to extract the secure bits with nearly 100% success. Further analysis shows that swapping the x and y axes for the LH data reproduces the curve for the HL case, effectively reducing the number of independent measurements by half. These results suggest that machine learning tools could exploit this property for enhanced detection performance, although such methods are unnecessary here since the LH and HL cases are completely separable. The only effective defense against this attack remains the traditional approach: properly increase the noise temperature on the side with lower resistance, or equivalently, scale down the noise temperature on the higher-resistance side. All claims are confirmed through computer simulations.
The unconditionally (that is, information theoretically) secure Kirchhoff-law-Johnson-noise (KLJN) key exchanger operates with two pairs of resistors, one at Alice's side and another one at Bob's end.Whenever the cable resistance is not negligible, the original scheme leaks information to the eavesdropper (Eve).The typical way to reduce this leak is treating the exchanged key by privacy amplification.In the present work our goal is to reduce the leak earlier, during the bit exchange level.So far, the common belief has been that the resistance values in Alice's/Bob's pairs should be very different for an easy separation of the bit values to yield low bit error probability in the key exchange.However, such situation is helping Eve during all the currently known types of attacks owing to higher information leak.We explore the possibility of enhancing the security by narrowing the difference between the resistance values.The impact on security and the cost of reduced communication speed are demonstrated for the Bergou-Scheuer-Yariv attack.
The statistical fluctuations of the mean-square noise voltages measured at Alice’s and Bob’s ends in the KLJN scheme are used to implement a binary classifier for a new type of wire-resistance-based attack. The data are plotted on a two-dimensional graph, where the x- and y-axes represent the mean-square voltages at Alice’s and Bob’s ends, respectively. When the wire resistance is non-zero, the data form distinct lines for the LH and HL cases, allowing Eve to extract the secure bits with nearly 100% success. Further analysis shows that swapping the x and y axes for the LH data reproduces the curve for the HL case, effectively reducing the number of independent measurements by half. These results suggest that machine learning tools could exploit this property for enhanced detection performance, although such methods are unnecessary here since the LH and HL cases are completely separable. The only effective defense against this attack remains the traditional approach: properly increase the noise temperature on the side with lower resistance, or equivalently, scale down the noise temperature on the higher-resistance side. All claims are confirmed through computer simulations.
A new attack against the Kirchhoff-Law-Johnson-Noise (KLJN) key distribution system is introduced. The attack is based on 1) Utilizing the dc-voltage-source - which we put at Alice’s end in our mathematical modeling of the attack-that could exist due to errors, imbalances, Electromagnetic interference and etc.2) On studying the number of samples per bit in the security key that the measured Alice/Bob voltages exceeds or falls below a threshold voltage, respectively. The threshold voltage is the average between dc voltages across low and high resistors- generated by a dc-voltage source. We count the number of samples the voltage at Bob’s end (containing both the noise and dc components) exceeds the threshold voltage and how many times it falls below the threshold to judge whether the resistor can be guessed as low or high for every cycle. Also for a pre-specified key-length we count the number of high resistance estimations per bit –at Bob’s end-according to the previous criterion and the non-successful estimations per bit to judge the final guessed resistor value at Alice’s end and Bob’s end, where if we have more bits with most of its measured samples are above the threshold voltage then we will predict Bob’s resistance as high resistance, otherwise we predict Bob’s resistance to be low resistance. The Simulation was conducted and the attack proved that it is successful unless the temperature increased dramatically to ranges more than a threshold temperature ~ Kelvin that increases when the number of samples per bit increases.
We demonstrate the security vulnerability of an ideal Kirchhoff–Law–Johnson–Noise key exchanger against transient attacks. Transients start when Alice and Bob connect a wire to their chosen resistor at the beginning of each clock cycle. A transient attack takes place during a short duration of time, before the transients reflected from the end of Alice and Bob mix together. The information leak arises from the fact that Eve (the eavesdropper) monitors the cable and analyzes the transients during this time period. We will demonstrate such a transient attack, and then, we introduce a defense protocol to protect against the attack. Computer simulations demonstrate that after applying the defense method the information leak becomes negligible.