© 2026 Aqora Quantum S.A.S.

Terms Privacy Legal Notice

On the Insecurity of the Flip-KLJN Secure Key Exchange Scheme

Christiana Chamon

Published Oct 21, 2025• 0 citations

Abstract

The Flip-KLJN scheme, recently proposed as an enhancement to the Kirchhoff-Law-Johnson-Noise (KLJN) secure key exchange protocol, claims to enable secure bit exchange in the previously discarded LL and HH states by introducing a bit-flipping mechanism. We show this claim is invalid. Because the mean-square noise voltages of the LL and HH states are distinguishable, they uniquely reveal both parties' resistor choices. The proposed flip operation is a deterministic bit permutation, equivalent to a NOT gate, and therefore cannot generate new entropy. Once the HL/LH case-which holds the only true security-is cracked, the entire flip mapping is exposed, retroactively and prospectively. This means that the security of all LL and HH states depends solely on a single secret: the current mapping of the bit flip schedule. Should an attacker learn this mapping (even just once), every corresponding LL and HH bit-past and future-is immediately compromised, reducing the security of half the message to only a one-bit secret. Hence the scheme does not expand secrecy, only the key length, and functions as a poor deterministic key expander, similar to the LIJK "noise-free" system previously analyzed by Kish (2024). Flip-KLJN therefore fails to provide information-theoretic security and cannot serve as a true secure key exchanger.

DOI: 10.36227/techrxiv.176106534.44552079/v1

Related papers

Random Number Generator Attack against the Kirchhoff-Law-Johnson-Noise Secure Key Exchange Protocol

Christiana Chamon, Shahriar Faysal Ferdous, László B. Kish

This paper introduces and demonstrates two new attacks against the Kirchhoff-Law-Johnson-Noise (KLJN) secure key exchange scheme. The attacks are based on random number generators with compromised security. First we explore the situation in which Eve knows the seed of both Alice's and Bob's random number generators. We show that in this situation Eve can crack the secure key bit within a fraction of the bit exchange period even if her current and voltage measurements have only a single bit of resolution. In the second attack, we explore the situation in which Eve knows the seed of only Alice's random number generator. We show that in this situation Eve can still crack the secure key bit but she needs to use the whole bit exchange period for the attack. The security of the KLJN key exchange scheme, similarly to other protocols, necessitates that the random number generator outputs are truly random for Eve.

arXiv:2005.10429•May 21, 2020•2 citations

Flip-KLJN: Random Resistance Flipping for Noise-Driven Secure Communication

Recep Akif Taşçı, Ibrahim Yildirim, Ertuğrul Başar

The information-theoretically (unconditionally) secure Kirchhoff-law-Johnson-noise (KLJN) bit exchange protocol uses two identical resistor pairs with high (H) and low (L) resistance values, driven by Gaussian noise generators emulating Johnson noise with a high common temperature. The resulting mean-square noise voltage on the wire connecting Alice and Bob has three levels: low (L/L), intermediate (H/LorL/H), and high (H/H), and secure key sharing is achieved at the intermediate level (L/HorH/L). This paper introduces the Flip-KLJN scheme, where a pre-agreed intermediate level, such asH/L, triggers a flip of the bit map value during the bit exchange period. For Eve, the bit map flips appear random. Thus, the formerly discardedH/HandL/Lsituations can also have a pre-agreed bit value mapping, which flips together with the original bit mapping. Thus, Flip-KLJN doubles the key rate and ensures that all three levels on the wire are indistinguishable for Eve. Bit error probabilities are addressed through analytic calculations and computer simulations.

arXiv:2504.08367•IEEE Transactions on Communications•Jul 3, 2025•9 citations

Performance analysis of the “intelligent” Kirchhoff-Law-Johnson-Noise secure key exchange

Janusz Smulko

The Kirchhoff-Law-Johnson-Noise (KLJN) secure key distribution system provides a way of exchanging information theoretic secure keys by measuring the random voltage and current through the wire connecting two different resistors at Alice's and Bob's ends. Recently new advanced protocols for the KLJN method have been proposed with enhanced performance. In this paper we analyze the KLJN system and compare with “intelligent” KLJN (iKLJN) scheme. This task requires the determination of the applied resistors and the identification of the various superpositions of known and unknown noise components. Some statistical tools to determine how the duration of the bit exchange window (averaging time) influences the performance of secure bit exchange will be explored.

International Journal of Modern Physics Conference Series•Jan 1, 2014•3 citations

Performance Analysis of the "Intelligent" Kirchhoff-Law–Johnson-Noise Secure Key Exchange

Janusz Smulko

The Kirchhoff-law–Johnson-noise (KLJN) secure key distribution system provides a way of exchanging theoretically secure keys by measuring random voltage and current through the wire connecting two different resistors at Alice's and Bob's ends. Recently new advanced protocols for the KLJN method have been proposed with enhanced performance. In this paper, we analyze the KLJN system and compare with the "intelligent" KLJN (iKLJN) scheme. That task requires determination of the applied resistors and identification of various superpositions of known and unknown noise components. Some statistical tools will be explored to determine how the duration of the bit exchange window (averaging time) influences the performance of the secure bit exchange.

Fluctuation and Noise Letters•Jul 10, 2014•30 citations

Deterministic Random Number Generator Attack Against the Kirchhoff-Law-Johnson-Noise Secure Key Exchange Protocol

Christiana Chamon, Shahriar Ferdous, Laszlo B. Kish

This paper demonstrates the vulnerability of the Kirchhoff-Law-Johnson-Noise (KLJN) secure key exchanger to compromised random number generator(s) even if these random numbers are used solely to generate the noises emulating the Johnson noise of Alice’s and Bob’s resistors. The attacks shown are deterministic in the sense that Eve’s knowledge of Alice’s and/or Bob’s random numbers is basically deterministic. Moreover, no statistical evaluation is needed, except for rarely occurring events of negligible, random waiting time and verification time. We explore two situations. In the first case, Eve knows both Alice’s and Bob’s random noises. We show that, in this situation, Eve can quickly crack the secure key bit by using Ohm’s Law. In the other situation, Eve knows only Bob’s random noise. Then Eve first can learn Bob’s resistance value by using Ohm’s Law. Therefore, she will have the same knowledge as Bob, thus at the end of the bit exchange period, she will know Alice’s bit.

arXiv:2012.02848•Fluctuation and Noise Letters•Apr 7, 2021•18 citations

PERFORMANCE ANALYSIS OF THE "INTELLIGENT" KIRCHHOFF-LAW-JOHNSON-NOISE SECURE KEY EXCHANGE

Janusz Smulko

The Kirchhoff-Law-Johnson-Noise (KLJN) secure key distribution system provides a way of exchanging theoretically secure keys by measuring random voltage and current through the wire connecting two different resistors at Alice's and Bob's ends. Recently new advanced protocols for the KLJN method have been proposed with enhanced performance. In this paper we analyze the KLJN system and compare with the intelligent KLJN (iKLJN) scheme. That task requires determination of the applied resistors and identification of various superpositions of known and unknown noise components. Some statistical tools will be explored to determine how the duration of the bit exchange window (averaging time) influences the performance of the secure bit exchange.

•Jun 1, 2014•0 citations

The Flip-KLJN scheme, recently proposed as an enhancement to the Kirchhoff-Law-Johnson-Noise (KLJN) secure key exchange protocol, claims to enable secure bit exchange in the previously discarded LL and HH states by introducing a bit-flipping mechanism. We show this claim is invalid. Because the mean-square noise voltages of the LL and HH states are distinguishable, they uniquely reveal both parties' resistor choices. The proposed flip operation is a deterministic bit permutation, equivalent to a NOT gate, and therefore cannot generate new entropy. Once the HL/LH case-which holds the only true security-is cracked, the entire flip mapping is exposed, retroactively and prospectively. This means that the security of all LL and HH states depends solely on a single secret: the current mapping of the bit flip schedule. Should an attacker learn this mapping (even just once), every corresponding LL and HH bit-past and future-is immediately compromised, reducing the security of half the message to only a one-bit secret. Hence the scheme does not expand secrecy, only the key length, and functions as a poor deterministic key expander, similar to the LIJK "noise-free" system previously analyzed by Kish (2024). Flip-KLJN therefore fails to provide information-theoretic security and cannot serve as a true secure key exchanger.

This paper introduces and demonstrates two new attacks against the Kirchhoff-Law-Johnson-Noise (KLJN) secure key exchange scheme. The attacks are based on random number generators with compromised security. First we explore the situation in which Eve knows the seed of both Alice's and Bob's random number generators. We show that in this situation Eve can crack the secure key bit within a fraction of the bit exchange period even if her current and voltage measurements have only a single bit of resolution. In the second attack, we explore the situation in which Eve knows the seed of only Alice's random number generator. We show that in this situation Eve can still crack the secure key bit but she needs to use the whole bit exchange period for the attack. The security of the KLJN key exchange scheme, similarly to other protocols, necessitates that the random number generator outputs are truly random for Eve.

The information-theoretically (unconditionally) secure Kirchhoff-law-Johnson-noise (KLJN) bit exchange protocol uses two identical resistor pairs with high (H) and low (L) resistance values, driven by Gaussian noise generators emulating Johnson noise with a high common temperature. The resulting mean-square noise voltage on the wire connecting Alice and Bob has three levels: low (L/L), intermediate (H/LorL/H), and high (H/H), and secure key sharing is achieved at the intermediate level (L/HorH/L). This paper introduces the Flip-KLJN scheme, where a pre-agreed intermediate level, such asH/L, triggers a flip of the bit map value during the bit exchange period. For Eve, the bit map flips appear random. Thus, the formerly discardedH/HandL/Lsituations can also have a pre-agreed bit value mapping, which flips together with the original bit mapping. Thus, Flip-KLJN doubles the key rate and ensures that all three levels on the wire are indistinguishable for Eve. Bit error probabilities are addressed through analytic calculations and computer simulations.

The Kirchhoff-Law-Johnson-Noise (KLJN) secure key distribution system provides a way of exchanging information theoretic secure keys by measuring the random voltage and current through the wire connecting two different resistors at Alice's and Bob's ends. Recently new advanced protocols for the KLJN method have been proposed with enhanced performance. In this paper we analyze the KLJN system and compare with “intelligent” KLJN (iKLJN) scheme. This task requires the determination of the applied resistors and the identification of the various superpositions of known and unknown noise components. Some statistical tools to determine how the duration of the bit exchange window (averaging time) influences the performance of secure bit exchange will be explored.

The Kirchhoff-law–Johnson-noise (KLJN) secure key distribution system provides a way of exchanging theoretically secure keys by measuring random voltage and current through the wire connecting two different resistors at Alice's and Bob's ends. Recently new advanced protocols for the KLJN method have been proposed with enhanced performance. In this paper, we analyze the KLJN system and compare with the "intelligent" KLJN (iKLJN) scheme. That task requires determination of the applied resistors and identification of various superpositions of known and unknown noise components. Some statistical tools will be explored to determine how the duration of the bit exchange window (averaging time) influences the performance of the secure bit exchange.

This paper demonstrates the vulnerability of the Kirchhoff-Law-Johnson-Noise (KLJN) secure key exchanger to compromised random number generator(s) even if these random numbers are used solely to generate the noises emulating the Johnson noise of Alice’s and Bob’s resistors. The attacks shown are deterministic in the sense that Eve’s knowledge of Alice’s and/or Bob’s random numbers is basically deterministic. Moreover, no statistical evaluation is needed, except for rarely occurring events of negligible, random waiting time and verification time. We explore two situations. In the first case, Eve knows both Alice’s and Bob’s random noises. We show that, in this situation, Eve can quickly crack the secure key bit by using Ohm’s Law. In the other situation, Eve knows only Bob’s random noise. Then Eve first can learn Bob’s resistance value by using Ohm’s Law. Therefore, she will have the same knowledge as Bob, thus at the end of the bit exchange period, she will know Alice’s bit.

The Kirchhoff-Law-Johnson-Noise (KLJN) secure key distribution system provides a way of exchanging theoretically secure keys by measuring random voltage and current through the wire connecting two different resistors at Alice's and Bob's ends. Recently new advanced protocols for the KLJN method have been proposed with enhanced performance. In this paper we analyze the KLJN system and compare with the intelligent KLJN (iKLJN) scheme. That task requires determination of the applied resistors and identification of various superpositions of known and unknown noise components. Some statistical tools will be explored to determine how the duration of the bit exchange window (averaging time) influences the performance of the secure bit exchange.