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arXiv:2005.11002 · Fluctuation and Noise Letters

Alternating (AC) Loop Current Attacks Against the KLJN Secure Key Exchange Scheme

Mutaz Y. Melhem, Christiana Chamon, Shahriar Ferdous, Laszlo B. Kish

Published Mar 18, 2021• 16 citations
arXivHTMLPDF

Abstract

Recently, several passive and active attack methods have been proposed against the Kirchhoff–Law–Johnson–Noise (KLJN) secure key exchange scheme by utilizing direct (DC) loop currents. The DC current attacks are relatively easy, but their practical importance is low. On the other hand, parasitic alternating (AC) currents are virtually omnipresent in wire-based systems. Such situations exist due to AC ground loops and electromagnetic interference (EMI). However, utilizing AC currents for attacks is a harder problem. Here, we introduce and demonstrate AC current attacks in various frequency ranges. The attacks exploit a parasitic/periodic AC voltage-source at either Alice’s or Bob’s end. In the low-frequency case, the procedure is the generalized form of the former DC ground-loop-based attack. In the high-frequency case, the power density spectrum of the wire voltage is utilized. The attack is demonstrated in both the low and the high-frequency situations. Defense protocols against the attack are also discussed.

DOI
10.1142/s0219477521500504

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Recently, several passive and active attack methods have been proposed against the Kirchhoff–Law–Johnson–Noise (KLJN) secure key exchange scheme by utilizing direct (DC) loop currents. The DC current attacks are relatively easy, but their practical importance is low. On the other hand, parasitic alternating (AC) currents are virtually omnipresent in wire-based systems. Such situations exist due to AC ground loops and electromagnetic interference (EMI). However, utilizing AC currents for attacks is a harder problem. Here, we introduce and demonstrate AC current attacks in various frequency ranges. The attacks exploit a parasitic/periodic AC voltage-source at either Alice’s or Bob’s end. In the low-frequency case, the procedure is the generalized form of the former DC ground-loop-based attack. In the high-frequency case, the power density spectrum of the wire voltage is utilized. The attack is demonstrated in both the low and the high-frequency situations. Defense protocols against the attack are also discussed.
In this study, a new attack against the Kirchhoff-Law-Johnson-Noise (KLJN) key distribution system is explored. The attack is based on utilizing a parasitic voltage-source in the loop. Relevant situations often exist in the low-frequency limit in practical systems, especially when the communication is over a distance, or between different units within an instrument, due to a ground loop and/or electromagnetic interference (EMI). Our present study investigates the DC ground loop situation when no AC or EMI effects are present. Surprisingly, the usual current/voltage comparison-based defense method that exposes active attacks or parasitic features (such as wire resistance allowing information leaks) does not function here. The attack is successfully demonstrated and proposed defense methods against the attack are shown.
A new attack against the Kirchhoff-Law-Johnson-Noise (KLJN) secure key distribution system is studied with unknown parasitic DC-voltage sources at both Alice's and Bob's ends.This paper is the generalization of our earlier investigation with a single-end parasitic source.Under the assumption that Eve does not know the values of the parasitic sources, a new attack, utilizing the current generated by the parasitic dc-voltage sources, is introduced.The attack is mathematically analyzed and demonstrated by computer simulations.Simple defense methods against the attack are shown.The earlier defense method based solely on the comparison of current/voltage data at Alice's and Bob's terminals is useless here since the wire currents and voltages are equal at both ends.However, the more expensive version of the earlier defense method, which is based on in-situ system simulation and comparison with measurements, works efficiently.
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.
This study addresses a new question regarding the security of the Kirchhoff-Law-Johnson-Noise (KLJN) scheme compromised by DC sources at Alice and Bob: What is the impact of these parasitic sources on active attacks, such as the man-in-the-middle (MITM) attack, or the current injection attack if Alice and Bod did not eliminate these sources? The surprising answer is that the parasitic DC sources actually increase the security of the system because, in the case of the MITM attack, they make easier to uncover the eavesdropping than the ideal situation. In some of the cases, Eve can fix this deficiency but then the problem gets reduced to the original MITM attack to which the KLJN scheme is immune, it is already proven earlier.
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.