Comments on the "Generalized" KLJN Key Exchanger with Arbitrary Resistors: Power, Impedance, Security

TL;DR

This paper critically examines the security and power flow of a generalized KLJN key exchange system with multiple resistors, revealing vulnerabilities and proposing modifications to improve security but with inherent trade-offs.

Contribution

It analyzes the security flaws of the VMG-KLJN scheme with 4 arbitrary resistors and introduces a new zero-power protocol with limited resistor flexibility to enhance security.

Findings

VMG-KLJN system is less secure than original KLJN due to parasitic capacitance and inductance attacks.

A new zero-power KLJN protocol with 3 resistors maintains security against certain passive attacks.

Modified KLJN systems can mitigate some vulnerabilities but at the cost of reduced resistor arbitrariness.

Abstract

In (Nature) Science Report 5 (2015) 13653, Vadai, Mingesz and Gingl (VMG) introduce a new Kirchhoff-law-Johnson-noise (KLJN) secure key exchanger that operates with 4 arbitrary resistors (instead of 2 arbitrary resistance values forming 2 identical resistor pairs in the original system). They state that in this new, VMG-KLJN, non-equilibrium system with nonzero power flow, the security during the exchange of the two (HL and LH) bit values is as strong as in the original KLJN scheme. Moreover, they claim that, at practical conditions, their VMG-KLJN protocol "supports more robust protection against attacks". First, we investigate the power flow and thermal equilibrium issues of the VMG-KLJN system with 4 arbitrary resistors. Then we introduce a new KLJN protocol that allows the arbitrary choice of 3 resistors from the 4, while it still operates with zero power flow during the exchange of single bits by utilizing a specific value of the 4th resistor and a binary temperature set for the exchanged (HL and LH) bit values. Then we show that, in general, the KLJN schemes with more than 2 arbitrary resistors (including our new protocol mentioned above) are prone to 4 new passive attacks utilizing the parasitic capacitance and inductance in the cable, while the original KLJN scheme is naturally immune against these new attacks. The core of the security vulnerability exploited by these attacks is the different line resistances in the HL and LH cases. Therefore, on the contrary of the statement and claim cited above, the practical VMG-KLJN system is less secure than the original KLJN scheme. We introduce another 2, modified, non-equilibrium KLJN systems to eliminate the vulnerability against some - but not all - of these attacks. However the price for that is the loss of arbitrariness of the selection of the 4th resistor and the information leak still remains greater than zero.