  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.
