Electrical engineering is often overshadowed by other STEM fields. Computer Science is cooler, and physics has the aura of the Faustian quest for the most fundamental truths science can uncover. Yet, this discipline produced a quite remarkable bit of research with profound implications for Quantum Information Science. It is not very well publicized. Maybe that is because it’s a bit embarrassing to the physicists and computer scientists who are heavily vested in Quantum Cryptography?
After all, the typical, one-two punch elevator-pitch for QIS is entirely undermined by it. To recap, the argument goes likes this:
- Universal Quantum Computing will destroy all effective cryptography as we know it.
- Fear not, for Quantum Cryptography will come to your rescue.
Significant funds went into the latter. And it’s not like there isn’t some significant progress, but what if all this effort proved futile because an equally strong encryption could be had with far more robust methods? This is exactly what the Kish Cypher protocol promises. It has been around for several years, and in a recent paper, Laszlo Bela Kish discusses several variations of his protocol that he modestly calls the Kirchhoff-Law-Johnson-(like)-Noise (KLJN) secure key exchange – although otherwise it goes by his name in the literature. A 2012 paper that describes the principle behind it can be found here. The abstract of the latter makes no qualms about the challenge to Quantum Information Science:
It has been shown recently that the use of two pairs of resistors with enhanced Johnson-noise and a Kirchhoff-loop—i.e., a Kirchhoff-Law-JohnsonNoise (KLJN) protocol—for secure key distribution leads to information theoretic security levels superior to those of a quantum key distribution, including a natural immunity against a man-in-the-middle attack. This issue is becoming particularly timely because of the recent full cracks of practical quantum communicators, as shown in numerous peer-reviewed publications.
There are some commonalities between quantum cryptography and this alternative, inherently safe, protocol. The obvious one is that they are both key exchange schemes; The more interesting one is that they both leverage fundamental physics properties of the systems that they are employing. In one case, it is the specific quantum correlations of entangled qubits, in the other, the correlations in classical thermodynamic noise (i.e. the former picks out the specific quantum entanglement correlations of the systems density matrix, the latter only requires the classical entries that remain after decoherence and tracing of the density matrix).
Since this protocol works in the classical regime, it shouldn’t come as a surprise that the implementation is much easier to accomplish than when having to accomplish and preserve an entangled state. The following schematic illustrates the underlying principle:
The recipient (Bob) connects the wire at random in predefined synchronicity with the sender (Alice). The actual current and voltage through the wire is random, ideally Johnson noise. The resistors determine the characteristic of this voltage, Bob can determine what resistor Alice used because he knows which one he connected, but the Fluctuation Dissipation Theorem ensures that wire-tapping by an attacker (Eve) is futile. The noise characteristics of the signal ensure that no information can be extracted from it.
Given that the amount of effort and funding that goes into Quantum Cryptography is substantial (some even mock it as a distraction from the ultimate prize which is quantum computing), it seems to me that the fact that classic thermodynamic resources allow for similar inherent security should give one pause. After all, this line of research may provide a much more robust approach to the next generation,”Shor safe”, post quantum encryption infrastructure.