Quantum Cryptography Made Obsolete?

The background story.

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:

  1. Universal Quantum Computing will destroy all effective cryptography as we know it.
  2. 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:

Core of the KJLN secure key exchange system. Alice encodes her message by connecting these two resistors to the wire in the required sequence. Bob, on the other hand, connects his resistors to the wire at random.

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.

4 thoughts on “Quantum Cryptography Made Obsolete?

  1. Thanks again for bringing an interesting one into the foreground. I think when we look back on this period people will begin to appreciate that sticking the word “Quantum” in front of everything is not a guaranteed path to success! At some point the scales will fall from people’s eyes and they will realize that science and engineering proceed by a path of continuous improvement. Electrical engineering deals directly with the single largest value force around (electromagnetism). That is the one trillion dollar force! Gravity may keep our feet on the ground but you cannot (yet) engineer gravity. However, QED is thought of by physicists as “Ho Hum”, we done that. Really? The theory gives infinity to pretty much everything and has to be savagely beaten by mathematical trickery to give perhaps six or so landmark “results”. At some point the physics community will wake up and realize the work has not even started. We have still to make the engineers version of QED. The REALLY USEFUL theory. The one which makes sense and guides you directly to fresh engineering insight. I am glad to see this cryptography dark horse coming through. Perhaps it will shake up the Zealots so they become a little less cocksure about the value of endlessly cryptic quantum phenomenology with no real engineering results. Nature is actually made of very particular stuff and QED describes 99% of what matters. So work on it!

  2. i. This issue is becoming particularly timely because of the recent full cracks of practical quantum communicators, as shown in numerous peer-reviewed publications.

    So what would prohibit potential hackers from trying to attack the practical implementations of Kish Cypher Protocol (KCP) or variants? Before quantum key distribution (QKD) systems were built, quantum cryptography was also being advertised as an `unconditionally secure’ cryptographic primitive. However, vulnerabilities in practical quantum cryptosystems were exposed and exploited. And they arise due to:
    1. operational deficiencies, or
    2. inherent limitations of the components used to construct the physical device, and
    3. failings in the hardware/software implementation.
    The blinding attacks, calibration-loophole exploit, wavelength-dependent beamsplitter attack and Trojan-horse breaches performed on different quantum key distribution (QKD) systems are some examples.

    Agreed that the system in KCP seems simpler than a typical QKD system, but field installations can bring some totally-un-thought-of dependencies into the picture. These may then enable attacks of type 1 and 2; and a type 3 attack is obviously what ails any cryptographic system, be it classical or quantum, so it applies here as well.

    So on the one hand we have fully functional quantum cryptosystems and on the other an idea of how to perform KCP. I do think it is a bit indiscreet then to make a comparison.

    ii. 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

    Sorry, but I spotted two rather stark ironies in the above sentence:
    1. If KCP/KLJN was indeed such a wonderful idea then how come, even after 8 years of being propounded, one has hardly heard of any group worldwide working on implementations/extensions/rigorous security proofs. The 21st century heralded the massive-information age, so I don’t think it could have been due to the lack of dissemination.
    In contrast, I believe one could easily find a dozen or more research groups worldwide, that are <8 years old, that have been engaged in the research on quantum cryptography.

    2. Notice the list of references in the paper you have linked. Out of the 25 citations, only 4 do not contain Kish as an author. Sure, incremental research is the oft-practiced model these days — as it (usually) does make lot of sense apart from being financially practical — so citing your own work is quite natural. But having more than EIGHTY percent of your own publications in a paper could, in my opinion, only happen when:
    a) no one else in the world owns this magical machine/device that you do
    b) no one else in the world cares for what you do, or
    c) your scientific practices are poor (plain narcissism/not happy in
    citing rivals, even when they ought to be…)

    As I understand, all the research on building cryptographic schemes based on classical thermodynamic noise has so far been composed of mainly theoretical ideas. Also, in my opinion, the first idea is always the toughest, but if & when it is realized as a major breakthrough, then it naturally invites/attracts more brains to invest time & energy to further it (and obviously, brains come cheap but machines, especially almost-magical ones, don’t). But this unfortunately doesn’t seem to have happened in the present case, and given my earlier point, I would think either b) or c) is more likely than a).

    Finally, to make myself clear, I am not advocating that QKD is necessarily more secure than Kirchhoff-Law-Johnson-(like)-Noise key exchange. I admit that I didn’t have the time to read this paper or any of its predecessors, nonetheless, it is certainly possible that the KLJN-type protocols COULD rival/better all currently-known QKD protocols. And I also agree that funding in quantum cryptography research has certainly been unduly favorable despite the actual progress achieved so far being short of the promises made in the early days. But regardless of that, I think that as of now, both protocols ought to be tested not only on their on-paper merits but also their experimental achievements. And KCP clearly doesn’t have anything to show in the latter category, so before raising provocative questions, let’s wait until the battles with hackers commence.

    1. Jay Ann, you are raising some very good questions. The reason why I worded this title so provocatively is because I would like to flush out why this protocol has received comparatively little attention.

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