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Quantum Cryptography Outperformed By Classical Technique

The secrecy of a controversial new cryptographic technique is guaranteed, not by quantum mechanics, but by the laws of thermodynamics, say physicists

  • June 14, 2012

Quantum cryptography has had a bad couple of years. For a decade or so, we were promised the capability to send messages with absolute secrecy guaranteed by the laws of physics. At least in theory.

In practice, however, things turned out a little differently.  In 2010, a team at the University of Toronto in Canada announced that they had successfully hacked a commercial quantum cryptography system. The problem was not the theory but the practical limitations of the equipment used to carry out this kind of communication and the loopholes this introduces. 

Then, earlier this year, a UK-based team showed that these kinds of practical limitations can never be overcome entirely since there is no way to prove beyond doubt  that any machine is not compromised (unless it is used only once and then thrown away).

So rather than being perfect, quantum cryptography turns out to be just ‘pretty good’, a standard that is perfectly acceptable for most people and one that very much looked as if it  was the best we ever can hope for.

Now quantum cryptography has a rival. Today, Laszlo Kish at the Texas A&M University in College Station and a few pals outline another way to send information, which they claim guarantees complete security. 

Once again the secrecy is guaranteed by the laws of physics but instead of quantum mechanics, Kish and co say the second law of thermodynamics provides the necessary underwriting. That’s the same law that prohibits perpetual motion machines powered by heat from the environment.

The idea is straightforward. Alice wants to send Bob a message via an ordinary wire. At each end of the wire, there are two different resistors that correspond to a 0 or 1. 

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 crucial part of this set up is that the actual current and voltage through the wire is random, ideally Johnson noise. The essential features of this noise are determined by the combination of resistors at each end. This noise is public–anybody can see or measure it.

Now here’s the clever bit. Bob knows which resistor he connected to the wire and so can work out which resistor Alice must have connected. 

But  Eve, who is listening in to the publicly available noise, does not know which resistor was connected at each end and cannot work it out either because the laws of thermodynamics prevent the extraction of this information from this kind of signal.

What’s more, any kind of active attack that might interrogate the resistors at each end always introduces energy into  the system that Alice and Bob can easily spot. That allows them to guarantee the secrecy, even when they send only a single bit.

Of course, there are certain practical limitations to any kind of experimental set up. However, Kish and co have measured the information leakage that this causes  and say it amounts to no more than 0.19 per cent. 

They say, perhaps a little harshly,  that this compares favourably to commercial quantum cryptography systems that have been hacked completely.

This is certainly an interesting approach that is attracting some attention: the paper on the arXiv is an invited talk for the 5th IEEE Workshop on Soft Computing Applications in August,  

However, the claim from Kish and co that this new approach is unconditionally secure is an ambitious one, which  they spend a significant part of the paper attempting to justify.

The idea of using the laws of physics to guarantee the secrecy of a message is as old as the hills. But the approach is only as good as the laws you choose. 

For example,  the laws governing the bulk properties of iron offer pretty good protection, which is why iron safes are a common way of protecting information. But anybody who knows how  to break iron can access these secrets.

How good is the protection offered by the laws of thermodynamics? At first glance it looks pretty solid. That’s demonstrated by the fact that nobody has been able to find a way of consistently extracting energy from the environment.

But there are various ways of extracting energy from the environment under certain localised conditions. A couple of years ago we looked at this experiment which uses Brownian motion to push a bead up a staircase. It doesn’t break the laws of thermodynamics but certainly  bends them. Could it be that Kish and co’s approach is vulnerable to similar kinds of attack?

Various groups have tried to find a flaw in the method, all them unsuccessful. If it turns out that it does off unconditional security, there will be a large group of investors in quantum cryptography wondering whether they should ask for their money back. 

Ref: arxiv.org/abs/1206.2534 : Information Theoretic Security By The Laws Of Classical Physics

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