Waiting for the Quantum Leap

For the longest time I have the suspicion that quantum cryptography, although a neat idea, is overrated. I was keeping an eye into developments (see previous posts) just in case. currently my impression is that, with the current, technology, QC is an expensive proposition for the added value it provides. It looks like I am in good company on this. In the October issue of Wired, Bruce Schneier writes a commentary piece where he asserts:

While I like the science of quantum cryptography — my undergraduate degree was in physics — I don’t see any commercial value in it. I don’t believe it solves any security problem that needs solving. I don’t believe that it’s worth paying for, and I can’t imagine anyone but a few technophiles buying and deploying it. Systems that use it don’t magically become unbreakable, because the quantum part doesn’t address the weak points of the system.

Security is a chain; it’s as strong as the weakest link. Mathematical cryptography, as bad as it sometimes is, is the strongest link in most security chains. Our symmetric and public-key algorithms are pretty good, even though they’re not based on much rigorous mathematical theory. The real problems are elsewhere: computer security, network security, user interface and so on.

Moreover I have a nagging question about the fundamental tenet of quantum cryptography. The principle is that Alice and Bob will know for sure that Eve is eavesdropping in their channel because their bits will be changing as required by the uncertainty principle. Eve may be out of luck in getting the secrets as Bob and Alice will certainly decide not to exchange them in her presence. However, the mischievous Eve may decide that she is quite happy with only preventing the exchange. I will call this a denial of channel attack by which Eve can prevent Alice and Bob to exchange any secret until the police figures out where she is tapping the quantum line and force her to stop. Eve-hacker can now start a cat and mouse chase, that judging from the record on netting hackers by the internet police, is lopsided on Eve’s favor.

A mathematical note aside, Schneier mentions in his article the Bennet-Brassard and key reconciliation algorithms used by quantum cryptography. In a paper written with A. Bruen and D. Wehlau we gave rigurous proof of convergence for the Bennet-Bessete-Brassard-Salvail and Smolin (BBBSS92)method. These results and more about quantum cryptography also appear on the our book.


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