Sunday, August 7, 2011

Stealing quantum keys

PhysicsWorld reports:
A straightforward and effective way for eavesdroppers to copy secret keys from quantum cryptographic systems has been tested by physicists in Germany. The technique does not require intercepting the quantum code itself, but merely sending a series of well timed weak light pulses to blind the receiver's detectors and then listening to a public, unencrypted message between sender and receiver. ...

This approach is used in a number of commercial encryption devices, and has been used in a limited number of financial and administrative transactions. However, while uncrackable in principle, it has been shown by various research groups to be vulnerable in practice, particularly because of limitations of Bob's single-photon detectors. Last month, for example, Christian Kurtsiefer of the National University of Singapore and colleagues used bright light to "blind" the avalanche photodiodes within the detectors, allowing them to manipulate Bob's measurements and steal the key without revealing their presence.
Quantum cryptography is supposed to be provably secure encryption based on the physics of quantum mechanics. But it is not, and it is useless. Every commercial system has been broken, and there is no practical use for any of them. This is just the latest break.

A typical exaplanation of quantum cryptography starts with:
Nobody understands quantum theory. - Richard Feynman, Nobel prize-winning physicist

Electromagnetic waves such as light waves can exhibit the phenomenon
of polarization, in which the direction of the electric field vibrations
is constant or varies in some definite way. A polarization filter is a
material that allows only light of a specified polarization direction to
pass. If the light is randomly polarized, only half of it will pass a
perfect filter.

According to quantum theory, light waves are propagated as discrete particles known as photons.
No, quantum theory does not say that light is propagated as discrete particle. The theory used to be known was wave mechanics, because everything in the theory propagates as waves, not particles. The light only appears discrete when it is observed, ie, when it is absorbed or emitted.

A polarization filter does not allow only light of a specified polarization direction to pass. In quantum lingo, the filter does an observation on the spin of the photons, thereby changing its spin. If you think of the filter as blocking some light and letting other light pass unchanged, then you will get conclusions that are contradicted by simple experiments.

Unpolarized light is not randomly polarized. Saying that light is randomly polarized suggests that it is a mixture of photons, each of which is polarized in some particular direction. But that is not what ordinary light like sunlight is.

Explanations of quantum mechanics seem to always say these nonsensical things, and then they quote Feynman in order to justify saying some incomprehensible. Yes, Feynman said that, but he only meant that he does not understand the theory as well as he would like.

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