Slashdot Mirror
Quantum Cryptography Broken, and Fixed
schliz writes in with research out of Sweden in which researchers showed that, looking at a quantum cryptographic system as a whole, it was possible for an eavesdropper to extract some information about the QC key, thus reducing the security of the overall system. The team then proposed a cheap and simple fix for the problem. "The advanced technology was thought to be unbreakable due to laws of quantum mechanics that state that quantum mechanical objects cannot be observed or manipulated without being disturbed. But a research team at Linköping University in Sweden claim that it is possible for an eavesdropper to [get around the limitations] without being discovered. In a research paper, published in the international engineering journal IEEE Transactions on Information Theory (abstract), the researchers propose a change in the quantum cryptography process that they expect will restore the security of the technology."
The title of their paper is "Security Aspects of the Authentication Used in Quantum Cryptography." That would make an awesome title for a book that aimed to cover every single security aspect of the authentication used in QC, but not a paper that simply points out that (duh!) you shouldn't allow the eavesdropper to see the key.
python -c "x='python -c %sx=%s; print x%%(chr(34),repr(x),chr(34))%s'; print x%(chr(34),repr(x),chr(34))"
Lasing is a quantum effect. If they weren't positively blase, we would probably call them quantum lasers, and then you would be in trouble.
Nerd rage is the funniest rage.
Just like the last time, the laws of quantum physics still work and it is still impossible to observe a quantum system without altering it. The researchers found that the classical authentication protocols that prevent man-in-the-middle attacks were insufficient.
This bit from the article sounds like they just added an initialization vector (see wikipedia for definition):
"The researchers propose an additional, non-quantum exchange of a small amount of random bits that are separate from the quantum key."
There was an interesting book on cryptography which I loaned to a friend, that surmised that the law of cryptography which state that every code can be broken is now defunct due to quantum cryptography.
This in effect means that the science of cryptography has met its end in terms of development.
Like the game of checkers, there are no more moves to make.
At the time of publication (2002?), the longest distance an encrypted quantum message sent and received was approximately 50kms and considered to be impossible to break.
Don't be apathetic. Procrastinate!
But how do you transmit that pad between parties?
That is exactly the point of quantum cryptography. The cryptographic key is the one time pad, negotiated between two parties, using superposition (and in some cases entanglement) in order to come to agreement on the pad and at the same time detect evesdroppers.
1. Alice sends the key to Bob, in the open, unencrypted, but using a random base-4 encoding. There are two states for a 1 bit and two states for a 0 bit.
2. Bob reads the key, but, due to the random encoding, he can read only half of it (you can read only if the receiver is in the same state as the sender), so Bob sees some random subset of the bits. This random subset is the key. Alice does not know which subset this is.
3. Bob transmits the configuration he used to read the stream back to Alice. Alice compares the configuration to her own configuration for sending data and derives which bits Bob saw. They now both know the key.
It is impossible to read the bits without changing them, in which case Bob will see something different from what was sent, so the keys won't match.
It is also impossible to derive the key from the configuration that is sent back by Bob because it only specifies how the bits were read, not what the bits were.
This is, of course, vulnerable to a man-in-the-middle attack, however.
The thing is you can only accurately read about 50% of the photons.
When Eve reads the message changes to 50% correct, 50% incorrect.
When Bob gets the photons his 50% will consist of 25% correct and 25% incorrect ones. (assuming true randomness)
When Alice and Bob compare there keys they will see the discrepancy.
Then the 1 and 0 are XORs with the message and then the result is sent.
http://en.wikipedia.org/wiki/Quantum_cryptography#Polarized_photons_-_Charles_H._Bennett_and_Gilles_Brassard_.281984.29
My Transformation Website
Kindle Books http://www.catprog.org/rev
Interactive CYOA http://www.catprog.org/st
Eve cannot read the stream because 0s and 1s are sent, shall we say, in two coordinate systems (bases) randomly chosen by Alice. The receiver, be it Bob or Eve, cannot in principle measure these basis, only guess them (randomly). If you guess right, you correctly receive 0 or 1. If you guess wrong, you receive garbage. After the transmission Alice and Bob tell each other (over a classical channel) the bases they chose for each bit, and they discard the bits for which they chose different bases. Then they check (and discard) some subset of bits for discrepancies. If Eve was measuring the stream during the transmission, she would inevitably introduce errors by wrongly guessing some bases. Therefore, if error rate is higher than a certain threshold, Alice and Bob conclude that their communication was eavesdropped and discard the transmission altogether.
Then there's the separate question of Eve messing with the classical communication between Alice and Bob, but AFAIK it has also been successfully dealt with.
See http://www.mai.liu.se/~jalar/qkg/faq.html?lang=en