Security: 'Half-Quantum' Cryptography Promises Total Security; quantum-encrypted key only

'Half-Quantum' Cryptography Promises Total Security
New Scientist (10/21/07) Marks, Paul

Many cryptographers believed that the only way to achieve complete security when transmitting information was to use quantum cryptography to share the key used for encryption. However, researchers say they can achieve the same level of security even if one party stays in the world of classical physics. In conventional quantum cryptography, a sender, dubbed Alice, generates a string of 0s and 1s and encodes them using a photon polarized in either the computational "basis" in which 0 and 1 are represented by vertical and horizontal polarizations, or in diagonal bases in which 1 and 0 are represented by 45 degree and negative 45 degree polarizations. When the photons arrive at their destination, the receiver, dubbed Bob, chooses either the computational or diagonal bases to measure each one, telling Alice which he has chosen. If the chosen basis is wrong, Alice tells Bob to discard that bit. The bits that are guessed correctly form the secret key. If an eavesdropper intercepts any photons, the stream is interrupted and Bob's ability to read a number of the photons he might have read correctly is destroyed. The increase in unreadable photons tells Bob the communication channel has been compromised. Researchers at the Israel Institute of Technology in Haifa and the University of Montreal have demonstrated that only Alice needs to be quantum-equipped. Alice encodes the bits as usual, though Bob can only use the computational basis. Bob randomly measures some of the received photons and returns the rest to Alice untouched. The bits read in the computational basis form the key. The system is still secure because anyone eavesdropping does not know which photons will be returned to Alice unmeasured.
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