QUANTUM CRYPTOGRAPHYClassic CryptographyQuantum vs. ClassicSlide 4Fundamental ConceptsSlide 6How it works?BB84Quantum Key DistributionVerification of QCReferencesQUANTUM CRYPTOGRAPHYNarayana D KashyapSecurity through UncertaintyCS 265Spring 2003Classic CryptographyKeys by Transposition and SubstitutionStrength mainly on Long Keys KDCMathematical Concepts Allows the eavesdropper in principle to measure physical properties without disturbing them.Quantum vs. ClassicPhysics instead of MathExchange of information – very secure in a strong sense.Laws of physics guarantee (probabilistically) that the secret key exchange will be secure.The ACT OF MEASUREMENT is an integral part of quantum mechanics, not just a passive, external process as in Classic Crypto.Fundamental ConceptsQuantum Channel along with Quantum theoryHeisenberg Uncertainty Principle – certain pairs of physical properties are complementary.Any effort to monitor the channel necessarily disturbs the signal in some detectable way. The uncertainty principle is used to build secure channel based on Quantum properties of light.Photons & PolarizationHow it works?Bob, informed about polarization being either 0 or 90 (rectilinear), can find out by his photomultiplier as to how it was sent.Such an apparatus is useless for distinguishing 45 or 135 (diagonal) photons - unless the apparatus is turned 45 degrees.Rectilinear and Diagonal polarizations are complementary properties.BB84Bennett and Brassard proposed in 1984 Quantum channel – Polarized PhotonsPublic channel – Normal MessagesAlice = Sender & Bob = receiverAgreed before hand that 90 and 45 are 1’s 0 and 135 are 0’s 1 1 0 0 1 AliceBobAliceBobQuantum Key DistributionVerification of QCAlice and Bob to compare the "parity"-evenness or oddness of a publicly agreed on random subset containing about half the bits in their data. Alice could tell Bob, "I looked at the 1st, 3rd, 4th, 9th,...996th and 999th of my 1,000 bits of data, and they include an even number of l's." It suffices to repeat the test 20 times, with 20 different random subsets, to reduce the chance of an undetected error to less than one in a million.ReferencesQuantum Cryptographyhttp://www.cyberbeach.net/~jdwyer/quantum_crypto/quantum1.htmQuantum Cryptography Tutorialhttp://www.cs.dartmouth.edu/~jford/crypto.htmlOxford Quantum Computation Group
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