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UVA CS 588 - Lecture 22

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Slide 1MenuVisual CryptographyKey + CiphertextPerfect Cipher?Perfect CipherAuthentication for remote votingDoing Encryption without ComputersRemote Voting SystemAuthentication by TransparencyQuantum CryptographyQuantum Physics for DummiesHeisenberg’s Uncertainty PrincipleQuantum CashPhoton PolarityPhoton StreamSlide 17Bank Verifies BillCounterfeiting Quantum CashPerfect Security?Guessing One +45º PhotonQuantum Key DistributionSlide 23Slide 24Detecting PhotonsFinding Correct GuessesWhat about Eve?Eve is CaughtIs this practical?Slide 30Movie TeaserWhat’s in the “Sneakers” Black Box?Quantum ComputingCryptographic Hashing AttacksCharge28 April 200528 April 2005CS588 Spring 2005CS588 Spring 2005David EvansDavid Evanshttp://www.cs.virginia.edu/evanshttp://www.cs.virginia.edu/evansPhun with Phun with PhotonsPhotons2CS588 Lecture 22Menu•Visual Cryptography•Quantum Cryptography•Quantum Computing (very briefly)•Cryptographic Hashing Attacks–Boyd and Isabelle3CS588 Lecture 22Visual Cryptography•Can we quickly do a lot of XORs without a computer?•Yes:0:1: Key Ciphertext Key Ciphertext.5 probability .5 probability4CS588 Lecture 22Key + Ciphertext Key Ciphertext Key Ciphertext+ ++ += 0= 15CS588 Lecture 22Perfect Cipher? Key Ciphertext Key Ciphertext.5 probability .5 probabilityPlaintext016CS588 Lecture 22Perfect Cipher Key Ciphertext Key Ciphertext.5 probability .5 probabilityPlaintext01P (C = | M = 0) = .5 P (C = | M = 1) = .5P (C = | M = 0) = .5 P (C = | M = 1) = .5Yes!==7CS588 Lecture 22Authentication for remote voting•Remote voting offers convenience –69% votes cast by mail in 2001 in state of Washington•Electronic voting is cheaper and faster–More secure?–New problems: virus, worm, spoofing, denial of service•Mutual authentication–Voter authenticated to server–Server authenticated to voterNathanael Paul, David Evans, Avi Rubin and Dan Wallach. Workshop on Human-Computer Interaction and Security Systems. 6 April 2003 http://www.cs.virginia.edu/evans/pubs/remote-voting.html8CS588 Lecture 22Doing Encryption without Computers•Can’t trust voters to have trustworthy computers–Viruses can tamper with their software•Need to do authentication in a way that doesn’t depend on correctness of user’s software•Lorenz cipher: use XOR to encrypt–Is there a way to do lots of XOR’s without a computer?9CS588 Lecture 22Remote Voting SystemEk (k1)Ek(kn)Ek(k2)……STEP 1 keyskiSki = “AQEGSDFASDF”SSTEP 2STEP 3 – if ki valid…STEP 4Key: AQEGSDFASDFki =client machine client machineEach voter is sent a key, ki10CS588 Lecture 22Authentication by Transparency11CS588 Lecture 22Quantum Cryptography12CS588 Lecture 22Quantum Physics for Dummies•Light behaves like both a wave and a particle at the same time•A single photon is in many states at once•Can’t observe its state without forcing it into one state•Schrödinger’s Cat–Put a live cat in a box with cyanide vial that opens depending on quantum state–Cat is both dead and alive at the same time until you open the box13CS588 Lecture 22Heisenberg’s Uncertainty Principle“We cannot know, as a matter of principle, the present in all its details.”Werner Heisenberg, 1920sIf you can’t know all the details about something you can’t copy it.Bits are easy to copy; photons are impossible to copy.14CS588 Lecture 22Quantum CashStephen Wiesner, late 60s:“I didn’t get any support from my thesis advisor – he showed no interest in it at all. I showed it to several other people, and they all pulled a strange face, and went straight back to what they were already doing.”(Quoted in Singh, The Code Book)15CS588 Lecture 22Photons have “spin”: V H +45º -45ºPhoton PolarityVertical filter:100% of V photons 50% of +45º photons (become V photons) 50% of -45º photons (become V photons) 0% of H photonsHorizontal filter:100% of H photons 50% of +45º photons (become H photons) 50% of -45º photons (become H photons) 0% of V photons16CS588 Lecture 22Photon StreamVertical filter:100% of V photons 50% of +45º photons (become V photons) 50% of -45º photons (become V photons) 0% of H photonsCan’t tell differencebetween V and +45ºand –45º photons17CS588 Lecture 22Quantum CashUncertainty Principal BankUncertainty Principal Bank$10000$10000$10000$10000$10000$10000$10000$10000In Dice We TrustIn Dice We TrustUnique ID258309274917392Spinning PhotonsRichard FeynmanRichard FeynmanSafecracker, Father of Quantum ComputingSafecracker, Father of Quantum Computing18CS588 Lecture 22Bank Verifies BillUnique ID258309274917392Spinning PhotonsUncertainty PrincipalID AmountPhotons… … …258309274917392$10000V-45H+45+45V… … …Bank aligns filters according to expected values. If photons onbill all pass through filters, the bill is valid.19CS588 Lecture 22Counterfeiting Quantum Cash•To copy a bill, need to know the photons.•Counterfeiter can guess, but loses information. Physics says there is no way to measure the spins without knowing them!20CS588 Lecture 22Perfect Security?•Bill photons: V (¼), +45 (¼), -45 (¼), H (¼)•Guess V-filter: passes 100% of V photons, ½ of +45 and ½ of -45–p (M = V | passes V filter) = .25 / (.25 + (.5 * .25) + (.5 * .25)) = .25/.5 = .5If photon passes, counterfeiter can guess it is a V photon, right ½ of the time. If photon doesn’t pass, guess it’s a H photon, right ½ of the time.–p (M = +45 | passes V filter) = .25•Actually a bit more complicated – can guess some photons wrong, and 50% chance bank won’t notice.21CS588 Lecture 22Guessing One +45º Photon•Passes through V-filter (.5)–Counterfeiter guesses V-photon–Passes through Banks +45 filter (.5)–.25 chance of getting it right•Doesn’t passes through V-filter (.5)–Counterfeiter guesses H-photon–Passes through Banks +45 filter (.5)–.25 chance of getting it right•Probability of not getting caught = .5•Forge bill with 6 photons = 1/26; use more photons for more valuable bills.22CS588 Lecture 22Quantum Key Distribution23CS588 Lecture 22Quantum Key Distribution•Charles Bennett (1980s)•Use quantum physics to transmit a key with perfect secrecy•Alice sends a stream of random photons•Bob selects random filters to try and guess photons•After, they communicate over insecure channel to figure out which bits were transmitted correctly24CS588 Lecture 22Quantum Key Distribution1. Alice generates a


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