1 / 19

Computer Security

Learn about Quantum Cryptography's use in secure communication, the BB84 Protocol, detection of eavesdroppers, and the Uncertainty Principle in Quantum Mechanics.

jvickers
Télécharger la présentation

Computer Security

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Computer Security “Quantum Cryptography” including Quantum Communication Quantum Computing

  2. Quantum Communication • NOT used to encrypt data! • Goal, instead, is to detect eavesdroppers • Can be used to exchange a private key

  3. Uncertainty Principle • In quantum mechanics, certain pairs of properties of particles cannot both be known simultaneously, e.g., • Position and momentum of an electron (Heisenberg) • If a measurement determines (with precision) the value of one of the properties, then the value of the other cannot be known

  4. Photon Spin (Polarization) • Photons can be given either “rectilinear’’ or “diagonal’’ spin as they travel down a fiber. • Rectilinear: or • Diagonal: or • Measuring rectilinear spin with a rectilinear filter yields polarization of photon. • blocked

  5. measurerectilinear • measurediagonal • destroys state What if the wrong filter is used? • or • (blocked) • (equal probability)

  6. Quantum Key Exchange • The goal is to enable Alice and Bob to agree on a private key, even in the face of an eavesdropper, Eve. • Like Diffie-Hellman, the protocol is still susceptible to a “man-in-the-middle” attack. • But unlike Diffie-Hellman, the protocol does not depend on the difficulty of computing discrete logarithms or any other computational problem.

  7. BB84 Protocol (Bennet and Brassard) • bit encoding: 0 1 0 1 • Alice sends Bob a stream of photons randomly polarized in one of 4 polarizations: • Bob measures the photons in random orientationse.g.: x + + x x x + x (orientations used) \ | - \ / / - \ (measured polarizations) 1 0 1 1 0 0 1 1 (encoded bit values) • Bob tells Alice in the open what orientations he used, but not what bit values he measured • Alice sends Bob in the open a list of positions at which the orientations are correct

  8. Detecting an Eavesdropper • Alice selects some subset of k of the shared bits and reveals them to Bob in the open. • If Bob notices any differences, then Eve must have changed a bit by guessing the wrong polarization when eavesdropping. • Eve has little hope of guessing the same polarization as Bob all k times. Each measurement has a ¼ chance of changing a bit value. The probability of not changing any values is (3/4)k – which can be very small if k is chosen large enough

  9. In the “real world” • In April 2014 China began installing a 2000-kilometer quantum communications link between Beijing and Shanghai • In August 2016 China launched the Quantum Science Satellite (QUESS) and plans to test quantum entanglement over large distances, and quantum key exchange

  10. Quantum Computers • The state of a computer consists of the contents of its memory and storage, including values of registers (including the program counter), memory, disk contents, etc. • In a conventional computer each memory “unit” holds one value (e.g., 0 or 1) at a time. Computation consists of a sequence of state transitions. • But in a quantum computer, a memory unit holds a “superposition” of possible values.

  11. Qubit (somewhat simplified) • A single quantum “bit” which is 1 with probability p and 0 with probability 1-p. • When measured, outcome is either 0 or 1. • Measuring a qubit changes its value! If outcome is 0, p is set to 0, if outcome is 1, p is set to 1. • A qubit could be implemented using a photon to carry a horizontal or vertical polarization.

  12. Quantum Entanglement • Suppose two bits have value 00 with probability ½ and 11 with probability ½. If the bits are separated and measured at different locations, the measurements must yield the same values. E.g., if first measurement is 0, second must also be 0. • Entanglement also allows multiple states (e.g., 00 vs. 11) to be acted on simultaneously. • Difficulty in building a quantum computer is maintaining quantum entanglement in the face of environmental noise (quantum decoherence).

  13. More on Qubits • A qubit is a superposition of two basis states, and (representing values 0 and 1), which can be thought of as north and south poles of a unit sphere. • I.e., qubit is , where v0 and v1 are complex numbers such that |v0|2 + |v1|2 = 1. (|v0|2 and |v1|2 are probabilities of qubit being 0 or 1) • https://commons.wikimedia.org/wiki/File:Bloch_sphere.svg • can be written as

  14. Quantum Gates • Qubits are manipulated with quantum logic gates. • Gates are just multiplications by unitary matrices. • Hadamard matrix maps to and to • i.e., gate operation is • This gate randomizes a basis state to have equal chance of being measured 0 or 1.

  15. Factoring Large Primes • In 1994 Peter Shor showed that a quantum computer can factor a number n in O(log3 n) time. • A similar result holds for solving the discrete logarithm problem. • If a large-enough quantum computer can be built, then RSA and Diffie-Hellman key-exchange will no longer be secure. • (But largest number factored with this algorithm as of 2015 was 21!)

  16. Details of Shor’s Algorithm • Pick a random number 1 < a < n • If a is a factor of n (i.e., GCD(a,n) > 1), what a lucky guess! • Use a quantum circuit to find smallest r > 1 such that ar = 1 mod n, i.e., find the order of a • If r is odd go back to step 1 • If ar/2 = -1 mod n go back to step 1 • GCD(ar/2 + 1, n) and GCD(ar/2 - 1, n) are factors of n • Example: n = 15, a = 7, r = 4 • a1 = 7 mod n, a2 = 4 mod n, a3 = 13 mod n, a4 = 1 mod n • a4/2+1 = 50, a4/2-1 = 48 • GCD(50,15) = 5, GCD(48,15) = 3

  17. Analysis of Shor’s Algorithm • Reduces factoring to order finding. Uses quantum computation to solve order finding. • After step 4, r is even, ar/2 ≠ 1 mod n. Proof: Otherwise r was not smallest positive integer such that ar = 1 mod n. Thus ar/2 - 1 ≠ 0 mod n. • After step 5, ar/2 ≠ -1 mod n. Thus ar/2 + 1 ≠ 0 mod n. • Observe that (ar/2 - 1)(ar/2 + 1) = ar – 1 = 0 mod n • So (ar/2 - 1)(ar/2 + 1) = kn for some integer k. • But neither (ar/2 - 1)or(ar/2 + 1) is a multiple of n. • Therefore by the prime factorization theorem, one contains p and the other contains q.

  18. Analysis of Shor’s Algorithm • What is the probability that if a is chosen at random in step 1, r (the order of a) is odd? • Claim: If the order of a is odd, the order of –a is even. • Proof: Let r’ be the order of –a, and suppose r’ is odd. Then ar’=(-1*-a)r’=-1*(-a)r’=-1 mod n. If r’ < r, then the order of a is r=2r’, and hence r is even, a contradiction. If r’ > r, then (-a)r=-1 mod n, so r’ = 2r, and hence r’ is even, another contradiction. • Thus at most half of the elements between 2 and n-1 have odd order.

  19. Controversial Quantum Computer • D-Wave Systems, Inc., purports to build a quantum computer based on a 128-qubit chipset. • No convincing demonstration of speed-up over conventional computer yet. • Unresolved debate about whether there is actually quantum entanglement among the qubits. (Evidence seems to be leaning towards yes?)

More Related