1. COMPSCI 290.2: 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. What if the wrong filter is used?or
(blocked)
(equal probability)
measure diagonal
measure rectilinear
destroys state

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:
Alice sends Bob a stream of photons randomly polarized in one of 4 polarizations:
Bob measures the photons in random orientations e.g.: x + + x x x + x (orientations used) \ | - \ / / - \ (measured polarizations)
(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)
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, what a lucky guess!
Use a quantum circuit to find smallest r > 1 such that ar = 1 mod n
If r is odd or 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. 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?)