Quantum Key Distribution

1. Quantum Key Distribution Yet another method of generating a key.

2. Two major methods BB84 E91 Device between Alice and Bob generates entangled photons. Alice and Bob read photons as orthogonal or diagonal. They compare notes on which they chose. Keep results from the same ones. Alice generates entangled photons: orthogonal or diagonal. Bob reads photon as orthogonal or diagonal. They compare notes on which they chose. Keep results from the same ones.

3. Information reconciliation. • Cascade Protocol: • Divide Key into blocks. • Using public channel, check blocks for parity. • If parity violated, binary search. • If error is from previous block that matched parity, that block has two errors. • Recursively done, thus name. • Errors found, yet Eve gains more knowledge.

4. Privacy Amplification. • If key is found to have been compromised. • Use a universal hash function to cut key to shorter length. • Shortening amount determined by amount Eve likely has found.

5. Methods of Attack • Intercept and resend. • Man in the Middle • Photon Number Splitting. • Hacking. • Denial of Service.

6. Intercept and Resend • Eve reads photons sent to Bob in orthogonal or diagonal. • Resends photon in same state she read. • 25% error rate found between Alice and Bob: 50% chance of right angles, if wrong, 50% chance that Bob reads it right anyways. • Pd=1-(3/4)n where n is number of bits. • In order for Pd=0.999999999, n≥72.

7. Man in the Middle • When there is no authentication, quantum systems cannot tell friend or foe any better than classical systems. • If Alice and Bob have no shared secret, they cannot make a secure line.

8. Photon number splitting • Only works on BB84. • Photons sent via laser pulses. • Laser pulses are .2 photons, actual numbers follow Poisson. • Many 0s, some 1s, some 2+ photon pulses. • Eve reads off singles of the 2+ photon pulses.

9. Hacking • Equipment tampered with to not generate actual random numbers. • Trojan Horse: shine light on Alice’s emitter to find her state. • Faked State • Phase remapping • Time Shift

10. Denial of Service • Without any lines of communication, no key can be made. • Not a security issue so much as a communication issue.

11. Overall value of QKD • Worthwhile for instantaneous generation of key over a distance. • Otherwise, one-use keys just as secure. • QKD currently feasible up to 148km in fiber-optic. • Could be used to generate keys between servers. • Still subject to brute-force hacking.