Quantum Key Distribution

1
Quantum Key Distribution

• Yet another method of generating a key.

2
Two major methods

• BB84

• E91

• 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.

• 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.

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.
• Pd1-(3/4)n where n is number of bits.
• In order for Pd0.999999999, n72.

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 Alices 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.