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1. Quantum Cryptography
Ranveer Raaj Joyseeree & Andreas Fognini
Alice
Bob
Eve

2. Classical Algorithms
1. Asymmetrical (public-key) cryptosystems:
- First implementationnn RSA (Ronald Rivest, Adi Shamir, and Leonard Adleman) 1978
- Very convenient, Internet
- Idea is based on computational complexity f(x) = y, x = ?.
- rely on unproven assumptions
Private Public
Message
Message Encrypted message

3. Classical Algorithms
Classical Algorithms
2. Symmetrical (secret-key) cryptosystems:
- only provably secure cryptosystem known today
- not handy, key as long as message
- key only valid for one transmission
- how to send the key in a secure manner?
M: 1 0 1 0 1 0 1 0
K: 1 0 0 0 1 1 1 0
S: 0 0 1 0 0 1 0 0
Distribute key over secure channel
M
M S
S: 0 0 1 0 0 1 0 0
K: 1 0 0 0 1 1 1 0
M: 1 0 1 0 1 0 1 0
XOR XOR

4. Quantum Cryptography: The BB84 Portocol
Ingredients: 1) One photon no copying,
2) Two non orthonormal bases sets
3) Insecure classical channel; Internet
What it does: Secure distribution of a key, can't be used to send messages
How it works:
50% correlated
Physikalische Blätter 55, 25 (1999)

5. Copy machine: e.g.
Eve's copy machine
50% decrease
in correlation!
Alice and Bob recognize
attack from error rate!

6. Conclusion
 Quantum cryptography means just the
exchange of keys
 Actual transmission of data is done with
classical algorithms
 Alice & Bob can find out when Eve tries to
eavesdrop.

7. Hacking Quantum Key Distribution systems
 QKD systems promise enhanced security.
 In fact, quantum cryptography is proveably
secure.
 Surely one cannot eavesdrop on such systems,
right?

8. Hacking QKD systems
 Security is easy to prove while assuming
perfect apparatus and a noise-free channel.
 Those assumptions are not valid for practical
systems e.g. Clavis2 from ID Quantique and
QPN 5505 from MagiQ Technologies.
 Vulnerabilities thus appear.

9. Hacking by tailored illumination
 Lydersen et al. (2010) proposed a method to
eavesdrop on a QKD system undetected.
 The hack exploits a vulnerability associated with
the avalanche photo diodes (APD‘s) used to
detect photons.

10. Avalanche photo diodes
 Can detect single photons when properly set.
 However, they are sensitive to more than just
quantum states.

11. Modes of operation of APD’s
 Geiger and linear modes

12. Geiger mode
 VAPD is usually fixed and called bias voltage and in Geiger mode, Vbias > Vbr.
 An incident photon creates an electron-hole pair, leading to an avalanche of carriers
and a surge of current IAPD beyond Ith. That is detected as a click.
 Vbias is then made smaller than Vbr to stop flow of carriers. Subsequently it is restored
to its original value in preparation for the next photon.

13. Linear mode
 Vbias < Vbr.
 Detected current is proportional to incident optical power Popt.
 Clicks again occur when IAPD > Ith.

14. Operation in practical QKD systems
 Vbias is varied as shown such that APD is in Geiger mode only
when a photon is expected
 That is to minimize false detections due to thermal fluctuations.
 However, it is still sensitive to bright light in linear mode.

15. The hack in detail
 Eve uses an intercept-resend attack.
 She uses a copy of Bob to detect states in a random basis.
 Sends her results to Bob as bright light pulses, with peak power
> Pth, instead of individual photons.
 She also blinds Bob‘s APD‘s to make them operate as classical
photodiodes only at all times to improve QBER.

16. The hack in detail
 C is a 50:50 coupler used in phase-encoded QKD systems.
 When Eve‘s and Bob‘s bases match, trigger pulse from Eve constructively interferes
and hits detector corresponding to what Eve detected.
 Otherwise, no constructive interference and both detectors hit with equal energy.
 Click only observed if detected current > Ith.

17. The hack in detail
 Clicks also only observed when Eve and Bob have
matching bases.
 This means Eve and Bob now have identical bit values
and basis choices, independently of photons emitted
by Alice.
 However, half the bits are lost in the process of
eavesdropping.

18. Performance issues?
 Usually, transmittance from Alice to Bob < 50%.
 APDs have a quantum efficiency < 50%.
 However, trigger pulses cause clicks in all cases.
 Loss of bits is thus compensated for and Eve stays
undetected

19. Other methods
 Method presented is not the only known exploit.
 Zhao et al. (2008) attempted a time-shift attack.
 Xu et al. (2010) attempted a phase remapping attack.

20. Conclusion
 QKD systems are unconditionally secure, based on
the fundamental laws of physics.
 However, physical realisations of those systems
violate some of the assumptions of the security proof.
 Eavesdroppers may thus intercept sent messages
without being detected.

21.  Rev Mod Phys 74, 145 (2002)
 Physikalische Blätter 55, 25 (1999)
 Nature Photonics 4, 686 (2010)
 Experimental demonstration of phase-remapping attack in a
practical quantum key distribution system. Xu et al. (2010)
 Hacking commercial quantum cryptography systems by tailored
bright illumination. Lydersen et al. (2010)
 Quantum hacking: Experimental demonstration of time-shift
attack against practical quantum-key-distribution systems. Zhao
et al. (2008).
Used Material