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Quantum Cryptography. Alice. Bob. Eve. Ranveer Raaj Joyseeree & Andreas Fognini. Classical Algorithms. 1. Asymmetrical (public-key) cryptosystems:. Message. Encrypted message. Message. Public. Private. - First implementationnn RSA (Ronald Rivest, Adi Shamir, and Leonard Adleman) 1978

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Quantum Cryptography




Ranveer Raaj Joyseeree & Andreas Fognini

Classical Algorithms

1. Asymmetrical (public-key) cryptosystems:


Encrypted message




- 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

Classical Algorithms

Classical Algorithms

2. Symmetrical (secret-key) cryptosystems:

Distribute key over secure channel






- 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?

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)

Eve's copy machine

Copy machine:


50% decrease

in correlation!

Alice and Bob recognize

attack from error rate!


  • 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.

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?

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.

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.

Avalanche photo diodes

  • Can detect single photons when properly set.

  • However, they are sensitive to more than just quantum states.

Modes of operation of APD’s

  • Geiger and linear modes

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.

Linear mode

  • Vbias < Vbr.

  • Detected current is proportional to incident optical power Popt.

  • Clicks again occur when IAPD > Ith.

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.

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.

The hack in detail

  • Cis 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.

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.

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

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.


  • 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.

Used Material

  • Rev Mod Phys 74, 145 (2002)

  • PhysikalischeBlätter 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).