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Practical attacks on quantum key distribution systems


Quantum cryptography allows guaranteely secure distribution of a secret key over an open optical channel.  The security against eavesdropping is based on the known laws of physics and is confirmed by rigorous theoretical proofs.  However, the model of legitimate users' equipment used in the proofs has so far been limited.  The proofs have assumed idealized models of optical and electrooptical components in legitimate users' setups and have omitted some component imperfections.  These omitted imperfections, as it has been shown, open possibilities for successful attacks.

Most quantum cryptosystems today contain two or more single photon detectors.  In this talk, I will consider two non-idealities of single photon detectors, and how Eve can exploit them.  The first non-ideality is a dependence of relative efficiency of '0' detector versus '1' detector on an external parameter controllable by Eve (e.g., timing of the incoming pulses).  The second non-ideality is a saturation behavior of a passively-quenched avalanche photodiode, where it becomes completely blinded by a moderately strong light.  I illustrate both imperfections with experimental data, show how Eve can construct successful attacks using them, and present some calculations on how strong the non-ideality should be to allow for a successful attack.  I also consider countermeasures legitimate users could devise.