The Research On Hacking And Defence In Practical Quantum Key Distribution Systems

Quantum key distribution (QKD), as the core of quantum cryptography which is theconnection of quantum mechanics and classical cryptography, admits two remote partiesto share unconditional secret key. Although the unconditional security of QKD is guaran-teedbythequantummechanicsandhasbeenprovedintheorybasedonsomeassumptions,the practical system is imperfect which will violate some assumptions. Generally speak-ing, any deviation between the standard security analysis and the practical system can beexploited by Eve to spy the secret key. In this dissertation, we study the quantum attackand defence for the practical QKD system, and then the main results are listed as follow-ing:1. We established a two-way QKD system, and distribute the key between Alice andBob who are connected by50km and100km fiber respectively. This system is verystable and the error rate is very low. At the same time, we replace the faraday mirrorwhich is used in most two-way QKD system with a ring composed by a polarizationbeam splitter and a faraday rotator, thus our system has two advantages: first, Alicecan use the polarization dependent phase modulator to encode her information (ingeneral two-way QKD system, Alice must use the polarization independent phasemodulator which is not the commercial setup); second, our system is immune fromthe passive faraday mirror attack.2. The faraday mirror (FM) plays a very important role in maintaining the stabilityof two way plug-and-play QKD system. However, the practical FM is imperfect,which will not only introduce additional quantum bit error rate (QBER) but alsoleave a loophole for Eve to spy the secret key. In this dissertation, we propose apassive faraday mirror attack in two way QKD system based on the imperfectionof FM. Our analysis shows that, if the FM is imperfect, the dimension of Hilbertspace spanned by the four states sent by Alice is three instead of two. Thus Evecan distinguish these states with a set of POVM operators belonging to three di-mensional space, which will reduce the QBER induced by her attack. Furthermore,a relationship between the degree of the imperfection of FM and the transmittanceof the practical QKD system is obtained. The results show that, the probability that Eve loads her attack successfully depends on the degree of the imperfection ofFM rapidly, but the QBER induced by Eve’s attack changes with the degree of theimperfection of FM slightly.3. Phase randomization is a very important assumption in the BB84QKD system witha weak coherent source. Thus an active phase modulator is needed to randomizethe phase of source. However, it is hard to check whether the phase of source israndomized totally or not in practical QKD systems. In this dissertation, a partiallyrandom phase attack is proposed to exploit this imperfection. Our analysis showsthat Eve can break the security of a two-way QKD system by using our attack, evenif an active phase randomization is adopted by Alice. Furthermore, the numericalsimulation shows that in some parameter regimes, our attack is immune to the one-decoy-state method.4. In order to ensure the phase of source is totally randomized and remove the par-tially random phase attack, a monitored active phase randomization in the practicalQKD system is necessary. In this dissertation, a stable and monitored active phaserandomization scheme for the one-way and two-way QKD system is proposed anddemonstrated in experiments. Furthermore, our scheme gives an easy way for Aliceto monitor the degree of randomization in experiments. Therefore, we expect ourscheme to become a standard part in future QKD systems due to its secure signifi-cance and feasibility.5. Thephasecodingquantumcryptographicschemeusingthehomodynedetectionandweak coherent state provides the simplest continuous variable quantum key distri-bution scheme from the experimental side. However, the inherent loss of practicalsystem will not only increase the bit error rate (BER) but also affect the security offinal key. In this dissertation, we propose a single-photon-detection attack, then thesecurity of final key will be compromised in some parameter regimes. Our resultsshow that the BER induced by Eve can be lower than the inherent BER inducedby the loss of system in some parameter regimes. Furthermore, our attack gives themaximalcommunicationdistanceofthisschemeforgivenexperimentalparameters.