Unconditional Security of Continuous-Variable Quantum Cryptography

This thesis deals with a detailed study of the unconditional security of Continuous-Variable (CV) Quantum Key Distribution (QKD). We consider general communication architectures based on both point-to-point and end-to-end principle. For the first case we develop an extensive analysis of the unconditional security of both one-way and two-way protocol under several eavesdropping conditions. We describe an effective post-processing strategy, to apply to one-way communications, that allows to neutralise general coherent attacks. This result motivates us to formulate the conjecture that the "de Finettization" of the classical data, typically adopted in the asymptotic limit of many signals exchanged in order to reduce the attacks from a general coherent to a collective one, may not be necessary for Gaussian one-way communication. For what it concerns two-way protocols we show that after the reduction of the general attack using the de Finetti symmetrization, two-mode coherent attacks are the optimal eavesdropping. Our cryptanalysis shows that the parties can exploit a strategy, in combination with the results obtained on the security of one-way protocols against two-mode coherent attacks. In fact we prove explicitly that two-way communication with Gaussian continuous variables is immune to general coherent attacks. This is possible exploiting the random opening and closing of the circuit by Alice (ON/OFF switching), in the limit of many uses of the channel. We then conclude that collective attacks represent, strictly, the best strategy available to Eve. We have then studied the general security of one-way protocol considering not just lossy channel, but extending the cryptanalysis to all physically allowed canonical forms attacks. Finally we extended the study of thermal CV-QKD to two-way communication at different frequencies, in the framework of optimal collective attacks. In the last part of this thesis we focused on CV-QKD considering a modern end-to-end configuration. We then extend CV quantum cryptography to a network configuration. We develop a detailed cryptoanalysis of a communication scheme based on an untrusted relay, assisting the parties during the preparation of the secret key. We find the optimal eavesdropping and prove, both theoretically and experimentally, the feasibility of high-rate CV-QKD over metropolitan distances with of-the-shelves devices.