Practical Issues in Quantum Cryptography

Practical Issues in Quantum Cryptography Feihu Xu Master of Applied Science Graduate Department of Electrical & Computer Engineering University of Toronto 2012 Cryptography plays a key role in our life ranging from computer passwords to electronic commerce to national military security. The most widely used modern encryption algorithm is the public-key algorithm. However, the security of all public-key algorithms relies on unproven computational assumptions. Hence, there is a potential loophole for a fast algorithm to compromise their security. Quantum cryptography or quantum key distribution (QKD), on the other hand, is an unbreakable encryption algorithm. In principle, QKD can provide unconditional security based on the fundamental laws of quantum physics. Unfortunately, real-life implementations of a QKD system may contain overlooked imperfections and thus violate the practical security of QKD. It is vital to explore these imperfections. In this thesis, I study two practical imperfections in quantum cryptography: i) Security loophole in QKD system because of imperfect quantum state preparation; ii) How to generate high-speed truly random numbers. i) Discovering security loophole in a commercial QKD system: One key assumption in QKD is that the sender (Alice) can prepare the required quantum states without errors. However, such an assumption may be violated in a practical QKD system. I perform a proof-of-principle experiment to demonstrate a technically feasible quantum attack that exploits such a security loophole in a commercial QKD system. The attack I utilize is called phase-remapping attack. ii) Generating high-speed truly random numbers: An essential element in QKD is a quantum random number generator (QRNG), which can generate true randomness by exploiting the indeterminism of quantum mechanics. However, due to the difficulties of measuring quantum effects in real setups, most approaches to QRNG are limited in speed. Here, I propose and experimentally demonstrate an ultrafast QRNG at a rate over 6 Gb/s, which is based on the quantum phase fluctuations of a laser. Moreover, I consider a potential adversary who has partial knowledge of the raw data and discuss how one can rigorously remove such partial knowledge with post-processing.