Quantum Key Distribution Post-processing

Quantum Key DistributionIn October 2019 Google claimed to have achieved quantum supremacy. This means that quantum computers are now capable of solving tasks that are impossible for classical computers to solve in a feasible time. Since before this goal was achieved, the growth of QC originated another problem: most classical cryptographic systems rely on heavy computational tasks, which are considered impossible to be solved in a reasonable time by classical computers, but can now be solved by quantum computers. Those systems include key exchange protocols and encryption protocols that are fundamental for modern communications. Researchers then started to study a new cryptography branch called Post Quantum Cryptography (PQC), which includes the sets of algorithms and protocols that can be considered secure even when facing quantum computers. Quantum cryptography is one of the main solutions developed to resist quantum computing. The idea is to use quantum computing to implement some cryptographic protocols both for quantum computers and classical ones. Quantum Key Distribution (QKD) is one of the solutions developed for key exchange. This is done by using quantum channels to implement key exchange between separate machines. Those protocols heavily rely on post-processing algorithms to achieve reliable security. One of the main challenges is finding efficient methods for error correction (ECC) and privacy amplification (PA). Those are the two main pillars of QKD post-processing and most of the metrics related to QKD protocols depend on them. For this reason, a lot of research has been done on both of those subjects. Error correction protocols are requested for two main reasons, the first one is the natural tendency of quantum bits to collapse into unwanted states due to their unstable nature and the fact that quantum channels are noisy. The second reason is that QKD algorithms always have to consider the possibility of having an eavesdropper (Eve) measuring some information traveling on a quantum channel, this resulting in adding extra errors due to Eve measurements. Many protocols have been developed for ECC, such as Winnow, LDPC and Cascade protocol, which is the most common and has proved to be efficient in QKD environments such as the BB84, E91 and B92 protocols. Considering the possibility of an eavesdropper acquiring knowledge on the data exchange brings with it the problem of how to make exchanged keys secure even if part of them is disclosed to an unwanted third party. Privacy amplification aims to solve this issue by applying classical cryptographic algorithms to the data. The drawback of this operation is the loss of part of the data that results in shortening the final key. Also, privacy amplification has been the bottleneck in terms of the speed of the whole QKD process until now, since it requires very high computational power. The thesis is aimed at analyzing the main ECC and PA protocols and proposing an efficient solution for QKD post-processing, implementing and testing the main algorithms to adapt them to a simulative platform developed to test QKD in a softwarized environment.