Oana Graur

• Physical Layer Security (PHYSEC) uses the inherently random and reciprocal nature of physical (wireless) channels as a source for the generation of symmetric keys at the physical layer. The channel measurements of the two users involved in the key agreement, from which the keys are derived, are corrupted by independent noise components. This leads to key discrepancies which need to be reconciled between the two parties involved in the key agreement before the keys can be used for encryption and authentication. The focus of this thesis is the key reconciliation step of PHYSEC, with particular emphasis on the two main classes of reconciliation schemes: those based on introducing guard bands during the quantization process and those based on error-correcting codes. To that end, we first investigate the effect that the choice of the quantization method and associated parameters has on the key agreement rate and on the security of the system. Our findings show that for medium to high SNRs, good reconciliation performance can be achieved with guard-based methods without compromising security. When the legitimate users experience low SNRs, however, we have found guard-based methods to be unsuitable when used as the sole reconciliation method. This is because, in order to achieve the target reconciliation performance at low SNRs, they would require large guard-band widths, which would have a negative impact not only on the efficiency and key generation rate but also on security by providing an advantage to potential eavesdroppers. We propose a hybrid reconciliation method that combines guard bands with error-correcting codes which we specifically designed to achieve good performance at low SNRs. As a final result, we provide several Low-Density Parity-Check (LDPC) code ensembles with a Multi-Edge-Type (MET) structure, which we have specifically designed for wireless key reconciliation.