Security Analysis for BB84 Key Distillation

Key distillation, also referred to as classical post-processing, plays a pivotal role in Quantum Key Distribution (QKD) protocols. Key distillation encompasses numerous subroutines, making the analysis of its overall security implications potentially challenging for those outside the research community. In this paper, we elucidate the role of the key distillation phase in QKD from a security standpoint. We begin by analyzing the different components of the key distillation phase individually, followed by an examination of the process as a whole. We then calculate the bit strength of the produced key, assuming that an attacker is executing an intercept and resend attack. For our analysis, we employ a practical key distillation implementation linked to a decoy state BB84 protocol as a case study. Our findings suggest that the security of the final key, post the key distillation phase, hinges on several factors. These include the theoretical security of the implemented subroutines, the total information leakage throughout the process, and the choices of subroutine parameters. Given these assumptions, we can distill 287 secure bits for every 1000 bits that undergo the key distillation procedure.