A simpler security proof for 6-state quantum key distribution

Six-state Quantum Key Distribution (QKD) achieves the highest key rate in the class of qubit-based QKD schemes. The standard security proof, which has been developed since 2005, invokes complicated theorems involving smooth Renyi entropies. In this paper we present a simpler security proof for 6-state QKD that entirely avoids Renyi entropies. This is achieved by applying state smoothing directly in the Bell basis. We furthermore show that the same proof technique can be used for 6-state quantum key recycling.