Security proof for quantum key recycling with noise

Quantum Key Recycling aims to re-use the keys employed in quantum encryption and quantum authentication schemes. We consider QKR protocols where classical information is embedded in qubit states. A partial security analysis for such protocols was done in [LS2018]. In the current paper we introduce a number of small protocol modifications and provide a security proof. Our proof is based on a computation of the statistical distance between the real quantum state of the system and a state in which the keys are completely secure. This is a non-asymptotic result. It also determines how much privacy amplification is needed as a function of the bit error rate. It turns out that less privacy amplification is needed than suggested by the min-entropy analysis in [LS2018].