Upper bounds on the leakage of private data and an operational approach to Markovianity

We quantify the consequences of a private key leakage and private randomness generated during quantum key distribution. We provide simple lower bounds on the one-way distillable key after the leakage has been detected. We also show that the distributed private randomness does not drop by more than twice the number of qubits of the traced-out system. We further focus on irreducible private states, showing that their two-way distillable key is nonlockable. We then strengthen this result by referring to the idea of recovery maps. We further consider the action of a special case of side channels on some of the private states. Finally, we connect the topic of (non-)Markovian dynamics with that of hacking. In particular, we show that an invertible map is non-CP-divisible if and only if there exists a state whose the key witnessed by a particular privacy witness increases in time. This complements the recent result of J. Kolodyn ' ski et al. [Phys. Rev. A 101, 020303(R) (2020)] where the log-negativity was connected with the (non-)Markovianity of the dynamics.