  A central claim in quantum cryptography is that secrecy can be proved
rigorously, based on the assumption that the relevant information-processing
systems obey the laws of quantum physics. This claim has recently been
challenged by Bernstein (arXiv:1803.04520). He argues that the laws of physics
may also entail an unavoidable leakage of any classical information encoded in
physical carriers. The security claim of quantum key distribution would then be
vacuous, as the computation of the final secret key would leak its value.
However, as we explain in this short note, Bernstein's reasoning is based on a
too "classical" understanding of physics. It follows from known theorems about
fault-tolerant quantum computation that quantum physics avoids his conclusion.
