A well-known feature of quantum mechanics is the secure exchange of secret bit strings which can then be used as keys to encrypt messages transmitted over any classical communication channel. It is demonstrated that this quantum key distribution allows a much more general and abstract access than commonly thought. The results include some generalizations for the Hilbert space version of quantum key distribution, but base upon a general non-classical extension of conditional probability. A special state-independent conditional probability is identifed as (...) origin of the superior security of quantum key distribution and may have more profound implications for the foundations and interpretation of quantum mechanics, quantum information theory, and the philosophical question what actually constitutes physical reality. (shrink)