Quantum cryptography makes it possible to expand a short shared key (of e.g.256 bits[1]) into an arbitrary long shared key. The novelty of quantumcryptography is that whenever a spy tries to eavesdrop the communication hecauses disturbances in the transmission of the message. Ultimately thisunavoidable disturbance is a consequence of Heisenberg's uncertainty principlethat limits the joint knowledge of complementary observables. Now, a problemremains: in order to initialize quantum key distribution, Alice and Bob need toshare a short shared key in order to be able to identify each otherunambiguously. Therefore a trusted courier is needed. We propose in this papera solution to the trusted courrier problem that was inspired by quantumcryptography. The idea is to encode the key that Alice sends to Bob intounstable nuclei in such a way that the message gets revealed only after thecourrier has delivered it to Bob. In this approach, we replace Heisenberguncertainties by another type of uncertainty, that characterizes the knowledgeof the time at which an unstable nucleus decays. As we shall show, this schememakes it possible to refresh a key even in the case that we do not trust thecourier who carries the key.
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