  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 quantum
cryptography is that whenever a spy tries to eavesdrop the communication he
causes disturbances in the transmission of the message. Ultimately this
unavoidable disturbance is a consequence of Heisenberg's uncertainty principle
that limits the joint knowledge of complementary observables. Now, a problem
remains: in order to initialize quantum key distribution, Alice and Bob need to
share a short shared key in order to be able to identify each other
unambiguously. Therefore a trusted courier is needed. We propose in this paper
a solution to the trusted courrier problem that was inspired by quantum
cryptography. The idea is to encode the key that Alice sends to Bob into
unstable nuclei in such a way that the message gets revealed only after the
courrier has delivered it to Bob. In this approach, we replace Heisenberg
uncertainties by another type of uncertainty, that characterizes the knowledge
of the time at which an unstable nucleus decays. As we shall show, this scheme
makes it possible to refresh a key even in the case that we do not trust the
courier who carries the key.
