Cryptographic encryption scheme for solving the trusted courier problem based on metastable excited nuclei

Abstract

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.