Monogamy of entanglement as a necessary and sufficient condition for safe QKD in any physical theory

We show that the monogamy of entanglement is a sufficient phenomenon in every physical theory, if the quantum key distribution is to be safe on the grounds of such theory. To do so we present the QKD protocol that is safe in any physical theory under the assumption of the monogamous entanglement only. The necessity of this condition is also discussed.