Entanglement is a well-known resource in quantum information, in particularit can be exploited for quantum key distribution (QKD). In this paper we definea two-way QKD scheme employing GHZ-type states of three qubits obtaining anextension of the standard E91 protocol with a significant increasing of thenumber of shared bits. Eavesdropping attacks can be detected measuringviolation of the CHSH inequality and the secret key rate can be estimated in adevice-independent scenario.
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