  In this paper we set forth a novel connection between the Bennett 1992
protocol and a Bell inequality. This allows us to extend the usual
prepare-and-measure protocol to its entanglement-based formulation. We exploit
a recent result in the frame of device-independent quantum key distribution to
provide a simple, model-independent, security proof for the new protocol. The
minimum efficiency required for a practical implementation of the scheme is the
lowest reported to date.
