  Device-independent quantum key distribution aims to provide key distribution
schemes whose security is based on the laws of quantum physics but which does
not require any assumptions about the internal working of the quantum devices
used in the protocol. This strong form of security, unattainable with standard
schemes, is possible only when using correlations that violate a Bell
inequality. We provide a general security proof valid for a large class of
device-independent quantum key distribution protocols in a model in which the
raw key elements are generated by causally independent measurement processes.
The validity of this independence condition may be justifiable in a variety of
implementations and is necessarily satisfied in a physical realization where
the raw key is generated by N separate pairs of devices. Our work shows that
device-independent quantum key distribution is possible with key rates
comparable to those of standard schemes.
