  The laws of quantum mechanics allow unconditionally secure key distribution
protocols. Nevertheless, security proofs of traditional quantum key
distribution (QKD) protocols rely on a crucial assumption, the trustworthiness
of the quantum devices used in the protocol. In device-independent QKD, even
this last assumption is relaxed: the devices used in the protocol may have been
adversarially prepared, and there is no a priori guarantee that they perform
according to specification. Proving security in this setting had been a central
open problem in quantum cryptography.
  We give the first device-independent proof of security of a protocol for
quantum key distribution that guarantees the extraction of a linear amount of
key even when the devices are subject to a constant rate of noise. Our only
assumptions are that the laboratories in which each party holds his or her own
device are spatially isolated, and that both devices, as well as the
eavesdropper, are bound by the laws of quantum mechanics. All previous proofs
of security relied either on the use of many independent pairs of devices, or
on the absence of noise.
