quantum correlations are today quantified by quantum discord [@VedralRMP], a new quantity which has been studied in several contexts with various operational interpretations and applications, including work extraction [@Demon], quantum state merging [Merging1,Merging2]{}, remote state preparation [@Remote], quantum metrology [@Qmetrology], discrimination of unitaries [@Gu] and quantum channel discrimination [@Qillumination].

In this paper, we identify the basic role of quantum discord in one of the most practical tasks of quantum information, i.e., quantum key distribution (QKD) [@Gisin]. The claim that quantum discord must be non-zero to implement QKD is intuitive. In fact, quantum discord and its geometric formulation are connected with the concept of non-orthogonality, which is the essential ingredient for quantum cryptography. That said, it is still very important to characterize the general framework where discord remains the only available resource for QKD. Necessarily, this must be a scenario where key distribution is possible despite entanglement being absent.

Here we show that this general scenario corresponds to device-dependent (or trusted-device) QKD, which encompasses all realistic protocols where the noise affecting the devices and apparata of the honest parties is assumed to be trusted, i.e., not coming from an eavesdropper but from the action of a genuine environment. This can be 