scenario where the optimal key rate may outperform the coherent information and key distribution may occur in the complete absence of entanglement (in any form, distillable or bound) as long as discord is non-zero. As a matter of fact, any prepare and measure QKD protocol whose security is based on non-orthogonal quantum states can be recast into an entanglement-free device-dependent form which is based on a classical-quantum state, with non-zero discord transmitted through the channel.

This discord-based representation is secure as long as the extra system $P$ is truly inaccessible to Eve, i.e., Alice’s and Bob’s private spaces cannot be accessed. Such a condition fails assuming side-channel attacks, where no noise can be trusted and $P$ becomes part of Eve’s systems. In this case, the secret-key rates are again dominated by the coherent information, which means that entanglement remains the crucial resource for device-independent QKD. For both device-independent QKD and ideal QKD (where system $P$ is absent), discord still represents an upper bound to the optimal secret-key rates achievable in direct or reverse reconciliation, with non trivial cases where this bound becomes tight.

In conclusion, quantum discord is a necessary resource for secure QKD. This is particularly evident in device-dependent 