  In this article, we analyse the relationship between the Bell violation and
the secure key rate of entanglement assisted quantum key distribution (QKD)
protocols. Specifically, we address the question whether Bell violation is
necessary or sufficient for secure communication. We construct a class of
states which do not show Bell violation, however, which can be used for secure
communication after local filtering. Similarly, we identify another class of
states which show Bell violation but can not be used for generating secure key
even after local filtering. The existence of these two classes of states
demonstrates that Bell violation as an initial resource is neither necessary
nor sufficient for QKD. Our work therefore forces a departure from traditional
thinking that the degree of Bell violation is a key resource for quantum
communication and brings out the role of local filtering.
