  In this paper we investigate the problem of optimal cache placement under
secrecy constraints in heterogeneous networks, where small-cell base stations
are equipped with caches to reduce the overall backhaul load. For two models
for eavesdropping attacks, we formally derive the necessary conditions for
secrecy and we derive the corresponding achievable backhaul rate. In particular
we formulate the optimal caching schemes with secrecy constraints as a convex
optimization problem. We then thoroughly investigate the backhaul rate
performance of the heterogeneous network with secrecy constraints using
numerical simulations. We compare the system performance with and without
secrecy constraints and we analyze the influence of the system parameters, such
as the file popularity, size of the library files and the capabilities of the
small-cell base stations, on the overall performance of our optimal caching
strategy. Our results highlight the considerable impact of the secrecy
requirements on the overall caching performance of the network.
