  We consider the problem of secure communication over a noiseless 1-2-1
network, an abstract model introduced to capture the directivity characteristic
of mmWave communications. We focus on structured networks, which we refer to as
1-2-1 atomic networks. Broadly speaking, these are characterized by a source, a
destination, and three layers of intermediate nodes with sparse connections.
The goal is for the source to securely communicate to the destination in the
presence of an eavesdropper with unbounded computation capabilities, but
limited network presence. We derive novel upper and lower bounds on the secrecy
capacity of 1-2-1 atomic networks. These bounds are shown to be tighter than
existing bounds in some regimes. Moreover, in such regimes, the bounds match
and hence, they characterize the secrecy capacity of 1-2-1 atomic networks.
