  The existing cryptosystem based approaches for privacy-preserving consensus
of networked systems are usually limited to those with undirected topologies.
This paper proposes a new privacy-preserving algorithm for networked systems
with directed topologies to reach confidential consensus. As a prerequisite for
applying the algorithm, a structural consensus problem is formulated and the
solvability conditions are discussed for an explicitly constructed controller.
The controller is then implemented with encryption to achieve consensus while
avoiding individual's information leakage to external eavesdroppers and/or
malicious internal neighbors.
