  Semantic communications have emerged as a promising solution to address the
challenge of efficient communication in rapidly evolving and increasingly
complex Internet of Things (IoT) networks. However, protecting the security of
semantic communication systems within the distributed and heterogeneous IoT
networks is critical issues that need to be addressed. We develop a secure and
efficient distributed semantic communication system in IoT scenarios, focusing
on three aspects: secure system maintenance, efficient system update, and
privacy-preserving system usage. Firstly, we propose a blockchain-based
interaction framework that ensures the integrity, authentication, and
availability of interactions among IoT devices to securely maintain system.
This framework includes a novel digital signature verification mechanism
designed for semantic communications, enabling secure and efficient
interactions with semantic communications. Secondly, to improve the efficiency
of interactions, we develop a flexible semantic communication scheme that
leverages compressed semantic knowledge bases. This scheme reduces the data
exchange required for system update and is adapt to dynamic task requirements
and the diversity of device capabilities. Thirdly, we exploit the integration
of differential privacy into semantic communications. We analyze the
implementation of differential privacy taking into account the lossy nature of
semantic communications and wireless channel distortions. An joint
model-channel noise mechanism is introduced to achieve differential privacy
preservation in semantic communications without compromising the system's
functionality. Experiments show that the system is able to achieve integrity,
availability, efficiency and the preservation of privacy.
