  Semantic communication is an important participant in the next generation of
wireless communications. Enabled by this novel paradigm, the conventional
Internet-of-Things (IoT) is evolving toward the semantic IoT (SIoT) to achieve
significant system performance improvements. However, traditional wireless
communication security techniques for bit transmission cannot be applied
directly to the SIoT that focuses on semantic information transmission. One key
reason is the lack of new security performance indicators. Thus, we have to
rethink the wireless communication security in the SIoT. As such, in the paper,
we analyze and compare classical security techniques, i.e., physical layer
security, covert communications, and encryption, from the perspective of
semantic information security. We highlight the differences among these
security techniques when applied to the SIoT. Novel performance indicators such
as semantic secrecy outage probability (for physical layer security techniques)
and detection failure probability (for covert communication techniques) are
proposed. Considering that semantic communications can raise new security
issues, we then review attack and defense methods at the semantic level.
Finally, we present several promising directions for future secure SIoT
research.
