  In pervasive Internet of Things (IoT) applications, the use of short packets
is expected to meet the stringent latency requirement in ultra-reliable
low-latency communications; however, the incurred security issues and the
impact of finite blocklength coding on the physical-layer security have not
been well understood. This paper comprehensively investigates the performance
of secure short-packet communications in a mission-critical IoT system with an
external multi-antenna eavesdropper. An analytical framework is proposed to
approximate the average achievable secrecy throughput of the system with finite
blocklength coding. To gain more insight, a simple case with a single-antenna
access point (AP) is considered first, in which the secrecy throughput is
approximated in a closed form. Based on that result, the optimal blocklengths
to maximize the secrecy throughput with and without the reliability and latency
constraints, respectively, are derived. For the case with a multi-antenna AP,
following the proposed analytical framework, closed-form approximations for the
secrecy throughput are obtained under both beamforming and
artificial-noise-aided transmission schemes. Numerical results verify the
accuracy of the proposed approximations and illustrate the impact of the system
parameters on the tradeoff between transmission latency and reliability under
the secrecy constraint.
