  This paper presents a secrecy performance study of a wiretap communication
system with finite blocklength (FBL) transmissions over Rayleigh fading
channels, based on the definition of an average information leakage (AIL)
metric. We evaluate the exact and closed-form approximate AIL performance,
assuming that only statistical channel state information (CSI) of the
eavesdropping link is available. Then, we reveal an inherent statistical
relationship between the AIL metric in the FBL regime and the commonly-used
secrecy outage probability in conventional infinite blocklength communications.
Aiming to improve the secure communication performance of the considered
system, we formulate a blocklength optimization problem and solve it via a
low-complexity approach. Next, we present numerical results to verify our
analytical findings and provide various important insights into the impacts of
system parameters on the AIL. Specifically, our results indicate that i)
compromising a small amount of AIL can lead to significant reliability
improvements, and ii) the AIL experiences a secrecy floor in the high
signal-to-noise ratio regime.
