  This paper investigates the maximal secret communication rate over a wiretap
channel subject to reliability and secrecy constraints at a given blocklength.
New achievability and converse bounds are derived, which are uniformly tighter
than existing bounds, and lead to the tightest bounds on the second-order
coding rate for discrete memoryless and Gaussian wiretap channels. The exact
second-order coding rate is established for semi-deterministic wiretap
channels, which characterizes the optimal tradeoff between reliability and
secrecy in the finite-blocklength regime. Underlying our achievability bounds
are two new privacy amplification results, which not only refine the existing
results, but also achieve stronger notions of secrecy.
