  In this paper we provide secrecy metrics applicable to physical-layer coding
techniques with finite blocklengths over Gaussian and fading wiretap channel
models. Our metrics go beyond some of the known practical secrecy measures,
such as bit error rate and security gap, so as to make lower bound
probabilistic guarantees on error rates over short blocklengths both preceding
and following a secrecy decoder. Our techniques are especially useful in cases
where application of traditional information-theoretic security measures is
either impractical or simply not yet understood. The metrics can aid both
practical system analysis, and practical system design for physical-layer
security codes. Furthermore, these new measures fill a void in the current
landscape of practical security measures for physical-layer security coding,
and may assist in the wide-scale adoption of physical-layer techniques for
security in real-world systems. We also show how the new metrics provide
techniques for reducing realistic channel models to simpler discrete memoryless
wiretap channel equivalents over which existing secrecy code designs may
achieve information-theoretic security.
