The wiretap channel has been devised and studied first by Wyner, and subsequently extended to the case with non-degraded general wiretap channels by Csiszar and Korner. Focusing mainly on the Poisson wiretap channel with cost constraint, we newly introduce the notion of reliability and security functions as a fundamental tool to analyze and/or design the performance of an efficient wiretap channel system. Compact formulae for those functions are explicitly given for stationary memoryless wiretap channels. It is also demonstrated that, based on such a pair of reliability and security functions, we can control the tradeoff between reliability and security (usually conflicting), both with exponentially decreasing rates as block length n becomes large. Two ways to do so are given on the basis of concatenation and rate exchange. In this framework, the notion of the δ secrecy capacity is defined and shown to attain the strongest security standard among others. The maximized vs. averaged security measures is also discussed.