  Quantum key distribution (QKD) gradually has become a crucial element of
practical secure communication. In different scenarios, the security analysis
of genuine QKD systems is complicated. A universal secret key rate calculation
method, used for realistic factors such as multiple degrees of freedom
encoding, asymmetric protocol structures, equipment flaws, environmental noise,
and so on, is still lacking. Based on the correlations of statistical data, we
propose a security analysis method without restriction on encoding schemes.
This method makes a trade-off between applicability and accuracy, which can
effectively analyze various existing QKD systems. We illustrate its ability by
analyzing source flaws and a high-dimensional asymmetric protocol. Results
imply that our method can give tighter bounds than the
Gottesman-Lo-L\"utkenhaus-Preskill (GLLP) analysis and is beneficial to analyze
protocols with complex encoding structures. Our work has the potential to
become a reference standard for the security analysis of practical QKD.
