  Let $X$, $Y$ be two correlated discrete random variables. We consider an
estimation of $X$ from encoded data $\varphi(Y)$ of $Y$ by some encoder
function $\varphi(Y)$. We derive an inequality describing a relation of the
correct probability of estimation and the mutual information between $X$ and
$\varphi(Y)$. This inequality may be useful for the secure analysis of crypto
system when we use the success probability of estimating secret data as a
security criterion. It also provides an intuitive meaning of the secrecy
exponent in the strong secrecy criterion.
