  Information leakage in Wyner's wiretap channel model is usually defined as
the mutual information between the secret message and the eavesdropper's
received signal. We define a new quantity called "conditional information
leakage given the eavesdropper's received signals," which expresses the amount
of information that eavesdropper gains from his/her received signal. A benefit
of introducing this quantity is that we can develop a fast algorithm for
computing the conditional information leakage, which has linear complexity in
the code length $n$, while the complexity for computing the usual information
leakage is exponential in $n$. Validity of such a conditional information
leakage as a security criterion is confirmed by studying the cases of binary
symmetric channels and binary erasure channels.
