If a small block size, such as , is used, then the system is equivalent to a classical substitution
cipher. Such systems, as we have seen, are vulnerable to a statistical analysis of the
plaintext. This weakness is not inherent in the use of a substitution cipher but
rather results from the use of a small block size. If is sufficiently large and an
arbitrary reversible substitution between plaintext and ciphertext is allowed, then
the statistical characteristics of the source plaintext are masked to such an extent
that this type of cryptanalysis is infeasible.