  We reprove a result of Boppana and Lagarias: If Pi_2^P is different from
Sigma_2^P then there exists a partial function f that is computable by a
polynomial-size family of circuits, but no inverse of f is computable by a
polynomial-size family of circuits. We strengthen this result by showing that
there exist length-preserving total functions that are one-way by circuit size
and that are computable in uniform polynomial time. We also prove, if Pi_2^P is
different from Sigma_2^P, that there exist polynomially balanced total
surjective functions that are one-way by circuit size; here non-uniformity is
used.
