  One-way functions are used in modern cryto-systems as doortraps because their
inverse functions are supposed to be difficult to compute. Nonetheless with the
discovery of reversible computation, it seems that one may break a one-way
function by running a reversible computer backward. Here, we argue that
reversible computation alone poses no threat to the existence of one-way
functions because of the generation of ``garbage bits'' during computations.
Consequently, we prove a necessary and sufficient condition for a one-to-one
function to be a one-way in terms of the growth rate of the total number of
possible garbage bit configurations with the input size.
