  This work constructs a discrete random variable that, when conditioned upon,
ensures information stability of quasi-images. Using this construction, a new
methodology is derived to obtain information theoretic necessary conditions
directly from operational requirements. In particular, this methodology is used
to derive new necessary conditions for keyed authentication over discrete
memoryless channels and to establish the capacity region of the wiretap
channel, subject to finite leakage and finite error, under two different
secrecy metrics. These examples establish the usefulness of the proposed
methodology.
