  Rabi, Rivest, and Sherman alter the standard notion of noninvertibility to a
new notion they call strong noninvertibility, and show -- via explicit
cryptographic protocols for secret-key agreement ([RS93,RS97] attribute this to
Rivest and Sherman) and digital signatures [RS93,RS97] -- that strongly
noninvertible functions would be very useful components in protocol design.
Their definition of strong noninvertibility has a small twist (``respecting the
argument given'') that is needed to ensure cryptographic usefulness. In this
paper, we show that this small twist has a large, unexpected consequence:
Unless P=NP, some strongly noninvertible functions are invertible.
