  Consider a protocol in which Belinda seals a (classical) message. She gives
the resulting sealed message to Charlie, who can either unseal and read the
message or return it unopened to Belinda. If he returns it unopened, Belinda
should be able to verify that Charlie neither read the message nor made a copy
that would allow him to read it later. Such a protocol is impossible with
classical cryptography: Charlie can copy a message and do anything he likes to
that copy without damaging the original. With quantum cryptography, on the
other hand, the no cloning theorem implies that Charlie cannot simply copy a
message and unseal the copy.
  Abstract In this paper, I prove that any conventional quantum cryptographic
protocol can give at best a very weak security guarantee. However, quantum
cryptography in conjunction with classical functions that can only be inverted
by humans (i.e. CAPTCHAs) can potentially give exponential security.
