  Quantum cryptography is known for enabling functionalities that are
unattainable using classical information alone. Recently, Secure Software
Leasing (SSL) has emerged as one of these areas of interest. Given a target
circuit $C$ from a circuit class, SSL produces an encoding of $C$ that enables
a recipient to evaluate $C$, and also enables the originator of the software to
verify that the software has been returned -- meaning that the recipient has
relinquished the possibility of any further use of the software. Clearly, such
a functionality is unachievable using classical information alone, since it is
impossible to prevent a user from keeping a copy of the software. Recent
results have shown the achievability of SSL using quantum information for a
class of functions called compute-and-compare (these are a generalization of
the well-known point functions). These prior works, however all make use of
setup or computational assumptions. Here, we show that SSL is achievable for
compute-and-compare circuits without any assumptions.
  Our technique involves the study of quantum copy-protection, which is a
notion related to SSL, but where the encoding procedure inherently prevents a
would-be quantum software pirate from splitting a single copy of an encoding
for $C$ into two parts, each of which enables a user to evaluate $C$. We show
that point functions can be copy-protected without any assumptions, for a novel
security definition involving one honest and one malicious evaluator; this is
achieved by showing that from any quantum message authentication code, we can
derive such an honest-malicious copy-protection scheme. We then show that a
generic honest-malicious copy-protection scheme implies SSL; by prior work,
this yields SSL for compute-and-compare functions.
