Secure Software Leasing
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We introduce the notion of secure software leasing (SSL): this allows for an authority to lease software to a third party with the security guarantee that after the lease expires, the third party can no longer produce any functionally equivalent software (including the original software) that can be run on the same platform. While impossible to achieve classically, this opens up the possibility of using quantum tools to realize this notion. In this work, we initiate a formal study of this notion and present both positive and negative results under quantum hardness of classical cryptographic assumptions. (1) Negative Result: We prove that it is impossible to construct SSL schemes for an arbitrary class of quantum unlearnable functions. In particular, our impossibility result also rules out quantum copy-protection [Aaronson CCC'09] for any class of quantum unlearnable functions; resolving an open problem on the possibility of constructing copy-protection for arbitrary quantum unlearnable circuits. Our techniques also rule out the existence of quantum VBB for classical circuits, answering an open problem posed by [Alagic and Fefferman arXiv'16]. Along the way, we introduce a notion called de-quantizable circuits and present the first construction of this notion which may be of independent interest. (2) Positive Result: On the other hand, we show that we can realize SSL for a subclass of evasive circuits (that includes natural implementations of point functions, conjunctions with wild cards, and affine testers).
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