## CryptoDB

### Rolando L. La Placa

#### Publications

Year
Venue
Title
2021
EUROCRYPT
Formulating cryptographic definitions to protect against software piracy is an important research direction that has not received much attention. Since natural definitions using classical cryptography are impossible to achieve (as classical programs can always be copied), this directs us towards using techniques from quantum computing. The seminal work of Aaronson [CCC'09] introduced the notion of quantum copy-protection precisely to address the problem of software anti-piracy. However, despite being one of the most important problems in quantum cryptography, there are no provably secure solutions of quantum copy-protection known for {\em any} class of functions. We formulate an alternative definition for tackling software piracy, called quantum secure software leasing (QSSL). While weaker than quantum copy-protection, QSSL is still meaningful and has interesting applications in software anti-piracy. We present a construction of QSSL for a subclass of evasive circuits (that includes natural implementations of point functions, conjunctions with wild cards, and affine testers) based on concrete cryptographic assumptions. Our construction is the first provably secure solution, based on concrete cryptographic assumptions, for software anti-piracy. To complement our positive result, we show, based on cryptographic assumptions, that there is a class of quantum unlearnable functions for which QSSL does not exist. In particular, our impossibility result also rules out quantum copy-protection [Aaronson CCC'09] for an arbitrary class of quantum unlearnable functions; resolving an important open problem on the possibility of constructing copy-protection for arbitrary quantum unlearnable circuits.
2020
TCC
\noindent Knowledge extraction, typically studied in the classical setting, is at the heart of several cryptographic protocols. The prospect of quantum computers forces us to revisit the concept of knowledge extraction in the presence of quantum adversaries. \par We introduce the notion of secure quantum extraction protocols. A secure quantum extraction protocol for an NP relation $\rel$ is a classical interactive protocol between a sender and a receiver, where the sender gets as input the instance $\inst$ and witness $\witness$ while the receiver only gets the instance $\inst$ as input. There are two properties associated with a secure quantum extraction protocol: (a) {\em Extractability}: for any efficient quantum polynomial-time (QPT) adversarial sender, there exists a QPT extractor that can extract a witness $\witness'$ such that $(\inst,\witness') \in \rel$ and, (b) {\em Zero-Knowledge}: a malicious receiver, interacting with the sender, should not be able to learn any information about $\witness$. \par We study and construct two flavors of secure quantum extraction protocols. \begin{itemize} \item {\bf Security against QPT malicious receivers}: First we consider the setting when the malicious receiver is a QPT adversary. In this setting, we construct a secure quantum extraction protocol for NP assuming the existence of quantum fully homomorphic encryption satisfying some mild properties (already satisfied by existing constructions [Mahadev, FOCS'18, Brakerski CRYPTO'18]) and quantum hardness of learning with errors. The novelty of our construction is a new non-black-box technique in the quantum setting. All previous extraction techniques in the quantum setting were solely based on quantum rewinding. \item {\bf Security against classical PPT malicious receivers}: We also consider the setting when the malicious receiver is a classical probabilistic polynomial time (PPT) adversary. In this setting, we construct a secure quantum extraction protocol for NP solely based on the quantum hardness of learning with errors. Furthermore, our construction satisfies {\em quantum-lasting security}: a malicious receiver cannot later, long after the protocol has been executed, use a quantum computer to extract a valid witness from the transcript of the protocol. \end{itemize} \noindent Both the above extraction protocols are {\em constant round} protocols. \par We present an application of secure quantum extraction protocols to zero-knowledge (ZK). Assuming quantum hardness of learning with errors, we present the first construction of ZK argument systems for NP in constant rounds based on the quantum hardness of learning with errors with: (a) zero-knowledge against QPT malicious verifiers and, (b) soundness against classical PPT adversaries. Moreover, our construction satisfies the stronger (quantum) auxiliary-input zero knowledge property and thus can be composed with other protocols secure against quantum adversaries.

#### Coauthors

Prabhanjan Ananth (2)