Aegis: Privacy-Preserving Market for Non-Fungible Tokens

Abstract

Non-fungible tokens (NFTs) are unique non-interchangeable digital assets verified and stored using blockchain technology. Quite recently, there has been a surging interest and adoption of NFTs, with sales exceeding <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\$}$</tex-math></inline-formula> 10 billion in the third quarter of 2021. Given the public state of Blockchain, NFTs owners face a privacy problem. More precisely, an observer can trivially learn the whole NFT collections owned by an address. For some categories of NFTs like arts and game collectibles, owners can sell them for a profit. However, popular marketplaces trade NFTs using public auctions and direct offers. Hence, an observer can learn about the new owner and the NFT purchase price. To tackle those problems, we propose <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Aegis</monospace> ,(Aegis is a shield carried by Zeus and Athena. It is a symbol of protection.) a protocol that allows users to add privacy to their NFTs ownership. In <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Aegis</monospace> users can swap NFTs for payment amounts in fungible tokens while hiding the details (i.e., involved parties, the NFTs, and the payment amounts). One of the main properties of <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Aegis</monospace> is its complete compatibility with existing NFT standards. We design <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Aegis</monospace> by leveraging zkSNARK proof system and smart contracts. We build an open-source prototype and perform experiments to evaluate <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Aegis</monospace> 's performance.