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Private Smart Contracts

There are two main directions in which successors to Bitcoin differentiate themselves – increased functionality and increased privacy. Ethereum [4] is a prime example of the former, whereas Zcash [2] and Monero focus on the latter. There has been much interest recently in merging these two paths to create a unified system with the privacy guarantees of Zcash and the expressability of Ethereum. Smart contract, originally proposed by Nick Szabo [3] are mechanisms by which people can agree to a set of rules that will be automatically enforced. When built on top of a cryptocurrency, smart contracts can govern monetary relationships between people and also pay out rewards or enforce penalties. Ethereum allows users to specify smart contracts, but with the caveat that all of the code for these contracts must be public. While the vision of private smart contracts is easy to connect to, subtle issues arise. Firstly, there’s a good reason why Ethereum compromised on privacy and Zcash compromised on functionality: there are major technical hurdles to combining them. Perhaps more importantly, though, upon closer examination it’s not even clear what properties we would want from a private smart contract system – what exactly would we like to keep private and whom are we hiding it from? In this talk, we will categorize various privacy modes for private smart contracts. We will also differentiate between several computational models for smart contracts and discuss the technical feasibility of each model. We will present recent and ongoing work–both ours and that of others–towards building private smart contracts, and we will lay out the technical barriers that we still need to cross. We intend to provide the necessary background in the first part of the talk to stimulate an engaging discussion in the latter. We will conclude by opening up a conversation on the applications and tradeoffs of private smart contracts. What will they be useful for? In particular, what novel privacy enhancing technologies can we build using private smart contracts as a primitive? Lastly, we will discuss the tradeoffs of private smart contracts and the difficulty they pose for law enforcement. As the PETS community has faced these issues before in the context of TOR, they might have useful experience in thinking about these questions, and we welcome a candid discussion on both the benefits and tradeoffs of this technology.